Xiaoyan Liang (a) (b), Xuan Zhu (a), Jijun Zhang (a) (b), Chen Xie (a), Wenxuan Yang (a), Bo Zhang (a), Linjun Wang (a) (b), Jian Huang (a) (b), Jiahua Min (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 interface states of TeO2/CdZnTe affects the capacitance voltage characteristics of metal oxide semiconductor (MIS) structured CdZnTe devices. Based on numerical simulations, the interface states exhibit a capacitance hump in the depletion region of the Csingle bondV curve, which is influenced by substrate doping concentration, frequency, interface state density, and energy level position. According to microstructure and component analysis by high resolution transmission electron microscope and energy dispersive spectroscopy (HRTEM-EDS), MIS-CdZnTe devices with TeO2 passivation layer of 142–173 nm and limited oxygen diffusion region as interface layers were prepared by NH4F/H2O2 passivation and CMP/NaClO oxidation, respectively, and then used for experimental capacitance characteristic measurement. The results show that the hump capacitance of MIS-CdZnTe prepared by passivation gradually decreases with increasing frequency, consistent with the simulation results, but presents relatively gentle due to the actual interface states with multiple energy levels. Moreover, the influence mechanism of interface traps on Csingle bondV curve is analyzed using band plots. In addition, the interface state density of MIS-CdZnTe structure is calculated using conductance method, which indicates that the interface state density of MIS- CdZnTe structure prepared by CMP/NaClO method is less than 1/3 that of passivation method (2.11 × 1010 cm−2 eV−1), presenting no obvious hump capacitance was observed in its Csingle bondV curves at low frequencies. Therefore, passivation forms an uneven thickness oxide interface layer between CdZnTe and TeO2, resulting in a higher density of interface states. While CMP/NaClO treatment forms a limited oxygen diffusion zone and achieves a smooth and dense interface, which can reduce the interface recombination rate, leading to a smaller leakage current (0.067 nA/mm2 @ 100 V) for MIS-CdZnTe device, and improving spectral performance of the energy resolution from 11.4 % to 7.7 %.
## Introduction
Metal oxide semiconductor structures are widely used in microelectronics and optoelectronics, and the performance and stability of such devices depend heavily on the formation of insulating layers (natural or deposited). The interface states is that which lies between the semiconductor and insulator and is consisted mainly of unsaturated bonds (i.e., suspended bonds) that exist in the interfaces and affect the performance of the device by changing the periodic potential field on the crystal surface [1,2]. CdZnTe (CZT) crystals are important materials for the preparation of room-temperature X-ray and gamma-ray detectors because of their high atomic number, large forbidden band width and high resistivity, which are widely used in nuclear safety, environmental protection, medical imaging and astrophysics [3]. The TeO2 layer generated on the surface of CZT crystals is intended to reduce electronic noise and prevent crystal aging [[4], [5], [6], [7], [8], [9]]. However, the interface states existing between the TeO2 insulating layer and the CZT semiconductor can also have an important impact on the device [10]. While CMP treatment with oxidants can obtain finer CZT surfaces [11], which can serve as an effective means of surface treatment for high-performance X-ray and gamma ray CZT detectors [12].
In fact, the study of interfacial states of metal oxide semiconductor structures has had a great attraction in recent years [[13], [14], [15], [16]]. Pengkun et al. [17] studied the influence and origin of the interfacial states between MoS2 and HfO2. They found that the presence of sulfur vacancies leads to interfacial states that cause frequency dispersion in the accumulated layers in the Csingle bondV curve. Nisika et al. [18] employ an ultrawide-bandgap Ga2O3 film to insert between CdZnTeSe absorber and TiO2 electron transport layer, which passivates the electrically active interface trap states to reduce the density of interface defects, finally the dark current and response time of the passivated device are significantly reduced. Sukri Karatas et al. [[19], [20], [21]]. studied the interface states density of Schottky barrier diodes, heterojunctions, MS structures by Csingle bondV characterization.For the radiation detector material CZT, wet passivation [6,7] and oxygen plasma etching [4] have been reported for passivating the crystal surface to improve electrical performance, but the trap states present at the CZT/TeO2 interface have not been reported in detail so far. In particular, the effect of interface states on the capacitance-voltage (Csingle bondV) characteristics deserves further study because the Csingle bondV characteristics are sensitive to the interface trap distribution, which is beneficial for quantifying the interface density of states [22,23], improving the interface quality, and enhancing the device performance.
In the article, a MIS structure model is constructed through numerical simulation methods. In this model, the influence of CZT/TeO2 interface states on the capacitance voltage curve was studied by applying different bias voltages, substrate doping concentrations, frequencies, interface state densities, and energy level positions. Based on the simulation results, the capacitance voltage curve of MIS structured CZT devices measured under experimental conditions was analyzed. The interface state density between the oxide layer and the semiconductor was calculated using the conductance method, and the changes in interface state density caused by wet passivation and oxidation after mechanochemical polishing were compared, providing possibilities for improving device performance and stability.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/effect-of-interface-states-on-capacitance-voltage-characteristics-of-cdznte-crystals-after-surface-oxidation.html