AVS 49th International Symposium
    Plasma Science Thursday Sessions
       Session PS+TF-ThM

Paper PS+TF-ThM9
Mechanism of Hydrogen-Induced Crystallization of Amorphous Silicon Thin Films

Thursday, November 7, 2002, 11:00 am, Room C-103

Session: Plasma Enhanced Deposition
Presenter: S. Sriraman, University of California, Santa Barbara
Authors: S. Sriraman, University of California, Santa Barbara
E.S. Aydil, University of California, Santa Barbara
D. Maroudas, University of California, Santa Barbara
Correspondent: Click to Email
Hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) films are used in solar cells, displays, and imaging devices. The a-Si:H films undergo chemically-induced structural transformation from a state of disorder to order (crystallinity) when exposed to H atoms from an H@sub 2@ plasma at temperatures lower than those required for thermal annealing; exposure to an H@sub 2@ plasma is used as a post-deposition treatment step for plasma-deposited a-Si:H films. In addition, nc-Si:H films can be grown during plasma deposition by heavily diluting the SiH@sub 4@ feed gas with H@sub 2@. Though several hypotheses have been proposed, the mechanism behind the H-induced disorder to order transition still remains unclear. The atomic-scale processes behind this structural transition are analyzed through molecular-dynamics (MD) simulations of repeated H atom impingement on a-Si:H films. These films were grown through MD by repeatedly impinging SiH@sub 3@ radicals on an initially H-terminated Si(001)-(2x1) surface. The evolution of the Si-Si radial distribution function during H exposure of the film showed gradual appearance of peaks corresponding to the coordination shells of crystalline Si, indicating a transition from disorder to order. Detailed structural analysis after H exposure revealed the presence of a nanocrystalline region embedded within the amorphous Si matrix. The structural transformation is mediated by H atoms that diffuse into the a-Si:H film and insert into strained Si-Si bonds to form intermediate bond-centered H (Si-H-Si) configurations. This results in local structural relaxation of these strained Si-Si bonds. The energetics of H insertion into strained Si-Si bonds and its implications in the structural relaxation of the amorphous phase are discussed. The existence of bond-centered H(D) in a-Si:H films exposed to H(D) atoms from a H@sub 2@(D@sub 2@) plasma also was verified experimentally through in situ infrared spectroscopy.