Pulsed excimer laser ablation growth and characterization of Ba(Sn0.1Ti0.9)O3 thin films

Polycrystalline thin films of Ba(Sn_0.1Ti_0.9)O₃ were deposited on Pt coated silicon substrates by pulsed excimer laser ablation technique. The room temperature dielectric constant of the Ba(Sn_0.1Ti_0.9)O₃ films was 350 at a frequency of 100 kHz. The films showed a slightly diffused phase transition in the range of 275–340 K. The polarization hysteresis behavior confirmed the ferroelectric nature of the thin films. Remanent polarization (P_r) and saturation polarization (P_s) were 1.1 and 3.2 μC/cm², respectively. The asymmetric capacitance–voltage curve for Ba(Sn_0.1Ti_0.9)O₃ was attributed to the difference in the nature of the electrodes. Dispersion in both the real (ε′_r) and imaginary (ε″_r) parts of the dielectric constant at low frequencies with increase in temperature was attributed to space charge contribution in the complex dielectric constant.     

Effects of texture on the dielectric properties of Ba(Zr0.2Ti0.8)O3 thin films prepared by pulsed laser deposition

Ba(Zr_0.2Ti_0.8)O₃ (BZT) thin films were deposited on Pt(111)/Ti/SiO₂/Si(100) substrates by a pulsed laser deposition process. The BZT thin films directly grown on annealed and un-annealed Pt/Ti/SiO₂/Si substrates exhibited random and high (100) orientations, respectively. The dielectric constant of a 400-nm-thick BZT film with (100) orientation was 331, which was higher than that of a BZT film with random orientation (∼236). This result is attributed to the fact that the polar axis of the (100)-oriented films was more tilted away from the normal to the film surface than that of the randomly oriented films. Also, the tunabilities of BZT thin films with random and (100) orientations were ∼50% and ∼59% at an applied field of 400 kV/cm, respectively. Improved tunability has been attributed to the (100) texture of the film leading to an enhancement of the in-plane-oriented polar axis. PACS 77.22.-d; 77.55.+f; 77.80.-e; 77.84.-s     

Structural and optical properties of Ba(Zn1/3Ta2/3)O3 thin films deposited by pulsed laser deposition

Ba(Zn_1/3Ta_2/3)O₃, (BZT), is a high-k and low loss dielectric resonator material which finds applications in the area of microwave communication and related areas. While there are reports on its bulk properties, there is hardly any report on its thin films. We report here preparation of thin films of BZT deposited on to borosilicate and quartz substrates using the pulsed laser deposition (PLD) technique under different oxygen mixing percentage (OMP) and preliminary studies on their structural and optical properties. The EDAX spectra show that the deposited films exhibit the composition of the target. As-deposited films, whether on a glass or quartz substrate, were found to be amorphous. The refractive index and energy bandgap of the films were found to be around 1.5 in the dispersion free region (500–1500 nm) and 5.2–5.5 eV, respectively. The Dynamic Force Microscopy (DFM) study of the BZT films exhibited columnar growth and films deposited at higher OMPs showed the smaller grain size.     

Preparation of epitaxial compositionally graded (Ba1-xSrx)TiO3 thin films with enhanced dielectric properties

Epitaxial compositionally graded (Ba_1-xSr_x)TiO₃ (BST) (0.0x0.25) thin films were deposited on (100) LaAlO₃ substrates by pulsed laser ablation, the substrates having bottom electrodes made of 100-nm-thick conductive La_0.5Sr_0.5CoO₃ (LSCO). Extensive X-ray diffraction, rocking-curve, and -scan studies indicate that the graded films are (100)-oriented and exhibit good in-plane relationships of [010]_BST//[010]_LAO and [001]_BST//[001]_LAO. For the up-graded films with barium concentration (1-x) increasing across the film thickness in the direction from the film/substrate interface to the film surface, the full width at half maximum of the BST film (200) rocking curve and the surface roughness, examined by atomic force microscopy, were larger than those of the down-graded films with barium concentration decreasing from the film/substrate interface to the film surface. The dielectric properties of the graded films, measured using vertical structures, show that at room temperature, the dielectric constant (_r) and dissipation factor (cos) at 100 kHz were 380 and 0.013 for the up-graded films, and 650 and 0.010 for the down-graded films, respectively. The dielectric behavior was enhanced in the down-graded films, which was attributed to the fact that the pure BaTiO₃ layer in the down-graded BST films not only serves as a bottom layer but also acts as an excellent seeding layer for enhancing subsequent film growth, leading to better film crystallinity and larger grain sizes in the down-graded films. The graded BST films undergo a diffuse phase transition, giving a broad, flat capacitance-versus-temperature profile. With such a graded structure, it is possible to build a dielectric thin-film capacitor having a capacitance which has a low temperature dependence over a broad temperature regime. PACS 77.55.+f; 68.55.Jk; 81.15.Fg     

Electron tunneling into thin films of Y1Ba2Cu3O7

We have fabricated tunnel junctions on thin films of Y₁Ba₂Cu₃O₇ which were epitaxially grown by magnetron sputtering on (100) and on (110) oriented SrTiO₃ substrates. These junctions of the type Y₁Ba₂Cu₃O₇/barrier/Pb or In showed with high reproducibility in the conductance a gap-like structure with the maxima near±16mV. We supplied experimental arguments that this structure reflects properties of the quasi particle excitation spectrum of Y₁Ba₂Cu₃O₇. The gap-like structure was found to disappear atT _c mainly by weakening and not by a shift of the peaks to lower voltages.     

Dielectric and piezoelectric properties of the Ba0.92Ca0.08Ti0.95Zr0.05O3 thin films grown on different substrate

Lead free Ba_0.92Ca_0.08Ti_0.95Zr_0.05O₃ (BCZT) thin films were deposited on Pt/Ti/SiO₂/Si and LaNiO₃(LNO)/Pt/Ti/SiO₂/Si substrates by a sol–gel processing technique, respectively. The effects of substrate on structure, dielectric and piezoelectric properties were investigated in detail. The BCZT thin films deposited on LNO/Pt/Ti/SiO₂/Si substrates exhibit (100) orientation, larger grain size and higher dielectric tunability (64%). The BCZT thin films deposited on Pt/Ti/SiO₂/Si exhibit (110) orientation, higher Curie temperature (75 °C), better piezoelectric property (d₃₃ of 50 pm/V) and lower dielectric loss (0.02). The differences in dielectric and piezoelectric properties in the two kinds of oriented BCZT films should be attributed to the difference of structure, in-plane stress and polarization rotation in orientation engineered BCZT films.     

Nanosecond domain wall dynamics in ferroelectric Pb(Zr, Ti)O(3) thin films

Domain wall motion during polarization switching in ferroelectric thin films is fundamentally important and poses challenges for both experiments and modeling. We have visualized the switching of a Pb(Zr, Ti)O(3) capacitor using time-resolved x-ray microdiffraction. The structural signatures of switching include a reversal in the sign of the piezoelectric coefficient and a change in the intensity of x-ray reflections. The propagation of polarization domain walls is highly reproducible from cycle to cycle of the electric field. Domain wall velocities of 40 m s(-1) are consistent with the results of other methods, but are far less than saturation values expected at high electric fields.     

Optical study of Ba(Mn_xTi_(1-x)O_3) thin films by spectroscopic ellipsometry

Stoichiometric Ba(Mn_xTi_(1-x)O_3) (BMT)thin films with various values of x were deposited on Si(111)substrates by the sol-gel technique.The influence of Mn content on the optical properties was studied by spectroscopic ellipsometry(SE)in the UV–Vis–NIR region.By fitting the measured ellipsometric parameter(Ψand)with a four-phase model(air/BMT+voids/BMT/Si(111)),the key optical constants of the thin films have been obtained.It was found that the refractive index n and the extinction coefficient k increase with increasing Mn content due to the increase in the packing density.Furthermore,a strong dependence of the optical band gap Egon Mn/Ti ratios in the deposited films was observed,and it was inferred that the energy level of conduction bands decreases with increasing Mn content.     

Role of Nd/Ba disorder on the penetration depth of Nd(1+x)Ba(2-x)Cu(3)O(7-delta) thin films

We report on a study on the effect of Nd/Ba disorder on the ab-plane penetration depth of epitaxial Nd(1+x)Ba(2-x)Cu(3)O(7-delta) thin films. While in stoichiometric samples lambda(T) at low temperature is linear, Nd-rich films exhibit a quadratic law. For low Nd excess (x<0.04), a satisfying fit is obtained using the "dirty" d-wave model assuming that Nd ions at Ba sites act as strong scattering centers. At high x (x>0.15) the data are explained if Nd/Ba disorder becomes less effective as a source of scattering. The effect of localization has been discussed to account for the experimental results.     

Epitaxial perovskite oxide thin films on Ba(Ti,Zr)O3 substrates for strain-induced electric/magnetic property changes near room temperature

We have reported the epitaxial growth and the electric/magnetic properties of SrRuO₃ and (La_0.67Sr_0.33)MnO₃ thin films on Ba(Ti_0.925Zr_0.075)O₃ substrates. The dielectric constant and thermal expansion of Ba(Ti_0.925Zr_0.075)O₃ exhibited hysteretic jumps at the structural transition temperatures. Near the rhombohedral to orthorhombic structural transition temperature of Ba(Ti_0.925Zr_0.075)O₃, which occurs at ∼33 °C, we have observed a drastic jump of resistivity and magnetization of the thin films. These results suggest that epitaxial perovskite oxide thin films on Ba(Ti_0.925Zr_0.075)O₃ substrates should be quite useful for the strain-induced changes of physical properties near room temperature.     

Microstructures and dielectric properties of compositionally graded (Ba1-xSrx) TiO3 thin films prepared by pulsed laser deposition

Compositionally graded (Ba_1-xSr_x)TiO₃ (BST) (x:0.0∼0.25) thin films were grown on Pt (111)/TiO₂/SiO₂/Si (100) substrates using layer-by-layer pulsed laser deposition in the temperature range 550–650 °C. Both downgraded (Ba/Sr ratio varying from 100/0 at the bottom surface to 75/25 at the top surface) and upgraded (Ba/Sr ratio varying from 75/25 at the bottom surface to 100/0 at the top surface) BST films were prepared. Their microstructures were systematically studied by X-ray diffractometry and scanning electron microscopy. A grain morphology transition from large ‘rosettes’ (>0.30 μm) to small compact grains (70–110 nm) was observed in the downgraded BST films as the deposition temperature was increased from 550 to 650 °C. No such grain morphology transition was detected in the upgraded BST films. Dielectric measurements with metal electrodes revealed an enhanced dielectric behavior in the downgraded films. This enhancement is mainly attributed to the large compressive stress field built up near the interface between the downgraded film and substrate. Furthermore, the BaTiO₃ layer in the downgraded BST films not only serves as a bottom layer but also as an excellent seeding layer for enhancing the crystallization of the subsequent film layers in the downgraded films. Received: 10 December 2001 / Accepted: 12 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: 86-25/359-5535, E-mail: xhzhu@public1.ptt.js.cn     

Structural and optical properties of Bi3.25Nd0.75Ti3O12 ferroelectric thin films

Ferroelectric Bi_3.25Nd_0.75Ti₃O₁₂ (BNT) thin films were grown on (111)Pt/Ti/SiO₂/Si substrates by a chemical solution method. The films were composed of large rod-like grains. XRD and Raman spectroscopy measurements showed they were polycrystalline perovskite structure with a good crystallinity. Pt/BNT/Pt capacitors had been fabricated and showed good ferroelectricity. The optical constants (n, k) of BNT thin films in the wavelength ranges of 0.2–1.7 μm and 2.5–11.4 μm were obtained by spectroscopic ellipsometry measurements. The dispersion of the refractive index in the interband transition region followed the single electronic oscillator model. The optical band gap was found to be about 3.61 eV. PACS 77.84.-s; 78.20.Ci; 77.80.-e     

Dilatometrische und dielektrische Anomalien bei den Phasenumwandlungen in Ba(Ti1-xSnx)O3

The temperature dependence of the thermal expansion, the dielectric constant, the pyroelectric current and the remanent polarization in polycrystalline samples of Ba(Ti_1-xSn_x)O₃ was measured. The influence of the external electric field and Sn-substitution on the character of all phase transitions was investigated.     

Domain wall creep in epitaxial ferroelectric Pb(Zr(0.2)Ti(0.08)O(3) thin films

Ferroelectric switching and nanoscale domain dynamics were investigated using atomic force microscopy on monocrystalline Pb(Zr(0.2)Ti(0.8))O(3) thin films. Measurements of domain size versus writing time reveal a two-step domain growth mechanism, in which initial nucleation is followed by radial domain wall motion perpendicular to the polarization direction. The electric field dependence of the domain wall velocity demonstrates that domain wall motion in ferroelectric thin films is a creep process, with the critical exponent mu close to 1. The dimensionality of the films suggests that disorder is at the origin of the observed creep behavior.     

Physical and electrical characteristics of Ba(Zr0.1Ti0.9)O3 thin films under oxygen plasma treatment for applications in nonvolatile memory devices

In this study, the effects of oxygen gas plasma treatment on the surface of Ba(Zr_0.1Ti_0.9)O₃ (BZT) films are investigated. The influence of oxygen plasma treatment on the crystal structure is developed by X-ray diffraction patterns and on the electrical characteristics are measured by the Al/BZT/Pt capacitor (metal–dielectric–metal) structure. Experimental results show that the capacitance increases and the leakage current density decreases as the oxygen plasma is treated on the BZT films. These results clearly indicate that the electrical characteristics of BZT films have effectively improved by means of the oxygen plasma surface treatment process. PACS 77.84.-s; 81.15.Cd; 73.40.Qv; 77.22.Ej; 51.50.+v     

The dependence of dielectric properties on the thickness of (Ba,Sr)TiO3 thin films

In the dielectric (Ba,Sr)TiO₃ thin films, the correlation between the film thickness and the dielectric properties was investigated. The dielectric properties such as the dielectric constant (ε) and dielectric loss (tan δ) were measured using the capacitor geometry. As the film thickness increased, the dielectric constant also increased due to the reduction of the interfacial dead-layer effect. However, the dielectric loss did not show a monotonous variation with the increasing film thickness. It was found that the dielectric loss correlated well with the non-uniform distribution of local strain, as analyzed by X-ray diffraction, according to the Curie–von Schweidler relaxation law.     

Optical properties of thin epitaxial Ba0.8Sr0.2TiO3 films

The properties of nanodimensional (Ba_0.8, Sr_0.2)TiO₃ films on single-crystal magnesia substrates are studied. The films are applied by rf sputtering and grow in the layer-by-layer mode. The lattice parameters are measured by the X-ray diffraction method. The transmission of the films with different thicknesses is studied in the wavelength range 190?C1100 nm. When analyzing experimental optical parameters, additional relaxation parameters depicting a final lifetime of the oscillator are used to characterize the refractive index and absorption factor in the dispersion relation. Such an approach allows a more accurate approximation of experimental data.     

X-ray photoelectron spectroscopy study of disordering in Gd2(Ti1-xZrx)2O7 pyrochlores

The dramatic increases in ionic conductivity in Gd2(Ti1-xZrx)2O7 solid solution are related to disordering on the cation and anion lattices. Disordering in Gd2(Ti1-xZrx)2O7 was characterized using x-ray photoelectron spectroscopy (XPS). As Zr substitutes for Ti in Gd2Ti2O7 to form Gd2(Ti1-xZrx)2O7 (0.25 < x < or =0.75), the corresponding O 1s XPS spectrum merges into a single symmetric peak. This confirms that the cation antisite disorder occurs simultaneously with anion disorder. Furthermore, the O 1s XPS spectrum of Gd2Zr2O7 experimentally suggests the formation of a split vacancy.     

Ferroelectric characteristics of silicate-bound Bi4Ti3O12 thin films

40-nm-thick Bi₄Ti₃O₁₂ films have been deposited by spin coating with a hybrid precursor solution of bismuth-2-ethylhexanoate, titanium tetraisopropoxide and tetraethysilicate. The 500 °C-annealed thin film consists of Bi₄Ti₃O₁₂ grains bound by ultra-thin amorphous silicate layers. The film shows a high degree of crystallinity with random orientation and exhibits a structure-dependent propeller-like P–V hysteresis loop. The ultra-thin layer of amorphous silicate is found to have multiple functions of binder, compositional buffer and insulator, which results in an improvement of the electrical properties of the Bi₄Ti₃O₁₂-Bi₂O₃×SiO₂ thin films. PACS 77.84.-s; 68.37.-d; 81.15.-z     

Morphology and microstructure of all-epitaxial ferroelectric tri-layered (Bi,La)4Ti3O12/Pb(Zr0.4Ti0.6)O3/ (Bi,La)4Ti3O12 thin films on SrTiO3(011)

The morphology and microstructure of all-epitaxial (Bi,La)₄Ti₃O₁₂/Pb(Zr_0.4Ti_0.6)O₃/(Bi,La)₄Ti₃O₁₂ (BLT/PZT/BLT) tri-layered ferroelectric films, grown on (011)-oriented SrTiO₃ (STO) substrates by pulsed laser deposition, are investigated by transmission electron microscopy (TEM). X-ray diffraction and electron diffraction patterns demonstrate that the epitaxial relationship between BLT, PZT and STO can be described as ; . Cross-sectional TEM images show that the growth rate of BLT is nearly two times that for PZT at the same growth conditions, and 90° ferroelectric domain boundaries lying on {110} planes are observed in the PZT layer. The 90° ferroelectric domains in the PZT layer extend up to 600 nm in length. Long domains penetrate into the neighboring columnar grain through the columnar grain boundary, whereas others are nucleating at the columnar grain boundaries. The roughness of the PZT/BLT interfaces appears to depend on the viewing direction, i.e., it is different for different azimuthal directions. Planar TEM investigations show that the grains in the top BLT layer have a rod-like morphology, preferentially growing along the [110]_BLT direction. The grain width is rather constant at about 90 nm, whereas the length of the grains varies from 150 to 625 nm. These morphological details point to the important role the crystal anisotropy of BLT plays for the growth and structure of the tri-layered films. PACS 81.15-z; 68.37.Lp; 77.84.-s