Xuchen Wang a, Wei Dai a, Zhou Li a, Zhao Fu a, Linyue Liu a b, Xiangli Zhong a, Xiaoping Ouyang a b, Jinbin Wang a
a National-Provincial Laboratory of Special Function Thin Film Materials, School of Materials Science and Engineering, Xiangtan University, Xiangtan, 411105, Hunan, China
b Northwest Institute of Nuclear Technology, Xi'an, 710024, China
## Abstract
Passivation plays a crucial role in surface treatment, which could generate an oxide layer that significantly decreases the surface leakage current of
Cadmium Zinc Telluride (CdZnTe, CZT) detectors and enhancing their final performance. However, existing passivation methods are limited by the unevenness of the oxide layer and insufficient reduction in surface leakage current. In this paper, a novel two-step passivation method for CdZnTe is proposed, in which the first step uses potassium persulfate (K2S2O8) solution and followed by treatment with ammonium fluoride mixed with hydrogen peroxide (NH4F/H2O2) solution. The results indicate that the passivation method can produce a uniform oxide layer, and the surface leakage current was decreased by 77.76 %. In particular, the energy resolution was improved by 80 % after passivation. This is attributed to the fact that the two-step passivation method removes the highly conductive Te-rich layer, leaving only the tellurium oxide on the CdZnTe surface. Therefore, the two-step passivation method provides an effective passivation treatment process for preparing high-performance CdZnTe room-temperature radiation detector.
## Introduction
Cadmium zinc telluride (CZT) is a typical compound semiconductor material that is highly regarded for its potential as a room-temperature nuclear radiation detector due to its advantageous properties, including a high average atomic number, wide forbidden-band width, high resistivity, and stability at room temperature [[1], [2], [3], [4], [5]]. The energy resolution of CZT detectors is constrained by surface leakage current, and the surface treatment employed during the fabrication of CZT detectors plays a crucial role in influencing surface leakage current levels [[6], [7], [8]]. The surface treatment consists of two processes: chemical etching and surface passivation. The purpose of chemical etching is to eliminate the mechanical damage by bromine in methanol (Br-MeOH), but at the same time it generates an enriched layer of metallic tellurium (Te-rich layer) on the surface of CZT. This Te-rich layer is characterized by high conductivity and causes an increase in surface leakage current [[9], [10], [11]], and the extent of this increase is dependent on the quality of the CZT crystal and the degree of chemical etching. Among these factors, improper chemical etching can cause a more substantial increase in surface leakage current, thereby adversely impacting the performance of CZT detector [[12], [13], [14]].
Surface passivation mitigates the negative consequences associated with chemical etching, rendering it an essential process in the fabrication of CZT detectors. This process involves the interaction of passivants with the Te-rich layer to consume metallic tellurium and form an oxide layer, then effectively impeding the transmission of leakage current between the surface electrode and the high-conductivity Te-rich layer [[15], [16], [17]]. Consequently, surface passivation is extremely important to enhance the performance of CZT detectors, which relies on specific passivants to achieve its purpose. Currently, the most commonly used passivants are KOH solution and NH4F/H2O2 mixed solution [[18], [19], [20], [21]]. The choice of passivants can result in variations in performance after treatment. Nevertheless, these solutions present certain problems, such as incomplete removal of the tellurium-rich layer, inadequate oxide layer thickness, and surface atomic ratio imbalances. In particular, the presence of a residual Te-rich layer beneath the oxide layer in CZT detectors can have a detrimental impact on their electrical performance [[22], [23], [24], [25]]. To address the above issue, two main approaches can be taken. The first approach is to exploring new passivants, such as NaClO and other strong oxidizing agents or (NH4)2S and other sulfurizing agents [[26], [27], [28]]. However, these new passivants require special conditions such as heat or catalysts for passivation. Another approach involves developing a novel passivation method that combines two solutions, such as the two-step passivation method that initially utilizes KOH solution to remove the Te-rich layer followed by treatment with NH4F/H2O2 mixed solution [29], but the efficacy of the two-step passivation method has been underexplored, necessitating further enhancement.
In this paper, a novel two-step passivation method was investigated by combining two approaches. This method consists of treating the Te-rich layer with K2S2O8 solution followed by subsequent passivation with NH4F/H2O2 mixed solution. The impacts of several different passivation methods on surface leakage current were investigated, and surface composition of CZT detectors is evaluated. In addition, energy resolution tests were performed on CZT detectors prepared by a novel two-step passivation method.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/two-step-passivation-by-k2s2o8-and-nh4f-h2o2-solutions-for-improving-the-performance-of-cdznte-detectors.html