Defects in CdTe-Based Photodetectors
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ABSTRACT The photoluminescence spectra, defect composition and optoelectronic properties of chlorine doped CdTe monocrystals, thick layers and thin films are investigated. It is supposed that the complex defect (VCd-2CITe) is a neutral defect that causes high resistivity of Cl doped CdTe. This complex can dissociate into two charged defects (VCd-CITe) and ClTe at 300 K. Acentre (Vcd-CITe) is a stable defect that is responsible for p-type conductivity of Cl doped CdTe. Depending on the C1 concentration high resistivity, high photoconductivity or high p-type conductivity can be formed in CdTe that is only chlorine doped. INTRODUCTION Cadmium telluride has received much attention in the past as a suitable material for optoelectronic devices. Cadmium zinc telluride has emerged as a leading candidate material for room-temperature X-ray and gamma-ray detectors [1]. Mercury cadmium telluride is the most important material for infrared detectors [2]. Cadmium selenide telluride is a well material for visible and near-infrared photodetectors. CdTe has been chosen as the most promising material for large area low cost thin film solar cells [3]. Although the defect structure of CdTe has been widely investigated, the collected information is insufficient yet, some fundamental physical problems still remain unsolved [4], and the technology of CdTe-based devices has been characterisized as a black magic sometimes. B.K. Meyer et al. are identified the isolated cadmium vacancy Vcd and the tellurium vacancy VTe in CdTe by electron paramagnetic resonance [5]. VCd is a double acceptor with energy level at Ev + 0.47 eV, VTe is a single donor with energy level at Ev + 0.2 eV [6]. Information about the intrinsic host interstitials in CdTe is scarce. A deep level at Ev + 0.56 eV is assigned to a double charged donor Cdi [7]. Relaxation of the crystal lattice around dopant atom in CdTe can change their electronic levels in the bandgap and the elastic energy, which contributes to the energy needed to form a substitutional dopant [4]. Cadmium chloride is often used as a fluxing agent for the recrystallization of polycrystalline CdTe layers and for the synthesizing of solid solutions from the variable compounds. Due to this chlorine is most often used impurity in CdTe. Substitutional chlorine at the position of Te is a single donor with a binding energy of Ec - 14 meV [8]. Interstitial chlorine is supposed to be an acceptor [9]. Chlorine forms variable complex centers in CdTe. The most known is A-centre, a shallow acceptor (Vcd-CITe) at Ev + 120 meV [8]. (Vcd-2CIT,) is identified as an isoelectronic complex [10]. Acceptor levels at 80 meV [7] or at 45 meV [11] have also assigned to this complex. In this work the experimental results of thermal treatments of the chlorine doped CdTe monocrystals, thick layers and thin films are presented. The formed complex defects and the stability of these defects are discussed.
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Mat. Res. Soc. Symp. Proc. Vol. 607 @2000 Materials Research Society
EXPERIMENT 6N+ purity level CdTe powder or monocrystals
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