Excitation-Energy Dependence of Photoluminescence Bandwidth in $a$-${\mathrm{As}}_2$${\mathrm{S}}_3$: Observation of the Mobility Gap?

A new optical system incorporating a fast Ge photodiode has made possible the first measurements at short times of the low-energy side of the photoluminescence spectrum of $a$-${\mathrm{As}}_2$ ${\mathrm{S}}_3$. Two photoluminescence processes are separated by their dependence on excitation energy, $E_x$. A number of observations suggest that the first step in the high-$E_x$ process is the creation of delocalized excitations. The abrupt onset of this process leads to the intimation that it arises from transitions across the mobility gap.