Homoepitaxial diamond films were deposited on Ib(100) substrates using the homebuilt microwave plasma-assisted chemical-vapor-deposition (MPCVD) apparatus . In this study, effect of oxygen addition during diamond growth was investigated . Typical growth condition was as follows; total gas pressure, microwave power, methane concentration (flow ratio of CH4 to the total gas flow), oxygen concentration (flow ratio of O2 to the total gas flow), and substrate temperature were 120‒140 Torr, 1.2‒1.4 kW, 4‒10%, 0‒2%, and 950‒1050 °C, respectively. Microwave power density estimated from the injected microwave power divided by the plasma volume was 110‒180 W cm-3.
By optimizing growth conditions with higher oxygen concentration of 2%, high-purity homoepitaxial (100) diamond layers, typical nitrogen concentration of which was less than 1 ppb, were successfully grown. Cathodoluminescence mapping and 3D Raman measurements indicated a number density of dislocations in the homoepitaxial layer is typically 104-105 cm-2. Tungsten carbide Schottky electrodes fabricated on the non-doped diamond showed high blocking voltage of 2.2 kV. This diamond films growth technique is promising for designing high-performance quantum information devices.
The author would like to thank Dr. T. Yamamoto, Dr. S. Koizumi, Dr. K. Watanabe, Dr. H. Umezawa, Dr. S. Onoda, Dr. O. Milikofu, Prof. F. Jelezko, and Prof. J. Isoya for helpful discussion and supporting characterization.
 T. Teraji, T. Yamamoto, K. Watanabe, Y. Koide, J. Isoya, S. Onoda, T. Ohshima, L. J. Rogers, F. Jelezko, P. Neumann, J. Wrachtrup, and S. Koizumi, phys. stat. sol. (a) 212, 2365(2015).
 T. Teraji, J. Appl. Phys. 118, 115304(2015).