Chemical vapor deposition, characterization, and thermodynamic phase diagram calculations of sp2-boron nitride thin films

The objectives of the research presented in this Thesis have been to (1) advance the field of homo- and heteroepitaxial growth of sp²-hybridized boron nitride (sp²-BN) thin films via chemical vapor deposition (CVD) and (2) evaluate the usefulness of utilizing calculated equilibrium phase diagrams for the prediction of sp²-BN phases - namely hexagonal BN (h-BN) and rhombohedral BN (r-BN) – prepared by thermal CVD. The deposited sp²-BN thin films related to objective (1) were characterized to determine the effects of implemented CVD growth parameters on film growth rates, the substrate-film interfacial microstructure, bulk film microstructure and crystallinity, chemical composition and bonding characteristics, and surface morphology and roughness. The calculated equilibrium phase diagrams related to objective (2) were compared with sp²-BN CVD processes from experimental literature to determine favorable thermodynamic conditions for the deposition of sp²-BN and to discover discrepancies related to the kinetics of CVD growth and sp²-BN phase transformations.

The first project related to the achievement of objective (1) of this Thesis was the growth of nanocrystalline sp²-BN thin films on (0001) 4H-SiC substrates at 1030 ^oC via continuous flow and discontinuous flow-modulated CVD techniques using diborane (B₂H₆) and ammonia (NH₃) as the B- and N sources, respectively. The latter technique enabled observations of both the effect of hydrogen purge steps between precursor injections and the length of injection times for B₂H₆ on the stoichiometry and microstructure of the films. Stoichiometric BN was achieved in all films grown continuously within the N/B gas phase ratio range of 20 – 200; this was not observed for the discontinuously grown films unless both the B₂H₆ flow rate and the injection time were minimized. Cross-section transmission electron microscopy (TEM) of films grown both continuously and discontinuously at N/B = 200 and using short B₂H₆ injection times relative to that of NH₃ for the latter process route revealed the initial growth of ~4 nm thick partially ordered sp²-BN layers. A transition zone then formed containing randomly oriented polycrystalline grains. Excess B incorporated into the discontinuously grown films during long B₂H₆ injection times resulted in single layer mixtures of amorphous and sp²-BN without anyobserved ordering.

In the second project related to objective (1) of this Thesis, nanocrystalline sp²-BN thin films were grown for 2 and 4.5 hours on mechanically polished polycrystalline pyrolytic boron nitride (PBN) substrates at 1060 ^oC via CVD using B₂H₆ and NH₃ as the B- and N sources, respectively. The use of an N/B gas phase ratio of 200 resulted in sp²-BN thin films having a stoichiometric surface chemistry, as determined by x-ray photoelectron spectroscopy. Crosssectional TEM revealed the initial growth of ~4 nm thick partially ordered [0001] sp²-BN layers regardless of the crystallographic orientation of the sets of layers in the substrate. A transition zone then formed within the deposited films that contained randomly oriented polycrystalline grains and that culminated in the formation of protrusions. Atomic force microscopy verified an increase in roughening of the surfaces of the films with an increase in growth time and the associated thickness.

In the first project related to objective (2) of the Thesis, equilibrium thermodynamic calculations were performed to generate diagrams indicating the phase fields wherein either h- or r-BN can be deposited via chemical vapor deposition as a function of temperature, choice of B source and N/B ratio derived from NH₃ and the B-source. Similar diagrams calculated using experimental conditions employed by groups who have synthesized r-BN films revealed that both in experiment and equilibrium, the choice of B-source strongly affects the size of the singlephase field for r-BN and, in general, deposition of r-BN can be realized at temperatures more than 100^oC below that predicted by equilibria.

The presence of C in B(C₂H₅)₃ makes possible its incorporation into hexagonal boron nitride (h-BN) thin films grown during CVD under select process conditions. In the second project related to objective (2) of the Thesis, a series of CVD phase diagrams that indicate the regions of stability of the phases of h-BN, C and B₄C as a function of temperature and mole ratio of reactants (N/B) under set pressures and H₂ diluent-to-reactant mole ratios (H2/(B + N)) were calculated and analyzed for the TEB/NH3/H2 system. The equilibrium calculations showed that within the total pressure range of 0.01 -100 Torr, increasing the H₂ content in the TEB/NH₃/H₂ gas mixture tends to suppress the stability of C throughout a 400 – 1600° C temperature range, but it also suppresses the stability of h-BN at higher temperatures. Further, it was determined that increasing the total system pressure suppresses the occurrence of C as a second phase while expanding the stability of h-BN. The thermodynamic equilibrium results were compared to the results of experimental investigations involving the CVD synthesis of sp²-BN as a means to evaluate their usefulness as a guide for avoiding the co-deposition of C and B₄C with h-BN.