Time-resolved studies of electron-hole-droplet transport in Ge

The interaction of phonons with electron-hole droplets (EHD) in Ge is examined from two points of view: the damping of droplet motion in a force field and the macroscopic droplet motion produced by directed phonon fluxes. The mobility, μ=τm, of the droplets is measured using the strain-gradient method. The magnitude and temperature dependence of the scattering time τ are found to be in agreement with the phonon scattering model of EHD-momentum damping. The phonon-wind force that transports droplets at a velocity V is measurable via the drift relation F=Vμ. The phonon-wind force for a given cw laser excitation is calibrated and compared to that produced by pulsed-laser excitation. In the pulsed case, the wind persists for up to 4 {μ}s after a 100-ns laser pulse, indicating a storage of energy. Time-resolved images reveal a cloud shape very different from the steady-state cloud and show that the phonon storage is localized to within 300 {μ}m of the excitation point. With a simple phonon-wind model, the droplet-velocity data give quantitative information on the energy flux leaving the storage region, and thus the total stored energy. This work demonstrates that electron-hole droplets provide a contactless quantitative probe of nonequilibrium phonon dynamics in Ge at low temperature.