Effect of impurity incorporation on emission wavelength in cathodoluminescence spectrum image study of GaN pyramids grown by selective area epitaxy

Metal-organic chemical vapor deposition has been used for selective epitaxy of GaN pyramids ranging in size from over 1 μm to 500 nm in length at the base. Pyramids are terminated by { 1 1- 01 } crystal facets. The optical properties of the pyramids are investigated by cathodoluminescence (CL) in a scanning electron microscope. CL spectrum imaging reveals the pyramid apices to emit light at ∼363.4 nm corresponding to the emission wavelength of relaxed GaN. As the CL excitation is moved away from the apex a systematic redshift is observed. The redshift is ∼4 nm for pyramids with 3 μm base dimension and ∼2 nm for the 500 nm pyramids. The shift is attributed to diffusion of silicon and oxygen into the GaN pyramids due to SiO2 mask decomposition with negligible contribution from stress. The observations are backed by finite element simulations of diffusion and stress.