S. V. Biryukov

Surface acoustic waves in thin films of barium strontium titanate on magnesium oxide substrates

It is established that, using a thin barium strontium titanate (BST) film as the active element in a surface acoustic wave (SAW) device, it possible to double the working frequency of the converter due to the formation of a periodic domain structure in the BST film. A thin ferroelectric film device with a standard electrode structure is capable of effectively exciting the SAW second harmonic. Variation of the external polarizing voltage applied to the electrodes ensures field-controlled electromechanical feedback in the converter.

Polarization of thin barium-strontium titanate films by an external electric field

The behavior of polarization of a Ba0.8Sr0.2TiO3 thin film in an external electric field for a planar geometry of electrodes is described using a simple phenomenological model. The “van der Waals field” Ez formed by the substrate along the normal to the film surface is introduced for fitting to experimental data. This field is estimated as Ez = 1.1 × 108 V/m. The phase diagram of the Ba0.8Sr0.2TiO3 film in the Ez (misfit strain) at room temperature is analyzed.

Effect of growth mechanisms on the deformation of a unit cell and polarization reversal in barium–strontium titanate heterostructures on magnesium oxide

The effect of a growth mechanism on the unit cell strain and the related change in the properties of single-crystal Ba0.8Sr0.2TiO3 films grown on MgO substrates according to the Frank–van der Merwe and Volmer–Weber growth mechanisms is studied. The unit cell strain is shown to depend substantially on the film thickness and the growth mechanism. It is found that the same film–substrate pair can be used to vary stresses in the film from two-dimensional tensile to compressive stresses due to a change in the growth mechanism and the film thickness.

Dynamics of polarization reversal in thin PZT films

New experimental data on the dynamics of polarization reversal in thin PZT films prepared by rf sputtering are reported. The frequency and amplitude dependences of the hysteresis loop area and coercive field are investigated. It is shown that, at infralow frequencies, there exists a critical frequency at which the shape of the hysteresis loop changes stepwise. Theoretical analysis of the related problems is given.

Stress-induced domain structure formation in nanodimensional barium strontium titanate films

Thin heteroepitaxial films of BaxSr1−xTiO3 (BST) with x = 0.80 possessing high structural perfection have been grown by RF magnetron sputtering on MgO(001) substrates. The microstructure and crystallographic characteristics of the BST films of various thicknesses have been studied using electron microscopy and X-ray diffraction. It is established that there is a threshold thickness at which the BST crystal lattice parameter in the direction perpendicular to the substrate exhibits a sharp change.

Modified relations for calculating the dielectric constant of barium–strontium titanate nanofilms

The results of measuring the capacitances of planar capacitors are used to calculate the dielectric constants of barium–strontium titanate nanofilms. An expression is derived to calculate an adjustable coefficient to obtain adequate dielectric constants for 20-nm-thick ferroelectric films. The field of applicability of the adjustable coefficient is determined.

Piezoelectric activity of thin PTZ films

The manifestations of the piezoelectric activity of thin ferroelectric PZT films produced by high-frequency cathode sputtering of targets of a stoichiometric composition are studied. The films are found to have natural unipolarity. Possible causes of this phenomenon are discussed. A family of piezoelectric hysteresis loops is experimentally recorded at various frequencies and strengths of an applied bias field. The dependences obtained are compared to classical dielectric hysteresis loops.

Geometrical effects in nanodimensional epitaxial films of barium strontium titanate

The effect of the thickness of 6- to 950-nm-thick Ba0.8Sr0.2TiO3 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 A1(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.

Structure and Polarization Relaxation of Ba 0.5 Sr 0.5 Nb 2 O 6 /(001)Si Films

The structure and dielectric characteristics of strontium barium niobate thin films deposited on single-crystalline silicon substrates without buffer layers are studied. It is found that the c axis in these heterostructures runs largely normally to the substrate surface and the a and b axes are randomly oriented in the plane of the substrate. The polarization relaxation in such heterostructures is investigated. It is shown that the film–substrate interface in the heterostructures grown by rf cathode sputtering may contain a low amount of long-lived charged defects.

Specific features of the ferroelectric state in two-layer barium strontium titanate-based heterostructures

The structural properties of one- and two-layer heterostructures based on the barium–strontium titanate of various compositions deposited by the Frank–Van der Merve on a magnesium oxide substrate have been studied. The heterostructures have been prepared by the rf sputtering of the stoichiometric ceramic targets in a Plazma 50 SE deposition system. The principal difference of this method of deposition from known analogs is that the growth of single-crystal films occurs from a disperse oxide phase formed in the plasma of a high-current rf discharge during the ceramic target sputtering at the cluster level. The peculiarities of the manifestation of the ferroelectric state in the two-layer heterostructures when changing the sequence order of the films with various compositions of barium–strontium titanate.