KTaO3 Substrates Research Articles

Magnetoresistance and Hall effect in e-doped superconducting SrLaCuO thin films

We have epitaxially grown c-axis oriented SrxLa1-xCuO2 thin films by rf sputtering on KTaO3 substrates with χ = 0.12. The as-grown deposits are insulating and a series of superconducting films with various Tc(ρ=0) up to 26 K have been obtained by in-situ oxygen reduction. Transport measurements in the ab plane of these samples have been undertaken. We report original results on the temperature dependence of the Hall effect and on the anisotropic magnetoresistance (T ≥ Tc). We discuss the magnitude of upper critical fields and anisotropy, the Hall effect, which presents changes of sign indicative of the existence of two types of carriers, the normal state magnetoresistance, negative in parallel magnetic field, a possible signature of spin scattering. These properties are compared to those of hole-doped cuprates, such as BiSr(La)CuO with comparable Tc.

Single-crystal epitaxial thin films of SrFeO2 with FeO2 “infinite layers”

Single-crystal thin films of SrFeO2, which is an oxygen-deficient perovskite with “infinite layers” of Fe2+O2, were prepared by using CaH2 for low-temperature reduction of epitaxial SrFeO2.5 single-crystal films deposited on KTaO3 substrates. This reduction process, removing oxygen ions from the perovskite structure framework and causing rearrangements of oxygen ions, topotactically transforms the brownmillerite SrFeO2.5 to the c-axis oriented SrFeO2.

Epitaxy-stabilized n-type superconducting cuprates

We report the growth of n-type superconducting T′-(La,Ce)2CuO4 and infinite-layer (IL) (Sr,La)CuO2 thin films by means of molecular beam epitaxy (MBE). The bulk synthesis of T′-(La,Ce)2CuO4 and IL-(Sr,La)CuO2 requires complicated techniques: synthesis at low temperatures below 600 °C for the former and at high pressures above 3 GPa for the latter. This makes it difficult to grow bulk single crystals. We have found, however, that high-quality single-crystalline films of both compounds can be rather easily prepared by thin-film processes. Single-phase T′-(La,Ce)2CuO4 films can be obtained for a wide range of x (0.0 ≤ x ≤ ∼0.4). The best Tcend is over 30 K, which is the highest in the T′ family. For IL-(Sr,La)CuO2, by using KTaO3 substrates, high Tcend over 39 K and also metallic resistivity were achieved for the first time to our knowledge. We describe the key parameters in the growth and the properties of the resultant films.

Single-crystalline superconducting thin films of electron-doped infinite-layer compounds grown by molecular-beam epitaxy

Single-crystalline Sr1−xLaxCuO2 thin films of electron-doped infinite-layer compounds were grown by molecular-beam epitaxy. Crucial to our success was the use of KTaO3 substrates. The best film showed Tconset=41.5 K and Tczero=39.0 K, which is close to the highest Tconset of 43 K for the bulk value. The resistivity of the optimum-doped film exhibited metallic temperature dependence with a low resistivity of 320 μΩ cm at room temperature and 120 μΩ cm just above Tc.

Long-range ferroelectric interactions in KTaO3/KNbO3 superlattice structures

Symmetric superlattice structures consisting of alternating atomic-scale layers of KTaO3 and KNbO3 with variable periodicity were grown on KTaO3 substrates by pulsed laser deposition. The in-plane structure of KNbO3 closely matches that of the KTaO3 substrate, resulting in KTaO3/KNbO3 heterostructures that are uniformly strained in-plane without misfit dislocations. This strain imposes an in-plane KNbO3 lattice spacing identical to that of the KTaO3 substrate for the temperature range 30 °C<T<700 °C, and a tetragonal-to-tetragonal transition is observed whose phase transition temperature Tc depends on the KNbO3 layer thickness. The in-plane strain results in a significant increase in this ferroelectric-paraelectric Tc for superlattices with relatively thick KNbO3 layers (Tc=535 °C for a 17 nm thick layer, as compared to 435 °C for bulk KNbO3) and for K(Nb0.5Ta0.5)O3 random-alloy thin films. As the superlattice period decreases, a reduction of Tc is observed. For superlattices with periodicities of 50 Å or less, the Curie temperature is identical to that of the K(Ta0.5Nb0.5)O3 random-alloy film, indicating significant long-range ferroelectric coupling across the KTaO3 layers.

Domain populations in epitaxial ferroelectric thin films: Theoretical calculations and comparison with experiment

An equilibrium domain theory is used to calculate the thickness and temperature dependencies of relative domain populations in tetragonal ferroelectric thin films epitaxially grown on cubic substrates. The relative coherency strain between the film and substrate that governs domain configurations is evaluated with the account of strain accomodation caused by the generation of misfit dislocations at the growth temperature. The threshold coherency strain, which provides the energetic equivalency of the c and a single-domain films and plays an important role in predicting multidomain patterns, is computed as a function of temperature with the aid of the Landau–Ginsburg–Devonshire-type thermodynamic theory. Theoretical predictions are compared with the experimental measurements for PbTiO3 films grown on (001)-oriented MgO and KTaO3 substrates and found to be in good agreement in both the thickness and temperature dependence of the c-domain volume fraction in these epitaxial heterostructures.

Substrate dependence in the growth of epitaxial Pb1−xLaxTiO3 thin films

The metalorganic chemical vapor deposition (MOCVD) technique has been applied to the growth of epitaxial Pb1−xLaxTiO3 (PLT) thin films with x=0.28. By first introducing an initial TiO2 layer, three-dimensional epitaxial PLT films were grown on the (100) surface of MgO substrate. For both KTaO3 (100) and Al2O3 (0001) substrates, heteroepitaxy was achieved without the introduction of TiO2 as the initial, intervening layer between the PLT film and the substrate. On Al2O3 substrates, PLT films with a [111] preferred orientation were grown with a good epitaxial in-plane relationship.

The growth and properties of epitaxial KNbO3 thin films and KNbO3/KTaO3 superlattices

Potassium niobate (KNbO3) thin films and potassium niobate/tantalate (KNbO3/KTaO3) superlattices have been grown on KTaO3 (001) substrates by pulsed laser deposition. The thin-film structures were analyzed by Rutherford backscattering/ion-channeling techniques, x-ray θ–2θ and Φ scans, and both conventional and Z-contrast scanning transmission electron microscopy. Excellent film flatness and crystallinity are evidenced by these techniques. At room temperature, the KNbO3 films are characterized by an orthorhombic structure which differs from that of bulk KNbO3. The interfaces between the layers in the KNbO3/KTaO3 superlattice structures were found to be compositionally sharp on an atomic scale.

The Phase Diagram of Epitaxial BaTiO3 Films under Bi-Axial Stress

AbstractThe phase diagram of eptaxial BaTiO3 films in the coordinates of T and αs ( substrate thermal expansion coefficient) is constructed on the basis of the thermodynamic analysis which has been carried out earlier [1]. It is found that two new phase are formed under bi-axial stress. The temperature dependencies of polarization and unit cell parameters for the films on the KTaO3 and SrTiO3 substrates are also calculated.

Epitaxial heterostructures YBa2Cu3O7−δ/KTaO3 for microwave applications

Combinations of high TC superconducting and ferroelectric films may give rise to tunable, high-Q microwave components. c-axis oriented YBa2Cu3O7−δ films were grown by laser ablation on (001) and vicinically cut KTaO3 substrates and studied by x-ray diffraction and electron microscopy. Competitive superconducting properties were registered. YBa2Cu3O7−δ/KTaO3/YBa2Cu3O7−δ trilayers were deposited on silicon-on-sapphire buffered by CeO2/Y-ZrO2. The dielectric permittivity of KTO3 at 15–100 K decreased considerably when the layer was polarized by a dc voltage. A loss factor tan δ=0.007 was measured at 100 kHz and T=50–100 K.

Second harmonic generation in potassium niobate thin films

Green light produced by second harmonic generation has been observed in an epitaxial orthorhombic KNbO3 thin film planar waveguide produced by ion-beam sputter deposition on a (100)-oriented MgO single crystal substrate. A Nd:YLF laser beam, with a wavelength of 1.053 μm and ∼80 ps, 100 MHz pulses under mode-locked operation, was coupled into the waveguide using a rutile prism, and a green light streak 3–4 mm long was seen in the guide. The TM0 mode of the input beam was phase matched to the TE1 mode of the second harmonic for a film thickness of 2300 Å. Second harmonic generation was also observed in a nonwaveguided configuration on thicker (4600–6500 Å) films on both MgO and KTaO3 substrates.

Optical Properties of Epitaxial Plt Thin Films

AbstractMetalorganic chemical vapor deposition (MOCVD) was used to prepare epitaxial or highly oriented PLT (Pb1-xLaxTiO3) thin films with x in the range of 0.21 to 0.34. The growth of PLT films resulted in three-dimensional epitaxial heterostructures on (100) surface of the MgO and the KTaO3 substrates. The PLT film grown on the KTaO3 (100) substrate has a significantly lower minimum channeling yield compared to that on the MgO (100) substrate because of the smaller lattice mismatch. The thickness and the refractive indices in the wavelength range of 435 to 1,523 nm were measured by the prism coupling method. The measured film thickness of 570 nm was in good agreement with that from RBS measurements. The refractive index of PLT film is smaller than that of PbTiO3, its difference at 632.8 nm is about 2.5 %. The dispersion of the refractive index was well fitted to a Sellmeier dispersion formula.

Pulsed Laser Deposition of Epitaxial Sr(RuxSn1-x)O3 Thin Film Electrodes and KNbO3/Sr(RuxSnl-x)O3 Bilayers

AbstractSr(RuxSnl-x)O3 is proposed as a new conducting oxide for use in epitaxial multilayer structures. The Sr(Ru0.48Sn0.52)O3 composition exhibits an excellent lattice match with (100)-oriented KTaO3, and films of this composition grown by pulsed laser deposition on KTaO3, SrTiO3, and LaAIO3 substrates have been analyzed by X-ray diffraction, Rutherford backscattering/ion channeling, and resistivity measurements. Epitaxial KNbO3/Sr(Ru0 48Sn0.52)O3 bilayers have been successfully grown.

Epitaxial KNbO3 thin films on KTaO3, MgAl2O4, and MgO substrates

Epitaxial potassium niobate (KNbO3) thin films have been deposited on KTaO3 (100), MgAl2O4 (100), and MgO (100) substrates using ion-beam sputter deposition. X-ray-diffraction results show that KNbO3 films have orthorhombic (110) orientation on all three substrates. Rutherford backscattering channeling analysis of KNbO3 films on KTaO3, MgAl2O4, and MgO exhibits minimum scattering yields (χmin) of 7%, 9%, and 18% for the Nb peak, respectively. This illustrates how the quality of epitaxy improves as the lattice mismatch decreases. Prism-coupling measurements reveal near-bulk refractive indices of about 2.27 for TE modes and 2.22 for TM modes for films on each substrate.

Growth and Properties of PbTiO3/PLT Heterostructures

ABSTRACTFerroelectric superlattice structures composed of three-dimensionally epitaxial PbTiO3 and PLT thin films have been successfully grown on SrTiO3 substrates by metalorganic chemical vapor deposition. The modulation structures were confirmed by θ-2θ XRD, and the excellent in-plane orientational relationship between the superlattice film and the substrate by (100), (110), and (111)-pole figures. The φ-scans through the (110) and (111) reflections were used as additional evidence for three-dimensional epitaxy. The substrate dependence of the epitaxial orientation of PLT and PbTiO3 single-layered thin films was investigated. PbTiO3 thin films with very high crystalline perfection can be successfully grown on KTaO3 substrates.

A study of microstructures in ferroelectric Pb(Zr,Ti)O3 and KNbO3 thin films

Oxide ferroelectric thin films are receiving considerable attention due to their potential application in a wide range of devices including ferroelectric nonvolatile memory, dynamic random access memory (DRAMs) electrooptic phase modulators, and laser light source frequency doublers. However, the complexity of the oxide materials has resulted in a relatively slow development of structureprocessing property relationships. In this paper, we review in turn how the microstructures of epitaxial KNbO3 and Pb(Zr Ti)O3 have been significantly improved over a period of time.In the case of epitaxial KNbO3 on oxide single crystal substrates, defect types which need to be controlled include 221-type misorientations, low angle boundaries, multiposition domains, and inversion domains. Several of these defect types are controlled via the substrate surface preparation. The role of lattice mismatch between film and substrate on the formation of the low angle boundaries remains an unresolved issue. We are currently comparing via TEM the KNbO3(100) films on (100)MgO, (100)MGAl2O4, (100)NdGaO3 and (100)KTaO3 substrates; these substrates covering a range of values of lattice mismatch between 0.1% and 5.0%.

Liquid Phase Epitaxy of Na1−yKyTa1−xNbxO3 for Pyroelectric Applications

ABSTRACTThe epitaxial growth of para- and ferroelectric KTa1−xNbxO3 (KTN) thin films was achieved in an in-house built liquid phase epitaxy (LPE) apparatus. Films were grown from a KF/NaF/KTN solution on KTaO3 substrates. Growth rates of several μ/min were achieved at a growth temperature of 1200 K. Up to 150 μm thick, smooth KTN films with different compositions were grown and characterized. Almost perfect lattice matching was achieved by the addition of NaF to the KF flux. Partial replacement of K by Na did not result in a significant change of physical properties of KTN.The investigation of the dielectric properties revealed a sharp change of the dielectric constant at the ferroelectric phase transition. The ferroelectric domain structure was modified by poling at 1kV/cm. Pyroelectric studies yielded a pyroelectric coefficient of up to 5.2 mC/m2K while the frequency dependence of the pyroelectric response showed a maximum at 230 Hz.

TfC14. Growth of para- and ferroelectric epitaxial layers of KTa1−xNbxO3by liquid phase epitaxy

Abstract Para- and ferroelectric layers of KTa1−xNbxO3 (KTN) could be grown on pure and Sn-doped (100) KTaO3 substrates by liquid phase epitaxy from KF/KTN, K2MoO4/KTN and KTN/K2O solutions. Depending on the required layer thickness either dilute solutions of KTN and KF, K2MoO4 or a KTN melt with an excess of K2O were used. Lattice fitting could be achieved by the growth of KTN:Na layers. The epitaxially grown samples were analysed by optical microscopy, wavelength dispersive electron probe microanalysis, scanning electron microscopy, x-ray and dielectric measurements. Ferroelectricity at room temperature was demonstrated by hysteresis loop measurements.

Growth of monocrystalline thin films of Potassium-Tantalate-Niobate (KTN) by liquid-phase epitaxy

Thin monocrystalline films of KTa1−xNbxO3 have been grown on [100] and [110] KTaO3 substrates by liquid-phase epitaxy (LPE) from diluted KF-KCl fluxes. Average growth rates of 0.5–4 μm min−1 produced uniform film thicknesses from ∼ 3 to 30 μm. As-grown films on [100] KTaO3 were optically transparent and showed a flat surface, whereas in [110] orientation pronounced facets were formed. The films have been characterized by means of X-ray diffraction, microprobe analysis, scanning electron microscopy and other methods.

Epitaxial superconducting thin films of YBa2Cu3O7−x on KTaO3 single crystals

We have grown thin YBa2Cu3O7−x films on single-crystal (001) KTaO3 substrates by means of coevaporation of Y, BaF2, and Cu, followed by an ex situ anneal in wet oxygen. The films are epitaxial in three dimensions, consisting of grains with either the c or the a axis of the YBa2Cu3O7−x unit cell perpendicular to the substrate. Films with a composition close to the 1:2:3 cation ratio exhibit sharp superconducting transitions at 90 K and critical current densities in excess of 0.3 MA/cm2 in zero magnetic field at 77 K. In Cu-rich films, the Y2Ba4Cu8O16–x phase with a Tc of 80 K is formed in grains with the c axis perpendicular to the substrate.