SrTiO3 Films Research Articles

Effects of Ba2+/Sr2+ ratio on the microstructure, dielectric and ferroelectric properties of Ba x Sr(1−x)TiO3 films prepared by micro-arc oxidation

ABSTRACT BaxSr(1-x)TiO3 (BST) thin films are in situ deposited on a Ti matrix through the micro-arc oxidation technique, using a mixed aqueous solution of Ba(OH)2•8H2O, Sr(OH)2•8H2O and 0.05 M EDTA as the electrolyte. The influence of the Ba2+/Sr2+ ratio on the phase composition, microstructure and properties of the BST thin films is investigated. Ba0.6Sr0.4TiO3, Ba0.5Sr0.5TiO3, and Ba0.4Sr0.6TiO3 thin films have a cubic phase, while Ba0.8Sr0.2TiO3 and Ba0.7Sr0.3TiO3 thin films have a tetragonal phase. When the concentration of Ba2+ in the electrolyte surpasses that of Sr2+, the width of micro-arc holes and the surface roughness of the film grow as the Ba2+ concentration rises. Furthermore, the dielectric and ferroelectric properties of BST films are favorably connected with the surface morphology. When the Ba2+ concentration is 0.06 M, the film has the best compactness and smoothness. In addition, the highest capacitance and lowest dielectric loss are achieved.

Flexible Lead-Free Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 Dielectric Film Capacitor with High Energy Storage Performance.

Ferroelectric thin film capacitors have triggered great interest in pulsed power systems because of their high-power density and ultrafast charge–discharge speed, but less attention has been paid to the realization of flexible capacitors for wearable electronics and power systems. In this work, a flexible Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 thin film capacitor is synthesized on mica substrate. It possesses an energy storage density of Wrec ~ 62 J cm−3, combined with an efficiency of η ~ 74% due to the moderate breakdown strength (3000 kV cm−1) and the strong relaxor behavior. The energy storage performances for the film capacitor are also very stable over a broad temperature range (−50–200 °C) and frequency range (500 Hz–20 kHz). Moreover, the Wrec and η are stabilized after 108 fatigue cycles. Additionally, the superior energy storage capability can be well maintained under a small bending radius (r = 2 mm), or after 104 mechanical bending cycles. These results reveal that the Ba0.5Sr0.5TiO3/0.4BiFeO3-0.6SrTiO3 film capacitors in this work have great potential for use in flexible microenergy storage systems.

Electrical conductivity in oxygen-substituted SrTiO3-δ films

Enhancement of electrical conductivity in fundamentally insulating ABO3 perovskite oxide ferroelectrics is crucial for innovative applications in resistive switching, photovoltaics, and catalysis. One of the methods to raise conductivity in bulk crystals or ceramics relies on the possibility to remove and/or substitute oxygen atoms. Here, we explored this approach for thin films of the representative perovskite oxide SrTiO3. Small-signal AC conductivity was investigated in epitaxial and polycrystalline films, where oxygen vacancies (VO), nitrogen (N), or hydrogen (H) were introduced in situ during film growth. Hopping mechanism of conductivity was evidenced by the observed strong growth of AC conductivity with temperature, frequency, and AC voltage in all films. Small polarons were identified as charge carriers. Oxygen vacancies/substitutions were suggested to facilitate hopping probability by generating sites for carrier localization. Important ferroelectric devices were proposed to benefit from the revealed hopping conductivity owing to its unique increase with an electric field.

Polar Phonon Behaviour in Polycrystalline Bi-Doped Strontium Titanate Thin Films.

Strontium titanate-based materials with ferroelectric or relaxor-like properties have drawn vast attention as polar dielectrics for electronics and telecommunications. Here, we study the lattice dynamics in sol–gel-derived Sr1−1.5xBixTiO3 thin films with x = 0.0053 and 0.167, deposited on Al2O3 substrates, using a variable-temperature far-infrared spectroscopy in a transmittance mode. Bi doping, known to induce a low-frequency dielectric relaxation in SrTiO3 (ST) ceramics and films, due to off-centre dopant ion displacements generating electric dipoles, is shown to affect the polar phonon behaviour of thin films. We show that in weakly Bi-doped films, the low-frequency polar TO1 mode softens on cooling but less than in undoped ST. In heavily Bi-doped ST films, this mode displays no significant frequency variation with temperature from 300 to 10 K. The polar phonon behaviour of polycrystalline Bi-doped ST thin films is comparable with that of Bi-doped ST ceramics, which exhibit dielectric relaxations and harden soft-mode behaviour instead of the ferroelectric phase transition.

Reducing surface depletion of superconducting SrTiO3 films with EuTiO3 capping layers

Doped SrTiO3 thin films are of significant scientific interest for their superconducting, ferroelectric, and thermoelectric properties. Air-exposed thin films of doped SrTiO3 suffer from surface carrier depletion that makes it difficult to obtain thin, conducting films, especially at low doping densities and temperatures. Here, we show that thin (10 nm) EuTiO3 capping layers can prevent surface depletion and allow for conducting heterostructures with SrTiO3 films as thin as 10 nm. We also investigate the superconducting properties of EuTiO3 capped SrTiO3 films. We show that the superconducting transition temperature decreases systematically as the SrTiO3 film thickness is reduced, which is likely due to proximity effects.

Orientation competition growth and mechanism of SrTiO3 film on CeO2 layer

A series of SrTiO3 films were deposited on CeO2 layer by pulsed laser deposition at substrate temperature in the range of 300 °C–700 °C. It was found that the (110)-oriented and (200)-oriented SrTiO3 grains coexisted on the CeO2 layer. Although the (110)-oriented grains dominated during the deposition, the dominance was weakened with increasing substrate temperature. The competition mechanism of SrTiO3 grains along different orientations was explained in view of the diffusion energy and the evolution of dislocation density. The dependence of the film stoichiometry on the substrate temperature and the effects of residual contaminants in the chamber were also studied by X-ray photoelectron spectroscopy.

In situ study of the electronic structure of polar-to-polar SrTiO3/(000[formula omitted])ZnO heterointerface

The SrTiO3(STO)/ZnO heterointerface, which is widely used in the fabrication of novel optoelectronic devices, is a classical system combining functional perovskite oxides and wurtzite-structure semiconductor materials. The electronic structure of the heterointerface often plays a significant role in controlling the functions of novel devices. In this study, the electronic structure was explored using in situ photoemission spectroscopy and X-ray absorption spectroscopy. X-ray diffraction results showed the coexistence of (111)STO and (011)STO orientations for the STO film deposited on the ZnO-(0001-) substrate via pulsed laser deposition. High-resolution transmission electron microscopic results revealed two types of polar interfaces: [112-][101-](111)STO//[12-10][101-0](0001-)ZnO and [111][21-1-](011)STO//[102-1][101-0](0001-)ZnO. In situ photoemission spectroscopic results revealed downward band bending and the transformation of the valence states of Ti from 4+ to 3+, with extra electrons transferring to the hybridization states between O 2p and Ti t2g orbitals at the polar-to-polar STO/ZnO interface. We propose that the polar discontinuity drives the electron transfer to the STO/ZnO interface during the growth process. This study provides insight into the electronic structure of the STO/(0001-)ZnO heterointerface.

Giant step bunching on SrTiO3 thin films grown epitaxially on vicinal MgO (1 0 0) surfaces

We report here the controlled formation of giant step bunching in the epitaxial growth of SrTiO3 thin film on vicinal MgO (100) substrates by on-axis radio-frequency (RF)-magnetron sputtering. We have studied the evolution of the surface morphology of such bunched steps by tuning the post-deposition annealing temperature. We demonstrate that the surface periodicity and reorganization of SrTiO3 nanostructures is widely enhanced under controlled post-annealing conditions in addition to Mg content diffusion in the SrTiO3 films. A morphology of facetted islands and bunched steps with extremely smooth terraces were obtained by post-annealing the SrTiO3 thin films at high temperatures. After a double post-annealing treatment at 900 °C and 700 °C, step bunching with a bunched step of fifteen-atomic unit cells and large terraced structures of about 1 µm width has been observed. The formation of these bunches of self-organized SrTiO3 nanostructures is of potential interest as templates for the growth of many functional materials.

Heteroepitaxial registry and band structures at the polar-to-polar STO/ZnO(000[formula omitted]) interfaces

The interface between ZnO and SrTiO3 (STO) provides a paradigm for combining perovskite oxides and wurtzite-structure semiconductors. However, the heteroepitaxial and energy band structures of the polar-to-polar STO/ZnO(0001¯) interfaces has rarely been studied. In this study, it is shown that the STO films prepared on the ZnO(0001¯) substrate by pulsed laser deposition possess [011]STO and [111]STO azimuth orientations, which exhibit three- and two-fold rotation domains respectively, as demonstrated by X-ray diffraction and transmission electron microscopy. The band structures of the STO(011)/ZnO(0001¯) and STO(111)/ZnO(0001¯) interfaces are bending downward as shown in the I-V characteristic spectra and first principle calculations. These results are different from the single orientation ZnO films grown on STO-(011) and -(111) substrate, as reported in previous literatures, showing the heteroepitaxial asymmetry between STO/ZnO and ZnO/STO interfaces. This work presents an approach towards the physical modeling of combinations between perovskite oxides and wurtzite-structure semiconductors.

Effect of Oxygen Partial Pressure on Crystal Structure, Oxygen Vacancy, and Surface Morphology of Epitaxial SrTiO3 Thin Films Grown by Ion Beam Sputter Deposition

Epitaxial SrTiO3 (STO) thin films were grown on (001)-oriented LaAlO3 (LAO) substrates at 800 °C by an ion beam sputter deposition (IBSD). Oxygen partial pressure (PO2) was varied at 1.5 × 10−5, 1.5 × 10−4, and 1.5 × 10−3 Torr during the growth. The effects of PO2 on crystal structure, oxygen vacancy, and surface morphology of the STO films were investigated and are discussed to understand their correlation. It was found that PO2 played a significant role in influencing the crystal structure, oxygen vacancy, and surface morphology of the STO films. All STO films grew on the LAO substrates under a compressive strain along an in-plane direction (a- and b-axes) and a tensile strain along the growth direction (c-axis). The crystalline quality of STO films was slightly improved at higher PO2. Oxygen vacancy was favorably created in the STO lattice grown at low PO2 due to a lack of oxygen during growth and became suppressed at high PO2. The existence of oxygen vacancy could result in a lattice expansion in both out-of-plane and in-plane directions due to the presence of Ti3+ instead of Ti4+ ions. The surface roughness of the STO films gradually decreased and was nearly close to that of the bare LAO substrate at high PO2, indicating a two-dimensional (2D) growth mode. The results presented in this work provide a correlation among crystal structure, oxygen vacancy, and surface morphology of the epitaxial STO films grown by IBSD, which form a useful guideline for further study.

Electroluminescence from the light-emitting devices with erbium-doped SrTiO3 films on oxidized silicon substrate

We report on the visible and near-infrared (NIR) electroluminescence (EL) from the light-emitting devices (LEDs) with erbium-doped SrTiO3 (SrTiO3:Er) films on oxidized silicon substrate, namely, with the structure of SrTiO3:Er/SiO2/Si in abbreviation. It is found that the 900 °C-annealed SrTiO3:Er films are most desirable for the SrTiO3:Er/SiO2/Si structured LEDs. In the EL-enabling voltage range, the electron transportation through the SiO2 layer of SrTiO3:Er/SiO2/Si structured LED is revealed to be compliant with the trap-assisted tunneling (TAT) mechanism. Based on the energy band diagram of the SrTiO3:Er/SiO2/Si structured LED under the EL-enabling forward bias where the negative voltage is connected with Si substrate, it is believed that a number of conduction electrons in Si can tunnel into the conduction band of SiO2 layer via the aforementioned TAT mechanism and then jump down to the conduction band of SrTiO3 host. Because the conduction band offset of SiO2 and SrTiO3 host is larger than 3 eV, the conduction electrons reaching the SrTiO3 host gain considerably high energies so that they become the so-called hot electrons. The Er3+ ions incorporated into the SrTiO3 host are impact-excited by the hot electrons, ultimately emanating the characteristic visible and NIR light.

High temperature treatment of epitaxial nickel ferrite thin films: The way to bulk-like magnetic properties

An appropriate choice of growth conditions has made feasible fabrication of thin epitaxial NiFe2O4 / SrTiO3 films with parameters close to those of bulk nickel ferrite. Spin waves with a damping parameter as low as 10-2 and a very narrow ferromagnetic resonance line were observed in the samples subjected to post-growth high temperature annealing in oxygen. The analysis of electron and X-ray diffraction has shown substantial improvement of the crystalline quality of the annealed films. X-ray absorption spectroscopy and X-ray magnetic circular dichroism data simulation have revealed that the films exhibit a completely inverted spinel structure characterized by the maximal possible net magnetic moment.

Dry reforming of methane over Ni supported on LaMnO3 thin films

Thin films of LaMnO3 were deposited onto MgAl2O4 by Atomic Layer Deposition (ALD) and studied as supports for Ni in the Dry Reforming of Methane (DRM). Scanning Transmission Electron Microscopy (STEM) with Energy-Dispersive X-ray Spectroscopy (EDS) demonstrated that LaMnO3 covered the support uniformly, and X-ray diffraction (XRD) showed that the LaMnO3 films maintained their perovskite structure after 5 redox cycles at 1073 K. The Ni also remained well dispersed after 5 redox cycles at 1073 K. The LaMnO3-supported catalyst was more active than Ni on the unmodified MgAl2O4 and showed superior resistance against coke formation. Finally, the results for Ni on the LaMnO3 films are compared to previous results for Ni on LaFeO3, CaTiO3, SrTiO3, and BaTiO3 films.

Correlating surface stoichiometry and termination in SrTiO3 films grown by hybrid molecular beam epitaxy

Hybrid oxide molecular beam epitaxy (hMBE), a thin-film deposition technique in which transition metal cations are delivered using a metal-organic precursor, has emerged as the state-of-the-art approach to the synthesis of electronic-grade complex oxide films with a stoichiometric growth window. However, numerous questions remain regarding the chemical mechanisms of the growth process and the surface properties of the resulting films. To examine these properties, thin film SrTiO3 (STO) was prepared by hMBE using a titanium tetraisopropoxide (TTIP) precursor for Ti delivery and an elemental Sr source on annealed STO and Nb-doped STO substrates with varying TTIP:Sr flux ratios to examine the conditions for the reported stoichiometric growth window. The films were transferred in vacuo to an x-ray photoelectron spectroscopy system to study the surface elemental composition. Samples were examined using x-ray diffraction to compare our surface sensitive results with previously reported measurements of the bulk of the films in the literature. Ex situ studies by atomic force microscopy, scanning tunneling microscopy, and low-energy electron microscopy confirmed the presence of surface reconstructions and an Ehrlich–Schwoebel barrier consistent with A-site SrO termination. We find that a surface exhibiting a mixture of SrO and TiO2 termination or a full SrO termination is necessary to obtain stoichiometric adsorption-controlled growth. These results indicate that surface Sr is necessary to maintain the chemical equilibrium for stoichiometric growth during the hMBE process, which is important for the design of future interfacial systems using this technique.

Structural and optoelectronic properties of combining Nb-doped SrTiO3/ITO films on (0 0 1)-YSZ substrate

Functional oxides combined with conventional conductive films are competitive alternative candidates as electrode and electron transport layer. This work explored the film and interface structures and the optoelectronic properties of the combined films of Nb-doped SrTiO3 (Nb-STO) as a classical functional oxide and ITO. The combined films were deposited on a (001)-YSZ substrate via laser molecular beam epitaxy method. In-situ high-energy electron diffraction, X-ray diffraction, and transmission electron microscopy shew [011]Nb-STO azimuth orientation and a two-fold symmetry rotation of the Nb-STO layer. Interestingly, the Nb-STO film growth at 650 °C or higher was able to induce cracks in ITO film. Besides, the growth temperature of Nb-STO from 400 °C to 700 °C changed its structures from amorphousness to polycrystal, the Nb-STO/ITO interface relationships, sheet resistance (from 25 to 83 Ω/sq) and electron mobility (40 to 2 cm·V−1·s−1); however, this had little influence on the transmittance. This work provided an insight into the combination of functional oxides and ITO for the fabrication of the electron transport layer.

Superconductivity in magnetically doped SrTiO3

Doped SrTiO3 is a superconductor whose pairing mechanism is still not fully understood. The response of a superconductor to impurities has long been used to obtain insights into the nature of the superconducting state. Here, the superconductivity of SrTiO3 films that are doped or alloyed with different rare earth ions, which carry a magnetic moment, is investigated. It is shown that large concentrations (up to a few percent) of rare earth ions with unpaired f-electrons, such as Sm and Eu, do not reduce the superconducting critical temperature and critical fields. The finding is independent of whether the rare earth ion acts as a dopant or is an isovalent impurity. The interactions between the superconducting condensate and the magnetic dopants that could result in the observed insensitivity to magnetic impurities are discussed.

Ta-doped SrTiO3 epitaxial thin film: A promising perovskite for optoelectronics

SrTiO3 is a wide bandgap cubic perovskite oxide and displays many exotic properties, i.e., transparent conductivity, photocatalysis, metallicity, ferroelectricity, superconductivity, colossal magnetoresistance, two-dimensional electron gas, etc., due to the manipulations of defect chemistry and constituent elements via impurity doping. This paper reports on the intricacy of the structural and optoelectronic properties of the epitaxially stabilized 5 at. % Ta-doped SrTiO3 (001) thin films on LaAlO3 (001) substrates by systematically varying the growth temperature and oxygen partial pressure during the pulsed laser deposition process. The influences of Ta dopant and growth parameters on the epitaxial quality of these layers are understood by determining the dopant location and its concentration in the SrTiO3 lattice. The complex relationships of optical and electronic properties on growth parameters, dopant concentration, and single crystal quality of the films are demonstrated. The observed low resistivity (∼5 × 10−3 Ω cm) and high optical transparency (∼85%–90%) of optimized Ta-doped SrTiO3 films offer it as an exciting material for next generation transparent optoelectronics.

Emergent Midgap Excitons in Large‐Size Freestanding 2D Strongly Correlated Perovskite Oxide Films

Abstract2D materials exhibit strong excitonic effects due to low dimensionality and enhanced Coulomb interactions, resulting in fascinating many‐particle phenomena like excitons. Though perovskite is a classical type of material hosting abundant correlated electronic phases, freestanding 2D perovskite oxides are not easy to fabricate and yet to be extensively studied. Here the realization of large size (1 × 1 cm2) freestanding perovskite SrTiO3 films, which show unexpected excitonic photoluminescence (PL) spectra and carrier dynamics, is reported. Two pronounced broad PL peaks emerge in 2D freestanding SrTiO3 films at 2.34–2.4 and 1.8–1.9 eV, of which the 2.34–2.4 eV emission originates from self‐trapped excitons localized within TiO6 octahedra, and the 1.8–1.9 eV peak from Ti vacancies. The time‐resolved PL shows a remarkable enhancement of nonradiative Auger recombination through three‐particle process, in which electron–hole excitons transfer their kinetic energy to other free electrons or holes. The results demonstrate unique excitonic properties in 2D perovskite SrTiO3 films and unravel their potential for high‐performance optoelectronic devices.

Influence of Sr deficiency on structural and electrical properties of SrTiO3 thin films grown by metal\u2013organic vapor phase epitaxy

Homoepitaxial growth of SrTiO3 thin films on 0.5 wt% niobium doped SrTiO3 (100) substrates with high structural perfection was developed using liquid-delivery spin metal–organic vapor phase epitaxy (MOVPE). Exploiting the advantage of adjusting the partial pressures of the individual constituents independently, we tuned the Sr/Ti ratio of the gas phase for realizing, stoichiometric, as well as Sr deficient layers. Quantitative energy dispersive X-ray spectroscopy in a scanning transmission electron microscope confirm Sr deficiency of up to 20% in nominally off-stoichiometrically grown films. Our MOVPE process allows to grow such layers in phase pure state and without extended defect formation. Indications for oxygen deficiency could not be identified. Sr deficient layers exhibit an increased permittivity of ɛr = 202 and a larger vertical lattice parameter. Current–voltage characteristics (IVCs) of metal–oxide–semiconductor (Pt/SrTiO3/SrTiO3:Nb) structures reveal that Sr deficient SrTiO3 films show an intrinsic resistive switching with on–off ratios of three orders of magnitude at RT and seven orders of magnitude at 10 K. There is strong evidence that a large deviation from stoichiometry pronounces the resistive switching behavior. IVCs conducted at 10 K indicate a defect-based mechanism instead of mass transport by ion diffusion. This is supported by in-situ STEM investigations that show filaments to form at significant higher voltages than those were resistive switching is observed in our samples.

Bandgap engineering in SrTiO3 thin films by electronic excitations: A synchrotron-based spectroscopic study

Engineering the optical bandgap of metal oxides is essential for optoelectronic and photocatalytic applications. The present study reports the application of electronic excitations, induced by 125 MeV Ag ions, in tuning the bandgap of SrTiO3 films in the range of 2.93-3.78 eV and their correlation with the synchrotron-based X-ray diffraction pattern and X-ray absorption spectra (XAS). The pre-edge spectral features in the XAS at Ti K-edge show significant variation in the valence state implying the role of electronic excitations in modifying the local electronic structures mainly the distortion in the TiO6 octahedra and the ratio of area under t2g/eg. This study gives an insight about the role of structural properties and electronic structures in bandgap tuning.