Qunfang Wang a, Haofei Huang a, Ke Xu a, Keyun Gu a, Longhui Lin a, Azhati Lina a, Yiheng Yang a, Ke Tang a b c, Zilong Zhang d, Masaya Toda d, Linjun Wang a b c, Jian Huang a b c
a School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
b Zhejiang Institute of Advanced Materials, SHU, Jiashan 314113, China
c Shanghai Collaborative Innovation Center of Intelligent Sensing Chip Technology, Shanghai 201800, China
d Department of Mechanical Systems Engineering, Tohoku University, Sendai, Miyagi 980-8579, Japan
## Abstract
Cadmium Zinc Telluride (CdZnTe) is a highly effective semiconductor material, renowned for its exceptional properties. In this study, we employed the close-spaced sublimation method to grow high-quality CdZnTe thick films on sapphire substrates. The effects of growth temperature on the composition, surface defects, and structural and electrical properties of CdZnTe films were examined. The investigation identified the optimal temperature conditions for the production of thick CdZnTe films of high quality. These were found to be a sublimation temperature of 675°C and a substrate temperature of 450°C. Furthermore, we have produced CdZnTe thick-film radiation detectors based on sapphire substrates. The detectors demonstrated an energy resolution of 18.9 % when subjected to 59.54 keV 241 Am γ-ray irradiation. The results of our investigation suggest that these CdZnTe thick film detectors have the potential to be employed in future low-cost, large-area imaging applications.
## 1. Introduction
CdZnTe is an excellent compound semiconductor material, exhibiting a high average atomic number, high resistivity, and a wide tunable bandgap. These properties render it highly promising for applications in solar cells and room-temperature radiation detectors [1], [2]. The current state of research indicates that CdZnTe radiation detectors have significant potential for application in a number of fields, including medical imaging, environmental monitoring, space science, and high-energy physics [3], [4], [5]. Nevertheless, the challenge of producing large-area, high-quality CdZnTe single crystals has restricted the use of these crystals in flat panel imaging detectors. Consequently, there has been a significant focus on the development of low-cost and large-area CdZnTe thick film radiation detectors [6], [7], [8].
A variety of techniques may be employed for the preparation of CdZnTe thick films, including molecular beam epitaxy (MBE) [9], metal-organic chemical vapor deposition (MOCVD) [10], vacuum evaporation and physical vapor transport, close-spaced sublimation (CSS), and so forth. In contrast, the close-spaced sublimation (CSS) method offers several advantages, including a rapid growth rate, a straightforward preparation process, and a relatively low cost, which collectively make it an appropriate choice for the growth of large-area CdZnTe thick films [11].
The thermal expansion coefficient and lattice mismatch between the substrate and the film have a significant impact on the properties of the prepared film, and thus require careful consideration during the preparation process [12], [13], [14]. Accordingly, the selection of an appropriate substrate represents a critical aspect in the preparation of high-quality CdZnTe thick films. A variety of substrate materials have been employed in the preparation of CdZnTe films, including fluorine-doped tin oxide (FTO) [15], indium tin oxide (ITO) [16], and silicon (Si) [17]. However, FTO has a substantial lattice mismatch with CdZnTe. ITO is brittle and susceptible to cracking under the elevated temperatures necessary for thick film deposition, while Si exhibits a notable thermal expansion mismatch with CdZnTe [18]. It has been demonstrated that a substrate with high thermal conductivity and uniform temperature distribution is conducive to the growth of crystals [19]. Sapphire, with its high thermal conductivity (32 W/m·K at room temperature) [20] and stability, can similarly provide enhanced growth conditions for CdZnTe films, improving crystallinity and reducing defects. Fig. S1(Supporting information, SI) illustrates the atomic arrangements of c-plane sapphire and (111) CdZnTe. Furthermore, sapphire has a thermal expansion coefficient of 5.8 × 10−6/K [21], which is highly compatible with that of CdZnTe (5.5 × 10−6/K) [22], making it an optimal substrate material for the fabrication of high quality CdZnTe thick films.
In this study, CdZnTe thick films were prepared on sapphire sub strates using the close-spaced sublimation method. The impact of preparation temperature on the characteristics of CdZnTe thick films on sapphire substrates was subjected to a comprehensive examination. Additionally, CdZnTe thick-film radiation detectors based on sapphire substrates were fabricated. The performance of these CdZnTe radiation detectors was also evaluated.
## 2. Experimental
The preparation of CdZnTe thick films using the CSS method is illustrated in Fig. S2 (Supporting information, SI), which depicts a facile process. In order to fabricate CdZnTe thick films on sapphire substrates (20 × 20 × 2.2 mm3, length × width × height), high-purity (>99.9999 %) Cd0.9Zn0.1Te powder was employed. During the deposition process, the pressure was maintained at approximately 1 Pa using a mechanical pump with a substrate-source distance of 6 mm.
The growth temperature is a key factor affecting the quality of CdZnTe thick films prepared by CSS method. This work examines the impact of varying growth temperatures on the characteristics of CdZnTe thick films deposited on sapphire substrates. The specific process parameters are presented in Table 1.
Table 1. Key fabrication parameters for CdZnTe films on sapphire substrates.
Sample Sublimation source temperature (℃) Substrate temperature (℃) Temperature difference (℃) Growth time
(h)
1 625 425 200 3
2 650 450 200 3
3 675 475 200 3
4 700 500 200 3
5 675 450 225 3
6 675 425 250 3
7 675 400 275 3
The surface treatment of CdZnTe thick film has a significant impact on the performance of CdZnTe film radiation detectors. In this study, a mechanical polishing (MP) technique was developed for application to as-grown CdZnTe films with the objective of reducing surface roughness and defects in the thick film, thereby enhancing device performance. In our previous research, we employed a comparable MP surface treatment process for a CdZnTe film [23].
In order to prepare CdZnTe thick film radiation detectors, a 100 nm thickness co-sputtered ZnTe: A Cu film with low resistivity was employed as an electrode buffer layer between the Au electrode and the CdZnTe thick film. In our previous work, we employed a ZnTe:Cu/Au composite electrode for the CdZnTe thick film devices, which led to a notable enhancement in the ohmic contact properties of the electrode [24].
The structure and morphology of CdZnTe thick films were analysed using an X-ray diffractometer (XRD, D/MAX-2200V PC 3KW, Cu-Kα, λ = 1.5406 Å) and a scanning electron microscope (SEM, JEOL JSM7500F), respectively. The electrical properties of the CdZnTe films were investigated utilising a semiconductor device analyser (Keithley 4200A-SCS). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi 150 W, Al-Kα, the spot size = 500 μm) was employed to examine the elemental energy state in CdZnTe thick films. The photoluminescence properties of CdZnTe thick films were investigated using a photoluminescence spectroscopy system (PL, LE-SP-M300DA). The energy spectrum of the Cd1−xZnxTe film detector was analysed using an Ortec multichannel pulse height analyser (Ortec Model TRUMPPCI-2k MCA).
# 3. Results and discussion
3.1. Effect of growth temperature on the properties of CdZnTe thick films
The effect of growth temperature on the properties of CdZnTe thick films was investigated by maintaining a constant temperature gradient of 200°C between the sublimation source and the substrate. The impact of growth temperature on the surface morphologies of CdZnTe thick films on sapphire substrates were observed by SEM, as illustrated in Fig. 1(a)-(d). As the temperature of the sublimation source increased from 625°C to 675°C, the grain size of the CdZnTe thick films exhibited a gradual increase. Nevertheless, further increases in the sublimation source temperature resulted in a reduction in grain size, which is likely attributable to the significantly higher deposition rate at elevated temperatures [25]. The CdZnTe thick film prepared with sublimation source temperatures of 675°C and substrate temperatures of 475°C displays larger grain sizes.
Fig. 1. (a)-(d) SEM images of CdZnTe films on sapphire substrates at different growth temperatures: (a) 625℃-425℃; (b) 650℃-450℃; (c) 675℃-475℃; (d) 700℃-500℃. (e) XRD patterns of CdZnTe films on sapphire substrates at different growth temperatures. (f) I-V characteristics of CdZnTe films on sapphire substrates at different growth temperatures in logarithmic coordinates.
Fig. 1(e) illustrates the XRD pattern of a CdZnTe thick film on a sapphire substrate as a function of growth temperature under a fixed temperature difference of 200°C between the sublimation source and the substrate. All CdZnTe thick films display a distinctive diffraction peak in the vicinity of 24°, which correlates with the (111) plane of the zinc-blende CdZnTe structure (JCPDS No. 53–0555) [26]. The XRD spectra demonstrate that the CdZnTe thick films prepared at varying temperatures exhibit a high degree of (111) preferential orientation. The intensity of the (111) diffraction peak initially increases and subsequently decreases with an increase in growth temperature. When the temperature of the sublimation source and substrate are 675 °C and 475 °C, respectively, the CdZnTe thick film exhibits the strongest (111) diffraction peak.
The full width at half maximum (FWHM) of the (111) diffraction peak of the CdZnTe thick film can be obtained based on the XRD results, as demonstrated in Table S1 (Supporting information, SI). Table S1 also demonstrates that alterations in growth temperature result in shifts in the position of the (111) diffraction peak of the CdZnTe thick film. In samples with sublimation source temperatures of 625 °C, 650 °C, 675 °C and 700 °C, the (111) peaks are observed at 2θ = 23.84°, 24.03°, 24.13° and 23.86°, respectively. The lattice parameter of the CdZnTe thick film is found to be dependent upon the concentration of zinc present. The shift in the position of the diffraction peak may be attributed to the variation in the concentration of zinc in the CdZnTe thick films produced at different temperatures during the growth process [27]. As indicated by Eq. (1), the XRD outcomes can be employed to ascertain the Zn concentration in CdZnTe thick films:
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