Small- and strong-signal dielectric response in a single-crystal film of partially deuterated betaine phosphite

Poly- and single-crystal films of betaine phosphite deuterated to ∼20% have been grown by evaporation on NdGaO₃ (001) substrates with a preliminarily deposited planar interdigital structure of electrodes. The small-signal dielectric response in the 0.1–100.0-kHz frequency range has revealed a strong anomaly in capacitance upon the transition of the films to the ferroelectric state. Application of a bias field brings about suppression and a slight shift of the dielectric anomaly toward higher temperatures. The strong-signal dielectric response has been studied by the Sawyer-Tower method over the frequency range 0.06–3.00 kHz both in the para- and ferroelectric phases. In contrast to the case of a plane-parallel capacitor, in the planar structure studied, the dielectric hysteresis loops exhibit a very small coercivity at low frequencies, which grows with increasing frequency. This difference should be assigned to different domain structures formed in a planeparallel capacitor and in a planar structure in a saturating field. The growth of hysteresis with increasing frequency in a planar structure is considered to be associated with the domain wall motion.     

Dielectric properties of thin films of partially deuterated betaine phosphite with large- and small-block structures

Ferroelectric thin films of partially deuterated betaine phosphite (DBPI) have been grown by evaporation on NdGaO₃ substrates with a preliminarily deposited structure of interdigitated electrodes. The block structure of the films is a texture in which the polar axis b is oriented in the plane of the film and the a* axis is perpendicular to this plane. Typical dimensions of the single-crystal blocks in DBPI films substantially exceeds the distance between the interdigitated electrodes (d = 50 μm). However, DBPI films in which the block structure has characteristic dimensions of the order of d have also been grown. Investigations of the dielectric properties of the films have demonstrated that the dimensions of the block structure have little effect on the behavior of a small-signal dielectric response, which, in the phase transition region, is characterized by a strong anomaly of the capacitance of the structure at T = T _c and by a glass-like behavior of the capacitance C and dielectric loss tangent tanδ in the temperature range of 120–200 K. By contrast, the low-frequency strong-signal dielectric response (dielectric hysteresis loops) in the structures with small blocks differs significantly from that observed for large-block structures. The difference in the frequency behavior of the hysteresis loops in the large-block and small-block structures is associated with the limitation of motion of domain walls in the case of small blocks.     

Ferroelectric films of deuterated glycine phosphite: Structure and dielectric properties

Polycrystalline textured films of deuterated glycine phosphite consisting of single-crystal blocks with lateral dimensions ～(50–100) μm and a thickness d ～ (1–5) μm have been grown by evaporation on NdGaO₃(100) and α-Al₂O₃ substrates with preliminarily deposited interdigitated electrodes, as well as on Al substrates. The c* (Z) crystallographic axis in the blocks is normal to the film plane, and the a (X) axis and the polar axis b (Y) are oriented in the film plane. The temperature dependences of the capacitance of the structures measured with the interdigitated electrode system reveal a strong dielectric anomaly at the film transition to the ferroelectric state. The phase transition temperature T _c depends on the degree of deuteration D of the glycine phosphite. The maximum value T _c = 275 K obtained in the structures studied corresponds to a degree of deuteration of the glycine phosphite D ～ 50%. The frequency behavior of the dielectric hysteresis loops in glycine phosphite films differs radically from that of the previously studied films of deuterated betaine phosphite, which evidences that polarization switching in these structures proceeds by different mechanisms. It has been that application of a dc bias to the electrodes changes the shape of the dielectric hysteresis loops and shifts them along the electric field axis. The shift of the loops depends on the sign, magnitude, and time of application of the bias. Possible mechanisms underlying the induced unipolarity are discussed.     

Antiferroelectric films of deuterated betaine phosphate

Thin films of partially deuterated betaine phosphate have been grown by the evaporation on Al₂O₃(110) and NdGaO₃(001) substrates with a preliminarily deposited structure of interdigitated electrodes. The grown films have a polycrystalline block structure with characteristic dimensions of blocks of the order of 0.1–1.5 mm. The degree of deuteration of the films D varies in the range of 20–50%. It has been found that, at the antiferroelectric phase transition temperature T_c^afe = 100–114 K, the fabricated structures exhibit an anomaly of the electrical capacitance C, which is not accompanied by a change in the dielectric loss tangent tanδ. The strong-signal dielectric response is characterized by the appearance of a ferroelectric nonlinearity at temperatures T >T_c^afe, which is transformed into an antiferroelectric nonlinearity at T <T_c^afe. With a further decrease in the temperature, double dielectric hysteresis loops appear in the antiferroelectric phase. The dielectric properties of the films have been described within the framework of the Landau-type thermodynamic model taking into account the biquadratic coupling ξP²η² between the polar order parameter P and the nonpolar order parameter η with a positive coefficient ξ. The E–T phase diagram has been constructed.     

Ti-doping induced antiferroelectric to ferroelectric phase transition and electrical properties in Sm-PbZrO3 thin films

The antiferroelectric (Pb_0.985Sm_0.01) (Zr_1-xTi_x)O₃ (Ti-PSZO) thin films were synthesized on Pt(111)/Ti/SiO₂/Si substrates using a chemical solution deposition method. The films were crystallized in the perovskite phase with a preferential orientation along (111) direction. With Ti doping in PSZO, a gradual transformation from antiferroelectric to ferroelectric phase transition was noticed at room temperature owing to the Ti doping induced lattice distortion. The phase transition has been confirmed through the P - E hysteresis loops, X-ray diffraction (peak shifting), capacitance-voltage measurements, and Raman scattering analysis. The thin film with Ti = 0.15 doping displayed a ferroelectric behavior with high dielectric constant and large dielectric tunability of about 62%. Also, Ti doping altered the Curie temperature (T_c) and enhanced the order of dielectric diffuseness. It is believed that Ti-doping in PSZO is an effective way to induce an antiferroelectric - ferroelectric phase transition and to tailor the electrical characteristics of PSZO thin films.     

Behavior of a lead scandium tantalate ferroelectric in an ac electric field

The behavior of an incompletely ordered lead scandium tantalate ferroelectric (the degree of ordering s=0.8) in the region of the spontaneous ferroelectric transition is studied in ac electric fields by dielectric and optical methods. Dielectric nonlinearity of two types is observed. In fields above 2 kV/cm, the nonlinearity is caused by induced polarization effects and macrohysteresis behavior, whereas in weak fields, it is most probably associated with the dynamics of domain walls and/or phase boundaries.     

Phase transitions in the CN<Subscript>x</Subscript>-TiN system attendant on the variation of the bias voltage during ion-stimulated growth

The structure of amorphous CN_x-TiN films grown by ion-stimulated deposition at a bias voltage U = 200–500 V is studied by X-ray diffraction. As the bias voltage increases in the range U = 300–360 V, the CN_x-TiN films are shown to undergo a phase transition in the amorphous phase having different order scales (20–50 Å): this transition is related to an increase in the content of the fraction of medium-cell (4 Å) carbon clusters as compared to the fractions of clusters with large (8 Å) and small (2 Å) cells. Under these conditions, 80–150 Å crystalline clusters undergo the phase transition from the Ti₂C(N) carbide into graphite (C _g) and diamond (C_d); the last two phases are represented by 100-Å clusters.     

Leakage current, ferroelectric and structural properties in Pb1−xBaxTiO3 thin films prepared by chemical route

Single-phase perovskite structure Pb_1−xBa_xTiO₃ thin films (x=0.30, 0.50 and 0.70) were deposited on Pt/Ti/SiO₂/Si substrates by the spin-coating technique. The dielectric study reveals that the thin films undergo a diffuse type ferroelectric phase transition, which shows a broad peak. An increase of the diffusivity degree with the increasing Barium contents was observed, and it was associated to a grain decrease in the studied composition range. The temperature dependence of the phonon frequencies was used to characterize the phase transition temperatures. Raman modes persist above tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of breakdown of the local cubic symmetry by chemical disorder. The absence of a well-defined transition temperature and the presence of broad bands in some interval temperature above FE-PE phase transition temperature suggested a diffuse type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. The leakage current density of the PBT thin films was studied at different temperatures and the data follow the Schottky emission model. Through this analysis the Schottky barrier height values 0.75, 0.53 and 0.34 eV were obtained to the PBT70, PBT50 and PBT30 thin films, respectively.     

Effects of oxygen partial pressure on the ferroelectric properties of pulsed laser deposited Ba0.8Sr0.2TiO3 thin films

The Ba_0.8Sr_0.2TiO₃ thin films were grown on the Pt–Si substrate at 700 °C by using a pulsed laser deposition technique at different oxygen partial pressure (PO₂) in the range of 1–20 Pa and their properties were investigated. It is observed that the PO₂ during the deposition plays an important role on the tetragonal distortion ratio, surface morphology, dielectric permittivity, ferroelectric polarization, switching response, and leakage currents of the films. With an increase in PO₂, the in-plane strain for the BST films changes from tensile to compressive. The films grown at 7.5 Pa show the optimum dielectric and ferroelectric properties and also exhibit the good polarization stability. It is assumed that a reasonable compressive strain, increasing the ionic displacement, and thus promotes the in-plane polarization in the field direction, could improve the dielectric permittivity. The butterfly features of the capacitance–voltage (C–V) characteristics and the bell shape curve in polarization current were attributed to the domain reversal process. The effect of pulse amplitude on the polarization reversal behavior of the BST films grown at PO₂ of 7.5 Pa was studied. The peak value of the polarization current shows exponential dependence on the electric field.     

Nonlinear dielectric response in the mixed K0.91(NH4)0.09H2PO4 crystal

The influence of bias and variable electric fields on the dielectric response of the K_0.91(NH₄)_0.09H₂PO₄ single crystal has been studied in the vicinity of the ferroelectric phase transition temperature T _C. Below T _C, the nonlinear response is caused mainly by the domain mechanism. This is confirmed, in particular, by observations of chaotic oscillations in a series RLC-circuit containing the studied sample as a capacitor C and excited by a sinusoidal voltage. Peculiarities of the behavior of the dielectric nonlinearity are found near the Curie temperature, which are explained by the emergence of an intermediate heterophase state.     

Temperature dependent polariton measurements in BaTiO3

Polariton techniques are used to determine the temperature dependence of all three A₁ modes between room temperature and T_c = 134°C in the ferroelectric phase of BaTiO₃. The results shows that there is a large difference between the clamped dielectric constant, ?(0), determined from these results and ?(0) determined from microwave capacitance techniques. It is argued that, while soft modes in displacive ferroelectrics are the driving force for the transition, other oscillators play an important role in low frequency dielectric behavior.     

Ferroelectricity and structure of BaTiO3 grown on YBa 2Cu3O_{7 - \delta } thin films

We have investigated the crystal structure and the ferroelectric properties of BaTiO₃ thin films with YBa₂Cu₃O as the bottom and Au as the top electrode. Epitaxial heterostructures of YBa₂Cu₃O and BaTiO₃ were prepared by dc and rf sputtering, respectively. The crystal structure of the films was characterised by X-ray diffraction. The ferroelectric behaviour of the BaTiO₃ films was confirmed by hysteresis loop measurements using a Sawyer Tower circuit. We obtain a coercive field of 30 kV/cm and a remanent polarisation of 1.25 μC/cm². At sub-switching fields the capacitance of the films obeys a relation analogous to the Rayleigh law. This behaviour indicates an interaction of domain walls with randomly distributed pinning centres. At a field of 5 MV/m we calculate a 3% contribution of the irreversible domain wall motion to the total dielectric constant. Received 24 June 1999 and Received in final form 27 August 1999     

Geometrical effects in nanodimensional epitaxial films of barium strontium titanate

The effect of the thickness of 6- to 950-nm-thick Ba_0.8Sr_0.2TiO₃ films epitaxially grown on (001)MgO on their ferroelectric properties is investigated. Raman scattering and X-ray diffraction data indicate a structural phase transition taking place at a film thickness of ≈70 nm, which changes drastically the lattice parameters of the film. Raman spectra taken of the films confirm that they are in the ferroelectric state and that their symmetry changes in going over a critical thickness, as revealed by a sharp displacement of the peaks corresponding to the A ₁(TO) and E(TO) components of the soft mode. At film thicknesses of <100 nm, the permittivity versus thickness dependence exhibits two peaks at thicknesses of ～18 and ～36 nm. Near the first peak, the dielectric nonlinearity is considerably higher than near the second one.     

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.     

Influence of domain structure on the nonlinear polarization properties of A 2 BX 4-group crystals

Harmonic analysis of repolarization in A ₂ BX ₄-group crystals in the ferroelectric phase near phase-transition temperature T _c is used to separate the contributions to the harmonic-component amplitudes in the output signal of a Sawyer-Tower circuit from charging the reference capacitor with capacitive and conductive current through such a crystal. Harmonic analysis of dielectric hysteresis loops obtained by placing a specimen in an electric field with harmonically changing strength reveals general regularities in the evolution of the domain walls of A ₂ BX ₄-group crystals. It is established that the domain structure affects the nonlinear polarization properties of our specimens. The dependence of the potential relief of ferroelectric ion motion in a harmonically changing electric field is plotted on the basis of the experimental results.     

A Raman and dielectric study of a diffuse phase transition in (Pb<Subscript>1-x</Subscript>Ca<Subscript>x</Subscript>)TiO<Subscript>3</Subscript> thin films

Dielectric and Raman scattering experiments were performed on polycrystalline Pb_1-xCa_xTiO₃ thin films (x=0.10, 0.20, 0.30, and 0.40) as a function of temperature. The results showed no shift in the dielectric constant (K) maxima, a broadening with frequency, and a linear dependence of the transition temperature on increasing Ca²⁺ content. On the other hand, a diffuse-type phase transition was observed upon transforming from the cubic paraelectric to the tetragonal ferroelectric phase in all thin films. The temperature dependence of Raman scattering spectra was investigated through the ferroelectric phase transition. The temperature dependence of the phonon frequencies was used to characterize the phase transitions. Raman modes persisted above the tetragonal to cubic phase transition temperature, although all optical modes should be Raman inactive. The origin of these modes was interpreted in terms of a breakdown of the local cubic symmetry due to chemical disorder. The lack of a well-defined transition temperature and the presence of broad bands in some temperature interval above the FE–PE phase transition temperature suggested a diffuse-type phase transition. This result corroborates the dielectric constant versus temperature data, which showed a broad ferroelectric phase transition in these thin films. PACS 77.80.Bh; 77.55.+f; 78.30.-j; 77.80.-e; 68.55.-a     

Dielectric properties of deuterated ammonium Rochelle salt

The dielectric constant of deuterated ammonium Rochelle salt along the three crystallographic axes as well as the pyroelectric charge density developed on the crystal surface perpendicularly to the b-axis have been measured with a high temperature resolution. The para- to ferroelectric transition at T_c = ?159°C shows a thermal hysteresis of ΔT = 0.15°C. This and the discontinuities in the dielectric constant along all three axes show that the phase transition is of the first-order. No phase intermediate between the para- and the ferroelectric phases could be detected.     

Observation of ferroelectricity induced by defect dipoles in the strain-free epitaxial CaTiO3 thin film

CaTiO₃ is a well-known incipient ferroelectric material that does not undergo a ferroelectric phase transition in spite of the intriguing dielectric constant behavior. Especially, unlike a prototypical incipient ferroelectric SrTiO₃, the paraelectric state of CaTiO₃ cannot be easily destroyed by small perturbations, including cation doping and epitaxial strain. We present that a nearly strain-free epitaxial CaTiO₃ film grown at a low oxygen partial pressure exhibits polarization–voltage hysteresis loops and the distinct difference of piezoresponse force microscopy phase signals, implying that a ferroelectric phase is induced. Such results are shown even at room temperature. We suggest that the observed ferroelectric behavior in CaTiO₃ film comes from the defect dipoles composed of vacancies inside the film. Using electron-probe microanalysis and optical absorption spectra measurements, we found that CaTiO₃ film has considerable Ca and O vacancies, forming the localized defect state in electronic structure. This work highlights the importance of vacancies and their clusters, such as defect dipoles, in understanding the electronic properties of perovskite oxide thin films, including ferroelectricity.     

Absence of relaxor-like ferroelectric phase transition in (Pb,Sr)TiO3 thin films

We have performed dielectric and micro-Raman spectroscopy measurements in the 298–673 K temperature range in polycrystalline Pb_0.50Sr_0.50TiO₃ thin films prepared by a soft chemical method. The phase transition have been investigated by dielectric measurements at various frequencies during the heating cycle. It was found that the temperature corresponding to the peak value of the dielectric constant is frequency-independent, indicating a non-relaxor ferroelectric behavior. However, the dielectric constant versus temperature curves associated with the ferroelectric to paraelectric phase transition showed a broad maximum peak at around 433 K. The observed behavior is explained in terms of a diffuse phase transition. The obtained Raman spectra indicate the presence of a local symmetry disorder, due to a higher strontium concentration in the host lattice. The monitoring of some modes, conducted in the Pb_0.50Sr_0.50TiO₃ thin films, showed that the ferroelectric tetragonal phase undergoes a transition to the paraelectric cubic phase at around 423 K. However, the Raman activity did not disappear, as would be expected from a transition to the cubic paraelectric phase. The strong Raman spectrum observed for this cubic phase is indicative that a diffuse-type phase transition is taking place. This behavior is attributed to distortions of the perovskite structure, allowing the persistence of low-symmetry phase features in cubic phase high above the transition temperature. This result is in contrast to the forbidden first-order Raman spectrum, which would be expected from a cubic paraelectric phase, such as the one observed at high temperature in pure PbTiO₃ perovskite. PACS 78.30.-j; 77.80.Bh; 64.70.Kb; 68.55.-a; 77.22.-a; 77.55.+f     

Influence of isomorphous substitution of metal ion on the low-frequency dielectric dispersion in NH2(CH3)2Al1−xCrx(SO4)2⋅6H2O ferroelectrics

This paper is devoted to the study of the influence of metal ion isomorphous substitution on the ferroelastic-ferroelectric phase transition and dispersion caused by the motion of domain walls in dimethylammonium metal sulfate hexahydrate DMAAl_1?xCr_xS ferroelectric crystals (x = 0, 0.065, 0.2). It is shown that such a substitution significantly changes the phase transition temperature and parameters of the dielectric dispersion. These changes are explained in terms of interaction between the metal-hydrate complexes and DMA groups that carry the dipole moment and due to this they are responsible for the phase transitions and motion of the domain walls.