AVS 50th International Symposium
    Plasma Science and Technology Wednesday Sessions
       Session PS1-WeA

Paper PS1-WeA8
Investigating the Fundamental Mechanism of Surface Smoothening of Plasma-Deposited Amorphous Silicon Thin Films through Atomistic Simulations

Wednesday, November 5, 2003, 4:20 pm, Room 314

Session: Mechanisms in Plasma-Surface Interactions
Presenter: S. Sriraman, University of California, Santa Barbara
Authors: S. Sriraman, University of California, Santa Barbara
S. Agarwal, University of California, Santa Barbara
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 SiH@sub 4@ containing discharges are widely used in photovoltaic and flat-panel display technologies. Nevertheless, the deposition mechanism of a-Si:H films and the fundamental surface processes that determine the surface morphology during deposition are still not well understood. Under conditions of low SiH@sub 4@ dissociation in the plasma, the dominant precursor for deposition is the SiH@sub 3@ radical. The remarkable smoothness of the a-Si:H films grown under these conditions has been used to conclude that the deposition precursor is very mobile and that it can fill surface valleys after adsorbing onto the film. Using molecular-dynamics (MD), molecular-statics, and Monte Carlo methods, we studied the growth of a-Si:H on initially H-terminated Si(001)-(2x1) surfaces; the films were grown through MD simulations of repeated impingement of SiH@sub 3@ precursor. The relationship between the structure, H coverage, morphology, and reactivity of plasma deposited a-Si:H film surfaces was investigated. Surfaces of a-Si:H films grown with SiH@sub 3@ as the sole deposition precursor were found to be remarkably smooth due to a valley-filling mechanism where mobile precursors, such as SiH@sub 3@ and Si@sub 2@H@sub 6@, diffuse and react with dangling bonds present in surface valleys. Surface transport of these adsorbed species may be driven by the surface Si-Si bond strain distribution, as well as the surface reactivity and morphology. Mobility of the surface species maybe mediated through the formation of over-coordination defects as weakly chemisorbed SiH@sub 3@ diffuse on the surface. Our analysis of the mechanism of SiH@sub 3@ precursor diffusion on the c-Si and a-Si:H surfaces will be presented. In particular, we emphasize the role of Si-Si bond strains in mediating the valley-filling mechanism leading to smooth film surfaces and the identity of the mobile precursor state.