Compositional Inhomogeneity in AlGaN Multiple Quantum Wells Grown by Molecular Beam Epitaxy: Effect on Ultraviolet Light-Emitting Diodes

AlGaN-based multiple quantum well structures grown by plasma-assisted molecular beam epitaxy were investigated for their use in light-emitting diode (LED) structures. Initial testing was carried out on 20 pairs of AlxGa(1-x)N/AlyGa(1-y)N quantum well structures with different levels of compositional inhomogeneities in the alloy, introduced by appropriate choice of deposition conditions. It was observed that these conditions did not change the overall absorption edge, but generated a strong red-shifted photoluminescence peak. A series of LEDs with similar quantum well structures in the active region were grown, fabricated and tested. Our results indicate that deliberate introduction of compositional inhomogeneities in the quantum wells increased their electroluminescence intensity by an order of magnitude, but similar to the photoluminescence results, shifted the peak from 320 nm to 350 nm. Additionally, for these devices, no efficiency reduction was observed up to an injection of 60 mA for a device area of 0.25 mm(2). These improvements may be attributed to carrier trapping mechanisms in the AlGaN well layers at deliberately introduced deep potential minima, which are likely to arrest both their overflow and their transport to extended defect states. The growth conditions also improve the p-doping efficiency and reduce the probability of leakage of carriers into the p-type layers through defect states. However, the increased efficiency and reduced droop comes at the cost of a red-shift of the electroluminescence peak.