Elsevier

Acta Materialia

Volume 58, Issue 2, January 2010, Pages 457-463
Acta Materialia

Pulsed laser deposition-induced reduction of SrTiO3 crystals

https://doi.org/10.1016/j.actamat.2009.09.024Get rights and content

Abstract

We report a generic method for fast and efficient reduction of strontium titanate (SrTiO3, STO) single crystals by pulsed laser deposition (PLD) of thin-films. The reduction was largely independent of the thin-film material deposited on the crystals. It is shown that thermodynamic conditions (450 °C, 10−7 torr, 10–60 min), which normally reduce STO (0 0 1) substrates to roughly 5 nm into a crystal substrate, can reduce the same crystals throughout their 500 μm thickness when coupled with the PLD. In situ characterization of the STO substrate resistance during thin-film growth is presented. This process opens up the possibility of employing STO substrates as a back-gate in functional oxide devices.

Introduction

Strontium titanate, SrTiO3 (STO), is a model oxide system that shows a wide range of interesting properties such as ferroelectricity [1], superconductivity [2], resistive switching [3] and thermoelectricity [4]. The key to the observation of many of these properties has been the wide electronic tunability of STO through doping both with oxygen vacancies and with substitutional elements including (but not limited to) La, Nb, Fe, or Co at the Sr and Ti sites. Specifically, oxygen vacancy doping has been established as a procedure to tailor an enhanced electron effective mass in STO [5], [6], which has great implications in controlling the electrical, thermal and optical properties of this system. Additionally, interfaces of STO with other oxides can yield two-dimensional electronic gases that have been thought to have high mobility and thermoelectric power [7], [8]. Due to the extreme stability of STO under reducing conditions owing to the multivalency of Ti, introducing oxygen vacancies can be carried out to high concentrations (up to 10%) and has been conventionally achieved by annealing at very low pressures (<10−5 torr) for extended periods of time [6]. Oxygen-reduction indeed transforms STO from a wide bandgap (3.2 eV) insulator into a good conductor (<100  cm), and can be employed to induce a wide range of functionality in the material. In addition, the inadvertent reduction of STO must be accounted for when any low pressure processing is done with or on the surface of single crystals of the material.

It has been both speculated and observed (but never proven) in recent years both by other research groups [9], [10], [11], [12], [13] as well as by these authors [14], [15] that the pulsed laser deposition (PLD) process itself induces oxygen vacancies in substrates of (0 0 1)-oriented STO, especially when oxide films are deposited onto their surfaces at or below oxygen partial pressures of 10−6 torr. It is interesting to note that introduction of oxygen vacancies has been observed in STO substrates when oxide thin-films were grown by molecular beam epitaxy (MBE), although the thin-film source was primarily metal atoms alone and the stoichiometric oxide thin-film was formed by the oxygen fed by the substrates [16], [17]. The oxygen pressure during growth by PLD plays a crucial role in affecting the overall sample transport properties [18], and possible transport contributions from the STO substrate exposed to reducing conditions during PLD could explain why superconductivity [19] as well as magnetism [20] have been observed at low temperatures in thin-films and interfaces grown on these substrates. The creation of oxygen vacancies during PLD seems ostensibly unlikely due to the usually slow diffusion of oxygen in oxide single crystals [21] and short times under which PLD is performed (typically several minutes to an hour). However, we provide direct evidence that the PLD process coupled with thermodynamically oxygen-reducing conditions facilitates efficient reduction of STO (0 0 1) single-crystal substrates, as well as the first direct observation of the STO substrate reduction process through in situ measurement of the substrate resistance during the PLD growth of thin-films. In addition, STO is widely used as a common substrate for epitaxial growth of various oxide systems like bismuth ferrite (BiFeO3) [22], lead zirconate titanate (PZT) [23] and yttrium barium copper oxide (YBa2Cu3O7) [24], among hundreds of other functional oxide systems. Hence, the procedure herein of the fast reduction of STO substrates shall provide a generic route to build epitaxially grown devices made of oxides on a conducting substrate vis à vis back-gated transistors.

Section snippets

Experimental

We employed 5 mm × 5 mm × 0.5 mm STO (0 0 1) single-crystal substrates for our study, supplied by either Crystec GmbH or Surfacenet GmbH, which were screened to be single domain and for narrow rocking curve FWHM values (typically <0.1°) to ensure high quality and low defect densities. All substrates were cleaned by ultrasonication for 5 min each in trichloroethylene, acetone, and isopropanol successively. Films of various oxide materials (SrTiO3, Sr0.98La0.02TiO3, LaAlO3) were deposited via PLD to

Results and discussion

Reduction of strontium titanate is well-known to induce n-type carriers for electrical conduction [6], [25], [26]. An oxygen vacancy frees two electrons, which dopes the insulator to a semiconductor, and the multivalency of titanium (in moving from the +4 to +3 oxidation state) allows the accommodation of several atomic percent oxygen vacancies and resulting carrier concentrations greater than 1022 cm−3 [6]. The redox chemistry of such a process in standard Kröger–Vink notation is given by,12O2+V

Conclusions

Our results clearly demonstrate the remarkable efficiency with which the PLD process removes oxygen from this material – over 10 orders of magnitude faster than that of a high vacuum anneal and reported values of bulk diffusion. Surprisingly, this rate is also orders of magnitude faster than that for diffusion along dislocations in STO; a microscopic mechanism for this phenomenon is still not fully understood. As the paradigmatic single crystal for oxide thin-film growth due to its cubic

Acknowledgements

The authors acknowledge support from DOE Grant #DE-AC02-05-CH11231. J. Seidel acknowledges support from the Alexander von Humboldt Foundation.

References (37)

  • H. Ohta

    Mat Today

    (2007)
  • A.E. Paladino et al.

    J Phys Chem Solids

    (1965)
  • J.H. Haeni et al.

    Nature

    (2004)
  • J.F. Schooley et al.

    Phys Rev Lett

    (1964)
  • S. Karg et al.

    Appl Phys Lett

    (2006)
  • Scullin ML, Mukerjee S, Ravichandran J, Huijben M, Moore JE et al. submitted for...
  • H.P.R. Frederikse et al.

    Phys Rev

    (1964)
  • A. Ohtomo et al.

    Nature

    (2004)
  • H. Ohta et al.

    Nature Mater

    (2007)
  • A. Kalabukhov et al.

    Phys Rev B

    (2007)
  • W. Siemons et al.

    Phys Rev Lett

    (2007)
  • W. Siemons et al.

    Phys Rev B

    (2007)
  • M. Basletic et al.

    Nat Mater

    (2008)
  • G. Herranz et al.

    Appl Phys Lett

    (2009)
  • C. Yu et al.

    Appl Phys Lett

    (2008)
  • M.L. Scullin et al.

    Appl Phys Lett

    (2008)
  • A. Uedono et al.

    J Appl Phys

    (2002)
  • A. Uedono et al.

    J Appl Phys

    (2005)
  • Cited by (69)

    • Stoichiometry in epitaxial oxide thin films

      2022, Epitaxial Growth of Complex Metal Oxides
    • Substrate stoichiometry changes during pulsed laser deposition: a case study on SrTiO<inf>3</inf>

      2021, Acta Materialia
      Citation Excerpt :

      The combination of these results indicates that actual deposition induces changes of the substrate apart from illumination effects, which are still present. These seem to originate from processes competing with the illumination, e.g. from possible oxygen deficient deposition [11–13], thus fundamentally changing the evolution of the bulk resistance with an increasing number of pulses. Especially the strong scatter and the difference between in-plane and across-plane measurements suggest that this newly observed effect is surface related.

    • Functional domain walls: Concepts and perspectives

      2019, Solid State Physics - Advances in Research and Applications
    • Thermal conductivity degradation and recovery in ion beam damaged tungsten at different temperature

      2018, Journal of Nuclear Materials
      Citation Excerpt :

      Indeed, it was shown that the dislocation loops and voids formed during the ion implantation are nanometers in size, which are extremely challenging to directly observe [39]. Thermal conductivity characterization, however, provides an indirect and yet sensitive probe to gauge these atomic and nanometer scale defects [40,41], as both types of heat carriers (electrons and phonons) are sensitive to scattering by defects. Thermal measurements using the 3ω technique were carried out on both pristine (reference) and ion damaged W in the temperature range of 300–370 K.

    View all citing articles on Scopus
    View full text