Control of Valence States in ZnO by Codoping Method
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T. YAMAMOTO *, H. K.-YOSHIDA **
*Electronic and Photonic Systems Engineering Department, Kochi University of Technology, Tosayamada-cho, Kochi 782-8502, JAPAN, [email protected] "**CondensedMatter Physics Department, ISIR, Osaka University, Osaka 567-0047, JAPAN ABSTRACT We have investigated the electronic structures of p- or n-type doped ZnO based on ab initio electronic band structure calculations in order to control valence states in ZnO for the fabrication of low-resistivity p-type ZnO. We find unipolarity in ZnO; p-type doping using Li or N increases the Madelung energy while n-type doping using Al, Ga, In or F species decreases the Madelung energy. We have proposed materials design: codoping using N acceptors and reactive codopants, Al or Ga, enhances electric properties in p-type codoped ZnO. It has been already verified by experiments using the N acceptors and Ga reactive donor codopants. We find a very weak repulsive interaction between Li acceptors and the delocalization of the Li-impurity states for Lidoped ZnO, in contrast with the case of N-doped ZnO. On the other hand, we find the compensation mechanism by the formation of 0 vacancy in the vicinity of the Li-acceptor sites. We propose a group VII element, F species, as a promising candidate for use of the reactive codopant as for Li-doped ZnO in order to realize low-resistivity p-type ZnO. INTRODUCTION Zinc oxide (ZnO) with a wurtzite structure is a wide band gap (3.436 eV at 4.2K) semiconductor which has many applications such as piezoelectronic transducers, varistors, and highly optically transparent conducting films. Recent successes in producing large-area single crystals have opened up the possible applications in short-wave length light emitting devices [1,2]. The main advantage of ZnO as the light emitter is large exciton binding energy (60 meV). ZnO is also much more resistant to radiation damage than other common semiconducting materials, such as Si, GaAs, CdS, and even GaN; thus it should be useful for space applications. In order to develop not only the optoelectronic devices but also such large-scale applications, one important issue that should be resolved is the fabrication of low-resistivity p-type doped ZnO, as well as other wide band gap semiconductors such as ZnSe and GaN. The realization of the p-type ZnO will lead to the fabrication of a unique pn junction, a key structure in the semiconductor technology. ZnO is naturally an n-type semiconductor because of a deviation from stoichiometry due to the presence of intrinsic point defects such as 0 vacancies (Vo) and Zn interstitials (ZnR). We have found a substantial decrease in the Madelung energy of n-type ZnO with the excess of Zn originated in the formation of Vo and Zn1 compared with that of stoichiometric ZnO, which causes the stabilization of ionic charge distributions in n-type ZnO, from ab initio electronic band structure calculations [3]. Several authors reported the fabrication of low-resistivity n-type ZnO using group III elements, [4,5] Si, a Zn-substituting sp
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