Pulsed laser deposition-induced reduction of SrTiO3 crystals
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 mΩ 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,
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.
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