GaN layers re-grown on etched GaN templates by plasma assisted molecular beam epitaxy
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GaN layers re-grown on etched GaN templates by plasma assisted molecular beam epitaxy L. He, X. Gu, J. Xie, F. Yun, A. A. Baski, and H. Morkoç Department of Electrical Engineering, Virginia Commonwealth University, Richmond, Virginia 23284 ABSTRACT The growth of high-quality GaN by plasma assisted molecular beam epitaxy (MBE) is challenging, in part due to the constraint of heteroepitaxy since GaN substrates are not yet commercially available and isotropic nature of growth. Despite the large lattice and thermal mismatch between sapphire and GaN, the former is still the most commonly used substrate for the GaN-based optical devices at present. In this paper, we demonstrate a re-growth technique to obtain an improved quality GaN by MBE on GaN template on sapphire where the grossly defective regions have been removed. This GaN template is formed by MBE growth of GaN followed by wet chemical etching to selectively remove the defective region. Improved quality GaN was re-grown on such a template under Ga rich conditions to a thickness of about 1 micron. After re-growth, the surface of GaN is atomically smooth with spiral features in the short range. The low temperature PL of the re-grown GaN is superior to those of MBE GaN films directly on sapphire. Atomic force microscopy (AFM) images reveal a two-dimensional re-growth initiating in regions free of extended defects. The results show that the selectively etched GaN on sapphire can be used as a good template to improve the quality of GaN. INTRODUCTION Owing to its wide band gap and good thermal stability, GaN is of paramount interest for optoelectronic devices applications, such as laser diodes and light-emitting diodes in visible and ultraviolet spectrum, as well as for electronic devices [1-3]. The growth of highquality GaN by MBE is challenging, in part due to the constraint of heteroepitaxy. At present, sapphire is the mostly used substrate for the GaN-based devices. However, the large lattice mismatch is present between the GaN epitaxial layer and the foreign substrates, resulting in a large number of dislocations [4,5]. In addition, the thermal expansion coefficient of the GaN film differs from that of the foreign substrate, giving rise to stress and bowing during heteroepitaxial growth [6]. Accordingly, thin GaN layers on sapphire contain a very large dislocation density, in the range of 109~1010cm-2 [7]. While, the ubiquitous sapphire substrates may be acceptable for the time being, sooner than later, one or more of the following will be required: a high breakdown voltage, high thermal conductivity, reduced low and high frequency noise in devices, anomaly free device operation, and lattice and stacking match [8]. In this work, pseudo-defect-void GaN formed by plasma assisted molecular beam epitaxy (MBE) on sapphire was used as the template to improve the quality of the re-grown GaN layer, and the re-growth is believed to be homogeneous and initiated only in the regions that are free of large concentration of extended defects. EXPERIMENT DETAILS Both GaN
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