Growth of GaN on polycrystalline chemical vapor deposition (CVD) diamond prepared on Si was achieved by hydride vapor phase epitaxy (HVPE). If the polycrystalline CVD diamond is separated from the Si substrate and the side turned to Si is covered by either TiN or anodic Al oxide, this is found to facilitate deposition of good crystalline quality GaN films. Porous TiN in these experiments was formed by Ti evaporation and subsequent in situ nitridation in the HVPE reactor during pregrowth heating. The films showed double crystal x-ray (0002) rocking curve half width of 245 in. and a strong bandedge luminescence. Thick films self-separated from the substrate. When growing GaN on the Si side of the diamond substrate is covered with porous Al anodic oxide, the authors observed the formation of well defined (0001) GaN texture with the maximum angle between the c-axes of various grains lower than 6.5 degrees, a very low (120 in.) half-width of the (0002) rocking curves of individual grains and an intense bandedge luminescence. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3488616]