Polycrystalline GaN layers were grown on polycrystalline Mo substrates by gas-source molecular-beam epitaxy using an ion-removal electron-cyclotron-resonance radical cell. Electron transport across the GaN/Mo interface showed ohmic characteristics. Field emission measurement was carried out on the polycrystalline GaN/Mo samples as a function of growth temperature. It was found that the GaN grown at 700 °C showing the grain size of 400 nm observed by atomic force microscopy leads to the lowest turn-on electric field. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)