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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.

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...

Despite the recent progress in the development of III-Nitride semiconductor based ultraviolet light emitting diodes (UVLEDs), commercially available devices emitting at wavelengths shorter than 370 nm still suffer from poor wall plug efficiencies on the order of ~ 1%. One factor limiting the performance of these devices is the presence of a high density of dislocations that arise from the typical heteroepitaxial growth on c-plane sapphire substrates that result in non-radiative recombination. Previously we have reported on the growth of AlGaN active regions containing self-assembled nanometer scale compositional inhomogeneities (NCI-AlGaN) that demonstrate enhanced luminescence efficiency despite the presence of a large dislocation density [1]. This phenomenon is attributed to (1) the high density of NCI regions that improves the probability of carriers recombining radiatively within them rather than non-radiatively at a structural defect and (2) the subsequent concentration of carriers within the narrower band gap NCI regions that suppresses non-radiative recombination and enhances radiative efficiency due to the reduced radiative lifetime at high carrier density [2,3]. Nevertheless, time-resolved photoluminescence (TRPL) studies show that the reduction of dislocation density in these materials further improves the active region performance [4], a phenomenon that may be due to improved transport from the wider band gap matrix to the NCI and concomitant higher carrier concentration therein. Bulk AlN has significant advantages over conventional c-plane sapphire as a substrate for III-Nitride based UVLEDs including a low dislocation density (etch pit density

Time-resolved photoluminescence studies can provide useful information for the development of InGaN/GaN heterostructures for long wavelength visible emitters. In this paper, we present results of time-resolved photoluminescence from samples grown using two different approaches to achieve green emission from InGaN/GaN MQWs. In one approach, samples, with high indium incorporation, were grown on a high quality AlN substrate to achieve green emission. The resulting photoluminescence decay of the green luminescence is long-lived and non-exponential. Quantitative analysis showed that the decay has a stretched-exponential characteristic, typical of InGaN/GaN MQW with potential fluctuation along the growth plane. This carrier localization, in a structure with low defect density, proves to be an effective means to achieve green emission. In another approach, a piezoelectric Stark-like ladder effect is used. In this case, a methodical layer-by-layer growth homogeneity optimization process was adopted to achieve an optical transition below the electron to heavy-hole (e1hh1) transition when the quantum well is subjected to the strong piezoelectric polarization dipole. This approach has proven to be successful in achieving green luminescence on conventional sapphire substrates. The resulting photoluminescence decay at 14 K, of a sample grown by this approach, is single exponential and shorter in duration than the decay observed in the first approach. This exponential decay is consistent with previous AFM studies that revealed a homogeneous active region.

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.