Recombination dynamics of indirect excitons in hexagonal BN epilayers containing polytypic segments grown by chemical vapor deposition using carbon-free precursors

Abstract

Hexagonal (h) BN is a semiconductor that crystallizes in layers of a two-dimensional honeycomb structure. Since hBN exhibits high quantum efficiency (QE) near-band edge emission at around 5.8 eV in spite of the indirect bandgap, hBN has a potential for the use in deep-ultraviolet light emitters. For elucidating the emission dynamics of indirect excitons (iXs) in hBN, spatially and temporally resolved luminescence measurements were carried out on hBN epilayers grown using carbon-free precursors. In addition to major μm-side flat-topped (0001) hBN columnar grains, sub-μm-scale polytypic segments were identified, which were likely formed by certain growth instabilities. The hBN domains exhibited predominant emissions of phonon-assisted fundamental iXs at 5.7–5.9 eV and a less-pronounced 4.0-eV emission band. The photoluminescence lifetime (τPL) for the iX emissions was 54 ps, which most likely represents the midgap recombination lifetime (τMGR) for an iX reservoir. Because τPL did not change while the cathodoluminescence (CL) intensity increased with temperature above 100 K, both the immobile character of iXs and strong exciton–phonon interaction seem significant for procreating the high QE. The CL intensity and τPL of the 5.5 eV band monotonically decreased with temperature, indicating that τPL represents τMGR, most probably a nonradiative lifetime, around the real states. Equally significant emissions at 6.035 eV at 12 K and 6.0–6.1 eV at 300 K were observed from the polytypic segments, most probably graphitic bernal BN, which also exhibited negligible thermal quenching property.