Luminescence dynamics of indirect-bandgap sp 2 -bonded hexagonal and rhombohedral BN polytypes

Abstract Hexagonal boron nitride (hBN) is a semiconductor that crystallizes in layers of a two-dimensional honeycomb structure based on in-plane three s p 2 covalent bonds between boron and nitrogen, namely monolayer BN (mBN), which are connected by out-of-plane π bonds with a stacking sequence AA′. Since hBN exhibits high quantum efficiency near-band-edge emission at around 5.77 eV (215 nm) in spite of the indirect bandgap, layered BN polytypes have attracted attention as a far UVC light-emitting material, where UVC is an abbreviation for deep-ultraviolet lights with the wavelengths between 100 and 280 nm. Recently, syntheses and characterizations of layered BN polytypes such as mBN, AB-stacked graphitic BN (bBN), ABC-stacked rhombohedral BN (rBN), and even unstable AA-stacked BN have been reported, which included indispensable physical and electronic parameters. In this article, steady-state, temporary-resolved, and spatially-resolved luminescence properties of O 2 -annealed hBN microcrystals (MCs) and rBN epitaxial films containing bBN segments grown by chemical vapor deposition using carbon-free precursors are reviewed to explore the excellence of s p 2 -bonded, layered BN polytypes. The hBN MCs and rBN epilayers exhibited UVC emissions in the energy range of 5.7–6.0 eV that were identified as phonon replicas of indirect excitons (iXs) with photoluminescence lifetimes ( τ PL ) of a few tens to 54 ps, which most likely represent the midgap recombination lifetimes for each iX reservoir. Because τ PL did not change while the cathodoluminescence intensity increased with temperature above 100 K, i.e. the iX emissions exhibited a retrograde thermal quenching behavior, both high joint density of states of iXs and strong exciton-phonon interaction seem significant for procreating the high quantum efficiency. Equally significant emissions at 6.035 eV at 12 K and 6.0–6.1 eV at 300 K were observed from the bBN segments. The results shown in this article are of importance for promoting BN polytypes in practical application to UVC light-emitting devices.