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The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions.

AlGaN hexagonal nano-pyramids were formed on GaN/sapphire, AlN/sapphire and Al 0.5 Ga 0.5 N/sapphire using selective area heteroepitaxy. It is found that both interfacial strain due to the lattice mismatch and the growth conditions greatly impact growth rate, Al incorporation and facet quality of these AlGaN nanostructures. A correlation between the interfacial strain provided by the underlying template with growth of nanostructures was made, and attributed to the difference in surface diffusion properties as a function of strain. This was directly observed with the difference in the growth rates and Al incorporation of nanostructures when grown on various templates. The growth rates of nanostructures were found to be much higher when grown on an AlN template in comparison to that on a GaN template under the same growth conditions. Detailed characterization of the AlGaN nanostructures shows highly uniform composition. Further, multiple quantum well structures were grown within the nanostructures on different templates. The effect of strain was again visible with differences in emission properties of the quantum wells.

Freestanding hydride vapor phase epitaxy grown GaN substrates subjected to various polishing methods were characterized for their surface and subsurface conditions. Although different polishing processes led to very smooth surfaces as seen in atomic force microscopy and optical microscopy, they resulted in drastically different optical properties measured by photoluminescence (PL) and cathodoluminescence (CL). PL measurements showed a large increase in defect luminescence. There was also subsurface damage evidenced in CL imaging for one polishing process. Morphological and optical characteristics of an overgrown GaN thin film indicate great differences in the initial nucleation on the substrates subjected to different polishing processes even though the starting surface roughness was similar. This difference continued for the growth of a 405 nm light emitting diode (LED) structure. A PL lifetime reduction by factors of two and four, respectively, was seen for the band edge peak of the GaN layer and the quantum well peak of the LED structure as measured by time-resolved PL. This variability in device performance emphasizes the need for a greater understanding of the effects of starting surfaces, and subsurface characteristics, on the epitaxial growth.

Homoepitaxial growth of GaN epilayers on free-standing hydride vapor phase epitaxy (HVPE) GaN substrates offered a better control over surface morphology, defect density, and doping concentration compared to conventional heteroepitaxial growth. The FWHM of the (0 0 0 2) X-ray diffraction (XRD) rocking curve from homoepitaxial GaN was measured to be as low as 79 arcsec, much smaller than ∼230 arcsec for GaN grown on sapphire. Schottky diodes grown on GaN substrates exhibited sharper breakdown characteristics and much lower reverse leakage than diodes on sapphire. However, the homoepitaxial devices had poor scalability due to the presence of yield-killing defects originating from the substrate surface. Vertical InGaN/GaN light-emitting diodes (LEDs) on GaN substrates showed reduced series resistance and reverse leakage compared to lateral LEDs on sapphire. Wafer mapping demonstrated that the distribution of leaky homoepitaxial devices correlated well with that of macroscopic defects in the GaN substrates.

Recently it has been discovered that when growing Al x Ga 1− x N on low-defect-density bulk AlN substrates pseudomorphic layers can be achieved with a thickness far exceeding the critical thickness as given by the Matthews and Blakeslee model. For instance, the critical thickness of an Al x Ga 1− x N layer (with x =0.6) is about 40 nm thick. However we have been able to grow layers with this composition that are pseudomorphic with a thickness exceeding the critical thickness by more than an order of magnitude. This work defines the limits of pseudomorphic growth on low defect density, bulk AlN substrates to obtain low defect density, high-power UV LEDs.