Wide band-gap silicon-carbon alloys deposited by very high frequency plasma enhanced chemical vapor deposition

The use of Very High Frequency (VHF) Plasma Enhanced Chemical Vapour Deposition in a capacitive discharge is investigated to fabricate hydrogenated amorphous silicon carbon alloys, using silane and methane as silicon and carbon precursors respectively and hydrogen dilution of the gas mixture. The properties of samples differ significantly from what normally observed for rf deposition. A wide band gap material is obtained, with a carbon ratio ranging from 0.2 to 0.63. An energy gap up to 3.4 eV is measured, indicating a large sp3 content. The most interesting properties are observed using 90% hydrogen dilution and 350°C as substrate temperature. In this case, a Si-C bond concentration up to 6x1022 cm-3 was measured for stoichiometric samples, associated to a highly crosslinked structure and no detectable Si-CH3 bending signal. The role of hydrogen in determining the optical properties of the film is established and is shown to affect mainly the valence electron concentration. Basing on the free energy model, hydrogen bonding is observed to lie in between a random and chemically ordered configuration. The results are obtained at a deposition rate much larger than both rf and ECR deposition, and are associated to a limited gas consumption, both aspects being advantageous for practical applications. The large Si-C bond concentration, associated to a limited silicon and carbon hydrogenation, makes the VHF deposited a-SiC:H a good starting material for subsequent crystallisation.