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Abstract

This thesis presents studies of probability of heterogeneous recombination of oxygen and
hydrogen atoms on surfaces of different types of carbon-based materials. The probability of
heterogenous recombination is described with a quantity called recombination coefficient,
which is defined as the probability that an atom which impinges upon a solid surface
recombines. The recombination coefficient depends on the composition of the surface and
type of impinging atoms, but also on surface morphology, temperature, etc. While there are
several theoretical models of recombination of atoms on solid surfaces, the results –
probability of recombination – are complex functions dependant on many parameters that are
not always well known. Therefore they are applicable only to very well defined, crystalline
surfaces. In the case of amorphous carbon deposits (a-C and a-C:H), the surface is not well
defined and thus the models are not applicable and the recombination coefficients have to be
determined experimentally.
A frequent method of determining the recombination coefficient is the diffusion method
in which the subject of the study is the impact of the presence of the studied material to the
behaviour of density of neutral atoms in its vicinity. In the scope of the work presented in this
thesis the method used to determine the recombination coefficients was the spatial diffusion
method which is based on studying the impact of the studied material to the spatial
distribution of neutral atom density in its vicinity. The spatial distribution of the neutral atom
density was measured in a side vessel of the experimental chamber in which the studied
material was inserted. The basis of the mathematical model used in this thesis is the Smith's
diffusion model.
The source of oxygen and hydrogen neutral atoms was an inductively coupled
radiofrequency discharfe created at pressures between 30 Pa and 280 Pa in various mixtures
of argon and hydrogen or oxygen. The measured densities of oxygen and hydrogen atoms
ranged from 5 ⋅ 10[sup]20/m3 in 9 ⋅ 10[sup]21/m3. The neutral atom densities were measured with fibre
optic catalytic probes. An authomatised method for processing of the recorded data was
developed.
Recombination of various types of carbon based materials were measured:
recombination coefficients of graphie and amorphous carbon deposits (a-c and a-C:H). The
recombination coefficient of amorphous forms of carbon was found to be of the order of
magnitude of 10−3 for both hydrogen and oxygen atoms. The recombination coefficient of
graphite was found to increase with the increase of surface roughness. The lowest value of the
recombination coefficient of graphite for hydrogen atoms was by a factor of approximately 2
larger than the recombination coefficient of amorphous carbon. The difference in the values
of recombination coefficient is attributed to the differences in surface roughness. The
recombination coefficients of graphite for oxygen atoms were found to be an order of
magnitude larger than the recombination coefficients of amorphous carbon, which is
attributed to the influence of crystallinity.
Recombination coefficients of several other materials were also determined. The
method presented in this thesis was verified by comparing the measured values to results
reported by other authors.
The erosion of amorphous carbon deposits by oxygen atoms was also studied. Based on
the measured probabilities of the oxidative reaction and the heterogenous recombination the
latter was found to be a significantly more likely process than oxidation.