(370b) Kinetic Monte Carlo Simulations of Surface Growth during Plasma Deposition of Silicon Thin Films

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, SiH_x(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.