Intrinsic exciton states in indirect-band-gap GaAsxP1-x: Composition dependence and effects of impurities.

Abstract

The low-temperature (T=2 K) absorption and photoluminescence spectra of ${\mathrm{GaAs}}_{\mathrm{x}}$${\mathrm{P}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ (x\ensuremath{\le}0.46) are reported. The absorption spectrum for x\ensuremath{\ge}0.15 can be separated into the ${\mathrm{N}}_{\mathrm{X}}$ band (excitons bound to isoelectronic nitrogen impurities) and the intrinsic exciton band. The latter can be fitted to a sum of transitions into extended exciton states for which \ensuremath{\alpha}(E)\ensuremath{\propto}(E-${E}_{\mathrm{fx}}$${)}^{1/2}$ and a tail due to localized states which can be approximated by \ensuremath{\alpha}(E)\ensuremath{\propto}exp[(E-${E}_{s}$)/${E}_{0}$] for E\ensuremath{\le}${E}_{s}$. For x0.15, the (${\mathrm{N}}_{\mathrm{X}}$+LO)-phonon sideband overlaps the intrinsic absorption spectrum and the two contributions cannot be accurately separated. The luminescence spectra show the impurity-bound exciton bands and for xg0.15, the ${M}_{0}^{X}$ band which is due to intrinsic excitons localized by potential fluctuations. The ${M}_{0}^{X}$ band shape is analyzed in terms of exciton tunneling by acoustic-phonon-assisted transitions into terminal states. The high-energy cutoff of the ${M}_{0}^{X}$ band is identified as the excitonic mobility edge (ME). For x0.15, the ME is determined by monitoring the emission intensity under selective excitation into the intrinsic exciton tail. The composition dependence of the ME energy, ${E}_{\mathrm{ME}(\mathrm{x})}$, is analyzed in terms of a simplified Lifshitz model. It assumes that excitons which are localized by potential fluctuations have spatially overlapping wave functions. For a critical value of the density of exciton states, the wave function extends over the whole crystal. The calculated values of ${E}_{\mathrm{ME}(\mathrm{x})}$ which are based on this model fit well the experimental data in the composition range x\ensuremath{\ge}0.10 and follow the expression ${\mathit{E}}_{\mathrm{ME}}$(x)=1.963x+2.327(1-x)-0.194x(1-x) eV.