CAUSES OF SIH4 DISSOCIATION IN SILANE DC DISCHARGES

Hydrogenated amorphous-silicon (a-Si:H) film growth on glass fibers strung between discharge electrodes is used to measure the distribution of film-producing radicals in a silane dc discharge. The measured distribution, as well as film deposition rates on the electrodes, show that typically >80% of the depositing radicals are produced in the cathode sheath. Discharge models, confirmed by the spatial distribution of optical emission, rule out the possibility that this dissociation in the sheath is due to electron impact. Collisions of energetic ions and neutrals with silane are clearly implicated as the cause of this sheath dissociation. In contrast, due to much lower ion kinetic energies, almost all dissociation is due to electron collisions in the low-power rf discharges most commonly used for film production. In addition, the ratio of the number of Si atoms deposited on all surfaces to the total number of ions collected at the dc-discharge cathode is measured to be 30, demonstrating the dominance of neutral radical deposition in these discharges. © 1990 The American Physical Society.