Effect of the Pekar mechanism on the phonon-drag thermopower in p-type Si/Si1-xGex heterostructures -: art. no. 205304

We examine the effect of the Pekar mechanism on the coupling of two-dimensional (2D) holes with 3D acoustic phonons in p-type Si/Si1-xGex heterostructures by using the phonon-drag thermopower, S-g, as a probe. The origin of the Pekar coupling is due to the variation of the dielectric permittivity under strain in nanostructures characterized by strong confining electric fields. The hole-phonon interaction via the Pekar mechanism resembles that of piezoelectric coupling in polar materials and dominates over the deformation potential coupling at low temperatures. It is found that S-g shows a T-4 temperature dependence at low T (Bloch-Gruneisen regime) which is in good agreement with the experimental data reported recently by Possanzini [Phys. Rev. B 69, 195306 (2004)]. We perform detailed calculations of S-g by considering both a screened Pekar coupling and a screened deformation potential coupling in p-type Si/SiGe structures in the temperature range 0.25 < T < 6 K. The value of the deformation potential constant is taken to be 4 eV, in good agreement with previous studies. The strength of the Pekar coupling is determined by the magnitude of the surface electric field E-s. We treat E-s as an adjustable parameter in our calculations and we find good agreement with all the experimental data of S-g, available at the moment, by using a value for E-s of the order of 10(9) V/m. The possibility for the existence of such a large surface field in Si/SiGe is discussed.