Characterization of lattice mosaic of a -plane GaN grown on r -plane sapphire by metalorganic vapor-phase epitaxy
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Characterization of lattice mosaic of a-plane GaN grown on r-plane sapphire by metalorganic vapor-phase epitaxy K. Kusakabe 1, S. Ando 2 and K. Ohkawa 1, 3 1 Department of Applied Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo, Japan 162-8601 2 Department of Electrical Engineering, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo, Japan 162-8601 3 Exploratory Research for Advanced Technology (ERATO), Japan Science and Technology Agency (JST), Japan ABSTRACT Nonpolar a-plane GaN films were grown on r-plane sapphire substrates by atmospheric metalorganic vapor-phase epitaxy. The as-grown layers were studied by high-resolution x-ray diffraction. The a-plane GaN lattice mosaic is orientation dependent as determined by x-ray rocking curve (XRC) measurements. The tilt mosaic measured with the c-axis within the scattering plane (c-mosaic) was greater than the mosaic measured with the m-axis within the scattering plane (m-mosaic). The mosaic along both azimuths decreased and the c-mosaic/mmosaic ratio was increased with increase of growth temperature from 1050 oC to 1080 oC. The morphological transition was correlated to change in the a-plane GaN tilt mosaic measured by XRC.
INTRODUCTION III-V nitride semiconductors are very attractive materials for application to green-toultraviolet light emitters and high power electronic devices [1]. To date most of the reported nitride-devices were grown on the polar (0001) c-plane sapphire, SiC, or free-standing GaN substrates. Epitaxy over the polar orientation leads to undesirable polarization effects. Carrier separation within quantum wells significantly reduces internal quantum efficiency for radiative transition. In recent years, the interest of epitaxy of nonpolar heterostructures has increased remarkably. This reason is mainly free from the polarization effects in these structures. Ng reported that ( 11 2 0 ) a-plane GaN films were grown on ( 1102 ) r-plane sapphire substrates by molecular beam epitaxy [2]. Craven et al. reported the growth of a-plane GaN by metalorganic vapor-phase epitaxy (MOVPE) [3]. Haskell and co-workers also reported the a-plane GaN films grown by hydride vapor-phase epitaxy [4]. The epitaxial growth of a-plane GaN on r-plane sapphire is difficult. The surface is frequently rough and the crystal quality is poor [5]. Thus, the lattice mosaic related to structural quality should be investigated. In this article, the structural characterization of a-plane GaN films grown on r-plane sapphire substrates is discussed.
EXPERIMENTAL DETAILS Nonpolar a-plane GaN films were grown on r-plane sapphire substrates in an atmospheric two-flow MOVPE reactor [6-8]. The sapphire substrates were nominal r-plane wafers with ± 2°
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surface orientation tolerance and required no preparation process. The growth process was modelled after the two-step growth method that has become standard for the c-plane GaN growth [9]. The sapphire substrate was thermally cleaned prior to the growth. A low temperature GaN nuclea
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