Longhui Lin a, Haofei Huang a, Ke Xu a, Qunfang Wang a, Ke Tang a, Meng Cao a, Jian Huang a b c, Linjun Wang 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
## Abstract
In this work,
Cadmium Zinc Telluride (CdZnTe) thick films were grown using close-spaced sublimation (CSS), while Al2O3 films of varying thicknesses were deposited by atomic layer deposition (ALD) as surface passivation layers. The influence of the Al2O3 surface passivation layer on the structure, surface morphology, chemical composition and electrical performance of the CdZnTe thick films was analyzed using XRD, EDS, SEM, AFM, XPS and current–voltage (I-V) characterization systems. The results demonstrate that the surface passivation layer could significantly improves surface defects and reduces the surface leakage current (SLC) of CdZnTe thick films. The CdZnTe thick film radiation detector exhibited an improved energy resolution, decreasing from 18.6 % to 15.1 % with a 40 nm Al2O3, as measured using a 60 KeV 241Am γ-ray source.
Graphical abstract
## Introduction
CdZnTe is considered one of the most promising materials for radiation detectors due to its high average atomic number, large and adjustable bandgap, high resistivity, and strong resistance to X-rays and γ-rays[1], [2], [3], [4]. CdZnTe detectors have broad application prospects in fields such as nuclear medicine imaging, national defense security, deep space exploration, and high-energy physics[5], [6], [7].
The growth process of CdZnTe single crystals is complex, and as their size increases, defects are more likely to form, making it challenging to produce large, high-quality crystals[8], [9], [10]. In contrast, the CdZnTe thick film growth process is simpler and more cost-effective, addressing the challenges associated with single crystals and garnering increased attention in the field of detectors[11], [12], [13], [14].
Impurities, secondary phases, and surface quality can limit the performance of CdZnTe devices. In particular, surface quality significantly affects the leakage current of the device, leading to increased noise in the detector and hindering improvement in energy resolution [15]. To enhance the performance of CdZnTe thick film devices, surface treatment of thick films is necessary, which can be categorized into three types: physical mechanical polishing, chemical etching, and surface passivation treatment[16]. Physical mechanical polishing and chemical etching polishing aim to remove the rough surface layer of CdZnTe thick films, resulting in a smoother surface, reduced surface defects, and improved surface quality[17], [18], [19]. Surface passivation treatment is one of the most effective methods for improving the surface leakage current of CdZnTe thick film detectors. It reduces the surface leakage current and surface state density by eliminating the Te-rich phenomenon[20]. Surface passivation treatments are mainly divided into wet and dry passivation. Wet passivation is the most widely used method, involving solutions such as H2O2[21], KOH[22], NH4F/H2O2[23], and KOH-KCl + NH4F/H2O2[24] solutions. Despite its simplicity, the reaction between the passivation solution and the CdZnTe thick film surface is complex and not fully understood.
Dry passivation involves depositing a thin film of highly resistive material on the sample surface through physical processes. These high-resistance films saturate the dangling bonds on the surface and improve surface defects. Prettyman[25] performed oxygen ion passivation after forming a TeO2 layer over 20 nm on the surface, significantly reducing the surface leakage current. Chen[26] used oxygen atom post-injection to partially mask the surface of samples in advance for comparison, achieving a leakage current reduction of up to 79 %. Additionally, Mescher[27] used a combination of oxygen ions and sputter deposition of a 1000 Å SiNx layer, resulting in nearly a 20-fold improvement in leakage current. These findings suggest that dry passivation, unlike wet passivation, ensures the quality of the film and its interfacial reaction with the sample surface while providing superior surface passivation.
Nowadays, Al2O3 is widely regarded as a popular surface passivation layer material for enhancing the electrical performance of devices due to its high dielectric constant, hardness, large bandgap, high electron mobility, high breakdown field strength, and excellent thermal and chemical stability[28], [29], [30]. Numerous methods exist for preparing Al2O3 thin films, with atomic layer deposition (ALD) considered one of the most efficient for producing high-quality Al2O3 films[31]. Goldstein[32] demonstrated that ALD can grow Al2O3 thin films with excellent conformality and uniformity, allowing precise control of film growth at the atomic level.
ALD-deposited Al2O3 has proven effective in passivating CdTe-based materials for both solar cells and radiation detectors. In solar cells, it reduces surface recombination, improves carrier lifetimes, and enhances power conversion efficiency, while also providing environmental stability[33], [34], [35]. In radiation detectors, Al2O3 lowers leakage current, improves charge collection efficiency, and enhances energy resolution[36], [37], [38]. Overall, ALD Al2O3 significantly boosts the performance and durability of CdTe devices. There are two main methods for integrating metal electrodes after deposition of passivation layer in these devices. The first is directly depositing the metal electrode onto the passivated insulating layer, which reduces surface defects and enhances device performance[35], [39], [40]. The second method involves creating openings in the insulating layer, allowing the metal electrode to directly contact the semiconductor surface, thereby improving conductivity without compromising the passivation effect[36], [38], [41].
Additionally, Zanettini[41] achieved 55 nm Al2O3 surface passivation layer on CdZnTe single crystals via sputtering, resulting in surface leakage current values 10 to 100 times lower than those measured using the best wet passivation methods. However, there have been almost no reports on the deposition of Al2O3 passivation layer on the surface of CdZnTe thick film using ALD method and the effect of the passivation layer on the performance of CdZnTe thick film devices. Consequently, further in-depth research in this area is warranted.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/the-effect-of-al2o3-surface-passivation-layer-prepared-by-ald-method-on-the-performance-of-cdznte-thick-film-detectors.html