Substitutional doping defects in two-dimensional GaSe as potential single-photon emitters

Single-photon emitters (SPE) have attracted enormous attention because they are essential components of emerging quantum communication technology. The emission in the near-infrared (NIR) range is of special interest for fiber-based applications. In this work, we consider the substitution defects in GaSe as potential sources of such emission. We selected the X_Se and X_Ga defects where X = C, Si, Ge, N, P, and As (we use the common X_Y notation where X is the dopant and Y is the substituted atom). We first evaluated the stability of the defects by analyzing the defect formation energies and phonon spectra obtained through density functional theory-based calculations. After identifying the stable defects, we calculated their optical excitation spectra using the linear response GW and the Bethe-Salpeter equation (BSE) methods. We found that the defects such as C_Se, N_Se, Si_Se have sharp intense excitation peaks in the region of 0.8 - 1.5 eV. The analysis of the contributions of the independent quasiparticle states (eigenstates of GW) to the corresponding BSE eigenstates suggests that these excitations can result in the emission with dominating narrow zero-phonon lines accompanied with the low-intense and narrow phonon sidebands. We thus propose the defects to be promising SPEs in NIR range.