Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire.

A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on -plane (0001) versus -plane (112̅0) sapphire (α-AlO) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the -plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H etching of -plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to -plane, the -plane of sapphire does not alter substantially under a similar high temperature H environment, thus providing a more stable alternative substrate for high quality hBN growth. The E Raman mode full width at half-maximum (FWHM) for hBN deposited on -plane sapphire is 24.5 ± 2.1 cm while for hBN on -plane sapphire is 24.5 ± 0.7 cm. The lesser FWHM standard deviation on -plane sapphire indicates uniform stress distribution across the film due to reduced undulations on the surface. The photoluminescence spectra of the hBN films at 300 and 3 K, obtained on -plane and -plane sapphire exhibit similar characteristics with peaks at 4.1 and 5.3 eV reported to be signature peaks associated with defects for hBN films deposited under lower V/III ratios. The dielectric breakdown field of hBN deposited on -plane sapphire was measured to be 5 MV cm, agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, -plane sapphire provides a more chemically stable substrate.