engleski

Nanoscale investigation of physical properties of LaAlO_3/SrTiO_3 interfaces

The physical properties of many perovskite oxides are critically dependent on either changes of the stoichiometry or on pure electronic doping. These effects may become particularly significant at the interface between two different oxides because of the structural and electronic mismatches of both compounds. Usually, the disparity of their properties results in strain fields or in diverging electrical potentials that may drive to either local changes of chemical composition or to charge transfer effects at the interface. These interface effects may lead to the emergence of physical functionalities at the interface which may be different to those of bulk, adding a larger flexibility to tune physical properties and opening up promising prospects for multifunctional devices. One particularly intriguing example of multifunctional oxide is that of SrTiO_3. In its stoichiometric form is a band insulator, but for tiny amounts of some dopants, of the order of 10^17 cm^(-3), it becomes metallic with high mobility conduction at low temperature. Also recently it has been suggested that pure electron charge transfer may dope the interface between a polar oxide (as LaAlO_3) and SrTiO_3, leading also to a high-mobility conduction. To fully understand the properties of these interfaces, we have made an effort to combine different advanced characterization tools with special emphasis on the use of spatially resolved characterization techniques. One step in that direction is the direct determination of the carrier density profile through resistance profile mappings collected in cross-section LaAlO_3/SrTiO_3 samples with a conducting-tip atomic force microscope. We have found that, depending on specific growth protocols, the spatial extension of the high-mobility electron gas can be varied from hundreds of micrometers into SrTiO_3 to a few nanometers next to the LaAlO_3/SrTiO_3 interface. Our results reveal that both interface conduction and bulk conduction due to oxygen vacancies can occur depending on the growth and annealing process. By using positron annihilation spectroscopy in combination with low-temperature magnetotransport characterization we have studied the mechanisms of defect diffusion in SrTiO_3 in the regime where oxygen vacancies are relevant. We show from this study that oxygen vacancy diffusion may lead to an alternative way to carrier confinement below the Fermi wavelength. Our results emphasize the relevance of using advanced interface characterization tools able to both resolve spatially the physical properties and also to detect small changes of the stoichiometry at the interface of complex oxides.

oxide heterostructures; high mobility electron gas; oxygen vacancies; spintronics

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