The quasi two- dimensional electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge; another is the extreme sensitivity of its transport properties to electrostatic boundary conditions. This surface-interface coupling was previously exploited to modulate both carrier densities and mobilities of the q2DES through the controlled adsorption of polar solvents and by capping with different materials. In our recent work just published in Advanced Materials, we investigate in detail the chemical, electronic and transport properties of several LAO(1-2 uc)/STO samples capped with different metals (Ti, Ta, Co, Ni80Fe20 – NiFe -, Nb, Pt, Pd and Au) grown in a ultra-high vacuum (UHV) system combining pulsed laser deposition (to grow the LAO), sputtering (to grow the metal) and in situ X-ray photoemission spectroscopy (XPS). The results confirm that for several metals a q2DES forms at 1-2 uc of LAO. Additionally, XPS shows that the appearance of interfacial conductivity is accompanied by a partial oxidation of the metal, a phenomenon that is strongly linked with the q2DES properties and with the formation of defects in this system. In contrast, for noble metals, the q2DES does not form at low LAO thicknesses and instead the critical thickness is increased above 4 unit cells. We discuss the results in terms of a hybrid mechanism that incorporates both electrostatic and chemical effects.

Tuning up or down the critical thickness in LaAlO3/SrTiO3 through in situ deposition of metal overlayers
D.C. Vaz et al ; Adv. Mater. 10.1002/adma.201700486 (2017)