The Role of SiH 3 Diffusion in Determining the Surface Smoothness of Plasma-Deposited Amorphous Si Thin Films: An Atomic
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The Role of SiH3 Diffusion in Determining the Surface Smoothness of Plasma-Deposited Amorphous Si Thin Films: An Atomic-Scale Analysis Mayur S. Valipa1,2, Tamas Bakos1, Eray S. Aydil2, and Dimitrios Maroudas1 1 Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003-3110, U. S. A. 2 Department of Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA 93106-5080, U. S. A. ABSTRACT Device-quality hydrogenated amorphous silicon (a-Si:H) thin films grown under conditions where the SiH3 radical is the dominant deposition precursor are remarkably smooth, as the SiH3 radical is very mobile and fills surface valleys during its diffusion on the a-Si:H surface. In this paper, we analyze atomic-scale mechanisms of SiH3 diffusion on a-Si:H surfaces based on molecular-dynamics simulations of SiH3 radical impingement on surfaces of a-Si:H films. The computed average activation barrier for radical diffusion on a-Si:H is 0.16 eV. This low barrier is due to the weak adsorption of the radical onto the a-Si:H surface and its migration predominantly through overcoordination defects; this is consistent with our density functional theory calculations on crystalline Si surfaces. The diffusing SiH3 radical incorporates preferentially into valleys on the a-Si:H surface when it transfers an H atom and forms a Si-Si backbond, even in the absence of dangling bonds. INTRODUCTION Thin films of hydrogenated amorphous silicon (a-Si:H), grown by plasma deposition from silane (SiH4) containing discharges, are used widely in the fabrication of solar cells, thin-film transistors for flat panel displays, and detectors for medical imaging [1,2]. Film properties such as surface roughness and film crystallinity affect device performance significantly; these properties depend on the identities and fluxes of the reactive radicals impinging on the film surface during deposition, as well as the resulting radical-surface interaction mechanisms. Under conditions of low SiH4 dissociation in the plasma, the dominant deposition precursor is the SiH3 radical [3]. The remarkable smoothness of device-quality a-Si:H films [4,5] grown under these conditions is used to conclude that the SiH3 radical is very mobile on the a-Si:H surface [4,6] and can passivate dangling bonds (dbs) present in surface valleys of rough a-Si:H films during diffusion [5], leading to surface smoothening. However, on a-Si:H surfaces, dbs are distributed randomly on both hills and valleys [7], and the db surface coverage is low (0.001) [8,9]. Furthermore, the fundamental mechanisms underlying SiH3 radical migration on a-Si:H surfaces are not well understood and the corresponding activation barrier, Ea, for the radical’s surface diffusion is still not known. Consequently, the precise role of the SiH3 radical in determining the surface smoothness of a-Si:H films remains unclear. The aim of the present work is to elucidate the atomic-scale mechanisms of SiH3 diffusion on a-Si:H surfaces, through a fundamental analysis based on molecular-dy
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