(378e) First-Principles-Based Kinetic Modeling of Surface Growth During Plasma Deposition of Silicon Thin Films

Hydrogenated
amorphous silicon (a-Si:H) thin films are used extensively in the fabrication
of large-area electronic and optoelectronic devices. Some of their important
applications are as thin-film transistors for active-matrix displays,
multiple-junction photovoltaic solar cells, active pixels in X-ray imaging,
photoreceptors, and sensing devices. Growth of these a-Si:H thin films by
plasma enhanced chemical vapor deposition (PECVD) has been an active area of
research for over two decades. In spite of the substantial amount of
information that has been generated from experiments, the growth models
proposed so far have been able to neither explain fully the complex growth
process nor account for the surface composition and its temperature dependence,
as well as the relative roles of coordination defects in the growth of a-Si:H
thin films.

Predictive
modeling of surface growth, as determined by plasma-surface interactions during
PECVD, requires properly developed coarse-grained dynamical simulations capable
of capturing effectively surface length scales and deposition time scales.
Toward this end, we have carried out a computational study of the growth mechanisms
of plasma deposited a-Si:H thin films based on kinetic Monte Carlo (KMC)
simulations according to a transition probability database constructed by
first-principles density functional theory (DFT) calculations. Based on the
results of our study, we propose a comprehensive model of a-Si:H thin-film
growth by plasma deposition under conditions that make the silyl radical the
dominant deposition precursor. The transition probabilities for the various
kinetic events accounted for in the KMC simulations are based on DFT
calculations on the H-terminated Si(001)-(2x1) surface; this surface is
considered as a representative model of the local chemical environment and
atomic coordination on the growing film surfaces. The relevant surface
transport and reaction processes were identified in molecular-dynamics
simulations of SiH₃ radical impingement on the growth surface and
include SiH₃ surface 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. The DFT calculations have been conducted within
the generalized gradient approximation (GGA) and employed 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. Results are presented for two types
of KMC simulations. The first one employs a fully ab initio database of
activation energy barriers for the surface rate processes involved and is
appropriate for modeling the very early stages of growth. The second one uses
approximate rates for all the relevant processes to account properly for the
effects on the activation energetics of interactions between species adsorbed
at neighboring surface sites; it is appropriate for modeling later stages of
growth toward a steady state of the surface composition.

We
find that the relative roles of surface coordination defects are crucial in
determining the surface composition of plasma deposited a-Si:H films and should
be properly accounted for. Furthermore, we explain adequately the critical
roles of both dangling bonds and floating bonds during a-Si:H film growth and
their relative contributions to the growth process over a broad range of
substrate temperatures. From the set of possible surface rate processes, of
particular importance is the radical dissociative adsorption mediated by Si
floating bonds along the reaction pathway that forms silicon dihydrides on the
surface. The KMC predictions for the temperature dependence (over the range
from 300 K to 700 K) of the surface concentration of SiH_x(s) (x =
1,2,3) species, the surface hydrogen content, and the surface dangling-bond
coverage are compared with experimental measurements on a-Si:H films deposited
under operating conditions that render the SiH₃ radical the dominant
deposition precursor. The predictions of both KMC simulation types are
consistent with the reported experimental data, which are based on in situ
attenuated total reflection Fourier transformed infrared (ATR-FTIR)
spectroscopy. The agreement of our computational modeling predictions with the
experimental measurements is good quantitatively and excellent in terms of the
qualitative trends captured.