Excitonic Mobility Edge inGaAsxP1−x

Time-resolved, selectively excited fluorescence and resonant Raman scattering (RRS) are used to determine the nature of excitons in $\mathrm{Ga}{\mathrm{As}}_x{\mathrm{P}}_{1-x}(x<~0.07)$. All localized excitons, both intrinsic and those bound to isoelectronic nitrogen impurities, have the same $e-h$ exchange splitting into $J=1,2\mathrm{}$ levels. This allows measurement of the density of terminal states and their contribution to RRS by zone-edge $\mathrm{L}{\mathrm{O}}_X$ phonons. An abrupt vanishing of the density of terminal states and this RRS presents firm evidence of the existence of an excitonic mobility edge and determines its position to an accuracy of 1 meV.