Research ArticleMATERIALS SCIENCE

How heteroepitaxy occurs on strontium titanate

See allHide authors and affiliations

Science Advances  12 Apr 2019:
Vol. 5, no. 4, eaav0764
DOI: 10.1126/sciadv.aav0764
  • Fig. 1 Depiction of the STO surface and its behavior during epitaxial growth.

    (A) Bare STO (001) surface after following the standard etch-and-anneal procedure, exhibiting ~0.4-nm-high steps with both (13×13)R33.7° (RT13) and (2×2)R45.0° (RT2) surface reconstructions. (B) Schematic of LaTiO3 growth by oxide MBE following either the TiO2/LaO (T/L) or LaO/TiO2 (L/T) growth sequence at 700°C in a background of 1 × 10−7 torr O2.

  • Fig. 2 The atomic structures of STO surfaces.

    Fitted specular reflectivity curves (top) for the bare (001) STO substrates modeled using mixtures of the (13×13)R33.7° (RT13) (29) and (2×2)R45.0° (RT2) (37) reconstructions (below). DFT-constrained fit results (dashed lines) for the bare STO #1 (A) and STO #2 (B) substrates. The intensity is given by the square of absolute x-ray structure factor (|F|2). Points near the Bragg peaks were excluded from the fits.

  • Fig. 3 Specular reflectivity curves after deposition of LaO and TiO2 MLs for different deposition sequences.

    The results for T/L/STO #1 and L/T/STO #2 are shown in (A) and (B), respectively. Measurements were performed with x-ray energies of 15.0 keV (red) and 16.1 keV (blue) for scattering under NR and R conditions at the Sr K-edge, respectively. Note the nearly identical reflectivity curves following the two different deposition sequences, which indicate the rearrangement of atomic layers.

  • Fig. 4 Fitted specular reflectivity curves (top) for the final structures after the T/L/STO #1 and L/T/STO #2 depositions modeled as mixtures of the TTLT and TLT surface structures (bottom).

    Fit results are presented as dashed lines for the T/L/STO #1 (A) and L/T/STO #2 (B) final structures. Points near the Bragg peaks were excluded from the fits.

  • Fig. 5 Schematic illustration of the scattering geometry used in the time-resolved measurements of the specular and diffuse intensities during oxide MBE.

    The measurements were performed at the 0012 anti-Bragg peak during the deposition processes, where the scattered intensities were collected by a Pilatus 100K area detector (detector image is shown after background subtraction). The plane of the incident (ki) and scattered (kf) wave vectors is set to be roughly perpendicular to the step edges. Specular and diffuse components could be seen, and slice of the data along the in-plane momentum transfer (q||) direction is shown in the inset plot, where the diffuse scattering lobe has a peak position at q|| = q0. The in situ scattering data and ex situ atomic force microscopy (AFM) image (2 μm × 2 μm scan area) are not related and are shown for demonstration purposes only. a.u., arbitrary units.

  • Fig. 6 Time-resolved measurements of the diffuse and specular intensities at the 0012 anti-Bragg peak during oxide MBE.

    (A and B) Results for the T/L/STO #1 deposition sequence. (C and D) Results for the L/T/STO #2 deposition sequence. (E and F) False-color images of the scattered intensities, along with the timing diagrams indicating the deposited thin-film material. Solid and dashed vertical lines indicate the times at which the shutter was opened and closed, respectively. An empirical error function (erf) was fitted to each of the integrated specular intensities measured after the deposition of the final MLs to characterize the time scales of the relaxation behaviors [dashed black curves in (C) and (F)].

  • Fig. 7 Diffuse scattering fit parameters over the course of the final LaO ML deposition in the L/T/STO #2 deposition sequence.

    The peak position q0 and a measure of the peak width ξ for the diffuse lobe near the 0012 peak are extracted. The mean interisland distance, di-i = 2π/q0, is also plotted.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/4/eaav0764/DC1

    Section S1. AFM images of prepared (001) STO substrates

    Section S2. Details of the DFT-constrained fitting of specular reflectivity curves

    Section S3. Calculated specular reflectivity curves of model STO (001) surfaces

    Section S4. Calculated specular reflectivity curves of model thin-film structures

    Section S5. Details of the SXRD characterization of the bare (001) substrates

    Section S6. Details of the SXRD characterization of the surface structures after the T/L and L/T deposition sequences

    Section S7. Details of the SXRD characterization of the surface structure after 1-ML LaO deposition

    Section S8. Determining the optimal weighting between the experimental data and DFT-derived structures

    Section S9. Determining the fraction of the coherent sum for the T/L and L/T reflectivity data

    Section S10. Fit parameters for the diffuse scattering component after deposition of the initial LaO and TiO2 MLs

    Section S11. Ex situ AFM images taken after the T/L and L/T depositions

    Fig. S1. Surface morphologies of (001)-oriented STO substrates prepared using the etch-and-anneal method described in (2) by AFM.

    Fig. S2. Calculated specular reflectivity curves of single-layer SrO (1 × 1-S) and TiO2 (1 × 1-T) surface terminations and TiO2 DL (1 × 1-TT) surface termination.

    Fig. S3. Calculated specular reflectivity curves of several TiO2 DL STO (001) surface reconstructions under 15.0-keV NR conditions and 16.1-keV R conditions for absorption at the Sr K-edge.

    Fig. S4. DFT-derived structures of possible layer sequences after deposition of LaO (L) and/or TiO2 (T) ML(s) on a TiO2 DL–terminated STO (001) surface and their calculated specular reflectivity curves.

    Fig. S5. Fitted specular reflectivity curves taken after the LaO ML deposition in the T/L deposition sequence, in which the surface is modeled as a mixture of TLTL, TTL, TL, and TT model structures.

    Fig. S6. Construction of a Pareto frontier to determine the optimal weighting between the experimental data and DFT-derived structures.

    Fig. S7. Determination of the fraction of the coherent sum in fitting the specular reflectivity data for the T/L and L/T structures.

    Fig. S8. Ex situ AFM images taken after the T/L and L/T deposition sequences.

    Table S1. Measures of the deviations of the fitted atomic positions from the DFT-derived minimum-energy positions (zrms and Edist) and the goodness of fit (χ2) for the bare (001) STO substrates modeled using the RT13 and RT2 reconstructions.

    Table S2. Measures of the deviations of the fitted atomic positions from the DFT-derived minimum-energy positions (zrms and Edist) and the goodness of fit (χ2) for the surfaces resulting after the T/L and L/T deposition sequences, modeled using the TLTT and TLT model structures.

    Table S3. Measures of the deviations of the fitted atomic positions from the DFT-derived minimum-energy positions (zrms and Edist) and the goodness of fit (χ2) for the final structures modeled as mixtures of the TLTL, TTL, TL, and TT surface structures.

    Table S4. Fit parameters for the diffuse scattering component after depositing the initial LaO and TiO2 MLs on each (001) STO substrate (STO #1 and STO #2).

    References (5761)

  • Supplementary Materials

    This PDF file includes:

    • Section S1. AFM images of prepared (001) STO substrates
    • Section S2. Details of the DFT-constrained fitting of specular reflectivity curves
    • Section S3. Calculated specular reflectivity curves of model STO (001) surfaces
    • Section S4. Calculated specular reflectivity curves of model thin-film structures
    • Section S5. Details of the SXRD characterization of the bare (001) substrates
    • Section S6. Details of the SXRD characterization of the surface structures after the T/L and L/T deposition sequences
    • Section S7. Details of the SXRD characterization of the surface structure after 1-ML LaO deposition
    • Section S8. Determining the optimal weighting between the experimental data and DFT-derived structures
    • Section S9. Determining the fraction of the coherent sum for the T/L and L/T reflectivity data
    • Section S10. Fit parameters for the diffuse scattering component after deposition of the initial LaO and TiO2 MLs
    • Section S11. Ex situ AFM images taken after the T/L and L/T depositions
    • Fig. S1. Surface morphologies of (001)-oriented STO substrates prepared using the etch-and-anneal method described in (2) by AFM.
    • Fig. S2. Calculated specular reflectivity curves of single-layer SrO (1 × 1-S) and TiO2 (1 × 1-T) surface terminations and TiO2 DL (1 × 1-TT) surface termination.
    • Fig. S3. Calculated specular reflectivity curves of several TiO2 DL STO (001) surface reconstructions under 15.0-keV NR conditions and 16.1-keV R conditions for absorption at the Sr K-edge.
    • Fig. S4. DFT-derived structures of possible layer sequences after deposition of LaO (L) and/or TiO2 (T) ML(s) on a TiO2 DL–terminated STO (001) surface and their calculated specular reflectivity curves.
    • Fig. S5. Fitted specular reflectivity curves taken after the LaO ML deposition in the T/L deposition sequence, in which the surface is modeled as a mixture of TLTL, TTL, TL, and TT model structures.
    • Fig. S6. Construction of a Pareto frontier to determine the optimal weighting between the experimental data and DFT-derived structures.
    • Fig. S7. Determination of the fraction of the coherent sum in fitting the specular reflectivity data for the T/L and L/T structures.
    • Fig. S8. Ex situ AFM images taken after the T/L and L/T deposition sequences.
    • Table S1. Measures of the deviations of the fitted atomic positions from the DFT-derived minimum-energy positions (zrms and Edist) and the goodness of fit (χ2) for the bare (001) STO substrates modeled using the RT13 and RT2 reconstructions.
    • Table S2. Measures of the deviations of the fitted atomic positions from the DFT-derived minimum-energy positions (zrms and Edist) and the goodness of fit (χ2) for the surfaces resulting after the T/L and L/T deposition sequences, modeled using the TLTT and TLT model structures.
    • Table S3. Measures of the deviations of the fitted atomic positions from the DFT-derived minimum-energy positions (zrms and Edist) and the goodness of fit (χ2) for the final structures modeled as mixtures of the TLTL, TTL, TL, and TT surface structures.
    • Table S4. Fit parameters for the diffuse scattering component after depositing the initial LaO and TiO2 MLs on each (001) STO substrate (STO #1 and STO #2).
    • References (5761)

    Download PDF

    Files in this Data Supplement:

Navigate This Article