TEM Study on Defects in Epitaxial CdZnTe Films Deposited on (001)GaAs by Close-Spaced Sublimation
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TEM Study on Defects in Epitaxial CdZnTe Films Deposited on (001)GaAs by Close-Spaced Sublimation Junning Gao, Wanqi Jie, Lin Luo, Yanyan Yuan, Tao Wang, Shouzhi Xi, Hui Yu State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China. ABSTRACT A transmission electron microscopy (TEM) study on defects in a 30 ȝm-thick epitaxial CZT film deposited on (001)GaAs via close-spaced sublimation was performed. The epi-layer is of good quality without twins. Dislocations and stacking faults are mainly gathered near the interface. The dislocations are extrinsic either of Lomer edge or 60° type. Pseudo extrinsic stacking faults consisting of two independent and oppositely oriented extrinsic dislocations have been found both on the (111) and the (1 11) planes. L-shaped defects originated from the interface have been discovered. The near-interface-side of L is consisted of 3 compressed (111) planes and the lateral side is consisted of 3-4 misarranged (1 11) planes. This L-shaped defect is induced by the absence of a misfit dislocation at the intersection between L and the interface. INTRODUCTION CZT films have wide applications in radiation detectors, solar cells, and substrates for HgCdTe (MCT) epitaxy [1-3]. Recently, high quality epitaxial CZT films have been successfully deposited via a cost-effective close-spaced sublimation (CSS) process with the growth rate up to 1 ȝm/min, which is promising in the application of thick CZT films up to hundreds of microns for radiation detectors [4]. The study of defects in semiconductor films is very important because defects play important roles in the mechanical and electronic properties of these films. For example, dislocations control the material plasticity and give rise to energy levels in band gaps. Stacking faults can act as sinks for point defects [5, 6]. Therefore, a thorough understanding of the defect status of these CSS-deposited films is necessary in advancing their future applications. GaAs has been frequently used as the substrate for the epitaxy of CZT. However, the large lattice misfit between CZT and GaAs causes large strain in CZT epi-layers which has to be relaxed through the creation of misfit dislocations and planar defects. The misfit dislocations are present in both Lomer edge type (type A) and 60° type (type B), and the number of the type A dislocations increases with lattice misfit because it is more effective in relaxing strain than type B [7-10]. Besides, the formation energy for planar faults in CZT is low [6]. Therefore, the defects in CZT epi-layers have the characteristics that the quantities of the defects in epi-layers generally decrease away from the interface [10, 11], hillocks are easy to be formed due to the intersection of planar defects to the layer surface [8, 11, 12], and lamellar and double-position twins are frequently present especially in (111) epi-layers [13-15]. Besides, ordered CuPt- and CuAu-type structures have been found as well [16, 17]. Here,
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