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Density-functional calculations concerning the structure and stability of wurtzite AlN surfaces are presented. Specifically, (0001) and (0001¯) polar surfaces and (11¯00) and (112¯0) nonpolar surfaces are discussed in detail. Binding energies, migration pathways, and diffusion barriers for relevant adatoms such as Al, Ga, and N on these polar and nonpolar surfaces are determined. The calculation indicates low diffusion barrier for Al adatom on Al terminated (0001) surface, whereas the N adatom seems to have lower diffusion barrier on N terminated (0001¯) surfaces. A strong anisotropy was observed for diffusion behavior for Al adatom on (11¯00) and (112¯0) surfaces in the [112¯0] and [0001] directions, respectively.

Ultraviolet (UV) studies in astronomy, cosmology, planetary studies, biological and medical applications often require precision detection of faint objects and in many cases require photon-counting detection. We present an overview of two approaches for achieving photon counting in the UV. The first approach involves UV enhancement of photon-counting silicon detectors, including electron multiplying charge-coupled devices and avalanche photodiodes. The approach used here employs molecular beam epitaxy for delta doping and superlattice doping for surface passivation and high UV quantum efficiency. Additional UV enhancements include antireflection (AR) and solar-blind UV bandpass coatings prepared by atomic layer deposition. Quantum efficiency (QE) measurements show QE > 50% in the 100–300 nm range for detectors with simple AR coatings, and QE ≅ 80% at ~206 nm has been shown when more complex AR coatings are used. The second approach is based on avalanche photodiodes in III-nitride materials with high QE and intrinsic solar blindness.

We report on a novel scheme of substrate engineering to obtain high-quality GaN layers on Si substrates. Ion implantation of an AlN∕Si substrate is performed to create a defective layer that partially isolates the III-nitride layer and the Si substrate and helps to reduce the strain in the film. Raman spectroscopy shows a substantial decrease in in-plane strain in GaN films grown on nitrogen implanted substrates. This is confirmed by the enhancement of the E2 (TO) phonon frequency from 564 to 567cm−1 corresponding to 84% stress reduction and substantial decrease in crack density for a 2-μm-thick GaN film. GaN films grown on implanted AlN∕Si substrate have better optical properties and smoother surface morphology as compared to nonimplanted AlN∕Si substrate.