Vertical strain and doping gradients in thick GaN layers

We report on spatially-resolved low-temperature luminescence and Raman experiments on a 220-μm-thick GaN layer grown on sapphire by hydride vapor phase epitaxy. Our measurements reveal that the peak position of the near-band-gap luminescence strongly depends on the distance to the substrate interface. The luminescence shifts continuously to lower energies with decreasing distance but a strong blue shift occurs directly at the interface. We correlate these effects with the inhomogeneous free-carrier distribution and the strain gradient found by our Raman experiments.