Band mixing and ambipolar transport by surface acoustic waves in GaAs quantum wells

The interaction of strong surface acoustic wave (SAW) fields with the electronic band structure of GaAs quantum wells (QW’s) is investigated using spatially resolved photoluminescence (PL) spectroscopy. The optical studies are accompanied by $\mathbf{k}\cdot \mathbf{p}$ and tight-binding (TB) calculations of the SAW effects on the electronic band structure. The SAW induces a time-dependent coupling between the heavy- (hh) and light-hole (lh) states in the valence band of the QW’s, which leads to an anticrossing of their energy levels for high SAW intensities. The coupling alters the strength and polarization of the optical transitions and can be reproduced by calculations of the optical transition matrix elements. Spatially resolved PL measurements of the SAW-induced ambipolar transport of electrons and holes provide evidence of a reduction of the transport efficiency for high SAW fields, which is attributed to a decrease of the hole mobility as the hh and lh levels approach each other. This conclusion is supported by TB calculations that show a significant enhancement of the heavy-hole effective mass under these conditions. In addition, the mobility may also be reduced by the squeezing of the wave functions towards the QW interfaces induced by strong piezoelectric fields, which makes the transport more sensitive to potential fluctuations induced by interface roughness and defects in the barrier layers.