Structure change of TiO2-terminated SrTiO3(001) surfaces by annealing in O2 atmosphere and ultrahigh vacuum (original) (raw)
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Guideline to atomically flat TiO2-terminated SrTiO3(001) surfaces
Surface Science, 2018
The use of SrTiO 3 (001) substrates with single TiO 2-or SrO-terminated surface is often required in order to grow high quality epitaxial thin films. Many surface preparation recipes can be found in the literature, based on different etching solutions (acid, neutral and basic pH) and combined with different annealing treatments. In this work, we compare three different methods usually used to obtain atomically flat TiO 2-terminated SrTiO 3 surfaces in which water, aqua regia (HCl-HNO 3) and BHF (NH 4 F-HF) solutions are used as reactive agent. Atomic force microscopy measurements (recorded in topography, phase detection and Kelvin-probe force modes) show that using the BHF solution leads to a reliable selective dissolution of SrO on the substrate surface, thus allowing to efficiently obtain atomically flat single TiO 2-terminated surfaces. The aging of the resulted surfaces as well as the possibility of layer-by-layer deposition atop is also discussed.
Quasi-ideal strontium titanate crystal surfaces through formation of strontium hydroxide
Applied Physics Letters, 1998
In recent years, well-defined and nearly perfect single crystal surfaces of oxide perovskites have become increasingly important. A single terminated surface is a prerequisite for reproducible thin film growth and fundamental growth studies. In this work, atomic and lateral force microscopy have been used to display different terminations of SrTiO3. We observe hydroxylation of the topmost SrO layer after immersion of SrTiO3 in water, which is used to enhance the etch-selectivity of SrO relative to TiO2 in a buffered HF solution. We reproducibly obtain perfect and single terminated surfaces, irrespective of the initial state of polished surfaces and the pH value of the HF solution. This approach to the problem might be used for a variety of multi-component oxide single crystals. True two-dimensional reflection high-energy electron diffraction intensity oscillations are observed during homo epitaxial growth using pulsed laser deposition on these surfaces.
Surface morphology determined by (0 0 1) single-crystal SrTiO 3 termination
Physica C-superconductivity and Its Applications, 2000
The terminating layer of a perovskite (0 0 1) ABO 3 crystal, with A as an alkaline earth metal and B as a transition metal, in¯uences the characteristics of the surface. The morphology during thermal treatment of the surface and epitaxial growth are determined by the composition of the surface, i.e., whether it is BO 2 , AO, or a mixed-terminated surface. Since the morphology of a thin ®lm is the result of the evolution of the surface during deposition, control from the very ®rst layer is essential for smooth ®lms. In the case of perovskite materials, this means control of the terminating layer. Here, we study the SrTiO 3 surface as a model system and the eects of the composition of the terminating layer, determined by the treatment and/or deposition of SrO and TiO 2 , on homo-epitaxial and hetero-epitaxial growth of SrCuO 2 and YBa 2 Cu 3 O 7 . A single-terminated surface turns out to be optimal with respect to a smooth morphology and perfect epitaxy of high-T c superconducting materials. Ó
Study and characterization of SrTiO3 surface
AIP Conference Proceedings, 2018
The two-dimensional electron gas (2DEG) at oxides interfaces and surfaces has attracted large attention in physics and research due to its unique electronic properties and possible application in optoelectronics and nanoelectronics. The origin of 2DEGes at oxide interfaces has been attributed to the well known "polar catastrophe" mechanism. On the other hand, recently a 2DEG was also found on a clean SrTiO 3 (001) surface where it is formed due to oxygen vacancies. However, these 2DEG systems have been until now found mostly on atomically perfect crystalline samples usually grown by pulsed laser deposition or molecular beam epitaxy i.e. samples which are difficult to be prepared and require specific experimental conditions. Here, we report on the fabrication of SrTiO 3 thin films deposited by magnetron sputtering which is suitable for mass-production of samples adapted for nanoelectronic applications. The characterisation of their structural and electronic properties was done and compared to those of SrTiO 3 single crystals. XRD patterns and SEM micrography show that the deposited films are amorphous and their structure changes to polycrystalline by heating them at 900 • C. Photoemission spectroscopy (XPS and UPS) was used to study the electronic properties of the films and the crystal. In both, we observe the 2DEG system at Fermi level and the formation of Ti 3+ states after heating the surface at 900 • C.
Surface structure of SrTiO3(001)
Physical Review B, 2007
We report on the structural determination of the surface of TiO 2 -terminated SrTiO 3 ͑001͒ using surface x-ray diffraction. The detailed analysis of two surface diffraction data sets are presented, one ͑cold͒ taken at room temperature in vacuum, and the other ͑hot͒ under typical conditions used for thin film growth. 49 different combinations of possible surface terminations are described for the cold structure, from which the final structure was chosen, consisting of a weighted mixture of a ͑1 ϫ 1͒ relaxation and ͑2 ϫ 1͒ and ͑2 ϫ 2͒ reconstructions, simultaneously present at the surface. The structures are best modeled by a TiO 2 -rich surface similar to that proposed by Erdman et al. ͓Nature ͑London͒ 419, 55 ͑2002͔͒. The reconstructions are energetically favorable according to density functional theory. They disappear within several minutes upon heating to the hot conditions, forming a termination very similar to the cold ͑1 ϫ 1͒, but more puckered and higher in energy. Six additional models, suggested by direct methods and the literature, to describe the hot surface are also discussed. Direct methods confirm the TiO 2 -rich termination and the atomic positions of the hot surface. The atomic coordinates for the two TiO 2 -rich surfaces exhibit significant displacements down to three unit cells, which may have important implications on possible surface ferroelectric phenomena in SrTiO 3 . Surface energy considerations suggest a temperature-induced order-disorder transition, produced by a mixing of the ͑2 ϫ 1͒ and ͑2 ϫ 2͒ reconstructions, to form the hot pseudo ͑1 ϫ 1͒ structure. Electrostatic stability arguments provide circumstantial support for the experimentally determined TiO 2 -rich surfaces.
Physical Review B, 2007
Strontium titanate, SrTiO 3 , a widely used substrate material for electronic oxide thin film devices, has provided many interesting features. In a combination with a similar oxide material, LaAlO 3, it has recently received great interest. It was suggested that two-dimensional electron gas is formed at the interface between SrTiO 3 and LaAlO 3, resulting in high electrical conductivity and mobility. In this report we demonstrate that the transport properties in those heterostructures are very sensitive to the deposition parameters during thin film growth. Using cathode-and photoluminescence studies in conjunction with measurements of electrical transport properties and microstructure we show that the electronic properties observed at a LaAlO 3 /SrTiO 3 interface can be explained by oxygen reduced SrTiO 3. In addition, we demonstrate that oxygen can be pushed in and out of the sample, but that re-oxygenation of an initially oxygen depleted LaAlO 3 /SrTiO 3 heterostructure is partly prevented by the presence of the film. 2 Strontium titanate, SrTiO 3 , is a very versatile material. It is a cubic perovskitestructured dielectric material with a wide band-gap of about 3.2 eV at 300 K in its stoichiometric composition 1,3. Its transparency and dielectric properties have made SrTiO 3 one of the most frequently used single crystal substrates for electronic oxide thin film devices. A prominent feature of SrTiO 3 is the possibility to control its surface composition at the atomic level. Singly terminated SrTiO 3 is an excellent choice as a starting block for growth of atomically sharp oxide heterointerfaces 4,5. The possibility of charge discontinuity between SrTiO 3 and another similar perovskite, LaAlO 3, has recently been proposed 6. Uncompensated charge may appear at the interface because of different formal valences in the ionic limit of Ti (4+) and La (3+) in these materials. As a result, highly interesting electrical properties like the formation of a two-dimensional electron gas was suggested. However, the properties of SrTiO 3 itself can easily be modified with a small compositional change. By replacing only a small fraction of Sr with an alkaline earth metal (like, e.g., Ba or Ca) the material can become ferroelectric with T C up to 136 K 7. A small addition of Nb, La or Ta makes the material highly (n-type) conducting with a charge carrier concentration of 10 19 cm-3 8,9 and superconducting below 0.4 K 10. Another method to modify the material properties is to reduce the stoichiometric SrTiO 3 and introduce oxygen deficiencies (δ in SrTiO 3-δ). There are mainly three different ways to create oxygen deficient SrTiO 3. One way is to anneal stoichiometric crystals at high temperature (800ºC-1200ºC) in vacuum 9 or in the presence of titanium or hydrogen 11. Alternatively, reduced STO 3-δ films can be deposited at low oxygen pressure, hence creating inherently oxygen depleted SrTiO 3-δ 12. It has also been shown that it is possible to make oxygen deficient layers of SrTiO 3-δ by Ar-ion bombardment 13,15 .
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2013
Epitaxial strontium titanate (STO) films have been grown by atomic layer deposition (ALD) on Si(001) substrates with a thin STO buffer layer grown by molecular beam epitaxy (MBE). Four unit cells of STO grown by MBE serve as the surface template for ALD growth. The STO films grown by ALD are crystalline as-deposited with minimal, if any, amorphous SiO x layer at the STO-Si interface. The growth of STO was achieved using bis(triisopropylcyclopentadienyl)strontium, titanium tetraisopropoxide, and water as the coreactants at a substrate temperature of 250 C. In situ x-ray photoelectron spectroscopy (XPS) analysis revealed that the ALD process did not induce additional Si-O bonding at the STO-Si interface. Postdeposition XPS analysis also revealed sporadic carbon incorporation in the as-deposited films. However, annealing at a temperature of 250 C for 30 min in moderate to high vacuum (10 À6-10 À9 Torr) removed the carbon species. Higher annealing temperatures (>275 C) gave rise to a small increase in Si-O bonding, as indicated by XPS, but no reduced Ti species were observed. X-ray diffraction revealed that the as-deposited STO films were c-axis oriented and fully crystalline. A rocking curve around the STO(002) reflection gave a full width at half maximum of 0.30 6 0.06 for film thicknesses ranging from 5 to 25 nm. Cross-sectional transmission electron microscopy revealed that the STO films were continuous with conformal growth to the substrate and smooth interfaces between the ALD-and MBE-grown STO. Overall, the results indicate that thick, crystalline STO can be grown on Si(001) substrates by ALD with minimal formation of an amorphous SiO x layer using a four-unit-cell STO buffer layer grown by MBE to serve as the surface template. V
Structural properties of ultrathin SrO film deposited on SrTiO3
Science and Technology of Advanced Materials
The role of epitaxial strain and chemical termination in selected interfaces of perovskite oxide heterostructures is under intensive investigation because of emerging novel electronic properties. SrTiO 3 (STO) is one of the most used substrates for these compounds, and along its < 001 > direction allows for two nonpolar chemical terminations: TiO 2 and SrO. In this paper, we investigate the surface morphology and crystal structure of SrO epitaxial ultrathin films: from 1 to about 25 layers grown onto TiO 2-terminated STO substrates. X-ray diffraction and transmission electron microscopy analysis reveal that SrO grows along its ½111 direction with a 4% out-of-plane elongation. This large strain may underlay the mechanism of the formation of self-organized pattern of stripes that we observed in the initial growth. We found that the distance between the TiO 2 plane and the first deposited SrO layer is 0:27ð3Þ nm, a value which is about 40% bigger than in the STO bulk. We demonstrate that a single SrO-deposited layer has a different morphology compared to an ideal atomically flat chemical termination.