Ultra-high Temperature Growth of Layered Hexagonal Boron Nitride on Sapphire by Molecular Beam Epitaxy

Hexagonal boron nitride (hBN) films were grown by ultra high temperature molecular beam epitaxy (MBE) on insulating sapphire substrates. Characterization of the films reveals smooth, highly crystalline and layered hBN grown on the substrate at ultra-high substrate thermocouple temperatures
$(> 1600^{\circ}\mathrm{C})$
and low impinging boron flux; films grown under different conditions exhibited rough and polycrystalline morphology. Chemical analysis by x-ray photoelectron spectroscopy (XPS), combined with analysis of in-situ reflection high energy electron diffraction (RHEED) patterns suggest the presence of a thin interfacial layer of aluminum nitride between the bulk sapphire substrate and the hBN layers caused by sapphire surface nitridation during early stages of growth. Scanning transmission electron microscopy (STEM) analysis confirmed the uniform and layered morphology of the hBN films as well as the nitridated sapphire surface. Absorption spectroscopy demonstrated a sharp absorption peak at ~6.1 eV, characteristic of hBN. Tauc's method allows the bandgap of the material to estimated from the absorption spectrum; a value of ~5.8 eV was obtained for the indirect bandgap of hBN, close to the 5.9 eV reported value. This work represents a step toward the integration of hBN into existing, mature epitaxial platforms and devices.