Rapid Bimolecular and Defect-Assisted Carrier Recombination in Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is a wide, indirect bandgap semiconductor that holds great promise for optoelectronic devices in the ultraviolet and mid-infrared spectral regimes. The efficiency of optoelectronic devices is dominated by the dynamic behavior of photogenerated carriers. Here we report on the dynamics of photoexcited free carriers in exfoliated ¹⁰B-enriched (99%) hBN at room temperature. Through implementation of ultrafast ultraviolet-pump–infrared-probe transient transmission spectroscopy, we identify two characteristic recombination rates. Initially, at high free carrier density, the pump fluence dependence is bimolecular with a characteristic rate constant of ∼2.0 × 10–⁷ cm³/s. This is followed by an exponential recombination of the free carriers at a rate of ∼2.3 × 10⁹ s–¹, which we assign to the influence of the impurities and defects in the lattice. These initial results offer insight into the radiative recombination processes for deep ultraviolet optoelectronic devices and toward realizing active control of mid-IR nanophotonic responses.