controlled 1.55-µm InAs Quantum Dots on InP Nanopyramids

alexandria.tue.nl

controlled 1.55-µm InAs Quantum Dots on InP Nanopyramids

Distribution, Number, and Polarization

Control of Site-ong>controlledong> ong>1.55ong>-ong>µmong> ong>InAsong>

ong>Quantumong> ong>Dotsong> on InP Nanopyramids

PROEFSCHRIFT

ter verkrijging van de graad van doctor

aan de Technische Universiteit Eindhoven,

op gezag van de rector magnificus, prof.dr.ir. C.J. van Duijn,

voor een commissie aangewezen door het College voor Promoties

in het openbaar te verdedigen

op dinsdag 8 november 2011 om 14.00 uur

door

Jiayue Yuan

geboren te Yangzhou, China


Dit proefschift is goedgekeurd door de promotor:

prof.dr. P.M. Koenraad

Copromotor:

dr. R. Nötzel

A catalogue record is available from the Eindhoven University of Technology

Library.

Yuan, Jiayue

Distribution, Number, and Polarization Control of Site-ong>controlledong> ong>1.55ong>-ong>µmong>

ong>InAsong> ong>Quantumong> ong>Dotsong> on InP Nanopyramids / by Jiayue Yuan. – Eindhoven,

Technische Universiteit Eindhoven, 2011. – Proefschrift.

ISBN: 978-90-386-2809-7

NUR: 926

Subject headings: metal-organic vapor phase epitaxy / selective-area epitaxy /

III-V semiconductors / semiconductor quantum dots / optical properties /

photoluminescence

The work presented in this thesis was carried out in the group of Photonics

and Semiconductor Nanophysics, at the Department of Applied Physics of the

Eindhoven University of Technology, Eindhoven, The Netherlands.

The work was supported by the Smart Mix Programme of the Netherlands

Ministry of Economic Affairs and the Netherlands Ministry of Education,

Culture and Science.

Printed by the printing service of the Eindhoven University of Technology.


To my dear wife, Yanting


Contents

1 Introduction.................................................................................................1

1.1 General introduction to quantum dots...................................................2

1.2 Applications of quantum dots.................................................................6

1.3 Scope of this thesis..................................................................................8

2 Epitaxial ong>Quantumong> ong>Dotsong> and Related Nanostructures ....................15

2.1 III-V compound semiconductors...........................................................16

2.1.1 Zinc-blende structure ................................................................16

2.1.2 Crystallographic planes ............................................................16

2.2 Growth method of self-assembled quantum dots ................................18

2.2.1 Stranski-Krastanow growth mode............................................18

2.2.2 Droplet epitaxy ..........................................................................20

2.3 Site-ong>controlledong> quantum dots on patterned templates........................21

2.3.1 Truncated pyramids ..................................................................21

2.3.2 V-grooves....................................................................................22

2.3.3 Nano-holes .................................................................................23

2.4 Summary ...............................................................................................25

3 Fabrication and Characterization Techniques .................................29

3.1 Substrate patterning: design and processing ......................................30

3.1.1 Optical lithography ...................................................................30

3.1.2 Electron beam lithography........................................................32

3.2 Sample growth ......................................................................................35

3.2.1 Metal-organic vapor phase epitaxy...........................................35

3.2.2 Selective-area epitaxy ...............................................................36

3.3 Structural characterization ..................................................................38

3.3.1 Atomic force microscopy............................................................38

3.3.2 Scanning electron microscopy...................................................39

3.4 Optical characterization .......................................................................41

3.4.1 Macro-photoluminescence spectroscopy ...................................41

3.4.2 Micro-photoluminescence spectroscopy....................................42

3.5 Summary ...............................................................................................44

4 ong>InAsong> ong>Quantumong> Dot Growth on Planar InP (100)................................47

4.1 Introduction...........................................................................................48

4.2 As/P exchange reaction in ong>InAsong>/InP materials....................................48

4.3 Effect of an ultrathin GaAs interlayer.................................................50

4.4 Experimental details ............................................................................52

4.5 Results and discussion..........................................................................53

4.5.1 GaAs interlayer thickness.........................................................53

4.5.2 Growth temperature..................................................................53

i


4.5.3 Group V/III ratio........................................................................55

4.5.4 Low-temperature InP cap layer thickness ...............................57

4.6 Summary ...............................................................................................60

5 Impact of Base Size and Shape on Formation Control of

Multifaceted InP Nanopyramids ................................................................65

5.1 Introduction...........................................................................................66

5.2 Experimental details ............................................................................66

5.3 Results and discussion..........................................................................67

5.3.1 Arrangement and (relative) size of pyramid top facets ...........67

5.3.2 Side facets competition for pyramids evolution .......................70

5.4 Summary ...............................................................................................73

6 Distribution, Number, and Polarization Control of Siteong>controlledong>

ong>1.55ong>-ong>µmong> ong>InAsong> ong>Quantumong> ong>Dotsong> on InP Nanopyramids ..........77

6.1 Introduction...........................................................................................78

6.2 Experimental details ............................................................................79

6.3 Results and discussion..........................................................................79

6.3.1 Distribution and number control of quantum dots..................79

6.3.2 Positioning four, three, two, and a single quantum dot ..........83

6.3.3 Controlling polarization anisotropy of quantum dots..............84

6.4 Summary ...............................................................................................89

7 Integration of Position Controlled ong>InAsong> ong>Quantumong> ong>Dotsong> into

Planar InP Structures ..................................................................................93

7.1 Introduction...........................................................................................94

7.2 Experimental details ............................................................................94

7.3 Results and discussion..........................................................................95

7.3.1 Regrowth around pyramids for active-passive integration .....95

7.3.2 Optical properties of integrated single quantum dot...............99

7.4 Summary .............................................................................................101

List of Abbreviations...................................................................................105

Summary........................................................................................................107

List of Publications .....................................................................................109

Acknowledgements......................................................................................113

Curriculum Vitae .........................................................................................115

ii


Chapter 1

Introduction

Semiconductor quantum dots (QDs), known as “artificial atoms”, are zerodimensional

nanostructures whose dimensions range from nanometers to tens

of nanometers. Their size is smaller than the de Broglie wavelength of

electrons, therefore quantum effects are manifested in the quantum dots. As a

result of quantum confinement, the energy states of electrons and holes are

discrete, like those of atoms. Within the past two decades after high-quality

QDs were experimentally realized they quickly entered the forefront of cutting

edge research leading to exciting physics and applications for the next

generation of quantum devices.

1


1.1 General introduction to quantum dots

Semiconductor materials have been extensively used in the last century

mainly as bulk compounds for conventional devices developed in the modern

electronics industry [1, 2]. The physical limitations of the downsizing of

electronic devices require the scientists, however, to develop new micro- and

nanoscale structures where the dimensionality of the structure plays the key

role for its functionality. In the new era of nanotechnology, quantum wells

(QWs), quantum wires (QWRs), and quantum dots (QDs), confining carriers

in one-, two-, and three-dimensions, respectively, have been explored as the

basic units exhibiting quantum properties of the materials. This requires

excellence in material processing, i.e, crystal growth of heterostructures

formed by elemental or compound semiconductors in ultra clean

environment. Epitaxial growth techniques such as molecular beam epitaxy

(MBE), metal-organic vapor phase epitaxy (MOVPE), and chemical beam

epitaxy (CBE) are commonly used to fabricate these high quality quantum

structures.

Semiconductor heterostructures composed of group III/V elements are

widely used in telecommunication applications. Their opto-electronic

properties make them ideal candidates for quantum functional devices based

on quantum size effects. ong>Quantumong> wells (QWs), which are used in lasing and

sensing applications, were the first quantum structures developed by

inserting a small band gap material, for instance GaAs, in a large band gap

material such as AlAs, where the thickness of the former is comparable to or

less than the de Broglie wavelength of the charge carriers in the bulk (order

of 10 nm for electrons in GaAs) [3]. The charge carriers are confined, in this

case, in one dimension with only discrete energy levels in this direction, but

are free to move in the other two directions. Based on this principle quantum

wires (QWRs) and quantum dots (QDs) are developed representing one- and

zero-dimensional quantum structures where the charge carriers are confined

in two- and three- dimensions, respectively, causing different density of

states and, thus, quantum mechanical properties of the structures. The

properties such as electron-hole interactions, spins of electron/holes, light

particles (photons), and lattice vibrations (phonons), hence, can be

manipulated in the material composed of single or ensembles of

nanostructures.

The number of electronic states per unit volume and energy is referred as

the density of states. In low-dimensional structures where the dimensions are

smaller than the de Broglie wavelength, quantization of the energy levels

takes place, which restricts the motion of electrons unlike the situation in

bulk where electrons are free to move in three dimensions.

Fig. 1.1 shows the energy dependence of the density of states due to the

different dimensionality of quantum confinement for bulk, QWs, QWRs, and

QDs. For bulk systems, the density of states, g(E), is continuous and g(E) is

2


proportional to E 1/2 . For QWs where electrons are confined in one dimension,

g(E) is constant for each subband. For QWRs g(E) is proportional to E -1/2 and

for QDs three-dimensional confinement is established and a Dirac -function

density of states is obtained.

The modification of the density of states due to the materials

dimensionality is responsible for many of the unique optical properties of

QDs and related devices, including higher materials and differential gain of

lasers, lower threshold current densities and a smaller temperature

sensitivity in their operation.

g(E)

3-D

(bulk)

g(E)

g(E) α E 1/2

2-D

(quantum well)

g(E) constant

E

E

g(E)

1-D

(quantum wire)

g(E)

g(E) α E -1/2

0-D

(quantum dot)

g(E) α δ(E)

E

E

Figure 1.1: The density of states of a three-, two-, one-, and zero-dimensional structure as

function of the energy. Images adapted from Ref. [4].

Another driving force for the study of the properties of zero-dimensional

systems is coming from the electronics industry. For electronic devices, there

was a dramatic development of the materials size scalability for integrated

circuits (IC) as reflected by Moore’s Law, which describes the general trend of

the number of transistors per microchip doubling approximately every two

years, shown in Fig. 1.2. However, this trend is predicted to be restricted

around 2020s due to the physical limitations of lithographic processes, thus,

strong alternative efforts for downsizing of electronic devices and materials to

prolong the trend have began.

3


Figure 1.2: Transistor counts for integrated circuits plotted against their dates of

introduction. The curve shows Moore's law - the doubling of transistor counts every two years

[5, 6].

The unique properties of semiconductor materials rely on their different

energy bandgaps and lattice constants, shown in Fig. 1.3, and the ability to

tune the material compositions and emission wavelengths leading to the

creation of tailored material properties and applications. For most of III-V

semiconductor materials (compounds of group III and V elements in the

periodic table such as GaAs, ong>InAsong>, and InP), it is well known that they have a

direct bandgap and therefore play the major role in the field of optoelectronic

devices. The GaAs and InP based materials and their compounds (ong>InAsong>,

InGaAs, InGaAsP, etc) are the most common materials used in telecom

applications covering the emission wavelengths of the 1.3 ong>µmong> and ong>1.55ong> ong>µmong>

regions.

4


Figure 1.3: Bandgap energy and lattice constant of various III-V semiconductors at room

temperature [7].

The lattice constant of semiconductor materials plays a major role in the

fabrication mechanisms of QDs. The most frequent technique for QD

fabrication is the growth of a material with larger lattice constant and

smaller band gap energy on top of a material with smaller lattice constant

and higher band gap energy. Due to the lattice mismatch, hence, strain,

three-dimensional (3-D) islands, namely QDs develop after a certain critical

thickness is reached. As an example ong>InAsong> quantum dots in a matrix of GaAs

are introduced here. The quantum dots are grown in the Stranski-Krastanow

growth mode [8] in which a thin layer of ong>InAsong> is grown on top of GaAs. The

lattice constant of ong>InAsong> is larger as compared to that of GaAs. Consequently,

when the thickness of the ong>InAsong> layer reaches a certain critical thickness

(typically 1.7 monolayers), a strain induced transition from two-dimensional

layer-by-layer growth to three-dimensional island growth occurs. This results

in the formation of self-assembled quantum dots on top of a thin wetting

layer. Fig. 1.4 shows the atomic force microscopy (AFM) image of uncapped

ong>InAsong>/GaAs QDs. The sizes of the QDs are ~70 nm in diameter and the dots

5


have a height of ~5 nm. However, the size, shape and composition of the dots

can be tuned by varying the growth conditions.

1 x 1 ong>µmong> 2

Figure 1.4: Atomic force microscopy (AFM) image of uncapped self-assembled ong>InAsong>/GaAs

quantum dots. The scan filed is 1 × 1 ong>µmong> 2 . Image adapted from Ref. [9].

1.2 Applications of quantum dots

The unique properties of QD structures allow the emergence of new

semiconductor applications with increased performance as well as new device

functionalities. Most of the expected improvements in device performance

originate from the change of the density of states, as discussed in section 1.1.

Based on these physical properties, various device applications have been

attempted using QDs in the active layer. Because of the delta-function like

density of states and the strong electron and hole confinement in QDs, they

offer a low and temperature-insensitive threshold current density for lasing

[10, 11]. The emission wavelength of such QD laser devices can be tuned to

1.3-ong>1.55ong> ong>µmong> compatible with telecommunication wavelengths for which the

attenuation in optical fibers is minimum. Fig. 1.5 shows the advancements in

6


the reduction of the threshold current density of bulk, QW, and QD lasers in

recent years showing the superior characteristics of QD lasers over their bulk

and QW counterparts [12].

Figure 1.5: Decrease of the threshold current density of semiconductor heterostructure

lasers with different dimensionality of the active layer. Image adapted from Ref. [12].

The use of QDs as the basic unit in single photon sources [13, 14] is also

an attractive application for metrology [15], quantum key distribution [16],

and linear optical quantum computing [17]. The QDs can be incorporated into

standard optoelectronic device structures allowing electrical operation of the

single photon sources and their integration into optical fiber communication

system.

However, QDs grown in Stranski-Krastanow (S-K) mode are distributed

randomly on the substrate surface with the density of ~10 10 cm -2 . In order to

isolate the single QD emission for advanced quantum functional devices such

as nanolasers and single photon sources, precise position and number control

of a few down to a single QD is required. This can be realized by pre-defined

QD nucleation on truncated pyramids formed by selective area growth in

dielectric mask openings and has been demonstrated for the ong>InAsong>/GaAs

material system by metal-organic vapor phase epitaxy (MOVPE) [18] and

molecular beam epitaxy (MBE) [19] and for the ong>InAsong>/InP material system by

7


chemical beam epitaxy (CBE) [20] and MOVPE [21]. In addition, control of

the QD shape and consequently linear polarization of the emission is crucial

for entangled photon sources [22]. They require symmetric QDs with zero

degree of polarization (DOP) and, hence, vanishing fine structure splitting

(FSS). This makes ong>InAsong>/InP QDs particularly promising exhibiting an about

10 times smaller FSS for a certain shape than ong>InAsong>/GaAs QDs [23]. Various

approaches to control the polarization of QDs have been demonstrated

employing (111) oriented substrates [24], thermal annealing [25], and inplane

magnetic fields [26].

Moreover, 2-D photonic crystals (PCs) containing QDs in order to create

active nanocavities [27, 28] coupled to ultra compact optical waveguides is

another achievement in semiconductor physics. In such QD-nanocavities the

charge carriers and light particles are both confined in 3-D structures

enabling the development of nanoscale integrated optical systems and

employing the optical properties of single QDs [29]. In addition, control of the

QD distribution is highly desirable to maximize the overlap with the photon

field of a specific optical mode with certain size and shape [30, 31]. Therefore,

PCs with QDs are promising candidates for advanced quantum functional

devices. From a more fundamental point, such structures are also of interest

for studies of the modification of the QD spontaneous emission rate through

the Purcell effect, and the study of quantum optics [32].

In order to develop such novel applications, high-quality QD structures

showing distinct opto-electronic properties, i.e., PL emission with narrow

linewidths and distinct energies from single QDs, have to be developed. This

can be accomplished by improving the structural properties, such as size,

shape and composition which have substantial influences on the QD optical

properties. In addition, the distribution, number, and polarization control of

site-ong>controlledong> QDs is still another challenge for device applications at

telecom wavelengths.

Moreover, several other applications of semiconductor QDs have been

proposed and demonstrated such as solar cells [33], mid-infrared

photodetectors [34], and QDs coupled to the surface plasmon resonance of

metal nanocrystals for quantum nano photonics [35].

1.3 Scope of this thesis

The scope of this thesis concerns the growth, structural and optical properties

of site-ong>controlledong> ong>1.55ong>-ong>µmong> ong>InAsong> quantum dots on truncated InP nanopyramids

grown by selective-area metal-organic vapor phase epitaxy (MOVPE). The

overviews regarding low-dimensional nanostructures, particularly QDs,

including their fundamental properties and applications are introduced in

Chapter 1.

8


In Chapter 2, an introduction to the physics background of epitaxial

quantum dots (QDs) and related nanostructures is given. The material

properties of III-V compound semiconductors including the zinc-blende

structure and the crystallographic planes are discussed. The two most

common methods to grow self-assembled quantum dots namely the Stranski-

Krastanow (S-K) growth mode and droplet epitaxy are addressed. To

motivate the study of site-ong>controlledong> quantum dots in this thesis, the predefined

QD formation on patterned templates including truncated pyramids,

V-grooves, and nano-holes is presented. In this thesis, the approach to

achieve site-ong>controlledong> ong>1.55ong>-ong>µmong> ong>InAsong> QDs by pre-defined QD formation on

truncated InP pyramids is applied.

In Chapter 3, we provide an overview of the experimental techniques to

fabricate and characterize the InP pyramids containing ong>InAsong> QDs. To achieve

the site-ong>controlledong> QDs on pyramids by selective area epitaxy, substrate

patterning is required. The pattern design for the samples processed by

optical lithography and electron beam lithography (EBL) is discussed. An

introduction to the sample growth using selective-area metal-organic vapor

phase epitaxy (SA-MOVPE) is given. The structural and optical

characterization techniques are explained.

Chapter 4 demonstrates ong>InAsong> QD growth on planar InP (100) by MOVPE

with a thin GaAs interlayer. The structural and optical properties of the QDs

are investigated when varying the GaAs interlayer thickness, growth

temperature, and group V/III ratio and related to As/P exchange, the goal

being to obtain high-quality QDs with emission wavelength around ong>1.55ong>-ong>µmong>.

Furthermore, the optical quality of the ong>InAsong> QDs depending on the InP

capping procedure is evaluated. The thickness of the low-temperature grown

(Low-T) InP cap layer directly on the QDs is crucial for the QD

photoluminescence (PL) peak wavelength and efficiency when followed by

high-temperature capping. With increase of the Low-T cap layer thickness,

the PL peak redshifts and the efficiency increases up to a thickness of 8 nm

after which the PL peak wavelength stays constant and the efficiency

strongly decreases. This behavior is attributed to the balance between

stability of the QDs and defect diffusion toward the QDs.

In Chapter 5, we report the impact of base size and shape on the

evolution control of multifaceted InP (100) nanopyramids grown by selectivearea

MOVPE. The pyramid top surfaces are composed of a (100) center facet

surrounded by high-index {103} and {115} facets. Their arrangement and

(relative) size depend on the size and shape of the pyramid top area. For a

certain shape, only the (100) facet remains below a critical size of the top

area. The arrangement and (relative) size of the top facets in turn are

governed by the {110} and {111} side facets whose area (ratio) depends on the

pyramid base size and shape. This self-consistently determines the ratio of

the (100) top facet area and the sum of the {110} and {111} side facet areas as

well as the height of the pyramids.

9


In Chapter 6, distribution, number, and polarization control of siteong>controlledong>

ong>InAsong> QDs on InP pyramids are reported. The QDs preferentially

nucleate on the high-index facets determining position and distribution. The

QD number is reduced with shrinking top surface size. Positioning of four,

three, two, and a single QD is realized depending on the top surface shape

and size. Sharp emission from a single QD is observed at ong>1.55ong> ong>µmong>.

Furthermore, with increasing growth temperature the QDs elongate causing

strong linear polarization of the photoluminescence. With reduced pyramid

base/pyramid top area/QD number, the degree of polarization decreases,

which is attributed to the symmetric pyramid top, reaching zero for single

QDs grown at lower temperature. This control of linear polarization is

important for entangled photon sources operating in the ong>1.55ong>-ong>µmong> wavelength

region.

Finally, integration of position ong>controlledong> ong>InAsong> QDs into planar InP

structures is presented in Chapter 7. A smooth surface morphology is

obtained at elevated regrowth temperature due to suppression of threedimensional

growth on the pyramids. The height differences are less than 30

nm after nominal 700 nm InP regrowth at 640 °C. Most important, the

integrated QDs maintain good optical quality after regrowth for the

realization of integrated nanophotonic devices and circuits operating at

telecom wavelengths.

10


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13


Chapter 2

Epitaxial ong>Quantumong> ong>Dotsong> and Related

Nanostructures

This chapter gives an introduction to the physics background of epitaxial

quantum dots (QDs) and related nanostructures. The first section discusses

the material properties of III-V compound semiconductors including the zincblende

structure and the crystallographic planes. The second section

introduces two typical methods to grow self-assembled quantum dots namely

the Stranski-Krastanow growth mode and droplet epitaxy. To motivate the

study of site-ong>controlledong> quantum dots in this thesis, the pre-defined QD

formation on patterned templates including truncated pyramids, V-grooves,

and nano-holes is discussed in the third section.

15


2.1 III-V compound semiconductors

2.1.1 Zinc-blende structure

The III-V semiconductor materials are composed of a mixture of group III (in

this work In and Ga) and group V (in this work As and P) elements in the

Periodic Table of Elements. These III-V compound semiconductors crystallize

in the zinc-blende structure, as shown in Fig. 2.1. The crystal structure is

similar to that of diamond and silicon: each atom in the lattice is surrounded

by four nearest neighbors which lie with equal distance at the corners of a

tetrahedron. This lattice can be constructed from two interpenetrating facecentered

cubic (fcc) sublattices A and B, which are translated by a quarter of

a body diagonal (with respect to each other), i.e. a/4 [111], where a is the

lattice constant. The III-V lattice is formed in the same way, except that the

nearest-neighbor points are occupied by the elements from the different

groups. Sublattice A contains group III atoms, while the group V elements

are located in the sublattice B.

Figure 2.1: Schematic illustration of the zinc-blende structure with the lattice constant a.

Red and black balls correspond to the group III atoms and group V atoms, respectively.

2.1.2 Crystallographic planes

The crystallographic planes of a crystal are related to the atomic

arrangement in the cubic lattice. Fig. 2.2 shows the schematic views of the

atomic arrangement of the (111), (110), and (100) planes of the III-V

compound semiconductor. These arrangements are the same as those for the

16


diamond lattice except that the two different kinds of atoms (III and V)

occupy alternate positions.

(a) (111) cleavage

(b) (110) cleavage

(c) (100) cleavage

Figure 2.2: Schematic views of the atomic arrangements of the (a) (111), (b) (110), and (c)

(100) planes of the III-V compound semiconductor.

In the case of zinc-blende crystals, the III-V compounds have two

different {111} planes, named as (111)A and (111)B. The A plane consists of

only group III atoms while the B plane contains only group V atoms, as seen

in Fig. 2.2 (a). The spacing between these two planes is different. Along the

[111] direction, planes A and B follow each other and the distance between A

and B is larger than that between B and A. Due to the surface polarity of

these two planes, there is an attractive electrostatic force which makes it

difficult to separate along the {111} planes [1]. However, the {110} planes are

composed of equal numbers of III and V atoms, so there will be no overall

electrostatic force between the planes. The principal cleavage in the zincblende

crystals is found to be in a plane parallel to (110). The cleavage can

not occur parallel to {100} planes due to the doubly bonded atoms with small

spacing, as shown in Fig. 2.2 (c).

Fig 2.3 shows the basic directions and planes relative to the (100)

substrate. The {110} and {111} facets are important planes for nanostructure

formation. The angle between the {110} facets and the (100) surface is 45°

17


and that between the {111} facets and the (110) surface is 54.7° [2]. The

formation of the facets and their arrangement to form InP nanopyramids by

selective-area metal-organic vapor phase epitaxy (SA-MOVPE) is discussed in

Chapter 5.

Figure 2.3: The definition of the crystallographic directions and planes relative to the (100)

substrate.

2.2 Growth method of self-assembled quantum dots

2.2.1 Stranski-Krastanow growth mode

The Stranski-Krastanow (S-K) growth mode follows the characteristics of 3-D

island growth with a thin 2-D layer beneath when a layer of material with

larger lattice constant is epitaxially grown on top of a material with smaller

lattice constant. The important term to determine the S-K growth mode is

the lattice mismatch between the two epitaxial layers. For instance, the

common combination of ong>InAsong> and GaAs has a lattice mismatch of 7%,

whereas the ong>InAsong>/InP system has a lattice mismatch of 3%. The growth

initially progresses with a pseudomorphic growth mode forming a strained 2-

D film, the wetting layer. This causes the formation of compressive strain in

the layer, as described in Fig. 2.4. When this strain accumulates in the film

and reaches a certain amount at the critical thickness, the growing material

matrix experiences a transition from a 2-D layer-by-layer growth to a 3-D

18


island growth. Naturally, the critical thickness is determined by the selection

of materials of the heteroepitaxial systems.

In flux

As flux

wetting layer formation

ong>InAsong> QD formation through

strain relaxation

ong>InAsong> wetting layer

GaAs

GaAs

GaAs

Figure 2.4: Schematic illustration of ong>InAsong> quantum dot formation on GaAs substrate in the

Stranski-Krastanow growth mode.

Fig. 2.5(a) shows the atomic force microscopy image of typical ong>InAsong>/GaAs

(100) quantum dots grown in the S-K growth mode by molecular beam

epitaxy (MBE) having a surface density of 5 × 10 10 cm -2 . The cross-sectional

transmission electron microscopy (X-TEM) image of a single dot is shown in

Fig. 2.5(b). The QDs have a base width of 20 nm and height of 7 nm. The QD

size, shape, composition, and density can be tuned by changing the growth

parameters during growth. The QDs grown in the S-K growth mode are

indeed defect-free giving rise to a high quality of the structural and optical

properties. However, due to the self-assembly process, size fluctuations and

randomly distributed nucleation sites are unavoidable.

(a)

5 × 10 10 cm -2

(b)

1 μm

Figure 2.5: (a) Atomic force microscopy image of ong>InAsong> quantum dots grown on GaAs

substrate. (b) Cross-sectional transmission electron microscopy image of a single ong>InAsong>

quantum dot grown on GaAs substrate. Images adapted from Ref. [3].

19


2.2.2 Droplet epitaxy

For the direct formation of QDs, a novel growth method called droplet epitaxy

was developed in 1990s by Koguchi et al. [4]. Compared with the island

formation based on the S-K growth mode, the droplet epitaxy is useful for the

formation of QDs not only in lattice-mismatched but also in lattice-matched

systems such as GaAs/AlGaAs [5]. Moreover, the droplet epitaxy approach

provides new opportunities to fabricate novel configurations of quantum- and

nanostructures, such as 3-D quantum ring structures [6] and InGaAs QD

molecules [7].

The process of droplet epitaxy in the MBE chamber consists of forming

numerous group III element liquid metal droplets such as In on the substrate

surface first by supplying their molecular beams. Droplets form based on the

Volmer-Weber growth mode because the binding energy between adatoms is

greater than that of the adatoms and the substrate surface atoms. The

succeeding exposure of the group III droplets to a group V molecular beam

such as As4 leads to the formation of ong>InAsong> nanocrystals, shown in Fig. 2.6.

This transition process and interaction between group III and V materials is

typically known as “crystallization”.

Another advantage of the droplet epitaxy is the possibility of the

fabrication of QD structures without a wetting layer by controlling the

stoichiometry of the substrate surface just before the deposition of group-III

element droplets. The self-assembled QDs grown by the droplet epitaxy are

composed of pure materials, whereas the S-K QDs often exhibit intermixing.

However the crystalline and optical qualities of the QDs are degraded due to

the required low deposition temperatures, therefore, post-growth annealing is

essential to restore the crystalline quality.

In flux

In droplet formation

As flux

ong>InAsong> QD formation through

droplet crystalization

GaAs

GaAs

GaAs

Figure 2.6: Schematic illustration of ong>InAsong> quantum dot formation on GaAs substrate by the

droplet epitaxy method.

20


2.3 Site-ong>controlledong> quantum dots on patterned

templates

2.3.1 Truncated pyramids

Self-organized semiconductor quantum dots (QDs) have brought enhanced

performance to lasers and optical amplifiers due to their discrete energy

states, which is of particular importance for operation in the ong>1.55ong>-ong>µmong> telecom

wavelength region [8, 9]. These devices rely on QDs grown in the S-K mode

which are distributed randomly on the substrate surface. For advanced

quantum functional devices such as nanolasers and single photon sources,

however, precise site control of QDs is required.

Figure 2.7: SEM images of the ong>InAsong> QD(s) formed on top of truncated GaAs pyramids by

selective-area MOVPE. The width of the top (100) facet is (a) 35 (growth saturated), (b) 77,

(c) 115, and (d) 220 nm. Images adapted from Ref. [10].

21


This can be realized by pre-defined QD nucleation on truncated pyramids

formed by selective area growth in dielectric mask openings and has been

demonstrated for the ong>InAsong>/GaAs material system by Hahn et al. [10]. Fig. 2.7

shows the formation of site-ong>controlledong> ong>InAsong> QD(s) grown on truncated GaAs

pyramids by metal-organic vapor phase epitaxy (MOVPE). The deposited

ong>InAsong> amount is 1.7 monolayers for all samples. When the growth of the GaAs

pyramids is almost saturated (pinch-off) with a width of 35 nm [Fig. 2.7(a)],

the top (100) facet is not clearly defined for the QD formation. With

increasing the width of the (100) facet from 77 nm to 115 nm, and to 220 nm,

single [Fig. 2.7(b)], double [Fig. 2.7(c)], and six [Fig. 2.7(d)] QD(s) are formed,

respectively. In this thesis, the approach to achieve site-ong>controlledong> ong>1.55ong>-ong>µmong>

ong>InAsong> QDs by pre-defined QD formation on truncated InP pyramids is used.

2.3.2 V-grooves

Another approach to form site-ong>controlledong> QDs in V-grooves (inverted

pyramids) has been demonstrated by Hartmann et al. [11], as shown in Fig.

2.8. By this technique, prior to the epitaxial growth, the (111)B GaAs

substrates are patterned with arrays of inverted pyramids using wet

chemical etching through resist masks prepared by photolithography or

electron beam lithography. The preferential chemical etching exposes slowlyetched

{111}A crystallographic planes that define the facets of the inverted,

tetrahedral pyramids. Subsequent growth of a multilayer structure using

MOVPE forms a QD heterostructure within each inverted pyramid. In this

case, for MOVPE growth on nonplanar (111)B GaAs substrates patterned

with inverted tetrahedral pyramids, the side-walls composed of near-{111}A

facets exhibit a more efficient rate of metalorganic precursors decomposition

than the bottom facets [(111)B for the pyramids]. This causes a higher growth

rate on the sidewall facets, which leads to a shrinking of the width of the

bottom facets.

Under typical growth conditions, the growth rate on the GaAs (111)B

facet is negligible as compared with that on the {111}A facets until the system

reaches a self-limited profile. Then the growth rates become equal, i.e., the

growth rate on (111)B is the same as that on the sidewalls due to capillarity

contributions. After the system has reached a self-limited profile, growth of

an InGaAs layer is performed exhibiting a thickening at the bottom compared

to the sidewalls. A lens shaped QD is formed at the bottom of the pyramid

due to the combination of growth rate anisotropy and capillarity effects [11-

15].

22


(a) (b) (c)

Figure 2.8: Fabrication steps of site-ong>controlledong> quantum dots in V-grooves (inverted

pyramids): schematic illustration (lower panel) and scanning electron microscopy images

(upper panel). (a) Substrate patterning with arrays of inverted pyramids. (b) MOVPE growth

of the quantum dot heterostructures. (c) Substrate removal and formation of upright

pyramidal quantum dot heterostructures. Images adapted from Ref. [12].

2.3.3 Nano-holes

Another attempt to fabricate site-ong>controlledong> QDs in nano-holes was

demonstrated by Nakamura et al. [16]. The pre-patterned GaAs (100)

substrates with two-dimensional (2D) hole arrays were fabricated by electron

beam lithography and reactive ion etching. As shown in Fig. 2.9, the holes

were arranged in a square lattice with center-to-center distance of 200 nm.

The diameter and depth of the holes were 80 nm and 25 nm, respectively. The

sample structure commenced with four In 0.4 Ga 0.6 As layers grown at 530 °C

followed by two ong>InAsong> layers grown at 500 °C. The amounts of first, second,

third, and fourth In0.4Ga0.6As QD layers were 13, 11.5, 5.8, and 5.5 ML,

respectively. The final two ong>InAsong> QD layers were grown with the same

thickness of 1.4 ML. Between each layer, a 20-nm-thick spacer layer (10-nm-

GaAs/5-nm-Al 0.4 Ga 0.6 As/5-nm-GaAs) was deposited at 500 °C. Fig. 2.10 shows

transmission electron microscopy (TEM) images of the vertical alignment of

the laterally ordered ong>InAsong> and InGaAs QD arrays stacked on the 2D hole

arrays. This vertical alignment is due to the strain field created by the

underlying QDs. An extended review of this growth technique is given in Ref.

[17].

23


Figure 2.9: (a) Schematic diagram and (b) AFM image of the pre-patterned substrate with a

two-dimensional hole array. Images adapted from Ref. [16].

Figure 2.10: (a, b) Transmission electron microscopy (TEM) images of stacked ong>InAsong> and

InGaAs QD arrays at different magnifications. Images adapted from Ref. [16].

24


2.4 Summary

In this chapter, an introduction to the epitaxial quantum dots (QDs) and

related nanostructures was given. The material properties of III-V compound

semiconductors including the zinc-blende structure and crystallographic

planes were discussed. Two typical methods to grow self-assembled quantum

dots using the Stranski-Krastanow growth mode and droplet epitaxy were

explained. To motivate the study of site-ong>controlledong> quantum dots in this

thesis, the pre-defined QD formation on patterned templates including

truncated pyramids, V-grooves, and nano-holes was discussed.

25


Bibliography

[1] S. Adachi, Physical properties of III-V semiconductor compounds, (John

Wiley & Sons, Chichester, 1992).

[2] R. M. Bozorth, Phys. Rev. 26, 390 (1925).

[3] P. Bhattacharya and Z. Mi, Proc. of IEEE 95, 1723 (2007).

[4] N. Koguchi and K. Ishige, Jpn. J. Appl. Phys. 32, 2052 (1993).

[5] J. G. Keizer, J. Bocquel, P. M. Koenraad, T. Mano, T. Noda, and K.

Sakoda, Appl. Phys. Lett. 96, 062101 (2010).

[6] T. Mano, T. Kuroda, S. Sanguinetti, T. Ochiai, T. Tateno, J. Kim, T.

Noda, M. Kawabe, K. Sakoda, G. Kido, and N. Koguchi, Nano. Lett. 5,

425 (2005).

[7] J. H. Lee, Zh. M. Wang, N. W. Strom, Yu. I. Mazur, and G. J. Salamo,

Appl. Phys. Lett. 89, 202101 (2006).

[8] R. Nötzel and J. E. M. Haverkort, Adv. Funct. Mater. 16, 327 (2006).

[9] H. Wang, J. Yuan, P. J. van Veldhoven, T. de Vries, B. Smalbrugge, E. J.

Geluk, E. A. J. M. Bente, Y. S. Oei, M. K. Smit, S. Anantathanasarn,

and R. Nötzel, Electron. Lett. 44, 522 (2008).

[10] C.-K. Hahn, J. Motohisa, and T. Fukui, Appl. Phys. Lett. 76, 3947

(2000).

[11] A. Hartmann, L. Loubies, F. Reinhardt, and E. Kapon, Appl. Phys. Lett.

71, 1314 (1997).

[12] E. Kapon, in Lateral Alignment of Epitaxial ong>Quantumong> ong>Dotsong>, edited by O.

G. Schmidt (Springer, Berlin Heidelberg New York, 2007), p. 591.

[13] E. Pelucchi, M. Baier, Y. Ducommun, S. Watanabe, and E. Kapon, Phys.

Status. Solidi. b 238, 233 (2003).

[14] M. H. Baier, S. Watanabe, E. Pelucchi, and E. Kapon, Appl. Phys. Lett.

84, 1943 (2004).A. Hartmann, L. Loubies, F. Reinhardt, and E. Kapon,

Appl. Phys. Lett. 71, 1314 (1997).

26


[15] Q. Zhu, K. F. Karlsson, E. Pelucchi, and E. Kapon, Nano. Lett. 7, 2227

(2007).

[16] Y. Nakamura, O. G. Schmidt, N. Y. Jin-Phillipp, S. Kiravittaya, C.

Müller, K. Eberl, H. Gräbeldinger, and H. Schweizer, J. Cryst. Growth

242, 339 (2002).

[17] S. Kiravittaya, H. Heidemeyer, and O. G. Schmidt, in Lateral Alignment

of Epitaxial ong>Quantumong> ong>Dotsong>, edited by O. G. Schmidt (Springer, Berlin

Heidelberg New York, 2007), p. 489.

27


Chapter 3

Fabrication and Characterization

Techniques

This chapter gives an overview of the experimental techniques to fabricate and

characterize the InP pyramids containing ong>InAsong> quantum dots (QDs) studied

in this thesis. To achieve the site-ong>controlledong> QDs on pyramids by selective area

epitaxy, substrate patterning is required. The first section discusses the

pattern design for the samples processed by optical lithography and electron

beam lithography (EBL). The second section gives an introduction to the

sample growth using selective-area metal-organic vapor phase epitaxy (SA-

MOVPE). The third section describes the structural characterization

techniques including atomic force microscopy (AFM) and scanning electron

microscopy (SEM). For the optical characterization the macro- and microphotoluminescence

(PL) spectroscopy techniques are discussed in the forth

section.

29


3.1 Substrate patterning: design and processing

3.1.1 Optical lithography

The fabrication of InP pyramids containing ong>InAsong> QDs is a multi-step, time

consuming procedure. The inherent key to achieve the site control is the first

step: substrate patterning. It mainly consists of optical lithography and

electron beam lithography (EBL) processes.

Optical lithography is a process used to transfer the pattern from a mask

onto the substrate. The Ultra-Violet (UV) light source is used to expose the

photoresist in unmasked areas. Fig. 3.1 shows the schematic drawing of the

pattern aligned in a quarter of a two-inch InP wafer. After optical

lithography and reactive ion etching (RIE) processes, the markers and 24

writing fields (WF) with the size of 190 × 160 ong>µmong> for the further EBL process

are created.

OF

2 mm

WF11

WF24

IF

3 mm

WF00

WF12

Globe markers

Figure 3.1: Schematic drawing of the pattern aligned in a quarter of a two-inch InP wafer.

The 24 writing fields (WF00-WF24) are defined for the EBL process.

30


The steps of the substrate patterning by optical lithography are

schematically shown in Fig. 3.2. The InP (100) substrate is cleaned in an

oxygen plasma at 300 W for 10 minutes and etched in H 3 PO 4 :H 2 O (1:10)

solution for 2 minutes to remove the oxide layer [Fig. 3.2(a)]. After rinsing

the wafer in deionized (DI) water and blowing dry with nitrogen, a 100 nm

thick SiNx layer is deposited at 300 °C using plasma-enhanced chemicalvapor

deposition (PECVD) [Fig. 3.2(b)]. After that, the sample is cleaned

again and a thin film of primer (HMDS) is deposited to improve the adhesion

of the photoresist. Then, a layer of positive HPR504 photoresist is spin-coated

and soft baked at 100 °C [Fig. 3.2(c)]. Subsequently, the sample is exposed

with Mask-I under the UV light for 3.3 seconds [Fig. 3.2(d)]. The illuminated

parts of the resist are removed by developing in PLSI: H2O (1:1) solvent for

75 seconds [Fig. 3.2(e)]. The next step is to transfer the patterning from the

resist to the SiNx layer by performing reactive ion etching (RIE) [Fig. 3.2(f)].

The remaining photoresist is stripped in oxygen plasma at 300 W for 30

minutes [Fig. 3.2(g)]. Thus, the writing fields (190 × 160 ong>µmong>) for the further

EBL are produced in this optical lithography process.

Mask-II

Mask-I

Markers

Figure 3.2: Schematic drawing of the processing steps for fabrication of the patterned

substrate by optical lithography.

31


An additional dipping in 1% HF solution for 1 or 2 seconds removes any

residues on the sample surface. In the second optical lithography process

[Figs. 3.2(h)-3.2(l)], eight markers at the four corners of each writing field

[Fig. 3.3] are defined with Mask-II under the UV exposure. The InP in the

markers areas is etched by RIE for 16 minutes, resulting in an etched depth

of 1 ong>µmong> for the clear visibility in the subsequent EBL process.

3.1.2 Electron beam lithography

Electron beam lithography (EBL) refers to a maskless lithographic process

that uses a focused beam of electrons to form the patterns needed for

material deposition on (or removal from) the wafer. The primary advantage of

EBL is that it works in a way to beat the diffraction limit of light and enables

small features in the nanometer regime. It offers higher patterning resolution

than the optical lithography, allowing flexible modification of the designed

patterns.

Fig. 3.3 shows the mask openings created in a writing field (190 × 160

ong>µmong>) by EBL and RIE processes. Different opening shapes are fabricated such

as circular, triangular, square, and elliptical. The center-to-center distance is

10 ong>µmong>. Fig. 3.4 illustrates the schematic drawing of the processing steps for

fabrication of the mask openings by EBL.

First, the sample is cleaned in the oxygen plasma at 300 W for 10

minutes and etched in H 3 PO 4 :H 2 O (1:10) solution for 2 minutes to remove the

oxide layer [Fig. 3.4(a)]. After dipping in 1% HF solution, the electron beam

resist ZEP520 is spin-coated onto the sample using a rotation speed of 5000

rpm for 40 seconds [Fig. 3.4(b)]. The resist layer is then soft baked at a

ramped-up temperature from 100 to 150 °C for 4 minutes and subsequently

hard baked at 200 °C for 2 minutes.

Second, the resist is exposed by the electron beam using an acceleration

voltage of 30 kV and a dose of 70 µC/cm 2 [Fig. 3.4(c)]. The pattern is

developed by immersing the sample in ZEP developer for 1 minute and MIKB

stopper for 45 seconds [Fig. 3.4(d)].

After a short cleaning step in the oxygen plasma at 100 W for 1.5

minutes, the unmasked SiNx layer is etched with RIE [Fig. 3.4(e)] and finally

the ZEP resist is completely removed in the oxygen stripper at 300 W for 30

minutes [Fig. 3.4(f)]. Finally, the prepatterned substrate for sample growth is

cleaned in H3PO4:H2O (1:10) solution for 2 minutes and rinsed in DI water to

remove any surface contaminants.

32


190 ong>µmong>

Markers

160 ong>µmong>

SiN x

InP substrate

Circular

Triangular Square Elliptical

10 ong>µmong>

Figure 3.3: The mask openings created in a writing field (190 × 160 ong>µmong>) by EBL and RIE

processes. Different opening shapes are fabricated such as circular, triangular, square, and

elliptical. The center-to-center distance is 10 ong>µmong>.

33


Figure 3.4: Schematic drawing of the processing steps for fabrication of the mask openings

created in a writing field (190 × 160 ong>µmong>) by electron beam lithography.

34


3.2 Sample growth

3.2.1 Metal-organic vapor phase epitaxy

For successful semiconductor device fabrication the epitaxial growth process

must be ong>controlledong> very accurately regarding both layer composition and

thickness. For advanced nanostructures control even at the submonolayer

level is required with high uniformity over the entire wafer surface and

reproducible from run-to-run. Importantly, the electronic and optical

epitaxial materials quality must be excellent to ensure good device

performance.

Metal-organic vapor phase epitaxy (MOVPE), also called metal-organic

chemical vapor deposition (MOCVD), is one of the most important epitaxial

growth techniques for fabricating the III-V semiconductor heterostructures.

The advantage of MOVPE growth is the flexibility of the vapor sources and

the ability to grow mass products which makes this technique very suitable

for the industry. Epitaxial structures for practical InP-based devices, such as

high electron-mobility transistors (HEMTs) and optoelectronics devices, are

grown primarily by MOVPE.

The MOVPE systems use gaseous (vapor phase) source materials, which

are transported by a carrier gas (commonly Hydrogen). The gas flow in

MOVPE is viscous and the chemicals reach the substrate by diffusion

through a stagnant boundary layer. Reactor pressures range from 10 mbar

(Low-pressure MOVPE) to 1000 mbar (atmospheric pressure MOVPE), i.e. no

ultra-high vacuum is required. To avoid gas turbulences, low-pressure

horizontal or vertical reactor systems are used. A typical reactor for research

work is composed of a radio-frequency (RF) or halogen lamp heated susceptor

in a cold-wall quartz reactor. Trimethylindium (TMIn) and trimethylgallium

(TMGa) are generally used as group III precursors. For group V precursors,

tertiarybutylarsine (TBAs) and tertiarybutylphosphine (TBP) are becoming

popular as alternative sources to traditional group V hydrides (AsH 3 and

PH 3 ) because they pyrolyse more rapidly at lower temperature, and are less

toxic. Table 3.1 lists the chemical formulas of the precursors used in MOVPE

growth. Moreover, the handling of these liquid source bubblers is much

easier, and low V/III ratios and low-temperature growth can be achieved.

Decomposition of the group III metal organic compounds (TMIn, TMGa) and

the group V hydrides (TBAs, TBP) occurs by partial pyrolysis in the gas

phase and further dissociation on the heated substrate surface. The material

flux into the reactor is regulated by electronic mass flow controllers or by

pressure controllers, while valves are used for gas switching.

In atmospheric or low-pressure MOVPE, the group III alkyls in the gas

stream of H 2 are already partially dissociated. They then diffuse through a

stagnant boundary layer above the heated substrate and further dissociation

yields the atomic group III elements. These migrate into the appropriate

35


lattice sites and deposit epitaxially by capturing a group V atom, either at the

heated substrate surface or in the gas stream. For usual growth

temperatures, the growth rate is limited by the diffusion rate of group III

alkyls through the boundary layer. Fig. 3.5 depicts the schematic drawings of

the growth mechanisms in MOVPE. An extended review of this growth

technique is given in Ref. [1].

Abbreviation Chemical name Chemical formula

TMIn trimethyl-indium (CH 3 ) 3 In

TMGa trimethyl-gallium (CH3)3Ga

TBAs tertiarybutyl-arsine (CH 3 ) 3 CAsH 2

TBP tertiarybutyl-phosphine (CH3)3CPH2

Table 3.1: The chemical formulas of the source materials used in MOVPE growth.

(a)

(b)

Figure 3.5: Schematic drawings of (a) the basic processes inside the MOVPE system and (b)

the growth kinetics involved in MOVPE. Images adapted from Ref. [2].

3.2.2 Selective-area epitaxy

In selective-area epitaxial growth, the term of selective means that deposition

of material takes place only in pre-defined opened areas and that the

remaining part of the masked surface ideally is free from material deposition.

This can be achieved by using a surface layer, usually a dielectric mask such

as a thin film layer (typically 50-100 nm) of SiNx or SiO 2 on which the

precursors do not decompose. Patterning of the dielectric layer is usually

36


processed by photolithography and etching (wet chemical or reactive ion

etching).

In selective-area metal-organic vapor phase epitaxy (SA-MOVPE), there

are three contributions to the growth in the opening region, including the

surface migration, lateral vapor phase diffusion, and vertical vapor phase

diffusion, as shown in Fig. 3.6. The vertical vapor phase diffusion primarily

supplies the source materials for the growth. The lateral vapor phase

diffusion comes from the re-entering of growth III source materials from the

mask back to the mass diffusion (boundary) layer which eventually diffuse

laterally to a growth region due to the concentration gradient. The surface

migration arises from source material migrating on the surface of the

dielectric mask region to the opening with exposed semiconductor layer.

On the prepatterned substrate, the non-uniform depletion of the gas

species in the boundary layer leads to concentration gradients in the region

above the mask and that above the semiconductor. It affects the growth rate

as well as the material properties of the grown layer in the opening different

to a planar and bare substrate in the same growth environment. In general,

the local growth rate is enhanced with decreasing the opening size and

increasing the mask area surrounding the opening due to the gas phase

diffusion and surface migration [3].

Figure 3.6: Principles of the surface migration and the vapor phase diffusion in SA-MOVPE.

37


3.3 Structural characterization

3.3.1 Atomic force microscopy

An Atomic Force Microscopy (AFM) is a tool to visualize the surface

morphology of the nanostructures. The advantages include the ambient

operational conditions, stability, and relative large scan speeds, providing an

easy and fast characterization process. It consists of a cantilever with a small

sharp tip (typical radius of ~ 20 nm) at its end. The AFM relies on the forces

between tip and sample. In order to observe these forces, it is necessary to

place the tip close to the surface of the sample. The forces are not measured

directly, but the deflection of the cantilever is measured. A laser beam shines

on the back of the cantilever and is reflected during the scan. The reflected

beam goes to an array of photodiodes which is used as position detector. A

feedback mechanism is employed to adjust the tip-to-sample distance to

maintain a constant force between the tip and the sample. By scanning the

probe across the surface and measuring the voltage applied to the piezo a

topographic image of the surface is obtained.

Fig. 3.7 shows three different operation modes used in AFM: (a) contact

mode, (b) non-contact mode, and (c) tapping mode. In contact mode the tip is

in contact (area of repulsive force) with the surface, either a constant height

of the cantilever or a constant force between tip and surface can be used. In

this mode there is the risk of damaging the sample and due to the

deformation of the tip and the sample no atomic resolution is obtainable, but

it is very fast. In non-contact mode the cantilever oscillates at its resonance

frequency above the surface (attractive force between tip and surface), this

mode is very slow but high resolution is obtainable. The tapping mode, also

called “intermittent contact mode”, combines the advantages of the previous

two techniques. Here the cantilever oscillates at a frequency slightly below

the resonance frequency and tips the surface, the amplitude ranges from 20

to 100 nm. The oscillation of the cantilever is kept constant with the aid of a

feedback loop.

In the tapping mode there are several possibilities to get information

about the sample. Either the height of the cantilever can be measured with

the laser and adjusted with the piezoelectric crystal and displayed by using a

specific color for each height or the amplitude of the oscillating cantilever can

be measured with the same laser. The amplitude changes due to the

interaction between the tip and the sample. In this thesis the height and

amplitude measurements in tapping mode AFM in air are used to

characterize the uncapped quantum dots and related nanostructures.

38


Figure 3.7: Three operation modes used in AFM: (a) contact mode, (b) non-contact mode, and

(c) tapping mode.

3.3.2 Scanning electron microscopy

The basic principle of scanning electron microscopy (SEM) is to scan the

sample with a high energetic, collimated electron beam and at the same time

collect the electrons emanating from the sample surface via a detector. It is

mainly used to study topography of the samples. The interaction of an

electron beam with surface features results in a contrast which is translated

into the image by dedicated software and electronics. A typical SEM system

consists of an electron gun, usually of the tungsten-filament emission type,

generating electrons and accelerating them up to the energy of ~ 40 keV. Two

or more condenser lenses then focus the electron beam with a very fine focal

spot size of 0.4 nm to 5 nm. The electrons interacting with the sample

dissipate their energy, resulting in secondary electron emissions from the

sample surface, and some of the inelastically scattered electrons are

backscattered out of the sample. Scanning coils scan the electron beam over

the surface area and a detector in the vacuum chamber collects the secondary

electrons for imaging. The contrast in SEM images originates from surface

topography and chemical composition of the sample.

In addition, the sample surface has to be clean in order to avoid any

contamination of the system operating at high vacuum. It is also required to

be conductive to avoid charging. Non-conductive samples are normally precoated

with a thin metallic film such as gold for reducing the charging.

In this thesis work, a JEOL-7500FA SEM operating in the secondary

electron detection mode is used. Fig. 3.8 shows the SEM images of (a) the

globe markers, (b) the writing field markers, (c) the square-based InP

pyramids containing ong>InAsong> QDs, and (d) the waveguide structures connected

to the writing field (190 × 160 ong>µmong>) which, however, have no function for the

experiments.

39


(a)

(b)

Depth: 1 ong>µmong>

(c)

(d)

Waveguides

Writing field

(190 × 160 ong>µmong>)

Figure 3.8: Top-view SEM images of (a) the globe markers and (b) the writing field markers

created by optical lithography and RIE etching. The InP etching depth is 1 ong>µmong>. (c) Side-view

SEM image of the square-based InP pyramids containing ong>InAsong> QDs. (d) Top-view SEM image

of the waveguide structures connected to the writing field (190 × 160 ong>µmong>).

40


3.4 Optical characterization

3.4.1 Macro-photoluminescence spectroscopy

Photoluminescence (PL) spectroscopy is used to study the optical properties of

the semiconductor materials. It consists of using an excitation source, mostly

a laser light at a fixed wavelength, and measuring the spectral distribution of

the luminescence. The laser light with higher energy than the bandgap of the

semiconductor creates electron-hole pairs which then recombine via either a

radiative (luminescence) or a non-radiative recombination process. PL

spectra correspond to the intensity of the emitted light as a function of the

wavelength (or emission energy).

Fig. 3.9 shows the macro-photoluminescence setup used to analyze the

optical properties of a large ensemble of nanostructures. The excitation light

is provided by a Nd:YAG continuous-wave laser, emitting at 532 nm (~ 2.33

eV) with a power of 25 mW. Through multiple mirrors the laser light arrives

onto the sample surface. The sample is mounted in a helium cryostat under

high vacuum (< 10 -5 mbar), allowing the measurement temperature to be

ong>controlledong> between 5 K and room temperature (RT). In order to decrease the

excitation power density, optional neutral density (ND) filters are introduced

in the excitation path. The PL from the sample is collected by a

monochromator for spectral analysis. In front of the monochromator a filter

blocks the reflected laser light, and a lens focuses the PL onto the entrance

slit of the monochromator. The monochromator disperses the PL using a

grating mounted on a rotatable grating stage. The rotatable grating stage

contains three different gratings: a low resolution grating with 200

grooves/mm and a blaze wavelength of 1.7 ong>µmong>, a medium resolution grating

with 300 grooves/mm and a blaze of 2 ong>µmong>, and a high resolution grating with

600 grooves/mm and a blaze of 1.6 ong>µmong>. The dispersed light is focused onto

either an InGaAs photodiode array detector (detection range of 0.8-1.6 ong>µmong>) or

an InSb single channel detector (detection range of 1.2-3 ong>µmong>). Both detectors

are cooled to -100 °C by liquid nitrogen.

41


Figure 3.9: Schematic drawing of the macro-photoluminescence system. A green laser

excites the sample mounted in the helium flow cryostat. The PL from the sample is dispersed

by a single monochromator and detected by either an InGaAs or InSb detector.

3.4.2 Micro-photoluminescence spectroscopy

In order to study the photoluminescence (PL) of individual nanostructures a

high spatial resolution of the optical setup is required. In microphotoluminescence

spectroscopy a microscope objective with high numerical

aperture (NA) is used for collecting the emitted photons. Fig. 3.10 shows the

setup of micro-photoluminescence spectroscopy. The sample is mounted in a

helium cryostat under high vacuum (< 10 -5 mbar), allowing the measurement

temperature to be kept between 5 K and RT. The sample is excited by a

continuous-wave laser operating at 635 nm with a power of 2.8 mW. In order

to decrease the excitation power density, optional neutral density (ND) filters

are introduced in the excitation path. The laser is focused by a microscope

42


objective (100×, NA = 0.5) to a spot size of ~ 1 ong>µmong> in diameter. An external

illuminator (green LED) is used for imaging on the CCD camera. By precisely

adjusting the XYZ stage a clear image of the sample together with the

focused laser spot is observed by the CCD camera. The PL is collected by the

same objective lens and a beam splitter (a cold mirror), and then is dispersed

by a 0.25 m single monochromator and detected by an InGaAs photodiode

array. The detector is cooled to -100 °C by liquid nitrogen during the

measurement.

Figure 3.10: Schematic drawing of the micro-photoluminescence setup. A red laser excites

the sample mounted in the helium flow cryostat. Excitation and detection of the PL are

through a microscope objective with a spatial resolution of ~ 2 ong>µmong>. The PL is dispersed by a

0.25 m single monochromator and detected by an InGaAs photodiode array.

43


3.5 Summary

In this chapter, we have described the mask design for the substrate

patterning. The relevant processing steps to fabricate the pre-patterned

substrates including optical lithography and electron beam lithography were

discussed. The selective-area MOVPE growth process was explained. A brief

overview of the AFM and SEM techniques for structural characterization was

given. For the optical characterization the setups for macro- and microphotoluminescence

(PL) spectroscopy were discussed.

44


Bibliography

[1] G. B. Stringfellow, Organicmetallic vapor phase epitaxy: theory and

practice, (Academic Press, 2 nd edition, London, 1999).

[2] G. J. Davies, J. S. Foord, and W. T. Tsang, Chemical beam epitaxy and

related techniques, (John Wiley & Sons, Chichester, 1997).

[3] D. Zhou, Lateral positioning and wavelength control of InP based

quantum wires and dots, (ISBN-13: 978-90-386-2232-3, 2007).

45


Chapter 4

ong>InAsong> ong>Quantumong> Dot Growth on Planar

InP (100)

ong>InAsong> quantum dots (QDs) are grown on planar InP (100) by metal-organic

vapor phase epitaxy (MOVPE) with a thin GaAs interlayer beneath the QDs.

The QDs’ structural and optical properties are investigated when varying the

GaAs interlayer thickness, growth temperature, and group V/III ratio and

related to As/P exchange, the goal being to obtain high-quality QDs with

emission wavelength around ong>1.55ong>-ong>µmong>. Furthermore, the optical quality of the

ong>InAsong> QDs depending on the InP capping procedure is evaluated. The thickness

of the low-temperature grown (Low-T) InP cap layer directly on the QDs is

crucial for the QD photoluminescence (PL) peak wavelength and efficiency

when followed by high-temperature capping. With increase of the Low-T cap

layer thickness, the PL peak redshifts and the efficiency increases up to a

thickness of 8 nm after which the PL peak wavelength stays constant and the

efficiency strongly decreases. This behavior is attributed to the balance

between stability of the QDs and defect diffusion toward the QDs. 1

1 Based on J. Cryst. Growth 315, 102 (2011) and J. Cryst. Growth 318, 570 (2011)

47


4.1 Introduction

Self-assembled semiconductor quantum dots (QDs) grown in the Stranski-

Krastanov (S-K) growth mode have demonstrated unique physical properties

and great potential for optoelectronic device applications [1]. ong>InAsong>/InP

quantum dots are ideally suited for optical devices operating in the ong>1.55ong>-ong>µmong>

wavelength region for applications in fiber based telecommunication systems

[2-4]. Controlling the emission wavelength of ong>InAsong>/InP QDs is a critical issue

due to the presence of As/P exchange during ong>InAsong> QD formation [5, 6]. This

results in too big QDs with too long emission wavelength, rough interfaces,

and photoluminescence (PL) line broadening [5, 7]. In order to efficiently

suppress the As/P exchange reaction, an ultrathin GaAs interlayer was

introduced on InGaAsP underneath the ong>InAsong> QDs [8-10].

In this chapter, we present the successful control of the emission

wavelength of ong>InAsong> QDs in the ong>1.55ong>-ong>µmong> wavelength region at room

temperature by inserting a GaAs interlayer underneath the QDs directly on

InP. For optimized growth parameters including the GaAs interlayer

thickness, growth temperature, and group V/III ratio, high quality ong>InAsong> QDs

on InP (100) are achieved.

Moreover, we report the effect of the thickness of the first lowtemperature

grown (Low-T) InP cap layer on the ong>InAsong> QD photoluminescence

(PL) wavelength and efficiency when followed by high-temperature (High-T)

capping. With increase of the Low-T cap layer thickness, the PL peak

redshifts and the efficiency increases up to a thickness of 8 nm after which

the PL peak wavelength stays constant and the efficiency strongly decreases.

This optimum Low-T cap layer thickness is attributed to the balance between

stability of the QDs during heat-up and defect diffusion toward the QDs

during High-T capping.

The As/P exchange reaction during ong>InAsong>/InP nanostructure growth is

discussed in section 4.2. Section 4.3 introduces an effective way to suppress

the As/P exchange and tune the emission wavelength to the ong>1.55ong> ong>µmong> region

by inserting an ultrathin GaAs interlayer underneath the ong>InAsong> QDs. The

experimental details and characterization techniques are described in section

4.4. In section 4.5, a systematic discussion of the results based on the

dependence on the different growth parameters is given. Finally, section 4.6

summarizes this chapter.

4.2 As/P exchange reaction in ong>InAsong>/InP materials

Despite the low mismatch (3.2%) between the ong>InAsong> and InP materials, it is

well-known that the difficulties to control the self-assembly of ong>InAsong>/InP QDs

partly arise due to the complexity of the dot formation mechanism associated

with the chemical reactions at the surface. For typical growth conditions, the

P atoms on the InP surface are easily exchanged by As atoms (As/P exchange

48


eaction, shown in Fig. 4.1), deteriorating the interface quality of

heterostructures [11-13], when the InP surface is exposed to an As ambient.

It has been shown that the As/P exchange reaction might be enhanced at

high temperature and high As flux rate [14-16]. Moreover, the As/P exchange

reaction on the InP surface alone was used for the formation of ong>InAsong> dots

[17]. In the case of ong>InAsong>/InP self-assembled QDs, the local variation of the

strain field around the dots may cause the As/P exchange reaction even more

complicated, significantly altering the kinetic processes of the self-assembled

QD formation. It has been shown that the production of excess ong>InAsong> (more

amount of ong>InAsong> than provided from the sources) is due to to the As/P

exchange reaction [5, 18] shifting the emission wavelength of QDs to beyond

1.6 ong>µmong> at room temperature (RT) [19].

Apart from the PL redshift, the As/P exchange reaction is the main

reason for rough interfaces, nonuniform layers, and PL broadening for

ong>InAsong>/InP QDs [5, 7, 11, 20]. Attempts to control the QD emission wavelength

by thin capping and annealing or double capping usually resulted in

enhanced size fluctuations producing multiple peaks in PL measurements

[21, 22]. However, the dot height could be reduced in this way and an

emission wavelength of 1.5 ong>µmong> was achieved.

(a)

(b)

P

In

P

In

InP

P desorption

As

InP

(c)

Segregated In migrates

InP

Figure 4.1: Schematic illustration of the As/P exchange reaction during (a) InP surface

exposed to As flux, As and P substitution occurs, (b) P desorption, and (c) segregation of In

adatoms floating on the surface.

49


4.3 Effect of an ultrathin GaAs interlayer

In order to suppress the As/P exchange reaction and consume the segregated

surface In layer, Gong et al. [8] proposed and demonstrated an effective

method to reproducibly tune the emission wavelength of ong>InAsong>/InP (100) QDs

grown by chemical-beam epitaxy (CBE) in the ong>1.55ong>-ong>µmong> wavelength region by

the insertion of an ultrathin (zero to 2.0 MLs) GaAs interlayer between the

ong>InAsong> QDs and the InGaAsP buffer layer. As a function of thickness, the GaAs

interlayer effectively suppresses the As/P exchange reaction, which

continuously reduces the QD height to tune the emission wavelength from

above 1.6 ong>µmong> to below ong>1.55ong> ong>µmong> at RT.

(a) (b) (c)

(g)

(d) (e) (f)

(h)

1 μm

Figure 4.2: AFM images of the ong>InAsong> QDs on lattice-matched InGaAsP with thin GaAs

interlayers between the ong>InAsong> QD layer and InGaAsP buffer. The GaAs layer thickness is (a)

0.0, (b) 0.3, (c) 0.8, (d) 1.2, (e) 1.9, and (f) 2.5 MLs. The height contrast is 10 nm for (a)-(c) and

5 nm for (d)-(f). (g) PL spectra at 4.8 K of the corresponded ong>InAsong> QDs on lattice-matched

InGaAsP with thin GaAs interlayers with different thicknesses. (h) The dependence of the

PL peak wavelength and linewidth on the GaAs layer thickness. [8]

The AFM images of 3.2 MLs ong>InAsong> QD layers grown on top of 100 nm

InGaAsP lattice-matched to InP substrates and the corresponding PL spectra

are shown in Fig. 4.2 for different GaAs interlayer thickness. The suppression

of the As/P exchange reaction can be understood from the chemical bond

50


strength differences between As and P to group-III elements. In-As bonds

(with the bond strength of 48.0 kcal/mol) are slightly more stable than In-P

bonds (47.3 kcal/mol) [23], favoring the replacement of In-P by In-As bonds,

i.e., substitution of P by As atoms on InP. On the contrary, the Ga-P bond

(54.9 kcal/mol) and Ga-As bond (50.1 kcal/mol) are more stable than the In-As

and In-P bonds, thus suppressing As/P exchange for GaAs terminated

surfaces. Moreover, the GaAs layer consumes the free segregated In adatoms

floating on the surface, thereby reducing the amount of In migrating toward

the apex of the ong>InAsong> islands. An ultrathin GaP interlayer was also

demonstrated for suppressing the As/P exchange reaction during ong>InAsong>/InP

QD formation [24], and similar experiments with inserted GaAs interlayers

were performed by metal-organic vapor phase epitaxy (MOVPE) [9].

GaAs interlayer

ong>InAsong> QD

Figure 4.3: (a) X-STM image of four layers of ong>InAsong>/InGaAsP/InP (100) QDs without GaAs

interlayer (L1) and with 0.5 (L2), 1.0 (L3), and 1.5 ML (L4) GaAs interlayers. (b) Total area

of all the QDs found after scanning 1 ong>µmong> in each layer as a function of the thickness of the

GaAs interlayer. [25]

51


Figure 4.3(a) shows the cross-sectional scanning tunneling microscopy (X-

STM) image of four layers of ong>InAsong>/InGaAsP/InP (100) QDs without GaAs

interlayer (L1) and with 0.5 (L2), 1.0 (L3), and 1.5 ML (L4) GaAs interlayers.

The presence of almost pure ong>InAsong> QDs without intermixing of Ga and P

inside the ong>InAsong> QDs was confirmed. Most important, the integrated crosssectional

area of the ong>InAsong> QDs [Fig. 4.3(b)] decreases significantly with

increasing thickness of the GaAs interlayer, indicating the suppressed As/P

exchange [25].

In this thesis, the insertion of an ultrathin GaAs interlayer beneath the

ong>InAsong> QDs serves as a basic technique used for the wavelength tuning into the

ong>1.55ong> ong>µmong> telecom wavelength region.

4.4 Experimental details

The samples were grown on the semi-insulating InP (1 0 0) substrates,

misoriented 2° toward (1 1 0), by metal-organic vapor phase epitaxy

(MOVPE) using trimethyl-indium (TMI), trimethyl-gallium (TMG),

tertiarybutyl-arsine (TBA), and tertiarybutyl-phosphine (TBP) as gas

sources. The reactor pressure was 100 mbar and the total reactor flow was

15000 SCCM (SCCM denotes standard cubic centimeter per minute). The

sample structure commenced with a 100 nm InP buffer followed by a 1-3

monolayers (ML) GaAs interlayer, a single layer of ong>InAsong> QDs grown with

varied TBA flow, 5 seconds growth interruption in TBA, and an upper 100

nm InP layer (for PL). The growth temperature was varied between 490 and

515 °C and the amount of ong>InAsong> for QD formation was 1.8-2 ML. The TBA flow

rate was between 0.5 SCCM (partial pressure of 0.00048 mbar) and 3 SCCM

(partial pressure of 0.0029 mbar). The V/III flow ratios for GaAs and ong>InAsong>

growth were 1.17 and 0.62 for the TBA flow rate of 3 SCCM. After growth of

the InP buffer, the GaAs interlayer was directly grown followed by 45 seconds

growth interruption for flushing out residual TMG to avoid Ga incorporation

into the ong>InAsong> QDs [9].

In order to study the effect of the low-temperature InP cap layer

thickness on the ong>InAsong> QD PL, the sample structure commenced with a

100 nm InP buffer followed by 2 ML ong>InAsong> QDs with a 2 ML GaAs interlayer

underneath grown at 490 °C. The TBA flow rate was 3 SCCM. Then a Low-T

InP cap layer was immediately grown also at 490 °C. The thickness was 5, 8,

10, and 50 nm in different samples. After that the sample was heated up to

585 °C and a 70 nm High-T InP cap layer was grown. For reference a sample

with only 50 nm Low-T capping was also grown. In a second set of samples

another 800 nm InP layer was additionally overgrown at 585 °C (O-High-T).

This mimics cladding layer growth or regrowth needed for full device

structures, as discussed in Chapter 7.

The morphology of the ong>InAsong> QDs plus GaAs interlayer was analyzed by

tapping mode atomic force microscopy (AFM) in air. The optical properties

52


were studied by PL at room temperature (RT) using a Nd:YAG laser (532 nm

line) as excitation source with power density of 256 mW/cm 2 .

4.5 Results and discussion

4.5.1 GaAs interlayer thickness

Figure 4.4(a-c) shows the AFM images of 2 ML ong>InAsong> QDs grown at 490 °C

with (a) 1 ML, (b) 2 ML, and (c) 3 ML GaAs interlayers. The TMI and TBA

flow rates are 200 SCCM (partial pressure of 0.0046 mbar) and 3 SCCM

(partial pressure of 0.0029 mbar). With increasing GaAs interlayer thickness,

the QD size (average QD height and diameter are given in the images)

together with the QD density decrease and the PL intensity increases before

it decreases. Figure 4.4(d) shows that the PL intensity increases 2 times for 2

ML GaAs and 2.7 times for 3 ML GaAs as compared with 1 ML GaAs. The PL

peak wavelength blueshifts from 1750 nm (1 ML GaAs) to 1700 nm (2 and 3

ML GaAs), as shown in the inset of Fig. 4.4(d). This reveals suppression of

As/P exchange during ong>InAsong> growth by the GaAs interlayer on InP, as

previously shown on InGaAsP [9, 10], resulting in better QD quality and

smaller size, before the quality of the QDs worsens for too large GaAs

interlayer thickness, i.e., tensile strain. In addition to the absence of Ga

intermixing in the almost pure ong>InAsong> QDs shown by X-STM [26], intermixing

would rather result in an increase of the QD size with GaAs interlayer

thickness though also in a PL blueshift.

4.5.2 Growth temperature

Figure 4.5(a-c) shows the AFM images of 2 ML ong>InAsong> QDs grown at (a) 490, (b)

500, and (c) 515 °C with 1 ML GaAs interlayers. The TMI and TBA flow rates

are 200 SCCM (partial pressure of 0.0046 mbar) and 3 SCCM (partial

pressure of 0.0029 mbar), respectively. With increasing growth temperature

from 490 to 515 °C, the ong>InAsong> QD height and base diameter significantly

increase, hence, the PL peak wavelength continuously redshifts from 1756

nm for 490 °C to 1796 nm for 515 °C, as seen in Fig. 4.5(d). This is consistent

with the fact that at higher temperature, P atoms are more easily desorbed

from the surface due to the high P vapor pressure and the As/P exchange

reaction is greatly enhanced along the periphery of the ong>InAsong> QDs on InP,

leading to free In adatoms migrating toward the apex of the QDs and

subsequent binding with As for excess ong>InAsong> formation [5]. Therefore, the

observed change in QD size confirms the occurrence and increase of

thermally activated As/P exchange at higher growth temperature. In

addition, the QD density is considerably reduced at increased growth

temperature, being 6.7 × 10 9 cm -2 at 515 °C, which is related to the larger In

adatom migration length at elevated growth temperature.

53


(a) (a)

(b) (c)

1 μm

Av. Diameter: 53 nm

Av. Height: 5.5 nm

Av. Diameter: 50 nm

Av. Height: 4.0 nm

Av. Diameter: 44 nm

Av. Height: 3.1 nm

PL Intensity (arb. units)

(d)

RT

PL peak wavelength (nm)

1800

1700

1600

0 1 2 3

GaAs thickness (MLs)

1ML GaAs

2ML GaAs

3ML GaAs

1200 1500 1800 2100

Wavelength (nm)

Figure 4.4: AFM images (1 × 1 ong>µmong> 2 ) of 2 ML ong>InAsong> QDs grown at 490 °C with (a) 1, (b) 2, and

(c) 3 ML GaAs interlayers for TBA flow rate of 3 SCCM. The height contrast is 20 nm in all

images. (d) PL spectra taken at room temperature of 2ML ong>InAsong> QDs with 1, 2, and 3 ML

GaAs interlayers. The inset shows the QD PL peak wavelength versus GaAs interlayer

thickness.

54


(a) (b) (c)

1 μm

Av. Diameter: 53 nm

Av. Height: 5.5 nm

Av. Diameter: 65 nm

Av. Height: 7.0 nm

Av. Diameter: 82 nm

Av. Height: 8.0 nm

PL Intensity (arb. units)

(d)

500 °C

RT

490 °C

500 °C

515 °C

1400 1600 1800 2000

Wavelength (nm)

Figure 4.5: AFM images (1 × 1 ong>µmong> 2 ) of 2 ML ong>InAsong> QDs grown at (a) 490, (b) 500, and (c) 515

°C with 1 ML GaAs interlayers for TBA flow rate of 3 SCCM. The height contrast is 20 nm in

all images. (d) PL spectra taken at room temperature of 2 ML ong>InAsong> QDs grown at 490, 500,

and 515 °C with 1 ML GaAs interlayers for TBA flow rate of 3 SCCM.

4.5.3 Group V/III ratio

In Fig. 4.6(a-c), AFM images of 2 ML ong>InAsong> QDs grown at 490 °C with 1 ML

GaAs interlayers for TBA flow rates of (a) 0.5, (b) 1, and (c) 3 SCCM are

shown. With reduction of the TBA flow rate from 3 SCCM (partial pressure of

0.0029 mbar) to 1 SCCM (partial pressure of 0.00097 mbar) and to 0.5 SCCM

(partial pressure of 0.00048 mbar), the QD PL peak wavelength blueshifts

from 1765 nm to 1603 nm and to 1443 nm. The PL intensity drops to 80% for

1 sccm TBA supply and to 24% for 0.5 SCCM TBA supply as compared with 3

SCCM TBA supply, as shown in Fig. 4.6(d). This shows reduced As/P

55


exchange for lower TBA supply but also decreasing QD quality. The increase

of the QD diameter for lower TBA flow is attributed to a larger In adatom

migration length. Even so, the QD PL peak wavelength shortens due to the

reduced QD height owing to the reduced As/P exchange for lower TBA flow

rate, most probably also reducing the height of the QDs penetrating into the

InP layer underneath.

(a)

(b)

(c)

1 μm

Av. Diameter: 65 nm

Av. Height: 4.5 nm

Av. Diameter: 60 nm

Av. Height: 4.9 nm

Av. Diameter: 53 nm

Av. Height: 5.5 nm

PL Intensity (arb. units)

(d)

PL peak wavelength (nm)

1800

1600

3 SCCM

1 SCCM

1400

0.5 SCCM

0 1 2 3

RT

TBA flow rate (SCCM)

1200 1500 1800 2100

Wavelength (nm)

Figure 4.6: AFM images (1 × 1 ong>µmong> 2 ) of 2 ML ong>InAsong> QDs grown at 490°C with 1 ML GaAs

interlayers for TBA flow rates of (a) 0.5, (b) 1, and (c) 3 SCCM. The height contrast is 20 nm

in all images. (d) PL spectra taken at room temperature of 2 ML ong>InAsong> QDs grown at 490°C

with 1 ML GaAs interlayers for TBA flow rates of 0.5, 1, and 3 SCCM. The inset shows the

QD PL peak wavelength versus TBA flow rate.

56


In the insets in Fig. 4.7, AFM images of 2 ML ong>InAsong> QDs for 200 SCCM

(partial pressure of 0.0046 mbar) TMI supply (left) and of 1.8 ML ong>InAsong> QDs

for 180 SCCM (partial pressure of 0.0042 mbar) TMI supply (right) are

shown. The growth temperature is 490 °C, the TBA flow rate is 3 SCCM, and

the GaAs interlayer thickness is 1 ML. The QDs exhibit the same average

height of 5.5 nm, resulting in the same PL peak wavelength. The PL

intensity increases 1.4 times for 180 SCCM TMI supply as compared with

200 SCCM TMI supply, confirming better QD quality for larger group V/III

ratio.

TMI 200 SCCM

TMI 180 SCCM

PL Intensity (arb. units)

Av. Diameter: 53 nm

Av. Height: 5.5 nm

RT

1 μm

Av. Diameter: 50 nm

Av. Height: 5.5 nm

TMI 180 SCCM

TMI 200 SCCM

1000 1500 2000 2500

Wavelength (nm)

Figure 4.7: PL spectra taken at room temperature of 2 and 1.8 ML ong>InAsong> QDs grown at 490

°C with 1 ML GaAs interlayers for TMI flow rates of 200 and 180 SCCM, and TBA flow rate

of 3 SCCM. Insets: AFM images (1 × 1 ong>µmong> 2 ) of 2 and 1.8 ML ong>InAsong> QDs with 1 ML GaAs

interlayers grown at 490 °C for TMI flow rates of 200 (left) and 180 SCCM (right).

4.5.4 Low-temperature InP cap layer thickness

The application of ong>InAsong>/InP QDs relies on achieving precise control of the QD

dimension, size, and composition in order to control the QD optical properties.

To this end, various techniques have been proposed: introduction of interface

layers [8, 9, 24], insertion of growth interruption times [27], rapid thermal

57


annealing [28], and double capping [29, 30] which is of particular importance

when the QDs are incorporated in device structures.

Here, we report the effect of the thickness of the first low-temperature

grown (Low-T) InP cap layer on the ong>InAsong> QD photoluminescence (PL)

wavelength and efficiency when followed by high-temperature (High-T)

capping. After 2 ML ong>InAsong> QDs with a 2 ML GaAs interlayer were grown at

490 °C, a Low-T InP cap layer was immediately grown at same temperature.

The thickness was 5, 8, 10, and 50 nm in different samples. After that the

sample was heated up to 585 °C and a 70 nm High-T InP cap layer was

grown. The scheme of the sample structure is shown in Fig. 4.8(a). For

reference a sample with only 50 nm Low-T capping was also grown. In a

second set of samples another 800 nm InP layer was additionally overgrown

at 585 °C (O-High-T). This mimics cladding layer growth or regrowth needed

for full device structures, as discussed in Chapter 7. The scheme of this

structure is shown in Fig. 4.8(b).

Figure 4.8: Schematic overview of the sample structures (a) before and (b) after overgrowth.

The InP Low-T cap layer was grown at 490 °C with different thickness of 5, 8, 10, and 50 nm

for different samples. The InP High-T cap layer with a thickness of 70 nm was grown at

585 °C. The O-High-T layer with a thickness of 800 nm was grown at 585 °C.

58


Figure 4.9 shows the PL spectra of the samples with different Low-T cap

layer thicknesses and 70 nm High-T capping before (solid colored lines) and

after (dotted colored lines) O-High-T overgrowth. The PL of the reference

sample with only 50 nm Low-T capping (at 490) is also shown (black line).

The PL peak of the 5 nm Low-T cap layer sample (solid blue line) decreases

1/3 times with blue shift to 1575 nm, compared with the sample grown at 490

°C. The PL peak increases 1.9 times with slight red shift to 1715 nm (solid

red line) when the low-T cap layer thickness is 8 nm compared with the

sample grown at 490 °C. For the low-T cap layer thickness of 10 nm the PL

peak decreases 0.9 times with red shift to 1720 nm (solid green line), similar

to that for the thickness of 50 nm, compared with the sample grown at 490

°C.

Figure 4.9: PL spectra of the QDs taken at room temperature before (solid lines) and after

(dotted lines) overgrowth with Low-T (490 °C) cap layer thicknesses of 5 (blue), 8 (red), 10

(green), and 50 (purple) nm and a 70 nm High-T (585 °C) cap layer. The black curve is the PL

from the QDs with only 50 nm Low-T capping (at 490).

59


This behavior is explained as follows: a sufficiently thick Low-T cap layer

is required to stabilize the QDs during heating up for High-T capping. If the

thickness is too small, the QDs are not fully capped and dissolve. Due to the

resulting height reduction the emission wavelength shortens. If the Low-T

cap layer, however, is too thick, the number of defects incorporated during

growth at such a low temperature increases. During the High-T capping

these defects can diffuse toward the QDs, degrading their optical quality.

This degradation becomes even stronger with larger High-T cap layer

thickness, i.e., longer growth time, as is seen in the reduced efficiency of the

PL after overgrowth of the 800 nm InP layer (dotted lines with the same

colors as the corresponding solid lines). The optimum Low-T cap layer

thickness for small PL wavelength shift and the highest efficiency is still

around 8 nm while no PL emission is observed for the 50 nm thick Low-T cap

layer. As Low-T capping at the QD growth temperature is unavoidable the

finding of the optimum layer thickness is important for the incorporation of

the QDs in device structures where upper layers are grown at higher

temperatures to guarantee high crystal quality.

4.6 Summary

In conclusion, we have grown ong>InAsong> QDs on planar InP (100) substrates by

MOVPE with an ultra-thin GaAs interlayer underneath the QDs. A wide

wavelength range and emission at ong>1.55ong> ong>µmong> at room temperature were

accessed by solely varying the thickness of the GaAs interlayer. The

continuous wavelength tuning was attributed to the suppression of As/P

exchange during ong>InAsong> QD growth. For optimized growth conditions, i.e., 2 ML

GaAs, 490 °C growth temperature, 1.5 SCCM (partial pressure of 0.0015

mbar) TBA supply, and 180 SCCM (partial pressure of 0.0042 mbar) TMI

supply, high quality ong>InAsong> QDs emitting in the ong>1.55ong>-ong>µmong> telecom wavelength

region were achieved.

We investigated the effect of the thickness of first low-temperature grown

(Low-T) InP cap layer on the ong>InAsong> QD PL wavelength and efficiency when

followed by high-temperature (High-T) capping. With increase of the Low-T

cap layer thickness, the PL peak redshifted and the efficiency increased up to

a thickness of 8 nm after which the PL peak wavelength stayed constant and

the efficiency strongly decreased. This optimum Low-T cap layer thickness

was attributed to the balance between stability of the QDs during heat-up

and defect diffusion toward the QDs during High-T capping which is crucial

when the QDs are incorporated in device structures.

60


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63


Chapter 5

Impact of Base Size and Shape on

Formation Control of Multifaceted

InP Nanopyramids

We report the impact of base size and shape on the evolution control of

multifaceted InP (100) nanopyramids grown by selective-area metal-organic

vapor phase epitaxy (MOVPE). The pyramid top surfaces are composed of a

(100) center facet surrounded by high-index {103} and {115} facets. Their

arrangement and (relative) size depend on the size and shape of the pyramid

top area. For a certain shape, only the (100) facet remains below a critical size

of the top area. The arrangement and (relative) size of the top facets in turn

are governed by the {110} and {111} side facets whose area (ratio) depends on

the pyramid base size and shape. This self-consistently determines the ratio of

the (100) top facet area and the sum of the {110} and {111} side facet areas as

well as the height of the pyramids. 1

1 Based on J. Appl. Phys. 106, 124304 (2009)

65


5.1 Introduction

The selective area growth [1, 2] of III-V semiconductor nanopyramids has

been widely studied in the GaAs based material system by metal-organic

vapor phase epitaxy (MOVPE) [3-6], and in the InP based material system

by chemical beam epitaxy (CBE) [7] and MOVPE [8, 9]. The nanopyramids

have been employed as templates for the position ong>controlledong> growth of ong>InAsong>

quantum dots (QDs) [8, 10], thus enabling a wide range of applications in the

field of future quantum functional devices [11-14].

In this chapter, we study the impact of the size and shape of the base of

multifaceted InP nanopyramids on their evolution control during MOVPE.

The arrangement and (relative) size of the facets forming the top surface of

the pyramids depend on the size and shape of the pyramids top area which,

in turn, are governed by the {110} and {111} side facets whose area (ratio)

depends on the pyramids base size and shape. This directly determines the

ratio of the (100) top facet area and the sum of the {110} and {111} side facet

areas as well as the height of the pyramids. Our study is the basis for the

position and distribution control of ong>InAsong> QDs (Chapter 6) required for

efficient QD nanolasers and single photon sources emitting around ong>1.55ong> ong>µmong>

in combination with submicrometer-scale active-passive integration (Chapter

7) for their implementation in photonic integrated circuits.

Section 5.2 describes the experimental details and characterization

techniques. The impact of base size and shape on formation control of the

multifaceted InP nanopyramids is presented in section 5.3. The conclusions

are summarized in section 5.4.

5.2 Experimental details

A 100 nm thick SiNx mask layer was deposited on the semi-insulating InP

(100) substrates, 2° misorientated towards (110), by plasma-enhanced

chemical-vapor deposition (PECVD). The openings in the SiNx layer were

created by electron beam lithography (EBL) and reactive ion etching (RIE).

The openings were arranged in a square lattice with center-to-center distance

of 10 ong>µmong>. Three characteristic shapes were fabricated: square with side

length along [001] (square-[001]), square with side length along [01-1]

(square-[01-1]), and circular. The side lengths or diameters of the openings

were varied between 500 nm and 1.5 ong>µmong>. Prior to growth, the samples were

cleaned three times in oxygen plasma and H3PO4:H2O (1:10) solution to

remove any surface contaminants. Selective area growth was carried out by

low-pressure MOVPE using trimethyl-indium (TMI) and tertiarybutylphosphine

(TBP), diluted in H2, as source materials. The growth temperature

was 610 °C, the growth rate was 18.39 nm/min in unmasked areas, and the

reactor pressure was reduced to 75 Torr to enhance the In adatom surface

migration length for well-defined pyramid formation. The total growth time

66


was 4.13 minutes. The morphology of the InP pyramids was characterized by

tapping mode atomic force microscopy (AFM) in air.

5.3 Results and discussion

5.3.1 Arrangement and (relative) size of pyramid top facets

Figure 5.1 shows the AFM images together with schematic drawings of the

different truncated InP pyramids for various size and shape of the mask

openings, i.e., pyramids base: (a), (b) squares with side along [001], (c), (d)

squares with side along [01-1], and (e), (f) circulars. The total top surface of

the large pyramids is composed of a (100) central facet and high-index {103}

and {115} facets around. The small pyramids only exhibit a (100) top facet.

The pyramids side walls are bound by {110} and {111} side facets. The facets

are identified from their inclinations with respect to the substrate surface

determined by AFM line scans [15].

Figure 5.1: AFM images of the truncated InP pyramids together with schematic drawings

for various base shapes: (a), (b) squares with side along [001], (c), (d) squares with side along

[01-1], and (e), (f) circulars. The dashed white lines in (a), (b) indicate the directions of the

line scans shown in Fig. 5.3. The scan fields are 2 × 2 ong>µmong> 2 .

67


In detail, square-[01-1] and circular based pyramids are bound by four

{110} and four {111} side facets with, however, different area ratio determined

by the pyramids base size and shape. Square-[001] based pyramids are bound

by four {110} side facets. As the {103} and {115} top facets are connected to

the {110} and {111} side facets, respectively, and surround the (100) central

facet, the arrangement and (relative) size of the pyramids top facets depend

on the size and shape of the pyramids top area and base. This is summarized

in Fig. 5.2 where the ratio of the (100) facet area and the total top surface

area is plotted as a function of the top surface area for various shapes. The

ratio is largest for square-[001] pyramids and smallest for square [01-1]

pyramids. This indicates that the ratio is governed by the appearance of {115}

facets, being largest when they are suppressed. The ratio sharply increases

for top areas between 0.3 and 0.7 ong>µmong> 2 , indicated by the arrow in Fig. 5.2. This

indicates the suppression of the high-index facets below a certain size of the

pyramid top area. This is evaluated by the AFM line profiles, shown in Figs.

5.3(a) and 5.3(b), along [001] across the square-[001] based pyramids as

indicated in Figs. 5.1(a) and 5.1(b) by the dashed white lines.

Figure 5.2: Ratio of (100) top facet area divided by the total pyramid top surface area as a

function of the top surface area for various base shapes: Square with side along [001]

(Square-[001]), square with side along [01-1] (Square-[01-1]), and circular. The dashed and

solid lines indicate the general trends. The black arrow indicates the area of pyramid top

surface between 0.3-0.7 ong>µmong> 2 .

68


The large pyramid exhibits an extended {103} facet between the {110} and

{100} facets with angles between the facets of 153 and 165°, respectively. For

the small pyramid, the {110} and (100) facets are directly connected with an

angle between the facets of 136°. The suppression of the high-index facets

with reduced top area is understood by a competition between surface energy

and edge energy to minimize the total surface energy [16, 17]. Even if the

surface energy of the high-index facets is larger than that of the (100) top

facet, the introduction of two edges with larger angles in the presence of highindex

facets, lowers the total surface energy compared to the case of direct

connection of the (100) top facet with the {110} side facets, resulting in a

sharper edge with smaller angle, contributing a larger edge energy. Below a

certain size of the high-index facets for reduced top area, however, the two

edges approach each other and the formation of a single edge becomes

beneficial, resulting in the disappearance of the high-index facets.

Figure 5.3: Cross-sectional AFM line profiles of square-based pyramids with side along [001]

taken along [001] with (a) 0.85 and (b) 0.74 ong>µmong> side length.

69


5.3.2 Side facets competition for pyramids evolution

The evolution of the pyramids sidewalls is governed by the competition

between the {111} and {110} facets for a given base shape. In Fig. 5.4, their

area ratio for various base shapes is plotted as a function of the area of the

pyramids base. There is a clear trend that the relative area of the {111} facets

decreases compared to that of the {110} facets with reduction of the base size.

This decrease is stronger, the larger the area ratio is, given by the base

shape. The ratio of the areas of the {111} and {110} facets is directly related to

the ratio of the areas of the {115} and {103} facets on the pyramids top and

therefore consistent with Fig. 5.2 showing the ratio of the area of the (100)

top facet and the total top area being larger for smaller relative area of the

{115} facets.

Figure 5.4: Ratio of the {111} and {110} side facet areas versus area of the pyramid base for

various base shapes: square with side along [001] (Square-[001]), square with side along [01-

1] (Square-[01-1]), and circular. The solid lines indicate the general trends.

70


The relative area of the {111} and {110} side facets is also directly related

to the ratio of the (100) top facet area and the total sidewall area shown in

Fig. 5.5, as well as the pyramids height as function of the area of the

pyramids base, shown in Fig. 5.6. The angle between the {111} facets and the

(100) surface is 54.7° and that between the {110} facets and the (100) surface

is 45° [18]. Hence, the larger the fraction of the {111} facets is, given by the

base shape, the steeper, on average, are the pyramids sidewalls. Therefore,

the relative area of the (100) top facet is larger (of course the area of the (100)

top facet and therefore the area ratio plotted in Fig. 5.5 reduces for all shapes

with reducing base area), and the height is smaller due to the smaller

relative growth rate enhancement (of course the height plotted in Fig. 5.6

increases for all shapes with reducing base area due to the increasing growth

rate enhancement).

Figure 5.5: Ratio of the (100) facet area and the sum of the {110} and {111} facet areas

versus area of the pyramid base for various base shapes: Square with side along [001]

(Square-[001]), square with side along [01-1] (Square-[01-1]), and circular. The solid lines

indicate the general trends.

71


Figure 5.6: Height of the truncated pyramids versus area of the base for various base

shapes: Square with side along [001] (Square-[001]), square with side along [01-1] (Square-

[01-1]), and circular. The solid lines indicate the general trends and the dashed line

represents the layer thickness on unmasked substrates.

The results plotted in Figs. 5.4 and 5.5 can be related to the total surface

energy, when neglecting the contribution of the high-index facets due to their

relatively small areas. The surface energies of the (111) and (-1-1-1) facets

are 6.2 and 4.4 eV/nm 2 , respectively, and the surface energy of the (110) facet

is 5.5 eV/nm 2 [17]. For the symmetric pyramids this would favor the

formation of {111} side facets rather than {110} side facets. However, in this

case the relative area of the (100) top facet, having the largest surface energy

of 6.2 eV/nm 2 , is larger. Therefore, {110} rather than {111} side facets

preferably form, most pronounced for small pyramids which are close to

pinch-off, to minimize the total surface energy.

72


5.4 Summary

In summary, we have reported the impact of base size and shape to control

the evolution of multifaceted InP nanopyramids grown by selective-area

metal-organic vapor phase epitaxy. Large base sizes lead to truncated

pyramids with the top surfaces composed of a (100) center facet and naturally

formed {103} and {115} facets around. In contrast, small base sizes lead to

formation of only a (100) top facet. The arrangement and (relative) size of the

facets are governed by the size and shape of the pyramids top area. The

arrangement and (relative) size of the top facets in turn are governed by the

{110} and {111} side facets whose area (ratio) depends on the pyramids base

size and shape. This directly determines the ratio of the (100) top facet area

and the sum of the {110} and {111} side facet areas, related to the

minimization of the total surface energy, as well as the height of the

pyramids. These findings are the basis to control the distribution (on the

high-index facets for large top area) and number (on the (100) top facets for

small top area) of ong>InAsong> quantum dots deposited on the InP nanopyramids [8].

73


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75


Chapter 6

Distribution, Number, and

Polarization Control of Siteong>controlledong>

ong>1.55ong>-ong>µmong> ong>InAsong> ong>Quantumong>

ong>Dotsong> on InP Nanopyramids

Distribution, number, and polarization control of site-ong>controlledong> ong>InAsong>

quantum dots (QDs) on InP pyramids grown by selective-area metal-organic

vapor phase epitaxy (MOVPE) is reported. The top surface of the pyramids is

composed of a (100) facet and high-index facets aside. The arrangement of the

facets is governed by the shape of the pyramid base and top surface area. The

QDs preferentially nucleate on the high-index facets determining position and

distribution. The QD number is reduced with shrinking top surface size.

Positioning of four, three, two, and a single QD is realized depending on the

top surface shape and size. Sharp emission from a single QD is observed at

ong>1.55ong> ong>µmong>. Furthermore, with increasing growth temperature the QDs elongate

causing strong linear polarization of the photoluminescence. With reduced

pyramid base/pyramid top area/QD number, the degree of polarization

decreases, attributed to the symmetric pyramid top, reaching zero for single

QDs grown at lower temperature. This control of linear polarization is

important for entangled photon sources operating in the ong>1.55ong>-ong>µmong> wavelength

region. 1

1 Based on Appl. Phys. Lett. 94, 143103 (2009) and Appl. Phys. Lett. 98, 201904 (2011)

77


6.1 Introduction

Self-organized semiconductor quantum dots (QDs) have brought enhanced

performance to lasers and optical amplifiers due to their discrete energy

states, which is of particular importance for operation in the ong>1.55ong>-ong>µmong> telecom

wavelength region [1, 2]. These devices rely on QDs grown in the Stranski-

Krastanow mode which are distributed randomly on the substrate surface.

For advanced quantum functional devices such as nanolasers and single

photon sources, however, precise position and number control of a few down

to a single QD is required. This can be realized by pre-defined QD nucleation

on truncated pyramids formed by selective area growth in dielectric mask

openings [3-7] and has been demonstrated for the ong>InAsong>/GaAs material system

by metal-organic vapor phase epitaxy (MOVPE) [4, 8, 9] and molecular beam

epitaxy (MBE) [6] and for the ong>InAsong>/InP material system by chemical beam

epitaxy (CBE) [5] and MOVPE [3]. Moreover, for efficient nanolasers and

single photon sources employing high-quality microcavities such as disks,

rings, or photonic crystal defects, control of the QD distribution is highly

desirable to maximize the overlap with the photon field of a specific optical

mode with certain size and shape [10, 11].

In addition, control of the QD shape and consequently linear polarization

of the emission is crucial for entangled photon sources [7, 12]. They require

symmetric QDs with zero degree of polarization (DOP) and, hence, vanishing

fine structure splitting (FSS). This makes ong>InAsong>/InP QDs particularly

promising exhibiting about 10 times smaller FSS for a certain shape than

ong>InAsong>/GaAs QDs [13]. Various approaches to control the polarization of QDs

have been demonstrated employing (111) oriented substrates [14], thermal

annealing [15], and in-plane magnetic fields [16].

In this chapter, we report the control of position and distribution of ong>InAsong>

QDs on truncated InP pyramids grown by selective-area MOVPE. The shape

of the pyramid base determines the facets and their relative sizes on the

pyramid top surface, resulting in position and distribution control of the QDs.

The QD number, for a specific shape of the pyramid top surface, is ong>controlledong>

by the area down to a single QD emitting at ong>1.55ong> ong>µmong>.

Furthermore, we present a different approach to control the shape and

polarization of site-ong>controlledong> multiple and single ong>InAsong> QDs on InP pyramids.

The size of the QDs increases with increasing growth temperature and the

QDs elongate causing strong linear polarization of the photoluminescence

(PL). Importantly, the DOP decreases with reduced pyramid base, i.e.,

pyramid top area, i.e., QD number induced by the symmetric pyramid top,

reaching zero for single QDs grown at lower temperature.

Section 6.2 describes the experimental details and characterization

techniques. The results and discussion of distribution, number, and

polarization control of site-ong>controlledong> ong>InAsong>/InP QDs is presented in section

6.3. Section 6.4 summarizes this chapter.

78


6.2 Experimental details

A 100 nm thick SiNx mask was deposited on the InP (100) substrates by

plasma-enhanced chemical-vapor deposition (PECVD) followed by defining

the mask openings by electron beam lithography (EBL) and reactive ion

etching (RIE). The openings were arranged in a square lattice with center-tocenter

distance of 10 ong>µmong>. Selective-area MOVPE was performed using

trimethyl-indium (TMI), trimethyl-gallium (TMG), tertiarybutyl-phosphine

(TBP), and tertiarybutyl-arsine (TBA) as precursors. The truncated InP

pyramids were grown at 610 °C with a growth rate of 18.39 nm/min in

unmasked areas. On the pyramid top, 3 monolayers (ML) ong>InAsong> QDs were

grown at 490 and 515 °C which were capped by 70 nm InP. A 1.5 ML GaAs

interlayer was inserted underneath the QDs to tune the QD emission

wavelength into the ong>1.55ong>-ong>µmong> telecom region [17]. The surface morphology of

uncapped ong>InAsong> QDs on InP pyramids was characterized by atomic force

microscopy (AFM) in air. The low-temperature micro-PL spectroscopy of the

capped QDs was performed by exciting the samples, mounted in a He-flow

cryostat, with a continuous-wave He-Ne laser operating at 635 nm.

Excitation and detection of the PL were through a microscope objective with a

spatial resolution of ~ 2 ong>µmong>. The PL was dispersed by a 0.25 m single

monochromator and detected by an InGaAs photodiode array. For the

polarization analysis, a linear polarizer followed by a quarter-wave plate

were inserted in front of the monochromator. The polarization-dependent PL

from 34 multiple QD and 26 single QD samples was measured for statistical

analysis.

6.3 Results and discussion

6.3.1 Distribution and number control of quantum dots

Based on the previous study of truncated InP pyramids formation discussed

in Chapter 5, we investigate more pyramids base shapes for the QD

distribution and number control. Figures 6.1(a)-6.1(d) show the AFM images

of the truncated InP pyramids with (a) square, (b) circular, (c) elliptical and

(d) triangular mask openings, i.e. pyramid base, together with schematic

drawings. In general, the pyramid side walls are bound by {110} and {111}

facets and the pyramid top surfaces are composed of a (100) facet and highindex

{103} and {115} facets around, determined from AFM line scans [18].

The shape of the top surfaces and the (relative) size of the facets are

determined by the shape of the pyramid base and the area of the top surfaces,

as summarized in Fig. 6.2 showing the ratio of the sum of the areas of the

{103} and {115} facets divided by the total top surface area as a function of the

top surface area for various shapes. For all base shapes, the relative size of

the {103} and {115} facets increases with the area of the top surface and below

79


an area of 0.2-0.3 ong>µmong> 2 the facets do not form. For a certain top surface area,

the relative size of the {103} and {115} facets is, for instance, larger for

elliptical-base pyramids (long axis along [01-1]), indicated by the blue line in

Fig. 6.2, than for square-base pyramids (side along [001]), indicated by the

red line in Fig. 6.2, suggesting the presence of extended {115} facets

(suppressed for the square-base pyramids) to increase the ratio.

Figure 6.1: AFM images of the truncated InP pyramids together with schematic drawings

for various base shapes: (a) square, (b) circular, (c) elliptical, and (c) triangular. The scan

fields are 2 × 2 ong>µmong> 2 .

80


Figure 6.2: Sum of the areas of the {103} and {115} facets divided by the total pyramid top

surface area as a function of the top surface area for various shapes: Hexagonal, square with

side along [01-1], circular, square with side along [001], elliptical with long axis along [011],

elliptical with long axis along [01-1], and triangular. Blue and red lines indicate the trends

for the elliptical (long axis along [01-1]) and square (side along [001]) base.

The size and shape of the pyramid top surfaces govern the distribution

and number of the ong>InAsong> QDs grown on the pyramids top, shown in Fig. 6.3.

The distribution of the QDs is [Figs. 6.3(a) and 6.3(b)] square for square base,

[Fig. 6.3(c)] triangular for triangular base, [Figs. 6.3(d) and 6.3(e)] circular for

circular base, and [Figs. 6.3(f) and 6.3(g)] elliptical for elliptical base with the

QDs aligned close to the edge of the top surfaces. This is due to the

preferential nucleation of the QDs on the high-index facets. Similar to ong>InAsong>

QDs on GaAs pyramids [8], it is believed that the high-index facets allow

optimal strain relaxation.

When changing the shape of the pyramid base from circular to elliptical,

the relative size of the {103} and {115} facets is reduced with the aspect ratio,

as shown in Figs. 6.3(f) and 6.3(g), where the aspect ratio is increased from

1.4 to 2. This changes the QD distribution from elliptical-ring-like to an oval

ring.

81


For reduced pyramid top area/base size, the shape of the QD distribution

is maintained with reduced extension, shown in Figs. 6.3(d) and 6.3(e) for

circular base. For all pyramid base shapes, the QD number is reduced with

reduced pyramid top area, which is summarized in Fig. 6.3(h). This is

consistent with the reduced size of the {103} and {115} facets on which the

QDs nucleate. The difference in QD number for the various shapes is directly

related to the relative size of the {103} and {115} facets. The QD number is

highest for elliptical-base pyramids with larger relative size [blue line in Fig.

6.3(h)], and lowest for square-base pyramids with lower relative size [red line

in Fig. 6.3(h)].

Figure 6.3: AFM images of ong>InAsong> QDs grown on the truncated InP pyramids top for [(a) and

(b)] square, (c) triangular, [(d) and (e)] circular, and [(f) and (g)] elliptical base. The scan

fields are 2 × 2 ong>µmong> 2 . (h) QD number as a function of the pyramid top surface area for various

shapes: Hexagonal, square with side along [01-1], circular, square with side along [001],

elliptical with long axis along [011], elliptical with long axis along [01-1], and triangular.

Blue and red lines indicate the trends for the elliptical (long axis along [01-1]) and square

(side along [001]) base.

82


6.3.2 Positioning four, three, two, and a single quantum dot

With the decrease of the top surface area, only the (100) facets form, as

discussed above, with rounded corners, and the QDs uniformly nucleate there

on. Close to pinch-off, four, three, two, and single QDs are positioned

around/at the center, with the QD number being related to the pyramid base

shape. Four for elliptical base, three for triangular base, two for hexagonal

base, and a single QD for circular base (with more than 60% reproducibility

mainly limited by the lithography accuracy), shown in Figs. 6.4(a)-6.4(d).

Hence, it is the combination of the top (100) facet shape and size that

determines the number of QDs.

High optical quality of the QDs is revealed by the micro-PL spectrum

taken at 5 K of a single QD on a circular-based pyramid, shown in Fig. 6.4(e).

The broad emission around 1200-1300 nm is attributed to the ong>InAsong> wetting

layer on the pyramid side facets. The single sharp peak at 1542 nm with

resolution-limited linewidth of 2 nm stems from the single position ong>controlledong>

ong>InAsong> QD.

Figure 6.4: AFM images of the InP pyramids close to pinch-off together with the magnified

images of the ong>InAsong> QDs on top for (a) elliptical, (b) triangular, (c) hexagonal, and (d) circular

base. The scan fields are 1 × 1 ong>µmong> 2 and 0.2 × 0.2 ong>µmong> 2 . (e) Micro-PL spectrum taken at 5.0 K

from a single capped ong>InAsong> QD on a circular-based InP pyramid.

83


6.3.3 Controlling polarization anisotropy of quantum dots

To control the QD shape and consequently the linear polarization of the

emission, QD(s) on circular-based pyramids were analyzed in detail. Figures

6.5(a)-6.5(f) show the AFM images (2 × 2 ong>µmong> 2 ) of the multiple and single ong>InAsong>

QDs grown on the top of truncated InP pyramids at [(a)-(c)] 490 °C and [(d)-

(f)] 515 °C. The diameters of the circular bases range between 1.5 and 0.7 ong>µmong>.

For higher growth temperature the QDs strongly elongate along the [01-1]

direction and number and size increase.

Figure 6.5: AFM images (2 × 2 ong>µmong> 2 ) of the multiple and single ong>InAsong> QDs grown at [(a)-(c)]

490 °C and [(d)-(f)] 515 °C on the InP pyramids with base diameters varying between 1.5 and

0.7 ong>µmong>.

The average QD width along [01-1] [Fig. 6.6(a)] increases from 50 nm for

the QDs grown at 490 °C to 75 nm for the QDs grown at 515 °C with a

broader distribution while the average width along [011] [Fig. 6.6(b)] of 50

nm remains almost unchanged. The increase of the QD number and size for

higher growth temperature is attributed to the enhanced thermally activated

As/P exchange during QD growth, which also accounts for the broader size

distribution. Hence, the amount of ong>InAsong> incorporated into the QDs is

84


increased, even in the presence of the GaAs interlayer which partly

suppresses excess ong>InAsong> formation. This is consistent with the fact that P

atoms are more easily desorbed from the surface due to the high P vapor

pressure and the As/P exchange reaction is greatly enhanced along the

periphery of the ong>InAsong> QDs. This leads to free In adatoms migrating toward

both the apex and side of the QDs and the subsequent binding with As for

excess ong>InAsong> formation [19]. The enhanced QD elongation along [01-1] at

higher growth temperature is attributed to the increased adatom surface

migration length which is generally larger along [01-1] and the increased QD

size leading to a shape transition from round to elongate [20, 21].

Figure 6.6: Histograms of (a) width along [01-1] and (b) width along [011] of the QDs grown

at 490 and 515 °C on the pyramids with the largest base area.

85


Micro-PL spectra taken at 5 K of the capped multiple and single QDs

grown at 490 °C are shown in Fig. 6.7. The PL spectrum of the multiple QDs

on the pyramid with the largest base is centered at 1465 nm with a full width

at half maximum (FWHM) of 34 nm while the PL line of the single QD is at

1471.4 nm with the FWHM of 0.6 nm limited by the spectrometer resolution.

For the higher QD growth temperature of 515 °C, the PL spectrum of the

multiple QDs is centered at 1495 nm with a broader FWHM of 59 nm while

the PL line of the single QD is at 1483 nm. This PL red shift and broader

FWHM are in agreement with the larger size and less uniform size

distribution of the QDs grown at higher temperature.

The excitation power dependent integrated PL intensity of a typical

single QD grown at 490 °C is plotted in the inset of Fig. 6.7. The integrated

PL intensity increases linearly with a slope of 1.08 indicating high optical

quality of the single QD with a low carrier scattering rate by impurities and

crystalline defects [22, 23] and negligible carrier losses due to nonradiative

recombination in the single QD and InP nanopyramid [24, 25]. At higher

excitation power > 10 µW, the QD emission saturates, leading to a reduced

slope of the integrated PL intensity.

Figure 6.7: Micro-PL spectra taken at 5 K of the multiple and single ong>InAsong> QDs grown at 490

°C. The inset shows the integrated PL intensity at 5 K of a single ong>InAsong> QD grown at 490 °C

as a function of the excitation power.

86


The shape anisotropy of the QDs results in distinct linear polarization of

the PL. Figures 6.8(a) and 6.8(b) shows the polar plot of the PL peak

intensity (I) of the multiple (on the pyramids with the largest base) and

single ong>InAsong> QDs grown at 490 and 515 °C. 0° designates the PL polarized

along [01-1] and 90° the PL polarized along [011]. The angular dependence of

the PL intensity is fitted by I = I[011] sin 2 θ + I[01-1] cos 2 θ, where θ is the

polarization angle. Most obvious for the QDs grown at 515 °C, the PL is

polarized along [01-1] which is the direction of QD elongation [26, 27]. Most

important, the DOP = (I[01-1] - I[011]) / (I[01-1] + I[011]) × 100% at the PL peak

position reduces with reduced pyramid base, i.e., reduced pyramid top area

and QD number, plotted in Fig. 6.8(c). The DOP for the multiple QDs on the

pyramid with the largest base area of 1.58 ong>µmong> 2 [Fig. 6.5(d)] is 38%. This is

comparable with polarization measurements of ong>InAsong> QD ensembles on GaAs

truncated pyramids [28]. The DOP decreases monotonically down to 17% for

the single QD on the pyramid with the smallest base area of 0.48 ong>µmong> 2 [Fig.

6.5(f)]. This is attributed to an increasing influence of the shape of the

shrinking pyramid top area, approaching the symmetric (100) top facet with

diamond-like boundary, on the QD shape rendering it more symmetric. For

the QDs grown at 490 °C, which are generally more symmetric, the slight

DOP for the multiple QDs is completely eliminated for the single QD.

It is difficult to quantitatively relate the DOP to the FSS which is the

figure of merit for realizing entangled photon sources and within the spectral

resolution the FSS cannot be resolved directly. In general, for ong>InAsong>/InP QDs

the FSS mainly depends on the QD shape [13]. The FSS from the intrinsic

Dresselhaus term [29] due to the bulk inversion asymmetry is a minor factor

because of the small lattice mismatch (3%) and the strong confinement of

holes. Moreover it has been reported that the FSS is directly related to the

DOP [14-16]. Hence, zero DOP implies the sought for zero FSS for our single

symmetric ong>InAsong> QDs grown at reduced temperature on the InP pyramids

close to pinch-off.

87


Figure 6.8: Polar plot of the polarized PL peak intensity for (a) multiple and (b) single ong>InAsong>

QDs grown at 490 and 515 °C. The solid lines represent fits to the experimental data by

I = I[011] sin 2 θ + I[01-1] cos 2 θ. (c) Degree of polarization (DOP) of the QD PL peak intensity

versus the area of pyramid base for the QDs grown at 490 and 515 °C. The red and blue

dashed lines are guides to the eye.

88


6.4 Summary

In conclusion, position and distribution control of ong>1.55ong>-ong>µmong> ong>InAsong> QDs on

truncated InP pyramids grown by selective-area MOVPE was achieved. The

top surface of the truncated InP pyramids was composed of a (100) facet and

{103} and {115} facets aside. The arrangement of the facets and their relative

sizes were determined by the shape of the mask openings and the top surface

area. The arrangement of the {103} and {115} facets allowed the precise

position and distribution control of the QDs. The realization of four, three,

two, and a single QD revealed the effectiveness of the shape and size of the

pyramid top surface for QD number control.

In addition, we have studied the shape and polarization control of siteong>controlledong>

multiple and single ong>InAsong> QDs on InP (100) pyramids. The QD size

increases with elevated growth temperature and the QDs strongly elongate

causing pronounced linear polarization of the PL. Most important the degree

of polarization reduces with reduced pyramid base/pyramid top area/QD

number due to increasing influence of the symmetric pyramid top on the QD

shape, reaching zero for single QDs grown at lower temperature. This is

important for the realization of single and entangled photon sources

operating in the ong>1.55ong>-ong>µmong> telecom wavelength region.

89


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92


Chapter 7

Integration of Position Controlled

ong>InAsong> ong>Quantumong> ong>Dotsong> into Planar InP

Structures

Position-ong>controlledong> ong>InAsong> quantum dots (QDs) are integrated into planar InP

structures by employing selective-area growth of InP pyramids and regrowth

by metal-organic vapor phase epitaxy (MOVPE). A smooth surface morphology

is obtained at elevated regrowth temperature due to suppression of threedimensional

growth on the pyramids. The height differences are less than 30

nm after nominal 700 nm InP regrowth at 640 °C. Most important, the

integrated QDs maintain good optical quality after regrowth for the

realization of integrated nanophotonic devices and circuits operating at

telecom wavelengths. 1

1 Based on J. Appl. Phys. 108, 104308 (2010)

93


7.1 Introduction

Photonic integration shows a clear exponential increase in component density

similar to Moore’s law in electronics [1]. For further increase, in particular

quantum dot (QD) gain material is appealing due to low transparency

currents, very broad gain spectra, and the possibility of deep etching without

device degradation. Extended cavity Fabry–Pérot QD lasers have already

been demonstrated based on established Butt-joint active-passive integration

technology [2]. Pushing photonic integration technology to the fundamental

limits regarding component size, complexity, and power consumption will

require position control of QDs in small numbers, down to a single QD and

novel integration techniques allowing device operation at the few/single

electron and photon levels. Among the various approaches for QD position

control based on growth on patterned substrates [3-6] and selective area

growth [7-13] we have chosen the latter. The deposition of QDs on selectively

grown pyramids allows not only QD position and number control but also the

control of the QD distribution through the pyramids base shape [12] for

matching it, e.g., to the optical mode of a particular photonic crystal

nanocavity.

In this chapter, we demonstrate the integration of position ong>controlledong>

ong>InAsong> QDs embedded in submicron-size InP pyramids into planar structures

by regrowth of InP in metal-organic vapor phase epitaxy (MOVPE). Growth

conditions are first optimized for nominal 250 nm InP regrowth and then

transferred to thicker layers for large-scale regrowth. A smooth surface

morphology is obtained at elevated regrowth temperature due to suppression

of three-dimensional growth on the pyramids. The height differences are less

than 30 nm after optimized nominal 700 nm InP regrowth at 640 °C and

there is no growth on the pyramids top. Most important, good optical quality

of the integrated QDs is maintained for the realization of integrated

nanophotonic devices and circuits operating at telecom wavelengths.

Section 7.2 describes the experimental details. The active-passive

integration of site-ong>controlledong> ong>InAsong> QDs into the planar InP structures is

discussed in section 7.3. The conclusions are summarized in section 7.4.

7.2 Experimental details

For substrate patterning a 100 nm SiNx mask layer was deposited on the InP

(100) substrates by plasma-enhanced chemical-vapor deposition. A field of

openings in the mask was processed by electron beam lithography and

reactive ion etching. The center-to-center distance of the openings was 10 ong>µmong>.

The truncated InP pyramids were grown at 610 °C by selective-area MOVPE.

Then the temperature was lowered to 500 °C for growth of a GaAs interlayer

and the ong>InAsong> QDs. After QD formation, an 8 nm InP capping layer was

immediately grown at 500 °C, as investigated in Chapter 4. Then the

94


temperature was raised to 610 °C and another 40 nm InP cap layer was

deposited. The detailed growth parameters can be found in Refs. 12 and 13.

The SiNx mask was completely removed in 10% hydrofluoric acid (HF) for 2.5

minutes and then rinsed in distilled (DI) water. Prior to the regrowth, the

samples were cleaned three times in oxygen plasma for 10 minutes with

power of 300 watt and 10% phosphoric acid (H3PO4) to remove any surface

contaminants. The regrowth of 250 or 700 nm InP was performed at

temperatures between 550 and 640 °C. The surface morphology of the

uncapped ong>InAsong> QDs on the InP pyramids top, the fully capped pyramids

(CPs), and the regrown pyramids (RPs) was examined by tapping mode

atomic force microscopy (AFM) in air. The optical properties of the capped

and regrown QDs were studied by micro-photoluminescence (micro-PL)

spectroscopy with the samples mounted in a He-flow cryostat. The PL was

excited by the 632.8 nm line of a He-Ne laser through an optical microscope

objective, which also served to collect the PL. The PL was dispersed by a

single monochromator and detected by a cooled InGaAs linear array detector.

7.3 Results and discussion

7.3.1 Regrowth around pyramids for active-passive

integration

Figure 7.1 shows the AFM images of the uncapped InP pyramids and CPs

before and after InP regrowth. The circular shaped mask opening has a

diameter of 700 nm. Fig. 7.1(a) reveals a single ong>InAsong> QD on the InP pyramid

close to pinch-off, Fig. 7.1(b) the CP after capping the QD, and Figs. 7.1(c)-

7.1(h) the RPs after deposition of nominal 250 nm InP at the growth

temperatures of 550, 585, 600, 615, and 630 °C, respectively. Fig. 7.2(a)

depicts the corresponding line profiles taken along [01-1] and [001], offset by

the thickness of the planar layers around the pyramids. The vertical solid

black, dotted red, and dashed blue lines indicate the pyramids center, edge,

and the planar areas. In Fig. 7.2(b) the regrown layer thicknesses at the

pyramids center, edge, and of the planar layers aside are plotted as a

function of regrowth temperature; in Fig. 7.2(c) the extension of the laterally

grown layers around the pyramids along [01-1] and [001], and in Fig. 7.2(d)

the angle of the sidewalls in the [01-1] and [001] directions between the

laterally and planar grown layers. Before regrowth, the height and diameter

of the capped pyramid are 510 and 850 nm, respectively. After regrowth, with

increasing temperature from 550 to 630 °C:

(i). The regrown layer thickness at the pyramid center reduces from

460 to 0 nm.

(ii). The layer thickness at the pyramid edge decreases from 930 to

260 nm.

95


(iii).

(iv).

(v).

The thickness of the planar layer around the pyramids increases

from 0 to 230 nm, close to the thickness at the pyramid edge.

The extensions of the laterally grown layer around the pyramid

along [01-1] and [001] reduce and become equal.

The angles of the sidewalls in the [01-1] and [001] directions

between the laterally and planar grown layers reduce and

become equal.

Figure 7.1: AFM images of (a) truncated pyramid with QD on top, (b) capped pyramid (CP),

and (c)-(h) regrown pyramids (RPs) with regrowth temperatures of [(c) and (d)] 550 °C, (e)

585 °C, (f) 600 °C, (g) 615 °C, and (h) 630 °C.

96


Figure 7.2: (a) AFM line profiles taken along [01-1] and [001] of the CP and RPs for different

regrowth temperatures, offset by the thickness of the planar layers around the pyramids.

The vertical solid black, dotted red, and dashed blue lines indicate the pyramids center, edge,

and the planar areas. (b)-(d) Temperature dependence of the (b) regrown layer thickness at

the pyramids center, edge, and of the planar layers aside, (c) extension of the laterally grown

layers around the pyramids along [01-1] and [001], and (d) angle of the sidewalls in the [01-1]

and [001] directions between the laterally and planar grown layers.

Therefore, with increase of the regrowth temperature there is a clear

transition from three-dimensional asymmetric growth entirely on the

pyramid to two-dimensional symmetric lateral and planar growth around the

pyramid leading to planarization.

97


The growth rate enhancement at the pyramid edge is attributed to

adatoms migrating from the pyramid side facets to the planar area and is

reduced when planar growth dominates. The strong asymmetry of the threedimensional

growth at low temperature is due to the different growth rates

on the pyramid facets. The growth rate on the (111) B facets is the largest,

leading to strong extension along [01-1], most probably due to its surface

energy being the lowest [13]. This growth asymmetry is also reflected in the

different and relatively steep sidewall angles of the laterally grown layer

around the pyramid. At high temperature, faceting becomes less pronounced

and symmetric lateral and planar growth dominate, which is also seen in the

almost identical and reduced sidewall angles.

To perform larger-scale regrowth, the nominal InP layer thickness is

increased to 700 nm and the growth temperature to 640 °C. Fig. 7.3 shows

the AFM image of the regrown structure around a pyramid with base size of

1.5 ong>µmong> together with the line profile, including that of the CP. There is no

growth on top of the pyramid. The growth rate enhancement at the pyramid

edge is reduced to 30 nm which is the same as the height difference of the

laterally and planar grown layers. The extension of the laterally grown layer

is increased to 5 ong>µmong> with an overall very smooth surface morphology. The PL

efficiency of the regrown InP is high, comparable to that of planar layers.

This confirms good crystal quality and indicates a low level of nonradiative

recombination centers which is required for low absorption of the passive

structure in photonic integrated circuits.

Figure 7.3: AFM image of the optimized large-scale RP with base size of 1.5 ong>µmong> together

with the line profile, including that of the CP. Inserted arrow indicates the profile place and

direction.

98


7.3.2 Optical properties of integrated single quantum dot

Figure 7.4(a) shows the micro-PL spectrum of an integrated single QD after

regrowth taken at 5 K with 13.6 nW laser excitation power. The PL peak is

centered at 0.8478 eV (1462 nm) with a full width at half maximum (FWHM)

of 308 µeV (0.5 nm). This is close to the resolution limit of our setup and most

probably larger than the intrinsic linewidth due to pronounced broadening

with excitation power already observed in this regime, as shown in Fig.

7.4(b). When the excitation power is increased from 13.6 nW up to 136 µW

the PL linewidth increases up to 6 nm. This is attributed to accumulating

charge around the QD with a statistically fluctuating distribution affecting

the QD energy levels. Hence the smallest FWHM of 0.5 nm we can measure

at the lowest excitation power (13.6 nW) limited by the sensitivity of our PL

setup is likely not the intrinsic dephasing related to linewidth of the highquality

single QD.

Figure 7.4: (a) Micro-PL spectrum of an integrated single QD after regrowth. (b) Excitation

power dependent micro-PL spectra. P0 = 13.6 nW. Resolution of monochromators is indicated.

99


To definitely make sure that the emission stems from a single QD,

temperature dependent PL spectra are taken, shown in Fig. 7.5(a). The

excitation power is 136 µW in order to trace the emission up to sufficiently

high temperatures. With increasing temperature the PL peak redshifts from

1456 nm at 5 K to 1478 nm at 120 K, plotted in Fig. 7.5(b), following the

change of the ong>InAsong> bandgap, typical for single QDs. The thermal quenching of

the PL intensity is probably due to the thermal escape of carriers from the

QD [14, 15]. The FWHM does not change much below 50 K (from 6 to 7 nm),

while there is a strong increase for higher temperatures. This again is typical

for single QDs also on planar substrates due to the change over from acoustic

to optical phonon scattering [16, 17].

Figure 7.5: (a) Temperature dependent micro-PL spectra and (b) plot of the QD PL peak

position and FWHM versus temperature.

100


7.4 Summary

In summary, we have demonstrated the planarization of selectively grown

submicron size InP pyramids containing integrated ong>InAsong> QDs by regrowth of

InP in MOVPE. Taking advantage of the two-dimensional lateral and planar

growth around the pyramids at elevated growth temperatures, a smooth

surface morphology was obtained. The height differences were less than 30

nm after optimized nominal 700 nm InP regrowth at 640 °C and there was

negligible growth on the pyramids. Most important, good optical quality of

the integrated QDs was maintained after regrowth paving the way for the

realization of integrated nanophotonic devices and circuits operating at

telecom wavelengths.

101


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103


104


List of Abbreviations

QD

QW

QWR

MBE

MOVPE

CBE

PECVD

EBL

UV

RIE

AFM

SEM

X-STM

PL

TMIn, TMI

TMGa, TMG

TBAs, TBA

TBP

SCCM

ML

FWHM

DOP

CP

RP

quantum dot

quantum well

quantum wire

molecular beam epitaxy

metal-organic vapor phase epitaxy

chemical beam epitaxy

plasma-enhanced chemical-vapor deposition

electron beam lithography

ultra-violet

reactive ion etching

atomic force microscopy

scanning electron microscopy

cross-sectional scanning tunneling microscopy

photoluminescence

trimethyl-indium

trimethyl-gallium

tertiarybutyl-arsine

tertiarybutyl-phosphine

standard cubic centimeter per minute

monolayer

full width at half maximum

degree of polarization

capped pyramid

regrown pyramid

105


106


Summary

This thesis demonstrates the growth, structural and optical characterizations

of site-ong>controlledong> ong>1.55ong>-ong>µmong> ong>InAsong> quantum dots (QDs) on truncated InP

nanopyramids grown by selective-area metal-organic vapor phase epitaxy

(MOVPE). This is the essential prerequisite for applications of such

nanostructures in future quantum functional devices, such as nanolasers and

single photon sources, operating in the technologically important ong>1.55ong>-ong>µmong>

telecom wavelength region.

The overviews regarding low-dimensional nanostructures, particularly

epitaxial QDs, including their fundamental properties and applications were

introduced. The material properties of III-V compound semiconductors

including the zinc-blende structure and crystallographic planes were

discussed. Two typical methods to grow self-assembled QDs using the

Stranski-Krastanow (S-K) growth mode and droplet epitaxy were explained.

To motivate the study of site-ong>controlledong> QDs in this thesis, the pre-defined

QD formation on patterned templates including truncated pyramids, V-

grooves, and nano-holes was discussed. In this thesis, the approach to achieve

site-ong>controlledong> ong>InAsong> QDs by pre-defined QD formation on truncated InP

nanopyramids is applied.

ong>InAsong> QD growth on planar InP (100) by MOVPE with a thin GaAs

interlayer was primarily studied. A wide wavelength range and emission at

ong>1.55ong> ong>µmong> at room temperature were accessed by solely varying the thickness of

the GaAs interlayer. The continuous wavelength tuning was attributed to the

suppression of As/P exchange during ong>InAsong> QD growth. For optimized growth

conditions, i.e., 2 ML GaAs, 490 °C growth temperature, 1.5 SCCM (partial

pressure of 0.0015 mbar) TBA supply, and 180 SCCM (partial pressure of

0.0042 mbar) TMI supply, high quality ong>InAsong> QDs emitting in the ong>1.55ong>-ong>µmong>

telecom wavelength region were achieved.

Moreover, we investigated the effect of the thickness of the first lowtemperature

grown (Low-T) InP cap layer on the ong>InAsong> QD PL wavelength

and efficiency when followed by high-temperature (High-T) capping. With

increase of the Low-T cap layer thickness, the PL peak redshifted and the

efficiency increased up to a thickness of 8 nm after which the PL peak

wavelength stayed constant and the efficiency strongly decreased. This

optimum Low-T cap layer thickness was attributed to the balance between

stability of the QDs during heat-up and defect diffusion toward the QDs

during High-T capping which is crucial when the QDs are incorporated in

device structures.

The impact of base size and shape to control the evolution of multifaceted

InP nanopyramids grown by selective-area MOVPE was reported. Large base

sizes lead to truncated pyramids with the top surfaces composed of a (100)

center facet and naturally formed {103} and {115} facets around. In contrast,

small base sizes lead to formation of only a (100) top facet. The arrangement

107


and (relative) size of the facets are governed by the size and shape of the

pyramids top area. The arrangement and (relative) size of the top facets in

turn are governed by the {110} and {111} side facets whose area (ratio)

depends on the pyramids base size and shape. This directly determines the

ratio of the (100) top facet area and the sum of the {110} and {111} side facet

areas, related to the minimization of the total surface energy, as well as the

height of the pyramids. These findings are the basis to control the

distribution (on the high-index facets for large top area) and number (on the

(100) top facets for small top area) of ong>InAsong> QDs deposited on the InP

nanopyramids. The realization of four, three, two, and a single QD revealed

the effectiveness of the shape and size of the pyramid top surface for QD

number control.

In addition, we have studied the shape and polarization control of siteong>controlledong>

multiple and single ong>InAsong> QDs on InP (100) pyramids. The QD size

increases with elevated growth temperature and the QDs strongly elongate

causing pronounced linear polarization of the PL. Most important the degree

of polarization reduces with reduced pyramid base/pyramid top area/QD

number due to increasing influence of the symmetric pyramid top on the QD

shape, reaching zero for single QDs grown at lower temperature. This is

important for the realization of single and entangled photon sources

operating in the ong>1.55ong>-ong>µmong> telecom wavelength region.

Finally, integration of position ong>controlledong> ong>InAsong> QDs into planar InP

structures was presented. We have demonstrated the planarization of

selectively grown submicron size InP pyramids containing integrated ong>InAsong>

QDs by regrowth of InP in MOVPE. Taking advantage of the twodimensional

lateral and planar growth around the pyramids at elevated

growth temperatures, a smooth surface morphology was obtained. The height

differences were less than 30 nm after optimized nominal 700 nm InP

regrowth at 640 °C and there was negligible growth on the pyramids. Most

important, good optical quality of the integrated QDs was maintained after

regrowth paving the way for the realization of integrated nanophotonic

devices and circuits operating at telecom wavelengths.

108


List of Publications

Journal papers

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, Jia Wang, Tjibbe de

Vries, Barry Smalbrugge, Chaoyuan Jin, Peter Nouwens, Erik Jan Geluk,

Andrei Yu. Silov, and Richard Nötzel, Controlling polarization anisotropy of

site-ong>controlledong> ong>InAsong>/InP (100) quantum dots, Appl. Phys. Lett. 98, 201904

(2011).

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, and Richard Nötzel, ong>InAsong>

quantum dot growth on planar InP (100) by metalorganic vapor-phase epitaxy

with a thin GaAs interlayer, J. Cryst. Growth 315, 102 (2011).

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, and Richard Nötzel,

Effect of low-temperature InP cap layer thickness on ong>InAsong> quantum dot

photoluminescence, J. Cryst. Growth 318, 570 (2011).

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, Tjibbe de Vries, Barry

Smalbrugge, Erik Jan Geluk, and Richard Nötzel, Planarization of InP

pyramids containing integrated ong>InAsong> quantum dots and their optical

properties, J. Appl. Phys. 108, 104308 (2010).

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, and Richard Nötzel,

Impact of base size and shape on formation control of multifaceted InP

nanopyramids by selective area metal organic vapor phase epitaxy, J. Appl.

Phys. 106, 124304 (2009).

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, Torsten Rieger, Peter

Nouwens, Tom J. Eijkemans, Tjibbe de Vries, Barry Smalbrugge, Erik Jan

Geluk, and Richard Nötzel, ong>1.55ong> ong>µmong> ong>InAsong> quantum dot distribution on

truncated InP pyramids and regrowth by selective area epitaxy, IOP Conf.

Series: Materials Science and Engineering 6, 012004 (2009).

Hao Wang, Jiayue Yuan, Torsten Rieger, René P. J. van Veldhoven, Peter

Nouwens, Tom J. Eijkemans, Tjibbe de Vries, Barry Smalbrugge, Erik Jan

Geluk, and Richard Nötzel, Distribution control of ong>1.55ong> ong>µmong> ong>InAsong> quantum

dots down to small numbers on truncated InP pyramids grown by selective

area metal organic vapor phase epitaxy, Appl. Phys. Lett. 94, 143103 (2009).

109


Hao Wang, Jiayue Yuan, P. J. van Veldhoven, T. de Vries, E. Smalbrugge, E.

J. Geluk, E. A. J. M. Bente, Y. S. Oei, M. K. Smit, S. Anantathanasarn, and

R. Nötzel, Butt joint integrated extended cavity ong>InAsong>/InP (100) quantum dot

laser emitting around ong>1.55ong> ong>µmong>, Electron. Lett. 44, 522 (2008).

Papers in progress

Jiayue Yuan, Chaoyuan Jin, Matthias Skacel, Adam Urbańczyk, René P. J.

van Veldhoven, Andrea Fiore, and Richard Nötzel, Coupling of ong>InAsong> quantum

dots to the plasmon resonance of In nanocrystal by droplet epitaxy, to be

submitted.

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, and Richard Nötzel,

Thermal stability of InP nanopyramids, to be submitted.

Jia Wang, Jiayue Yuan, Frank W. M. van Otten, and Richard Nötzel,

Combining selective area growth and self-organized strain engineering for

site-ong>controlledong> local ong>InAsong>/InP quantum dot arrays, accepted for publication in

J. Cryst. Growth.

International conference proceedings

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, and Richard Nötzel, ong>InAsong>

quantum dot growth on planar InP (100) by metalorganic vapor-phase epitaxy

with a thin GaAs interlayer, Physics @ Fundamental Research on Matter

(FOM), Veldhoven, The Netherland, January 18-19, 2011. (Poster)

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, and Richard Nötzel,

Formation of site-ong>controlledong> ong>InAsong>/InP quantum dots and their integration

into planar structures, Conference on Optoelectronic and Microelectronic

Materials and Devices, The Australian National University, Canberra,

Australia, December 12-15, 2010. (Poster)

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, and Richard Nötzel,

Lateral arrangements of size and number ong>controlledong> ong>1.55ong>-ong>µmong> ong>InAsong> quantum

dots on InP nanopyramids, Proceedings of the 15th Annual Symposium of the

IEEE Photonics Benelux Chapter, Delft, The Netherlands, November 18-19,

2010. (Oral talk)

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, and Richard Nötzel,

Effect of low-temperature InP cap layer thickness on ong>InAsong> quantum dot

photoluminescence, The Sixteenth International Conference on Crystal

Growth and The Fourteenth International Conference on Vapor Growth and

Epitaxy (ICCG-16/ICVGE-14), Beijing, China, August 8-13, 2010. (Poster)

110


Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, and Richard Nötzel,

Distribution, size, and number control of position-ong>controlledong> ong>InAsong> quantum

dots on InP nanopyramids, The Sixteenth International Conference on

Crystal Growth and The Fourteenth International Conference on Vapor

Growth and Epitaxy (ICCG-16/ICVGE-14), Beijing, China, August 8-13, 2010.

(Oral talk)

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, and Richard Nötzel, ong>InAsong>

quantum dot growth on planar InP (100) by metalorganic vapor-phase epitaxy

with a thin GaAs interlayer, 15th International Conference on Metal Organic

Vapor Phase Epitaxy (ICMOVPE-XV), Hyatt Regency, Lake Tahoe, USA,

May 23-28, 2010. (Poster)

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, and Richard Nötzel,

Distribution and number control of ong>1.55ong>-ong>µmong> ong>InAsong> quantum dots on truncated

InP pyramids grown by selective area metalorganic vapor-phase epitaxy, 15th

International Conference on Metal Organic Vapor Phase Epitaxy (ICMOVPE-

XV), Hyatt Regency, Lake Tahoe, USA, May 23-28, 2010. (Oral talk)

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, and Richard Nötzel, ong>1.55ong>-

ong>µmong> ong>InAsong> quantum dot number and size control on truncated InP pyramids

and integration by selective area epitaxy, Proceedings of SPIE Vol. 7610

[7610-37]. ong>Quantumong> ong>Dotsong> and Nanostructures: Synthesis, Characterization,

and Modeling VII. SPIE Photonic West, San Francisco, California, USA,

January 23-28, 2010. (Oral talk)

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, and Richard Nötzel, ong>1.55ong>-

ong>µmong> ong>InAsong> quantum dot distribution control on truncated InP pyramids and

regrowth by selective area epitaxy, Physics @ Fundamental Research on

Matter (FOM), Veldhoven, The Netherland, January 19-20, 2010. (Poster)

Richard Nötzel, Nut Sritirawisarn, Ekber Selçuk, Hao Wang, and Jiayue

Yuan, Lateral ordering, position, and number Control: tackling today’s

challenges in quantum dot materials research, 2009 Collaborative Conference

on Interacting Nanostructures, San Diego, USA, November 9-13, 2009.

(Invited talk)

Richard Nötzel, Nut Sritirawisarn, Ekber Selçuk, Hao Wang, and Jiayue

Yuan, ong>Quantumong> dots for future nanophotonic devices: lateral ordering,

position, and number Control, 2009 IEEE Photonics Society Annual meeting,

Belek-Antalya, Turkey, October 4-8, 2009. (Invited talk)

111


Richard Nötzel, Nut Sritirawisarn, Ekber Selçuk, Hao Wang, and Jiayue

Yuan, Tackling today’s challenges in quantum dot materials research: lateral

ordering, position, and number control, SemiconNano 2009, Anan, Japan,

August 9-14, 2009. (Invited talk)

Hao Wang, Jiayue Yuan, René P. J. van Veldhoven, and Richard Nötzel,

Distribution control of ong>1.55ong> ong>µmong> ong>InAsong> quantum dots down to small numbers on

truncated InP pyramids grown by selective area metal organic vapor phase

epitaxy, SPIE Optics + Photonics, San Diageo, California, USA, August 2-6,

2009. (Poster)

Jiayue Yuan, Hao Wang, and Richard Nötzel, Distribution control of ong>1.55ong> ong>µmong>

ong>InAsong> quantum dots down to small numbers on truncated InP pyramids grown

by selective area metal organic vapor phase epitaxy, International Nano-

Optoelectronics Workshop (iNow), Stockholm and Berlin, August 1-15, 2009.

(Oral talk and poster, Best student paper, the third place)

Jiayue Yuan, Hao Wang, René P. J. van Veldhoven, Torsten Rieger, Peter

Nouwens, Tom J. Eijkemans, Tjibbe de Vries, Barry Smalbrugge, Erik Jan

Geluk, and Richard Nötzel, ong>1.55ong> ong>µmong> ong>InAsong> quantum dot distribution on

truncated InP pyramids and regrowth by selective area epitaxy, Symposium

K, E-MRS 2009 Spring Meeting, Strasbourg, France, June 8-12, 2009. (Oral

talk)

Hao Wang, Jiayue Yuan, Torsten Rieger, P. J. van Veldhoven, Peter

Nouwens, Tom J. Eijkemans, Tjibbe de Vries, Barry Smalbrugge, Erik Jan

Geluk, and Richard Nötzel, Distribution and number control of ong>InAsong> quantum

dots emitting around ong>1.55ong> ong>µmong> on truncated InP pyramids grown by selective

area metal organic vapor phase epitaxy, Symposium VIII-Emerging

Semiconductor Technologies, Semicon, Shanghai, China, March 17-19, 2009.

(Oral talk)

Hao Wang, Jiayue Yuan, P. J. van Veldhoven, T. de Vries, E. Smalbrugge, E.

J. Geluk, E. A. J. M. Bente, Y. S. Oei, M. K. Smit, S. Anantathanasarn, and

R. Nötzel, Butt-joint integrated ong>InAsong>/InP quantum dot Fabry-Perot laser

devices emitting at 1.5 ong>µmong>, 14th European Conference on Integrated Optics,

ECIO 08, Eindhoven, The Netherlands, June 11-13, 2008. (Oral talk)

112


Acknowledgements

Research is, as many other things in life, made up by team efforts. Writing

this acknowledgement part is a real pleasure, it means that a lot has been

accomplished and I would like to thank a number of people who helped and

encouraged me during my PhD project.

First of all, I would like to express my deep gratitude to Prof.Dr. Richard

Nötzel, my daily supervisor, my main support through all those years. I

would like to thank him here for the immensity of knowledge he transferred

to me on the way to become a good physicist. Dear Richard, we did it! Thank

you for opening this wonderful project to me, I must say that working with

you has made a fruitful collaboration. I really appreciate all the scientific

discussions we have had throughout these four years. Thank you for being

available and patient to me all the time. Keeping eyes on my progresses, you

have always been ready to redirect me in right track when necessary. Your

continuous scientific support, both in material growth and optical

characterization, is highly acknowledged. In particular, I sincerely wish you

and your family all the best in Madrid, Spain.

Then, I would like to thank my first promoter Prof.Dr. Paul Koenraad for

his valuable comment and support. Dear Paul, it was always nice talking to

you and your positive attitude and enthusiasm helped me quite a lot.

Throughout your guidance on the assessment talks and other occasions, the

four years PhD study in PSN group went smoothly as planned. Thank you

very much for reading the thesis and giving the critical remarks.

Of course I would like to thank some academic staff members in the

group for their helps and discussions: Dr. Andrei Silov, Dr. Rob van de

Heijden, and Prof.Dr. Andrea Fiore. In particular, I want to express my

sincere gratitude to Dr. Andrei Silov who assisted me a lot in optical

measurements. I really appreciated his advices and encouragement which

have made it possible to go through one tough peer-reviewed paper.

Moreover, it was really funny to interact with you for many jokes during the

coffee time. Thank you also for constantly giving me suggestions and support

to my future career.

I am also very grateful to the reading committee members: Prof.Dr.

Sebastian Lourdudoss, Prof.Dr. Stefano Sanguinetti, and Prof.Dr. Meint Smit

for evaluating my PhD thesis. I greatly appreciate your critical and valuable

comments which improve the quality of my thesis very well.

My sincere thankfulness also goes to Dr. Hao Wang who helped me quite

a lot in experiments. I must say that it was very easy and enjoyable to work

with Dr. Hao Wang. Through him, I got familiar with MOVPE, cleanroom

processings, and characterization techniques. Most important, lots of nice

collaborations and scientific discussions lead to several beautiful results in

this thesis. I gratefully admire his professional attitude towards the scientific

work. Thank you for not only keeping me on a good track during my research,

113


ut also for being my friend. I would like to delivery my best wishes to you for

every possible success in Australia.

It was extremely a great pleasure for me to work with our group’s

technicians. My special thanks to René van Veldhoven for the MOVPE

assistances. Through him, the results presented in thesis were made possible

due to the high quality samples growth in our MOVPE reactor. I thank Tom

Eijkemans for his technical assistance in using AFM and PL setups. It was

really funny to interact with you for many comedy imitations. I thank Frank

van Otten and Rian Hamhuis for discussions on the material growth. I also

would like to thank Peter Nouwens for helping me with SEM and EBL, Jos

van Ruijven for helping with vacuum systems and helium issues, and

Martine van Vlokhoven for electronics support. My acknowledgements also go

for the technicians from OED group: Tjibbe de Vries, Barry Smalbrugge, and

Erik Jan Geluk for training and assisting me a lot of processings in the

cleanroom.

I am indebted to my colleagues, at Photonics and Semiconductor

Nanophysics (PSN) group, for providing me a nice environment to learn and

grow. I would like to thank Margriet van Doorne and Annebee Langenhuizen

for the administrative matters. I am especially grateful to some of my excolleagues:

Cem, Ekber, Nut, Jan, Niek, Harm, Junji, Arjan, Mehmet, and

Murat. I am also quite happy of working with Matthias, Adam, Chaoyuan,

Jia, Tian, Zili, Bowen, Saeedeh, Leonardo, Francesco, Döndü, Stenven, Joost,

Salman, and Joris. I wish them a lot of successes in their research and future

career. In addition, a special thank goes to all my friends in Eindhoven for

the good times we spent together.

I am also very grateful to the close collaborations I have had with thesis

students and I would like to give many thanks to Torsten Rieger and Paul

Scheer van Maaswaal for their contributions to my PhD project.

I can not, of course, end without thanking my parents and my wife. I

would like to express my great gratitude to my parents for supporting me

spiritually with constant encouragements throughout my whole life and

education. I think that you will be very proud of my success in Eindhoven,

The Netherlands. At last, I also would like to give my greatest thanks to my

wife, Yanting who is constantly there for me with her unconditional love and

support during all these years. Thank you so much for showing great interest

in my “ong>Quantumong> ong>Dotsong>” and always inspiring me to hold the positive and

optimistic attitude towards my research. It is to them I dedicate this

dissertation.

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Curriculum Vitae

Jiayue Yuan, born on 21 st of June 1983 in Yangzhou, Jiangsu Province, China

2007-2011:

• Ph.D. in Physics, in Group of Photonics and Semiconductor

Nanophysics, Department of Applied Physics, Eindhoven University of

Technology (TU/e), Eindhoven, The Netherlands

• Thesis title: Distribution, Number, and Polarization Control of Siteong>controlledong>

ong>1.55ong>-ong>µmong> ong>InAsong> ong>Quantumong> ong>Dotsong> on InP Nanopyramids

• Supervisor and promoter: Prof.Dr. Richard Nötzel and Prof.Dr. Paul

Koenraad

2005-2007:

• M.Sc. in Photonics, in School of Information and Communication

Technology, Royal Institute of Technology (KTH), Stockholm, Sweden

• Thesis title: Observation and Analysis of Voids in Buried Arrayed

Waveguide Gratings

• Supervisor and promoter: Dr. Fredrik Olsson and Prof.Dr. Sebastian

Lourdudoss

2001-2005:

• B.Sc. in Communication Engineering, in Department of Electrical

Engineering, Northwestern Polytechnical University, Xi’an, China

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