Ion and radical reactions in the silane glow discharge deposition of a-Si:H films

A mass spectrometric analysis of the positive ions and neutral products in a silane glow discharge has been performed. The active species, created by dissociation, disproportionation, and ion-molecule reactions are mainly SiH₂, SiH₃, and H. A calculation of the distribution of the SiH _n ⁺ ions shows that the silane concentration monitors the abundance of SiH ₃ ⁺ . The diffusional transport of radicals toward the discharge-tube walls can explain the observed deposition rates. The study of SiH₄-SiD₄ and SiH₄-D₂ plasmas emphasizes several reactions which modify the free-radical populations depending on the discharge conditions: disproportionation, termination, recombination, and abstraction. Heterogeneous reactions have also been observed: etching of the film by H atoms and direct incorporation of hydrogen in the growing film. A general scheme for the plasma deposition mechanism is proposed.