Strong photoluminescence emission from polycrystalline GaN layers grown onW, 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-cyclot ron-resonance radical cell. X-ray diffraction rocking curves showed prefere ntial GaN(0002) or GaN(10-11) orientations. The grain sizes ranged from 100 to 800 nm. Strong photoluminescence (PL) emission without yellow luminesce nce was observed from these polycrystalline GaN layers. At 77 K, PL peaks a t 3.46 and 3.26 eV were observed, and their temperature dependence fit a si mple relation based on the number of phonons. The higher-energy peak probab ly was due to the free excitonic transition in hexagonal GaN. (C) 2001 Amer ican Institute of Physics.