1 Exciton and interband optical transitions

Near band gap photoluminescence (PL) of hBN single crystal has been studied at cryogenic temperatures with synchrotron radiation excitation. The PL signal is dominated by the D-series previously assigned to excitons trapped on structural defects. A much weaker S-series of self-trapped excitons at 5.778 eV and 5.804 eV has been observed using time-window PL technique. The S-series excitation spectrum shows a strong peak at 6.02 eV, assigned to free exciton absorption. Complementary photoconductivity and PL measurements set the band gap transition energy to 6.4 eV and the Frenkel exciton binding energy larger than 380 meV. 2 Hexagonal boron nitride (hBN) is an anisotropic wide band gap semiconductor, constituted of graphite-like sheets with an hexagonal structure. It is nowadays one of the most promising materials for developing far UV light emitting devices.[1-3] Its electronic and optical properties have been the subject of many publications in the last decade. The understanding of electronic band and exciton structure of hBN is also a particularly important issue since it is considered as a reference system for BN nanotubes. [4-7] According to th