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.
Polycrystalline GaN layers were grown on polycrystalline Mo substrates by gas-source molecular-beam epitaxy using an ionremoval electron-cyclotron-resonance radical cell. The GaN/Mo interface exhibited ohmic characteristics. The atomic force microscopy image for the polycrystalline GaN surface revealed the grain structure with a grain size of 300-600 nm and a tip-like structure around the grains. Field emission measurement was carried out for the polycrystalline GaN/Mo samples. The turn-on electric field was as low as 10.6 V/µm at an emission current density of 10 −8 A/cm 2 . Field enhancement factors estimated from the Fowler-Nordheim plot in low and high electric field regions are as large as 200 and 610, respectively.
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