Polarity-Controlled GaN/AlN Nucleation Layers for Selective-Area Growth of GaN Nanowire Arrays on Si(111) Substrates by Molecular Beam Epitaxy
Matthew D. Brubaker, Shannon M. Duff, Todd E. Harvey, Paul T. Blanchard, Alexana Roshko, Aric W. Sanders, Norman A. Sanford, Kristine A. Bertness
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 Gapolar templates produced slow-growing pyramidal structures bounded by (11̅02) 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.