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Mid-ultraviolet LEDs grown on AlN substrates with a range of quantum efficiencies and wavelengths spanning 250 to 280 nm have been investigated by time-resolved photoluminescence and electroluminescence. Through scaling of room temperature internal quantum efficiencies across all devices, radiative and nonradiative lifetimes are also estimated. General trends observed include an increase in PL lifetime for longer wavelength and higher external quantum efficiency devices, consistent with the increase in the estimated nonradiative life-time with increasing wavelength. Despite these trends, the external quantum efficiency of the devices increases only weakly with increasing wavelength from 258 to 279 nm, suggesting that optimization of radiative lifetime and injection efficiency also play an important role. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Temperature dependent time-resolved photoluminescence is used to study the development of active regions for optoelectronic devices employing AlGaN nanostructures for deep-UV emission. The changing importance of dislocation versus point defects and their relationship to different forms of carrier localization are discussed. The results presented suggest that AlGaN nanostructure development for deep-UV emitters require both point defect/impurity suppression for improved efficiency and lower dislocation density for improved interface quality. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

High quality Alx>0.5Ga1−xN layers were grown on (0001) sapphire substrates by metal organic chemical vapor deposition utilizing an AlN nucleation layer. The growth conditions of the nucleation layer were observed to have a large impact on both the surface morphology and the crystalline quality of the AlxGa1−xN layers. Low temperature nucleation layers with an optimum thickness of ∼15nm gave the best quality AlxGa1−xN layers. Additionally the annealing of the nucleation layer was seen to have a dramatic effect on the crystalline quality of the overgrown AlxGa1−xN layers. The quality of the films as measured by atomic force microscopy and x-ray diffraction was largely independent of the V/III ratios of the AlxGa1−xN layers, although Al incorporation and growth rate were affected. Two distinct regimes were identified for the Al incorporation in the overgrown AlxGa1−xN as a function of ammonia flow rate; the Al incorporation first decreases with increase in the ammonia flow rate and then increases with a cont...

We report on the selective area heteroepitaxy and facet evolution of AlGaN nanostructures on GaN/sapphire substrate using various mask materials. We also report on the challenges associated with selection of an appropriate mask material for selective area heteroepitaxy of AlGaN with varying Al composition. The shape and the growth rate of the nanostructures are observed to be greatly affected by the mask material. The evolution of the AlGaN nanostructures and Al incorporation were studied exhaustively as a function of growth parameters including temperature, pressure, NH3 flow, total alkyl flow, and TMAl/(TMAl+TMGa) ratio. The growth rate of nanostructures was reduced drastically when higher Al percentage AlGaN nanostructures were grown. The growth rates were increased for higher Al percentage AlGaN using a surfactant, which resulted in a high-quality pyramidal structure. As indicated by high-resolution x-ray diffraction and cathodoluminescence spectroscopy, the composition of Al in the AlGaN nanostructure is significantly different from that of a thin film grown under the same growth conditions.

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.

We report on the application of a new scheme of substrate engineering in Si (111) substrate to obtain high quality GaN layers on Si substrates. An ion implanted defective layer is formed in the Si substrate that partially, crystallographically and mechanically isolates the III-Nitride layer and Si substrate which results in reduction of strain in the Nitride film. Nitrogen ion implantation energies of 60-75 keV and ion doses of 1×1016 cm–2-2×1016 cm–2 were applied to AlN buffer layers of 55 nm thick grown on Si (111). The experimental results show an increase in the crack separation of over 1 mm for 2 µm thick GaN film grown on implanted 55 nm AlN/Si complex substrate as seen under optical microscope. Moreover the GaN films grown on implanted AlN/Si substrate showed smoother surface morphology (RMS roughness of 0.41 nm) as compared to that grown on non-implanted AlN/Si substrates (RMS roughness of 1.50 nm). Results of etch pit density and XRD measurements show significant reduction in dislocations in III-Nitride layers. Our data shows a strong dependence of GaN quality on the implantation conditions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on development of smooth a-plane GaN without the use of lateral epitaxial overgrowth. Effects of GaN buffer growth conditions, namely V/III ratio and temperature, on the quality of high temperature a-plane GaN layers on r-plane sapphire substrates have been studied. Optimization of the buffer growth conditions resulted in surfaces of 2 µm thick overgrown a-plane GaN at high temperature with less pit defects, the presence of which is reported as a common occurrence for a-plane GaN grown on r-plane sapphire. The surfaces were smooth with RMS roughness of less than 2 nm for a 5 µm x 5 µm AFM scanning area. We will present the effect of V/III ratio in low temperature buffer growth and qualitative growth mechanism model that has been constructed based on the structural and morphological study of the layers grown at various stages. Layers were characterized using AFM, SEM, and HRXRD. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultra-low dislocation density aluminum nitride is a very promising substrate for many device structures based on the III-V nitride system. A better lattice and thermal expansion match than foreign substrates such as SiC or sapphire make AlN the substrate of choice for heteroepitaxial growth of AlGaN alloys, especially those with high aluminum concentration. In this paper, we show crystalline and chemical characterization of native aluminum nitride substrates, as well as characterization of AlGaN epilayers with 40 and 50% concentration of aluminum. The observation of atomic steps in atomic force microscope scans of the bare substrates and epilayers, as well as the narrow Full Width at Half Maximum (FWHM) measured on X-ray diffraction, are an indication of both the good surface preparation of the substrates and quality of the epilayers. An estimation of defect density on epilayers grown by Migration Enhanced Metal Organic Chemical Vapour Deposition (MEMOCVD) resulted in mid 10 6 /cm 2 .

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.

We report on the growth and characterization of a novel, III-nitrides based, tunable hyperspectral photodetector. The proposed device structure consists of AlxGa1–xN multilayer heterostructure with Al composition of 0 < x

Density functional theory calculations were carried out to study the binding energies and diffusion barriers for various adatoms on AlN and GaN (0001) surfaces. The binding energies and potential energy surfaces were investigated for Al, Ga, and N adatoms on both Al (Ga) terminated and N terminated (0001) surfaces of AlN (GaN). Calculations for the diffusion paths and diffusion energy barriers for Al, Ga, and N adatoms on AlN and GaN were performed. It was found that the N adatom on N terminated AlN and GaN surfaces faces a high diffusion barrier due to strong N-N bond. The Al and Ga adatom on Al (Ga) terminated AlN (GaN) surfaces showed lower diffusion barriers due to the weak metallic bonds. However, the diffusion barrier for an Al adatom was always larger than that of a Ga adatom on any surface. The surfaces were also subjected to a hydrostatic compressive and tensile strain in the range of 0 to 5% to investigate the effect of strain on diffusion barriers. The diffusion energy barrier for N adatom on N terminated AlN and GaN surfaces decreased when the strain state was changed from tensile to compressive. In contrast, Al and Ga adatoms show continuous increase in diffusion barriers from tensile to compressively strained Al (Ga) terminated AlN (GaN) surfaces.

The effect of edge and screw dislocations on the electrical and optical properties of n-type Al0.34Ga0.66N is investigated. It is found that edge dislocations strongly affect the electrical properties of n-type Al0.34Ga0.66N. Both free carrier concentration and mobility decrease with increasing edge dislocation density. Edge dislocations also enhance nonradiative recombination, which is indicated by decreasing near-band-edge UV as well as parasitic blue photoluminescence. The UV/blue ratio is found to be independent of the edge dislocation density but strongly depends on the Si doping concentration.