Direct imaging of local strain relaxation along the {1(1)under-bar01} side facets and the edges of hexagonal GaN pyramids by cathodoluminescence microscopy

Self-organized hexagonal GaN pyramids of 5 mu m width and covered by the six {1101} side facets have been grown selectively by MOVPE on patterned GaN (0001)/sapphire substrates on top of a 2 mu m thick GaN (0001) layer. The GaN pyramids are investigated by spatially resolved cathodoluminescence. Besides a narrow luminescence peak at 355 nm, originating from the 2 mu m thick GaN layer, an additional broad luminescence band is observed from the GaN pyramids around a wavelength of 357 nm. A strong energy shift is found along the {1101} pyramidal facets and directly visualized by monochromatic CL images and linescans. The effect of thermal strain due to the mismatch of thermal expansion coefficient of the various layers involved is analyzed by varying the temperature from 5 K to room temperature. A strong impact of the free carrier concentration on the local band gap and its temperature dependence has been observed. (C) 1998 Elsevier Science B.V. All rights reserved.