The effect of ion-surface and ion-bulk interactions during hydrogenated amorphous silicon deposition

The ion-bombardment induced surface and bulk processes during hydrogenated amorphous silicon (a-Si:H) deposition have been studied by employing an external rf substrate bias (ERFSB) in a remote Ar-H2-SiH4 expanding thermal plasma (ETP). The comparison of the ETP chem. vapor deposition without and with ERFSB enables us to identify some important ion-surface and ion-bulk interactions responsible for film property modifications. Employing ERFSB creates an addnl. growth flux and the low energetic ions deliver an extra 5-10 eV per Si atom deposited at typical deposition rates of 10-42 A/s which is a sufficient ion dose to modify the film growth. It is demonstrated that the extra surface and bulk process during a-Si:H growth, induced by the addnl. ion bombardment, provide an extra degree of freedom to manipulate the a-Si:H microstructure. An ion-film interaction diagram is introduced, which is used to discriminate ion-surface interactions from ion-bulk interactions. According to this ion-film interaction diagram, the a-Si:H grown with ERFSB can be roughly classified in three phases. In phase I the only ion-surface process activated is Si surface atom displacement. In phase II also ion-induced Si bulk atom displacement is sufficiently activated, whereas in phase III ion-induced Si atom sputtering is significant. Phase I is characterized by a redn. in the nanosized void d., a redn. in defect d., and an improvement of the photoresponse. We find that the Si surface displacement is the process responsible for various improvements of the material properties via the enhanced surface migration. Phase II is characterized by an enhancement of vacancy incorporation. In accordance with the introduced ion-film interaction diagram, the Si atom bulk displacement process is responsible for the incorporation of addnl. vacancies. Phase III is characterized by the decrease in growth flux and the increase in void d. The significant contribution of ion-sputtering processes is responsible for the effects obsd. in phase III. [on SciFinder (R)]