Shulei Wang a, Xiangzhi Cao a, Chen Xie a, Jijun Zhang a, Xiaoyan Liang a, Linjun Wang b, Zheren Xu a, Xiaolong Song a, Panhui Qiu a
a School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China
b Zhejiang Institute of Advanced Materials, SHU, Jiashan, 314113, China
## Abstract
In this work, platinum (Pt) electrodes were deposited on CdZnTe wafers by electroless H2PtCl4 solution and subsequently annealed at different temperature from 323 K to 473 K. Focused ion beam (FIB) imaging, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), current-voltage (I–V) analysis and energy spectra under 241Am irradiation were adopted to investigate the effect of low-temperature rapid annealing on interface structure, electrical and carrier transport properties of Pt/CdZnTe samples. Single-crystal PtTe layer with [100] orientation and thickness of 20 nm was found in the interface between electroless Pt and CdZnTe wafer. The leakage current of the Pt/CdZnTe detectors at 200 V decreased from 212 nA cm−2 to 101 nA cm−2 after low-temperature rapid annealing, due to a decrease of voltage drop across the PtTe interface layer. The energy spectra and carrier transport performance of Pt/CdZnTe detectors suggested that the optimal annealing temperature was 423 K. The energy spectra of Pt/CdZnTe detector under 241Am showed that the FWHM was improved from 23.3% to 14.4%, while the full charge collection and mobility lifetime product of electrons increasing from 611 to 625 and 3.17 × 10−3 to 3.76 × 10−3 cm2 V−1 s−1, respectively.
## Introduction
Direct conversion semiconductor detectors used to detect X-ray and gamma-ray have been studied for almost 30 years [1]. The CdZnTe detector, with its excellent electron transport properties and low leakage current at room temperature (300 K), is considered one of the most promising X-ray and gamma-ray detector materials in many applications, such as nuclear medicine imaging and environmental monitoring [[2], [3], [4]] [[2], [3], [4]] [[2], [3], [4]]. The contact interface layer will have a direct influence on the leakage current, electric field distribution and carrier transport performance of the detectors [5,6]. The Pt electrode has a high work function (∼5.6eV), which is convenient for blocking contact with CdZnTe to limit the leakage current.
There are several deposition methods for Pt electrodes on CdZnTe: thermal evaporation, sputtering and electroless deposition. Electroless deposition has been shown not only to form stronger adhesion [7,8], but also to produce better energy resolution. Lee [9] et al. compared the effects of different contact methods (thermal evaporation, electroless deposition) and different electrode materials (Au, Pt) on the performance of CdZnTe detectors and found that electroless Pt electrode has better electrical properties. Bettelli [10] et al. found that Pt electrode has the effect to improve collection efficiency of hole and decrease the “polarization effect”, making them more suitable for photon-counting X-ray detectors in high-flux conditions (>106 photons/mm2).
The annealing process is usually applied to improve the properties of metal contacts on CdZnTe detectors. Kim [11] et al. found that low-temperature (200°C) annealing could reduce the leakage current of Au/CdZnTe devices due to the oxidation of Te in the interfacial layer to high resistance TeO2 or CdTeO3. Shi [12] et al. found that rapid-annealing enhanced the interdiffusion between Ti/Au electrodes and CdZnTe crystals, thus improving the adhesion of Ti/Au electrodes and reducing the leakage current of detectors. Rejhon [13] et al. found that the change in Schottky barrier height of Au/CdTe due to low-temperature annealing was responsible for the change of bulk current, while the surface leakage current was affected by TeO2 layer thickness. However, it is also reported that low-temperature annealing will cause higher leakage currents [[14], [15], [16]] [[14], [15], [16]] [[14], [15], [16]]. Wang [16] et al. found that Au/p-CdZnTe had excellent ohmic property with the increase of leakage currents after 2 h annealing at 333 K. Therefore, the impact mechanisms of low-temperature annealing on the leakage current of Au/CdZnTe (CdTe) is still controversial. Meanwhile, the effect of low-temperature annealing on the electroless Pt/CdZnTe needs further studies.
In this work, the contact interface, electrical and carrier transport properties of electroless Pt electrodes on CdZnTe were investigated. The study is focused on the mechanics of the properties of leakage current and energy spectrum after low-temperature rapid annealing treatment of Pt/CdZnTe detectors. The results will further the understanding of the electroless Pt contact on CdZnTe crystal and improvement in the fabrication of CdZnTe detectors.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/effect-of-low-temperature-rapid-annealing-on-structural-and-electrical-properties-of-electroless-platinum-contacts-on-cdznte-detectors.html