B. Park a b, Y. Kim b c, J. Seo a b, K. Kim d
a Dept. of Health and Safety Convergence Science, Korea University, Seoul, 02841, South Korea
b Interdisciplinary Program in Precision Public Health, Korea University, Seoul, 02841, South Korea
c Marine Radioactivity Monitoring Group, Korea Marine Environment Management Corporation, Busan, 48931, South Korea
d School of Health and Environmental Science, Korea University, Seoul, 02841, South Korea
## Abstract
Parylene coating was adopted on CdZnTe (CZT) detector as a mechanical protection layer after wet passivation with hydrogen peroxide (H2O2) and ammonium fluoride (NH4F). Wet chemical passivant lose their effectiveness when exposed to the ambient conditions for a long time. Parylene coating could protect the effectiveness of passivation, by mechanically blocking the exposure to the ambient conditions. Stability of CZT detector was tested with the measurement of leakage current density and response to radio-isotopes. When the enough thickness of parylene (>100 μm) is adopted, parylene is a promising protection layer thereby ensuring the performance and long-term stability of CZT detectors.
## 1. Introduction
Cadmium Zinc Telluride (CZT) is a commercial material for X-ray and gamma-ray detectors operating at room temperature [1]. However, there are still some issues that need to be addressed to further improve detector performance. One of them is corresponding with the long-term stability after passivation [2].
Selective etching property of Br-based etchants leaves highly conductive Te-rich layers on the surface. These Te-rich layers act as a charge trapping center and degrade CZT detector performance. The most common way to solve this problem is to passivate the Te-rich layers through a wet or dry passivation process. In both passivation processes, Te atoms in Te-rich layers chemically react with oxygen and are then converted to TeO, TeO2, and TeO3. Typical wet passivation processes need chemical compounds rich in the oxygen, such as hydrogen peroxide (H2O2), ammonium fluoride (NH4F) or ammonium sulfide ((NH4)2S) [[2], [3], [4], [5], [6], [7]]. The main disadvantage of wet passivation is that chemicals are not pure compared to CZT material (6–7N purity). It can therefore contaminate the surface of CZT detector. Dry passivation requires thermal treatment with high temperature in oxygen atmosphere. When the CZT material is exposed to 200 °C for more than 20 min, it loses its original high resistivity. Moreover, Te oxides initially form a stable state in both processes, however it gradually destabilizes over time in the ambient environment due to reacting with moisture and carbon dioxide [2].
To solve the problem, we added the parylene coating process after the conventional passivation processes. Parylene is an organic polymer with high resistivity, dielectric strength, and is waterproof. Moreover, it can form a uniform film on the surface of any sample despite having any unusual geometry, such as holes or bumps [8,9]. These are why the parylene has widely been used as a protective coating for challenging electronics, aerospace, and medical applications. Here, parylene coating was tested as itself or mechanical protection layer after wet passivation with hydrogen peroxide and ammonium fluoride.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/effectiveness-of-parylene-coating-on-cdznte-surface-after-optimal-passivation.html