Boron-carbon (BxC) thin films enriched in 10B are potential neutron converting layers for 10Bbased solid-state neutron detectors given the good neutron absorption cross-section of 10B atoms in the thin film. Chemical Vapour Deposition (CVD) of such films faces the challenge that the maximum temperature tolerated by the aluminium substrate is 660 °C and low temperature CVD routes for BxC films are thus needed. This thesis presents the use of two different organoboron precursors, triethylboron –B(C2H5)3 (TEB) and trimethylboron – B(CH3)3 (TMB) as single-source precursors for CVD of BxC thin films. The CVD behaviour of TEB in thermal CVD has been studied by both BxC thin film deposition and quantum chemical calculations of the gas phase chemistry at the corresponding CVD conditions. The calculations predict that the gas phase reactions are dominated by ?-hydride eliminations of C2H4 to yield BH3. In addition, a complementary bimolecular reaction path based on H2-assisted C2H6 elimination to BH3 is also present at lower temperatures in the presence of hydrogen molecules. A temperature window of 600 – 1000 °C for deposition of X-ray amorphous BxC films with 2.5 ? x ? 4.5 is identified showing good film density (2.40 – 2.65 g/cm3) which is close to the bulk density of crystalline B4C, 2.52 g/cm3 and high hardness (29 – 39 GPa). The impurity level of H is lowered to < 1 at. % within the temperature window. Plasma chemical vapour deposition has been studied using TMB as single-source precursor in Ar plasma for investigating BxC thin film deposition at lower temperature than allowed by thermal CVD and further understanding of thin film deposition process. The effect of plasma power, total pressure, TMB and Ar gas flow on film composition and morphology are investigated. The highest B/C ratio of 1.9 is obtained at highest plasma power of 2400 W and TMB flow of 7 sccm. The H content in the films seems constant at 15±5 at. %. The B-C bond is dominant in the films with small amount of C-C and B-O bonds, which are likely due to the formation of amorphous carbon and surface oxidation, respectively. The film density is determined as 2.16±0.01 g/cm3 and the internal compressive stresses are measured to be <400 MPa.

Thin films of Boron Nitride (BN) and Boron Carbide (BC) possess properties that make them attractive for various applications. Epitaxially grown BN exhibits potential for optoelectronic devices, as piezoelectric materials, and graphene technology. Epitaxial BC is a semiconductor that could allow bandgap tuning and has potential applications in thermoelectric and optoelectronic devices. Both BN and BC material systems, generally deposited using chemical vapour deposition (CVD), are limited by the lack of control in depositing epitaxial films. In my thesis work, I have studied the evolution of various crystal phases of BN and BC and the factors that affect them during their CVD processes. I deposited and compared the growth of BN on Al2O3 (0001), (11 2 over bar 0), (1 1 over bar 02) and (10 1 over bar 0) substrates and used two organoboranes as boron precursors. Only Al2O3(11 2 over bar 0) and Al2O3 (0001) rendered crystalline films while the BN growth on the remaining substrates was X-ray amorphous. Furthermore, the less investigated Al2O3(11 2 over bar 0) had better crystalline quality versus the commonly used Al2O3 (0001). To further understand this, I studied crystalline BN thin films on an atomic scale and with a time evolution approach, uncovering the influence of carbon on hexagonal BN (h-BN). I showed that h-BN nucleates on both substrates but then either polytype transforms to rhombohedral-BN (r-BN) in stages, turns to less ordered turbostratic-BN or is terminated. An increase in local carbon content is the cause of these changes in epitaxial BN films during CVD. From the time evolution, we studied the effect of Al2O3 modification on h-BN nucleation during CVD. The interaction between boron and carbon during BN growth motivated studies also on the BxC materials. BxC was deposited using CVD at different temperatures on 4H-SiC(0001) (Si-face) and 4H-SiC(000 1 over bar) (C-face) substrates. Epitaxial rhombohedral-B4C (r-B4C) grew at 1300 °C on the C-face while the films deposited on the Si-face were polycrystalline. Comparing the initial nucleation layers on both 4H-SiC substrates on an atomic scale we showed that no interface phenomena are affecting epitaxial r-B4C growth conditions. We suggest that the difference in surface energy on the two substrate surfaces is the most plausible reason for the differences in epitaxial r-B4C growth conditions. In this thesis work, I identify the challenges and propose alternative routes to synthesise epitaxial BN and B4C materials using CVD. This fundamental materials science work enhances the understanding of growing these material systems epitaxially and in doing so furthers their development.

Boron nitride thin films can be deposited on different substrates using techniques such as plasma deposition, ion beam deposition and reactive sputter deposition.