Chemistry Reference
In-Depth Information
and (5.4) modifies to
dn
ads
dt
=
n
0
t
−
n
0
t
chemisorption
.
j
σ
ads
n
0
(
1
−
)
−
(5.8)
This model yields a temperature-dependent chemisorption rate: If the substrate tem-
perature increases, the rate of thermal desorption from the weakly adsorbed state
increases (see (5.4) and (5.5)) leading to a decrease of
. Consequently, the growth
rate decreases, because it is directly proportional to the coverage in the weakly
adsorbed state. This is typical for expitaxial growth of crystalline silicon [4]. The
growth precursors adsorb on terraces on the surface and diffuse to surface steps,
where they are trapped, promoting film growth. At high surface temperatures, the
thermal desorption becomes too fast compared with the time the precursors need
to reach the chemisorption sites at the steps via surface diffusion. In consequence,
the growth rate decreases with increasing substrate temperature. Additional surface
reactions, like etching of the growing film as well as etching of the weakly adsorbed
state, can be described as in (5.6).
5.4 SURFACE DIFFUSION
Impinging species, adsorbing into a weakly adsorbed state, may not be able to
thermalize with the surface upon impact, but diffuse on the surface as
hot precursors
[5]. This has several consequences for the interpretation of the macroscopic growth
rate. The surface diffusion in the
hot precursor
state can be incorporated into the
chemisorption model. A typical example for adsorption via a
precursor state
is the
adsorption of atomic hydrogen on hydrogen-terminated silicon surfaces [6-9].
Themigrationofatomsormoleculesalongthesurfaceisoneofthemostimportant
elementary steps of gas-surface interaction, reactive or nonreactive. The main reason
for surface diffusion is the crystal structure and chemical composition of the substrate.
Depending on the structure of the crystal face, diffusion coefficients may vary by
orders of magnitude. Diffusion rates parallel to steps are greater than diffusion rates
perpendicular to them. Both diffusion along the surface and diffusion into the near-
surface layers of the solid are possible. In the latter case, one speaks of absorption or
bulk diffusion.
5.5 ENERGY ACCOMMODATION
Plasma particles striking the solid surface exhibit a wide energy spectrum. In addi-
tion to low-energetic neutrals (atoms, molecules, radicals), there also exist particles
with higher energy (
E
1 eV) that can be distributed on the different excitation
modes.
If the energy of an incoming particle is too high for entering an adsorption
process, or if the energy is too small for sputtering or implantation, respectively, or if
it finds no suitable surface site, the particle will simply be reflected at the surface. Due
to the energy transfer at such elastic scattering the substrate is heated until a thermal
