Layer number dependent optical properties of multilayer hexagonal BN epilayers

Deep ultraviolet photoluminescence emission spectroscopy has been employed to probe the layer number dependent near band-edge transitions above 5 eV in multilayer hexagonal boron nitride (h-BN) epilayers grown by metal-organic chemical vapor deposition. Two emission lines near 5.30 and 5.47 eV were resolved at 10 K. These two emission lines share similar spectroscopic features, and their energy peak separation is nearly independent of the number of layers. The observed energy separation of ∼172 meV coincides well with the in-plane phonon vibration mode, E_2g, having an energy of 1370 cm⁻¹ (∼172 meV). The results suggested that the emission line at ∼5.30 eV and ∼5.47 eV are a donor-acceptor-pair transition and its one E_2g phonon replica, respectively. When the number of layers decreases from 100 to 8, the emission peak positions (E_p) of both emission lines blueshifted monotonically, indicating the dimensionality effects on the optical properties of h-BN. The layer number dependence of E_p can be described by an empirical formula, which accounts for the variations of the energy bandgap and activation energies of impurities with the number of layers. The results revealed that the impurity activation energies and the carrier-phonon coupling strength increase as the dimensionality of h-BN scales from thick layer to monolayer, suggesting that it is more difficult to achieve conductivity control through doping in monolayer or few-layer h-BN than in thick h-BN.