Molecular beam epitaxy of two-dimensional semiconductor BiI₃films exhibiting sharp exciton absorption

Bismuth triiodide (BiI3) is a two-dimensional (2D) layered semiconductor with a fairly narrow bandgap (∼2 eV) and strong excitonic absorption near the band edge. Being endowed with a large absorption coefficient in the visible range, BiI3 attracts a growing interest as a potential compound for thin-film optoelectronic devices. Although various growth techniques have been employed so far, thin films with a high enough crystallinity for the solid assignment of the exciton resonances have never been synthesized. Here, we demonstrate molecular beam epitaxy of high-quality BiI3 films by employing a thin buffer layer deposited at a low temperature (30 °C) to make high temperature (70 °C) growth possible. The films fabricated by this two-step method realize a single-crystalline structure and atomically flat surfaces. The absorption spectra of the films exhibit strong and sharp excitonic resonances near the band edge, indicating the high crystallinity of the films comparable to the bulk single crystals. The presently established fabrication process should be widely applicable to other 2D halide thin films and heterostructures, providing ideal platforms for observing unprecedented emergent phenomena.