Zero-field thermopower of a thin heterostructure membrane with a two-dimensional electron gas

We study the low-temperature thermopower of micrometer-sized free-standing membranes containing a two-dimensional electron system. Suspended membranes of 320 nm thickness, including a high-electron-mobility structure in the Hall bar geometry of 34 mu m length are prepared from GaAs/AlGaAs heterostructures grown by molecular-beam epitaxy. Joule heating on the central region of the membrane generates a thermal gradient with respect to the suspension points where the membrane is attached to cold reservoirs. Temperature measurements on the membrane reveal strong thermal gradients due to the low-thermal conductivity. We measure the zero-field thermopower and find that the phonon-drag contribution is suppressed at low temperatures up to 7 K.