LOCAL STRAIN DISTRIBUTION OF HEXAGONAL GAN PYRAMIDS

Self-organized hexagonal GaN pyramids of 5 mu m width and covered by s ix {1 (1) under bar 0 1} side facets were investigated by spatially re solved cathodoluminescence and micro-Raman spectroscopy. Beside a narr ow luminescence peak at 355 nm, originating from the 2 mu m thick GaN layer, an additional broad luminescence band was observed from the GaN pyramids around a wavelength of 357 nm. A strong energy shift is foun d along the {1 (1) over bar 0 1} pyramidal facets and directly visuali zed by monochromatic CL images and linescans. The effect of the therma l strain due to the mismatch of the thermal expansion coefficient of t he various layers involved was analyzed by micro-Raman spectroscopy an d by varying the temperature from 5 K to room temperature. A strong im pact of the free-carrier concentration on the local band gap and its t emperature dependence was found. (C) 1998 Elsevier Science B.V. All ri ghts reserved.