Condensation of Free Excitons into Electron-Hole Drops in Pure Germanium

Abstract

A simple theory is given to justify the existence, in intrinsic semiconductors, of electron-hole drops (EHD) resulting from the condensation of free excitons. It is shown that such a condensation is favored by a multivalley band structure. The critical density ${n}_{c}$ of electron-hole pairs in EHD is then determined in pure Ge from luminescence experiments and it is compared to other data. This study shows that ${n}_{c}$ is of the order of 2 \ifmmode\times\else\texttimes\fi{} ${10}^{17}$ ${\mathrm{cm}}^{\ensuremath{-}3}$x in this material. We also investigate the light emission of EHD in uniaxially stressed Ge. The results obtained are interpreted from the stress variation of ${n}_{c}$ and they are shown to be consistent with the EHD theoretical model. In addition, we study, in the same material, the temperature dependence of the EHD total lifetime. Finally, we briefly discuss the case of Si with respect to the EHD model.