Compositionally graded relaxed AlGaN buffers on semipolar GaN for mid-ultraviolet emission

In this Letter, we report on the growth and properties of relaxed, compositionally graded AlxGa1-xN buffer layers on freestanding semipolar (20 (2) over bar1) GaN substrates. Continuous and step compositional grades with Al concentrations up to x = 0.61 have been achieved, with emission wavelengths in the mid-ultraviolet region as low as 265 nm. Coherency stresses were relaxed progressively throughout the grades by misfit dislocation generation via primary (basal) slip and secondary (non-basal) slip systems. Threading dislocation densities in the final layers of the grades were less than 10(6)/cm(2) as confirmed by plan-view transmission electron microscopy and cathodoluminescence studies. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4757423]