AbstractsChemistry

Cubic boron nitride (cBN) is a synthetic and an intrinsically super-hard material with the second highest hardness and thermal conductivity next to diamond. Cubic BN with its tetrahedral sp3 structure is isostructural and isoelectronic to diamond. Diamond is far the most extreme material but cBN surpasses diamond in some properties. Unlike diamond, cBN is chemically inert to molten ferrous materials and resistant to oxidation up to 1200 °C at atmospheric conditions. These properties make cBN more attractive than diamond in many mechanical and tribological applications. Cubic BN has the widest bandgap (6.2 ± 0.2 eV) among III-V semiconductors, high electron mobility and hole mobility. In contrast to diamond it maintains high resistivity even at extreme temperatures. It can be doped for both p- and n-type conductivities and is piezoelectric. Therefore cBN is a potential candidate for construction of high-temperature, high power and high-speed electronic devices operating in harsh environment. Although cBN films are very attractive they have not been implemented into practical applications because of many accoutered problems like poor quality, low phase purity, high compressive stress (up to 20 GPa), poor adhesion to the substrates, small deposited area and limited film thickness (≤200 nm). These cBN properties suggest that the development of novel technologies for synthesis of high-quality, large-area and thick cBN films is very challenging. This thesis presents a viable route towards the practical applications of cBN coatings through better understanding of the nucleation and growth behavior of the deposited cBN thin films. The aforementioned limitations have been overcome by the introduction of fluorine chemistry mediating by a complex He-Ar-N2-BF3-H2 plasma induced in an electron cyclotron resonance (ECR) system. This deposition method enables the preparation of thick cBN films (>1 μm) with low internal stress and over large areas of silicon (Si) substrates. Fourier transform infrared (FTIR) spectroscopic examination shows that the films are composed of >80 % cBN phase. Detail analysis of BN structures employing high resolution transmission electron microscopy (HRTEM) and transmission electron energy loss spectroscopy (EELS) revealed that a pure cBN layer is formed on top of an initial graphitic BN layer which accounts for the insignificant hexagonal BN (hBN) signal in FTIR spectra. The characteristic transverse optical (TO) and longitudinal optical (LO) phonons modes of cBN, generally appearing in the spectra of cBN crystals synthesized by high pressure high temperature (HPHT) methods, were found in our cBN films which are the indicatives of large cBN crystallites. It was found that the ion bombardment, gas composition, substrate temperature and growth time significantly affect the phase purity and crystallinity of cBN films. However, the growth of cBN still follows the typical pattern with a graphitic precursor, amorphous/turbostratic BN (aBN/tBN) at the Si substrate interface. This work however illustrates that…