| dc.description.abstract | There is a growing interest in synthesizing new materials with unique
mechanical properties like hardness or electrical and optical properties. For this
purpose, Boron-Carbon-Nitrogen (BCN) ternary phase diagram promises new
materials with potentially unique properties, such as variable band gap
semiconductors or phases with extreme hardness. On the other hand, the
physical or mechanical properties of these new BCN materials strongly depend
on the chemical environment of the atoms and their atomic structure. In this
thesis, atomic structure and chemical environment of the atoms in BCN thin
films were investigated. BCN films were synthesized by Reactive Magnetron
Sputtering (RMS) technique from a B4C target. Various process parameters of
synthesis were changed during deposition, such as the substrate bias, substrateto-target
distance and N2 flow.
The effect of process parameters are investigated with respect to their
fundamental effects on the growing BCN films. Several sets of experiments
were planned and conducted in order to gain insight as per their effect on the
final chemistry and atomic structure. The characterization of the chemical
composition of the films was done using data from Infrared Spectroscopy, Raman Spectroscopy, X-ray Photoelectron Spectroscopy, X-Ray Diffraction,
and Electron Energy Loss Spectroscopy. Also, electron transparent thin crosssections
from the BCN films were prepared using focused-ion beam technique
for conducting High Resolution Transmission Electron Microscopy analysis for
the verification of atomic structure.
In the first series, named B series, the energy is supplied to growing film by
applying a radio frequency generated d.c. bias on the substrate. Magnitude of
the applied bias was changed throughout the series. In the second and third
series, namely P and D series, the effect of substrate-to-target distance was
investigated. In these series, BCN and BN films were deposited on substrates
that were located at different distances from the target surface. In sub-series,
effect of, i) the magnitude of applied bias, ii) type of applied substrate bias on
the chemistry of the BCN films were scrutinized. In addition, the effect of
atomic composition on the bonding preferences was studied. For this purpose, a
series of BCN films were r.f. sputter deposited from B4C target with different
N2 flow rate at the process gas.
After the careful analysis of the data from mainly the spectroscopic techniques,
several important results were obtained. First, a prevailing bonding preference,
i.e. phase segregation, was observed in the films deposited regardless of the
process parameters used, such that a dominant presence of B-N and C-C or C-N
bonding were observed in the films. Furthermore, increasing the substrate bias
or decreasing the substrate-to-target distance resulted in the atomic ordering and
layered (turbostratic) BCN films. Examination of the spectroscopic data in
detail also indicated that the individual layers were made out of separate
domains of h-BN like and graphitic like carbon regions, which supports the
phase-segregation assertion. Two main regimes are identified for the growth of BCN films;
thermodynamically or kinetically controlled regimes. BCN films synthesized
with large substrate bias or close to target surface were overall more ordered as
the adatoms arriving on the substrate surface had enough energy to diffuse and
find energetically most favorable sites. Such a case could be termed as
thermodynamically controlled regime. In the opposite case, where adatoms
were in a diffusion-limited environment, the final chemistry and structure was
dictated by the kinetics. However, the prevalence of B-N bonding in both cases,
and failure to observe hybridized chemistry suggests that bonding energy
consideration is the major deciding factor for the chemistry of BCN films.
As a conclusion, the work presented herein suggests that phase segregation in
BCN films reveal as an innate character, while hybridization is not observed in
the process parameter space explored. The main reason for this is the relative
energies of the B-N and C-C bonding. | en_US |
