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ong>Effectong> ong>ofong> ong>theong> ong>oxygenong> ong>pressureong> on ong>theong> microstructure and optical properties ong>ofong>

ZnO films prepared by laser molecular beam epitaxy

Changzheng Wang a,

, Zhong Chen b , Haiquan Hu a , Dong Zhang a

a School ong>ofong> Physics Science and Information Engineering, Liaocheng University, Liaocheng 252059, Shandong Province, PR China

b School ong>ofong> Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore

article info

Article history:

Received 9 April 2009

Received in revised form

16 July 2009

Accepted 18 July 2009

PACS:

68.55. a

71.55.Gs

74.25.Gz

81.16.Mk

Keywords:

Zno films

Laser molecular beam epitaxy

Oxygen ong>pressureong>

Microstructure

Optical properties

1. Introduction

abstract

Since ong>theong> UV emission ong>ofong> ZnO films was discovered by Tang

et al. [1], ZnO has received considerable attention in ong>theong> recent

years [2–5] due to its potential properties. As one promising

metal-oxide material in ong>theong> semiconductor field ZnO is a wide and

direct gap semiconductor (3.37 eV) with hexagonal wurtzite

structure and has an exciton binding energy ong>ofong> 60 meV which is

larger than ong>theong> ong>theong>rmal energy at room temperature [6], thus

easily leading to lasing action based on excitons recombination

even above room temperature. Meanwhile, its high chemical,

ong>theong>rmal stability and abundance make it an attractive material for

a wide variety ong>ofong> applications, such as ultraviolet (UV) emitters

and detectors, surface acoustic wave (SAW) devices, gas sensors

and transparent conducting electrodes [7]. So far ong>theong>re were a few

techniques to prepare ZnO thin films, including metal organic

chemical vapor deposition (MOCVD) [8,9], pulsed laser deposition

(PLD) [4,10], sol–gel [11,12], atomic layer deposition [3], magnetron

Corresponding author. Tel.: +86 635 8237201; fax: +86 635 8258864.

E-mail address: wcz102@sjtu.org (C. Wang).

0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.physb.2009.07.165

ARTICLE IN PRESS

Physica B 404 (2009) 4075–4082

Contents lists available at ScienceDirect

Physica B

journal homepage: www.elsevier.com/locate/physb

A series ong>ofong> ZnO films were prepared on ong>theong> Si (10 0) or glass substrate at 773 K under various ong>oxygenong>

ong>pressureong>s by using a laser molecular beam epitaxy system. The microstructure and optical properties

were investigated through ong>theong> X-ray diffraction, Raman spectrometer, scanning electron microscope,

ultraviolet–visible spectrophotometer and spectrong>ofong>luorophotometer. The results showed that ZnO thin

film prepared at 1 Pa ong>oxygenong> ong>pressureong> displayed ong>theong> best crystalinity and all ZnO films formed a

columnar structure. Meanwhile, all ZnO films exhibited an abrupt absorption edge near ong>theong> wavelength

ong>ofong> 380 nm in transmission spectra. With increasing ong>theong> ong>oxygenong> ong>pressureong>, ong>theong> transmission intensity

changed non-monotonically and reached a maximum ong>ofong> above 80% at 1 Pa ong>oxygenong> ong>pressureong>, based on

which ong>theong> band gaps ong>ofong> all ZnO films were calculated to be about 3.259–3.315 eV. Photoluminescence

spectra indicated that ong>theong>re occurred no emission peak at a low ong>oxygenong> ong>pressureong> ong>ofong> 10 5 Pa. With ong>theong>

increment ong>ofong> ong>theong> ong>oxygenong> ong>pressureong>, ong>theong>re occurred a UV emission peak ong>ofong> 378 nm, a weak violet emission

peak ong>ofong> 405 nm and a wide green emission band centered at 520 nm. As ong>theong> ong>oxygenong> ong>pressureong> increased

furong>theong>r, ong>theong> position ong>ofong> UV emission peak remained and its intensity changed non-monotonically and

reached a maximum at 1 Pa. Meanwhile ong>theong> intensity ong>ofong> green emission band increased monotonically

with increasing ong>theong> ong>oxygenong> ong>pressureong>. In addition, it was also found that ong>theong> intensity ong>ofong> UV emission

peak decreased as ong>theong> measuring temperature shifted from 80 to 300 K. The analyses indicated that ong>theong>

UV emission peak originated from ong>theong> combination ong>ofong> free excitons and ong>theong> green emission band

originated from ong>theong> energy level jump from conduction band to OZn defect.

& 2009 Elsevier B.V. All rights reserved.

sputtering [13,14], and laser molecular beam epitaxy (L-MBE) [15].

Among ong>theong>m, L-MBE has been widely applied to ong>theong> growth ong>ofong>

high quality oxide thin films under high vacuum conditions.

Although ong>theong>re was a rapid development in ZnO films, recent

studies indicated that ong>theong>re are two bottlenecks to impede ong>theong> use

ong>ofong> ZnO films: (1) The efficiency ong>ofong> emission is low and ong>theong>re exist

some discrepancy in emission mechanism. For example, Lin et al.

[16] considered that ong>theong> green emission band corresponds to ong>theong>

electron transition from ong>theong> bottom ong>ofong> ong>theong> conduction band to ong>theong>

local level formed by oxide misplaced defects, while Jin et al. [17]

think ong>theong> green emission band originates from ong>theong> electron

transition from a shallow donor level formed by ong>oxygenong> vacancies

to a shallow acceptor level formed by Zn vacancies. (2) There were

no steady preparation methods ong>ofong> P-ZnO so far. In this paper, we

mainly probed ong>theong> first problem, i.e., ong>theong> emission mechanism ong>ofong>

ZnO films. It was well known that ong>theong> photoluminescence (PL) ong>ofong>

ZnO films has a close relationship with its microstructure and its

vacancies. However, ong>theong> microstructure and vacancies ong>ofong> ZnO

films depend strongly on ong>theong> ong>oxygenong> ong>pressureong> during ong>theong>

preparation ong>ofong> ZnO film. Therefore, in this paper we prepared

ZnO films with high quality by using L-MBE system and

investigated ong>theong> effect ong>ofong> ong>theong> ong>oxygenong> ong>pressureong> on ong>theong>


4076

microstructure and vacancies, and ong>theong>n tried to clarify ong>theong>

emission mechanism ong>ofong> ZnO films.

2. Experimental procedure

In our experiments, all ZnO films were prepared on ong>theong> single

crystal Si (10 0) or glass substrates at 773 K by using laser

molecular beam epitaxy (L-MBE) system. The laser source is KrF

excimer laser with a wavelength ong>ofong> 248 nm. The pulse ong>ofong> laser is

5 Hz and ong>theong> energy ong>ofong> per pulse is 100 mJ. The ZnO ceramic target

with a diameter ong>ofong> 40 mm and a purity ong>ofong> 99.99% was installed

parallel to ong>theong> substrate surface and ong>theong> distance between target

and substrate is about 5 cm. The base ong>pressureong> is about 5 10 6

Pa. During ong>theong> deposition, all KrF excimer laser pulses were

focused by a lens and were directed onto a ZnO ceramic target. In

order to improve ong>theong> crystal quality ong>ofong> ZnO films and to

compensate ong>theong> ong>oxygenong> atoms in ZnO films, ong>theong> ong>oxygenong> gas with

a purity ong>ofong> 99.999% was put into deposition chamber. The ong>oxygenong>

ong>pressureong>s chosen during deposition were 1 10 5 ,1 10 2 ,1,10

and 30 Pa, respectively. Both ong>theong> target and substrate rotated with

30 circles per minute in order to make ZnO films homogenous.

The deposition time kept 2 h for all samples.

The microstructure ong>ofong> ZnO films was investigated using both Xray

diffractometer (XRD) with Cu-Ka radiation (l ¼ 0.15406 nm)

and Raman spectrometer. The cross-section morphology ong>ofong> ZnO

films were characterized by using scanning electronic microscopy

(SEM). The room transmittance spectra ong>ofong> ZnO films were

measured by UV–Visible spectrophotometer and ong>theong> photoluminescence

(PL) spectra were measured with Edinburgh Instruments

FLS920 type Spectrong>ofong>luorophotometer in which ong>theong> excitation

wavelength was chosen as 325 nm, and a 450 W Xe-lamp was

used as ong>theong> exciting light source.

3. Results and discussion

3.1. Structure and morphology

Fig. 1 showed ong>theong> dependence ong>ofong> XRD spectra ong>ofong> ZnO films

prepared on Si substrate at 773 K on various ong>oxygenong> ong>pressureong>s. It

could be found that ZnO films formed wurtzite (B4) structure and

ong>theong>re occurred a strong (0 0 2) diffraction peak at about 2y ¼ 34.51

for all ZnO films. At a low ong>oxygenong> ong>pressureong> ong>ofong> 10 5 Pa, ong>theong> intensity

ong>ofong> (0 0 2) diffraction peak was weak and no oong>theong>r diffraction peak

occurred. As ong>theong> ong>oxygenong> ong>pressureong> increased, ong>theong> intensity ong>ofong> (0 0 2)

Fig. 1. XRD spectra ong>ofong> ZnO films prepared at various ong>oxygenong> ong>pressureong>s.

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C. Wang et al. / Physica B 404 (2009) 4075–4082

diffraction peak increased and ong>theong>n decreased, reaching a

maximum at ong>theong> middle ong>oxygenong> ong>pressureong> ong>ofong> 1 Pa. Meanwhile,

ong>theong>re occurred a weak (0 0 4) diffraction peak at 1 Pa ong>oxygenong>

ong>pressureong>. However, ong>theong> intensity ong>ofong> (0 0 4) diffraction peak was

very weak compared with that ong>ofong> (0 0 2) diffraction peak, implying

that ZnO films have strong (0 0 2) texture and grew along [0 0 2]

orientation. It was well known that ong>theong> intensity ong>ofong> diffraction

peak have a close relationship with both ong>theong> crystal quality and

ong>theong> thickness ong>ofong> films. Therefore it is not appropriate to judge ong>theong>

crystal quality ong>ofong> ZnO films only from ong>theong> intensity ong>ofong> (0 0 2)

diffraction peak. Anoong>theong>r important factor judging ong>theong> crystal

quality is full width at half maximum (FWHM) ong>ofong> rocking curve ong>ofong>

(0 0 2) diffraction peak for ZnO films, through which we can easily

judge ong>theong> crystal quality ong>ofong> ZnO films prepared at various ong>oxygenong>

ong>pressureong>s. The smaller ong>theong> FWHM, ong>theong> better ong>theong> crystal quality.

Accordingly, we measured ong>theong> rocking curve ong>ofong> (0 0 2) diffraction

peak for ZnO films prepared at various ong>oxygenong> ong>pressureong>s, as

shown in Fig. 2. It could be seen from Fig. 2 that only a broad peak

occurred in ong>theong> rocking curve and no double peak occurred,

indicating that all ZnO films grew only along [0 0 2] orientation

[18], which was in agreement with XRD results. In addition, it

could be found that as ong>theong> ong>oxygenong> ong>pressureong> increased ong>theong> FWHM

ong>ofong> ong>theong> rocking curve ong>ofong> (0 0 2) diffraction peak decreased first and

ong>theong>n increased, reaching a minimum ong>ofong> about 3.361 at 1 Pa ong>oxygenong>

ong>pressureong>. This indicated that ZnO film prepared at 1 Pa ong>oxygenong>

ong>pressureong> had ong>theong> best crystal quality among all ZnO films.

Meanwhile, it was clear from Fig. 1 that ong>theong> diffraction angle ong>ofong>

all ZnO (0 0 2) diffraction peaks shifted to higher angle compared

with ong>theong> standard XRD spectrum ong>ofong> ZnO power (2y ¼ 34.431) [19],

implying that ong>theong> lattice constant c changed for all ZnO films.

These changes were mainly caused by ong>theong> residual stress ong>ofong> ZnO

films. Therefore, ong>theong> residual stress also affects significantly ong>theong>

structure and properties ong>ofong> ZnO films to some extent and it is

important to disclose ong>theong> residual stress ong>ofong> ZnO films. For ZnO

films with wurtzite structure, ong>theong> residual stress can be obtained

by following formula [20]:

s ¼ 2c2 13

c33ðc11 þ c12Þ c c0

2c13 c0

where c i,j (i, j ¼ 1, 2, 3) stands for ong>theong> elastic constants in different

orientations. c0 is ong>theong> lattice constant ong>ofong> ZnO films without defects

and c is ong>theong> lattice constant ong>ofong> ZnO film in our experiments, which

can be gained by ong>theong> following formula:

1 4

¼

d2 3

h 2 þ hk þ k 2

d 2

þ l2

c 2

2d sin y ¼ l ð3Þ

where h ¼ 0, k ¼ 0, l ¼ 2 for ZnO (0 0 2) diffraction peak, ong>theong>refore,

c is expressed as follows:

c ¼ 2d002 ¼ l

ð4Þ

sin y

where d 002 is ong>theong> interplaner spacing ong>ofong> ZnO (0 0 2) crystal plane. l

and y are X-ray wavelength and ong>theong> Bragg diffraction angle,

respectively.

Substituting ong>theong> value ong>ofong> ci,j (i, j ¼ 1, 2, 3) in Eq. (1) with

c 11 ¼ 208.8 GPa, c 12 ¼ 119.7 GPa, c 13 ¼ 104.2 GPa, c 33 ¼ 213.8 GPa

[21], we could obtain ong>theong> residual stress s in ZnO film by ong>theong>

following formula:

s ¼ 233

c c0

c0

According to Eq. (5), we calculated ong>theong> residual stress s in

various ZnO films, as shown in Fig. 3. Fig. 3 also showed ong>theong>

ð1Þ

ð2Þ

ð5Þ


Fig. 2. Rocking curve ong>ofong> (0 0 2) diffraction peak for ZnO films prepared at various

ong>oxygenong> ong>pressureong>s.

changes ong>ofong> lattice constant (c) corresponding to ong>theong> changes ong>ofong> ong>theong>

residual stress in various films. It was apparent that ong>theong> all ZnO

films exhibited tensile stresses [22,23], which was in agreement

with ong>theong> results ong>ofong> Hong [23]. In ong>theong> ZnO film with a wurtzite

structure, ong>theong> residual film stress in ong>theong> direction being

perpendicular to ong>theong> surface originated from a C-axis strain.

Basically, this stress included an intrinsic stress originating from

ong>theong>rmal mismatch between films and substrates and anoong>theong>r

intrinsic stress originating from ong>theong> film structure. Since all ZnO

films were prepared with same substrate temperature, ong>theong>

intrinsic stress originating from ong>theong>rmal mismatch between

films and substrates must have almost same value for all ZnO

films. However, it was clear that ong>theong> intrinsic stress changed nonmonotonically

with ong>theong> ong>oxygenong> ong>pressureong>. Therefore, we drew a

conclusion that ong>theong> intrinsic stress ong>ofong> ZnO films mainly originated

from ong>theong> film structure. The better ong>theong> crystal quality, ong>theong> smaller

ong>theong> intrinsic stress was. Meanwhile, it was worthy to note that ong>theong>

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C. Wang et al. / Physica B 404 (2009) 4075–4082 4077

Fig. 3. The lattice constant and in-plane stress ong>ofong> ZnO films prepared at various

ong>oxygenong> ong>pressureong>s.

intrinsic stress reached a minimum when ong>theong> ong>oxygenong> ong>pressureong> is

1 Pa, indicating that ZnO film has ong>theong> best crystal quality at 1 Pa

ong>oxygenong> ong>pressureong>.

In addition, Raman scattering can give some information about

ong>theong> crystal structure on ong>theong> scale ong>ofong> a few lattice constants and is

also a good method to evaluate residual stress within ZnO

crystallized films [24]. Therefore, we investigated ong>theong> Raman

spectra ong>ofong> ZnO films prepared on Si substrate at various ong>oxygenong>

ong>pressureong>s, as shown in Fig. 4. It could be seen that E 2 modes

corresponding to ZnO films were discovered besides some Raman

peaks corresponding to single crystal Si and no oong>theong>r Raman peak

corresponding to ZnO were discovered. E2 modes include E2 (low)

mode at about 99 cm 1 and E2 (high) mode at about 437 cm 1 .

Moreover E2 (low) mode can evaluate ong>theong> structure ong>ofong> ZnO films

and E2 (high) mode can evaluate residual stress within ZnO films.

For E2 (low) mode, it was clear from Fig. 4 that ZnO film exhibited

a dispersed E2(low) peak with low intensity at a low ong>oxygenong>

ong>pressureong> ong>ofong> 10 5 Pa, indicating that ZnO film has bad crystal

quality. As ong>theong> ong>oxygenong> ong>pressureong> increased, E2 (low) peak first

turned sharp and ong>theong>n turned dispersed, getting ong>theong> sharpest at

1 Pa ong>oxygenong> ong>pressureong>. At ong>theong> same time, ong>theong> intensity ong>ofong> E2 (low)

peak also reached ong>theong> largest value. These indicated that ZnO film

has ong>theong> best crystal quality at 1 Pa ong>oxygenong> ong>pressureong>, which was in

good agreement with XRD results. Meanwhile, it could be seen

from Fig. 4 that all ong>theong> position ong>ofong> E2 (high) peaks for all ZnO films

shifted to low frequency compared to single crystal ZnO [25],

indicating that all ZnO exhibited tensile stress [24,26]. The Raman

frequencies corresponding to E 2 (high) mode were listed in Table

1. It was worthy to note from Table 1 that Raman frequencies ong>ofong>

ZnO film at 1 Pa ong>oxygenong> ong>pressureong> have ong>theong> smallest difference from

that ong>ofong> single crystal ZnO, implying that ZnO film at 1 Pa ong>oxygenong>

ong>pressureong> has ong>theong> smallest tensile stress, which was in good

agreement with XRD results.

During deposition ong>ofong> ZnO films, thin films tend to lower ong>theong>ir

surface energy. As a result, ong>theong> grain with lower surface energy

will become larger as ong>theong> film thickness increases and ong>theong>n ong>theong>

thin films should have a preferential growth along a crystallographic

direction, which can be confirmed by ong>theong> SEM observation.

Fig. 5 showed ong>theong> cross-section morphologies ong>ofong> ZnO film

prepared at 1 and 10 Pa ong>oxygenong> ong>pressureong>, respectively. It could be

seen that ZnO films with ong>theong> thickness ong>ofong> about 800 nm formed

columnar grains, indicating that ZnO films grew in columnar form.

By comparison ong>ofong> ong>theong> XRD spectra, it could draw a conclusion that

ZnO films grew along ong>theong> [0 0 2] orientation which is

perpendicular to ong>theong> substrate surface.


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Fig. 4. Raman spectra ong>ofong> ZnO films prepared at various ong>oxygenong> ong>pressureong>s.

Table 1

Raman frequencies ong>ofong> ZnO films prepared at various ong>oxygenong> ong>pressureong>s.

Oxygen ong>pressureong> (Pa) E 2 (high) (cm 1 )

10 5

10 2

435.33

435.35

1 436.34

10 436.29

30 435.83

3.2. Optical properties

Fig. 6 showed transmission spectra ong>ofong> ong>theong> ZnO films prepared

on glass substrate at various ong>oxygenong> ong>pressureong>s. It was clear that all

ZnO films exhibited an apparent absorption edge at about 380 nm.

Below 380 nm, ong>theong> transmittance ong>ofong> all ZnO films was almost zero

and above 380 nm ong>theong> transmittance ong>ofong> all ZnO films increased

with increasing ong>theong> wavelength. Especially, ong>theong> transmittance ong>ofong>

all ZnO films can reach 80% or more in ong>theong> range ong>ofong> visible light.

With increasing ong>theong> ong>oxygenong> ong>pressureong>, ong>theong> transmittance ong>ofong> all ZnO

films first increased and ong>theong>n decreased in visible light range,

reaching a maximum for 1 Pa ong>oxygenong> ong>pressureong>, which was

attributed to ong>theong> changes ong>ofong> structure in ZnO films. At a low

ong>oxygenong> ong>pressureong> ong>ofong> 10 5 Pa, ong>theong>re were too many Zn atoms in films

and ZnO film exhibited a bad crystal quality (see Fig. 1), thus

resulting in low transmittance. As ong>theong> ong>oxygenong> ong>pressureong> increased,

ong>theong> stoichiometry ong>ofong> ZnO film drew near one gradually and ZnO

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Fig. 5. The cross-section morphologies ong>ofong> ZnO films (a) 1 Pa and (b)10 Pa.

Fig. 6. Transmission spectra ong>ofong> ong>theong> ZnO films prepared at various ong>oxygenong>

ong>pressureong>s.

film had ong>theong> better crystal quality, thus causing ong>theong> increment ong>ofong>

ong>theong> transmittance. With increasing ong>theong> ong>oxygenong> ong>pressureong> furong>theong>r,

ong>theong>re were too many ong>oxygenong> atoms in film. They eiong>theong>r formed


lots ong>ofong> defects in films to make ong>theong> crystal quality get bad or

aggregated into film surface, ong>theong>n resulting in ong>theong> decrease ong>ofong> ong>theong>

transmittance.

As a direct band gap semiconductor, ong>theong> band gap ong>ofong> ZnO film

is an important parameter. According to ong>theong> following formula,

ong>theong> band gap has a close relationship with an absorption

coefficient (a) and phonon energy (hg) [27].

a 2 ¼ Aðhg EgÞ ð6Þ

where Eg is ong>theong> optical band gap ong>ofong> thin film and A is a constant.

The absorption coefficient (a) could be calculated from ong>theong>

following equation [28]:

a ¼ 2:303log 1

T =d 0

where T is ong>theong> transmittance and d0 is film thickness and can be

obtained according to SEM cross-section morphologies ong>ofong> ZnO

film. Using formula (4) and (5), we calculated ong>theong> band gap ong>ofong> ZnO

films at various ong>oxygenong> ong>pressureong>s on ong>theong> basis ong>ofong> ong>theong> transmittance,

as shown in Fig. 7.

The calculated results showed that ong>theong> band gap ong>ofong> all ZnO

films were determined to about 3.259–3.315 eV, which was

smaller than ong>theong> direct band gap ong>ofong> ZnO (3.37 eV) at room

temperature. This result may be caused by ong>theong> band tail which is

composed ong>ofong> defect localized states both at ong>theong> bottom ong>ofong>

conduction band and ong>theong> top ong>ofong> ong>theong> valance band [29]. And ong>theong>se

defect localized states mainly resulted from ong>theong> non-stoichiometry

and ong>theong> lattice mismatch in ZnO films. On ong>theong> oong>theong>r hand,

some ong>oxygenong> vacancies or interstitial Zn atoms can also give rise

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C. Wang et al. / Physica B 404 (2009) 4075–4082 4079

ð7Þ

to ong>theong> decrease ong>ofong> ong>theong> band gap. As ong>theong> ong>oxygenong> ong>pressureong> increased,

ong>theong> band gap first increased and ong>theong>n decreased, reaching a

maximum ong>ofong> about 3.315 eV at 1 Pa ong>oxygenong> ong>pressureong>. This result

may be attributed to ong>theong> fact that ZnO film had ong>theong> best crystal

quality at 1 Pa ong>oxygenong> ong>pressureong>, thus resulting in ong>theong> fact that ong>theong>

band gap ong>ofong> ZnO film prepared at 1 Pa ong>oxygenong> ong>pressureong> was most

closely related to ong>theong> value ong>ofong> ong>theong> direct band gap ong>ofong> ZnO (3.37 eV)

at room temperature.

The origins ong>ofong> ong>theong> defect-related deep-level PL band have been

investigated for a long time. However, due to ong>theong> complexity ong>ofong>

ong>theong> microstructure ong>ofong> ZnO, ong>theong>re was still no satisfactory

consensus [16,17]. In order to clarify this discrepancy, we

investigated ong>theong> PL spectra ong>ofong> ZnO films prepared on Si substrate

at various ong>oxygenong> ong>pressureong>s, as shown in Fig. 8. From Fig. 8, one

could see that ong>theong> ong>oxygenong> ong>pressureong>s had an important effect on ong>theong>

PL spectra ong>ofong> ZnO films. When ZnO film was prepared at a low

ong>oxygenong> ong>pressureong> ong>ofong> 10 5 Pa, no emission peak occurred in PL

spectra. As ong>theong> ong>oxygenong> ong>pressureong> increased to about 10 2 Pa, ong>theong>re

occurred a strong UV emission peak with 378 nm, a weak violet

emission peak with 405 nm and a weak deep-level green emission

band centered at about 520 nm. As ong>theong> ong>oxygenong> ong>pressureong> increased

furong>theong>r, ong>theong>re occurred no oong>theong>r new emission peak in ZnO films

while ong>theong> intensity ong>ofong> above emission peak changed apparently. It

was clear that ong>theong> intensity ong>ofong> violet emission peak had almost no

change and ong>theong> intensity ong>ofong> green emission band increased

monotonically with ong>theong> ong>oxygenong> ong>pressureong>. Meanwhile, ong>theong>

position ong>ofong> UV emission peak had no change and ong>theong> intensity ong>ofong>

UV emission peak first increased and ong>theong>n decreased as ong>theong>

ong>oxygenong> ong>pressureong> increased, reaching a maximum at 1 Pa ong>oxygenong>

ong>pressureong>.

It was well known ong>theong>re are various defects in ZnO films, such

as ong>oxygenong> vacancies, zinc vacancies, interstitial ong>oxygenong>, interstitial

zinc, antisite ong>oxygenong> and so on [6]. When ZnO film was prepared

at 10 5 Pa, ong>theong> stoichiometry in ZnO films deviated significantly

from one and ong>theong>re were too many Zn atoms, thus resulting in ong>theong>

generation ong>ofong> many ong>oxygenong> vacancies during deposition. These

ong>oxygenong> vacancies can generate higher concentration ong>ofong> ong>theong>

carriers, thus resulting in ong>theong> Auger effect and ong>theong>n ong>theong> quenching

ong>ofong> ong>theong> light emission in ZnO films. As ong>theong> ong>oxygenong> ong>pressureong>

increased, ong>theong> number ong>ofong> ong>oxygenong> vacancies diminished and ong>theong>n

some light emission occurred in ZnO films, as shown in Fig. 8.

Fig. 7. The band gap ong>ofong> ZnO films prepared at different ong>oxygenong> ong>pressureong>s. Fig. 8. PL spectra ong>ofong> ZnO films prepared at various ong>oxygenong> ong>pressureong>s.


4080

From Fig. 8, one could calculate that ong>theong> phonon energy

corresponding to UV emission peak ong>ofong> 378 nm is 3.28 eV, which

was smaller than ong>theong> value ong>ofong> ong>theong> band gap ong>ofong> ZnO at room

temperature (3.37 eV). This result showed ong>theong> UV emission peak

could not originate from ong>theong> band–band transition between ong>theong>

valance band and conduction band that Wang et al. indicated [30].

Most researches [31,32] indicated that UV emission peak

originated from ong>theong> recombination ong>ofong> free excitons. ZnO has a

larger exciton binding energy (60 meV), which could assure more

efficient exciton emissions at room temperature. Moreover ong>theong>

free excitons have a close relationship with ong>theong> crystal quality ong>ofong>

ZnO films. At a low ong>oxygenong> ong>pressureong>, ZnO films had bad crystal

quality and had lots ong>ofong> defects. These defects could make a

quenching center ong>ofong> free excitons, thus resulting in a low

concentration ong>ofong> ong>theong> free excitons and a low intensity ong>ofong> UV

emission peak. As ong>theong> ong>oxygenong> ong>pressureong> increased, ong>theong> crystal

quality ong>ofong> ZnO films got better and ong>theong>re were fewer defects in

ZnO films, thus causing ong>theong> increment ong>ofong> ong>theong> concentration ong>ofong> free

excitons and ong>theong>n ong>theong> intensity ong>ofong> UV emission peak. When ong>theong>

ong>oxygenong> ong>pressureong> increased furong>theong>r, ong>theong> crystal quality ong>ofong> ZnO films

got worse again, thus resulting in ong>theong> worse crystal quality and

ong>theong>n ong>theong> decrease ong>ofong> ong>theong> intensity ong>ofong> UV emission peak. Therefore,

one could draw a conclusion that at 1 Pa ong>oxygenong> ong>pressureong> ong>theong> free

excitons in ZnO films had ong>theong> highest concentration, and ong>theong>n ong>theong>

intensity ong>ofong> UV emission peak also reached a maximum.

Most researchers considered that ong>theong> green emission band was

attributed to ong>theong> ong>oxygenong> vacancies in ZnO films [33,34]. In our

experiment, ong>theong> phonon energy was about 2.4 eV corresponding to

ong>theong> center ong>ofong> green emission band (520 nm), which was far

smaller than ong>theong> value ong>ofong> ong>theong> band gap ong>ofong> ZnO at room

temperature (3.37 eV). Therefore, according to calculated results

ong>ofong> Xu [35], ong>theong>re existed a few transition mechanisms to generate

ong>theong> green emission band in ZnO films: (1) ong>theong> electron transition

from shallow donor level formed by ong>oxygenong> vacancies to shallow

acceptor level formed by Zn vacancies; (2) ong>theong> electron transition

from ong>theong> bottom ong>ofong> ong>theong> conduction band to acceptor level formed

by misplaced ong>oxygenong> defects and (3) ong>theong> electron transition from

shallow donor level formed by interstitial Zn atoms to shallow

acceptor level formed by interstitial ong>oxygenong> atoms. It was well

known that with ong>theong> increment ong>ofong> ong>oxygenong> ong>pressureong> ong>theong> ong>oxygenong>

vacancies will decrease. So if ong>theong> first mechanism was right, ong>theong>

intensity ong>ofong> green emission band will decrease with ong>theong> increment

ong>ofong> ong>oxygenong> ong>pressureong>. As a fact, however, ong>theong> intensity ong>ofong> green

emission band increased (see Fig. 8) with ong>theong> ong>oxygenong> ong>pressureong>,

indicating that ong>theong> first mechanism is inappropriate in our

experiments. In addition, since ong>theong> radii ong>ofong> ong>oxygenong> ions is far

larger than that ong>ofong> Zn ions, ong>theong>re existed a very low probability

that ong>oxygenong> ions form interstitial ong>oxygenong> defects [36]. Moreover,

ong>theong> probability that atoms formed misplaced ones should be

larger than that those atoms formed interstitial ones according to

ong>theong>rmodynamic. Therefore, as ong>theong> ong>oxygenong> ong>pressureong> increased ong>theong>

concentration ong>ofong> ong>oxygenong> misplaced atoms would be far larger than

that ong>ofong> interstitial ong>oxygenong> atoms. Combining with Fig. 8, we could

draw a conclusion that green emission band should originate from

ong>theong> electron transition from ong>theong> bottom ong>ofong> ong>theong> conduction band to

acceptor level formed by oxide misplaced, which was in agreement

with ong>theong> results ong>ofong> Lin’s [36].

In Fig. 8, a weak violet emission peak occurred at about 405 nm

and its corresponding phonon energy was about 2.4 eV. Thus

according to Xu’s results [35] violet emission peak may originate

from ong>theong> electron transition from conduction band to shallow

acceptor level formed by Zn vacancies.

In addition, one could evaluate ong>theong> crystal quality and optical

properties according to ong>theong> ratio between ong>theong> intensity ong>ofong> UV

emission peak and ong>theong> intensity ong>ofong> deep-level emission peak

[37,38]. The larger ong>theong> ratio, ong>theong> better both ong>theong> crystal quality and

ARTICLE IN PRESS

C. Wang et al. / Physica B 404 (2009) 4075–4082

optical properties. On ong>theong> basis ong>ofong> Fig. 8, a series ong>ofong> ratios were

given, which are 2.52, 6.14, 10.22, 1.30, 0.23 for various ong>oxygenong>

ong>pressureong>s, respectively. Among ong>theong>m, ong>theong> ratio is ong>theong> largest at

1 Pa ong>oxygenong> ong>pressureong>, indicating that ZnO film had ong>theong> best crystal

quality and ong>theong> best optical properties. This was in good

agreement with above experimental results.

As mentioned above, UV emission peak originated from ong>theong>

recombination ong>ofong> free excitons. In order to disclose ong>theong> emission

characteristics ong>ofong> free excitons, we furong>theong>r investigated ong>theong>

relationship between ong>theong> PL spectra and measuring temperature,

as shown in Fig. 9. Fig. 9 showed ong>theong> dependence ong>ofong> PL spectra on

ong>theong> measuring temperature for ZnO films prepared on Si substrate

at 1 Pa ong>oxygenong> ong>pressureong>. It was remarkable that ong>theong>ir main

features ong>ofong> ong>theong> PL spectra were similar, in which all ong>theong>

luminescence spectra only consisted ong>ofong> ong>theong> UV emission peak

and no deep-level emission was found [31]. As ong>theong> measuring

temperature increased from 80 to 300 K, ong>theong> UV emission peak

underwent red-shifts from 370 to 378 nm, which could be

attributed to ong>theong> fact that ong>theong> band gap got narrow when ong>theong>

measuring temperature increased. Meanwhile, it was also found

that ong>theong> intensity ong>ofong> UV emission peak decreased gradually and

ong>theong> full width at half maximum obviously broadened when ong>theong>

measuring temperature increased from 80 to 300 K, which could

be explained as follows. At a low temperature, ong>theong> emission peak

was dominated by free excitons and bound excitons. With ong>theong>

increment ong>ofong> temperature ong>theong> bound excitons were ong>theong>rmally

ionized and ong>theong> carriers may take part in PL spectra at high

temperature, thus resulting in ong>theong> fact that ong>theong> intensity ong>ofong>

emission peak resulting from bound excitons decreased rapidly.

Meanwhile, ong>theong>rmally activated nonradiative recombination also

caused ong>theong> decrease ong>ofong> ong>theong> intensity ong>ofong> emission peak [31]. In

addition, it could be calculated that ong>theong> energy corresponding to

UV emission peak is about 3.359 eV at 80 K and is about 3.297 eV

at 300 K from our experiments, which had ong>theong> similar value with

ong>theong> results ong>ofong> some researches [39,40] that ong>theong> emission energy ong>ofong>

free excitons was about 3.358 eV at 80 K and about 3.299 eV at

300 K, implying that ong>theong> UV emission peak may originate from ong>theong>

recombination ong>ofong> free excitons.

In order to confirm ong>theong> mechanism ong>ofong> UV emission peak, we

investigated ong>theong> relationship between ong>theong> integral intensity ong>ofong> UV

emission peaks and measuring temperature. As a result, we found

ong>theong> integral intensity ong>ofong> UV emission peaks decreased monotonically

as measuring temperature increased, as shown in Fig. 10.

Fig. 9. The PL spectra ong>ofong> ZnO film measured at various temperatures.


Fig. 10. The relationship between ong>theong> integrated intensity ong>ofong> UV peaks and

measuring temperatures. The solid line was a fitting line.

According to ong>theong> results ong>ofong> Holtz et al. [41], ong>theong>re was a following

relationship between ong>theong> integral intensity ong>ofong> UV emission peaks

and measuring temperature:

I0

IðTÞ ¼

1 þ Aexpð E=kBTÞ

where E is ong>theong> activation energy ong>ofong> ong>theong> ong>theong>rmal quenching process,

kB is Boltzmann constant, I0 is ong>theong> emission intensity at 0 K, T is a

ong>theong>rmodynamic temperature and A is a constant.

Based on ong>theong> formula (8), we made a ong>theong>oretical fitting to ong>theong>

experimental data with a nonlinear least-squares method. The

solid line in Fig. 10 was ong>theong> ong>theong>oretical fit to ong>theong> experimental data.

As could be seen from Fig. 10 that ong>theong> fitting result showed E was

59.2 meV, which was similar to ong>theong> exciton binding energy ong>ofong>

60 meV in a bulk ZnO crystal, furong>theong>r indicating that ong>theong> UV

emission peak originate from ong>theong> recombination ong>ofong> free excitons.

4. Conclusions

In summary, we have investigated ong>theong> effect ong>ofong> ong>theong> ong>oxygenong>

ong>pressureong> on ong>theong> structure and optical properties ong>ofong> ZnO films and

found that ong>oxygenong> ong>pressureong> played a key role in ZnO films. At a

low ong>oxygenong> ong>pressureong> ong>ofong> 10 5 Pa, XRD showed that ZnO film had a

weak (0 0 2) diffraction peak and a large FWHM in rocking curve

and Raman spectra displayed a dispersed E 2 (low) mode, both ong>ofong>

which indicated that ZnO films had bad crystal quality. As a result,

ZnO films exhibited low transmittance and no emission peak in PL

spectra. As ong>theong> ong>oxygenong> ong>pressureong> increased, ZnO film displayed

strongest (0 0 2) diffraction peak and had a minimum ong>ofong> FWHM in

rocking curve at 1 Pa ong>oxygenong> ong>pressureong> and at ong>theong> same time E2

(low) peak turned sharpest and had highest intensity, showing

that ZnO film prepared at 1 Pa ong>oxygenong> ong>pressureong> had ong>theong> best

crystal quality. Accordingly, ong>theong> transmission intensity changed

non-monotonically as ong>theong> ong>oxygenong> ong>pressureong> increased, reaching ong>theong>

strongest transmission in visible light at 1 Pa ong>oxygenong> ong>pressureong>.

Meanwhile, ong>theong>re occurred a UV peak ong>ofong> 378 nm, a weak violet

peak ong>ofong> 405 nm and a wide green band centered at 520 nm in PL

spectra. With increasing ong>theong> ong>oxygenong> ong>pressureong>, ong>theong> intensity ong>ofong>

green emission band increased monotonically, and ong>theong> position ong>ofong>

UV peak did not shift while its intensity first increased and ong>theong>n

decreased, reaching a maximum at 1 Pa ong>oxygenong> ong>pressureong>. The

analysis showed that ong>theong> UV emission peak originated from ong>theong>

recombination ong>ofong> free excitons, violet peak originated from

ong>theong> energy level jump from conduction band to VZn defect and

ARTICLE IN PRESS

C. Wang et al. / Physica B 404 (2009) 4075–4082 4081

ð8Þ

ong>theong> green band originated from ong>theong> energy level jump from

conduction band to OZn defect. In addition, SEM observation

indicated that ZnO films grew with a columnar form, which

proved that ZnO film grew along ong>theong> [0 0 2] orientation perpendicular

to ong>theong> substrate surface.

Acknowledgments

This work was supported by ong>theong> Research Foundation ong>ofong>

Shandong Provincial Education Department ong>ofong> China (no. J08LI13)

and by ong>theong> Science Foundation for Distinguished Young Scientist

ong>ofong> Shandong Province (no. 2008BS04036) ong>ofong> China. The authors

also thank ong>theong> Shandong Provincial Educational Department to

provide a supporting foundation for College Teachers to research

at abroad.

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