Epitaxial BaTiO3 Films Research Articles

Tuning electrical conductivity, charge transport, and ferroelectricity in epitaxial BaTiO3 films by Nb-doping

The electrical conductivity, charge transport behavior, and ferroelectricity of epitaxial BaNbxTi1-xO3 films (BNTO, 0.0 ≤ x ≤ 0.5) prepared by pulsed laser deposition are investigated. It is found that Nb-doping can tune the conventional insulating BaTiO3 films from an insulating to highly conductive semiconducting or metallic state, resulting in a variation of the electrical conductivity of the BNTO films over 105. For x ≤ 0.25, the charge transport is dominated by the small polaron hopping mechanism, while the charge transport for x = 0.5 transits from the bipolaron to the small-polaron, and then the thermal phonon scattering mechanisms with increasing temperature. Interestingly, the piezo-force microscopy imaging reveals the presence of ferroelectricity in the properly Nb-doped conductive BNTO films (x ≤ 0.25) deposited in the presence of a small amount of oxygen (3 × 10−3 Pa). Our work provides additional technical roadmaps to manipulate the conductivity and charge transport behaviors in ferroelectric films, which will boost potential applications in future information storage, sensors, and photovoltaic devices.

Low Temperature Epitaxial Barium Titanate Thin Film Growth in High Vacuum CVD

Barium titanate is a promising perovskite material, exhibiting numerous materials properties advantageous for a range of applications, such as ferroelectricity, piezoelectricity, a high dielectric constant, and large electrooptic coefficients. It represents a potential candidate for the implementation of high bandwidth and low energy consuming modulators for optical communication in future integrated systems. Integration of the optical components with electronic circuits on silicon requires, however, suitable low temperature deposition processes for the growth of high quality epitaxial BaTiO3 films, which are still missing. For integration of novel materials into the complementary metal–oxide–semiconductor platform, a variety of prerequisites must be fulfilled; among them there is a strict limitation of the thermal budget of the growth process. Furthermore, high growth rates are desirable for enhanced throughput. Many chemical vapor deposition (CVD) processes allow growing thin films at high rates, but epitaxial BaTiO3 film formation typically requires high substrate temperatures. It is demonstrated that high vacuum CVD is capable of growing epitaxial barium titanate films on magnesium oxide, strontium titanate, and strontium titanate buffered silicon substrates at process temperatures of 400 °C and growth rates of up to 100 nm h−1 without the need of an additional annealing step; this is the lowest substrate temperature reported for a CVD processes.

High ferroelectric polarization in c-oriented BaTiO3 epitaxial thin films on SrTiO3/Si(001)

The integration of epitaxial BaTiO3 films on silicon, combining c-orientation, surface flatness, and high ferroelectric polarization is of main interest towards its use in memory devices. This combination of properties has been only achieved so far by using yttria-stabilized zirconia buffer layers. Here, the all-perovskite BaTiO3/LaNiO3/SrTiO3 heterostructure is grown monolithically on Si(001). The BaTiO3 films are epitaxial and c-oriented and present low surface roughness and high remnant ferroelectric polarization around 6 μC/cm2. This result paves the way towards the fabrication of lead-free BaTiO3 ferroelectric memories on silicon platforms.

Alternative interpretations for decreasing voltage with increasing charge in ferroelectric capacitors.

Recent claim on the direct observation of a negative capacitance (NC) effect from a single layer epitaxial Pb(Zr0.2,Ti0.8)O3 (PZT) thin film was carefully reexamined, and alternative interpretations that can explain the experimental results without invoking the NC effect are provided. Any actual ferroelectric capacitor has an interfacial layer, and experiment always measures the sum of voltages across the interface layer and the ferroelectric layer. The main observation of decreasing ferroelectric capacitor voltage (VF) for increasing ferroelectric capacitor charge (QF), claimed to be the direct evidence for the NC effect, could be alternatively interpreted by either the sudden increase in the positive capacitance of a ferroelectric capacitor or decrease in the voltage across the interfacial layer due to resistance degradation. The experimental time-transient VF and QF could be precisely simulated by these alternative models that fundamentally assumes the reverse domain nucleation and growth. Supplementary experiments using an epitaxial BaTiO3 film supported this claim. This, however, does not necessarily mean that the realization of the NC effect within the ferroelectric layer is impractical under appropriate conditions. Rather, the circuit suggested by Khan et al. may not be useful to observe the NC effect directly.

Influence of top contact noble metals on leakage current properties of epitaxial BaTiO3 film capacitors

Dielectric properties, ferroelectric hysteresis loops, and leakage current characteristics of an epitaxial BaTiO3 (BTO) film were investigated with La0.5Sr0.5CoO3 as bottom electrode and the different noble metal as top electrodes (Au, Ag and Pt, which were deposited on the same surface of a BTO film). The dielectric constants, dielectric loss, leakage currents and ferroelectric loops all exhibited the top-electrode-dependent behavior, which can be attributed to the different interface state. The J–V characteristics were analyzed with various transportation mechanisms in detail. Regardless of the top electrode materials, the leakage current characteristics were described by a modified Schottky contact model for all BTO film capacitors. It was proved that large space charge densities were the main contribution to the slim hysteresis loops. The potential of these interface charge dominated BTO film heterostructures in the application of high energy density storage capacitors was also demonstrated.

Ferroelectricity and Giant Bulk Photovoltaic Effect in BaTiO3 Films at the Nanoscale

The ferroelectric and photovoltaic properties of BaTiO3 films at the nanoscale are considered. The switching and scaling of the coercive field in the hetero epitaxial BaTiO3 films with thickness 3–50 nm are investigated in condenser. At the nanoscale there is competing of extrinsic (domain-driven) and intrinsic homogeneous (Landau-Ginzburg) mechanisms. The conversion efficiency of the Bulk Photovoltaic Effect at the nanoscale could be extremely large. It is experimentally shown for heteroepitaxial BaTiO3 films with thickness 20 and 40 nm. For nanoscaled BaTiO3 films the conversion efficiency is 4,5 orders of magnitude larger as compared to the bulk crystals.

Strain-controlled optical absorption in epitaxial ferroelectric BaTiO3 films

A lattice strain of 0.3%–1.3% is achieved in epitaxial tetragonal BaTiO3 films grown on (001)-oriented SrTiO3 single-crystal substrates. Our experimental studies of absorption spectra in the range of 0.74–9.0 eV demonstrate that epitaxy produces significant changes in the optical properties of the films compared with those of a reference polydomain BaTiO3 crystal: the absorption edge and the peak at 5 eV strongly blue-shift by 0.2–0.4 eV, the magnitude of the peak at 5 eV drops, and certain spectral features disappear, whereas the absorption peak at 8.5 eV remains unchanged. The observed behavior is attributed to ferroelectric polarization, which is enhanced by epitaxial strain in the films. Our results indicate that epitaxially induced variations of ferroelectric polarization may be used to tailor the optical properties of thin films for photonic and optoelectronic applications.

Absence of low temperature phase transitions and enhancement of ferroelectric transition temperature in highly strained BaTiO3 epitaxial films grown on MgO Substrates

Recently, a large enhancement in the ferroelectric transition temperature of several oxides is reported by growing the respective thin films on appropriate substrates. This phenomenon is correlated with high residual strain in thin films often leading to large increase in the tetragonality of their crystal structure. However, such an enhancement of transition temperature is usually limited to very thin films of ∼10 nm thickness. Here, we report growth of fully strained epitaxial thin films of BaTiO3 of 400 nm thickness, which are coherently grown on MgO substrates by pulsed laser deposition technique. Conventional high resolution x-ray diffraction and also the reciprocal space map measurements confirm that the film is fully strained with in-plane tensile strain of 5.5% that dramatically increases the tetragonality to 1.05. Raman measurements reveal that the tetragonal to cubic structural phase transition is observed at 583 K, which results in an enhancement of ∼200 K. Furthermore, temperature dependent Raman studies on these films corroborate absence of all the low temperature phase transitions. Numerical calculations based on thermodynamical model predict a value of the transition temperature that is greater than 1500 °C. Our experimental results are therefore in clear deviation from the existing strain dependent phase diagrams.

Space-charge dominated epitaxial BaTiO3 heterostructures

Epitaxial BaTiO3 thin films (∼300nm thick) with various conductive oxide bottom electrodes including (La0.5Sr0.5)CoO3, SrRuO3 and LaNiO3 were prepared on single-crystalline SrTiO3 substrates using a radiofrequency magnetron sputtering technique in an oxygen-rich atmosphere and at a deposition temperature of 700°C. The charge transport phenomena of these films were systematically investigated. Comparison of leakage current levels with respect to different BaTiO3–bottom electrode interfaces gave a direct proof of the p-type semiconducting behavior in the BaTiO3 films, whose I–V characteristics were well described by a modified Schottky contact model up to an applied field of ∼60MVm−1. By adapting this model, we obtained interface potential barriers, space-charge and free charge carrier densities, as well as depletion layer width and interface layer thickness for the epitaxial BaTiO3 films. The dominance of space-charges (∼1020cm−3) over intrinsic polarizations in these films well explained the apparent reduction in remnant polarization and tilting of the P–V hysteresis loops, as well as an enhanced dielectric strength. Further increase of the applied electric field will result in complete depletion of the BTO films. Consequently, charge transport in these films will become bulk-controlled. This prediction was validated by experimental J–V curves acquired for a broad range of applied electric field, where ionic conduction and Pool–Frenkel emission currents consecutively dominated in the high field (60–220MVm−1) and ultrahigh field (220–300MVm−1) regions. Finally, the potential of these lead-free heterostructures as high-energy density dielectric capacitors was demonstrated.

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.

Ferroelectric memristive effect in BaTiO3 epitaxial thin films

Epitaxial BaTiO3 (BTO) films have been grown on Nb-doped SrTiO3 (NSTO) (0 0 1) substrate. The ferroelectric resistive switching effect and memristive behaviours have been observed in Pt/BTO/NSTO heterostructures. It exhibits two resistance states under different polarization, and both current–voltage curves are nonlinear, slightly asymmetric and free of discontinuities, in contrast with what often occurs with other resistive switching devices. In addition, the resistance can be continuously tuned up to 1.3 × 105% by varying pulse voltage amplitudes for the ferroelectric memristor application. Moreover, the device exhibits excellent retention and reproducibility. The above phenomena can be qualitatively explained by considering the modulation of the width and height of the potential barrier at the BTO/NSTO interface by ferroelectric polarization.

Resistive switching in epitaxial BaTiO3 films grown on Nb-doped SrTiO3 by PLD

Abstract 100-nm-thick epitaxial BaTiO 3 (BTO) films were prepared on the Nb-doped SrTiO 3 (NSTO) single-crystal substrates by pulsed laser deposition. Bipolar resistive switching (RS) behavior was observed in the as-prepared BTO/NSTO samples, which was attributed to the formation and rupture of conductive filaments in the BTO films. The RS effect was suppressed after the samples were annealed in air due to the decrease in oxygen vacancies during annealing, which was confirmed by X-ray photoelectron spectroscopy (XPS) measurements. The current–voltage curves of the as-prepared samples show linear ohmic conduction at the low resistive state and space-charge-limited conduction (SCLC) at the high resistive state, respectively. SCLC dominated in the annealed samples.

Electrocaloric effect in BaTiO3: A first-principles-based study on the effect of misfit strain

We address the question of how the electrocaloric effect in epitaxial thin films of the prototypical ferroelectric BaTiO3 is affected by the clamping to the substrate and by substrate-induced misfit strain. We use molecular dynamics simulations and a first-principles-based effective Hamiltonian to calculate the adiabatic temperature change ΔT under different epitaxial constraints. Our results demonstrate that, consistent with phenomenological theory, clamping by the substrate reduces the maximum ΔT compared to bulk BaTiO3. On the other hand, compressive misfit-strain leads to a strong increase of ΔT and shifts the maximum of the electrocaloric effect to higher temperatures. A rather small compressive strain of –0.75% is sufficient to obtain a ΔT that is larger than the corresponding bulk value.

Polarization-Control of the Potential Barrier at the Electrode Interfaces in Epitaxial Ferroelectric Thin Films

Electrode interface is a key element in controlling the macroscopic electrical properties of the ferroelectric capacitors based on thin films. In the case of epitaxial ferroelectrics, the electrode interface is essential in controlling the leakage current and the polarization switching, two important elements in the read/write processes of nonvolatile memories. However, the relation between the polarization bound charges and the electronic properties of the electrode interfaces is not yet well understood. Here we show that polarization charges are controlling the height of the potential barriers at the electrode interfaces in the case of Pb(Zr,Ti)O3 and BaTiO3 epitaxial films. The results suggest that the height is set to a value allowing rapid compensation of the depolarization field during the polarization switching, being almost independent of the metals used for electrodes. This general behavior open a new perspective in engineering interface properties and designing new devices based on epitaxial ferroelectrics.

Erratum: Switching of ferroelectric polarization in epitaxial BaTiO3 films on silicon without a conducting bottom electrode

Erratum: Switching of ferroelectric polarization in epitaxial BaTiO3 films on silicon without a conducting bottom electrode

Switching of ferroelectric polarization in epitaxial BaTiO3 films on silicon without a conducting bottom electrode

Epitaxial growth of SrTiO₃ on silicon by molecular beam epitaxy has opened up the route to the integration of functional complex oxides on a silicon platform. Chief among them is ferroelectric functionality using perovskite oxides such as BaTiO₃. However, it has remained a challenge to achieve ferroelectricity in epitaxial BaTiO₃ films with a polarization pointing perpendicular to the silicon substrate without a conducting bottom electrode. Here, we demonstrate ferroelectricity in such stacks. Synchrotron X-ray diffraction and high-resolution scanning transmission electron microscopy reveal the presence of crystalline domains with the long axis of the tetragonal structure oriented perpendicular to the substrate. Using piezoforce microscopy, polar domains can be written and read and are reversibly switched with a phase change of 180°. Open, saturated hysteresis loops are recorded. Thus, ferroelectric switching of 8- to 40-nm-thick BaTiO₃ films in metal-ferroelectric-semiconductor structures is realized, and field-effect devices using this epitaxial oxide stack can be envisaged.

Effect of Strain on Barium Titanate Epitaxial Films

Landau-Ginzburg free energy expression with the normalized coefficients is used to elucidate the phase transition properties of strained ferroelectric films. In particular, we investigate the need to include higher order free energy terms for epitaxial strained BaTiO3 thin films. Our study reveals that the inclusion of eighth-order expression into the free energy is crucial in determining the phase transition of highly-strained BaTiO3 epitaxial films normally grown on thick cubic substrates. The phase transition is found to be second order but the unstrained film undergoes the first order phase transition. On the order hand, the calculation based on the usual sixth-order Landau-Ginzburg expression show that the films have no phase transition, which is contrary to the experimental observations.

Ultra-flat BaTiO3 epitaxial films on Si(001) with large out-of-plane polarization

Ferroelectric BaTiO3 is rarely used in monolithic Si devices due to the low quality of BaTiO3 films on Si, as polycrystallinity, degradation of bottom Pt electrodes, low polarization, and high roughness. Here, we overcome these limitations by using a buffer structure that combines yttria-stabilized zirconia, CeO2, and conducting LaNiO3. BaTiO3 films on the multilayered buffer, with total thickness of the buffer below 100 nm, are epitaxial, display remnant polarization of 6–10 μC/cm2, and have roughness of a few Å. These unprecedented properties pave the way to integrate ferroelectric BaTiO3 into Si platforms.

Effects of the depolarization field in a perforated film of the biaxial ferroelectric

The domain structure in a biaxial ferroelectric layer perforated by cylindrical channels has been investigated using the numerical simulation based on the phenomenological theory of ferroelectricity and the equations of electrostatics in the framework of the Gauss-Seidel iterative method. Both polar axes lie in the plane of the film, which is characteristic of thin epitaxial films of BaTiO3 and (Ba1 − xSrx)TiO3 on a MgO substrate. The calculations have been performed using the parameters of BaTiO3, which does not matter because of the qualitative character of the results: the electrostatic problem is two-dimensional and formally applies to infinitely thick layers rather than to thin layers. The primary attention has been paid to the systems containing sixteen channels. Two different orientations of the polar axes with respect to the lattice channels have been considered. It has been shown that, for these orientations, the domain structure has a different character: when the line with the minimum distance between the channels is perpendicular to the bisector of the angle between the polar axes, this structure contains a single channel in the repeating motif and a polarization vortex; when one of the polar axes is perpendicular to the line with the minimum distance between the channels, the situation is less clear. There are indications that the repeating motif of the domain structure in a system of many channels contains two channels and does not contain vortices. The strong influence of the electrodes on the domain structure in this case has been noted.

A comparative study of the BaTiO3 film and the BaTiO3/(Ba0.7Sr0.3)TiO3 superlattice using X-ray diffraction and raman spectroscopy

A comparative study of the BaTiO3 epitaxial film and the BaTiO3/(Ba0.7Sr0.3)TiO3 superlattice prepared by pulsed laser deposition on MgO(100) substrates has been performed. The complete parallel orientation of the film and the substrate has been established and the modulation period of the superlattice Λ = 28 nm, the parameters of the unit cell averaged over the period of the superlattice Λ, and the unit cell parameters of the layers forming the superlattice have been determined using the X-ray diffractometry data. According to the X-ray diffraction data, the unit cell of the BaTiO3 single-component film and the unit cell averaged over the period of the superlattice Λ are pseudocubic; however, an analysis of the polarization characteristics of the Raman spectra has suggested that the film and the superlattice have monoclinic symmetry (P x = P y ≠ 0, P z ≠ 0). The transformation of the components of the soft mode due to the deformation of the epitaxial layers forming the superlattice has been discussed.