Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions

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 meV Å−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.

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Figure 1: Built-in polarization across LaAlO3/SrTiO3 tunnel junction diodes.
Figure 2: Thickness-dependent built-in potential and interband tunnelling across polar LaAlO3.
Figure 3: Tuning the tunnelling current across LaAlO3 by tuning the SrTiO3 permittivity and charge density.
Figure 4: Capacitance measurements agree with J V and also reveal an induced dipole across the heterostructure.

References

  1. 1

    Goniakowski, J., Finocchi, F. & Noguera, C. Polarity of oxide surfaces and nanostructures. Rep. Prog. Phys. 71, 016501 (2008).

    ADS  Article  Google Scholar 

  2. 2

    Stengel, M. & Vanderbilt, D. Berry-phase theory of polar discontinuities at oxide–oxide interfaces. Phys. Rev. B 80, 241103 (2009).

    ADS  Article  Google Scholar 

  3. 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, 3222–3233 (1999).

    ADS  Article  Google Scholar 

  4. 4

    Tsukazaki, A. et al. Quantum hall effect in polar oxide heterostructures. Science 315, 1388–1391 (2007).

    ADS  Article  Google Scholar 

  5. 5

    Ohtomo, A. & Hwang, H. Y. A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface. Nature 427, 423–426 (2004).

    ADS  Article  Google Scholar 

  6. 6

    Kawasaki, M. et al. Atomic control of the SrTiO3 crystal surface. Science 266, 1540–1542 (1994).

    ADS  Article  Google Scholar 

  7. 7

    Nakagawa, N., Hwang, H. Y. & Muller, D. A. Why some interfaces cannot be sharp. Nature Mater. 5, 204–209 (2006).

    ADS  Article  Google Scholar 

  8. 8

    Tasker, P. W. The stability of ionic crystal surfaces. J. Phys. C 12, 4977–4984 (1979).

    ADS  Article  Google Scholar 

  9. 9

    Thiel, S., Hammerl, G., Schmehl, A., Schneider, C. W. & Mannhart, J. Tunable quasi-two-dimensional electron gases in oxide heterostructures. Science 313, 1942–1945 (2006).

    ADS  Article  Google Scholar 

  10. 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).

    ADS  Article  Google Scholar 

  11. 11

    Pentcheva, R. & Pickett, W. E. Electronic phenomena at complex oxide interfaces: Insights from first principles. J. Phys.: Condens. Mater. 22, 043001 (2010).

    ADS  Google Scholar 

  12. 12

    Siemons, W. et al. Origin of charge density at LaAlO3 on SrTiO3 heterointerfaces: Possibility of intrinsic doping. Phys. Rev. Lett. 98, 196802 (2007).

    ADS  Article  Google Scholar 

  13. 13

    Willmott, P. R. et al. Structural basis for the conducting interface between LaAlO3 and SrTiO3 . Phys. Rev. Lett. 99, 155502 (2007).

    ADS  Article  Google Scholar 

  14. 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. 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. 16

    Zener, C. A theory of electrical breakdown of solid dielectrics. Proc. R. Soc. A 145, 523–529 (1934).

    ADS  Article  Google Scholar 

  17. 17

    Simon, J. et al. Polarization-induced Zener tunnel junctions in wide-band-gap heterostructures. Phys. Rev. Lett. 103, 026801 (2009).

    ADS  Article  Google Scholar 

  18. 18

    Noguera, C. & Goniakowski, J. Polarity in oxide ultrathin films. J. Phys. Condens. Matter. (2008).

  19. 19

    Bykhovski, A., Gelmont, B., Shur, M. & Khan, A. Current–voltage characteristics of strained piezoelectric structures. J. Appl. Phys. 77, 1616–1620 (1995).

    ADS  Article  Google Scholar 

  20. 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, 4500–4505 (2002).

    ADS  Article  Google Scholar 

  21. 21

    Maurice, J-L. et al. Electronic conductivity and structural distortion at the interface between insulators SrTiO3 and LaAlO3 . Phys. Status Solidi 203, 2209–2214 (2006).

    ADS  Article  Google Scholar 

  22. 22

    Mi, Y. Y. et al. Epitaxial LaAlO3 thin film on silicon: Structure and electronic properties. Appl. Phys. Lett. 90, 181925 (2007).

    ADS  Article  Google Scholar 

  23. 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).

    ADS  Article  Google Scholar 

  24. 24

    Brinkman, A. et al. Magnetic effects at the interface between non-magnetic oxides. Nature Mater. 427, 493–496 (2007).

    ADS  Article  Google Scholar 

  25. 25

    Simon, J., Protasenko, V., Lian, C., Xing, H. & Jena, D. Polarization-induced hole doping in wide-band-gap uniaxial semiconductor heterostructures. Science 327, 60–64 (2010).

    ADS  Article  Google Scholar 

  26. 26

    Yu, E. T. et al. Schottky barrier engineering in III–V nitrides via the piezoelectric effect. Appl. Phys. Lett. 73, 1880–1882 (1998).

    ADS  Article  Google Scholar 

  27. 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).

    ADS  Article  Google Scholar 

  28. 28

    Posternak, M., Baldereschi, A., Catellani, A. & Resta, R. Ab initio study of the spontaneous polarization of pyroelectric BeO. Phys. Rev. Lett. 64, 1777–1780 (1990).

    ADS  Article  Google Scholar 

  29. 29

    Copie, O. et al. Towards two-dimensional metallic behaviour at LaAlO3/SrTiO3 interfaces. Phys. Rev. Lett. 102, 216804 (2009).

    ADS  Article  Google Scholar 

  30. 30

    Bell, C. et al. Dominant mobility modulation by the electric field effect at the LaAlO3/SrTiO3 interface. Phys. Rev. Lett. 103, 226802 (2009).

    ADS  Article  Google Scholar 

  31. 31

    Sze, S. M. Physics of Semiconductor Devices 3 edn (Wiley-Interscience, 2006).

    Google Scholar 

  32. 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).

    ADS  Article  Google Scholar 

  33. 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).

    ADS  Article  Google Scholar 

  34. 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).

    ADS  Article  Google Scholar 

  35. 35

    Müller, K. A. & Burkard, H. SrTiO3: An intrinsic quantum paraelectric below 4 K. Phys. Rev. B 19, 3593–3602 (1979).

    ADS  Article  Google Scholar 

  36. 36

    Liu, M., Kim, H. K. & Blachere, J. Lead–zirconate–titanate based metal/ferroelectric/insulator/semiconductor structure for nonvolatile memories. J. Appl. Phys. 91, 5985–5996 (2002).

    ADS  Article  Google Scholar 

  37. 37

    Miller, S. L. & McWhorter, P. J. Physics of the ferroelectric nonvolatile memory field effect transistor. J. Appl. Phys. 72, 5999–6010 (1992).

    ADS  Article  Google Scholar 

  38. 38

    Bickel, N., Schmidt, G., Heinz, K. & Müller, K. Ferroelectric relaxation of the SrTiO3(100) surface. Phys. Rev. Lett. 62, 2009–2011 (1989).

    ADS  Article  Google Scholar 

  39. 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).

    ADS  Article  Google Scholar 

  40. 40

    Bristowe, N. C., Artacho, E. & Littlewood, P. B. Oxide superlattices with alternating p and n interfaces. Phys. Rev. B 80, 045425 (2009).

    ADS  Article  Google Scholar 

  41. 41

    Ogawa, N. et al. Enhanced lattice polarization in SrTiO3/LaAlO3 superlattices measured using optical second-harmonic generation. Phys. Rev. B 80, 081106 (2009).

    ADS  Article  Google Scholar 

  42. 42

    Salluzzo, M. et al. Orbital reconstruction and the two-dimensional electron gas at the LaAlO3/SrTiO3 interface. Phys. Rev. Lett. 102, 166804 (2009).

    ADS  Article  Google Scholar 

  43. 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).

    ADS  Article  Google Scholar 

  44. 44

    Haeni, J. H. et al. Room-temperature ferroelectricity in strained SrTiO3 . Science 430, 758–761 (2004).

    Google Scholar 

  45. 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).

    ADS  Article  Google Scholar 

  46. 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).

    ADS  Article  Google Scholar 

  47. 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, 3517–3531 (2004).

    ADS  Google Scholar 

  48. 48

    Goniakowski, J. & Noguera, C. The concept of weak polarity: An application to the SrTiO3(001) surface. Surf. Sci. 365, 016501 (1996).

    Article  Google Scholar 

  49. 49

    Cao, Y., Zimmermann, T., Xing, H. & Jena, D. Polarization-engineered removal of buffer leakage for GaN transistors. Appl. Phys. Lett. 96, 042102 (2010).

    ADS  Article  Google Scholar 

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Acknowledgements

We thank J. H. Bardarson, M. Gajek and R. Dynes at UC Berkeley as well as X. Du at Stonybrook for discussions and comments on the manuscript. G.S-B. acknowledges support from the Japan Society for Promotion of Science (Award No. SP08057) and the US National Science Foundation (Award No. OISE0812816) as part of the 2008 E.A.P.S.I. fellowship program, under which this work was commenced. W.S. acknowledges support from the Dutch Organization for Scientific Research (NWO-Rubicon Grant). The work at Berkeley (R.R.) was supported by the US Department of Energy under contract No. DE-AC02-05CH1123. The work at Florida (A.F.H.) was supported by the US National Science Foundation under Grant No. 0404962.

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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 J V curves with J.R. and W.S. S.S. simulated the J V 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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Correspondence to Guneeta Singh-Bhalla.

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Singh-Bhalla, G., Bell, C., Ravichandran, J. et al. Built-in and induced polarization across LaAlO3/SrTiO3 heterojunctions. Nature Phys 7, 80–86 (2011). https://doi.org/10.1038/nphys1814

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