Development of diamond devices and recent progress in diamond growth

The growth of high-quality crystals is always the first priority when one considers using a new material for electrical devices. Furthermore, the doping process should be evaluated in terms of applicability to fabrication of semiconductor devices. We obtained smooth diamond epitaxial films grown at a high CH4/H-2 ratio by the microwave plasma chemical vapor deposition (CVD) method, and fabricated MESFETs on them for the first time in 1989. We developed a doping technique and demonstrated two key structures of diamond devices: MiS (metal-intrinsic-semiconductive) diamond structure and pulse-doped (delta-doped) structure. In the diamond devices with these structures, we realized both temperature stability and high conductance at room temperature, because carriers are injected or diffused from the highly doped region (degenerated region) to the intrinsic region. The intrinsic diamond also suppressed the junction leakage and raised the breakdown voltage of the devices. However, in order to further improve device characteristics, we should optimize the growth conditions and improve the crystal quality of diamond. Films grown on the (001) substrate at a high CH4/H-2 (=6%) ratio are microscopically rough, although they have no growth hillocks, as had been previously reported. On the other hand, films grown on the (001) well-oriented substrate at a low CH4/H-2 (=0.5-2%) ratio have growth hillocks, while the region around the hillocks is microscopically smooth. We applied the Burton-Cabrera-Frank (BCF) theory to study the growth conditions, and investigated the effects of substrate off-orientation on the surface morphology. Then we successfully obtained macroscopically and microscopically smooth films at a low CH4/H-2 (=2%) ratio on the (001) substrate with a 7 degrees off-angle toward <110>. Schottky diodes fabricated on them exhibited excellent I-V characteristics with a rectifying ratio of 10(10) at 5 V.