Plasma-assisted deposition of hydrogenated amorphous silicon (a-Si:H) thin films from silane-containing discharges is used extensively in large-area electronics, optoelectronics, and photovoltaics for fabrication of solar cells, thin-film transistors for flat panel displays, and detectors for medical imaging. Important properties of these films include their hydrogen content, surface composition, roughness, and crystallinity, which are governed by the interactions between the film surface and chemically reactive species arriving at the surface from the plasma. Prediction of surface and film properties, as determined by such plasma-surface interactions, requires properly developed coarse-grained dynamical simulations capable of capturing effectively surface length scales and deposition time scales. Toward this end, in this presentation, we report results of surface growth during Si plasma deposition according to kinetic Monte Carlo (KMC) simulations based on a first-principles database for radical-surface and adsorbed radical-radical interactions as identified from molecular-dynamics (MD) simulations of a-Si:H film growth.
We present KMC simulation results under growth conditions that render the SiH3 radical the dominant deposition precursor. The transition probabilities for the various kinetic events accounted for in the KMC simulations are based on first-principles density functional theory (DFT) calculations on the H-terminated Si(001)-(2x1) surface. The relevant surface transport and reaction processes have been identified in the MD simulations and include SiH3 diffusion, SiH3 chemisorption and insertion into Si-Si bonds, surface H abstraction reactions, surface hydride dissociation reactions, as well as SiH4 and Si2H6 desorption into the gas phase. In our DFT analysis, the H-terminated Si(001)-(2x1) surface is considered as a representative model of local chemical environment and atomic coordination on the growing film surfaces. Our DFT calculations are conducted within the generalized gradient approximation (GGA) and employ plane-wave basis sets, ultra-soft pseudopotentials, slab supercells, and the nudged elastic band method for determining optimal surface reaction/diffusion pathways and the corresponding activation energy barriers.
The predicted surface hydride, SiHx (x = 1,2,3), composition and overall hydrogen content are in very good agreement with experimental in situ composition measurements using attenuated total reflection Fourier transformed infrared (ATR-FTIR) spectroscopy on a-Si:H films deposited under similar growth conditions. The temperature dependence of the surface hydride composition and the trends that have been captured, over a range of deposition temperatures, are fully consistent with reported experimental observations. Si trihydrides are predicted to be the dominant surface hydride species at low temperatures (T ≤ 373 K), while dihydrides become dominant at 500 K and monohydrides become the most abundant surface species at higher temperatures (T ≥ 640 K). The surface hydrogen concentration is predicted to decrease drastically over the temperature range from 350 to 500 K and to remain almost constant over the range from 500 to 640 K. The surface dangling bond coverage is found to be practically independent of temperature and quite low, on the order of ~10-2.