Contact resistance tuning at metal / Nb:SrTiO$_3$ interfaces using LaAlO$_3$ interlayers

SrTiO$_3$ (STO) exhibits coexistence of high mobility electrons and possible unconventional superconductivity (SC) [1]. Transition metal (TM) contacts to epitaxial insulator/Nb doped STO (NSTO) structures are prototypically used to probe the electronic structure (ES) of n-type STO in tunneling experiments. However, the field dependent permittivity in STO at low temperatures makes it difficult to probe ES when the barrier height (BH) is large [2]. We show that the contact resistance $R_c$ across TM/NSTO interfaces (IF) can be effectively tuned by inserting a thin LaAlO$_3$ (LAO) interlayer (IL) between the TM and NSTO. Change of IL thickness from 0 to 2 u.c. in Co/LAO/NSTO (100) structures results in systematic reduction of $R_c$ by orders of magnitude, as evidenced by a transition from Schottky-type rectifying to nearly ohmic current-voltage curves. This is because the polar nature of the LAO surface, generating an IF dipole IL, lowers the Schottky BH. This is a useful method to optimize $R_c$ for tunneling experiments in doped STO with possible applications for SC and spintronics. It is also important that this effect fails for $ex$-$situ$ deposited TM and we discuss the distinction. [1]C. Richter et al., Nature 502, 528 (2013) [2]T. Susaki et al., Phys. Rev. B 76, 155110 (2007)