Substrate Modification during Chemical Vapor Deposition of hBN on Sapphire

A comparison of hexagonal boron nitride (hBN) layers grown by chemical vapor deposition on C-plane (0001) versus A-plane (112̅0) sapphire (α-Al₂O₃) substrate is reported. The high deposition temperature (>1200 °C) and hydrogen ambient used for hBN deposition on sapphire substantially alters the C-plane sapphire surface chemistry and leaves the top layer(s) oxygen deficient. The resulting surface morphology due to H₂ etching of C-plane sapphire is inhomogeneous with increased surface roughness which causes non-uniform residual stress in the deposited hBN film. In contrast to C-plane, the A-plane of sapphire does not alter substantially under a similar high temperature H₂ environment, thus providing a more stable alternative substrate for high quality hBN growth. The E_2g Raman mode full width at half-maximum (FWHM) for hBN deposited on C-plane sapphire is 24.5 ± 2.1 cm^-1 while for hBN on A-plane sapphire is 24.5 ± 0.7 cm^-1. The lesser FWHM standard deviation on A-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 C-plane and A-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 A-plane sapphire was measured to be 5 MV cm^-1, agreeing well with reports on mechanically exfoliated hBN flakes. Thus, under the typical growth conditions required for high crystalline quality hBN growth, A-plane sapphire provides a more chemically stable substrate.