The optical absorption and photoconductivity spectra of hexagonal boron nitride single crystals

Abstract

Recently, a strong cathodoluminescence at 215 nm (photon energy 5.765 eV) has been reported on pure HPHT hexagonal boron nitride (hBN) single crystals (K. Watanabe et al., Nature Mater. 3, 404 (2004)). For the first time, we measured the photocurrent and optical absorption spectra of these new hBN crystals in a broad spectral range 1–6.6 eV. Our measurements support the previously published results, especially that the single crystal hBN has a direct band gap about 5.95 eV, and a rather strong exciton binding energy that allows excitons to exist even at room temperature. We found three new opto-electronic transitions at 2.7, 3.6 and 5.4 eV and observed the oscillatory photoconductivity in 2.7–3.6 eV region.