Identification of quantum confined interband transitions in type-II ...

Identification of quantum confined interband transitions in type-II
InAs/GaSb superlattices using polarization sensitive photocurrent
spectroscopy
Nutan Gautam, Ajit Barve, and Sanjay Krishna
Citation: Appl. Phys. Lett. 101, 221119 (2012); doi: 10.1063/1.4767358
View online: http://dx.doi.org/10.1063/1.4767358
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v101/i22
Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 101, 221119 (2012)
Identification of quantum confined interband transitions in type-II InAs/GaSb
superlattices using polarization sensitive photocurrent spectroscopy
Nutan Gautam, Ajit Barve, and Sanjay Krishna a)
Center for High Technology Materials, Electrical and Computer Engineering Department,
University of New Mexico, Albuquerque, New Mexico 87106, USA
(Received 2 October 2012; accepted 29 October 2012; published online 29 November 2012)
We report on the use of polarization sensitive photocurrent spectroscopy for identifying the
participating transitions in type-II InAs/GaSb strained layer superlattice system. Transverse electric
and transverse magnetic photocurrents have been measured for both midwave infrared and
longwave infrared superlattices, and prominent features have been analyzed to identify different
interband transition energies and unambiguously predict the correct ordering of hole minibands.
The interband transition energies have also been confirmed with theoretical simulations using
empirical pseudopotential method. Order of the participating valence minibands has been
determined as: heavy-hole1, light-hole1 and light-hole2, with increase in hole energy. VC 2012
American Institute of Physics. [http://dx.doi.org/10.1063/1.4767358]
The past decade has witnessed the emergence of the type-
II 1,2 strained layer superlattice (T2SL) based on the InAs/
GaSb/AlSb 6.1 Å family. 3 Ever since their application for
infrared detection has been demonstrated, 4,5 significant advances
in epitaxial growth techniques, such as molecular beam
epitaxy (MBE) and a host of material characterization tools,
have enabled good control over the crystalline quality of these
mixed-cation mixed-anion superlattices. This has sparked dramatic
advances in the field of infrared detectors. For example,
the first superlattice focal plane array (FPA) in a 320 256
format was reported in 2005 in the mid-wave infrared region
(MWIR, k c 5 lm). 6 In the past five years, mega-pixel FPAs
have been demonstrated using InAs/GaSb T2SL 7,8 in the
more technologically challenging long wave infrared (LWIR,
k c 10 lm) with a noise-equivalent temperature difference
comparable to state of the art detectors based on mercury cadmium
telluride (MCT, HgCdTe) alloy. 9
Although the infrared detector applications of T2SL have
led to a significant technological progress, 10–12 the physics
behind the material system is not yet very well understood.
For example, the order of the quantum confined interband
transitions between hole levels and electron levels is unclear.
Even though it is generally accepted that the band-edge states
are dominated by a transition between the lowest confined
electron level in the conduction band (C1) and the highest
lying heavy hole level in the valence band (HH1), the
sequence of higher energy transitions has not been well established.
Different theoretical methods have been applied to
understand the bandstructure, electronic and optical properties
of superlattices. 5,13–16 There have been a number of contradicting
theoretical ventures at identifying the ordering of minibands
in the valence band. It has been suggested in the
literature that valence minibands are ordered as HH1, LH1,
HH2 with increasing hole energy, 17,18 while elsewhere the theoretically
suggested ordering is HH1, HH2, and LH1. 14,19,20
In this paper, we report polarization sensitive photocurrent
spectroscopy in InAs/GaSb T2SL photodiodes, which
a) Author to whom correspondence should be addressed. Electronic mail:
skrishna@chtm.unm.edu.
has enabled us to unambiguously identify the hole-confined
levels in both MWIR and LWIR T2SLs. The band-edge transition
is dominated by the transverse electric (TE) polarization
photocurrent, indicating a C1-HH1 transition as the
lowest energy transition. A clear dip is observed in the transverse
magnetic (TM) photocurrent spectrum, indicating the
onset of the higher order transition. TE and TM polarization
photocurrent spectra have been compared and different interband
transition energies have been extracted from theoretical
calculations using empirical pseudopotential method
(EPM). 16 After correlating the experimental observations
with the theoretical results, it has been concluded that the
quantum-confined interband transitions are ordered as C1-
HH1, C1-LH1, and C1-LH2, respectively, with the increase
in transition energy. This study can be helpful in designing
lasers based on InAs/GaSb T2SL for which the absorption
coefficient in different polarizations becomes crucial.
Recently, there have been efforts on integrating metallic
structures such as a plasmonic array 21 and metamaterials on
infrared photodetectors to enhance optical absorption and to
develop polarization selective detection scheme, and this
study will be important in designing such structures for
T2SL system. The method described here is quite simple and
easily applicable to a broad range of other material systems.
The InAs/GaSb MWIR and LWIR T2SL detectors are
grown in V80H MBE chamber equipped with cracker sources
for Sb and As. The detectors are grown on GaSb:Te substrate.
The reported MWIR, made of 10 ML InAs/10ML
GaSb SL, and LWIR, made of 14 ML InAs/7 ML GaSb,
detectors are photodiodes with PIN architecture. The bottom
contact layer is N-type doped with GaTe to the level of
2.8 10 18 cm 3 , while the top contact layer is P type doped
to the level of 2.8 10 18 cm 3 with Be. The absorber region
(I region) is non-intentionally doped (n.i.d). The schematics
of detector structures are shown in Figs. 1(a) and 1(b) for
MWIR and LWIR, respectively.
After epitaxial growth, the material is fabricated into
single pixel arrays consisting of 410 410 lm 2 devices. This
starts with inductively coupled plasma etching (ICP), followed
by deposition of Ti/Pt/Au to make an ohmic contact
0003-6951/2012/101(22)/221119/4/$30.00 101, 221119-1
VC 2012 American Institute of Physics
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221119-2 Gautam, Barve, and Krishna Appl. Phys. Lett. 101, 221119 (2012)
FIG. 1. Structure schematic of (a) MWIR
and (b) LWIR InAs/GaSb type-II strained
layer superlattice photodiode.
with top and bottom contact layers. One die of the fabricated
devices is then edge polished at 45 angle to couple polarized
light to the polished edge. The device is mounted inside
a cryostat, cooled with liquid nitrogen. An illustration of the
polarization of light and orientation of TE and TM polarization
directions with respect to the device under test has been
given in Fig. 2(a). Fig. 2(b) shows the device mounted inside
the cryostat and images of the device mounted on the chip
carrier on a 45 mount. Polarization of light in the plane of
the superlattice (x-y plane) is TE, while that along the growth
direction (z-axis) is TM. The glowbar source of Fourier
transform infrared (FTIR) spectrometer is used as the
excitation source for the measurements. The beam passes
through the KRS5 polarizer for selection of either s polarization
or p polarization. It is to be noted that the sample is
mounted inside the cryostat such that the polished edge is
normal to the incident light. A bias voltage is applied across
the device using a current preamplifier. Once excitation
beam falls on the device, electron-hole pairs are generated
leading to the photocurrent.
Once photocurrent has been measured for s and p polarized
beams, it is resolved into TE and TM component using
the following math. 22 If R p and R s are the photocurrents for p
and s polarized beams, respectively, then the photocurrent
due to TE polarization is same as R s . The photocurrent for
TM polarization can be obtained by using Eq. (1)
TE
TM ¼ 1
h i: (1)
2 R p
1
R s
FIG. 2. (a) Schematic of the structure of device under test and the orientation
of electric field with respect to the device. TE polarization is the polarization
of light in plane of T2SL while TM is in the growth direction. (b)
Picture of device mounted inside the cryostat for measurements at 77 K, and
a zoomed in image of the device mounted in the chip carrier at 45 angle.
Figs. 3(a) and 3(b) show the experimentally observed photocurrent
spectra. The unpolarized photocurrent spectrum for
10 ML InAs/10 ML GaSb MWIR T2SL photodetector has
been shown in Fig. 3(a), which shows a cutoff wavelength of
5.8 lm, at 77 K and a reverse bias of 50 mV. From the polarized
photocurrent spectrum shown in Fig. 3(b), we make
three key observations: (1) The TE photocurrent is higher
than the TM photocurrent for all the wavelengths; (2) the
TM absorption vanishes near the band-edge before the TE
absorption, and (3) there is a pronounced dip in the TM photocurrent
at 3.7 lm (0.335 eV), away from the band-edge of
the T2SL. Interband transition energies have been obtained
from theoretical simulations using EPM. The electronic
bandstructure is first calculated for bulk materials, such as
InAs and GaSb, using pseudopotential form factors for these
materials. Once bulk bandstructure and pseudopotential form
factors are known, the extension of EPM simulations to
T2SL is straightforward as has been described in Ref. 16.
Fig. 3(c) shows the calculated bandstructure and energy dispersion
relationship for MWIR for wave-vector in the plane
of T2SL (k xy ) as well as in the growth direction (k z ). Zero on
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221119-3 Gautam, Barve, and Krishna Appl. Phys. Lett. 101, 221119 (2012)
FIG. 3. (a) Unpolarized, (b) TE, and TM photocurrent spectra for MWIR
photodiode. (c) Calculated bandstructure of T2SL showing dispersion of
energy with respect to electron wavevector in plane (k xy ) and in the growth
direction (k z ) for MWIR T2SL.
the energy scale denotes the conduction band energy of InAs
at C point. As can be seen, bands are much more dispersive
in the plane of the T2SL as compared to the growth direction,
which is consistent with other simulation methods, such
as eight band k p. 19
It is known that for heavy hole transitions, the TE polarization
absorption stronger than the TM polarization absorption,
while, for light hole transitions, the TM polarization
absorption is stronger than the TE polarization absorption.
23,24 Figure 3(b) shows that near the band edge, highlighted
by the circle, TM polarization photocurrent is smaller
than the TE polarization photocurrent and it vanishes before
the TE response. This indicates that the band edge transition
is from the heavy hole (HH1) miniband in valence band to
the lowest conduction miniband (C1). It confirms with the
well-known fact that the first valence miniband is a heavy
hole band. The prominent dip in the TM photocurrent
FIG. 4. (a) Unpolarized, (b) TE, and TM photocurrent spectra for LWIR
photodiode. (c) Calculated bandstructure of T2SL showing dispersion of
energy with respect to electron wavevector in plane (k xy ) and in the growth
direction (k z ) for LWIR T2SL.
response (marked with an arrow in Fig. 3(b)) specifies the
onset of interband transition from second valence miniband
to C1 band at 0.35 eV. This transition energy matches well
with the theoretically predicted energy for transition from
V2 to C1 band. After the dip, there is a significant increase
in TM photocurrent response for higher energies. Also, the
rate of increase of TM photocurrent is higher than that of TE
photocurrent. This establishes that the second valence miniband
is a light hole (LH1) band. Theoretically predicted transition
energy from V3 to C1 is at 0.436 eV. From the
photocurrent measurements, this transition is observed at
0.46 eV, as marked by an arrow in Fig. 3(b). It can be seen
that for energies higher than this transition energies, the rate
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221119-4 Gautam, Barve, and Krishna Appl. Phys. Lett. 101, 221119 (2012)
of increase in TM photocurrent signal is higher than that of
the TE photocurrent, indicating that the third valence miniband
is a light hole (LH2) band as well. Other higher order
transitions cannot be seen as the transition energy becomes
comparable to the GaSb substrate absorption edge.
Similar measurements were conducted on a homojunction
LWIR detector consisting of 14 ML InAs/7ML GaSb as
the relative position and ordering of minibands can be significantly
different between MWIR and LWIR T2SL. Fig. 4(a)
shows the unpolarized photocurrent spectrum with a cutoff
wavelength of 11.5 lm, at 77 K, at a reverse bias of 50 mV.
Fig. 4(b) shows the polarization selective photocurrent spectra
for TE and TM polarizations. Interestingly, the polarization
sensitive photocurrent spectrum also shows the same
three features observed in the MWIR T2SL: a dominant TE
polarized absorption, zero TM response at the band-edge,
and a dip in TM polarized light at 5 lm, beyond which the
signal increases monotonically for lower wavelengths. Theoretically
calculated dispersion diagram has been shown in
Fig. 4(c). Figure 4(b) shows TE photocurrent spectrum
higher than TM photocurrent near the band edge indicating
that the band edge transition is HH1-C1 transition. The
prominent dip in TM spectrum is observed at 0.25 eV in Fig.
4(b), beyond which TM spectrum increases for higher energies.
Also, the rate of increase in TM photocurrent is higher
than the TE photocurrent. This concludes that the dip in TM
spectrum marks the onset of transition from second valence
miniband to C1 conduction and establishes that the second
valence miniband is a light hole (LH1) band. Third interband
transition occurs at 0.41 eV, as marked with an arrow in Fig.
4(b). A decrease and increase in TE and TM photocurrent
spectra, respectively, has been observed for energies higher
than this transition energy. This establishes that the third valence
miniband is a light hole (LH2) band as well.
In conclusion, polarization sensitive photocurrent spectroscopy
measurements on type-II InAs/GaSb superlattice
detectors have led to the identification and ordering of quantum
confined valence minibands for both MWIR and LWIR
material systems. It has been established in this work that the
ordering of the valence minibands is heavy-hole (HH1),
light-hole (LH1), light-hole (LH2), and heavy-hole (HH2),
with increasing energy, for both MWIR and LWIR T2SL.
The interband transition energies have been well supported
by theoretical calculations.
The authors would like to acknowledge the funding
from AFOSR FA9550-10-1-0113 and FA9550-09-1-0202.
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