Plasma-assisted physical vapor deposition of polycrystalline hexagonal boron nitride films

Two-dimensional hexagonal boron nitride (2D-hBN), also known as white graphene, is an electrically insulating wide-band-gap material with several emerging applications. However, most of the previous systematic work has focused deposition of cubic BN films, while the synthesis of wafer-scale hBN films with desired properties remains a challenge. Here, we present results of our ongoing systematic study of polycrystalline hBN films with thicknesses in a range of 100-6000 nm deposited by variants of reactive magnetron sputtering with a radiofrequency (RF) driven discharge. We describe how the plasma discharge characteristics and, hence, resultant major film properites can be controlled by the magnetron source design, the confining magnetic field, and process parameters such as the working gas pressure (controlling landing neutral atom ballistics and energetics), subtrate temperature (adatom mobility), and substrate bias (bombarding ion energy). Even without epitaxy, with substrates held close to room temperautre, hBN films are polycrystalline, characterized by a FWHM of the major E_2g Raman vibrational mode (1370 cm^-1) in the range of 40 – 100 cm^-1, depending on deposition conditions. The FWHM reduces to ~30 cm^-1 when a higher deposition temperature of 600-800 ^oC is used. Interestingly, all as-grown films are polycrystalline (turbostratic, with asymmetrically stacked layers) rather than amorphous even for room temperature and high deposition pressure of 50 mTorr characterized by low landing atom energetics. These film growth and characterization experiments are guided by results of in-situ plasma diagnostics, which greatly aids deposition process development.<br/>This work was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344.