Phonon-wind-induced anisotropy of the electron-hole droplet cloud in Ge

We report the results of experimental studies of the electron-hole droplet cloud in Ge and present a set of model calculations explaining the anisotropic features of the cloud in terms of a phonon-wind force. Luminescence images of the droplet cloud in Ge display several prominent features: broad $〈111〉$ lobes, sharp axial $〈100〉$ flares, and sharp {100} planar flares. The central lobes are identified with the LA-phonon component of the phonon-wind force, while the axial flares are associated with the slow TA component and the planar flares with the fast TA component. Thus, the effects of all three acoustic-phonon modes are observed in the cloud. The anisotropy of the calculated phonon-wind force is due to both the electron-phonon interaction and the ballistic phonon flux. The relative contributions of these factors are examined. Droplet trajectories under the influence of the phonon-wind force are calculated; these calculations illustrate the nonradial nature of the force. All of the general features of the observed anisotropic cloud are well accounted for by the anisotropies of the phonon wind.