GaAs (1 0 0) heterostructures

GaAs (1 0 0) heterostructures

Applied Surface Science 253 (2007) 8470–8473 www.elsevier.com/locate/apsusc Dependence of the microstructural properties on the substrate temperature...

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Applied Surface Science 253 (2007) 8470–8473 www.elsevier.com/locate/apsusc

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, Korea b Research Institute of Information Display, Division of Electronics and Computer Engineering, Hanyang University, 17, Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea c 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 Hg1xCdxTe epilayers [6–8]. However, since the growth of high-quality CdTe/GaAs heterostructures has inherent 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 highquality optoelectronic devices that can operate in the nearinfrared 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: [email protected] (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

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heterostructure is presented on the basis of the high-resolution TEM (HRTEM) and the SADP results.

electron microscope operating at 300 kV with a high-resolution pole piece.

2. Experimental details

3. Results and discussion

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 molybdenum 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 108 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 109 Torr. The source temperatures of the Cd and the Te sources for the CdTe epilayers were 195 and 300 8C, respectively, and the typical ˚ /s. The typical thickness growth rate was approximately 1.38 A 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 micrographs were obtained using a JEOL JEM 3010 transmission

Fig. 1 shows cross-sectional bright-field TEM images of the CdTe/GaAs heterostructures grown at various growth temperatures 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 heterostructures 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 heterointerfaces is attributed to the different transmission of the incident e-beam at heterointerfaces resulting from the different materials. Misfit dislocations existed at CdTe/GaAs heterointerface 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

Fig. 1. Cross-sectional bright field transmission electron microscopy images of the CdTe/GaAs heterostructures grown at various growth temperatures: (a) 300, (b) 310, (c) 320, and (d) 340 8C.

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

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.

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 ¯ zone axis; the CdTe/GaAs heterostructures grown at 340 8C along the ½110 (hkl)CdTe and (hkl)GaAs correspond to the CdTe and the GaAs indices, respectively.

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

˚ , which is larger determined from the SADP result, is 6.501 A 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  103 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 ˚ , respectively. and 6.501 A 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.

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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). References [1] Y.P. Chen, S. Sivananthan, J.P. Faurie, J. Electron. Mater. 22 (1993) 951. [2] J.P. Faurie, R. Sporken, Y.P. Chen, M.D. Lange, S. Sivananthan, Mater. Sci. Eng., B 16 (1993) 51. [3] S.K. Pandey, U. Tiwari, R. Raman, C. Prakash, V. Krishna, V. Dutta, K. Zimik, Thin Solid Films 473 (2005) 54. [4] A. Million, N.K. Dhar, J.H. Dinan, J. Cryst. Growth 159 (1996) 76. [5] Y.B. Hou, J.H. Leem, T.W. Kang, T.W. Kim, Appl. Surf. Sci. 151 (1999) 213. [6] Y.S. Ryu, B.S. Song, H.J. Kim, T.W. Kang, T.W. Kim, J. Mater. Res. 18 (2003) 257. [7] Y.S. Ryu, Y.B. Heo, B.S. Song, S.J. Moon, Y.J. Kim, T.W. Kang, T.W. Kim, Appl. Phys. Lett. 83 (2003) 3776. [8] N.V. Sochinskii, C. Reig, I. Mora-Sero´, J. Peraza, V. Mun˜oz, Thin Solid Films 381 (2001) 48. [9] Y. Nakamure, N. Otsuka, M.D. Lange, R. Sporken, J.P. Faurie, Appl. Phys. Lett. 60 (1992) 1372. [10] S.M. Sze, Modern Semiconductor Device Physics, John Wiley & Sons, New York, 1998. [11] N. Otsuka, L.A. Kolodziejski, R.L. Gunshor, S. Datta, R.N. Bicknell, J.F. Schetzina, Appl. Phys. Lett. 46 (1985) 860. [12] J.M. Ballingall, Appl. Phys. Lett. 48 (1986) 1273. [13] R. Srinivasa, M.B. Panish, H. Temkin, Appl. Phys. Lett. 50 (1991) 1441. [14] H.S. Lee, H.L. Park, T.W. Kim, J. Cryst. Growth 292 (2006) 10. [15] M. Jung, S.I. Mho, H.L. Park, Appl. Phys. Lett. 88 (2006) 133121. [16] J. Yin, Q. Huang, J. Zhou, J. Appl. Phys. 79 (1996) 3714.