Polarity-Controlled GaN/AlN Nucleation Layers for Selective-Area Growth of GaN Nanowire Arrays on Si(111) Substrates by Molecular Beam Epitaxy

We have demonstrated dramatic improvement in the quality of selective-area GaN nanowire growth by controlling the polarity of the underlying nucleation layers. In particular, we find that N-polarity is beneficial for the growth of large ordered nanowire arrays with arbitrary spacing. Herein, we present techniques for obtaining and characterizing polarity-controlled nucleation layers on Si (111) substrates. An initial AlN layer, which is demonstrated to adopt Al-(N-)polarity for N-(Al-)rich growth conditions, is utilized to configure the polarity of subsequently grown GaN layers as determined by piezoresponse force microscopy (PFM), polarity-dependent surface reconstructions, and polarity-sensitive etching. Polarity-dependent surface reconstructions observed in reflection high-energy electron diffraction (RHEED) patterns were found to be particularly useful for in situ verification of the nucleation layer polarity, prior to mask deposition, patterning, and selective-area regrowth of the GaN NW arrays. N-polar templates produced fast-growing nanowires with vertical m-plane side walls and flat c-plane tips, while Ga-polar templates produced slow-growing pyramidal structures bounded by (1 (1) over bar 02) r-planes. The selective-area nanowire growth process window, bounded by nonselective and no-growth conditions, was found to be substantially more relaxed for NW arrays grown on N-polar templates, allowing for long-range selectivity where the NW pitch far exceeds the Ga diffusion length.