Diamond Crystal Growth on Orientation-Controlled Carbon-Based Materials

Diamond is an excellent semiconductor material due to its wide band gap (5.45 eV), high mobility and high thermal conductivity. Diamond semiconductors are expected to be used in the construction of ultraviolet light-emitting devices, high-temperature devices and high-speed devices. Diamond have the potential to contribute to energy saving and miniaturization of various electrical products when diamond is widely used in practical applications. However, there are various problems in using diamond as a semiconductor. One of the problems is that size-limited and the high cost of the substrates in homoepitaxial growth used to fabricate high-quality diamond substrates. It is therefore preferable to establish a process for the fabricate of high-quality diamond substrate by heteroepitaxial growth. We used orientation-controlled graphite, an allotrope of diamond, as a substrate for diamond crystal growth. We present the results of our investigation into methods for diamond heteroepitaxial growth on graphite. Although there have been reported that of diamond crystal growth on graphite substrates after a pre-treatment. This is considered that this required pre-treatment because the crystal growth was on the plane of a honeycomb structure with hexagons lined up by carbon, and there were no bonding points. In the present study, crystals were grown without any pre-treatment process on the X-Y plane, where the configured by honeycomb structure, and on the Z plane, which intersects it at right angles and exposes the chemically active points as the dangling bonds. The effect of the orientation of the crystal growth substrate on diamond crystal growth was analyzed and the possibility of graphite substrates as diamond crystal growth substrates was examined. Here, it is considered that the dangling bonds on the Z-plane before crystal growth are terminated with hydrogen. In this study, polycrystalline diamond were fabricated on highly oriented pyrolytic graphite (HOPG) substrates using a microwave plasma chemical vapor deposition (MPCVD). The dependence of crystal size on raw gas ratio, deposition temperature, and substrate orientation was investigated. The raw gas ratio was fixed at CH 4 :H 2 = 0.5:199.5 and the deposition temperatures were set at 600°C, 700°C, 800°C, 900°C and 980°C. The deposition pressure was kept constant at 10 kPa. The diamond crystal size deposited in the XY plane exceeded that deposited in the Z plane. The largest difference in crystal size was observed at a temperature of 980°C and a methane flow rate of 0.5, the crystal size in the XY and Z planes was 1.3311μm 2 and 0.4232μm 2 , respectively, and the crystals in the XY plane were about three times larger than those in the Z plane. We attributed this change in crystal size due to the difference in substrate orientation to the distortion of part of the tetrahedral structure of diamond to match the hexagonal structure of graphite in the XY plane and the secondary bonding of the tetrahedral structure. In other words, although many active sites are exposed on the Z plane, the bonding distance of carbon atoms is not convenient for the generation of diamond crystals, and the hexagonal reticular structure of the XY plane is more convenient for the generation of crystals, so the diamond crystals on the XY plane are considered to have grown larger in these condition ranges. Figure 1