OBSERVATION OF PIEZOELECTRIC-LIKE BEHAVIOR IN COHERENTLY STRAINED B-DOPED(100)SIGE SI HETEROSTRUCTURES/

In the present work tile hybrid acoustic spectroscopy technique has be en used to demonstrate the conversion of a high frequency (HF) electri c field into acoustic waves and to provide the first direct observatio n of the piezoelectric effect in the SiGe/Si strained layer system. Th e sample was a p-type modulation doped Si0.88Ge0.12/Si heterostructure containing a two-dimensional hole gas with carrier sheet density 2 x 10(11) cm(-2) and a 4.2 K mobility of 10 500 cm(2) V-1 s(-1). It was e xcited with a 225.7 MHz high frequency pulse of 1 mu s duration and 0. 5 W peak power. The amplitude of the output acoustic signal was about 110 dB below the electromagnetic input signal at 77 K. We believe that the observed electric field-acoustic conversion is associated with th e non-centrosymmetric structure of the ordered unit cell of the strain ed SiGe alloy. Additionally we report on phonon Raman scattering at T = 300 K and hot hole Shubnikov-de Haas and zero magnetic field resisti vity behaviour in the same heterostructures in the temperature range o f 0.35 to 1.4 K. A broad band near 255 cm(-1) and a peak near 435 cm(- 1) have been attributed to a particular Si-Ge ordering within the allo y layer. The energy relaxation of the carriers has been measured and i s found to be dominated by a weakly screened piezoelectric coupled aco ustic-phonon mechanism, thereby providing further evidence of ordering in the SiGe alloy.