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Uniformity of lattice misfit strain was quantitatively evaluated in heteroepitaxial (Ba, Sr)TiO3 films which were deposited by radio-frequency magnetron sputtering on SrRuO3/SrTiO3 substrates. Due to lattice misfit strain, the (Ba, Sr)TiO3 films had a 3% longer c-axis than the inherent bulk of (Ba0.75Sr0.25)TiO3. As analyzed by Hall's theory, the uniformity of the strain was 0.3% of the c-axis (0.413 nm) and 9% of the elongation of the c-axis (0.015 nm). It was suggested by the comparative study of two specimens that there is a strong correlation between the quality of the ferroelectric hysteresis loop and the uniformity of the misfit strain in heteroepitaxial (Ba,Sr)TiO3 films.
The growth, structural and cathodoluminescent (CL) properties of europium activated yttrium oxide (Eu:Y2O3) thin films are reported. The Eu:Y2O3 films were grown in-situ using a pulsed laser deposition technique. Our results show that Eu:Y2O3 films can grow epitaxially on (100) LaAlO3 substrates under optimized deposition parameters. The epitaxial growth of Eu:Y2O3 films on LaAlO3, which has a lattice mismatch of ∼ 60 %, is explained by matching of the atom positions in the lattices of the film and the substrate after a rotation. CL data from these films are consistent with highly crystalline Eu:Y2O3 films with an intense CL emission at 611 nm.
We have controlled the nanoscale growth mechanism and surface morphology of YBa2Cu3O7 (YBCO) based high-Tc thin films and heterostructures, using miscut SrTiO3 substrates. On exact (001) SrTiO3 substrates, the YBCO films grow in a screw dislocation growth mode. The barrier layers (La6.4Sr1.6Cu8O20 and PrBa2Cu3O7) grown on top of such a YBCO film also show spiral growth features, indicating pseudomorphic growth. On miscut substrates (with miscut angle ≥ 4° toward ) the YBCO films grow by step-flow. However, the La6.4Sr1. 6Cu8O20 layers grown on such YBCO bottom electrodes, show a high degree of step bunching with rough surface. In contrast, the PrBa2Cu3O7 layers show clear step-terrace surface morphology similar to the underlying YBCO bottom electrode, suggesting the existence of periodic nanoscale steps at the S-N interface. These heterostructures can be used for the fabrication of SNS Josephson junctions to take advantage of the proximity effect coupling at the nanoscale steps at the interface.
The first room-temperature liquid compounds useful for the CVD of alkaline earth metalcontaining oxides were prepared by reacting metal (Mg, Ca, Sr, and Ba) beta-diketonates with novel polyamine ligands. The compounds are monomeric and can be completely flash-vaporized without leaving any non-volatile residue detectable at the parts-per-million level. A stable, solvent-free liquid mixture was formed by mixing new liquid barium, strontium and titanium compounds. CVD experiments using direct liquid injection of this liquid mixture deposited films of barium strontium titanate. This approach should also be applicable to the deposition of many other multicomponent oxides containing alkaline earth metals: ferroelectrics (strontium bismuth tantalate), metallic conductors (strontium vanadium oxide, lanthanum strontium cobalt oxide), phosphors (calcium tungstate), non-linear optical materials (beta-barium borate), magnetic oxides (barium ferrite), colossal magnetoresistive materials (lanthanum strontium manganese oxide), high Tc superconductors (yttrium barium copper oxide, bismuth calcium strontium copper oxide) and microwave dielectrics (barium magnesium tantalate).
(001)-oriented epitaxial SrBi2Nb2O9 thin films have been grown by pulsed laser deposition on (001) SrTiO3 and (001) LaAlO3—Sr2AlTaO6 substrates at optimized growth conditions. 4-circle x-ray diffraction, Rutherford backscattering spectrometry, and transmission electron microscopy reveal highly oriented epitaxial films. Atomic force microscopy indicates spiral growth for films grown on SrTiO3 and layer-by-layer growth for films grown on LaAlO3—Sr2AlTaO6.
This contribution describes the synthesis, characterization, and implementation of new lanthanide and main group metal-organic chemical vapor deposition precursors based on the 2,2-dimethyl-5-N-2-methoxyethylimino-3-hexanonato ligand system. The new homoleptic, fluorinefree, low melting, and highly volatile complexes are ideally suited for oxide MOCVD, and in many applications are superior to standard β-diketonates while maintaining ease of synthesis and low cost. This is explicitly demonstrated by the growth of high quality CeO2/YBa2Cu3O7-δ multilayers.
The effect of annealing on 3-dimensional lattice strain, crystallographic domain structure, magnetic and electrical properties of both 250 Å and 4000 Å thick epitaxial La0.8Ca0.2MnO3 (LCMO(x=0.2)) thin films grown on (001) LaAlO3 substrates have been studied. While short annealing time (∼2hrs. at 950 °C in oxygen of 1 atm. pressure) leads to anomalous increase of the peak temperature (Tp) and Curie temperature (Tc) above room temperature and that of the bulk material, longer annealing time (∼10 hrs.) restores the Tp and Tc to almost the same values as that of the as-grown films. Furthermore, as the annealing time is increased, the lattice strain relaxes with film's lattice parameter approaching the bulk value. In-plane and out-of-plane lattice parameters and strain states of the as-grown and annealed films were measured directly using normal and grazing incidence x-ray diffraction. A clear correlation is observed between Tp and perovskite unit cell volume for both the films. Tp is found to increase with the decrease of perovskite unit cell volume. This is attributed to the enhancement of overlap between Mn d orbitals and oxygen p orbitals leading to increased bandwidth and conductivity. Crystalline quality of the films as determined by the full width at half maximum (FWHM) of the x-ray rocking curves, improves with the annealing time. This work highlights the importance of controlling the 3-dimensional lattice strain for optimizing the properties of CMR films.
In order to develop an alternative buffer layer architecture using the sol-gel process to produce YBCO (YBa2Cu3O7-δ) coated conductors, Yb2O3 has been chosen as the candidate material. Buffer layers of Yb2O3 were epitaxially grown on biaxially textured-Ni (100) substrates by the sol gel process for the first time. The Yb2O3 precursor solution was prepared from an alkoxide sol-gel route in 2-methoxyethanol and was deposited on textured-Ni (100) substrates by either spin coating or dip coating methods. The amorphous film was then processed at 1160°C under flowing (96%)Ar/H2(4%) gas mixture for one hour. The Yb2O3 film exhibited a strong c-axis orientation on the Ni (100) substrates. The phi and omega scans indicated good in plane and out of plane orientations. The X-ray (222) pole figure showed a cube-on-cube epitaxy. High current YBCO films were grown on the Yb2O3 sol-gel buffered-Ni substrates.
The strain inhomogeneity and crystallographic domain structures and their influence on the electrical transport and magnetic properties of epitaxial colossal magnetoresistive La1-xCaxMnO3 (x = 0.2, 0.33) films have been studied as a function of film thickness and two types of (001) substrates, SrTiO3 and LaAlO3. Out-of-plane and in-plane lattice parameters were determined using normal and grazing incidence x-ray diffraction (GID), and least-squares fits of off-axis x-ray reflections. The lattice strain near the film surface as determined by GID appears to relax faster than that at the film interior as determined by the least-squares fit, indicating the presence of strain inhomogeneity in the films. The observed strain inhomogeneity appears to influence the magnetic and electrical transport properties. In particular the measured temperature dependent magnetization exhibits multiple transitions indicating a variation of Curie temperatures within the same sample. While the very thin films exhibit single out-of-plane domains, accompanied by a high crystalline coherence and smooth surfaces, strain relaxation in thicker films leads to mixed ( 001)T and (110)T textures, and increased mosaic spread and surface roughness. The films also exhibit electrically insulating “dead” layers about 100 – 200Å thick.
The increasing interest in scanning probe instruments (SPM) stems from the outstanding possibilities in measuring electric, magnetic, optical, and structural properties of surfaces and surface layers down to the molecular and atomic scale. For the inspection of ferroelectric materials both the scanning force microscope (SFM) and the scanning near-field optical microscope (SNOM) are promising techniques revealing information on the polarization vector and the electric field induced stress within a crystal. Polarization sensitive modes are discussed as is friction force microscopy, dynamic force microscopy (DFM) and voltage modulated SFM. From these measurements, 180° domain walls (c-domains) are resolved down to 4 nm, while 3-dimensional polarization mapping in ferroelectric BaTiO3 ceramics reveals a 25 nm resolution. On the other hand, non-contact DFM measurements in ultra-high vacuum are able to resolve ferroelectric surfaces down to the atomic scale. Then also the chemical heterogeneity at the sample surface is differentiated from ferroelectric domains down to a 5 nm lateral resolution, taking advantage of the short range chemical forces. SNOM in contrast probes the optical properties of ferroelectric crystals both in transmission and reflection. Here image contrast arises from changes in the refractive index between different domains as well as at domain walls. In addition, SPM instruments are used for the local modification of ferroic samples by applying a relatively high voltage pulse to the SPM tip. Domains with diameters down to 30 nm are thus created with the size depending on both the switching and material parameters.
Selection of a proper electrode for high dielectric material such as (Ba, Sr)TiO3 is a great concern because the deposition of BST requires a high temperature and an oxidizing atmosphere. In this study, we suggested the perovskite-type electrodes, which provide a structural match with the BST dielectric material, under the recognition that the high leakage current is associated with the structural mismatch between BST and the electrode. We studied the (Ca,Sr)RuO3 electrode of which the lattice parameter can be tuned to fit into BST by changing the Ca/Sr ratio. We also studied (Ba,Sr)RuO3 electrode which is not only structurally identical but also chemically similar to BST. In addition, the effect of doping in the BSR electrode was investigated to minimize the leakage current by proper modulation of the barrier height. The electrodes were directly deposited on an Si substrate and all the films in the experiments were deposited by RF magnetron sputtering technique. Electrical properties were measured from MIM structure. The main focus was to address the effect of Ca/Sr and Ba/Sr ratio variations in the electrodes on the resulting dielectric constant and the leakage current. The interface characteristics between the BST film and the electrode were examined in order to interpret the electrical properties of BST films.
Recent theoretical analyses  have suggested that the origin of the reduced Jc at grain boundaries in high-Tc superconductors may be band bending, which results in the depletion of mobile charge carriers at the boundaries. For this to occur in these p-type superconductors there must, by definition, be a high density of localized donor states in the boundary plane. Here we report a “structural unit” analysis of [0011 tilt grain boundaries in YBa2Cu 3O7-δ (YBCO) that indicates there may be a simple structural origin for such localized donor states.
The retention phenomena of purposely aligned micron-size domains (defined as “bits”) in Pb(Zr,Ti)O3 thin films were characterized by atomic force microscopy (AFM) combined with a lock-in amplifier. It is found that the retention loss occurs by “region by region” showing local variation of the rate of the loss. Furthermore, the total retention loss can be successfully described by an extended exponential decay, which implies a narrow distribution of the relaxation times of the domains. This probably comes from the fact that the micron-size bits consist a few hundreds of domains. Along with the characterization, the effects of the bit size and the poling time per unit area on the retention characteristics were investigated. Based on our observations, it is concluded that the retention time is proportional to both the poling time per unit area and the bit size. This trend is successfully explained by a kinetic model developed by our group.
We have observed Electron Beam Induced Current imaging of thin film ferroelectrics. The Electron beam irradiation of a thin ferroelectric film creates a local temperature gradient that induces a polarization gradient and therefore a local electric field. Although the temperature difference is small the gradient is on the order of thousands K/cm and results in a corresponding electric field of a few MV/cm. The thermally induced electric field drives the electron beam created carriers toward an electrode thus inducing an externally measurable current. Despite the very small carrier life time (<1 ns) in ferroelectrics, the induced electric field is strong enough to collect carriers from a few hundred nm depth before recombination. An EBIC gain of 5 to 20 was measured experimentally with BaTiO3 and LiTaO3 films on silicon substrates. This method is insensitive to charge traps and provides a resolution better than 1 μm.
Stress and deformation of PZT thin films deposited on silicon wafers due to thermal expansion during the annealing process are modeled using a 3-D shell element of ANSYS. Two different designs of PZT thin films on the wafer are modeled. The first design is a PZT/Pt/Ti/silicon dioxide/silicon wafer, which is used for making acoustic emission sensors. The second design is a PZT/Pt/Ti/silicon dioxide/silicon nitride/silicon dioxide/silicon wafer, commonly used in fabrication of cantilever beams. For the design without the silicon nitride layer, the thermal stress of the PZT film is 298MPa, Pt 1280MPa, Ti 647MPa, the silicon dioxide layer is 228MPa, and the silicon wafer is 0.41–1.67MPa. For the design with silicon nitride, the thermal stresses are: PZT 301MPa, Pt 1280MPa, Ti 651MPa, silicon dioxide 226MPa, silicon nitride 416MPa, silicon dioxide 226MPa, and silicon wafer 1.05–4.23MPa. The residual stress of the PZT film is measured at 200–25OMPa for the design without silicon nitride, and 336MPa for the design with silicon nitride. Comparisons of the thermal stress with the tensile or proof stress of material for each layer indicate that thermal stress of the PZT film is slightly greater than its bulk tensile stress, that of Pt film is five times greater than its bulk tensile stress, and that of Ti film is approximately equal to its bulk tensile stress. The thermal stresses of silicon dioxide, silicon nitride, and silicon wafer layers are far smaller than their proof stresses.
Lattice defects present in PLD-grown, epitaxial SrRuO3 thin films on (001) SrTiO3 substrates are analyzed by high resolution transmission electron microscopy (HRTEM). Before the preparation of TEM samples, the electrical resistivity of films grown at different substrate temperatures was determined. Films grown at 775 °C exhibited a low electrical resistivity of only 200 μΩcm. They were found to be of orthorhombic structure and contained only few lattice defects. Films grown at 700 °C showed a high electrical resistivity of 1400 μΩcm. They were of cubic lattice symmetry, while films grown at temperatures above 800 °C showed resistivities between 300 and 900 μΩcm. The latter films mainly consist of an orthorhombic-cubic phase mix and involve lattice defects of high density, such as twins and antiphase boundaries (APBs). These defects are mainly located in between the islands and obviously contribute to the high film resistivity observed. For example, the APBs contain an extra single SrO layer, which is certainly insulating. Moreover, Ru vacancies are present in these films.
In this paper we investigate the microstructural accommodation of nonstoichiometry in (BaxSr1-x)Ti1+yO3+Z thin films grown by chemical vapor deposition. Films with three different (Ba+Sr)/Ti ratios of 49/51 (y=0.04 in the notation of the formula above), of 48/52 (y = 0.08) and of 46.5/53.5 (y=0.15), were studied. High-resolution electron microscopy is used to study the microstructure of the BST films. High-spatial resolution electron energy-loss spectroscopy (EELS) is used to reveal changes in chemistry and local atomic environment both at grain boundaries and within grains as a function of titanium excess. We find an amorphous phase at the grain boundaries and grain boundary segregation of excess titanium in the samples with y=0.15. In addition, EELS is also used to show that excess titanium is being partially accommodated in the grain interior. Implications for the film electrical and dielectric properties are outlined.
Multicomponent oxide films are needed to meet the increasing demands of the electronics industry. Three main methods that involve oxidation of a metallic substrate are thermal, anodic and plasma oxidation. Today we do not have an adequate fundamental physical-chemical model of how multicomponent oxides evolve on alloys under these oxidizing conditions to design a wide range of materials for electronic devices. The three methods will be discussed in terms of physical/chemical parameters that influence the chemical nature and structure of the resulting oxides. By using surface studies of the oxidation behavior of numerous metals and alloys we have been able to delineate the factors which are most important to the oxide formation process and provide insight into the prediction of oxide layer structures. The electrochemical processes that occur during the materials reaction with a chosen environment will be used to discuss the physical and chemical mechanisms involved. Intrinsic and extrinsic electric fields will be shown to influence the chemical and structural nature of the resulting oxide structures. Examples will be presented from a number of metal and alloy systems that have been examined in our laboratory. These include Al, Ti, Zr, Nb, Mn, Cu and Ni and some of their selected alloys. The models that have developed from these studies are providing some predictive power in how the complex oxide overlayer will be chemically speciated and on its structure.
Single phase, (001) oriented SrBi2Ta2O9(SBT) thin films have been deposited on (100) MgO substrates using pulsed laser deposition(PLD) technique. In order to study the influence of the growth conditions on the microstructure and stoichiometry, SBT film growth was carried out under different deposition conditions, using an Kr:F excimer laser (248 nm, 30 ns FWHM). Effect of laser fluence(0.75–2 J/cm2), frequency of the laser pulse (4–10 Hz), substrate temperature (700–850°C), oxygen partial pressure(150–450 mTorr) have been studied using X-ray diff-raction(XRD), Scanning Electron Microscope(SEM), X-ray energy dispersive analysis (EDAX), atomic force microscopy(AFM). It was found that small variation in growth temperature and oxygen pressure has a large influence on the average grain size (50–180 nm) and surface roughness (0.14–0.3 nm) respectively. Outgrowths on the film surface were observed at high substrate temperatures, high laser energy and with higher frequencies (>6) of the laser pulse. Highly c-axis oriented SBT thin films with homogeneous surface morphology and with an average surface roughness of 0.14 nm were deposited at 750 °C, 260 mTorr O2 pressure and 1.25 J/cm2 laser fluence. These films have a tangent loss around 0.05 and dielectric constant around 417 with a tunability of 4% in the microwave frequency region (1–20 GHz).
This work reports on the spectroscopic properties of europium ions intercalated in a vanadium pentoxide matrix, as well as the effects of thermal treatment on its structural and electronic properties. X-ray diffraction, TG/DTA, ESR, luminescence as well as XANES were used in order to characterize the intercalation compound. For the as grown samples, the intercalation of Eu3+ is observed to have little effect in the V2O5 lamellar structure, however the inter-planar spacing grows from 1.19 nm to 1.33nm for a 10% Eu3+ content. The luminescence for the as grown samples are dominated by the host, the same happening with the ESR spectra. However ESR data show that the presence of Eu3+ in the host induces the partial reduction of vanadium to V4+ For samples treated above 873K the luminescence excitation spectra presents a new shoulder at 300 nm, which when excited shows emissions bands typical of the Eu3+ In ESR the same thermal treatments are marked by the appearance of a new broad line assigned to ferromagnetic EuO, as well as the partial oxidation of V4+ to V5+ This result may be correlated to suppression of Eu3+ luminescence due to the presence of water in the neighborhood of Eu3+.