Fe2+in GaP studied by Fourier-transform emission and absorption spectroscopy

We present an optical Fourier-transform-infrared photoluminescence (PL) and absorption study of the ${}_{}{}^5{}_{}{}^{}$ ${\mathit{T}}_2$ ${\mathrm{\leftrightarrows }}^5$E internal 3d transitions of ${\mathrm{Fe}}^{2+}$ in GaP. We analyze in detail the four zero-phonon lines of ${\mathrm{Fe}}^{2+}$ which appear at about 3300 ${\mathrm{cm}}^{\mathrm{-}1}$ in GaP. A fine structure, which originates from different iron isotopes, is resolved. Detailed PL and absorption spectra between 2500 and 4200 ${\mathrm{cm}}^{\mathrm{-}1}$ reveal many features in the Stokes and anti-Stokes phonon sidebands. With the help of temperature-dependent absorption measurements we are able to set up the complete level scheme for the internal 3d transitions of ${\mathrm{Fe}}^{2+}$ in GaP. We compare the experimental results to theoretical values obtained by crystal-field theory including spin-orbit coupling.