Abstract: A synthesis of phosphorus-doped diamond by a microwave plasma method using a volatile hydrocarbon and hydrogen mixed therewith, as a reaction gas, wherein phosphorus is used as a dopant, and hydrogen bonded to the phosphorus is dissociated so that the phosphorus is introduced into diamond as an impurity without being bonded to hydrogen.

Abstract: An apparatus for synthesizing diamond, which comprises a reaction chamber, a means for supplying to the reaction chamber a gas mixture comprising a hydrogen gas and an organic compound decomposable by a plasma to form a diamond or a gas mixture comprising a hydrogen gas, an inert gas and said organic compound, a means of conducting a microwave of frequency larger than 300 MHz to the reaction chamber to generate a microwave plasma in the gas mixture and a substrate held by a substrate holder to locate in the microwave plasma and on which diamond is to be formed, wherein the microwave conducting means is designed to introduce the microwave to the reaction chamber from a plurality of directions.

Abstract: A method for preparing needle-like, fibrous or porous diamond, or an aggregate thereof, which comprises subjecting diamond synthesized by a chemical vapor deposition method or a plasma-assisted vapor deposition method, or a film of such diamond, to plasma treatment in a gas stream containing oxygen, carbon dioxide, steam, hydrogen, a halogenated hydrocarbon or a halogenated carbon, or to thermal oxidation treatment in a gas stream containing oxygen, carbon dioxide or steam, for etching.

Abstract: A method for synthesizing diamond wherein hydrogen gas which has passed through a micro-wave non-electrode discharge and mixed with hydrocarbon gas, or a mixture gas consisting of hydrocarbon and hydrogen after its passing through a micro-wave non-electrode discharge, is introduced onto the surface of a substrate heated to a temperature of from 300.degree. to 1300.degree. C. to decompose hydrocarbon in its energetically activated state for the diamond deposition.