Chengjie Feng a, Jiahua Min a, Xiaoyan Liang a, Jijun Zhang a, Linjun Wang a b, Yue Shen a
a School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China
b Zhejiang Institute of Advanced Materials, SHU, Jiashan, 314113, China
## Abstract
The surface leakage current (SLC) of CdZnTe is an important factor affecting detection performance. In this work, we established a two-dimensional model of metal-semiconductor-metal (MSM) structured CdZnTe with a low-resistance layer on the surface. The composition of SLC and the distribution of the internal electric field are analyzed by simulation. The study found that the change of electrode size will change the contribution of SLC from cathode, anode and side surfaces. With the increase of electrode size, the contribution of SLC to total leakage current decreases gradually, and the internal electric field distribution becomes more uniform. According to the conclusions of simulation, we have proposed a simple and accurate method to estimate the bulk resistivity and SLC of CdZnTe. In addition, combined with a series of experiments, we have also summarized several methods which can reduce the contribution of the SLC in MSM structured CdZnTe detector in order to improve energy resolution.
## Introduction
Cadmium zinc telluride (CZT) is considered to be the most promising material for room temperature nuclear radiation detector due to its high band gap, high resistivity, high mobility lifetime product of electron, large average atomic mass, wide energy detection range and wide operating temperature range [1]. It has already widely been used in medical, aerospace, nuclear physics and other fields.
The leakage current is the primary factor affecting the energy resolution of CZT detector [2]. Generally, it is believed that the leakage current can be divided into two parts: bulk leakage current and surface leakage current (SLC). CZT detector without proper surface treatment has large SLC, which will seriously affect the performance of the detector. We believe that there are two main sources of SLC. The first is the mechanical damage layer caused by surface mechanical polishing [3]. The second is the non stoichiometric Te-rich layer after etching process [4,5]. Both of them will lead to a huge increase in the total leakage current of the device. Due to different crystal quality and treatment processes, this growth can reach 1–3 orders of magnitude [[6], [7], [8]]. Although there are accurate methods to measure SLC nowadays, there is still no detailed analysis about the composition of SLC and the proportion of SLC and bulk leakage current in CZT devices.
We know that passivation is a common and useful way to reduce SLC of CZT detector [[9], [10], [11], [12]]. However, in a large number of experiments, we found that the bad stability of passivation effect by solution method, and the current after passivation is often the same or bigger than that of the mechanically polished sample [6,12,13]. Therefore, it is necessary to explore the composition and transmission of SLC in detail to help optimize the process further.
At present, there are two main methods to measure the SLC: parallel strip electrode method and guard ring electrode method [[14], [15], [16], [17]]. The parallel strip electrode method can qualitatively compare the resistivity between different CZT surfaces, but can not directly measure the overall SLC of detector. The better method is using guard ring electrode, which can separate the bulk current and SLC of the device, so as to accurately calculate the bulk resistivity. However, due to the limitation of their own process, the distance between electrodes should be very small (less than 100 μm generally [16,[18], [19], [20]]). Both methods need to use lithography technology and test platform such as probe station and dark box, so the practical operation process is very complicated.
In this work, we use semiconductor device simulation software to simulate the CZT photoconductive detector with MSM structure at high precision to explore the SLC and internal electric field. According to the simulation conclusions, we propose a simple and accurate method to measure bulk resistivity of CZT wafer, and summarize several methods to reduce SLC of MSM structure detector.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/investigation-on-method-of-reducing-surface-leakage-current-of-the-cdznte-photoconductive-detector-with-msm-structure.html