AVS 51st International Symposium
    Thin Films Thursday Sessions
       Session TF-ThM

Paper TF-ThM2
Atomic-Scale Analysis of SiH@sub 3@ and H Surface Diffusion on Plasma-Deposited Amorphous Silicon Thin Films

Thursday, November 18, 2004, 8:40 am, Room 303C

Session: Modeling & Fundamentals in Thin Film Deposition
Presenter: D. Maroudas, University of Massachusetts, Amherst
Authors: M.S. Valipa, University of California, Santa Barbara
T. Bakos, University of Massachusetts, Amherst
E.S. Aydil, University of California Santa Barbara
D. Maroudas, University of Massachusetts, Amherst
Correspondent: Click to Email
Hydrogenated amorphous silicon (a-Si:H) thin films grown by plasma-assisted deposition from silane-containing discharges are used widely in the fabrication of solar cells and flat panel displays. Film properties, such as surface roughness and film crystallinity, depend on the mobility of reactive species that impinge on the film surface during deposition. Surface transport of SiH@sub 3@ radicals and H atoms is particularly important in determining film smoothness and affecting film crystallinity, respectively. This presentation focuses on detailed atomic-scale analysis of diffusion of SiH@sub 3@ and H on the a-Si:H surface. Using molecular-dynamics (MD) simulations of repeated impingement of SiH@sub 3@ radicals on the growth surface, we modeled the deposition of a-Si:H films on crystalline Si (c-Si) substrates. MD-grown a-Si:H film surfaces were found to be remarkably smooth due to valley-filling mechanisms mediated by diffusion of SiH@sub 3@, the mobile precursor, and resulting in passivation of dangling bonds present in surface valleys or at valley edges. Surface migration of SiH@sub 3@ is driven by the Si-Si bond strain distribution on the surface, which is strongly coupled with the surface morphology and reactivity. In addition, exposure of MD-grown a-Si:H films to H atoms from an H@sub 2@ plasma leads to formation of nanocrystalline regions in the films. Analysis of MD trajectories revealed that H atoms diffuse on the a-Si:H surface and into the bulk film and insert into strained Si-Si bonds; this leads to local structural relaxation and triggers disorder-to-order transitions. We also report results of first-principles density functional theory (DFT) calculations performed using c-Si surfaces as representative models of atomic bonding at film growth surfaces. The DFT results are consistent with the mechanisms of SiH@sub 3@ and H transport identified by the MD simulations and provide quantitative predictions of the migration energetics.