Yamada Hideaki1
AIST1
Variety of material constants of diamond are the bests amongst those of other competitive materials and attract researchers to realize several future applications. Many indexes of diamond to characterize performance as power devices are better than those of, for example, Si, SiC, and GaN, owing to its wide bandgap, large breakdown field strength and high thermal conductivity. This suggests potential to realize extreme improvements in efficiency of power control in, for example, Electric-Vehicles, power plants and its transmission principally. Especially, this extremely high thermal conductivity is expected to enlarge range of applications of other semiconductor devices with relatively low thermal conductivity, for example, GaN- and Ga<sub>2</sub>O<sub>3</sub>-based ones. On the other hand, quantum information, which can be controlled by electronic states in specific color centers of diamond, for example, Nitrogen-Vacancy (NV) centers, is known to be treated under steady state with coherence time which is longer than those of others. This may realize high performance quantum computers, information transmission equipment with high security, and variety of quantum sensors with high resolution and wide dynamic range.<br/><br/>To realize such fascinating industrial applications, it is indispensable to establish the way to produce single-crystalline diamond wafers with sufficient quality and size under acceptable cost. Artificial diamond was firstly reported by using high-pressure-high-temperature method, in which stable state of diamond is realized. This method is nowadays adopted to produce mechanical tools worldwide. On the other hand, it is considered practically difficult to realize inch size crystals for industrial applications by using this method. On the other hand, chemical vapor deposition (CVD) was also found to be the other way to obtain diamond crystals artificially. Several CVD methods with variety of radical sources have been proposed and proofed to be possible methods, for example, flames, hot-filaments, and plasmas. Among several plasma sources, microwave plasma (MWP) CVD has been widely adopted to grow diamond crystals especially for electronics and quantum applications because of its relatively high plasma and power-density without electrode near the top surface of the substrate. Especially, developments in techniques of crystal growth by using MWPCVD during recent 20 years realized remarkable market entry of artificial diamond as gemstones.<br/><br/>We have realized techniques to prepare free standing wafers with less loss, inch-sized wafers, and bulk diamond with thickness of several millimeters. To realize them, we tried to optimize the crystal growth technique by changing chamber shape, applying pulse mode discharge, superimposed microwave, accompanied with numerical simulation of the plasma to understand growth environment. In addition to the technique of the crystal growth, processing technique is one of the important issues to prepare diamond wafers, because diamond is hardest material with cleavage characteristics similar to Si. Recently, we confirmed that plasma-assisted polishing could realize very fine surface with less processing damages.<br/><br/>Now, we are going to apply such wafers for thermal management of electronic devices with high power density, and devices which can be applied to harsh environments. In the future, this material is expected to fulfill its true potential in the fields of 1) Power electronics, and 2) Quantum applications. These applications require reduction of dislocation density and control of impurity concentrations. We are now trying to solve these subjects by using plasma processing.