J. Levoska

Thermal oxidation of porous silicon: Study on structure

The structural changes of porous silicon (PS) samples during oxidation are investigated and analyzed using various microscopy techniques and x-ray diffraction. It is found that the surface roughness of oxidized PS layers increases with the oxidation at 200–400°C and decreased at 600–800°C. At 800°C a partially fused surface is observed. The oxide formed on the wall of porous silicon skeleton is amorphous. The shifts of Si(400) peaks are observed in the x-ray diffraction patterns, which are correlated to the lattice deformation induced by thermal expansion coefficient mismatch between the grown SiO2 and the residual Si, and to the intrinsic stress caused by the Si–O bonds at the Si–SiO2 interface. These explanations are supported by thermomechanical modeling using three-dimensional finite element method.

Dielectric properties of pulsed laser deposited films of PbMg1/3Nb2/3–PbTiO3 and PbSc1/2Nb1/2O3–PbTiO3 relaxor ferroelectrics

Highly oriented perovskite films of PbMg1/3Nb2/3–PbTiO3 and PbSc1/2Nb1/2O3–PbTiO3 with compositions near the morphotropic phase boundary were formed by pulsed laser deposition on La0.5Sr0.5CoO3/MgO (100). The dielectric properties of the films were studied over the frequency range of 100 Hz–1 MHz in the temperature range 20–350 °C. The room temperature polarization and dielectric permittivity of 250-nm-thick films were close to those in bulk ceramics. The films exhibited relaxor-type behavior with thermal hysteresis and with the temperatures of the dielectric peaks corresponding to those in the bulk. The width of the transition in the films was larger than in bulk ceramics. The properties of the films were found to be influenced by the small grain size rather than by the interfaces of the films.

Relaxor behavior of pulsed laser deposited ferroelectric (Pb1−xLax)(Zr0.65Ti0.35)O3 films

The dielectric behavior of pulsed laser deposited ferroelectric (Pb1−xLax)(Zr0.65Ti0.35)O3 films (PLZT x/65/35, x=0–9.75 at. %) has been studied experimentally. Epitaxial stoichiometric PLZT films were formed on a pulsed laser deposited layer of La0.5Sr0.5CoO3 (LSCO) on MgO (100) single-crystal substrates. The dielectric permittivity and loss tangent of the resulting heterostructures were measured in the temperature range of 20–350 °C at a frequency of 100 Hz–1 MHz. A peak around 130–350 °C was observed in the dielectric permittivity versus temperature curves. The peak exhibited a relaxor type behavior. Its position was a nonmonotonic function of the La content and depended on the microstructure of the film. The broadening of the peak of the dielectric permittivity was larger than that in the ceramic PLZT and it also depended on the La content and microstructure of the film. The broadening depended on the temperature and frequency ranges: master curves of the normalized dielectric permittivity versus norm...

Pulsed-laser deposition of diamond-like carbon: Relations between laser fluence, velocity of carbon ions, and bonding in the films

In the pulsed-laser deposition process, high intensity laser pulses expel material from a solid target and form expanding plasma near the solid surface. The expansion of the plasma produces a forward-directed beam of ionized and neutral species with typical energies of 1–100 eV. In this study, amorphous diamond-like carbon (DLC) thin films were deposited onto silicon substrates at room temperature using an XeCl excimer laser (wavelength 308 nm, pulse length 20 ns) with laser fluences in the range 5–45 J cm−2, on a pyrolytic graphite target. The effect of laser fluence on the velocity and kinetic energy distribution of carbon ions was measured by time-of-flight (TOF) spectrometry using a system based on a Faraday cup with biased grids, and a multichannel plate based particle detector. We have found high kinetic energies, up to 500 eV, for expelled atomic species. In order to study the effect of the energy of the arriving ions on the structure of DLC, the bonding of carbon atoms in films, deposited under si...

Experimental studies and modeling of Pb–Zr–Ti–O film growth in pulsed laser deposition

The growth rate and composition of films deposited by laser ablation of Pb(Zr0.65Ti0.35)O3, both in vacuum and in ambient oxygen and argon with laser fluences in the range 0.3–3.0 J/cm2, were studied experimentally with using energy dispersive x-ray analysis. The film growth rate increased in vacuum with an increase in laser fluence and demonstrated two modes of behavior upon adding gas: decreased at low laser fluence and increased nonmonotonously at high laser fluence. Deposition in vacuum resulted in Pb-deficient films with the Pb/Ti ratio decreasing with an increase in laser fluence, while deposition in a gas resulted in a strong increase of Pb content with the Pb/Ti ratio increasing faster under strong laser irradiation. Changes in the film growth rate and composition were similar upon adding either oxygen or argon. No noticeable changes in the spatial distribution of the film growth rate and composition were found in a gas ambient with respect to a vacuum ambient. The obtained experimental results we...

Application of the interface capacitance model to thin-film relaxors and ferroelectrics

The interface capacitance model is briefly revised. It is emphasized that this is a device model related to the boundary condition for polarization and unable to explain the thickness evolution of the ferroelectric properties. The model can be applied to extract the properties of the film from those measured in the capacitor. The interface parameters are found to be temperature independent. The coupling of the film with the interface and the electrode is shown to result in the measured frequency dependent permittivity totally different from that of the film.

Preparation of Bi-Pb-Sr-Ca-Cu-O high Tc superconducting material via oxalate route at various pH values

Abstract A method for coprecipitation of Bi-Pb-Sr-Ca-Cu-O superconducting material via the oxalate route is presented. Bismuth, lead, strontium, calcium and copper nitrates and oxalic acid are used as starting materials. The precipitation of precursor from these materials is studied at different pH values The compositions of the coprecipitated powders are determined by direct current plasma atomic emission spectrometry. Two useful pH regions for precipitation are found. The first is a low pH region between 2 to 4 and the second is a high pH region of over 10. The precursors and sintered bulk samples are investigated by TEM, SEM/EDS and XRD. Resistivity measurements are carried out. The composition of the decomposed precursor is found to be homogeneous. The decomposed precursor has a particle size of around 300 nm. The sintered material consists of approximately 50% of the high T c phase and shows zero resistance at 104 K.

Dielectric relaxation and polar phonon softening in relaxor ferroelectric PbMg1/3Ta2/3O3

S. Kamba, ∗ D. Nuzhnyy, S. Veljko, V. Bovtun, J. Petzelt, Y.L. Wang, J. Levoska, M. Tyunina, J. Macutkevic, N. Setter, and J. Banys Institute of Physics ASCR, v.v.i. Na Slovance 2, 182 21 Prague 8, Czech Republic Ceramics Laboratory, Swiss Federal Institute of Technology EPFL CH-1015 Lausanne, Switzerland Microelectronics and Materials Physics Laboratories, University of Oulu, PL 4500, FIN 90014 Oulun Yliopisto, Finland Semiconductor Physics Institute, A. Gostauto 11, LT-2600 Vilnius, Lithuania Faculty of Physics, Vilnius University, Sauletekio 9, LT-10222 Vilnius , Lithuania (Dated: February 1, 2008)Relaxor ferroelectric PbMg1/3Ta2/3O3 ceramics and thin films were investigated by means of broad-band dielectric, time-domain terahertz (THz), and Fourier-transform infrared (IR) spectroscopy in the frequency range 100 Hz–90 THz at temperatures 100–490 K; the THz and IR spectra were studied from 20 to 900 K. A diffused and strongly temperature dependent peak in the complex permittivity is caused by a dielectric relaxation due to the dynamics of polar clusters. The relaxation appears below the Burns temperature Td in the THz range; it slows down on cooling through the microwave and MHz range and anomalously broadens. The shortest and longest relaxation times of the distribution of relaxation times follow Arrhenius and Vogel–Fulcher laws, respectively. The degree of the B-site order has only a small influence on the parameters of the dielectric relaxation and almost no influence on the phonon parameters. Below Tm≅180 K the distribution of relaxation frequencies becomes broader than our experimental spectral...

A numerical simulation method for the laser‐induced temperature distribution

A numerical simulation approach for the evaluation of temperature distribution in a substrate during the laser processing is developed. The explicit finite‐difference solution of the heat (diffusion) equation is chosen as the starting point in order to simply retain general nonlinearity, physical transparency, and flexibility of the procedure. The method is tested with a simulation of silicon substrate irradiated by cw argon laser, and the results are compared with those from experiments and exact analytical results, where available. Deficiences due to the omission of the nonlinearity, i.e., the temperature dependency of the material properties, in the results of approximative analytical approaches are discussed.

Unstable state in epitaxial films of sodium niobate

Epitaxial perovskite-type films of sodium niobate were grown by pulsed laser deposition, and their crystal structure and dielectric properties were experimentally studied. The dielectric permittivity, inverse permittivity and its derivative, and dielectric hysteresis were analyzed and compared with those of antiferroelectric lead zirconate films. At temperatures in the interval of 80–600 K, the state of the sodium niobate films with relaxed misfit strain is interpreted as an unstable coexistence of antiferroelectric and ferroelectric phases. The origin of the ferroelectric phase is discussed.