Nano-selective-area growth of group III-nitrides on silicon and conventional substrates
Bonanno, Peter L.
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We developed and evaluated NSAG techniques for Group III-Nitrides as a way to mitigate the various difficulties with this material system (high defect density, threading dislocations, phase separation and graining, etc.), to bring these material systems to the nanoscale, and to allow the use of inexpensive silicon substrates. To that end, we used optimized NSAG of GaN, InGaN, AlGaN, and BGaN grown on GaN/Sapphire and AlN/Si(111) templates, evaluating the effectiveness of our NSAG techniques on each template by comparing our results to those obtained from planar (non-SAG) growth. We also investigated the engineering of microtemplates by coalescing NSAG GaN structures and comparing surface properties and subsequent epilayer growth to that of normal planarly grown GaN. Across the board, NSAG was selective and, when compared to planar growth, consistently resulted in higher quality material with fewer dislocations. NSAG of GaN on AlN/Si(111) resulted in defect-free nanopyramids, 90% of which were single crystal. By coalescing nanostructures into a microtemplate, we produced an InGaN top layer with 7 times the optical emission intensity as InGaN grown simultaneously on non-NSAG planar GaN. NSAG InGaN nanopyramids grown on GaN/Sapphire templates were used to make PIN-based solar cells that produced current 3 orders of magnitude greater than their planar counterparts, and which had 20 times greater IV ratios at ±1 V. We then leveraged this newly-won know-how with our previous success growing NSAG GaN on AlN/Si(111) to produce InGaN nanopyramids on AlN/Si(111) with no defect band and 50% stronger luminescence than in 2D growth. These nanopyramids were highly uniform, single-crystal, dislocation-free, and free from phase clustering effects and other nonuniformities found in planar growth. With additional effort, we achieved a maximum InN composition of 33%, with NSAG material showing four times better emission characteristics than planar material on the same substrate. Additionally, we found that mask margin affected InN composition and therefore emission wavelength of our nanopyramids, and that by using different mask geometries on the same template, we can create single-growth-step multi-color micropixels. In our most current iteration, we produced both green and red-emitting material in one growth step. Lastly, we achieved NSAG BGaN nanopyramids on both AlN/Si(111) and GaN/Sapphire. As expected, we found unmasked field growth of BGaN to be of a much lower quality on AlN/Si(111) than on GaN, but also found that the former benefitted much more from NSAG, owing to its low BGaN nucleation rate. As with NSAG of GaN, BGaN nanopyramids on AlN/Si(111) were single crystal to the extent that nucleation occurred once per aperture, which happened in more than 90% of the apertures. On AlN/Si(111), XRD and CL showed BN composition to be between 1.3 and 2.0%, and the nanopyramids on both substrates exhibited smooth sidewalls.