Atomic Force Microscopy Study of GaN Grown on Al 2 O 3 (0001) by LP-MOVPE
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Atomic Force Microscopy Study of GaN Grown on Al2O3 (0001) by LP-MOVPE K. Xu1, D. H. Lim1, B. L. Liu1, X. L. Du1, G. H. Yu1, A. W. Jia1,2, and A. Yoshikawa1,2 Center for Frontier Electronics & Photonics, Chiba University-Venture Business Laboratory 2 Department of Electronics & Mechanical Engineering, Chiba University 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan 1
K. Takahashi Department of Media Science, Teikyo University of Science and Technology 2525 Yatsuzawa, Uenohara, Kitatsurugun, Yamanashi 409-0193, Japan
ABSTRACT There is currently a high degree of interest in understanding the diverse mechanisms that determine the growth morphology of epitaxial films. To understand and control over these mechanisms, it is essential to know how the growth mechanisms are correlated with morphologies. In GaN MOVPE processes, growth temperature has a remarkable effect on the film morphologies and properties. In present work, the temperature dependency of surface morphology of GaN epilayers grown by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) was studied using atomic force microscopy. It was demonstrated that dislocations strongly influence the growth mechanisms and the morphology of the films. Three growth modes were identified at different growth temperature ranged from 1030°C to 1100°C, which are, (1) spiral growth dominated by dislocation with screw type, (2) monolayer step flow, and (3) nucleation-assisted step flow growth. The significant effect of dislocation on the surface morphologies and growth mechanisms may be attributed to the high dislocation density and higher activation energy for a step overcoming the dislocation pinning in GaN. Because few dislocations will be introduced under monolayer step flow growth mode, we can infer the useful information about GaN initial growth.
INTRODUCTION
Due to their superior material properties, GaN and related III-nitrides are the best candidates for short wavelength optical emitters and detectors as well as for high temperature and high power electronic devices [1, 2, 3]. As for the epitaxial growth of GaN by MOVPE, it is not easy to get high quality film mainly due to the large lattice mismatch between GaN and most frequently used sapphire substrate [4]. Usually, there are large numbers of parameters to optimize before we can get high quality GaN film. Atomic microscopy study of GaN surface morphology has received much attention because it can provide us very useful information involving the film growth process and mechanism. AFM observation has revealed that GaN surface was mediated with threading dislocations with the Burgers vector of screw type or mixed type. However, it is still not well described how the threading dislocations affect the growth process, especially how the threading dislocation mediated surface morphology will be evolved with growth temperature. For CVD processes, the deposition temperature is an important factor controlling the growth mode [5,6], providing step flow growth at sufficiently high temperatures, and layer by layer and
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