Preparation of Sapphire for High Quality III-Nitride Growth

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Internet Journal Nitride Semiconductor Research

Preparation of Sapphire for High Quality III-Nitride Growth J. Cui1, A. Sun1, M. Reshichkov1, F. Yun1, A. Baski1 and H. Morkoç1 1Virginia

Commonwealth University,Department of Electrical Engineering and Physics Department,

(Received Friday, June 2, 2000; accepted Friday, July 21, 2000)

We developed a unique preparation technique to eliminate surface damage on the c-plane of sapphire and render it atomically flat. AFM images of c-plane sapphire annealed at 1380 °C for 1hour show terrace-like features with about 0.2 µm long terraces. The GaN layers grown by MBE on annealed sapphire have [0 0 2] symmetric and [1 0 4] asymmetric full width at half maximum (FWHM) of about 60 and 132 arcsec, respectively. This compares with 408 and 600 arcsec, respectively, for GaN grown on sapphire having gone through conventional chemical cleaning.

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Introduction

In recent years the III-V nitrides have attracted much attention [1] because of the potential to produce hightemperature and high-power electronic devices [2], as well as bright blue light-emitting diodes (LEDs) [3], lasers [4] and solar blind UV photovoltaic detectors [5]. [a] Motivations for GaN based high-temperature, high-power electronics are due to GaN’s high electronsaturation velocity, wide bandgap, high thermal conductivity and the availability of heterojunctions. Essential in exploiting all these applications is high-quality III-V nitrides. One of the bottlenecks that have hindered the development of GaN is the lack of a suitable substrate material that is lattice matched to and thermally compatible with GaN. Due to the lack of an ideal substrate, nearly all III-V nitride materials are grown on sapphire despite its poor structural and thermal match to III-Nitrides. The predilection towards sapphire substrates is due to its availability, low cost, and its ease of handling and improving quality. Sapphire is also stable at high temperatures (about 1000 °C), a property that is required for epitaxial growth using the various deposition techniques commonly employed for GaN [7], [8]. In addition, its transparence to visible light makes it ideal for some detector application. The most commonly used orientation of sapphire for GaN, AlN and InN growth is the (0001) orientated basal plane, but the (21(-3)1), (1101), (1120) planes have also been explored. A mechanical– chemical-polish involving many process steps is employed by sapphire manufacturers which produces a reasonable overall flatness of the surface, but leaves

excessive damage and visible scratches caused by the grit used. It has been established that the crystal structure of epitaxial GaN is strongly influenced by not only the substrate material and its orientation, but to a great extent the surface preparation of the substrate. An ideal cplane sapphire surface for epitaxy would have bi-layer steps leading to the realm where all the terrace surfaces have the same surface polarity so that stacking mismatch boundaries as well as other defects in the ensuing nitrid