The origin of deep-level impurity transitions in hexagonal boron nitride

摘要

Deep ultraviolet photoluminescence (PL) emission spectroscopy has been employed to investigate the origin of the widely observed deep level impurity related donor-acceptor pair (DAP) transition with an emission peak near 4.1 eV in hexagonal boron nitride (h-BN). A set of h-BN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) under different ammonia (NH_3) flow rates to explore the role of nitrogen vacancies (V_N) in the deep-level transitions. The emission intensity of the DAP transition near 4.1 eV was found to decrease exponentially with an increase of the NH_3 flow rate employed during the MOCVD growth, implying that impurities involved are V_N. The temperature-dependent PL spectra were measured from 10 K up to 800 K, which provided activation energies of ～0.1 eV for the shallow impurity. Based on the measured energy level of the shallow impurity (～0.1 eV) and previously estimated bandgap value of about 6.5 eV for h-BN, we deduce a value of ～2.3 eV for the deep impurity involved in this DAP transition. The measured energy levels together with calculation results and formation energies of the impurities and defects in h-BN suggest that V_N and carbon impurities occupying the nitrogen sites, respectively, are the most probable shallow donor and deep acceptor impurities involved in this DAP transition.