Theoretical study of the interactions of SiH2 radicals with silicon surfaces

Abstract

Silylene (SiH2) radicals created by electron impact dissociation of silane in reactive gas discharges can play an important role in plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH2 radicals with a variety of crystalline and amorphous silicon surfaces based on atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds is shown to exert the strongest influence on the radical-surface reaction mechanisms and the corresponding reaction probabilities. The SiH2 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is found to react with a probability of approximately 50%. The SiH2 radical attaches itself to the surface either by forming two bonds with Si atoms of adjacent dimers in the same dimer row or through Si–Si bonds with one or both atoms of a surface dimer. In addition, the SiH2 radical can attach itself in the trough between dimer rows, forming two Si–Si bonds with second-layer Si atoms. The energetics and dynamics of these surface reactions are analyzed in detail. A reaction probability of approximately 70% is calculated for SiH2 radicals impinging on surfaces of hydrogenated amorphous silicon (a-Si:H) films with varying concentrations of hydrogen. Recent experimental measurements have reported a 60% loss probability for the SiH2 radical on the reactor walls through laser induced fluorescence. The experimentally obtained reaction probability falls within the range for the sticking coefficients on the H-terminated and amorphous film surfaces as determined by our atomistic calculations. Molecular-dynamics (MD) simulations of a-Si:H film growth by repeated impingement of SiH2 radicals have revealed adsorption reactions at early stages to occur with similar energetics as the corresponding reactions of isolated radicals on crystalline surfaces. The reaction probability of SiH2 on a-Si:H films deposited through MD simulations is approximately 30%. Finally, it is found that the SiH2 radical is much more mobile on surfaces of a-Si:H films than on crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.

Time-resolved cavity ringdown study of the Si and SiH3 surface reaction probability during plasma deposition of a-Si:H at different substrate temperatures

“…Moreover, direct measurements of ␤ for the silane radicals have been carried out by means of the various diagnostics mentioned in Sec. I, revealing ␤ values of ϳ0.2-0.3, 6,8,[10][11][12][13]20,25 [40][41][42][43][44] and molecular dynamics simulations, 41,[45][46][47][48] which address the surface reactions at the atomistic scale. For SiH 3 , only indirect information is available about the dependence of ␤ on the substrate temperature from the early study of Matsuda et al 6 This study, though, suffers from the uncertainty that the measured ␤ values are averaged overall plasma species.…”

“…This trend is in agreement with the observations from molecular dynamics (MD) simulations. [45][46][47][48] To get an indication of the relative importance of the different radicals to a-Si: H film growth, the Si growth flux due to a specific radical, ⌫ Si (i.e., the number of Si atoms deposited per second by the specific radical) can be estimated from the value of ␤ and the density n in the plasma: 54 …”

Molecular dynamics simulation of ion bombardment on hydrogen terminated Si(001)2×1 surface

“…Such successful comparisons included the structure and energetics of gas-phase hydrogen-containing silicon species (SiHx, xϭ1,2,3), [45][46][47] crystalline and amorphous Si bulk phases, and surfaces, as well as mechanisms and energetics of reactions of SiHx (xϭ1,2,3) with Si surfaces. [30][31][32][33] Newton's equations of motion were integrated using the velocity Verlet algorithm. 48 The time step used in our simulation was 0.1 fs.…”

“…[25][26][27][28][29] In addition, systematic MD investigations have been carried out to understand interactions between reactive neutral species and silicon surface. [30][31][32][33] A number of groups have used MD to study the effect of energetic species during film deposition. 34 -36 Kitabatake et al simulated direct ion-beam deposition using 10 eV Si ions.…”

Full simulation of silicon chemical vapor deposition process

“…This is about the whole flow filed in the reactor, that is, the spatial distribution of velocity, energy and density from the viewpoint of reactor size. These are evaluated by Navier-Stokes equations [1] " [31 or the DSMC method [4] " [6 l The other is the study of the micro-scale analysis and this contains intermolecular potential and collision dynamics from the viewpoint of molecular size [7)[81 . In the actual CVD process these physical phenomena strongly interact each other and the multi-scale analysis must be needed as shown in Fig.l.…”