Peiyang Sun a, Yue Shen a, Yibo Kuang a, Dandan Wen a, Jian Huang a, Feng Gu a, Jiahua Min a, Linjun Wang a b
a School of Materials Science and Engineering, Shanghai University, China
b Zhejiang Institute of Advanced Materials, SHU, Jiashan, 314113, China
## Abstract
CdZnTe films with less defects and high quality were fabricated on aluminum-rich AlN (Al–AlN) transition layers by RF magnetron sputtering technique. The Al doping and regulation effect on defect states, structure and photoelectrical performance of CdZnTe films were minutely investigated by XRD, HRTEM, SEM, XPS, Photoluminescence and Photoelectric characteristics. Results strongly suggested the self-doping process of Al from Al–AlN transition layers, which helped to compensate the defect density of Cd vacancy in CdZnTe films, thus improving crystallization, response performances and resistivity of Al-doped CdZnTe films. Electrical characterization implied the formation of good ohmic contact between electrodes and Al-doped CdZnTe films. Under 254 nm UV light illumination, the surface dark current of optimal Al-doped CdZnTe film was 25 times smaller than that of non-doped film, and rise and decay times were 3.4 and 2.8 times faster, indicating the feasibility of high quality film detectors for solar-blind UV detection.
## Introduction
CdZnTe is compound semiconductor material with the advantages of low leakage current and wide band gap in the application of optoelectronic devices, solar blind detectors, x-ray and gamma-ray radiation detectors, which can operate at room temperature [1,2]. Compared with single crystals, CdZnTe films have remarkable characteristics, which are mainly reflected in the convenient fabrication procedure, low cost, fast growth rate [3] and feasibility of preparation of large size detector [4,5]. The researchers have developed several methods to prepare CdZnTe films, such as molecular beam epitaxy (MBE) [6], mental-organic chemical vapor deposition (MOCVD) [7], close-space sublimation [8], magnetron sputtering [9], electrochemical deposition [10], liquid-phase epitaxy [11] and electron beam deposition [12] etc. Among the various processes for preparing CdZnTe films, magnetron sputtering technology prepared CdZnTe films with uniform stoichiometry and good quality at relatively easy control [13]. X. Gao et al. deposited CdZnTe films with smooth surface and uniform composition by RF magnetron sputtering method [14]. A.S. Pugalenthi et al. found higher substrate temperature was beneficial to improve crystallization and optical properties of CdZnTe films by magnetron sputtering [15]. K. Kim et al. deposited polycrystalline CdZnTe thick films by thermal evaporation method, and the resistivity decreased rapidly along with the increase of defect concentration in CdZnTe films [16]. CdZnTe films inevitably have many inherent defects, resulting in low resistivity and poor μτ, which are unfavorable for detector applications. X. Wan et al. reported that CdZnTe epitaxial films were annealed in Te2 atmosphere to compensate for Cd vacancies (VCd), improving the electron mobility lifetime product (μτ)e obviously [17]. However, G. Rajesh et al. observed photo induced electrical bistability of sputter deposited CdZnTe films, which explained by the fact that photo induced defect sites provided additional charge carriers and enhanced conductive state of the films in presence of light, indicating the defects related application in optoelectronic devices [18].
Introduction of compensating impurities is thought to be an effective method to solve the native defects (VCd) and plays an important role in improving detection performance of CdZnTe crystals. The doped elements are usually groups I, III (In, Al) IV (Ge, Pb), V (Bi) and VII (Cl, I) [[19], [20], [21]]. R. Nan et al. grew CdZnTe crystals with introducing Al as a donor impurity to pin the Fermi energy level near the middle of band gap, and thus increased the resistivity [22,23]. V. Carcelén et al. studied the resistivity of CdZnTe crystals grown by doping Bi was increased,which reduced significantly the concentration of vacancies of Cd and Te [24]. Yang ge et al. reported the resistivity was increased three orders and the stabilization period of leakage current was reduced dramatically by introducing In element into CdZnTe crystals [25]. Yasir Zaman et al. characterized In and Pb co-doped CdZnTe crystals with resistivity up to 1.8 × 1010 Ω cm and μτ of 5.12 × 10−4 cm2/V is higher than In doped CdZnTe [26,27].
Inspired by the defect regulation of doping elements in CdZnTe crystals, researchers proposed to optimize photoelectric properties of CdZnTe films by doping elements, such as In, B, P, Cu and Al. Z. Zhu et al. prepared Cu-doped Cd0.6Zn0.4Te thin films that had suitable transmittance to meet the need for tandem solar cells [28]. J. Bie et al. investigated the introduction of In element to compensate for Cd vacancies and other impurity acceptors and improve the current densities of CdZnTe films [29]. D. Zeng et al. sputtered Al on the CdZnTe films and heated, the Al atoms occupied the position of intrinsic Cd vacancy as donor so that CdZnTe films exhibited lower leakage current (2.3 × 10−8 A) [9]. M. Ali et al. observed phosphorus implanted CdZnTe devices with a relatively high conductivity was beneficial to obtain larger short circuit current for Photo-Voltaic (PV) applications [30].
This work focuses on the preparation and characterization of high quality CdZnTe films by RF magnetron sputtering for solar-blind UV detection. The effects of Al doping on the structural and photoelectrical properties of CdZnTe films were investigated. All test results proclaim that doped Al can move into the lattice of CdZnTe films by thermal diffusion on the sputtering process, while optimal doped Al acts as donor to compensate for Cd vacancies, reducing the defect concentration in CdZnTe films most notably.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/al-doping-and-defect-regulation-of-sputtered-cdznte-films-based-on-al-aln-transition-layers.html