Low-defect and Stress-Free AlN(0001) Nanoprisms and Microrods Selectively Grown on Micro-Patterned C-Sapphire Substrate by Plasma-Assisted Molecular Beam Epitaxy

This paper describes different growth modes of AlN layers on micro-cone patterned c-sapphire substrates (μ-PSSs) using plasma-assisted molecular beam epitaxy. Ordered arrays of AlN nanoprisms and microrods were selectively grown on the tips of μ-PSS's microcones according to a bottom-up formation mechanism using sequential migration enhanced and metal-modulated epitaxy (MME) under metal-rich growth conditions at 820 °C. Transmission electron microscopy revealed structurally perfect AlN regions above the tips of the μ-PSSs, which initiate as inverted nanopyramids with {1011¯} side faces, evolving into hexagonal nanoprisms with orientations of {11¯00} and (0001) for side and top surfaces, respectively. The diameter and height of these ordered hexagonal nanoprisms, which have a 60% probability of nucleating, were about 1 μm. Long-term MME growth of these nanoprisms in both vertical and lateral directions led to the formation of AlN(0001) microrods with a maximum possible diameter of two micrometers and a height of up to 6 μm. Atomic force microscopy revealed a mixed step-flow and 2D nucleation growth mechanism for the flat tops of these AlN nanoprisms and microrods with an average surface roughness of 1–2 monolayers. Micro-Raman spectroscopy demonstrated narrow E2 (high) linewidths of 3.8 and 4.2 cm−1 for essentially stress-free AlN nanoprisms and microrods, respectively.