Judith Gabel (PhD thesis): Interface Engineering of Functional Oxides: A Photoemission Study

Abstract

Due to their complex chemical structure transition metal oxides display many fascinating properties which conventional semiconductors lack. For this reason transition metal oxides hold alot of promise for novel electronic functionalities. Just as in conventional semiconductor heterostructures, the interfaces between different materials play a key role in oxide electronics. The textbook example is the (001) interface between the band insulators LaAlO₃ and SrTiO₃ at which a two-dimensional electron system (2DES) forms. In order to utilize such a 2DES inprospective electronic devices, it is vital that the electronic properties of the interface can be controlled and manipulated at will. Employing photoelectron spectroscopy as well as electronic transport measurements, this thesis examines how such interface engineering can be realized in the case of the LaAlO₃/SrTiO₃ heterostructure:
By photoemission we manage to unambiguously distinguish the different mechanisms by which SrTiO₃ can be doped with electrons. An electronic reconstruction is identified as the driving mechanism to render stoichiometric LaAlO₃/SrTiO₃ interfaces metallic. The doping of theLaAlO₃/SrTiO₃ heterointerface can furthermore be finely adjusted by changing the oxygen vacancy (VO) concentration in the heterostructure. Combining intense x-ray irradiation with oxygen dosing, we even achieve control over the VO concentration and, consequently, the doping in the photoemission experiment itself.
Exploiting this method, we investigate how the band diagram of SrTiO₃-based heterostructures changes as a function of the VO concentration and temperature by hard x-ray photoemission spectroscopy. With the band bending in the SrTiO₃ substrate changing as a function of the VO concentration, the interfacial band alignment is found to vary as well. The relative permittivity of the SrTiO₃ substrate and, in particular, its dependence on temperature and electric field is identified as one of the essential parameters determining the electronic interface properties. That is also why the sample temperature affects the charge carrier distribution. The mobile charge carriers are shown to shift toward the SrTiO₃ bulk when the sample temperature is lowered. This effect is, however, only pronounced if the total charge carrier concentration is small. At highcharge carrier concentrations the charge carriers are always confined to the interface, independentof the sample temperature.
The dependence of the electronic interface properties on the VO concentration is also investigated by a complementary method, viz. by electronic transport measurements. These experiments confirm that the mobile charge carrier concentration increases concomitantly to the VO concentration. The mobility of the charge carriers changes as well depending on the VO concentration. Comparing spectroscopy and transport results, we are able to draw conclusions about the processes limiting the mobility in electronic transport. We furthermore build a memristor device from our LaAlO₃/SrTiO₃ heterostructures and demonstrate how interface engineering is used in practice in such novel electronic applications.
This thesis furthermore investigates how the electronic structure of the 2DES is affected by the interface topology: We show that, akin to the (001) LaAlO₃/SrTiO₃ heterointerface, an electronic reconstruction also renders the (111) interface between LaAlO₃ and SrTiO₃ metallic. The change in interface topology becomes evident in the Fermi surface of the buried 2DES which is probed by soft x-ray photoemission. Based on the asymmetry in the Fermi surface, we estimate the extension of the conductive layer in the (111)-oriented LaAlO₃/SrTiO₃ heterostructure. The spectral function measured furthermore identifies the charge carriers at the interface as large polarons.

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