Peter Kr. Petrov

Techniques for microwave measurements of ferroelectric thin films and their associated error and limitations

This paper examines the problem of evaluating the microwave properties of thin ferroelectric films patterned as planar capacitors. Two types of microwave measurements of ferroelectric thin films are considered: reflection- and resonance-type measurements. Algorithms are presented for evaluation of capacitance-permittivity and dielectric loss. Using sensitivity analysis, the error and limitations associated with these measurements are estimated. The end result is a series of formulae that use the network analysers measurement data to calculate the capacitance-permittivity, the dielectric loss and the associated error.

Improved SrTiO3 thin films using oxygen relaxation technique

The oxygen relaxation technique for thin-film growth is explored. In this technique, films are deposited as nanoscale multilayered structures. After the deposition of each layer, the deposition process is stopped, the sample is cooled down slowly in situ in an oxygen-rich atmosphere to a temperature of 250°C, then it is heated up to the set deposition temperature for growth of the next layer. The results show that by using the oxygen relaxation technique, it is possible to improve the electrical properties and the microstructure of SrTiO3 films keeping the surface smooth and free from pinholes and pores. The oxygen relaxation technique is more efficient for smaller film thickness repetition.

Enhanced electrical properties of ferroelectric thin films by ultraviolet radiation

Ferroelectric films in the paraelectric phase exhibit two undesirable properties: hysteresis in the voltage-capacitance characteristics and a significant relaxation time of the capacitance. Our experiments show that suppression of both of these is achieved by using UV radiation with wavelengths corresponding to the material forbidden gap. Experimentally we also observed UV radiation induced modulation of thin film permittivity without an applied electric field. The observed phenomena are believed to have the same origin: UV light generates nonequilibrium charge carriers that screen out local electric field induced by defects and interfaces inside ferroelectric thin films and change films effective dielectric properties.

Evaluation of Residual Stress in Thin Ferroelectric Films Using Grazing Incident X-Ray Diffraction

A comparative study of three methods for thin film, in-plane residual strain evaluation is presented. Thin films of SrTiO3 and BaxSr(1 - x)TiO3 deposited using pulsed laser deposition on LaAlO3 and MgO substrates were investigated. It is shown that the most precise way to evaluate thin film, in-plane residual strain is by using Grazing Incidence X-ray diffraction.

Shift of phase transition temperature in strontium titanate thin films

The results of structural and electrical characterizations of SrTiO3 thin films deposited onto MgO and LaAlO3 substrates by pulsed laser deposition technique are presented. The influence of substrate and annealing procedure on the crystalline structure and dielectric properties of these ferroelectric thin films are investigated. The obtained experimental data are analyzed in terms of the Landau theory taking into account the room-temperature lattice mismatch of the ferroelectric and substrates as well as the difference in their thermal expansion. It is shown that the behavior of the SrTiO3 thin films could not be attributed to the effect of the film/substrate mechanical coupling. As a possible nature of the observed behavior one can consider the non-stoichiometry of the film composition caused by the chemical contact of the film with the substrate and by the annealing.

Sintering Behaviour of BaxSr1-xTiO3

This paper deals with the sintering behaviour of Barium Strontium Titanate (BST). Two methods were examined; method 1: BaTiO3 + SrTiO3 source powders; method 2: BaCO3 + SrCO3 + TiO2 source powders. Commercially available 50 nm BST (x = 0.60) powder was used to investigate the effect of particle size on activation energy (Ea). Discs were sintered in a Netzsch 402C Dilatometer and Ea for the powders were obtained by applying the Arrhenius expression. The results indicate powders made from both methods display a varying trend in Ea over a range of x. The commercial powder yielded a higher Ea compared with powder made from method 1.

Tuneable Two-Pole One-Dielectric-Resonator Filter with Elliptic Characteristics

A frequency tuneable microwave filter based on a dual mode dielectric resonator has been developed. Tuning of the filter was realised by using metal plate located over the dielectric resonator and moved along the resonator axis by a piezoelectric bimorph. The main parameters of the filter were: central frequency in the middle of the range of tuning—2.35 GHz, bandwidth for 1dB level—∼ 0.5%, range of tuning—∼ 10%. The insertion losses in the whole tuning range were below ∼ 1 dB.

Structural Investigation of Thin SrTiO3 Films Grown on MgO and LaAlO3 Substrates

Structural investigations of SrTiO3 thin films deposited by pulsed laser ablation onto MgO and LaAlO3 substrates are presented. The residual strain along the c-axis (the growth direction) was evaluated with conventional X-ray diffraction. Evaluation of the in-plane strain was accomplished with grazing-incidence X-ray diffraction. The unit cell of the STO film on LAO substrate had an orthorhombic structure with a coexistence of compressive strain along a-axis and tensile strain along b-axis. The film on an MgO substrate was tetragonal, stretched along the c-axis and compressed in-plane. The elemental composition of the samples was investigated using an INCA system for X-ray elemental analysis attached to a Hitachi S-4300 SEM. A comparison of the residual strain and stoichiometric ratio between as-deposited films and films annealed for four hours at 1100°C in flowing oxygen is presented.

Tunable end-coupled ferroelectric-gap filters based on barium strontium titanate capacitors

Tunable filters are desirable components for modern RF and microwave communication systems. It is required that the subsystems be easily integrated, reconfigurable, reproducible and small. Microstrip filters are good candidates for meeting these requirements. These can be made tunable by using ferroelectric interdigital capacitors to couple half wavelength resonators and applying an electrostatic field using a DC voltage. Based on work previously undertaken on ferroelectric capacitors end coupled band pass filters have been modelled. It is shown that the effects of varying the ferroelectric gap capacitors within the expected tuning range will be a modification in the centre frequency of a band pass filter of around 20%.

TUNABLE 4-POLE PIEZOELECTRIC FILTER BASED ON TWO DIELECTRIC RESONATORS

ABSTRACT A frequency tuneable microwave filter based on two dual mode dielectric resonators has been developed. Tuning of the filter was realised by using metal plates located over the dielectric resonators and moved along the resonator axis by piezoelectric bimorphs. The main parameters of the filter were: central frequency in the middle of the range of tuning −2.9 GHz, bandwidth for 1dB level ≈0.5%, range of tuning ≈140 MHz. The insertion losses in the whole tuning range were below ≈q2 dB.