Hetero-epitaxial GaN films in general have very high concentration of defects and at the same time exhibited broad photoluminescence peaks at energies below the band-gap energy (3.4eV), i.e., the so-called yellow and blue luminescence centered at 2.2 and 2.9eV, respectively. The origin of these photoluminescence peaks was commonly attributed to defects. A study was made of the relationship of the yellow and blue luminescence, to the defect structure, by using a combination of photo-etching and photoluminescence. Nominally undoped (n-type) GaN layers were grown by metal organic chemical vapor deposition. By photo-etching the perfect material between defects was removed and defect-related nanocolumns were formed. Photoluminescence measurements of samples etched to various degrees allow the identification of the different photoluminescence features. A fully etched sample exhibited no photoluminescence signature related to any of the band edge features and blue luminescence was quenched completely, while the yellow luminescence was attenuated only to the degree to which the volume amount of GaN was removed. Such behavior suggested that defects causing yellow luminescence were not related to dislocations but were rather homogenously distributed throughout the layer in the form of point-like defects. The decrease of the blue luminescence intensity at 4.4K with excitation dose (bleaching) and the simultaneous increase in yellow luminescence intensity for as-grown samples and the lack of such effects for etched samples imply a direct connection between the defect centers responsible for yellow luminescence and blue luminescence.

Statistical Photoluminescence of Dislocations and Associated Defects in Heteroepitaxial GaN Grown by Metal Organic Chemical Vapor Deposition. L.Macht, J.L.Weyher, A.Grzegorczyk, P.K.Larsen: Physical Review B, 2005, 71[7], 073309 (4pp)