Different Crystal Growth Mechanisms of Si(001)-(2 × 1):H during Plasma-Enhanced Chemical Vapor Deposition of SiH₃ and SiH₂ Radicals: Tight-Binding Quantum Chemical Molecular Dynamics Simulations

Abstract

We use tight-binding quantum chemical molecular dynamics to investigate the crystal growth mechanisms of H-terminated Si(001)-(2 × 1) during plasma-enhanced chemical vapor deposition of SiH3 and SiH2 radicals. We find that crystal growth by SiH3 radical deposition consists of two stages: (1) the first SiH3 radical abstracts a surface-terminating H atom and produces a dangling bond, and (2) a second SiH3 radical is adsorbed on the dangling bond. Thus, at least two SiH3 radicals are required for generating a new Si–Si bond. Interestingly, during SiH2 deposition, a SiH2 radical can be directly adsorbed onto a H-terminated site without H abstraction by another SiH2 radical. Thus, one SiH2 radical is sufficient for generating a new Si–Si bond. This SiH2 radical crystal growth mechanism is different from the SiH3 radical mechanism. The direct adsorption process consists of a two-step chemical reaction: (1) the SiH2 radical abstracts a surface-terminating H atom and produces a dangling bond and a SiH3 radical, and (2) the SiH3 radical is adsorbed on the dangling bond. In addition, the crystal growth rate for SiH2 radicals is higher than that for SiH3 radicals, because generating one new Si–Si bond requires either a single SiH2 radical or two SiH3 radicals. However, our simulations reveal that SiH2 deposition produces defective Si thin films because many dangling bonds are formed during crystal growth. Compared with SiH2 deposition, SiH3 deposition should therefore produce Si thin films of higher quality.

The reason why thin-film silicon grows layer by layer in plasma-enhanced chemical vapor deposition

“…1(c) and (d) ). This is in agreement with our previously proposed “abstraction-adsorption” mechanism 22 . At this point, we define the terms lower layer and upper layer for referring to the layers of Si atoms terminated by H atoms.…”

“…Long-distance diffusion of weakly physisorbed SiH 3 radicals is therefore unlikely. We also found from tight-binding quantum chemical MD simulations that the initial growth of thin-film Si follows an “abstraction-adsorption” mechanism 21 22 , where a DB is generated on the surface via H abstraction by a SiH 3 radical and the next SiH 3 radical is adsorbed onto the generated DB. Thus, although experimental results have found a relationship between surface diffusion and layer-by-layer growth 23 24 , the diffusion mechanism of SiH 3 radicals has not yet been confirmed, and the previously proposed diffusion mechanisms thus remain controversial.…”

“…Our calculations find that, in addition to the adsorption of SiH 3 radicals via an abstraction-adsorption mechanism 22 , the DBs on the hydrogenated surface exhibit three important behaviors: (a) DBs migrate from a Si-H 2 in the upper layer to a Si-H 3 site in the same layer; (b) DBs migrate from a Si-H 2 site in the upper layer to a Si-H site in the lower layer; and (c) DBs remain on step sites in the lower layer for a long duration. Figure 4 shows a model for the layer-by-layer growth mechanism based on the above results.…”

Origin of Chemical Order in a-SixCyHz: Density-Functional Tight-Binding Molecular Dynamics and Statistical Thermodynamics Calculations

“…Thus, the number of atoms in the unit cell changes during CVD simulations. We have used the above algorithm in CVD simulations of a-Si:H and plasma etching simulations of a SiO 2 substrate and validated it by comparing our QCMD results with both DFT and experimental results. − , We calculate five trajectories of the impingement of a total of 80 CH 3 and SiH 3 radicals for up to 160.0 ps with each [CH 3 ]/([CH 3 ] + [SiH 3 ]) fraction.…”

Detection and characterization of singly deuterated silylene, SiHD, via optical spectroscopy

“…Modelling the role of SiH 2 in the fabrication of amorphous silicon thin films and polycrystalline silicon has also attracted significant interest. [1][2][3][4][5][6] Motivated primarily by the desire to garner a fundamental understanding of the properties of this simplest of silicon containing polyatomic molecules, and in part by the desire to develop a real time, in situ SiH 2 monitoring scheme, there have been numerous reported experimental and theoretical [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] studies of gas-phase SiH 2 , and to a lesser extent SiD 2 . In addition, SiH 2 is predicted to be abundant in circumstellar envelopes of carbon rich stars 49 and has been tentatively identified 50,51 via the detection of the 1 11 -0 00 pure rotational transition.…”