Ultra-thin boron nitride films by pulsed laser deposition: Plasma diagnostics, synthesis, and device transport
This work describes, for the first time, a pulsed laser deposition (PLD) technique for growth of large area, stoichiometric ultra-thin hexagonal and amorphous boron nitride for next generation 2D material electronics. The growth of boron nitride, in this case, is driven by the high kinetic energies and chemical reactivities of the condensing species formed from physical vapor deposition (PVD) processes, which can facilitate growth over large areas and at reduced substrate temperatures. The use of optical emission spectroscopy during plasma growth provides insight into chemistry, kinetic energies, time of flight data, and spatial distributions within a PVD plasma plume ablated from a boron nitride (BN) target by a KrF laser at different pressures of nitrogen gas. Time resolved spectroscopy and wavelength specific imaging were used to identify and track atomic neutral and ionized species including B +, B*, N+, N*, and molecular species including N2*, N2+, and BN. Formation and decay of these species formed both from ablation of the target and from interactions with the background gas were investigated and provided insights into fundamental growth mechanisms of continuous, amorphous boron nitride thin films. By selectively choosing substrates that can facilitate epitaxial hexagonal growth, synthesis of ultra-thin, few-layer hexagonal boron nitride ( h-BN) was possible using the PLD technique. This process permits growth of thin, polycrystalline h-BN at 700°C, a much lower temperature than that required by traditional growth methods, most typically chemical vapor deposition (CVD). Analysis of the as-deposited films reveals epitaxial-like growth on the nearly lattice matched HOPG substrate, resulting in a nanocrystalline h-BN film with grain sizes of approximately 5 nm, and amorphous BN (a-BN) on the non-lattice matched sapphire substrates, both with film thicknesses of 1.5–2 nm. Stoichiometric films with boron-to-nitrogen ratios of unity were achieved by adjusting the background pressure within the deposition chamber and the distance between the target and substrate. Conductive atomic force microscopy (C-AFM) measurements of electron tunneling behavior depict a uniform The reduction in deposition temperature and formation of stoichiometric, large-area h-BN films by PLD provides a process that is easily scaled-up for two-dimensional dielectric material synthesis and also presents a possibility to produce very thin and uniform a-BN. Little is known as to how the degree of crystallinity, surface roughness, and other properties of the Graphene device performance including electron and hole mobility, as well as Dirac point, were substantially influenced by the presence of the dielectric material. In few-layer graphene films transferred to traditional h-BNSiO2 substrates, as well as SiO2 substrates coated with 5 nm a-BN, the transport properties in graphene were significantly suppressed in comparison to the annealed nanocrystalline h-BN, presumably due to increased scattering events. The weak van der Waals terminated h-BN films suppressed these scattering events from the presence of high energy surface optical phonon modes and lack of Coulombic scattering. A two-fold improvement in average hole mobility and a Dirac point shift from > 60V to approximately 3.5V indicate that the influence of hexagonal crystallinity in the channel material is vital for high performance graphene devices. Thermal conductivity measurements of as-deposited a-BN and annealed h-BN were made possible by a nanofabricated freestanding bridge configuration, where the enhanced surface diffusion allowed for 100 nm h-BN grain formation at 600°C. Infrared microscopy and a one-dimensional heat transport model were used to measure both structural configurations of a-BN and h-BN to have inplane thermal conductivities of 5 W m−1 K−1 and 65 W m−1 K−1, respectively. In this study, the amorphous boron nitride was investigated as a means to enhance thermal conductance at dielectric/metal interfaces. Due to the atomic-scale roughness, covalently terminated bonding, as well as the high Debye temperature of BN, engineering a high contact thermal conductance is possible using different deposition techniques as a function of metal Debye temperatures. A high thermal conductance of 130 MW m−2 K−1 was measured on an a-BN/aluminum metal interface when the Al was prepared under the proper DC conditions, with a reduction in almost 50% when prepared in sub-optimum conditions using high power impulse magnetron sputtering (HIPIMS). (Abstract shortened by ProQuest.)
Fisher, Purdue University.
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