Ion-induced effects on grain boundaries and a-Si:H tissue quality in microcrystalline silicon films

Abstract

Microcrystalline silicon films have been deposited by means of the remote expanding thermal plasma. The effect of ion bombardment on the microcrystalline silicon film properties has been investigated by applying an RF bias to the deposition substrate. The application of the RF substrate bias resulted in the formation of an additional plasma in front of the substrate holder. Neither the SiH4 depletion nor the growth flux was significantly enhanced upon substrate biasing, which suggests that (the composition of) the growth precursor flux is unaffected and that the ion-film interaction mechanisms were responsible for the observed material changes. Moderate bias conditions (i.e., dc bias voltages up to ∼70 V) led to an improved grain boundary passivation and densification of the amorphous silicon tissue, as concluded from the analysis of the infrared Si-Hx stretching modes. These improvements have been ascribed to ion-induced Si surface atom displacement, which enhances the surface diffusion length of the growth precursors. More-energetic ion bombardment (i.e., under applied dc bias voltages of ∼60 V and higher) resulted in enhanced (di)vacancy incorporation via ion-induced Si bulk atom displacement. The film crystallinity was found not to be affected by the ion bombardment, although a reduced crystallite size was observed under ion bombardment conditions where Si bulk displacement had been sufficiently activated. The extent of the ion-film interaction mechanism has been enhanced by increasing the ion-to-Si deposition flux ratio. Under specific ion bombardment conditions, i.e., dc bias voltage in the range of 40–70 V and ion/Si arrival rate ∼0.20, microcrystalline silicon films have been obtained which, on the basis of the Si-Hx stretching modes, are qualified as solar-grade, i.e., the intergranular space is filled with a dense amorphous silicon tissue which completely passivates the crystalline grain boundaries.