Introduction of localized spin-state transitions in the optical absorption spectrum of Cr-doped GaN

Using a hybrid density functional, we study the electronic, magnetic, and neutral excitations of the paradigmatic Cr-doped GaN solid. Our results indicate that the $\ensuremath{\approx}1$% doped wurtzite GaN crystal retains the semiconducting nature of the host, associated with a magnetic moment of 3 ${\ensuremath{\mu}}_{B}$. As a consequence of Cr doping, additional hybridized bands are formed within the intrinsic band gap of GaN, leading to a considerable band gap narrowing by 1.4 eV and low-energy optical transitions. Our results indicate dark Cr $d\text{\ensuremath{-}}d$ transitions at very low energy (0.5--0.7 eV), followed by bright transitions in the visible energy range (1.8--2.2 eV). Characterization of the electron-hole pairs suggests that the latter originates from the internal transitions among hybridized ${\mathrm{Cr}}_{d}\text{--}{\mathrm{N}}_{p}$ states. With the existence of these additional optical features within the visible energy window, one can expect an enhancement to the photoelectric conversion efficiency of GaN upon Cr doping, in addition to applications in spintronics.