Yang Li a b, Xinlei Zhang c, Xin Wan a b, Yajie Liu a b, Gangqiang Zha b, Kun Cao a b, Wanqi Jie a b
a Key Laboratory of Radiation Detection Materials and Devices, Ministry of Industry and Information Technology, China
b School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
c School of Physics and Information Technology, Shaanxi Normal University, Xi'an, Shaanxi, 710119, China
## Abstract
Thick CdZnTe epitaxial film is a promising alternative for X-ray detectors, especially in the case of large-area imaging application. One of the key issues in growing thick heteroepitaxial film with high crystallinity is to reduce the mismatch dislocations that appear at the interface between CdZnTe epilayer and the substrate. In this paper, we prepared CdZnTe films without and with ZnTe buffer layers on GaSb (001) substrates by the closed space sublimation. All preparation conditions for ZnTe buffer layers, CdZnTe epilayers on ZnTe buffers and GaSb substrates, as well as the pretreatment of GaSb surface have been optimized through tremendous experiments. The results of electron backscatter diffraction and transmission electron microscope observation prove that ZnTe buffer layers suppress (
)M twins extending from GaSb. The deposition temperature for inserting ZnTe buffer about 495 K is optimized for getting smooth ZnTe surface, on which high crystalline quality CdZnTe epilayer with a narrower full-width at half-maximum (85 arcsec) of double crystal X-ray rocking curve for (004) reflection and lower intensity ratio IDcomplex/I(D0, h) of low-temperature photoluminescence spectroscopy were produced.
## Introduction
X- and γ-ray detectors fabricated by ternary compound semiconductor CdZnTe possess the advantages of low leakage current, high detection efficiency, good carrier transport property and room temperature applications [1, 2], and therefore widely concerned in the fields of medical imaging and nuclear radiation inspection [3, 4]. Vertical gradient freezing (VGF), vertical Bridgman (VB), high pressure Bridgman (HPB) and traveling heater method (THM) have been used for bulk crystal growth of detector grade CdZnTe. In recent years, an emerging technology is to produce large area high-quality CdZnTe epitaxial films with low-cost by vapor phase deposition, with the target application of large area X-ray imaging like digital radiography (DR) [5] and photon counting computerized tomography (CT) [6].
The preparation methods of CdZnTe films include molecular beam epitaxy (MBE) [7], hot wall epitaxy (HWE) [8], metal organic chemical vapor deposition (MOCVD) [9], closed space sublimation (CSS) [10], etc. Among them, CSS is more probable for the growth of CdZnTe thick films up to hundreds of micrometers due to its high growth rates and low preparation costs [11]. CdZnTe thick films with the thickness of 1.1 mm were reported [12], but the fabricated detector showed the large electronic noise and low charge collection efficiency (CCE), and only 31% energy resolution (ER) for 241Am@59.5 keV radiation, much poorer than the commercial detector made from bulk crystals (ER < 5%) [13]. It is believed that high density defects like dislocations and micro twins in CdZnTe epilayer [14] acted as current leakage path [15] and charge traps [16], which increase the dark current and reduce the CCE. Lowering the dislocation density in CdZnTe films, especially near CdZnTe/GaAs interface, is therefore the precondition of obtaining high-performance CdZnTe film detectors.
Dislocations in CdZnTe films are initiated by the large lattice mismatch between CdZnTe films and the widely used GaAs substrates. The lattice mismatch between CdZnTe and GaAs is as large as 14%, therefore, dislocations and small-angle grain boundaries inevitably form at the interface during epitaxial growth. Methods of selection deposition [17], insertion of buffer layer [18] and multi-step growth [19] have been used to control the early nucleation process near the growth interface and further the expansion of defects. Cao [20] decreased the full width of half maximum (FWHM) of the double crystal X-ray rocking curve (DCXRC) of CdZnTe film from 306 to 136 arcsec by a two-step CSS method. However, FWHM of DCXRC for bulk CdZnTe crystals generally reaches below 50 arcsec [21]. We found the lattice mismatch between GaSb and CdZnTe is only 5.6%, which can be a preferable substrate for the growth of high-quality CdZnTe films, with which good energy resolution of 17% for 241Am@59.54 KeV γ-ray was obtained in our previous study [22]. However, the FWHM of DCXRC for CdZnTe/GaSb film was still as high as 173 arcsec and the resolution needs further improvement.
In this study, we used ZnTe buffers to improve the crystalline quality of CdZnTe films. Meanwhile, the mechanism of suppression of dislocation and twin defects in CdZnTe/ZnTe/GaSb structure will be discussed.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/improvement-of-crystallinity-of-cdznte-epilayers-on-gasb-substrates-by-znte-buffer-layer.html