We report improvement of the quality of bulk zinc-oxide (ZnO) substrates by thermal annealing in oxygen atmosphere. The high quality substrates for epitaxy are described in terms of smooth surfaces with atomic structures and high crystallinity without mosaicity. We employ thermal annealing techniques with temperature range from 800degreesC to 1100degreesC to investigate quality of annealed ZnO substrates. As-supplied ZnO substrates have 3.1 nm of surface roughness and 48.6 arcs of width of X-ray rocking curves. The surface morphology dramatically changes to be smooth surfaces with increase in annealing temperature. Thermal annealing at 1000degreesC of ZnO substrates changes from island surfaces to atomically smooth surfaces. By thermal annealing at 1100degreesC, surfaces with atomic step and terrace are obtained with 0.4 nm of surface roughness measured by atomic force microscopy. For crystallinity estimated by X-ray rocking Curves, ZnO Substrates show remarkable improvement with increasing temperature. Long diffraction tails due to damaged surface layers disappear by annealing at 900degreesC. The narrowest width of (0002) X-ray rocking curves of ZnO is obtained of 17.2 arcs with annealing at around 1100degreesC. We find that thermal annealing at around 1000degreesC is useful technique to improve the quality of ZnO substrates for ZnO homoepitaxy. (C) 2004 Elsevier B.V. All rights reserved.