Local defects in two-dimensional gallium sulfide as potential single-photon emitters: First-principles evaluation

The single-photon emitters (SPEs) are essential components of emerging quantum communication technology. It has been shown that the local defects in the wide-bandgap semiconductors can be sources of sharp zero-phonon lines (ZPLs) of emission, which is favorable for applications. Most of the reported SPEs emit photons in the visible light region, while fiber-based quantum communication applications require sources of near-infrared (NIR) emission. Therefore, the purpose of this paper is to search for defects which are promising candidates for SPEs in the NIR region by applying computational methods. To this end, based on an educated guess, 12 local doping substitutional defects in two-dimensional (2D) gallium sulfide were selected for consideration. The selected defects are ${Y}_{X}$, where $X=\mathrm{Ga}$ or S, and $Y=\mathrm{C}$, Si, Ge, N, P, As. Here, $X$ stands for a substituted element and $Y$ for a dopant. To evaluate the stability of the defects, their formation energy, binding energy of the dopants, and phonon spectra were calculated within the density functional theory (DFT) approximation. Three defects, ${\mathrm{Ge}}_{\mathrm{Ga}}, {\mathrm{N}}_{\mathrm{Ga}}$, and ${\mathrm{P}}_{\mathrm{Ga}}$, are found to have imaginary-frequency vibration modes, indicating their dynamical instability. They were removed from further consideration. A preliminary analysis of the DFT densities of electronic states (DOS) revealed six remaining defects, ${\mathrm{As}}_{\mathrm{Ga}}, {\mathrm{Ge}}_{\mathrm{S}}, {\mathrm{Si}}_{\mathrm{S}}, {\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$, with DOS favorable for formation of local optical excitations in the NIR region. Next, for these six defects, the state-of-the-art linear response GW and the Bethe-Salpeter equation (BSE) methods were used to obtain their electronic structures, in terms of the independent quasiparticle (IQP) states, and optical excitation spectra, in terms of the frequency-dependent dielectric function and the dipole optical transition oscillator strength. The calculation results indicate that the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects have intense sharp excitation peaks in the NIR region. The analysis of the contributions of the IQP states to the corresponding BSE eigenstates brings us to the conclusion that these excitations can result in emission with dominating narrow ZPL peaks. I thus propose here the ${\mathrm{C}}_{\mathrm{S}}, {\mathrm{N}}_{\mathrm{S}}$, and ${\mathrm{P}}_{\mathrm{S}}$ defects in 2D GaS as promising candidates for SPEs in the NIR emission region.