Ran Jiang a, Yang Kang a b, Kun Cao a, Xin Wan a, Heming Wei a, Renying Cheng a, Xue Tian a, Gangqiang Zha a b, Tingting Tan a
a School of Materials Science and Engineering, Northwestern Polytechnical University, Xi'an, Shaanxi, 710072, China
b Research and Development Institute of Northwestern Polytechnical University in Shenzhen, Shenzhen, Guangdong, 518057, China
## Abstract
Cadmium Zinc Telluride (CdZnTe or CZT) materials, favored for their physical superiority in optoelectronic applications, can be efficiently produced as large-area epitaxial film via close space sublimation (CSS), enhancing potential in flat-panel detector imaging. However, the application of CdZnTe epitaxial film in medical imaging equipment is limited by their lower resistivity, prolonged response times, and diminished sensitivity. By elevating the substrate temperature, we enhanced the number and depth of reverse sublimation pits on the surface of the CdZnTe epitaxial film, established an approximately 100 nm Zn-rich layer, broadened the bandwidth, and minimized electrode injection. Additionally, we observed an aggregation of dislocations at the edges of the reverse sublimation pits, which promoted surface recombination and reduced leakage current. These modifications significantly increased the film's resistivity to 1011 Ω cm. Further, they notably decreased the rise and drop times to 2 m s and 4 m s, respectively. Integrated with thin-film transistors (TFTs), the optimized CdZnTe epitaxial film now distinctly differentiate between air, plastics, and metals under X-ray examination. This improved performance of X-ray flat panel detectors (FPDs) based on CdZnTe epitaxial film is promising for the development of next-generation X-ray detection systems.
## Introduction
Since the 1970s, compound semiconductors such as CdZnTe, belonging to the group II-VI compounds, have garnered significant attention for their unique properties. These include small size, tunable forbidden bandwidth, low noise, high resolution, and rapid energy response rate [[1], [2], [3], [4]], rendering them particularly suitable for the detection of hard X-ray and γ-ray at room temperature within the energy range of 10 keV to 1 MeV [5]. In the meantime, CdZnTe has attracted considerable interest owing to its improved carrier transport performance and superior Detective Quantum Efficiency (DQE) of detectors [6,7]. Furthermore, CdZnTe detectors have seen notable advancements in various applications including aerospace, medical imaging, security check and nuclear safety, offering substantially higher energy and spatial resolution compared to scintillator detectors [[8], [9], [10]].
CdZnTe epitaxial film offers several advantages over standard melt or solution technique CdZnTe crystals, including large area, low cost, and fast-growth rate [1,4,11,12]. As a result, they can reduce the cost of splicing, which is a common process in X-ray panel detectors. Nonetheless, stable photocurrent plateau, short response time, and minimal dark current are essential characteristics of high-quality X-ray panel detectors [12,13]. The resistivity of the CdZnTe epitaxial film obtained from the recent studies is relatively low, range from 108 to 109 Ω cm, which may lead to detectors exhibiting high dark current. Moreover, the film's extended light response time about 3s–5s, renders it unsuitable for use in medical imaging equipment. Additionally, the photocurrent of the film fluctuates around the mean value and fails to remain stable at the same dose [[14], [15], [16], [17]].
To continually enhance the quality of the film, it is essential to refine the procedural parameters and compare various gas phase growth techniques [18]. In this study, CdZnTe epitaxial films were grown using the Close Space Sublimation (CSS) method. Amin et al. [19] assert that the growth rate and quality of the CSS method depend on several factors including the source-substrate distance, source-substrate temperature, pressure, deposition time, temperature, and gas composition in the deposition chamber. Consequently, substrate temperature plays a pivotal role in determining the quality of the epitaxial films, a topic that has been extensively studied both domestically and internationally [20,21]. Doroody et al. [22] were the pioneers in growing CdTe films on ultra-thin (100 μm) Schott D263T glass substrates using a high-temperature deposition process, specifically short distance sublimation (CSS), and investigated the optimal substrate deposition temperature for growing CdTe films on Indium Tin Oxide (ITO). Li et al. [23] attempted to grow CdZnTe polycrystalline films on TFT circuit boards and ITO/TFT substrates using close-distance sublimation (CSS) technology. However, the substrate temperatures required for growth exceeded the thermal tolerance of the TFT, causing the CdZnTe polycrystalline films to crack and detach. Wu et al. [24] examined the impact of substrate temperatures on the crystalline quality of CdZnTe polycrystal films, noting that the grain size of the films varied with the substrate temperature. To date, there are few reports on the effect of substrate temperature on the quality of CdZnTe single crystal films grown on GaAs substrates. A comprehensive understanding of how the growth process impacts the microscopic defects on the surface of CdZnTe epitaxial film is imperative. Additionally, elucidating the mechanism through which this process influences carrier transport in CdZnTe epitaxial film is crucial for systematically analyzing the optoelectronic performance of detectors and laying the groundwork for enhancing the quality of digital photographic current-integrated imaging in the future.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/optimized-substrate-temperature-for-high-quality-cdznte-epitaxial-film-in-x-ray-flat-panel-detectors.html