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.

Single crystals of hexagonal boron nitride (hBN) have recently been envisioned for electronic, optoelectronic, and nanophotonic applications. In this study, the production of large-scale, high-quality hBN single crystals via precipitation from a new solvent composed of Fe and Cr was demonstrated to be viable at atmospheric pressure. The clear and colorless crystals have a maximum domain size of around 2 mm and a thickness of around 200 μm. The Raman spectra and photoluminescence emission spectra demonstrate that the crystals produced with this solvent are pure hBN phase, and with low defect and residual impurity concentrations. The use of an Fe−Cr mixture provides a lower cost alternative to the more common Ni−Cr solvent for growing large hBN of comparable quality.

In this paper, the optical and electrical properties of Mg-doped AlN nanowires are discussed. At room temperature, with the increase of Mg-doping concentration, the Mg-acceptor energy level related optical transition can be clearly measured, which is separated about 0.6 eV from the band-edge transition, consistent with the Mg activation energy in AlN. The electrical conduction measurements indicate an activation energy of 23 meV at 300 K-450 K temperature range, which is significantly smaller than the Mg-ionization energy in AlN, suggesting the p-type conduction being mostly related to hopping conduction. The free hole concentration of AlN:Mg nanowires is estimated to be on the order of 10 16 cm À3 , or higher. V

The optical properties of catalyst-free AlN nanowires grown on Si substrates by molecular beam epitaxy were investigated. Such nanowires are nearly free of strain, with strong free exciton emission measured at room temperature. The photoluminescence intensity is significantly enhanced, compared to previously reported AlN epilayer. Moreover, the presence of phonon replicas with an energy separation of ∼100 meV was identified to be associated with the surface-optical phonon rather than the commonly reported longitudinal-optical phonon, which is further supported by the micro-Raman scattering experiments.

Photoluminescence spectroscopy has been employed to probe the near band-edge transitions in hexagonal BN (h-BN) epilayers synthesized under varying ammonia flow rates. The results suggest that the quasi-donor-acceptor pair emission line at 5.3 eV is due to the transition between the nitrogen vacancy and a deep acceptor, whereas the 5.5 eV emission line is due to the recombination of an exciton bound to a deep acceptor formed by carbon impurity occupying the nitrogen site. By growing h-BN under high ammonia flow rates, nitrogen vacancy related peaks can be eliminated and epilayers exhibiting pure free exciton emission have been obtained.