A review of the preparation of carbon nitride films

Using a semiempirical rule based on a range of existing hard materials, Liu and Cohen predicted that some of the crystalline forms of C3N4 might have values of bulk modulus close to, or even greater than, that of diamond. Subsequent theoretical work has supported this result, although modifying which of the crystal structures had the most extreme properties. Based on the assumption that hardness is primarily determined by the bulk modulus, a large number of groups initiated research to try to make this ‘harder-than-diamond’ material. Although today there are doubts as to the validity of this assumption, the ability to synthesise materials with such a large bulk modulus is of great interest because such substances can be expected to have extreme properties, such as thermal conductivity, compression strength, acoustic velocity, etc. A review is presented of the methods used, and the results obtained, by a variety of groups in their attempts to prepare carbon nitride films. The preparation techniques have been somewhat arbitrarily divided into the following sections: (1) atmospheric-pressure chemical processes, including pyrolysis and explosive shock; (2) ion-beam deposition, including cathode arc; (3) laser techniques; (4) chemical vapour deposition processes, including direct-current (DC) arc, hot-filament chemical vapour deposition (HFCVD) and plasma-enhanced chemical vapour deposition (PECVD); and (5) reactive sputtering, with and without deliberate substrate biasing. We have also included a section describing some of the problems associated with the analysis of carbon nitride films. A brief description of the techniques is provided and we discuss some of the problems associated with the characterisation methods that have been used in the analysis of the deposited material. A somewhat speculative set of conclusions is presented which we hope will be of assistance to colleagues working in this field.