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.