Exciton spectra and layer decomposition in MoSi2N4/WSi2N4 heterostructures

Excitons in van der Waals heterostructures having interlayer or intralayer types are responsible for their optical absorption properties. Here, we systematically investigate the band alignment and excitons in the ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}/{\mathrm{WSi}}_{2}{\mathrm{N}}_{4}$ heterostructure using ab initio $GW$ calculations and the Bethe-Salpeter equation. The quasiparticle energy spectrum shows that the valence band maximum and conduction band minimum are from different layers, and hence the band alignment is type II in this heterostructure. However, unlike normal type-II heterostructures with interlayer excitons at the lowest energy, the ${\mathrm{MoSi}}_{2}{\mathrm{N}}_{4}/{\mathrm{WSi}}_{2}{\mathrm{N}}_{4}$ heterostructure behaves like a type-I heterostructure, in which the optical excitation at the lowest energy is the intralayer exciton while the interlayer exciton is higher in energy. We develop a macroscopic dielectric model explaining the atypical exciton feature as a result of the large layer distance and the small exciton Bohr radius in this heterostructure. In addition, we find that the interlayer exciton is dark due to the absence of band hybridizations between the two layers, and the energy and oscillator strength of the intralayer exciton are almost independent of the stacking configuration. These results enrich the exciton mechanism in 2D layer materials and are beneficial for the design of optoelectronic devices based on moir\'e heterostructures.