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Electrostatic tuning of LaAlO3-SrTiO3 interface devices

Smink, A.E.M. (2014) Electrostatic tuning of LaAlO3-SrTiO3 interface devices.

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Abstract:We report transport measurements on structured LaAlO3-SrTiO3 interface devices with a SrCuO2-SrTiO3 capping layer and Au top gate electrode. The properties of the 2DEG at the interface were tuned by an applied top gate voltage. These properties included the carrier density, the average global carrier mobility, and the longitudinal resistance in a magnetic field. The quality and reproducibility of the devices was strongly improved in this work. The structuring hard mask layer was insulating in all samples, the average global mobility was increased by an order of magnitude, and the gate leakage current was comparable to the lowest value in literature, at most. An electrostatic model was devised to describe the effects of gating on the 2DEG. Qualitative features, such as an increase in the carrier density due to the presence of a top gate electrode, were predicted and confirmed by measurements. The capacitance equation of the top-gated 2DEG was found to have additional terms with respect to the equation in the parallel plate capacitor model. These terms were related to the density of states of proposed band diagrams in literature and the shape of the confinement potential at the interface. Strong indications of the validity of this capacitance model were found by experiment. The measurements also laid bare some anomalous effects that could not directly be explained. The measured emergence of a resistance upturn, accompanied by a large positive magnetoresistance at low temperature, could not be explained by the Kondo model, or by the magnetoresistance related to spin-momentum locking alone. The magnetoresistance of the samples with an applied top gate was also anomalous and could not be described by weak antilocalization theory, size effects or classical magnetoresistance alone. In a broader perspective, the development of top-gate tunable, structured high-mobility LaAlO3-SrTiO3 devices is a major step towards the creation of a quantum point contact in an oxide heterostructure. The fabrication of such a device could lead to the first observation of (quantum) ballistic transport in a strongly correlated electron system like LaAlO3-SrTiO3, enabling the study of new phenomena arising in such devices.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Nanotechnology MSc (60028)
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