Electronic structure of Cd, In, Sn substitutional Defects in GaSe
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Electronic structure of Cd, In, Sn substitutional Defects in GaSe Zsolt Rak1, Subhendra D Mahanti1, Krishna C Mandal2, and Nils C Fernelius3 1 Physics and Astronomy, Michigan State University, East Lansing, MI, 48824 2 EIC Laboratories, Inc, 111 Downey Street, Norwood, MA, 02062 3 AFRL/MLPSO, Wright-Patterson Air Force Base, Dayton, OH, 45433 ABSTRACT Ab initio electronic structure calculations within density functional theory have been carried out in pure GaSe and GaSe doped with substitutional impurities (Cd, In and Sn) at the Ga site in order to understand the nature of the defect states and how they depend on the nominal valence of these three impurities. We find that Cd impurity introduces a defect state located between 0.1 - 0.18 eV above the valence band, in good agreement with photoluminescence peaks seen at 0.13 eV and 0.18 eV. Using both experimental and theoretical effective mass parameters we show that effective mass model fails to describe these acceptor states. Sn changes the single particle density of states (DOS) near the bottom of the conduction band, and gives rise to resonant states deep in the valence band. In, on the other hand, behaves like Ga, it does not make noticeable change in the DOS of the host GaSe crystal. INTRODUCTION Gallium Selenide (GaSe) is a wide band gap semiconductor, which has been studied extensively for its unique properties associated with its layered structure. Because of its large nonlinear optical coefficient (d22=75 pm/V), GaSe is an excellent non-linear optical material [1]. One of the important applications of nonlinear optical effects is the generation of broadband midinfrared electromagnetic waves and detection of coherent broadband THz radiation. Recently, Liu et al. reported broadband terahertz wave detection using GaSe including EIC grown crystals [2-3]. Shi et al have used GaSe to generate tunable, narrowband THz radiation using a difference-frequency technique [4]. The dielectric function and momentum relaxation time of carriers in GaSe single crystals were investigated using terahertz time-domain spectroscopy (THz-TDS) [5]. GaSe also finds application in the area of radiation detectors and since the first report by Manfredotti et al. in 1974, GaSe has been studied by several groups due to easy fabrication, high resistance for radiation damage, and high temperature operation [6-12]. It has room temperature resistivity of 103-109 Ω⋅cm [6]. The large leakage current for pure GaSe made it unsuitable for efficient radiation detectors. However when doped with Ge or Sn it has high resistivity [13]. Yamazaki et al. reported 3-7 orders of magnitude decrease in leakage current and improvement in energy resolution by doping with 0.005-0.1 at. % Ge [11-12]. The crystal structure of GaSe is extremely interesting; Ga and Se layers can be stacked in different ways, which leads to different poly-types (β,γ, δ or ε). These polytypes are wide band gap semiconductors with room temperature band gaps ranging from 1.996 eV (for ε and γ), 2.026 eV (for δ) to 2.046 eV (f
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