SrTiO3 Buffer Layer Research Articles

Structural study and ferroelectricity of epitaxial BaTiO3 films on silicon grown by molecular beam epitaxy

Integration of epitaxial complex ferroelectric oxides such as BaTiO3 on semiconductor substrates depends on the ability to finely control their structure and properties, which are strongly correlated. The epitaxial growth of thin BaTiO3 films with high interfacial quality still remains scarcely investigated on semiconductors; a systematic investigation of processing conditions is missing although they determine the cationic composition, the oxygen content, and the microstructure, which, in turn, play a major role on the ferroelectric properties. We report here the study of various relevant deposition parameters in molecular beam epitaxy for the growth of epitaxial tetragonal BaTiO3 thin films on silicon substrates. The films were grown using a 4 nm-thick epitaxial SrTiO3 buffer layer. We show that the tetragonality of the BaTiO3 films, the crystalline domain orientations, and SiO2 interfacial layer regrowth strongly depend on the oxygen partial pressure and temperature during the growth and on the post-deposition anneal. The ferroelectricity of the films, probed using piezoresponse force microscopy, is obtained in controlled temperature and oxygen pressure conditions with a polarization perpendicular to the surface.

Phonon and magnon excitations in Raman spectra of an epitaxial bismuth ferrite film

An epitaxial film of bismuth ferrite BiFeO3 on a MgO(001) single-crystal substrate has been prepared by pulsed laser deposition using SrTiO3 and SrRuO3 buffer layers. At room temperature, the polarization characteristics of the Raman spectra of the BiFeO3 film under study suggest a monoclinic symmetry. The high-temperature (295–1100 K) investigations of the Raman spectra have been performed in the frequency range 20 cm−1 < ν < 1600 cm−1. Particular attention has been paid to the high-frequency region with a band observed at 610 cm−1, which corresponds to the maximum density of states of the magnon branch at the Brillouin zone boundary, and an intense band in the second-order Raman spectra with the maximum at ∼1250 cm−1, which corresponds to the density of states of two-magnon excitations. It has been found that the intensity of the band at ∼1250 cm−1 decreases linearly with an increase in the temperature and, above 650 K, this band is absent. The extrapolation of the temperature dependence of the integrated intensity of the band at 1250 cm−1 suggests that this film undergoes an antiferromagnetic phase transition at a temperature of ∼670 K.

Ink-jet printing of SrTiO3 buffer layers from aqueous solutions

In this work, fully a-axis oriented SrTiO3 thin films were synthesized by ink-jet printing of water-based precursor inks. The developed precursor solution or ‘ink’ was optimized in terms of rheology, leading to the ejection of single droplets showing a maximum contact angle of 12° on (100) oriented single crystal LaAlO3 substrates. By using the appropriate ink-jet deposition parameters and thermal treatment, well-textured and dense SrTiO3 films of 130 nm thickness were obtained. The biaxial texture is maintained up to the surface of the films, leading to the formation of (h00)-oriented terraces. As shown by transmission electron microscopy, excellent texture transfer was achieved from the SrTiO3 film to the YBa2Cu3O7 − δ layer deposited by pulsed laser deposition. Outstanding superconducting properties were obtained with critical current densities up to 3.6 MA cm−2 in self-field at 77 K, demonstrating that these sustainable SrTiO3 films meet the requirements to be used as growing template for high quality superconducting coatings.

(Invited) Resistive Switching Characteristics and Controllable Quantized Conductance in Single-Crystal Anatase TiO2 on Si (001)

Resistive switching in single crystal anatase TiO2 grown by atomic layer deposition on Si (001) with an epitaxial single crystal SrTiO3 buffer layer is characterized and analyzed. Although switching occurs via a valence-change mechanism involving motion of oxygen vacancies, electrical characteristics highly reminiscent of electrochemical metallization memories – low leakage current in the high-resistance state, highly linear current-voltage characteristics in the low-resistance state, and a very high on/off current ratio – are observed. We postulate that these characteristics may be associated with formation of Ti n O2n-1 Magnéli or Magnéli-like nanophases in the low-resistance state. In addition, quantized conductance is observed in the low-resistance state, with the number of conducting ballistic channels being highly controllable via application of different compliance currents during the memory SET process. For compliance currents of 10-200µA, low resistance state conductances corresponding to 1 to 4 conductance channels, controllable to within a single quantum of conductance, are observed.

X-ray analysis of strain distribution in two-step grown epitaxial SrTiO3 thin films

Epitaxial SrTiO3 (STO) thin films were grown on (001)-oriented LaAlO3 (LAO) substrates using a two-step growth method by ion beam sputter deposition. An STO buffer layer was initially grown on the LAO substrate at a low temperature of 150 °C prior to growing the STO main layer at 750 °C. The thickness of the STO buffer layer was varied at 3, 6, and 10 nm, while the total film thickness was kept constant at approximately 110 nm. According to x-ray structural analysis, we show that the STO buffer layer plays an essential role in controlling the strain in the STO layer grown subsequently. It is found that the strains in the STO films are more relaxed with an increase in buffer layer thickness. Moreover, the strain distribution in two-step grown STO films becomes more homogeneous across the film thickness when compared to that in directly grown STO film.

Effect of “symmetry mismatch” on the domain structure of rhombohedral BiFeO3 thin films

Considerable work has focused on the use of epitaxial strain to engineer domain structures in ferroic materials. Here, we revisit the observed reduction of domain variants in rhombohedral BiFeO3 films on rare-earth scandate substrates. Prior work has attributed the reduction of domain variants to anisotropic in-plane strain, but our findings suggest that the monoclinic distortion of the substrate, resulting from oxygen octahedral rotation, is the driving force for variant selection. We study epitaxial BiFeO3/DyScO3 (110)O heterostructures with and without ultrathin, cubic SrTiO3 buffer layers as a means to isolate the effect of “symmetry mismatch” on the domain formation. Two-variant stripe domains are observed in films grown directly on DyScO3, while four-variant domains are observed in films grown on SrTiO3-buffered DyScO3 when the buffer layer is &amp;gt;2 nm thick. This work provides insights into the role of the substrate—beyond just lattice mismatch—in manipulating and controlling domain structure evolution in materials.

The influence of SrTiO3 buffer layer on ferroelectric properties of Al-doped BaTiO3 thin films

Fabrication of Al0.01Ba0.99TiO3 (Al-BTO) thin films on SrTiO3/MgO/TiO2/SiO2/Si (STO/MgO) and MgO/TiO2/SiO2/Si (MgO) substrates were comparatively prepared using pulsed laser deposition (PLD). The structures of the prepared thin films were studied using X-ray diffraction (XRD) and field emission scanning electron microscopy (FE-SEM). The SrTiO3 buffer layer between Al-BTO thin film and MgO substrate enhanced the ferroelectric, electrical properties. Moreover, the P-E loops showed a dramatic enhancement in the polarization values from 18 μC/cm2 at 600 kV/cm to 52 μC/cm2 at 780 kV/cm for Al-BTO thin films on SrTiO3/MgO and MgO substrates, respectively. In addition, the dielectric SrTiO3 buffer layer decreased the leakage current from 3 × 10−3 to 12 × 10−6 A/cm2 and increased breakdown electric field from 10 to 30 kV/cm. The electric properties revealed that, the dielectric SrTiO3 buffer layer also decreased the conductivity, mobility and carrier concentrations. The data of Al-BTO thin films on SrTiO3/MgO substrate exhibited remarkably adjusted polarization and leakage current indicating a good ferroelectric property for non-volatile ferroelectric random access memory (FERAM) applications.

Enhancement of Properties of Mn/Y Co-doped Ba0.67Sr0.33TiO3 Films on Oxide Electrode by Inserting a SrTiO3 Buffer Layer

Mn/Y co-doped Ba0.67Sr0.33TiO3 (Mn+Y: BST) thin films were deposited on LaNiO3 (LNO)/SiO2/Si substrates without and with SrTiO3 (STO) buffer layers by radio frequency (RF) magnetron sputtering technology. Influence of the STO buffer layer thickness on the properties of the Mn+Y: BST films was investigated. X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations revealed that all the films have uniform and crack-free surface with a perovskite structure. The Mn+Y: BST films with STO buffer layer of 20 nm showed the best electrical properties. The improved properties have been attributed to the STO buffer layer.

Ferroelectric enhancement of La-doped BaTiO3 thin films using SrTiO3 buffer layer

Pulsed laser deposition (PLD) was used to fabricate La0.01Ba0.99TiO3 (LBTO) thin films on MgO/TiO2/SiO2/Si substrates with and without SrTiO3 (STO) buffer layer. LBTO films deposited on STO layers exhibited enhanced ferroelectricity and decreased leakage current (9×10−7A/cm2 at 50kV/cm), conductivity, and Hall mobility, compared to those of LBTO films on MgO substrate. The remanent polarization (Pr) and coercive field (Ec) for LBTO thin films on STO buffered MgO substrate dramatically improved, 36.5μC/cm2 and ~220kV/cm, compared to those for LBTO thin films on MgO substrate, 3μC/cm2 and ~60kV/cm. The degradation of Pr and Ec after 105 switching test is less than 0.1% for LBTO thin films on STO buffered MgO substrate. This work demonstrates a route to a lead-free ferroelectric thin film for nonvolatile memories and electro-optic devices.

Enhanced dielectric properties from barium strontium titanate films with strontium titanate buffer layers

In order to enhance the permittivity and tunability of the dielectric component, a thin film dielectric composite consisting of a radio frequency sputtered SrTiO3 (STO) buffer layer and metalorganic solution deposited Mg-doped BaxSr1−xTiO3 (Mg-BST) thin film overgrowth was developed using affordable industry standard processes and materials. The effect of the STO buffer layer thickness on the dielectric response of the heterostructure was investigated. Our results demonstrate that the composite film heterostructure, evaluated in the metal-insulator-metal configuration Pt/STO/Mg-BST/Pt on sapphire substrate, with the thinner (9–17 nm) STO buffer layers possessed enhanced permittivity (εr ∼ 491) with respect to the thicker 41 nm buffer layer (εr ∼ 360) and that of a control Mg-BST film without a STO buffer layer (εr ∼ 380). Additionally, the composite film with the thinner buffer layers were shown to have low losses (tan δ ∼ 0.02), low leakage characteristics (J = 7.0 × 10−9 A/cm2), high breakdown voltage (VBR &amp;gt; 10 V), a large grain microstructure (∼125 nm), and smooth pin-hole free surfaces. The enhanced permittivity of the composite dielectric film resulted from three major factors: (i) the template-effect of the thin STO buffer layer on the thicker Mg-BST over-layer film to achieve a large grain microstructure, (ii) the low viscosity of the metallo-organic solution deposition (MOSD) solution, which ensured heterogeneous nucleation of the Mg-BST overgrowth film on the surface of the STO buffer layer, and (iii) minimization of the low permittivity grain boundary phase (TiO2−x phase). The dielectric response of the BST can be explained using a thermodynamic model taking into account interlayer electrostatic and electromechanical interactions. Additionally, Mg doping of the BST enabled low loss and low leakage characteristics of the heterostructure. The large permittivity, low loss, low leakage characteristics, and defect free surfaces of the composite dielectric heterostructure promote tunable device miniaturization and hold the potential to enable enhanced electromagnetic coupling in ferromagnetic/high permittivity dielectric heterostructures, which in turn would facilitate the realization of integrated charge mediated voltage controlled magnetic radio frequency/microwave communication devices.

Heteroepitaxial growth and multiferroic properties of Mn-doped BiFeO3 films on SrTiO3 buffered III–V semiconductor GaAs

Epitaxial Mn-doped BiFeO3 (MBFO) thin films were grown on GaAs (001) substrate with SrTiO3 (STO) buffer layer by pulsed laser deposition. X-ray diffraction results demonstrate that the films show pure (00l) orientation, and MBFO (100)//STO(100), whereas STO (100)//GaAs (110). Piezoresponse force microscopy images and polarization versus electric field loops indicate that the MBFO films grown on GaAs have an effective ferroelectric switching. The MBFO films exhibit good ferroelectric behavior (2Pr ∼ 92 μC/cm2 and 2EC ∼ 372 kV/cm). Ferromagnetic property with saturated magnetization of 6.5 emu/cm3 and coercive field of about 123 Oe is also found in the heterostructure at room temperature.

Coaction and distinguishment of converse piezoelectric and field effects in La0.7Ca0.3MnO3/SrTiO3/0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 heterostructures

The volatile and nonvolatile electroresistances related to the converse piezoelectric induced strain and ferroelectric field effects are improved in La0.7Ca0.3MnO3/SrTiO3/0.68Pb(Mg1/3Nb2/3)O3-0.32PbTiO3 heterostructures by inserting a SrTiO3 buffer layer. Due to the coaction of the strain and field effects, the tri-resistance states are observed, and the relative contributions of the two effects on the resistance changes can be quantitatively distinguished by a programmable control of the polarization electric fields in “ON” and “OFF” modes, respectively. Our results indicate that the well-designed heterostructure exhibits potential for application in multifunctional devices.

Interface-engineered resistive switching in Ag/SrTiO3/Nd0.7Ca0.3MnO3/YBa2Cu3O7 heterostructures

For Ag/Nd0.7Ca0.3MnO3/YBa2Cu3O7 (Ag/NCMO/YBCO) heterostructures, we investigate effects of an SrTiO3 (STO) buffer layer inserted into the Ag/NCMO interface upon the room-temperature resistive switching. In comparison with the non-buffered (Ag/NCMO/YBCO) structure, the insertion of the STO buffer layer can greatly enhance the electric-field-induced-resistance (EPIR) effect. The STO-buffered (Ag/STO/NCMO/ YBCO) device can be switched on-and-off between the higher to lower resistance states at an EPIR ratio of 253% with pulsed voltage ±1.5 V and 405% with pulsed voltage ±3.0 V. The enhanced EPIR ratio is attributed to a combined effect of the migration of oxygen vacancies near the interface and ferroelectric polarization of the STO buffer.

C-axis-oriented growth of anatase TiO2 thin films on glass substrate with SrTiO3/TiN template

Uniaxially c-axis-oriented anatase TiO2 thin films were prepared on glass substrates using a SrTiO3/TiN multilayer template. A (100)-oriented TiN layer with a thickness of >100nm served as a seed to introduce atomic regularity onto the surface of amorphous glass. A SrTiO3 buffer layer was used to provide a lattice-matched surface for heteroepitaxy of anatase. X-ray diffraction (XRD) measurements confirmed highly (001)-oriented growth of anatase TiO2 thin films on this SrTiO3/TiN template.

Influence of SrTiO3 Buffer Layer on the Ferroelectric Properties of Bi2NiMnO6 Thin Film

Double perovskite Bi2NiMnO6 (BNMO) thin films with and without SrTiO3 (STO) buffer layer have been prepared on Pt (111)/Ti/SiO2/Si(100) substrates by a chemical solution deposition method. The microscopic surface morphology, ferroelectric, and leakage current have been investigated. It is shown that the STO buffer layer can improve the remnant polarization slightly, and reduce the coercive electric field from 100 to 36 kV/cm. When the positive voltage was applied on top Au electrode of the BNMO/STO/Pt thin film capacitors, the leakage current was controlled by ohmic conduction. When the negative voltage was applied on bottom Pt electron, the leakage current of BNMO/STO thin film was limited by the traps and the density of carriers at high electric field.

In-plane dielectric properties of epitaxial Ba0.7Sr0.3TiO3 thin films grown on GaAs for tunable device application

We have epitaxially deposited ferroelectric Ba0.7Sr0.3TiO3 (BST) thin films grown on GaAs substrate via SrTiO3 buffer layer by laser molecular beam epitaxy. Structural characteristics of the heterostructure were measured by various techniques. The in-plane dielectric properties of the heteroepitaxial structure under different applying frequency were investigated from −190 to 90 °C, indicating Curie temperature of the BST film to be around 52 °C. At room temperature, the dielectric constant of the heterostructure under moderate dc bias field can be tuned by more than 30% and K factor used for frequency agile materials is found to be close to 8. Our results offer the possibility to combine frequency agile electronics of ferroelectric titanate with the high-performance microwave capabilities of GaAs for room temperature tunable device application.

Growth and properties of GdTiO3 films prepared by hybrid molecular beam epitaxy

The paper reports on the thin film growth of a protoptype Mott insulator, ferrimagnetic GdTiO3, using shuttered molecular beam epitaxy. Substrates were (001) (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT), with and without epitaxial SrTiO3 buffer layers, respectively. It was found that on bare LSAT, the starting monolayer was crucial for stabilizing the GdTiO3 perovskite phase. The quality of the films was evaluated using structural, electric, optical and magnetic characterization. Structural characterization showed that the GdTiO3 layers were free of pyrochlore impurity phases and that the lattice parameter was close to what was expected for coherently strained, stoichiometric GdTiO3. The room temperature film resistivity was 7Ωcm and increased with decreasing temperature, consistent with Mott insulating characteristics. The Curie temperature was 30K and a small coercivity was observed at 2K, in good agreement with bulk GdTiO3 properties reported in the literature.

Enhanced memory window of Au/BaTiO3/SrTiO3/Si(001) MFIS structure with high c-axis orientation for non-volatile memory applications

Metal–ferroelectric–insulator–semiconductor (MFIS) structures with BaTiO3 (BTO) as a ferroelectric film and SrTiO3 (STO) as an insulating buffer layer were fabricated on p-type Si(001) substrates using an ion beam sputter deposition technique. The effect of out-of-plane orientation on the electrical properties of the MFIS structures, including leakage current density and memory window behavior, were studied using the growth of the BTO ferroelectric film on Si substrate buffered by highly c-axis-oriented and random-oriented STO buffer layers. The experimental results show that the out-of-plane orientations of the BTO films were almost identical to those of the STO buffer layers. The MFIS structure with a high c-axis orientation exhibited a maximum clockwise capacitance-voltage memory window of 1.17 V with a low leakage current of 1.05×10−7 A/cm2 at an applied voltage of 4 V, which is a significant improvement compared to the MFIS structure with a random orientation. The difference in the electrical properties of the MFIS structures with both types of orientation is discussed in detail. The results obtained from this study indicate that the Au/BTO/STO/p-Si MFIS structure with high c-axis orientation has good potential for use in non-volatile memory applications.

Effects of SrTiO3 buffer layer on structural and electrical properties of Bi3.15Nd0.85Ti3O12 thin films prepared by a chemical method

Ferroelectric Bi3.15Nd0.85Ti3O12 (BNT) thin films have been grown on Pt/Ti/SiO2/Si substrates at 750°C by a chemical solution deposition method using SrTiO3 (STO) as a buffer layer. The influence of STO buffer layer on the phase and microstructure of BNT thin films was examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electrical properties were investigated both for BNT thin films with and without STO buffer layer. The results showed that STO buffer layer strongly influenced the microstructure and electric properties of BNT thin films. BNT ferroelectric thin films with STO buffer layer exhibited the good crystallization behavior, the enhanced fatigue characteristics and excellent leakage current properties. This indicates that the introduction of the STO buffer layer prevents the interfacial diffusion and charge injection between BNT thin films and the substrate effectively and improves the interface quality.

Strain induced ionic conductivity enhancement in epitaxial Ce0.9Gd0.1O2−δ thin films

Strained epitaxial Ce0.9Gd0.1O2−δ (CGO) thin films are deposited on MgO(001) substrates with SrTiO3 (STO) buffer layers. The strain in CGO epitaxial thin films is induced and controlled by varying the thickness of STO buffer layers. The induced strain is found to significantly enhance the in-plane ionic conductivity in CGO epitaxial thin films. The ionic conductivity is found to increase with decrease in buffer layer thickness. The tailored ionic conductivity enhancement is explained in terms of close relationships among epitaxy, strain, and ionic conductivity.