Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions

Journal name:
Nature Physics
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Ionic crystals terminated at oppositely charged polar surfaces are inherently unstable and expected to undergo surface reconstructions to maintain electrostatic stability. Essentially, an electric field that arises between oppositely charged atomic planes gives rise to a built-in potential that diverges with thickness. Here we present evidence of such a built-in potential across polar LaAlO3 thin films grown on SrTiO3 substrates, a system well known for the electron gas that forms at the interface. By carrying out tunnelling measurements between the electron gas and metallic electrodes on LaAlO3 we measure a built-in electric field across LaAlO3 of 80.1 meVÅ−1. In addition, capacitance measurements reveal the presence of an induced dipole moment across the heterostructure. We foresee use of the ionic built-in potential as an additional tuning parameter in both existing and future device architectures, especially as atomic control of oxide interfaces gains widespread momentum.

At a glance


  1. Built-in polarization across LaAlO3/SrTiO3 tunnel junction diodes.
    Figure 1: Built-in polarization across LaAlO3/SrTiO3 tunnel junction diodes.

    a, Tunnel junctions are formed between thermally evaporated circular metallic electrodes on LaAlO3 and the electron gas, as described in the Methods section. bJV curve measured at 10K for a 20u.c. sample compared to curves calculated within the direct (black lines) and Zener tunnelling (blue line) models as labelled, and described in Supplementary Section SI. cJ versus LaAlO3 thickness, dLAO, for all samples at V =−0.1V showing a clear peak at 20u.c. (7.56nm). Calculated curves for direct tunnelling (dark blue) and Zener tunnelling (light blue) are labelled and shown. The inset provides a closer look at the negative bias region of JV for several thicknesses showing that J does not scale monotonically with thickness for a given V (see the dotted line at V =−0.1V).

  2. Thickness-dependent built-in potential and interband tunnelling across polar LaAlO3.
    Figure 2: Thickness-dependent built-in potential and interband tunnelling across polar LaAlO3.

    a, A schematic band diagram of the LaAlO3/SrTiO3 interface at zero bias (black outline) for LaAlO3 thicknesses much less than the critical thickness, dLAOcr, for both small positive applied bias (+V, green outline on top sketch) and small negative applied bias (−V, green outline on bottom sketch). Unlike typical capacitors where bending across the dielectric is induced by the electrode workfunction difference alone, Vm, in LaAlO3 an intrinsic built-in potential, Vi, adds to the band bending. Following conventions used for the polar nitrides19, the bending in LaAlO3 reflects the ionic built-in potential as well as bending resulting from the Pt and SrTiO3 chemical potential difference, as described in detail in the text and in Supplementary Fig S2. In treating the LaAlO3 as a wide-bandgap insulator with a midgap Fermi level, we have not included any curvature that may appear in the LaAlO3 bands owing to metal-induced gap states. b, Our measurements suggest that at dLAOcr, the valence and conduction band of LaAlO3 align at the Fermi level giving rise to Zener tunnelling across the LaAlO3 bandgap for −V. The band diagrams shown reflect this observation. The metal and electron gas remain pinned at the Fermi level whereas the potential across LaAlO3 increases with thickness. Given the excellent screening and small skin depth of Pt on the left, bending in the electrode is not shown and the band alignments give the appearance of changing. At the SrTiO3/LaAlO3 interface, assuming the SrTiO3 is a semiconductor, a strong change in charge density is expected with increasing thickness but not observed23, and hence not shown. Thus, screening at the LaAlO3/SrTiO3 interface may be aided by a dipole layer in SrTiO3 (refs 33, 34). We also note that surface reconstructions, if present, add a degree of ambiguity to determining precise band alignments15. In Supplementary Section SI and Fig. S2 we provide an in-depth discussion on thickness-dependent band bending across LaAlO3 and the resulting band diagrams.

  3. Tuning the tunnelling current across LaAlO3 by tuning the SrTiO3 permittivity and charge density.
    Figure 3: Tuning the tunnelling current across LaAlO3 by tuning the SrTiO3 permittivity and charge density.

    aJV curves for a 20u.c. LaAlO3 sample for several temperatures with theoretical fits to the direct tunnelling model (black). The inset schematically depicts how changes in band bending in the SrTiO3 interface region produce changes in the barrier height . bJ and the barrier heights ( ) extracted from fits to the direct tunnelling model for each of the JV curves shown in a as a function of temperature. Here JexpT. cJV for a 30u.c. LaAlO3 at 50K for several different positive and negative applied SrTiO3 backgate fields, Ebg. The effect is much more pronounced for negative rather than positive biases, with JexpEbg. (The lowest point deviates from the linear trend probably because it is near measurement limits.) d, Each of the JV curves shown in c was also fitted to the direct tunnelling model.  increases linearly with increasingly negative backgate fields whereas J decreases logarithmically.

  4. Capacitance measurements agree with JV and also reveal an induced dipole across the heterostructure.
    Figure 4: Capacitance measurements agree with JV and also reveal an induced dipole across the heterostructure.

    aCV curves for a 30u.c. sample measured at 10K and 10kHz are qualitatively similar to a metal–insulator–semiconductor capacitor curve with a ferroelectric contribution, or a metal–insulator-ferroelectric–semiconductor capacitor. The drop in CV occurs during the Zener tunnelling regime when carriers are introduced into the LaAlO3 bandgap, making it a leaky dielectric. b, The phase angle of the measured complex impedance as shown at several temperatures. A hysteresis appears near 100 K and increases with decreasing temperature. c,d, The hysteresis is frequency independent for frequencies below the RC roll-off limit (c), and the hysteresis window, ΔV, increases with the maximum applied voltage, Vmax, for a given sweep (d). bd together qualitatively provide indications of dipole switching at the interface in SrTiO3.


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Author information


  1. Department of Physics, University of California, Berkeley, California 94720, USA

    • Guneeta Singh-Bhalla,
    • Wolter Siemons &
    • Ramamoorthy Ramesh
  2. Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Guneeta Singh-Bhalla,
    • Jayakanth Ravichandran &
    • Ramamoorthy Ramesh
  3. Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8561, Japan

    • Guneeta Singh-Bhalla,
    • Christopher Bell,
    • Yasuyuki Hikita &
    • Harold Y. Hwang
  4. Department of Physics, University of Florida, Gainesville, Florida 32611, USA

    • Guneeta Singh-Bhalla &
    • Arthur F. Hebard
  5. Japan Science and Technology Agency, Kawaguchi, 332-0012, Japan

    • Christopher Bell &
    • Harold Y. Hwang
  6. Applied Science and Technology Graduate Group, University of California, Berkeley, California 94720, USA

    • Jayakanth Ravichandran
  7. Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, California 94720, USA

    • Sayeef Salahuddin
  8. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA

    • Ramamoorthy Ramesh


C.B. deposited the LaAlO3 films. G.S-B. prepared and measured the tunnel junctions with C.B., modelled the data with J.R. and analysed the JV curves with J.R. and W.S. S.S. simulated the JV curves within the non-equilibrium Green’s function approach. The manuscript was prepared by G.S-B. with assistance/input from C.B., J.R., W.S. and Y.H. H.Y.H., A.F.H. and G.S-B. contributed to conceptualizing the experiment. H.Y.H. provided insights and expertise on the LaAlO3/SrTiO3 interface, R.R. on ferroelectricity and A.F.H. on interpreting complex impedance.

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