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Boron-doped homo-epitaxial films were deposited by microwave plasma CVD on synthesized single-crystal diamonds. Boron concentration in the films was determined by the boron concentration in the reactant gases. The crystallinity of boron- doped films was found to be affected by the boron concentration in the reactant gases. The conductivity of boron doped films was determined by the concentration of boron, and the activation energy was nearly the same as for natural IIb diamonds. The effect of the surface orientation of substrates was found in the conductivity of highly doped films. The result of the Hall effect measurement indicated that the mobility of boron-doped diamond epitaxial films was 70 cm2/V·s even at 500°C.

Research and development have been carried out to apply the CVD diamond film to surface acoustic wave (SAW) devices. Several kinds of layered structures including a diamond layer have been investigated by the calculations and experiments, and it has been found that the diamond SAW has great advantages for the application of high-frequency SAW devices with high SAW velocity, small temperature coefficient and high power durability. Practical SAW devices have been successfully fabricated with ZnO/diamond/Si and SiO 2 /ZnO/diamond/Si structures whose characteristics are superior to those with conventional SAW materials.

ZnO thin film has been epitaxially grown on the (111) plane of the diamond substrate by rf magnetron sputtering at substrate temperature as low as 260 °C. The crystallinity was examined by x-ray diffraction and reflection high-energy electron diffraction. It was found that the smallest standard deviation angle estimated from the x-ray rocking curve of the ZnO(0002) peak was 0.27° whereas the mismatch of the lattice parameter between film and substrate is 28.8%. The epitaxial relationship between ZnO film and the diamond is determined as [112̄0] ZnO//[1̄01] diamond.

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