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Title: Study of strain effect to the LaAlO₃/SrTiO₃ heterostructure
Authors: Zhang, Fan
Degree: Ph.D.
Issue Date: 2019
Abstract: Oxide heterostructures have attracted a great deal of attention due to their potential for electronic devices applications and profound physics from transport properties to spintronics. Termed as one of the top 10 significant filed of breakthroughs by Science magazine in 2007, the LaAlO₃/SrTiO₃ two-dimensional electron gas (LAO/STO 2DEG) is becoming the most attractive topics as oxide heterostructures, where the interface structures have been proved to be crucial for their unique properties as 2DEG. To realize an interfacial transport property modulation at the LAO/STO heterostructure, many efforts were made through applying external stimulations like electric or magnetic field, UV light or surface modifications. Among those stimulations, mechanical strain is one of the most important physical factors that can intrinsically change a material's properties. In the LAO/STO heterostructure, mechanical strains can change the lattice geometry at the interface, break the lattice symmetry and thus influence the properties of the 2DEG systems. In this presentation, the effect of two types of strains on the LAO/STO 2DEG system will be introduced. The first type is a uniaxial strain induced through magnetostriction coupling. By driving a magnetostriction material with a magnetic field, dynamic uniaxial tensile or compressive strain can be applied on the LAO/STO heterostructure and induced anisotropic resistance increase or decrease at cryogenically low temperatures. This anisotropic strain results in symmetry breaking at the interface and induces further splitting of the electronic band structure and therefore produces different conductivities along the x and y in-plane directions. In particular, it is observed that along the strained direction the interface conductivity decreases by up to 70% under a tensile strain, while it increases by 6.8% under a compressive strain at 2 K. Through first principles calculation, it is found that the electron carrier densities and mobilities will be altered under both strain states. The second type is the bending induced inhomogeneous strain gradient, which can result in flexoelectricity across the heterostructure. The measured effective flexoelectric coefficient of the LAO film epitaxially grown on Nb doped STO substrate is determined to be thickness dependent and the 10 u.c. thick samples have the maximum average value of 46 μC/m, which is more than 4 orders larger than its intrinsic magnitude of bulk LAO. Influenced by the flexoelectric field, we detected that the interfacial resistance of LAO/STO heterostructure increased or decreased according to the sample bending direction. Through first principles calculation, it is found that the electron carrier density shows consistent change trend as the resistance change, showing that the resistance increases under a n-shape bending results from a carrier density decrease and vice versa. This further proves the capability of mechanical strains in modifying the oxide heterostructures' properties. Both the introduced approaches of mechanical strain engineering provide more freedom of control for transport properties of not only oxide heterostructures but also other thin film materials. These results broaden the horizon of study on dynamic strain modulation and flexoelectricity effect in many materials.
Subjects: Hong Kong Polytechnic University -- Dissertations
Oxides -- Electric properties
Electron gas
Pages: xxi, 112 pages : color illustrations
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