Electronic Structure of Li-Impurities in ZnSe.
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ELECTRONIC STRUCTURE OF Li-IMPURITIES IN ZnSe. T. OGUCHI*, T. SASAKI* AND H. KATAYAMA-YOSHIDA** *National Research Institute for Metals, Nakameguro, Meguro-ku, Tokyo 153, Japan "**Department of Physics, Tohoku University, Aramaki-Aoba, Sendai 980, Japan. ABSTRACT Electronic properties of ZnSe with a Li impurity are investigated with use of the localdensity-functional approach. The electronic structures are calculated for different impurity sites by taking the neighboring lattice relaxation into account. By comparing their total energies, the stability of the Li impurity in ZnSe is discussed. It is proposed that the Li impurity at the substitutional Zn site might be unstable to the tetrahedral interstitial site with an ionization of Li and a vacancy at the Zn site. INTRODUCTION Properties of Li impurity in zinc selenide (ZnSe) is recognized as a crucial problem in fabricating a p-type material for the blue light-emitting-diode. The experimental result shows that substitutional Li at the Zn site (Liz) seems to be unstable at room temperatures[ 1]. Although Liz is expected to work as a shallow acceptor, only little is known on the electronic structure of the Li impurity in ZnSe. For elemental semiconductors, detailed information on the electronic structure and the atomic configuration of various defects are now available from theoretical and experimental studies. Similar situation is being achieved for III-V semiconductors but less attention has been paid to II-VI systems. The electronic structure of an acceptor has been investigated successfully with the effective-mass theory in most semiconductors, particularly, Si and Ge. An application of the effective-mass theory to the electronic structure of an acceptor in ZnSe is probably doubtful since the effective Bohr radius of a shallow acceptor state calculated within the theory is 13.7A which is much larger than the corresponding observed value (4.7A) for the Na acceptor[2,3]. Chacham er al. have calculated the Lizn system using a cluster model and have obtained the activation energy of the acceptor level in good agreement with the experimental value[4]. However, no lattice relaxation has been taken into account in their calculation. In this Symposium, we present results of the electronic structure calculation for a Li impurity in ZnSe using the local-density-functional formalism taking the lattice relaxation into account and discuss the stability of the Li impurity. In particular, we focus on the following reaction: Lizn --> Lii + Vzn,
(1)
where Lii and Vzn denote a Li impurity at an interstitial site and a vacancy at the Zn site, respectively. In order to clarify the reaction of Eq. (1), the total energies of these systems have been calculated. MODEL AND METHODS We applied the local-density approximation of the density-functional theory to supercell model systems[5]; a defect is introduced into an fcc unit cell which contains eight Zn and eight Se atoms with the zinc-blend structure. The norm-conserving pseudopotential method without the spin-orbit interaction [6] w
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