Photoluminescence studies of GaN epilayer-nanocrystals grown on γ-LiAlO₂ substrate

We report the optical studies of the properties of M-plane GaN/c-plane GaN nanocrystal heterostructure on γ-LiAlO₂ substrate grown by plasma-assisted molecular beam epitaxy. In this structure, in addition to the M-plane epilayer, nanocrystals grown in c-direction could also be observed in the step edges of the M-plane GaN terraces and the hexagonal basis of the γ-LiAlO₂ substrate. X-ray diffraction (XRD) with peaks at 2θ=32.295°, 34.680° and 34.505° are attributed to the M-plane GaN, LiAlO₂ and c-plane GaN, respectively. Two peaks were observed in the photoluminescence spectra at low temperature. The peak at 3.33 to 3.35 eV is attributed to the emission from c-plane GaN nanocrystals and the peak at 3.50 eV is attributed to the emission from M-plane GaN epilayers. The relative intensity of these peaks is position-dependent. In the area with higher concentration of the GaN nanocrystals the emission for the nanocrystals is stronger and vice versa. Cathodoluminescence shows that the emission peak at 3.33-3.35 eV is originated from the nanocrystals GaN.