Optical properties of N-polar GaN: The possible role of nitrogen vacancy-related defects

Detailed comparison of optical quality of GaN layers grown homoepitaxially on bulk Ga-polar and N-polar substrates by plasma-assisted molecular beam epitaxy was performed. One order of magnitude lower photoluminescence (PL) intensity and decay time at room temperature was observed for layers grown on N-polar substrates, realized using gallium-rich conditions. This nonradiative recombination channel was very efficient also at liquid helium temperatures, what was evidenced by no improvement in PL time decay. Optical quality of GaN layer (PL intensity, lifetimes) grown on N-polar substrates was greatly improved by using nitrogen-rich growth conditions. We attribute this improvement to suppressed formation of nitrogen vacancy-related point defects. Finally, linewidth below 1 meV and lifetime above 0.1 ns (DX line, He temperature) were obtained for layers grown on N-polar substrate.