TiO2-terminated SrTiO3 Substrates Research Articles

Depth profile study of LaAl1‐xCrxO3/SrTiO3 (x = 0, 0.2, 0.6, and 1) using time of flight secondary ion mass spectrometry (TOF‐SIMS)

The conducting two‐dimensional electron gas (2DEG) behavior in LaAlO3 (LAO)/SrTiO3 (STO) and their associated mechanisms from various aspects have brought tremendous attention in the concerned area of research. To correlate the 2DEG behavior with their compositions, we have performed time of flight secondary ion mass spectrometry (TOF‐SIMS) depth profile analysis of thin films of Cr‐doped LAO/STO system as LaAl1‐xCrxO3 (x = 0, 0.2, 0.6 and 1) deposited over TiO2 terminated STO substrate, which includes two parent compounds LAO/STO (metallic) and LCO/STO (insulating). The uniform decrease of La and Al concentration at the interface of LAO/STO (metallic) system and in the contrary the nonuniformity of La and Cr concentration in LCO/STO (insulating) system have been highlighted. The uniform variation of ionic concentration at the interface of LAO/STO may increase the career concentrations to make the system metallic. The upward and downward diffusion at the interfaces of intermediate compositions varies differently from their parent ones due to the mixing of Al and Cr. Our results may help to understand the conducting nature of LAO/STO system for future developments and applications in such system.

Electro-optic effect in thin film strontium barium niobate (SBN) grown by RF magnetron sputtering on SrTiO3 substrates

Ferroelectric strontium barium niobate (SrxBa1−xNb2O6 or SBN) is a material with high electro-optic (EO) response. It is currently of interest in low voltage silicon-integrated photonics (SiPh). We have grown strongly textured SBN films with x = 0.65 by radio frequency sputtering on (100)-oriented SrTiO3 substrates where grains with mixed (310) and (001) out-of-plane orientation form. For these mixed orientation films, we observed a maximum effective EO coefficient of 230 pm/V using a transmission EO measurement geometry that is responsive only to the in-plane polarization component coming from the (310)-oriented grains. We also demonstrate that by growing SBN on TiO2-terminated SrTiO3 substrates, we can obtain predominantly (001)-oriented SBN films with out-of-plane polarization. Transmission EO measurements on such (001)-oriented films show a reduced effective Pockels coefficient of 88 pm/V, which is consistent with the overall ferroelectric polarization becoming out-of-plane. This work shows that controlling substrate termination is effective in controlling the grain orientation of SBN films grown on top and that one can readily integrate SBN films on SrTiO3-buffered Si for use in SiPh.

High-mobility electron gas in Sr2TiO4/SrTiO3 heterostructure engineered by vertical Ruddlesden–Popper faults

High-mobility electron gases in SrTiO3-based heterostructures have been mainly synthesized on single TiO2 terminated SrTiO3 substrates. Here, we show a high-mobility electron gas observed in a Sr2TiO4/SrTiO3 heterostructure based on an untreated SrTiO3 substrate with mixed termination, showing the mobility value up to 28 000 cm2 V−1 s−1 at 2 K. SrO–SrO vertical Ruddlesden–Popper faults in the Sr2TiO4 film provide escape channels for oxygen atoms under high temperatures, enabling the as-grown tuning of oxygen vacancies in the SrTiO3 layer during film deposition and reversible after-growth modulation in a thermal annealing process.

Two-step treatment to obtain single-terminated SrTiO3 substrate and the related difference in both LaAlO3 film growth and electronic property

A two-step treatment, first chemical etching then thermal treatment, is proposed to achieve an atomically flat and thermally stable TiO2-terminated SrTiO3 substrate. LaAlO3 films were then grown on those TiO2-terminated and as-received substrates. LaAlO3 films on the TiO2-terminated SrTiO3 substrate maintained the layer-by-layer growth mode with a sharp interface, while films on the as-received substrates easily underwent reconstruction adverse to the sharp interface. Both LaAlO3/SrTiO3 interfaces displayed metallic conductive behavior, while the difference in magnetotransport properties indicated the difference in origin for interface conductivity. Large positive magnetoresistance implied that the LaAlO3/as-received substrate interface was a 3D conductive interface dominated by oxygen vacancies. However, the annealed-LaAlO3/treated-substrate interface preserved intrinsic quasi-2D interface magnetism as evidenced by large negative magnetoresistance.

Highly conductive two-dimensional electron gas at the interface of Al2O3/SrTiO3

We create a two-dimensional electron gas at the Al2O3/SrTiO3/LaAlO3 heterostructures using pulsed laser deposition, which exhibits a decreasing sheet resistance with increasing growth temperatures of Al2O3 films. Structural characterizations of films are confirmed by cross-sectional transmission electron microscopy. Compared with these heterostructures with Al2O3 films deposited on pristine SrTiO3 and TiO2-terminated SrTiO3 substrates, the Al2O3/SrTiO3/LaAlO3 heterostructures are more conductive. X-ray photoelectron spectroscopy indicates the formation of oxygen vacancies at the SrTiO3 side of the interface, which results from the redox reactions by reducing SrTiO3 films. Furthermore, the existence of oxygen vacancies on the SrTiO3 side is verified by a blue-light emission.

Off-Stoichiometry Driven Carrier Density Variation at the Interface of LaAlO3/SrTiO3

The interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted enormous interests due to its rich physical phenomena, such as metallic nature, magnetism and superconductivity. In this work, we report our experimental investigations on the influence of the LAO stoichiometry to the metallic interface. Taking advantage of the oxide molecular beam epitaxy (MBE) technique, a series of high quality LAO films with different nominal La/Al ratios and LAO thicknesses were grown on the TiO2-terminated STO substrates, where systematic variations of the LAO lattice constant and transport property were observed. In particular, the sheet density can be largely reduced by nearly an order of magnitude with merely about 20% increase in the nominal La/Al ratio. Our finding provides an effective method on tuning the electron density of the two-dimensional electron liquid (2DEL) at the LAO/STO interface.

Selective growth of epitaxial Sr2IrO4 by controlling plume dimensions in pulsed laser deposition

We report that epitaxial Sr2IrO4 thin-films can be selectively grown using pulsed laser deposition (PLD). Due to the competition between the Ruddlesden-Popper phases of strontium iridates (Srn+1IrnO3n+1), conventional PLD methods often result in mixed phases of Sr2IrO4 (n = 1), Sr3Ir2O7 (n = 2), and SrIrO3 (n = ∞). We have discovered that reduced PLD plume dimensions and slow deposition rates are the key for stabilizing pure Sr2IrO4 phase thin-films, identified by real-time in-situ monitoring of their optical spectra. The slow film deposition results in a thermodynamically stable TiO2\\SrO\IrO2\SrO\SrO configuration at an interface rather than TiO2\\SrO\SrO\IrO2\SrO between a TiO2-terminated SrTiO3 substrate and a Sr2IrO4 thin film, which is consistent with other layered oxides grown by molecular beam epitaxy. Our approach provides an effective method for using PLD to achieve pure phase thin-films of layered materials that are susceptible to several energetically competing phases.

One dimensional electron gas at the LaAlO3/SrTiO3 interface and its transport properties

Quasi-one-dimensional electron gases (q1DEGs) have been obtained by fabricating LaAlO3 nanowires, using the technique of electrostatic spinning plus post annealing, above TiO2-terminated SrTiO3 substrate. The q1DEG exhibits an electronic transport behavior of variable range hopping with the one dimension characteristic. Visible light illumination produces a strong effect on transport process, depressing the resistance of the q1DEG by a factor up to 8. As expected, gating effect is weak at relative high temperatures, ∼3.2% at 150 K and 1.5% at 300 K under a back gate of 200 V. Aided by light illumination, however, the gating effect is 35-fold amplified, and the resistance increases under not only negative gates but also positive gates, different from the normal gating effect without illumination. Possible explanations for these phenomena are given.

Pulsed laser deposited LaInO3 thin films and their photoluminescence characteristics

Photoluminescence studies of pulsed laser deposition grown LaInO3 thin films on TiO2 terminated SrTiO3 substrate showed a broad-band blue emission peaking at 412nm, which is primarily attributed to the charge transfer transitions between oxygen and indium, inter (d–f) and intra (f–f) band transitions. The results are analysed based on the mechanism of absorption and emission within the d10 levels of In3+ ions. Broadening of the photoluminescence emission is observed with reduction in size of the crystallite owing to quantum confinement effects.

Influence of Oxygen Content on the Electronic Properties of the PrAlO3/SrTiO3 Interface

ABSTRACTWe have investigated the effect of oxygen pressure during growth (PO2) on the electronic and magnetic properties of PrAlO3 films grown on TiO2-terminated SrTiO3 substrates. The films are smooth, with flat terraces. Resistivity measurements show an increase in the sheet resistance as PO2 is increased from 10–3 to 10–4 torr, with an usual peak as a function of temperature for the sample grown in higher oxygen pressure. We measured a moderate positive magnetoresistance (MR) at low magnetic fields that evolves into a larger negative MR at high fields, for both PO2 samples. Hall effect data exhibit a complex temperature dependence that suggests a compensated carrier density. We observe behavior consistent with two different types of carriers at each of the two different interfaces.

Origin of the stabilization of the metastable tetragonal high-pressure phase in SrCuO2 thin films grown on SrTiO3 substrates by pulsed laser deposition

In this work we have systematically investigated the evolution of structure and stoichiometry in SrCuO2 films grown on TiO2-terminated SrTiO3 substrates as a function of the substrate temperature. Depending on the growth temperature SrCuO2/SrTiO3 films can exhibit either a pure tetragonal high-pressure phase, or a pure orthorhombic low-pressure phase, or a mixed phase. Our results indicate that at low substrate temperatures the non-equilibrium state of the growth process is responsible for the stabilization of the metastable tetragonal high-pressure structure in SrCuO2 thin films grown on (001) SrTiO3 substrates, whose lattice matches the metastable structure. In addition, at higher substrate temperatures thermodynamics become dominant over other factors and the SrCuO2 thin films are stabilized in the thermodynamically stable orthorhombic phase.

Suppression of the critical thickness threshold for conductivity at the LaAlO3/SrTiO3 interface.

Perovskite materials engineered in epitaxial heterostructures have been intensely investigated during the last decade. The interface formed by an LaAlO3 thin film grown on top of a TiO2-terminated SrTiO3 substrate hosts a two-dimensional electronic system and has become the prototypical example of this field. Although controversy exists regarding some of its physical properties and their precise origin, it is universally found that conductivity only appears beyond an LaAlO3 thickness threshold of four unit cells. Here, we experimentally demonstrate that this critical thickness can be reduced to just one unit cell when a metallic film of cobalt is deposited on top of LaAlO3. First-principles calculations indicate that Co modifies the electrostatic boundary conditions and induces a charge transfer towards the Ti 3d bands, supporting the electrostatic origin of the electronic system at the LaAlO3/SrTiO3 interface. Our results expand the interest of this low-dimensional oxide system from in-plane to perpendicular transport and to the exploration of elastic and inelastic tunnel-type transport of (spin-polarized) carriers.

The metallic interface between insulating NdGaO3 and SrTiO3 perovskites

Perovskite NdGaO3 (NGO) films, 2–20 unit cells in thickness, have been deposited epitaxially on {001} TiO2-terminated SrTiO3 substrates at different O2 pressures. The {001} NdGaO3/SrTiO3 (STO) interface becomes metallic as the NdGaO3 overlayer is more than 4 unit cells in thickness. The sheet carrier density is above 1013 cm–2 and temperature-independent from 300 down to 7 K. Similar metallic interface has also been achieved in {111} NdGaO3/SrTiO3. Post-annealing in O2 does not change the transport characteristics significantly. These indicate that oxygen vacancies may not have a predominant contribution to the observed interfacial conduction in NGO/STO heterostructures deposited at high oxygen pressure.

Epitaxial growth of atomically flat gadolinia-doped ceria thin films by pulsed laser deposition

Epitaxial growth of Ce0.8Gd0.2O2(CGO) films on (001) TiO2-terminated SrTiO3 substrates by pulsed laser deposition was investigated using in situ reflective high energy electron diffraction. The initial film growth shows a Stransky–Krastanov growth mode. However, this three-dimensional island formation is replaced by a two-dimensional island nucleation during further deposition, which results in atomically smooth CGO films. The obtained high-quality CGO films may be attractive for the electrolyte of solid-oxide fuel cells operating at low temperature.

Effect of various deposition conditions on the electrical properties of LAO/STO hetero interfaces

We have examined the effects of partial oxygen pressure and laser energy density on the electrical transport properties of thin LAO films grown on (100) TiO2-terminated SrTiO3 substrates. Films were grown by pulsed laser deposition monitored by in-situ reflection high-energy electron diffraction (RHEED). Layer-by-layer growth, as indicated by clear RHEED oscillations, can be obtained in a wide range of oxygen partial pressures from 10−6 to 5×10−2 mbar. Transmission electron microscopy (TEM) analysis shows that the interface is coherent and atomically sharp for all deposition conditions. The STO substrate is oxygen self reduced at an oxygen pressure of 10−6 mbar and the electrical properties of the interface are dominated by the presence of oxygen vacancies. By increasing the oxygen pressure above 10−4 mbar, the substrate itself is insulating but the interface still shows metallic conductivity. However, the interface becomes insulating at an oxygen pressure of 5×10−2 mbar. We also found that the interface exhibits insulator-to-metal transition by changing the laser fluence during the deposition of the film. The interface prepared at 5×10−2 mbar shows metallic conductivity at high fluence, above 3.5 J/cm2.

Step-induced uniaxial magnetic anisotropy of La:67Sr:33MnO3 thin films

The magnetic anisotropy of epitaxial La0.67Sr0.33MnO3 (LSMO) thin films on vicinal, TiO2-terminated SrTiO3 substrates is investigated. Atomic force microscopy shows a regular step-terrace structure on the LSMO surface which is a replication of the surface of the substrate. The films show in-plane uniaxial magnetic anisotropy at room temperature, with the easy axis along the step direction. At low temperature the films show biaxial crystalline anisotropy with easy axes along [110], and hard axes along the [100] direction of LSMO.