Surface Dynamics of Charge Transport in LaAlO₃/SrTiO₃with Time-Resolved Kelvin Probe Force Microscopy

Time-resolved Kelvin probe force microscopy (tr-KPFM) was used to map surface potential dynamics in LAO/STO under its critical thickness, where it acts as an insulator, as polarizing and depolarizing electric fields were applied to planar electrodes in ambient conditions. Two time constants, for polarization and relaxation (i.e., depolarization) each, were derived and were shown to be independent of the direction of electrical field, revealing the reproducibility and recovery of the surfaces. Ambient aging or recent electrical history did not change the surface kinetics as determined by tr-KPFM. During polarization, the faster time constant (∼3-6 s) is likely associated with the transport of H+ ions through the full length of the channel. The slower time constant (∼40-50 s), on the other hand, is observed across only half the channel width near the higher-potential─more positive─electrode, which is likely associated with an electrochemical reaction such as the dissociation of water. This slower time constant drops to ∼10-20 s toward the middle of the channel which could represent the transport of OH- ions. During relaxation, the fast time constant is consistent with polarization and is correlated to the same mechanism while the slower time constant is higher than it was during polarization and may represent the reabsorption of moisture from the atmosphere.