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Title: Non-conducting interfaces of LaAlO{sub 3}/SrTiO{sub 3} produced in sputter deposition: The role of stoichiometry

Abstract

We have investigated the properties of interfaces between LaAlO{sub 3} films grown on SrTiO{sub 3} substrates singly terminated by TiO{sub 2}. We used RF sputtering in a high-pressure oxygen atmosphere. The films are smooth, with flat surfaces. Transmission electron microscopy shows sharp and continuous interfaces with some slight intermixing. The elemental ratio of La to Al, measured by the energy dispersive X-ray technique, is found to be 1.07. Importantly, we find these interfaces to be non-conducting, indicating that the sputtered interface is not electronically reconstructed in the way reported for films grown by pulsed laser deposition because of the different interplays among stoichiometry, mixing, and oxygen vacancies.

Authors:
; ; ;  [1]; ;  [2];  [3]
  1. Kamerlingh Onnes Laboratorium, Leiden University, Leiden (Netherlands)
  2. National Centre for High Resolution Microscopy, Kavli Institute for Nanoscience, Delft Technical University, Lorentzweg 1, 2628 CJ Delft (Netherlands)
  3. Faculty of Science and Technology and MESA Institute for Nanotechnology, University of Twente, 7500 AE Enschede (Netherlands)
Publication Date:
OSTI Identifier:
22162807
Resource Type:
Journal Article
Resource Relation:
Journal Name: Applied Physics Letters; Journal Volume: 102; Journal Issue: 12; Other Information: (c) 2013 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ALUMINATES; ATMOSPHERES; ENERGY BEAM DEPOSITION; FILMS; INTERFACES; LANTHANUM COMPOUNDS; LASER RADIATION; MIXING; OXYGEN; PULSED IRRADIATION; SPUTTERING; STOICHIOMETRY; STRONTIUM TITANATES; SUBSTRATES; SURFACES; TITANIUM OXIDES; TRANSMISSION ELECTRON MICROSCOPY; VACANCIES; X RADIATION

Citation Formats

Dildar, I. M., Boltje, D. B., Hesselberth, M. H. S., Aarts, J., Xu, Q., Zandbergen, H. W., and Harkema, S. Non-conducting interfaces of LaAlO{sub 3}/SrTiO{sub 3} produced in sputter deposition: The role of stoichiometry. United States: N. p., 2013. Web. doi:10.1063/1.4798828.
Dildar, I. M., Boltje, D. B., Hesselberth, M. H. S., Aarts, J., Xu, Q., Zandbergen, H. W., & Harkema, S. Non-conducting interfaces of LaAlO{sub 3}/SrTiO{sub 3} produced in sputter deposition: The role of stoichiometry. United States. doi:10.1063/1.4798828.
Dildar, I. M., Boltje, D. B., Hesselberth, M. H. S., Aarts, J., Xu, Q., Zandbergen, H. W., and Harkema, S. 2013. "Non-conducting interfaces of LaAlO{sub 3}/SrTiO{sub 3} produced in sputter deposition: The role of stoichiometry". United States. doi:10.1063/1.4798828.
@article{osti_22162807,
title = {Non-conducting interfaces of LaAlO{sub 3}/SrTiO{sub 3} produced in sputter deposition: The role of stoichiometry},
author = {Dildar, I. M. and Boltje, D. B. and Hesselberth, M. H. S. and Aarts, J. and Xu, Q. and Zandbergen, H. W. and Harkema, S.},
abstractNote = {We have investigated the properties of interfaces between LaAlO{sub 3} films grown on SrTiO{sub 3} substrates singly terminated by TiO{sub 2}. We used RF sputtering in a high-pressure oxygen atmosphere. The films are smooth, with flat surfaces. Transmission electron microscopy shows sharp and continuous interfaces with some slight intermixing. The elemental ratio of La to Al, measured by the energy dispersive X-ray technique, is found to be 1.07. Importantly, we find these interfaces to be non-conducting, indicating that the sputtered interface is not electronically reconstructed in the way reported for films grown by pulsed laser deposition because of the different interplays among stoichiometry, mixing, and oxygen vacancies.},
doi = {10.1063/1.4798828},
journal = {Applied Physics Letters},
number = 12,
volume = 102,
place = {United States},
year = 2013,
month = 3
}
  • Sputter deposition of oxide materials in a high-pressure oxygen atmosphere is a well-known technique to produce thin films of perovskite oxides in particular. Also interfaces can be fabricated, which we demonstrated recently by growing LaAlO{sub 3} on SrTiO{sub 3} substrates and showing that the interface showed the same high degree of epitaxy and atomic order as is made by pulsed laser deposition. However, the high pressure sputtering of oxides is not trivial and number of parameters are needed to be optimized for epitaxial growth. Here we elaborate on the earlier work to show that only a relatively small parameter windowmore » exists with respect to oxygen pressure, growth temperature, radiofrequency power supply and target to substrate distance. In particular the sensitivity to oxygen pressure makes it more difficult to vary the oxygen stoichiometry at the interface, yielding it insulating rather than conducting.« less
  • We have studied the stoichiometry of epitaxial LaAlO{sub 3} thin films on SrTiO{sub 3} substrate grown by pulsed laser deposition as a function of laser energy density and oxygen pressure during the film growth. Both x-ray diffraction ({theta}-2{theta} scan and reciprocal space mapping) and transmission electron microscopy (geometric phase analysis) revealed a change of lattice constant in the film with the distance from the substrate. Combined with composition analysis using x-ray fluorescence we found that the nominal unit-cell volume expanded when the LaAlO{sub 3} film was La-rich, but remained near the bulk value when the film was La-poor or stoichiometric.more » La excess was found in all the films deposited in oxygen pressures lower than 10{sup -2} Torr. We conclude that the discussion of LaAlO{sub 3}/SrTiO{sub 3} interfacial properties should include the effects of cation off-stoichiometry in the LaAlO{sub 3} films when the deposition is conducted under low oxygen pressures.« less
  • Surface photovoltage (SPV) spectroscopy, which is a versatile method to analyze the energetic distribution of electronic defect states at surfaces and interfaces of wide-bandgap semiconductor (hetero-)structures, is applied to comparatively investigate heterostructures made of 5-unit-cell-thick LaAlO{sub 3} films grown either on TiO{sub 2}- or on SrO-terminated SrTiO{sub 3}. As shown in a number of experimental and theoretical investigations in the past, these two interfaces exhibit dramatically different properties with the first being conducting and the second insulating. Our present SPV investigation reveals clearly distinguishable interface defect state distributions for both configurations when interpreted within the framework of a classical semiconductormore » band scheme. Furthermore, bare SrTiO{sub 3} crystals with TiO{sub 2} or mixed SrO/TiO{sub 2} terminations show similar SPV spectra and transients as do LaAlO{sub 3}-covered samples with the respective termination of the SrTiO{sub 3} substrate. This is in accordance with a number of recent works that stress the decisive role of SrTiO{sub 3} and the minor role of LaAlO{sub 3} with respect to the electronic interface properties.« less
  • Cited by 1
  • SrTiO{sub 3} is not only of enduring interest due to its unique dielectric, structural, and lattice dynamical properties, but is also the archetypal perovskite oxide semiconductor and a foundational material in oxide heterostructures and electronics. This has naturally focused attention on growth, stoichiometry, and defects in SrTiO{sub 3}, one exciting recent development being such precisely stoichiometric defect-managed thin films that electron mobilities have finally exceeded bulk crystals. This has been achieved only by molecular beam epitaxy, however (and to a somewhat lesser extent pulsed laser deposition (PLD)), and numerous open questions remain. Here, we present a study of the stoichiometry,more » defects, and structure in SrTiO{sub 3} synthesized by a different method, high pressure oxygen sputtering, relating the results to electronic transport. We find that this form of sputter deposition is also capable of homoepitaxy of precisely stoichiometric SrTiO{sub 3}, but only provided that substrate and target preparation, temperature, pressure, and deposition rate are carefully controlled. Even under these conditions, oxygen-vacancy-doped heteroepitaxial SrTiO{sub 3} films are found to have carrier density, mobility, and conductivity significantly lower than bulk. While surface depletion plays a role, it is argued from particle-induced X-ray emission (PIXE) measurements of trace impurities in commercial sputtering targets that this is also due to deep acceptors such as Fe at 100's of parts-per-million levels. Comparisons of PIXE from SrTiO{sub 3} crystals and polycrystalline targets are shown to be of general interest, with clear implications for sputter and PLD deposition of this important material.« less