Strong photoluminescence emission from polycrystalline GaN layers grown on W, Mo, Ta, and Nb metal substrates

Polycrystalline GaN layers were grown on W, Mo, Ta, and Nb metal substrates by gas-source molecular-beam epitaxy using an ion-removal, electron-cyclotron-resonance radical cell. X-ray diffraction rocking curves showed preferential GaN(0002) or GaN(10–11) orientations. The grain sizes ranged from 100 to 800 nm. Strong photoluminescence (PL) emission without yellow luminescence was observed from these polycrystalline GaN layers. At 77 K, PL peaks at 3.46 and 3.26 eV were observed, and their temperature dependence fit a simple relation based on the number of phonons. The higher-energy peak probably was due to the free excitonic transition in hexagonal GaN.