Thickness-dependent excitonic properties of hexagonal boron-nitride thin films

We report the excitonic properties of hexagonal boron-nitride (h-BN) thin films as a function of thickness using deep-ultraviolet photoluminescence spectroscopy and spectroscopic ellipsometry for optical absorption analysis. The peak positions of the defect-induced donor-acceptor PL emissions and the band-to-band exciton absorption exhibit a blueshift with increasing thickness or an increase in the effective dielectric constant of the surrounding environment. The optical absorption spectra analyzed using the Elliott model, combined with renormalized band-gap calculations, reveal an exciton binding energy of 1.04 eV for monolayer h-BN, which decreases to 0.16 eV in h-BN single crystal due to environmental screening effects on the electron-hole interactions. Furthermore, both the band-gap value and Urbach energy decrease as the film thickness increases. The optical absorption coefficient derived from spectroscopic ellipsometry measurement of monolayer h-BN is directly compared with results from the first-principles GW plus Bethe-Salpeter equation calculations.