Electrical and optical properties of amorphous silicon carbide, silicon nitride and germanium carbide prepared by the glow discharge technique

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Abstract
Amorphous specimens of silicon carbide, silicon nitride and germanium carbide have been prepared by decomposition of suitable gaseous mixtures in a r.f. glow discharge. Substrates were held at a temperature T d between 400 and 800 K during deposition. In all three of the above materials the results of optical absorption and of d.c. conductivity measurements show a systematic variation with T d and with the volume ratio of the gases used. Electron microprobe results on silicon carbide specimens indicate that a wide range of film compositions can be prepared. The optical gap has a pronounced maximum at the composition Si00–32C0–68 where it is 2·8 eV for a sample deposited at T d = 500 K, but shifts to lower energies with increasing T d. The conductivity above about 400 K has a single activation energy approximately equal to half the optical gap and extended state conduction predominates if the silicon content exceeds 32%. If the latter is reduced, hopping transport takes over and it is suggested that the excess carbon in the network tends to bond in three-fold graphitic coordination. Absence of any obvious feature in the electronic properties at the stoichiometric composition SiC implies that there is little tendency towards compound formation in the glow discharge films. The present results are discussed in relation to measurements on specimens prepared by different methods.