Diamond is the hardest substance among the solid substances existing on the earth, electrically constitutes an insulating material whose heat conduction rate is highest at 30.degree. to 650.degree. C. (e.g., about five times that of copper), and is optically excellent in light transmission over a wide range extending from the ultraviolet region to the infrared region except a part of the infrared region.
Also, it is known that the doping of specific impurities causes the diamond to exhibit semi-conductor characteristics.
Due to such excellent properties useful in the many field of applications, the diamond is used as diamond paste, cutter, etc., by utilizing its hardness, for example. However, from the practical point of view the diamond has not been used in the technical field of electronic devices due to the fact that its synthetic method solely has recourse to the high-pressure process thus making it impossible to produce thin plate shaped diamond.
However, owing to the fact that the diamond has a wide band gap, if it is possible to produce a thin diamond film of the p-type or n-type semi-conductor by the doping of the proper impurities, they may be used as thermally stable materials, more particularly as materials suitable, for example, for the window member of solar cells and a high temperature operating semi-conductor in place of the existing semiconductor devices mainly composed of Si, GaAs and other materials which are limited in operating temperature. Moreover, the thin diamond film is expected as the most promising material for the passivation film of GaAs which is presently looked for earnestly.
In order to provide materials which meet these needs, various production methods of the artificial diamond-like thin film from the vapor phase have recently been studied and all of these method employing CVD methods (chemical vapor deposition method) have been said to be capable of producing the thin film by the plasma arc decomposition of a raw gas consisting of a mixed gas of a hydrocarbon such as methane or ethane and hydrogen or by the thermal decomposition of such raw gas on a tungsten filament heated to about 2000.degree. C. or over.
However, these methods involve extremely difficult factors from the standpoints of conditions, efficiency, etc., and also the industrialization of these methods, setting aside their use on a laboratory scale, is difficult in consideration of the cost and other factors.