Chemistry Reference
In-Depth Information
The surface loss probability of SiH
3
radicals on amorphous hydrogenated silicon
films has been measured to
β
=
0.25 by several authors with various techniques
[16-19]. This surface loss probability is also independent of substrate temperature
T
S
. However, the growth rate is temperature independent up to 350
◦
C, corre-
sponding to a sticking coefficient of
g
0.1 and increases by a factor of 2.5 above
this temperature, corresponding to a sticking coefficient of
g
=
0.25 [17]. It is
believed that the growth of a-Si:H films is due to the adsorption of SiH
3
radicals
at dangling bonds on the surface. Below 350
◦
C, the surface of the a-Si:H film
is hydrogen-terminated and incoming atomic hydrogen or SiH
3
radicals abstract
surface hydrogen to form H
2
or SiH
4
, respectively. The resulting dangling bonds
act as chemisorption sites for incoming SiH
3
radicals [27-30]. Above a substrate
temperature of
=
350
◦
C, hydrogen desorbs from SiH surface groups, creating new
open sites at the growing film surface, which act as additional adsorption sites
for incoming SiH
3
radicals. As a consequence, the growth rate increases with
increasing substrate temperature [17].
The amorphous hydrogenated silicon surface remains hydrogen oversaturated
during growth at typical growth temperatures of 250
◦
C, as known from infrared
absorption measurements [30,31]. The defect density at this growing film surface
is below 1% as measured by electron spin resonance measurements [32]. The
experimental observation of a very low density of dangling bonds at the surface
is in contradiction to the sticking coefficient of SiH
3
radicals of
∼
0.1. The stick-
ing coefficient of SiH
3
radicals of 0.1 would imply an average dangling bond
density at the surface of the order of 10%, if incoming SiH
3
radicals directly
chemisorb at an open bond upon impact. This problem was resolved by intro-
ducing a hot precursor state for SiH
3
prior to chemisorption at a dangling bond:
SiH
3
adsorbs in a hot precursor state and diffuses on the surface to an open bond,
where it chemisorbs. The fast surface diffusion of SiH
3
was also supported by
molecular dynamic simulation, which indicated a migration length on the surface
of
∼
20 Å [33].
This description of a-Si:H film growth is based on the paradigm that SiH
3
is
incorporated in the film only via chemisorption at dangling bond sites at the sur-
face. The adsorption of SiH
3
on deuterated a-Si:D surfaces has been studied by von
Keudell and Abelson [34] using in situ real-time infrared absorption spectroscopy.
By following the isotope exchange, they observed that SiH
3
directly inserts into
strained surface bonds at the growing film surface. The insertion reaction does
not refute the
hot precursor
model. However, the experimental observation of a
sticking coefficient of 0.1 for SiH
3
on the hydrogen saturated growing film sur-
face can be due to the fact that the density of strained bonds at the surface is
expected to be much higher compared to that of the dangling bonds (in the case
of the silicon (100) surface, one Si-Si bond per Si atom in the first monolayer is
strained). In that case, a reaction step based on the fast surface diffusion is no
longer necessary, in order to explain the observed sticking coefficient of 0.1. The
insertion model has been corroborated later by molecular dynamics modeling of
SiH
3
chemisorption [35]. Recently, the dangling bond density at a growing a-Si:H
film surface has been measured using cavity ring-down spectroscopy in reflection
mode by Kessels [36].
They showed that the steady-state dangling bond density is by orders of mag-
nitude too low to explain the observed absolute silicon growth rate. Based on these
results, the conventional model for a-Si:H film growth based on SiH
3
chemisorp-
tion at dangling bond sites has to be revised. Amorphous hydrogenated silicon
∼
