ZnO/GaN Heteroepitaxy
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ZnO/GaN Heteroepitaxy K.W.Jang1, D.C.Oh1, T.Minegishi1, H.Suzuki1, T.Hanada1, H. Makino2, M.W.Cho2, and T. Yao1,2 1 Center for Interdisciplinary Research, Tohoku University, Aramaki, Aoba-Ku, Sendai 980-8578, Japan 2 Institute for Materials Research, Tohoku University, Katahira 2-1-1, Aoba-Ku, Sendai 980-8577, Japan S.K.Hong Department of Materials Engineering, Chungnam National University, 220, Gung-dong, Yuseong-gu, Daejeon, 305-764, Korea ABSTRACT This paper presents the recent achievements of ZnO/GaN heteroepitaxy. The general controlling method and mechanism for the polarity of heteroepitaxial ZnO and GaN films by interface engineering via Plasma-assisted Molecular beam epitaxy(P-MBE) are introduced in a viewpoint of principle for polarity control. We propose the principle of crystal polarity: Crystal polarity can succeed at the heterointerface when no interface layer is formed, while an interface layer with inversion symmetry is formed, the crystal polarity is inverted at the heterointerfae. The effects of polarity on the interface, surface and bulk structure, and the structural and optical properties of ZnO/GaN epitaxy are also included. The polarity of GaN on ZnO is successfully controlled based on the proposed principle for control of crystal polarity. Additionally, the electronic characteristics such as electron concentration, band-line-up, and C-V characteristics of ZnO/GaN heterointerface are dicussed.
INTRODUCTION The heteroepitaxy system of ZnO-based II-VI oxide/GaN-based III-V nitride has unique features: (1) ZnO/GaN provides a close lattice matching heterostructure; (2) Type-II band alignment with variable band offset with alloy composition; (3) spontaneous polarization field exists regardless of lattice misfit, while piezoelectric field is generated by lattice misfit; (4) ZnO and GaN are eco-friendly materials. Owing to those properties, the ZnO/GaN heterostructure will provide unique applications in various fields. The purpose of this paper is to overview the heteroepitaxial growth of ZnO on GaN and GaN on ZnO and to present prospect of ZnO/GaN heteroepitaxy. The topics to be discussed will include: polarity control of ZnO/GaN
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heteroepitaxy; evaluation of ZnO substrates for GaN epitaxy; interface electronic properties of ZnO/GaN.
POLARITY CONTROL OF ZnO/GaN HETEROEPITAXY [1-5] The polarity can be different from the termination and the crystal polarity is a bulk property but the surface termination is a surface property. Consequently, there are four types of wurtzite ZnO {0001} surface structures: (1) Zn-polar surface with Zn-atom termination, (2) Zn-polar surface with O-atom termination, (3) O-polar surface with O-atom termination, and (4) O-polar surface with Zn-atom termination. Recently, we have reported success in controlling the growth of Zn- and O-polar ZnO films on GaN templates by changing pregrowth treatments. Since the GaN template grown by metal organic chemical vapor deposition (MOCVD) has Ga-polar nature, the growth of ZnO without causing oxidation would result in
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