Pseudomorphic growth of thick n-type AlxGa1−xN layers on low-defect-density bulk AlN substrates for UV LED applications

Recently it has been discovered that when growing Al_xGa_1−xN 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_xGa_1−xN 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.