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

Abstract

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 (11̅ 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.

Eutectic Formation, V/III Ratio, and Controlled Polarity Inversion in Nitrides on Silicon

“…That the Si‐containing IDB is planar suggests that the AlN layer had an initially planar growth surface, as expected for Al‐rich growth with a eutectic layer floating on the surface. The roughness of the AlN/GaN interface, which is characteristic of N‐rich AlN growth, and its lack of Si are consistent with the eutectic layer being completely incorporated into the AlN layer when the V/III ratio became N‐rich. Variations in the Si concentration laterally along the IDB, as indicated by the one linescan showing no Si increase at the IDB (Figure b), led to narrow regions where the polarity was not inverted.…”

“…A potential source of such clustering could be the formation of droplets, which have been frequently reported for Al‐rich growth conditions . Fluctuations in the Al–Si eutectic “layer” thickness and in the associated Si distribution may be either a direct result of or a precursor to droplet formation.…”

“…Thus, the ability to induce polarity inversion would be useful for obtaining metal‐polar films and nanowires. This may be especially important for nanowires, which are advantageous due to their lower defect densities, as spontaneously grown NWs are typically N‐polar, and because selective area nanowire growth on Si is facilitated by N‐polarity . In fact controlled polarity inversion has already been demonstrated both for epitaxial AlN layers and for spontaneously nucleated GaN nanowires using an oxidized interlayer.…”

“…Samples with AlN and GaN layers were grown on Si(111) substrates by plasma‐assisted MBE. The growth system and monitoring equipment were described previously . The nitrogen plasma source was different from that used in earlier studies and, based on the improved run to run reproducibility with this source, it was expected to have less variation in active nitrogen flux than that described before …”

GaN Nanowire MOSFET With Near-Ideal Subthreshold Slope

“…Compared with other NW formation approaches such as dry etching and vapor-liquid-solid (VLS) growth, the catalyst-free selective-area growth is beneficial to avoid defects from etch damage and impurities from the catalyst, while yielding good control of the wire diameter during growth. The detailed growth process and conditions can be found in [15] and [16]. The NWs used here were uniformly doped with Si at a concentration of 1 ± 0.1 × 10 18 cm −3 , as estimated by Raman spectroscopy using the method described in [17].…”