Depth resolved cathodoluminescence study of optical transitions in MOVPE grown hexagonal GaN

The cathodoluminescence (CL) depth profiling of the optical emissions from (0001) GaN/GaNbuffer/Al2O3 heterostructures has been performed at 77<T<300 K and electron beam energies (0.25<E-b<30 keV). At 1<E-b<2 keV the yellow luminescence (YL) band dominates the spectra in all the samples at all the temperatures. By increasing E-b, the yellow band to near band edge ratio first decreases and then increases again as the layer/buffer interface is approached. This last result is discussed on the basis of the different dependence of the yellow and near band edge emissions with the excitation conditions and is interpreted in terms of electrically active point defects trapped at the stress field of dislocations. CL spectra collected at 300 K with 0.25<Eb<1 keV shows solely the YL band; by decreasing the temperature, no emission peaks are observed between 6 and 150 K. Defect related recombinations from surface states are considered to be responsible for the YL emission at E-b<1 keV.