Far-infrared absorption study of exciton ionization in germanium

We use the strength of the far-infrared absorption of excitons at 3 me V as a measure of their concentration at a variety of temperatures and excitation densities. At low temperatures and densities the excitons are in equilibrium with electron-hole drops while in the region of 10°K they ionize to form an electron-hole plasma. A simple kinetic model shows that the location of the boundary on the phase diagram separating the high-temperature plasma from the exciton region depends on the electron-hole recombination rate. Different surface treatments appear to vary this rate, and in some cases a plasma density of ${10}^{15}$ ${\mathrm{cm}}^{-3\mathrm{}}$ can be obtained. In the absence of a strong plasma, however, an exciton concentration of 5 X ${10}^{15}$ can be built up with no evidence of a lowering of exciton binding energy.