Electrostatic charge accumulation versus electrochemical doping in SrTiO₃ electric double layer transistors

In electric double layer transistors with SrTiO₃ single crystals, we found distinct differences between electrostatic charge accumulation and electrochemical reaction depending on bias voltages. In contrast to the reversible electrostatic process below 3.7 V with a maximum sheet charge carrier density, n_S, of 10¹⁴ cm^-2, the electrochemical process causes persistent conduction even after removal of the gate bias above 3.75 V. n_S reached 10¹⁵ cm ^-2 at 5 V, and the electron mobility at 2 K was as large as 10 ⁴ cm² /V s. This persistent conduction originates from defect formation within a few micrometers depth of SrTiO₃.