Atomic and Electronic Structure of Symmetric Tilt Boundaries in ZnO
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Atomic and Electronic Structure of Symmetric Tilt Boundaries in ZnO Fumiyasu Oba, Shigeto R. Nishitani, Hirohiko Adachi, Isao Tanaka1, Masanori Kohyama2, and Shingo Tanaka2 Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto 606-8501, Japan 1 Department of Energy Science and Technology, Kyoto University, Sakyo, Kyoto 606-8501, Japan 2 Department of Materials Physics, Osaka National Research Institute, Agency of Industrial Science and Technology, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan ABSTRACT We have investigated the atomic and electronic structure of symmetric tilt boundaries in ZnO by a first-principles plane-wave pseudopotential method. Equilibrium boundary geometries with distorted- and dangling-bonds are obtained. Localized electronic states form mainly at the lower valence band and the bottom of the upper valence band owing to the bond disorder. However, the electronic states near the band gap are not significantly affected; deep states are not generated in the band gap. The small effects of the bond disorder on the electronic structure can be attributed to the band structure characteristic of ZnO. INTRODUCTION Grain boundaries in ZnO are interesting for the non-linear current-voltage characteristics [1-3]. These characteristics are widely utilized as varistors to protect electric circuits. The appearance of the characteristics is attributed to the formation of the double Schottky barrier based on the interfacial electronic states [4]. According to reports on the nonlinear characteristics, the generation of the interfacial states is likely to be associated with the presence of some impurities and/or excessive oxygen [5-13]. However, it should be important to understand the electronic structure originating from bond disorder at the grain boundaries itself, differentiating from extrinsic effects by the impurities and the excessive oxygen. In the present study, we investigate the stoichiometric configurations of the ZnO [0001] / ( 1 230) Σ=7 symmetric tilt boundary in order to clarify the effects of the bond disorder. The atomic and electronic structure of the boundary is determined by a first-principles pseudopotential method. As a result, we find that the electronic states in the vicinity of the band gap are not significantly affected by bond distortion and the presence of dangling-bonds. Deep electronic states are not recognized in the band gap even for the configuration with dangling-bonds. These results can be generally explained by the band structure characteristic of ZnO; deep interface states observed for ZnO varistors should not originate solely from bond disorder at stoichiometric boundaries. COMPUTATIONAL PROCEDURE The first-principles calculations were performed using a plane-wave pseudopotential method [14] within the local density approximation (LDA) [15]. Troullier-Martins type pseudopotentials [16] were used in the Kleinman-Bylander form [17] with the local components of s for Zn and p for O. The boundary geometries modeled in our previous
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