Correlation between microstructure and localized band gap of GaN grown on SiC

Transmission electron microscopy and localized electron energy loss (EEL) spectroscopy in the band gap energy regime were conducted in the GaN/3C-SiC system. Though grown on a cubic substrate, the GaN film exhibits hexagonal phases of two different orientation relationships with the substrate. One of these has transformed in parts into a cubic phase via sequential faulting. The band gap was determined in local regions of good crystallinity, in the proximity of stacking faults, dislocations, grain boundaries, and heterointerfaces. Extrapolation of the low EEL spectra gives a band gap value of 3.4 eV for the unfaulted hexagonal-close-packed (hcp) phase, which ties in with the predicted value. The band gap itself in the EEL spectra is heavily masked by near band edge states around 3.2 and 3.3 eV in particular in the faulted grains. Local variations in the midgap and near band edge density of states in the EEL spectra were found to relate to different amounts and kinds of microstructural defects in the GaN film. Accordingly photoluminescence measurements at 6 K do not reveal the bound exciton at around 3.47 eV, but instead suggest the presence of donor acceptor pairs at 3.27 eV. EEL spectra of the cubic portions reveal a tail of the scattering intensity, which can be fitted by a parabolic density of states curve extending to 3.2 eV, the predicted value of the cubic band gap. These transitions are, however, of surprisingly low intensity, considering the crystallographic perfection of the material in the cubic portions. (C) 1998 American Institute of Physics.