Field-driven tearing of electron-hole pairs in ultrathin Bi₂Te₃ films

Excitons, quasiparticles formed by Coulomb interactions between electron-hole pairs, can determine the optical properties of semiconductors and facilitate hybrid exciton-polariton quasiparticles. Recent studies have reported Bi₂Te₃ single crystals as a promising material for spatially indirect excitons, formed by surface electrons and bulk holes, which have a long lifetime. One promising route to further control excitonic behavior is to reduce the material’s dimensionality to the ultrathin limit, providing greater control over interactions via reduced screening or the integration of gating capabilities. However, challenges remain, particularly the intrinsic electron doping in ultrathin Bi₂Te₃ which prevents the launching of electron-hole excitations across the bandgap. In this work, we employed molecular beam epitaxy to grow high-quality Bi₂Te₃ thin films. We tuned the chemical potential near the charge-neutral point by adjusting the antisite defect density. Despite the high crystalline quality, time-resolved angle-resolved photoemission spectroscopy did not reveal clear excitonic signatures. We attribute this observation to the tearing of electron-hole pairs due to strong in-plane electric fields facilitated by the uneven doping, as well as out-of-plane fields which manifest as a surface photovoltage effect. Our findings provide insights for future material engineering to realize exotic nonequilibrium phenomena in topological insulators.