Resonant exciton in GaSe

Experimental evidence is given for the occurrence in $\mathrm{Ga}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ ($0\le x\le 0.2$) of a resonance between the free exciton associated with the direct band gap on the one hand and states near the indirect-conduction-band minimum on the other hand. This evidence includes the temperature dependence of the luminescence spectrum of $\mathrm{Ga}{\mathrm{S}}_x{\mathrm{Se}}_{1-x}$ for excitation energies lying above and below the exciton ground state, the excitation spectra of two characteristic luminescence lines, and the shape of the absorption line of the free exciton. The experimental results are in excellent agreement with a model of recombination kinetics proposed earlier for GaSe as well as with Fano's theory of the line shape of resonant states.