High mobile electron gas at LaAlO 3 /SrTiO 3 heterointerface
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High mobile electron gas at LaAlO3/SrTiO3 heterointerface Shanshan Su and Jeong Ho You Department of Mechanical Engineering, Southern Methodist University, Dallas TX ABSTRACT We calculated the mobility of two-dimensional electron gas along an n-type interface in LaAlO3/SrTiO3 heterostructure using the linearized Boltzmann equation. By solving the Schrödinger equation with the Poisson equation self-consistently, it was found that the interface remained non-conducting up to four unit cells of LaAlO3 film. For five or higher unit cells, the interface became conducting due to the significant overlap between the SrTiO3 conduction band and the LaAlO3 valence band. The electron gas was localized within 7 nm from the interface and multi-subbands were occupied. The calculated mobility matches reasonably well with available experimental data. It was found that the mobility is limited by the remote ionic charged layers in LaAlO3 at low temperature. At high temperature, the polar optical phonon was found to be the dominant scattering center. INTRODUCTION Heterostructures consisting of two perovskite band insulators, LaAlO3 (LAO) and SrTiO3 (STO) have received much attention due to the presence of high mobile two-dimensional electron gas (2DEG) at the interface [1]. This exciting conducting behavior has motivated many research activities in the past few years to understand the origin of 2DEG and high conductivity. Thiel et al.[2] found that the n-type LAO/STO interface becomes conducting with the LAO thickness above 4 unit cells (u.c.). Huijben et al.[3] found the critical separation distance between n- and p-type interfaces, below which the electron density increases with the separation distance. The existence of parallel electron-hole bilayer was observed when a STO capping layer was added on LAO films [4]. Density functional theory (DFT) calculations have been commonly employed to understand the origin of 2DEG. The calculated partial density of states in a LAO/STO/LAO slab indicates that with exceeding the critical LAO thickness of 4 u.c., electrons have transferred from the LAO valence band to the STO conduction band [5]. Pentcheva and Pickett [6] also found that the band gap closes with 5 u.c. of LAO, resulting in generating electrons at the interface and holes at the surface. However, the system size in DFT calculations is limited due to high computational cost and a reliable calculation method for the mobility in LAO/STO interface is currently unavailable. In this work, for the first time, we present the mobility calculation for the 2DEG along LAO/STO interface using the linearized Boltzmann equation. The system of interest contains an n-type interface with the various LAO film thicknesses of 1-30 u.c. on a 40 u.c.-thick STO substrate. We assume that the interface is defect-free and there are no oxygen vacancies. CALCULATION METHODS The mobility of the 2DEG along LAO/STO interface is calculated in two steps. First, energy levels and wavefunctions of electrons and holes are calculated by solving Schrödinger equation with P
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