The Growth of ZnO on CrN Buffer Layer Using Surface Phase Control by Plasma Assisted Molecular-beam Epitaxy
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The Growth of ZnO on CrN Buffer Layer Using Surface Phase Control by Plasma Assisted Molecular-Beam Epitaxy Jinsub Park1, Tsutomu Minegishi1, Seunghwan Park1, Inho Im1, Takahasi Hanada1, Soonku Hong2, Takenari Goto3, Meoungwhan Cho1,3, and Takafumi Yao1,3 1
Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan
2
Materials Science and Engineering,Chungnam National University, Daejeon, 305-764, Korea, Republic of
3
Center for Interdisciplinary Research, Tohoku University, Sendai, 980-8578, Japan
ABSTRACT Epitaxial ZnO films are successfully grown on Al2O3 substrates with phase controlled CrN buffer layer using Zn and O-plasma pre-exposures on CrN layers by plasma assisted molecular beam epitaxy (P-MBE). The Zn exposures on CrN layers prior to ZnO film growth result in the formation of rocksalt CrN without surface oxidation. On the other hand, the surface of the initially deposited CrN layers with rocksalt structure changes into hexagonal structured Cr2O3 after O-plasma exposure as confirmed by reflection high-energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HR TEM). Etching studies show that the ZnO films grown on CrN have +C polarity, while the polarity of ZnO on Cr2O3/CrN double buffer is –C polarity. The interdiffusion of Zn and Cr occurs at the ZnO/CrN interface, while the interdiffusion is negligible at the ZnO/ Cr2O3 interface. The interdiffusion of Cr and Zn can be suppressed by inserting a low-temperature ZnO buffer layer in between ZnO and CrN layers, which helps improve the crystal quality of ZnO layers grown with CrN buffer. INTRODUCTION ZnO is an attractive material for applications in ultraviolet optoelectronic devices owing to wide band gap of 3.37eV and a large exciton binding energy of 60meV at room temperature [1]. ZnO crystallizes in wurtzite structure and naturally has crystal polarity: Zn-polar (+C polar) and O-polar (-C polar). The polarity control of ZnO films is an important role in optical devices and has a crucial effect on doping efficiency [2]. It is possible to control the crystal polarity of ZnO films by engineering the interface between ZnO and substrates [3-5]. It has been reported so far that low temperature buffer layers are effective in reduction of defect densities caused by lattice misfit between ZnO and substrates [6-7]. This study will deal with the polarity control of ZnO films grown on phase controlled CrN buffer. CrN has a rock salt crystal structure with a lattice parameter of 0.293 nm. An expected epitaxy relationship for the heterostructure of ZnO/CrN/Al2O3(0001) is schematically shown in Fig. 1. Assuming this epitaxy relationship, the 18% lattice misfit between ZnO/Al2O3(0001) can be split into two heterostructures with smaller lattice misfits: ZnO/CrN (11%) and CrN/Al2O3 (6.5%)[8]. High temperature oxidation of CrN by O-plasma yields Croxides with various oxide phases, which can be controlled by oxidation conditions [9].
Figure 1. In-plane orientation relationship among ZnO, CrN, and c-Al2
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