  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.
