We have investigated the effect of crystalline defects existing in mirror-polished single-crystalline diamond substrates on the electronic quality of diamond films homoepitaxially grown on them. High-pressure/high-temperature-synthesized diamond substrates were exposed to quasi-electron-cyclotron-resonance (quasi-ECR) oxygen plasma to etch the substrate surface layers approximate to 1 mu m in thickness, where a substantial amount of polishing-process-induced crystalline defects were supposed to remain. Defects other than these were also expected to be etched out during the oxygen quasi-ECR process. Homoepitaxial diamond films were subsequently grown on them using a high-power microwave-plasma chemical-vapor-deposition (MPCVD) method. The crystalline quality of the grown films was characterized electronically using cathodoluminescence measurements. The results clearly verify that the proposed etching process for mirror-polished diamond substrates can effectively suppress defect formation in the MPCVD diamond films homoepitaxially grown on them. (c) 2005 Elsevier B.V. All rights reserved.