Tuning Up or Down the Critical Thickness in LaAlO3/SrTiO3 through In Situ Deposition of Metal Overlayers

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO₃ and SrTiO₃ has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LaAlO₃ thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. In this paper, the chemical, electronic, and transport properties of LaAlO₃/SrTiO₃ samples capped with different metals grown in a system combining pulsed laser deposition, sputtering, and in situ X-ray photoemission spectroscopy are investigated. The results show that for metals with low work function a q2DES forms at 1–2 uc of LaAlO₃ and is accompanied by a partial oxidation of the metal, a phenomenon that affects the q2DES properties and triggers the formation of defects. In contrast, for noble metals, the critical thickness is increased above 4 uc. The results are discussed in terms of a hybrid mechanism that incorporates electrostatic and chemical effects.

The possibility to tune up or down the critical thickness at which a 2D electron gas appears at the LaAlO₃/SrTiO₃ interface, by capping the LaAlO₃ with different metals is demonstrated. Besides electrostatic effects, X-ray photoemission spectroscopy reveals the importance of the partial oxidation of the metal on the properties of the gas.

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