Bright Luminescence from Indirect and Strongly Bound Excitons in h-BN.

Abstract

A quantitative analysis of the excitonic luminescence efficiency in hexagonal boron nitride (h-BN) is carried out by cathodoluminescence in the ultraviolet range and compared with zinc oxide and diamond single crystals. A high quantum yield value of ∼50% is found for h-BN at 10K comparable to that of direct band-gap semiconductors. This bright luminescence at 215nm remains stable up to room temperature, evidencing the strongly bound character of excitons in bulk h-BN. Abinitio calculations of the exciton dispersion confirm the indirect nature of the lowest-energy exciton whose binding energy is found equal to 300±50 meV, in agreement with the thermal stability observed in luminescence. The direct exciton is found at a higher energy but very close to the indirect one, which solves the long debated Stokes shift in bulk h-BN.