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Electrical transport and microstructure of interfaces between nm-thick films of various perovskite oxides grown by pulsed laser deposition (PLD) on TiO2- terminated SrTiO3 (STO) substrates are compared. LaAlO3/STO and KTaO3/STO interfaces become quasi-2DEG after a critical film thickness of 4 unit cell layers. The conductivity survives long anneals in oxygen atmosphere. LaMnO3/STO interfaces remain insulating for all film thicknesses and NdGaO3/STO interfaces are conducting but the conductivity is eliminated after oxygen annealing. Medium-energy ion spectroscopy and scanning transmission electron microscopy detect cationic intermixing within several atomic layers from the interface in all studied interfaces. Our results indicate that the electrical reconstruction in the polar oxide interfaces is a complex combination of different mechanisms, and oxygen vacancies play an important role.
This is a copy of the slides presented at the meeting but not formally written up for the volume.
Recent advances in oxide electronics are indebted to an increased understanding of the material properties at the atomic level. The complex chemical structure of oxides also offers more flexibility: various electronic properties from semiconductors to superconductors and multiferroics, can be tailored by proper materials combinations. The microscopic properties of perovskite oxide interfaces may have a strong impact on the electrical transport properties at and near these interfaces. Recently, the interface between two similar oxide materials, SrTiO3 and LaAlO3, has received much attention since it is suggested to give rise to a two-dimensional electron gas. The polar discontinuity due to different ionic valences of Sr and La can be one possible reason for the interfacial charging. However, other factors like strain in the LaAlO3 film, presence of oxygen vacancies in the SrTiO3 substrate and atomic inter-diffusion may also affect the interface properties. Here we present a detailed experimental investigation of optical, electrical and microstructural properties of heterointerfaces between SrTiO3 and LaAlO3. We found that the interface properties strongly depend on the oxygen pressure conditions during growth of LaAlO3 films. Cathode- and photoluminescence observed from LaAlO3 films grown at 10-6 mbar oxygen pressure was exactly the same as from oxygen reduced SrTiO3 substrates. Additionally, electrical Hall mobility follows the same power-law dependence similar to the one measured in oxygen reduced SrTiO3, suggesting that oxygen vacancies in SrTiO3 can in fact be responsible for the high conductivity at the interface. This is also confirmed on a microscopic level by the findings of local strain fields at the interface reaching 10 nm into the SrTiO3 substrate.
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