Transport properties of ultrathin SrTiO3 barriers for high-temperature superconductor electronics applications

Current transport through ultrathin SrTiO3 (STO) barriers has been studied systematically with respect to its dependence on barrier thickness d=2-30 nm, temperature T, and voltage V in state-of-the-art planar YBa2Cu3O7-x/SrTiO3/Au (YBCO/STO/Au) heterojunctions with c-axis oriented YBCO layer. We identified different transport regimes: Elastic tunneling was observed for samples with a nominal barrier thickness of 2 nm, which represents our experimental minimum for obtaining insulating transport characteristics. Already for slightly thicker STO barriers, resonant tunneling and hopping via a small number of localized states begins to dominate the transport behavior. For d>20 nm, a crossover to variable range hopping behavior is observed in the high-bias voltage regime as well as in the high temperature regime. A localization length of similar to0.46 nm indicating the spread of the localized states can be derived from these experiments. This value is close to the STO lattice constant and corresponds to a high density of localized states of n(L)similar to6x10(19)(eV)(-1) cm(-3). In a free electron tunneling model, this corresponds to an average tunnel barrier height of similar to0.4 eV. (C) 2003 American Institute of Physics.