Superconducting transition for the surface conducting layer and changes in the transport properties with electrochemical etching for various samples. (a) The T dependence of the RS values of the surface conducting layer (RSsurface), normalized to the value at 90 K. RSsurface was estimated from the change in the sheet conductance between VG values of 5 V and 0 V. Data for three VG cycles of two samples, labeled as LAO-1, CaF2-1 and CaF2-2 in (c), are shown. The RS-T curves just before the last etching cycle for the LAO and CaF2 samples are also plotted. The inset shows the RH values of the surface conducting layers of the LAO-1, CaF2-1 and CaF2-2 samples as a function of temperature. (b–d) Values of 1/RS at 140 K and 50 K and of RH at 50 K, respectively, as functions of the film thickness for the LAO and CaF2 samples. VG was changed from 5 V to 0 V once and twice during the etching of the LAO and CaF2 samples, respectively. (e) Schematic illustration of the evolution of a sample during etching. A surface layer is formed during etching and disappears with the removal of the gate bias.