Ballistic phonon-drag imaging in AlAs quantum wells

The phonon-drag voltage induced by acoustic phonons has been studied for the first time in AlAs quantum wells. The 15 nm wells are embedded in Al_0.45Ga_0.55As buffer layers grown on (001) GaAs wafers. Electrons in the wells couple strongly to phonons due to the large effective mass, leading to signals that are several orders of magnitude larger than those observed in similar experiments with GaAs heterostructures. Measurements of the drag voltage as a function of phonon source position (phonon-drag images) yield information about the polarization dependence of the electron-phonon coupling. The data is compared to numerical simulations including deformation potential and piezoelectric coupling. The experimental images show distinctive signals from slow transverse and longitudinal phonon modes that indicate strong deformation potential coupling. Additional modeling is needed to understand the source of the clear sign inversion of the signal near the center of the experimental images. Future studies at ³He temperatures will investigate quantum effects. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)