Chemistry and structure of sputter deposited boron-carbon-nitrogen thin films

Bengü, Erman
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Bilkent University
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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.

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