The effects of post-annealing technology on crystalline quality and properties of hexagonal boron nitride films deposited on sapphire substrates

Highly c-axis oriented hBN films were deposited onto sapphire substrates by a low-pressure chemical vapor deposition method. The influences of the post-annealing technology, including annealing temperatures (1500-1700 degrees C), annealing time (5-30 min), and annealing ambient (N-2, H-2, and Ar), on the crystalline quality and optical and photoelectric properties of deposited hBN films have been investigated. The post-annealing treatment can not only improve the ordering of hBN domains and remove the defects in hBN films, but also promote the growth of hBN grains and decrease the grain boundaries. The optimal crystalline quality was obtained when hBN films were annealed in N-2 ambient at 1700 degrees C for 10 min. As for the hBN films annealed at the optimum conditions, average longitudinal and transverse sizes of hBN grains are estimated to be respectively about 27 and 108 nm, and the intrinsic absorption edge and the optical bandgap are severally 215 nm and 5.79 eV. Moreover, the photoelectric responsibility of an ultraviolet photodetector based on the annealed hBN films, whose cutoff wavelength is less than 224 nm, was increased by 8 times.