Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions

Journal name:
Nature Physics
Volume:
7,
Pages:
80–86
Year published:
DOI:
doi:10.1038/nphys1814
Received
Accepted
Published online

Abstract

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

Figures

  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.

References

  1. Goniakowski, J., Finocchi, F. & Noguera, C. Polarity of oxide surfaces and nanostructures. Rep. Prog. Phys. 71, 016501 (2008).
  2. Stengel, M. & Vanderbilt, D. Berry-phase theory of polar discontinuities at oxide–oxide interfaces. Phys. Rev. B 80, 241103 (2009).
  3. Ambacher, O. et al. Two-dimensional electron gases induced by spontaneous and piezoelectric polarization charges in N- and Ga-face AlGaN/GaN heterostructures. J. Appl. Phys. 85, 32223233 (1999).
  4. Tsukazaki, A. et al. Quantum hall effect in polar oxide heterostructures. Science 315, 13881391 (2007).
  5. Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423426 (2004).
  6. Kawasaki, M. et al. Atomic control of the SrTiO3 crystal surface. Science 266, 15401542 (1994).
  7. Nakagawa, N., Hwang, H. Y. & Muller, D. A. Why some interfaces cannot be sharp. Nature Mater. 5, 204209 (2006).
  8. Tasker, P. W. The stability of ionic crystal surfaces. J. Phys. C 12, 49774984 (1979).
  9. Thiel, S., Hammerl, G., Schmehl, A., Schneider, C. W. & Mannhart, J. Tunable quasi-two-dimensional electron gases in oxide heterostructures. Science 313, 19421945 (2006).
  10. Segal, Y., Ngai, J. H., Reiner, J. W., Walker, F. J. & Ahn, C. H. X-ray photoemission studies of the metal–insulator transition in LaAlO3/SrTiO3 structures grown by molecular beam epitaxy. Phys. Rev. B 80, 241107 (2009).
  11. Pentcheva, R. & Pickett, W. E. Electronic phenomena at complex oxide interfaces: Insights from first principles. J. Phys.: Condens. Mater. 22, 043001 (2010).
  12. Siemons, W. et al. Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: Possibility of intrinsic doping. Phys. Rev. Lett. 98, 196802 (2007).
  13. Willmott, P. R. et al. Structural basis for the conducting interface between LaAlO3 and SrTiO3 . Phys. Rev. Lett. 99, 155502 (2007).
  14. Li, Y., Phattalung, S. N., Limpijumnong, S. & Yu, J. Oxygen-vacancy-induced charge carrier in n-type interface of LaAlO3 overlayer on SrTiO3: Interface versus bulk doping carrier. Preprint at http://arxiv.org/abs/0912.4805 (2009).
  15. Gu, X., Elfimov, I. S. & Sawatzky, G. A. The role of the band gaps in reconstruction of polar surfaces and interfaces. Preprint at http://arxiv.org/0911.4145 (2009).
  16. Zener, C. A theory of electrical breakdown of solid dielectrics. Proc. R. Soc. A 145, 523529 (1934).
  17. Simon, J. et al. Polarization-induced Zener tunnel junctions in wide-band-gap heterostructures. Phys. Rev. Lett. 103, 026801 (2009).
  18. Noguera, C. & Goniakowski, J. Polarity in oxide ultrathin films. J. Phys. Condens. Matter. (2008).
  19. Bykhovski, A., Gelmont, B., Shur, M. & Khan, A. Current–voltage characteristics of strained piezoelectric structures. J. Appl. Phys. 77, 16161620 (1995).
  20. Lim, S. G. et al. Dielectric functions and optical bandgaps of high-K dielectrics for metal–oxide–semiconductor field-effect transistors by far ultraviolet spectroscopic ellipsometry. J. Appl. Phys. 91, 45004505 (2002).
  21. Maurice, J-L. et al. Electronic conductivity and structural distortion at the interface between insulators SrTiO3 and LaAlO3 . Phys. Status Solidi 203, 22092214 (2006).
  22. Mi, Y. Y. et al. Epitaxial LaAlO3 thin film on silicon: Structure and electronic properties. Appl. Phys. Lett. 90, 181925 (2007).
  23. Bell, C., Harashima, S., Hikita, Y. & Hwang, H. Y. Thickness dependence of the mobility at the LaAlO3/SrTiO3 interface. Appl. Phys. Lett. 94, 222111 (2009).
  24. Brinkman, A. et al. Magnetic effects at the interface between non-magnetic oxides. Nature Mater. 427, 493496 (2007).
  25. Simon, J., Protasenko, V., Lian, C., Xing, H. & Jena, D. Polarization-induced hole doping in wide-band-gap uniaxial semiconductor heterostructures. Science 327, 6064 (2010).
  26. Yu, E. T. et al. Schottky barrier engineering in III–V nitrides via the piezoelectric effect. Appl. Phys. Lett. 73, 18801882 (1998).
  27. Pentcheva, R. & Pickett, W. E. Ionic relaxation contribution to the electronic reconstruction at the n-type LaAlO3/SrTiO3 interface. Phys. Rev. B 78, 205106 (2008).
  28. Posternak, M., Baldereschi, A., Catellani, A. & Resta, R. Ab initio study of the spontaneous polarization of pyroelectric BeO. Phys. Rev. Lett. 64, 17771780 (1990).
  29. Copie, O. et al. Towards two-dimensional metallic behaviour at LaAlO3/SrTiO3 interfaces. Phys. Rev. Lett. 102, 216804 (2009).
  30. Bell, C. et al. Dominant mobility modulation by the electric field effect at the LaAlO3/SrTiO3 interface. Phys. Rev. Lett. 103, 226802 (2009).
  31. Sze, S. M. Physics of Semiconductor Devices 3 edn (Wiley-Interscience, 2006).
  32. Susaki, T., Kozuka, Y., Tateyama, Y. & Hwang, H. Y. Temperature-dependent polarity reversal in Au–Nb:SrTiO3 Schottky junctions. Phys. Rev. B 76, 155110 (2007).
  33. Hikita, Y., Nishikawa, M., Yajima, T. & Hwang, H. Y. Termination control of the interface dipole in La0.7Sr0.3MnO3/Nb:SrTiO3 (001) Schottky junctions. Phys. Rev. B 79, 073101 (2009).
  34. Minohara, M., Yasuhara, R., Kumigashira, H. & Oshima, M. Termination layer dependence of Schottky barrier height for La0.6Sr0.4MnO3/Nb:SrTiO3 heterojunctions. Phys. Rev. B 81, 235322 (2010).
  35. Müller, K. A. & Burkard, H. SrTiO3: An intrinsic quantum paraelectric below 4K. Phys. Rev. B 19, 35933602 (1979).
  36. Liu, M., Kim, H. K. & Blachere, J. Lead–zirconate–titanate based metal/ferroelectric/insulator/semiconductor structure for nonvolatile memories. J. Appl. Phys. 91, 59855996 (2002).
  37. Miller, S. L. & McWhorter, P. J. Physics of the ferroelectric nonvolatile memory field effect transistor. J. Appl. Phys. 72, 59996010 (1992).
  38. Bickel, N., Schmidt, G., Heinz, K. & Müller, K. Ferroelectric relaxation of the SrTiO3(100) surface. Phys. Rev. Lett. 62, 20092011 (1989).
  39. Pentcheva, R. & Pickett, W. E. Avoiding the polarization catastrophe in LaAlO3 overlayers on SrTiO3(001) through polar distortion. Phys. Rev. Lett. 102, 107602 (2009).
  40. Bristowe, N. C., Artacho, E. & Littlewood, P. B. Oxide superlattices with alternating p and n interfaces. Phys. Rev. B 80, 045425 (2009).
  41. Ogawa, N. et al. Enhanced lattice polarization in SrTiO3/LaAlO3 superlattices measured using optical second-harmonic generation. Phys. Rev. B 80, 081106 (2009).
  42. Salluzzo, M. et al. Orbital reconstruction and the two-dimensional electron gas at the LaAlO3/SrTiO3 interface. Phys. Rev. Lett. 102, 166804 (2009).
  43. Vonk, V. et al. Interface structure of SrTiO3/LaAlO3 at elevated temperatures studied in situ by synchrotron X-rays. Phys. Rev. B 75, 235417 (2007).
  44. Haeni, J. H. et al. Room-temperature ferroelectricity in strained SrTiO3 . Science 430, 758761 (2004).
  45. Zubko, P., Catalan, G., Buckley, A., Welche, P. R. L. & Scott, J. F. Strain-gradient-induced polarization in SrTiO3 single crystals. Phys. Rev. Lett. 99, 167601 (2007).
  46. Yoshimatsu, K., Yasuhara, R., Kumigashira, H. & Oshima, M. Origin of metallic states at the heterointerface between the band insulators LaAlO3 and SrTiO3 . Phys. Rev. Lett. 101, 026802 (2008).
  47. Glinchuk, M. D. & Morozovska, A. N. The internal electric field originating from the mismatch effect and its influence on ferroelectric thin film properties. J. Phys.: Condens. Matter. 16, 35173531 (2004).
  48. Goniakowski, J. & Noguera, C. The concept of weak polarity: An application to the SrTiO3(001) surface. Surf. Sci. 365, 016501 (1996).
  49. Cao, Y., Zimmermann, T., Xing, H. & Jena, D. Polarization-engineered removal of buffer leakage for GaN transistors. Appl. Phys. Lett. 96, 042102 (2010).

Download references

Author information

Affiliations

  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

Contributions

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.

Competing financial interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to:

Author details

Supplementary information

PDF files

  1. Supplementary Information (900k)

    Supplementary Information

Additional data