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

Abstract

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 chemical 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 additional growth flux and the low energetic ions deliver an extra 5–10eV per Si atom deposited at typical deposition rates of 10–42Å∕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 additional 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 reduction in the nanosized void density, a reduction in defect density, 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 additional vacancies. Phase III is characterized by the decrease in growth flux and the increase in void density. The significant contribution of ion-sputtering processes is responsible for the effects observed in phase III.

Structural origins of intrinsic stress in amorphous silicon thin films

“…Fig. 4b (in agreement with results observed by Hamers,33,34 Smets,23 and Wank 35 indicates that increasing ion momentum results in an initially rapid decrease in film porosity in the tensile region, followed by a nearly invariant low porosity in the compressive region.…”

“…Finally, we observe that silicon monohydride content (Fig. 4b) increases continuously with ion momentum, suggesting that bulk particle implantation of hydrogenated Si (the mechanism supposed to be responsible for the silicon monohydride in the material 23 ) is occurring uniformly throughout our deposition range, and providing further evidence that bulk ion bombardment is indeed occurring throughout all deposition conditions, and lending credence to the theory that void collapse occurs from bulk bombardment even at low ion momentum levels. From this information we conclude that stress creation does indeed appear to be a balance between two separate phenomena responsible for tensile and compressive forces: In region I, the hydrogen void concentration is high, indicating that the level of void collapse is low, and thus the tensile stress is relatively low as well, responsible for the decrease in stress from the pure implantation model (dashed line) observed here.…”

“…III-A) as an explanation of the total stress state in our films, we do not preclude the possibility of bombarded hydrogen influencing the stress state -as the aforementioned model (to our knowledge) makes no claims of the origin of the varying hydrogen content in the films, the theories could in fact be complementary: implantation of SiH X radicals (as suggested in Ref. 23) would account for the correlation between compressive stress and hydrogen content observed in Ref. 27, and our analysis/model does not depend on the specific injected species (Si, H, or, most likely, a combination of the two) actually causing the induction of stress.…”

“…Based on models put forth by Windischmann 22 and Smets, 23 we are able to describe the observed stress behavior purely through ion interactions with the depositing film and correlate these descriptive models to the observed experimental data.…”

In situ crystallization kinetics studies of plasma-deposited, hydrogenated amorphous silicon layers

“…The total hydrogen content (C H ) was calculated from the Si-H wagging mode using a proportionality constant previously reported in literature. 15,26 A Philips X'Pert Pro system was used for the XRD measurements with Cu-Ka x rays selected with a graphite crystal monochromator. The information related to the MRO scale (3-6 Å to as high as 15-25 Å 20,27-30 ) were obtained by XRD linewidth analysis of the full-width-at-half-maximum (FWHM) of the lowest angle x ray scattering peak.…”