Fabrication of two-dimensional electron gas with high mobility in SrTiO_3 to develop novel quantum

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This project is aimed at realizing a two-dimensional electron gas in SrTiO_3. For that purpose, it is essential to fabricate heterostructures and to understand electronic structures at the heterointerfaces. We report our results related to this purpose.1. We have deposited LaAlO_3 films on SrTiO_3(100) substrates and probed the interface electronic structure by EELS. For SrO-terminated substrates, holes generated by the polar discontinuity were compensated by oxygen vacancies. This is the result of the removal of the band offset, which was also observed for an n-type interface originating from TiO_2-terminated substrates.2. We have probed the interface electronic structure at an interface of strontium titanate and transition-metal oxides by measuring junction properties. As a result, we have found a chemical potential shift by magnetic field at the interface of manganite, which modulates the junction properties. Precise analysis has revealed the importance of electron tunneling in the junction properties.3. We have observed the Schottky barrier height formed at SrRuO_3/Nb : SrTiO_3 interface by internal photoemission spectroscopy. As a result, the interface can be described by the Schottky-Mott model. We have found that this method can be generally applicable to other interfaces composed of transition-metal oxides.4. We have fabricated a quantum well structure by a Mott insulator and probed the electronic states. We have used lanthanum vanadate. As a result, we have found the existence of tetravalent vanadium, which does not exist in a bulk form, at the top layer of the vanadate due to a charge reconstruction by x-ray photoemission spectroscopy. In addition, a competing phase of trivalent vanadium and pentavalent vanadium is formed by adjusting growth conditions. We have also found tetravalent vanadium at the phase boundary by EELS.