  In this paper we present a thorough first-principles based density functional
theory study of the structural stability, electronic, magnetic, and optical
properties of pristine and doped gallium phosphide (GaP) monolayers. The
pristine GaP monolayer is found to have a periodically buckled structure, with
an indirect band gap of 2.15 eV. The doping by X (B, Al, In, C, Si, Ge, Sn, Zn,
Cd) at the Ga site, and Y (N, As, Sb, O, S, Se, Te, Zn, Cd) at the P site is
considered, and an indirect to direct transition is observed after doping by In
at the Ga site. For several cases, significant changes in the band gap are seen
after doping, while system becomes metallic when O is substituted at the P
site. The spin-polarized band structures are calculated for the monolayers with
doping-induced magnetism, and we find that for some cases a direct band gap
appears for one of the spin orientations. For such cases, we investigate the
intriguing possibility of spin-dependent optical properties. Furthermore for
several cases, the band gap is very small for one of the spin orientations,
suggesting the possibility of engineering half metallicity by doping. For the
layers with direct band gaps, the calculated optical absorption spectra are
found to span a wide energy range in the visible and ultraviolet regions. The
calculated formation energies of both the pristine and doped structures are
quite small, indicating that the laboratory realization of such structures is
quite feasible. On the whole, our results suggest that the doped GaP monolayer
is a material with potentially a wide range of applications in nanoelectronics,
spintronics, optoelectronics, solar cells, etc.
