Research ArticleCHEMICAL PHYSICS

Probing vacancy behavior across complex oxide heterointerfaces

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Science Advances  22 Feb 2019:
Vol. 5, no. 2, eaau8467
DOI: 10.1126/sciadv.aau8467
  • Fig. 1 The schematic of the measurement system and the structure of the multilayer oxide films.

    (A) Schematic of the high-temperature in situ potential scanning geometry. (B) Low-magnification HAADF image of the multilayer sample. (C and D) High-magnification HAADF images of interfaces between Nb:STO and first STO, first STO, and first YSZ layers, respectively. The insets show selected-area electron diffraction patterns of each layer. The yellow circles indicated by the arrow in (D) represent the polycrystallinity of the first YSZ layer. Images in (C) and (D) were collected from red and blue parts of (B), respectively.

  • Fig. 2 A comparison of in situ surface potential on STO/YSZ multilayer oxide films and thermally treated Nb:STO(001) substrate under room and elevated (500°C) temperatures.

    (A) Topographic (top) and surface potential (bottom) images and (B) extracted potential profile results collected with biases of 0 V at 500°C. A large potential variation occurs across the Nb:STO substrate and continues throughout the multilayer film. (C and D) Potential profiles collected from STO/YSZ multilayer oxide films with biases ranging from 0 to 3 V at room temperature (RT) (C) and at 500°C (D) suggest electronic- and ionic-dominant transport, respectively. (E and F) Potential profiles collected on the thermally treated Nb:STO(001) substrate with biases ranging from 0 to 3 V at room temperature (E) and at 500°C (F) remain consistently flat across the sample.

  • Fig. 3 The oxygen vacancy concentration distribution across STO/YSZ multilayer oxide films.

    (A) Illustrated schematic of the band diagram at the Nb:STO/STO interface. Here, the contact potential difference (CPD) measured by SSPM is used to determine the offset of the Fermi band from the conduction band, as a function of position in the z direction. (B) Surface potential profile collected in situ at 500°C (red) and resulting oxygen vacancy concentration distribution across the STO/YSZ multilayer oxide films (black). The oxygen vacancy concentration decreased from STO to YSZ near the interface and extends ~7 μm into the bulk Nb:STO substrate. The calculated oxygen vacancy concentration in YSZ films indicates an abrupt increase compared to the oxygen vacancy concentration in STO probed and derived from the surface potential (dashed red lines).

  • Fig. 4 The PLD induced substrate reduction and reoxidation process.

    (A) Deposited films scavenge oxygen from the substrate due to the limited oxygen exchange rate with the atmosphere in the PLD chamber. (B) Reduction of the substrate becomes more severe with the increasing thickness of the deposited films and the introduction of subsequent layers. (C) Mechanism of the formation of large oxygen vacancy annihilation region. After deposition, the oxygen incorporates back into the substrate through the multilayer films as the PLD chamber is backfilled with ~600 torr of oxygen and cooled from 750°C.

  • Fig. 5 The effects of annealing in oxidizing environments on the evolution of STO/YSZ multilayer surface potential profiles.

    The surface potential of the STO/YSZ multilayer films annealed at 600°C for 24 hours under ambient conditions, measured at 500°C in situ. The annealing process drastically reduces Embedded Image, and the potential profile becomes consistently flat across the Nb:STO substrate as compared to the as-prepared profiles of the same sample in Fig. 2D.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaau8467/DC1

    Section S1. Out-of-plane XRD diffraction pattern of the multilayer films

    Section S2. Sample geometry and preparation process after film deposition

    Section S3. Steady-state currents of multilayer films during in situ SSPM at 500°C

    Section S4. Oxygen vacancy concentration calculation

    Section S5. Oxygen vacancy concentrations of reference samples

    Section S6. Temperature profile during cooling process

    Section S7. In situ SSPM on YSZ/Nb:STO and STO/Nb:STO control samples

    Fig. S1. θ-2θ XRD pattern of STO/YSZ/STO/YSZ multilayer films on Nb:STO (001) substrate.

    Fig. S2. Cut/polish sequence, possible oxygen incorporation pathways, and scan geometry.

    Fig. S3. Current measured on multilayer films under 0 to 3 V at 500°C.

    Fig. S4. SSPM calibration data collected on different STO substrates.

    Fig. S5. The temperature versus time (cooling) curve of the sample upon chamber backfill.

    Fig. S6. SSPM, Nb:STO/YSZ film (10 mtorr), Nb:STO/STO film, Nb:STO/YSZ film (1 mtorr).

    References (5154)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. Out-of-plane XRD diffraction pattern of the multilayer films
    • Section S2. Sample geometry and preparation process after film deposition
    • Section S3. Steady-state currents of multilayer films during in situ SSPM at 500°C
    • Section S4. Oxygen vacancy concentration calculation
    • Section S5. Oxygen vacancy concentrations of reference samples
    • Section S6. Temperature profile during cooling process
    • Section S7. In situ SSPM on YSZ/Nb:STO and STO/Nb:STO control samples
    • Fig. S1. θ-2θ XRD pattern of STO/YSZ/STO/YSZ multilayer films on Nb:STO (001) substrate.
    • Fig. S2. Cut/polish sequence, possible oxygen incorporation pathways, and scan geometry.
    • Fig. S3. Current measured on multilayer films under 0 to 3 V at 500°C.
    • Fig. S4. SSPM calibration data collected on different STO substrates.
    • Fig. S5. The temperature versus time (cooling) curve of the sample upon chamber backfill.
    • Fig. S6. SSPM, Nb:STO/YSZ film (10 mtorr), Nb:STO/STO film, Nb:STO/YSZ film (1 mtorr).
    • References (5154)

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