Optical transitions via the deep O donor in GaP. I. Phonon interaction in low-temperature spectra

A detailed investigation of optical transitions via the deep O donor state in GaP is presented, with emphasis on evaluation of the influence of phonons (lattice relaxation) on the spectral behavior of cross sections. Very sensitive purely optical techniques, such as photoluminescence-excitation (PLE) or -quenching (PLQ) measurements on bulk material provide data on optical cross sections which are accurate enough to allow an unambigous evaluation of parameters for phonon interaction in optical transitions. The near-edge part of ${\sigma }_{p1\mathrm{}}^0\left(h\nu \right)$ shows clear phonon structure due to two phonon modes $\hslash {\omega }_1\approx 19$ meV and $\hslash {\omega }_2\approx 48$ meV with linear coupling strengths ${\lambda }_1=1.65\mathrm{}\pm 0.15$ and ${\lambda }_2=1.1\mathrm{}\pm 0.1$, respectively, giving a Franck-Condon shift ${\Delta }_{\mathrm{FC}}=85\mathrm{}\pm 5$ meV for the O donor in GaP. These values are found to be the same in the ${\mathrm{O}}^{+}$ state (${\sigma }_{p1\mathrm{}}^0$ spectra) and in the ${\mathrm{O}}^0$ state (radiative emission), which justifies the use of a linear model for the electron-phonon interaction. Further, the detailed agreement with the experimental spectra justifies a simple theoretical treatment within the framework of the adiabatic and Condon approximations. A method to separate out the electronic part ${\sigma }_{\mathrm{e}1}\left(h\nu \right)$ of the optical cross section using the knowledge of the phonon lineshape function has been developed, involving a simple deconvolution procedure of low-temperature experimental data. This electronic spectrum ${\sigma }_{\mathrm{e}1}\left(h\nu \right)$ is the appropriate one for comparison with theoretical models for photoionization cross sections. A simple effective-mass treatment of such cross sections is developed including effects of wave-function symmetry as well as the real band structure. A fit of this theoretical model to the electronic part of ${\sigma }_{p1\mathrm{}}^0\left(h\nu \right)$ gives a threshold energy 1.453 ± 0.002 eV at 1.5 K, which implies a band gap for GaP ∼ 14 meV higher than previously established. The spectral behavior of ${\sigma }_{n1\mathrm{}}^0\left(h\nu \right)$ at low temperature indicates strong effects of excited states on the O center, extending ≥ 35 meV below the continuum threshold.