Infrared Study of the Quasi-Two-dimensional Electron System at the Interface Between SrTiO3 and Crystalline or Amorphous LaAlO3

We have used grazing-angle infrared spectroscopy to detect the Berreman effect (BE) in the quasi-two-dimensional electron system (q-2DES) which forms spontaneously at the interface between SrTiO3 (STO) and a thin film of LaAlO3 (LAO). From the BE, which allows one to study longitudinal optical excitations in ultrathin films like the q-2DES, we have extracted at different temperatures its thickness, the charge density and mobility of the carriers under crystalline LAO (sample A), and the charge density under amorphous LAO (sample B). The latter quantity turns out to be higher than in sample A, but a comparison with Hall measurements shows that under amorphous LAO the charges are partly localized at low T with a low activation energy (about 190 K in units of k(B)) and are thermally activated according to a model for large polarons. The thickness of the q-2DES extracted from our spectra turns out to be 4 +/- 1 nm for crystalline LAO and 7 +/- 2 nm for amorphous LAO.