Shallow-Deep Core-Exciton Instability inSixGe1−xAlloys

Transmission and electron-yield measurements of the Si $L_{2,3}$ edge as a function of alloy composition $x$ in ${\mathrm{Si}}_x{\mathrm{Ge}}_{1-x}$ alloys reveal anomalous spectra for $x\approx 0.15$, the composition of the crossover of the conduction-band minima from $L$ in the Brillouin zone (for $x<\mathrm{}0.15\mathrm{}$) to $\Delta$ (for $x>\mathrm{}0.15\mathrm{}$). We interpret this as evidence for a sudden change of the ground state of the exciton from an extended effective-mass Wannier-type state for $x<\mathrm{}0.1\mathrm{}$, to a deep localized state near $x=0.15\mathrm{}$, and back to an extended state for $x>\mathrm{}0.2\mathrm{}$.