VERTICAL STRAIN AND DOPING GRADIENTS IN THICK GAN LAYERS

We report on spatially-resolved low-temperature luminescence and Raman experiments on a 220-mu m-thick GaN layer grown on sapphire by hydrid e 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 dire ctly at the interface. We correlate these effects with the inhomogeneo us free-carrier distribution and the strain gradient found by our Rama n experiments. (C) 1997 American Institute of Physics. [S0003-6951(97) 02343-7].