Xin Wan a, Yang Li a, Tingting Tan a, Yajie Liu a, Heming Wei a, Kun Cao a, Gangqiang Zha b
a School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shanxi, 710072, China
b Research and Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen, Guangdong, 518057, China
## Abstract
CdZnTe (CZT) epitaxial films were deposited on GaAs substrates by closed spaced sublimation. In order to reduce the adverse effects of point defects and dislocations in the film on its properties, CZT films were annealed at 400 °C in Te2 atmosphere at different times. The I–V test and energy spectrum show that the resistivity of CZT films increased from 109 Ω cm to 1010 Ω cm, and the electron mobility lifetime product (μτ)e increased from 10−5 cm2 V−1 to 10−4 cm2 V−1. According to the etch pit density and photoluminescence tests, the results show that annealing reduced the dislocation density from 3.28 × 105 cm−2 to 2.52 × 105 cm−2 in the CdZnTe films. In this study, the annealing process which can stably improve the properties of CdZnTe films is obtained, that is, annealing at 400 °C in Te2 atmosphere for 2–4 h.
## Introduction
As an II-VI compound semiconductor with excellent photoelectric properties, CdZnTe (CZT) is widely used in medical imaging, industrial nondestructive testing, space exploration, astrophysics and other fields [[1], [2], [3]]. With the development of large-area imaging detectors, the size requirements of CZT crystal for high-performance devices are also increasing. At present, the mature melt method is commonly used to grow CZT single crystal, but the yield and production efficiency of this method are low, which increases the cost and price of CZT crystal and limits its application. Due to the limitations of CZT crystal properties, such as low thermal conductivity, easy to form vacancies, dislocations, inclusions and precipitates, it is very difficult to grow large-size single crystals at the detector level [4,5]. In order to replace the CZT crystal with high cost and limited size, the preparation of CZT film has become a research hotspot in recent years. Compared with other film growth methods, the close-space sublimation not only has simple equipment structure, fast film growth rate, but also has low preparation cost and high quality.
However, even if CZT epitaxial film has a wide market prospect, it cannot avoid the formation of a large number of defects in the growth process, which seriously affects the photoelectric properties and carrier transport properties. CZT films were generally prepared by gas-phase methods such as MBE, MOCVD, and CSS [[6], [7], [8]]. The basic principle is that after the CZT polycrystalline source is heated and decomposed, it is combined into single crystal CZT on the epitaxial substrate, which has the same crystal structure as CZT. At present, GaAs is commonly used as the epitaxial substrate material of CZT films. One the one hand, GaAs is easy to obtain large area single crystal and low cost, on the other hand, GaAs has much higher electron mobility than other semiconductor materials, which can make CZT/GaAs detectors work efficiently.
Nevertheless, due to the different lattice constants between CZT and GaAs substrate, there is often lattice mismatch between them, which leads to the formation of threading dislocations, sub-grain boundaries and stacking faults [9,10]. In CZT crystals, the existence of dislocations often causes lattice distortion and becomes the center of scattering carries [11]. In addition, deep level defects are formed in semiconductors, which can capture carriers. Some point defects are also formed during the growth of the CZT films. For the point defects in CZT, its influence on the performance of CZT depends on the type of point defect [12,13]. Shallow level defects often affect the carrier concentration and conductivity type of CZT. For instance, the conductive type of CZT crystal grown with In or annealed in Cd atmosphere is usually n-type, which is caused by the shallow donor level InCd or Cdi [[14], [15], [16], [17]]. In contrast, deep level defects affect the transport performance of carriers, and electrons or holes are usually captured by them in the transport process. What's more, deep level defects compensate the excess shallow level defects in CZT, resulting in high resistance of CZT [[18], [19], [20], [21]]. Based on this mechanism, researchers make Te2 content higher than stoichiometric ratio in the CZT crystal grown by melt method, so as to form deep-level TeCd and obtain high resistance. Therefore, in order to obtain better photoelectric properties and carrier transport properties of CZT epitaxial films, we can start with regulating the type and concentration of point defects and dislocation density.
A number of studies have shown that various defects in materials can be controlled by annealing [[22], [23], [24]]. Li [25] found that the resistivity of CZT/GaAs was significantly increased from 1.64 × 109 Ω cm to 4.16 × 109 Ω cm, and the dislocation density reduced from 2.29 × 105 cm−2 to 1.56 × 105 cm−2 after annealing in Te2 atmosphere for 10 h at 350 °C. Tan [26] grew Ge/Si(001) epitaxial film by three-step method and annealed in H2 atmosphere after growth. The test results show that this growth method and annealing treatment jointly reduce the density of TDs in the film. Annealing treatment can not only affect the dislocation density in the material, but also affect the concentration of point defects, so as to improve the properties of the material. Chu [27] found that the concentration of TeCd increases with the increase of VCd concentration. In addition, using Cd/Zn combined annealing, by controlling the concentration of VCd, the conductivity type of crystal changes and high resistance is obtained during annealing.
Therefore, this study starts with regulating the point defects and dislocation in CZT epitaxial film, and uses annealing to improve its photoelectric properties and carrier transport properties. Furthermore, the change mechanism of defects in CZT epitaxial film during annealing can be obtained, which could guide the growth of CZT epitaxial film.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/effects-of-annealing-in-te2-atmosphere-on-photoelectric-properties-and-carrier-transport-properties-of-cdznte-films.html