Near band gap photoluminescence properties of hexagonal boron nitride

TL;DR

This study investigates near band-gap photoluminescence in hexagonal boron nitride, revealing two emissions linked to excitons and defect states, and discusses their behavior with excitation energy.

Contribution

It provides detailed spectral analysis of near band-gap luminescence in hexagonal boron nitride, identifying exciton and defect-related emissions and their excitation-dependent shifts.

Findings

Identification of two main emission bands at 5.5 eV and 5.3 eV.

Assignment of the 5.3 eV band to quasi donor-acceptor pair states.

Observation of a large blue shift in the 5.3 eV emission with increasing excitation energy.

Abstract

Near band-gap luminescence (hnu > 5 eV) of hexagonal boron nitride has been studied by means of the time- and energy-resolved photoluminescence spectroscopy method. Two emissions have been observed at 5.5 eV and 5.3 eV. The high-energy emission at 5.5 eV is composed of fixed sub-bands assigned to bound excitons at 5.47 eV, 5.56 eV and 5.61 eV. The non-structured low-energy emission at 5.3 eV undergoes a large blue shift (up to 120 meV) with a linear slope <1 with increasing excitation energy Eexc. At Eexc>5.7 eV, the band position is fixed and marks the transition from the Raman to the photoluminescence regime. We assign the 5.3 eV band to quasi donor-acceptor pair (q-DAP) states due to electrostatic band fluctuations induced by charged defects. The shift is explained by photo-induced neutralization of charged defect states. The absence of contribution to the q-DAP luminescence from exciton suggests the existence of a large exciton binding energy, which is qualitatively consistent with theoretical predictions.