Molecular beam epitaxy of GaN on lattice-matched ZrB 2 substrates using low-temperature GaN and AlN nucleation layers
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E8.36.1
Molecular beam epitaxy of GaN on lattice-matched ZrB2 substrates using low-temperature GaN and AlN nucleation layers Rob Armitage1, Kazuhiro Nishizono2, Jun Suda1, Tsunenobu Kimoto1 1 2
Kyoto University, Nishikyo-ku, Kyoto, Japan 615-8510 Kyocera Corporation, Sourakugunseikachou, Hikaridai, Kyoto, Japan 619-0237
ABSTRACT GaN epilayers have been grown by plasma-assisted molecular-beam epitaxy on ZrB2 substrates with close in-plane lattice match. Growth processes utilizing both low-temperature GaN (LTGaN) and AlN nucleation layers were investigated. The x-ray ω-scan widths for the optimized LT-GaN nucleation process were 400 and 750 arcsec for symmetric and asymmetric reflections, respectively. When using LT-GaN nucleation layers, the chemical incompatibility of ZrB2 results in a high dislocation density despite the in-plane lattice match. The epilayer polarity was N-polar for LT-GaN nucleation layers under all conditions investigated. For AlN nucleation layers, Gapolar epilayers were obtained under suitable conditions (Al-rich, lower nucleation temperatures) for nominal AlN thickness as low as 1 nm. From RHEED analysis it appears that a psuedomorphic Al wetting layer forms on the ZrB2 surface, and that using AlN as the nucleation layer may offer promise for reducing the epilayer defect density. INTRODUCTION ZrB2 is an interesting substrate for GaN epitaxy due to its very favorable in-plane lattice constant (0.3168 nm, exactly matched to Al0.24Ga0.76N) and thermal expansion coefficient (5.9×10-6 K-1, compared to 5.6×10-6 K-1 for GaN ) [1]. For applications in vertical current devices the low resistivity (5 µ Ω-cm) and good thermal conductivity (100-130 W/K-m) of ZrB2 are also advantages [1]. Moreover much progress has been made recently in bulk growth and polishing of ZrB2, and it is expected that substrates may be commercially available in the near future. Despite the desirable properties mentioned above, deposition of GaN directly on ZrB2 at typical MBE and MOCVD growth temperatures led to poor results [1]. At high temperatures a ZrN-like layer forms on the ZrB2 surface and the wetting of GaN is very poor [2]. A two-step method using a low-temperature nucleation layer of AlN or GaN yields much improved results, insofar as a smooth film with dislocation density similar to that of GaN-on-sapphire can be obtained [2,3,4]. Further research on the nucleation process is worthwhile to perhaps exploit the lattice-matched characteristic of ZrB2.and achieve GaN epilayers of lower dislocation density. In the present work we study low-temperature GaN (LT-GaN) and AlN nucleation layers for growth of high-temperature GaN epilayers on ZrB2 by rf-plasma MBE. A method to control the crystal polarity of the high-temperature epilayer by a suitable choice of the nucleation layer type (LT-GaN or AlN) is demonstrated. The influence of the metal/nitrogen flux ratio during initial nucleation on the material properties of the subsequent main epilayer is presented, and prospects for further reducing the defect density in MBE GaN g
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