The conductance confined at the interface of complex oxide heterostructures provides new opportunities to explore nanoelectronic as well as nanoionic devices. Herein we show that metallic interfaces can be realized in SrTiO 3 -based heterostructures with various insulating overlayers of amorphous LaAlO 3 , SrTiO 3 and yttria-stabilized zirconia films. On the other hand, samples of amorphous La 7/8 Sr 1/8 MnO 3 films on SrTiO 3 substrates remain insulating. The interfacial conductivity results from the formation of oxygen vacancies near the interface, suggesting that the redox reactions on the surface of SrTiO 3 substrates play an important role.

The discovery of two-dimensional electron gases at the heterointerface between two insulating perovskite-type oxides, such as LaAlO 3 and SrTiO 3 , provides opportunities for a new generation of all-oxide electronic devices. Key challenges remain for achieving interfacial electron mobilities much beyond the current value of approximately 1,000 cm 2 V -1 s -1 (at low temperatures). Here we create a new type of two-dimensional electron gas at the heterointerface between SrTiO 3 and a spinel g-Al 2 O 3 epitaxial film with compatible oxygen ions sublattices. Electron mobilities more than one order of magnitude higher than those of hitherto-investigated perovskite-type interfaces are obtained. The spinel/perovskite twodimensional electron gas, where the two-dimensional conduction character is revealed by quantum magnetoresistance oscillations, is found to result from interface-stabilized oxygen vacancies confined within a layer of 0.9 nm in proximity to the interface. Our findings pave the way for studies of mesoscopic physics with complex oxides and design of high-mobility all-oxide electronic devices.

Document VersionEarly version, also known as pre-print Link back to DTU Orbit Citation (APA):Chen, Y., Trier, F., Wijnands, T., Green, R. J., Gauquelin, N., Egoavil, R., ... Pryds, N. (2015). Extreme mobility enhancement of two-dimensional electron gases at oxide interfaces via charge transfer induced modulation doping. Nature Materials, 14(8) Supplementary Information, Fig. S1). For all samples, with the d-LAO film thickness up to 20 nm, atomic force microscopy (AFM) images show regular surface terraces with a step height of 0.4 nm (Fig. 1b), similar to that of the original STO substrate and indicating uniform film growth. Representative samples were investigated further by scanning transmission electron microscopy (STEM). Our subsequent spectroscopic measurements reveal dramatic electronic reconstruction in the LSMO-buffered samples. Firstly, different from the unbuffered d-LAO/STO sample where the 2DEG is coupled strongly to a large content of oxygen vacancies extending more than 3 nm deep into STO 24 , all buffered samples show a rather low content of Ti 3+ far below the detection limit of our in situ X-ray photoelectron

Oxygen vacancies play crucial roles in determining the physical properties of metal oxides, representing important building blocks in many scientific and technological fields due to their unique chemical, physical, and electronic properties. However, oxygen vacancies are often invisible because of their dilute concentrations. Therefore, characterizing and quantifying their presence is of utmost importance for understanding and realizing functional metal oxide devices. This, however, is oftentimes a non-trivial task. In this Perspective paper, we discuss the relevant regimes of concentrations and associated phenomena arising from oxygen vacancies. We then focus on experimental techniques available for observing oxygen vacancies at widely different levels of concentrations. Finally, we discuss current challenges and opportunities for utilizing oxygen vacancies in metal oxides.

ABSTRACT:The discovery of two-dimensional electron gases (2DEGs) in SrTiO 3 -based heterostructures provides new opportunities for nanoelectronics. Herein, we create a new type of oxide 2DEG by the epitaxial-strain-induced polarization at an otherwise nonpolar perovskite-type interface of CaZrO 3 /SrTiO 3 . Remarkably, this heterointerface is atomically sharp, and exhibits a high electron mobility exceeding 60,000 cm 2 V -1 s -1 at low temperatures. The 2DEG carrier density exhibits a critical dependence on the film thickness, in good agreement with the polarization induced 2DEG scheme.* Corresponding Author. Email:yunc@dtu.dk. Phone: +45 4677 5614.2 KEYWORDS: Complex oxide interfaces, oxide electronics, two-dimensional electron gases, strain induced polarization.Atomically engineered complex oxide heterostructures exhibit a variety of exotic interfacial properties because of strong interactions among the spin, charge, and orbital freedoms as well as lattice vibrations.One particular example is the emergence of high mobility two-dimensional electron gases (2DEGs) at the interface between two oxide insulators, 1,2 one of which is SrTiO 3 (STO), the basis material of oxide electronics. These complex oxide 2DEGs consist of strongly coupled electrons and give rise to a rich set of physical phenomena 3-5 , for example, superconductivity 6,7 , magnetism 8,9 , and tunable metal-insulator transitions on nanoscale, 10,11 providing new opportunities for nanoelectronics and mesoscopic physics. Under optimized conditions, the CZO films deposited by pulsed laser deposition (PLD) can be epitaxially grown on the (001) TiO 2 -terminated STO substrates within a layer-by-layer two-dimensional growth mode, as confirmed by the presence of periodic intensity oscillations of the reflection highenergy electron diffraction (RHEED) pattern monitored in-situ during film growth (Supporting information, Fig.S1). Both RHEED intensity oscillations and sharp RHEED patterns can persist up to a film thickness over 50 unit cells (uc), suggesting high quality film growth. A terrace surface of the grown heterostructure is detected by atomic force microscopy (AFM), which shows a regular step height of 0.4 nm (Fig. 1b). High-resolution X-ray diffraction (XRD) further confirms the epitaxial growth of the (001) We further investigated the atomic structure and interface chemistry of our CZO/STO heterostructures by an aberration corrected scanning transmission electron microscopy (STEM) in combination with electron energy-loss spectroscopy (EELS). Figure 2a shows a high-angle annular dark field (HAADF) STEM image of a CZO/STO sample with the CZO layer of approximately 50 uc (~20 nm). The CZO film is found to be coherent with the STO substrate with no obvious defects or dislocations at the interface. The averaged line profiles (Fig. 2b) Fig.1. In a similar zirconate system of SrZrO 3 /SrTiO 3 , the compressive strain has been reported to result in ferroelectricity in its superlattices. 27 Theoretical calculations by first principle density functional ...