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Applied Surface Science 253 (2007) 8470–8473

Dependence of the microstructural properties on

the substrate temperature in strained CdTe

(1 0 0)/GaAs (1 0 0) heterostructures

K.H. Lee a, J.H. Jung b, T.W. Kim b,*, H.S. Lee c, H.L. Park c

a Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Koreab Research Institute of Information Display, Division of Electronics and Computer Engineering, Hanyang University, 17,

Haengdang-dong, Seongdong-gu, Seoul 133-791, Koreac Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Korea

Received 2 February 2007; accepted 10 April 2007

Available online 21 April 2007

Abstract

CdTe thin films were grown on GaAs (1 0 0) substrates by using molecular beam epitaxy at various temperatures. The bright-field transmission

electron microscopy (TEM) images and the high-resolution TEM (HRTEM) images showed that the crystallinity of CdTe epilayers grown on GaAs

substrates was improved by increasing the substrate temperature. The result of selected-area electron diffraction pattern (SADP) showed that the

orientation of the grown CdTe thin films was the (1 0 0) orientation. The lattice constant the strain, and the stress of the CdTe thin film grown on the

GaAs substrate were determined from the SADP result. Based on the SADP and HRTEM results, a possible atomic arrangement for the CdTe/GaAs

heterostructure is presented.

# 2007 Published by Elsevier B.V.

PACS : 68. 37. Lp; 68. 55. Jk

Keywords: CdTe/GaAs heterostructure; Microstructural properties; Atomic arrangement

1. Introduction

The growth of high-quality CdTe thin films has attracted

much interest because of their applications in the areas of solar

energy conversion, gamma-ray detection, and electro-optic

modulation due to their low thermal noise and large absorption

coefficient [1–5]. CdTe epitaxial layers have been extensively

grown because CdTe thin films can be useful buffer layers for

the growth of Hg1�xCdxTe epilayers [6–8]. However, since the

growth of high-quality CdTe/GaAs heterostructures has inhe-

rent problems due to the large lattice mismatch (Da/a = 12.77%

at 25 8C), studies of the microstructural properties of CdTe/

GaAs heterostructures are very important for achieving high-

quality optoelectronic devices that can operate in the near-

infrared region of the spectrum [9]. In addition, studies of the

improvement on the microstructural properties play a very

* Corresponding author. Tel.: +82 2 2220 0354; fax: +82 2 2292 4135.

E-mail address: twk@hanyang.ac.kr (T.W. Kim).

0169-4332/$ – see front matter # 2007 Published by Elsevier B.V.

doi:10.1016/j.apsusc.2007.04.019

important role in enhancing device efficiency [10], and

systematic studies concerning the microstructural properties

of the CdTe/GaAs heterostructures dependent on the substrate

temperature are still necessary if high-quality heterostructures

are to be obtained. Even though some studies concerning the

orientation of the CdTe epitaxial films grown on GaAs (1 0 0)

substrates dependent on the preheating temperature have been

reported [11–15], very few works on the dependence of the

microstructural properties on the substrate temperature for

CdTe thin films grown on GaAs (1 0 0) substrates have been

performed [16].

This paper reports the dependence of the microstructural

properties on the substrate temperature of CdTe epitaxial films

grown on GaAs (1 0 0) substrates by using molecular beam

epitaxy (MBE) at various temperatures. Transmission electron

microscopy (TEM) and selected-area electron diffraction

pattern (SADP) measurements were performed to investigate

the microstructural properties of the CdTe thin films grown on

GaAs (1 0 0) substrates. A possible crystal for the CdTe/GaAs

K.H. Lee et al. / Applied Surface Science 253 (2007) 8470–8473 8471

heterostructure is presented on the basis of the high-resolution

TEM (HRTEM) and the SADP results.

2. Experimental details

Elemental Cd and Te with purities of 99.9999% were used

as the source materials and were precleaned by repeated

sublimation. Si-doped N-type (1 0 0) GaAs substrates were

degreased in trichloroethylene (TCE), etched in acetone, etched

in a Br-methanol solution, and rinsed in de-ionized water

thoroughly. As soon as the chemical cleaning process was

finished, the GaAs substrates were mounted onto a molybde-

num susceptor. Prior to CdTe thin-film growth, the GaAs

substrates were thermally cleaned at 600 8C for 5 min in situ in

the growth chamber at a pressure of 10�8 Torr. The depositions

of the CdTe epilayers were done on GaAs substrates by using

the MBE technique at substrate temperatures between 300 and

340 8C and at a system pressure of 10�9 Torr. The source

temperatures of the Cd and the Te sources for the CdTe

epilayers were 195 and 300 8C, respectively, and the typical

growth rate was approximately 1.38 A/s. The typical thickness

of the CdTe film was approximately 1 mm.

Cross-sectional TEM specimens were prepared by forming a

sandwich with epoxy, followed by mechanical cutting and

polishing with diamond paper to an approximately 30-mm

thickness, and then argon-ion milling at liquid-nitrogen

temperature to electron transparency. High-resolution micro-

graphs were obtained using a JEOL JEM 3010 transmission

Fig. 1. Cross-sectional bright field transmission electron microscopy images of the

310, (c) 320, and (d) 340 8C.

electron microscope operating at 300 kV with a high-resolution

pole piece.

3. Results and discussion

Fig. 1 shows cross-sectional bright-field TEM images of the

CdTe/GaAs heterostructures grown at various growth tem-

peratures of (a) 300, (b) 310, (c) 320, and (d) 340 8C. The

bright-field TEM images depict the top CdTe thin film and the

bottom GaAs substrates. The dislocations in the CdTe thin films

appears as lines in the images of the CdTe films, as shown in

Fig. 1. The defects, such as dislocations, in the CdTe thin films

decrease with increasing substrate temperature, indicative of

the improvement of the crystallinity of the CdTe thin film due to

the thermal effect.

Cross-sectional HRTEM images of the CdTe/GaAs hetero-

structures grown at various growth temperatures of (a) 300, (b)

310, (c) 320, and (d) 340 8C are shown in Fig. 2. The

appearance of the white contrast near CdTe/GaAs hetero-

interfaces is attributed to the different transmission of the

incident e-beam at heterointerfaces resulting from the different

materials. Misfit dislocations existed at CdTe/GaAs hetero-

interface resulting from the large lattice mismatch between the

CdTe thin film and the GaAs substrates. The numbers of the

defects in the CdTe thin film at the CdTe/GaAs heterointerfaces

decrease with increasing substrate temperature.

The SADP of the CdTe/GaAs heterostructures at 340 8C is

shown in Fig. 3. The incident beam directions of both the CdTe

CdTe/GaAs heterostructures grown at various growth temperatures: (a) 300, (b)

Fig. 2. Cross-sectional high-resolution transmission electron microscopy images of the CdTe/GaAs heterostructures grown at various growth temperatures: (a) 300,

(b) 310, (c) 320, and (d) 340 8C.

K.H. Lee et al. / Applied Surface Science 253 (2007) 8470–84738472

epilayer and the substrate are the ½1 1 0� zone axis. Electron

diffraction spots occur in pairs, with the large inside spot and

the smaller outside spot corresponding to the CdTe and the

GaAs, respectively. The difference of the spot size originates

Fig. 3. Electron-diffraction pattern from transmission electron microscopy of

the CdTe/GaAs heterostructures grown at 340 8C along the ½110� zone axis;

(hkl)CdTe and (hkl)GaAs correspond to the CdTe and the GaAs indices, respec-

tively.

from the more transmission of the incident e-beam to the CdTe

film. The diffraction pattern indicates that an epitaxial

orientation relationship is formed between the CdTe and the

GaAs in the CdTe/GaAs heterostructure. All of the CdTe (hkl)

planes are parallel to the GaAs (hkl) planes. The SADP of the

CdTe/GaAs heterostructure depicts that the orientation of the

CdTe epilayer is (1 0 0). The lattice constant of the c-axis for

CdTe (1 0 0) film grown on the GaAs (1 0 0) substrate,

Fig. 4. Schematic diagram of the (1 1 0) projection of the crystal structure for a

CdTe/GaAs heterostructure grown at 340 8C.

K.H. Lee et al. / Applied Surface Science 253 (2007) 8470–8473 8473

determined from the SADP result, is 6.501 A, which is larger

than that of the CdTe (1 0 0) bulk. The value of the the strain

for the CdTe layer in the direction perpendicular to the CdTe/

GaAS heterointerfaces is 3.09 � 10�3 and the angle between

the h1 1 0i and the h1 1 2i directions for the CdTe thin film is

58.418.A possible schematic diagram of the (1 1 0) projection of the

crystal structure for a CdTe/GaAs heterostructure grown at

340 8C described on the basis of the SADP and the HRTEM

results is shown in Fig. 4. Fig. 4 shows that the angles between

the ½1 1 0� and the ½1 1 2� directions for the GaAs substrate and

the CdTe layer are 548740 and 588410, respectively. The lattice

constants for the GaAs substrate and the CdTe layer are 5.6532

and 6.501 A, respectively.

4. Summary and conclusions

The TEM images showed that the crystallinity of the CdTe

epitaxial films grown on the GaAs (1 0 0) substrates was

improved with increasing substrate temperature due to the

thermal effect. The lattice constant and the horizontal stress of

the CdTe thin film were determined from the SADP results, and

a possible schematic diagram of the crystal structure for the

CdTe/GaAs heterostructure was proposed on the basis of the

SADP and HRTEM results. These observations can help

improve understanding of the microstructural properties of the

CdTe/GaAs heterostructures dependent on growth temperature,

and these results indicate that the crystallinity of CdTe epilayers

grown on GaAs substrates can be improved by changing the

substrate temperature.

Acknowledgement

This work was supported by the Korea Research Foundation

Grant funded by the Korean Government (MOEHRD, Basic

Research Promotion Fund) (KRF-2004-005-D00166).

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