B. Park a b, Y. Kim b c, J. Seo a b, J. Byun 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
Several cadmium zinc telluride (CZT) crystals were fabricated into radiation detectors using methods that included slicing, dicing, lapping, polishing, and chemical etching. A wet passivation with sodium hypochlorite (NaOCl) was then carried out on the Br-etched detectors. The Te-rich layer on the CZT surface was successfully compensated to the Te oxide layer, which was analyzed with X-ray photoelectron spectroscopy data of both a Br-etched crystal and a passivated CZT crystals. We confirmed that passivation with NaOCl improved the transport property by analyzing the mobility-lifetime product and surface recombination velocity. The electrical and spectroscopic properties of large volume detectors were compared before and after passivation, and then the detectors were observed for a month. Both bar and quasi-hemispherical detectors show an enhancement in performance after passivation. Thus, we could identify the effect of NaOCl passivation on large volume CZT detectors.
## 1. Introduction
Cadmium zinc telluride (CZT) is a promising material for X- and gamma-ray detectors operating at room temperature [1]. However, it has an unresolved problem related to its surface treatment. Chemical etching with bromine, which is one of the fabrications on CZT, has a selective etching rate on each atom of CZT, resulting in the formation of a Te-rich layer on the CZT surface [2]. The Te-rich layer causes high conductivity on the CZT surface and acts as a trapping center. A representative method to solve the problem caused by the Te-rich layer is wet passivation, which has been investigated by many researchers [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12]]. Research shows a passivation effect mainly on the planar type CZT (or CdTe-based) detectors. There are few reports of a passivation effect on large-volume CZT detector (i.e., bar type or quasi-hemispherical). However, the passivation effect on large volume detectors will be more effective than passivation effect on small detectors, as they have wider surfaces and as the effect of passivation works at the surface of a specimen.
The chemical compounds rich in oxygen are used for passivation on the Te-rich surfaces of CZT crystals. Potassium hydroxide [3], ammonium fluoride [[4], [5], [6], [7], [8], [9]], ammonium sulfide [10,11], and hydroperoxide [4,12] epitomize chemical passivants. The passivant was proven to reduce the conductivity of the Te-rich layer and increase the spectroscopic performance of CZT detectors. However, wet-passivated detectors show degraded properties after a long time [8]. The phenomenon would occur gradually because of the passivated- and unstable CZT surface being exposed to ambient air or moisture. Thus, a novel passivant with long-term stability and an effective passivation ability (oxidation) needs to be found.
In this study, we investigated the effect of sodium hypochlorite (NaOCl) on bar and quasi-hemispherical CZT detectors which could effectively show the influence of passivation because of their wide surface area.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/passivation-effect-on-large-volume-cdznte-crystals.html