Narrow Results

A nonlinear thermodynamic formalism is developed to calculate the pyroelectric property of epitaxial single domain ${\mathrm{\text{SrTiO}}}_3∕\mathrm{\text{Si}}$ heterojunctions by taking into account the thermal expansion misfit strain at different temperatures. It has been demonstrated that the crucial role was played by the contribution associated with the structure order parameter arising from the rotations of oxygen octahedral on pyroelectricity. A dramatic decrease in the pyroelectric coefficient due to the strong coupling between the polarization and the structure order parameter is found at ferroelectric ${\mathbf{\text{T}}}_{\mathrm{\text{F1}}}$–${\mathbf{\text{T}}}_{\mathrm{\text{F2}}}$ phase transition. At the same time, the thermal expansion mismatch between film and substrate is also found to provide an additional weak decrease of pyroelectricity. The analytic relationship of the out-of-plane pyroelectric coefficient and dielectric constant of ferroelectric phases by considering the thermal expansion of thin films and substrates has been determined for the first time. Our research provides another avenue for the investigation of the pyroelectric effects of ferroic thin films, especially, such as antiferroelectric and multiferroic materials having two or more order parameters.

Voltage control of magnetism in multiferroics, where the ferromagnetism and ferroelectricity are simultaneously exhibiting, is of great importance to achieve compact, fast and energy efficient voltage controllable magnetic/microwave devices. Particularly, these devices are widely used in radar, aircraft, cell phones and satellites, where volume, response time and energy consumption is critical. Researchers realized electric field tuning of magnetic properties like magnetization, magnetic anisotropy and permeability in varied multiferroic heterostructures such as bulk, thin films and nanostructure by different magnetoelectric (ME) coupling mechanism: strain/stress, interfacial charge, spin–electromagnetic (EM) coupling and exchange coupling, etc. In this review, we focus on voltage control of ferromagnetic resonance (FMR) in multiferroics. ME coupling-induced FMR change is critical in microwave devices, where the electric field tuning of magnetic effective anisotropic field determines the tunability of the performance of microwave devices. Experimentally, FMR measurement technique is also an important method to determine the small effective magnetic field change in small amount of magnetic material precisely due to its high sensitivity and to reveal the deep science of multiferroics, especially, voltage control of magnetism in novel mechanisms like interfacial charge, spin–EM coupling and exchange coupling.

The influence of free surface and depolarizing field on structural phase transitions in thin ferroelectric films from an ordered state to a disordered one is investigated. The dependences of the order parameter on the distance from the free film surface are calculated. It is shown that with the presence of the depolarizing field and in its absence, the effective thickness of the surface layer depends on the temperature. Nearby the phase transition point, the thickness increases indefinitely. Calculations considering depolarizing field showed that the phase transition points for the bulk ferroelectrics and the film under given boundary conditions coincide. Also shown that in the absence of depolarizing field with mixed boundary conditions, the film thickness does not affect the order parameter, and in presence of the field, this influence is observed.

Lead-free piezoelectric thin films of 0.055BaZrO₃–0.935(K${}_{0.45}$Na${}_{0.5}$Li${}_{0.05}$)NbO₃–0.01(Bi${}_{0.5}$Na${}_{0.5}$)TiO₃ (BZ–KNLN–BNT) were fabricated on (100)-LaNiO₃/SiO₂/Si substrates. The Pechini (polymeric precursor) method was carried out to prepare the complicated multi-component precursor solution. This method was suited for controlling the complex components accurately. The films crystallized at 700${}^{\circ }$C showed a (100)_pc-oriented perovskite structure, whose grain size was about 200nm and thickness was about 700nm. The Curie temperature $T$_c of the films was 292${}^{\circ }$C that was near to that of the bulk ceramics. The preparation method used in this work provided a possibility for the application of multi-component lead-free piezoelectric films.

The paper presents the structural and ferroelectric results for Sr${}_{1-x}$Ba_xBi₂Nb₂O₉($x=0.30$; 0.85) thin films, which were obtained by using dip-coating. The solutions containing the desirable ions were prepared from the powders of the previous studied ceramic samples. The films were deposited at room temperature on Fluorine-doped Tin Oxide (FTO) substrates and submitted to a heat treatment for crystallization. The films were characterized by using scanning microscopy electronic, energy dispersive spectroscopy and ellipsometry. Hysteresis ferroelectric loops were obtained, at room temperature, by using a Sawyer-Tower circuit at several frequencies. A well-defined grain structure was observed for both compositions. The energy dispersive spectroscopy (EDS) measurements revealed the presence of the corresponding elements from the chemical composition of the ceramic systems. The band-gap energy was around 3.3eV for both samples. Typical hysteresis loops for normal and relaxor ferroelectrics were obtained for $x=0.30$ and 0.85, respectively.

This paper presents the results of studies of the structure, microstructure, dependences of the piezoelectric properties on the electric field (dielectric hysteresis loops, reversible nonlinearity, deformation characteristics) and dielectric properties of the ceramic material PCR-13 (based on the PZT system) in the temperature range 300–900K and frequencies of an alternating electric field ($25÷2\cdot 10^6$) Hz. The character of the obtained dependences made it possible to attribute PCR-13 to ferro-hard materials. Using the HF cathodic sputtering method, PCR-13 thin films were fabricated on Si (001) substrates. It is shown that they are polycrystalline textured, while in comparison with bulk material, the film contains tensile stresses of the unit cell in the plane of the substrate and compresses in the perpendicular direction with the value ${\epsilon }_{33}=-0.018$. The capacitance-voltage characteristics of the Al/PCR-13/Si/Al heterostructure were studied. The reasons for the revealed patterns are discussed.

BiFeO₃ thin films were prepared using the chemical solution route on Pt/TiO₂/SiO₂/Si(100) substrates under different crystallization kinetics. The crystallization kinetic effects on the dielectric and electrical properties have been investigated. These properties included dielectric permittivity, electric modulus, electrical conductivity measurements as a function of the temperature (300–525 K) and frequency (10²–10⁶ Hz), and leakage current measurements electric field range ± 30 kV/cm at room temperature. The differences observed in conductivity and current density of the BiFeO₃ films were discussed in terms of possible defects induced by the crystallization kinetic. An anomalous relaxor-like dielectric behavior characterized by a broad maximum in the real dielectric permittivity as a function of temperature and the low-frequency dielectric dispersion has been observed. The nonexpected peaks in the real permittivity were accompanied by increasing at least four orders in the conductivity’s magnitude at high temperatures. The origin of the relaxor-like dielectric anomalies is discussed, suggesting that the dielectric permittivity peaks are artifacts due to carrier migration correlated to the onset of the Maxwell–Wagner effect.

We report the unexpectedly excellent dielectric properties of amorphous thin films with compositions in the Bi–Ti–O system. Films were deposited by RF magnetron reactive co-sputtering. In the composition range of 0.5 < $x$ < 0.7, amorphous ${{\text{Bi}}_{1-}}_x$${\text{Ti}}_x$${\text{O}}_y$ exhibits excellent dielectric properties, with a high dielectric constant, ${𝜀}_r$$\sim$ 53, and a dissipation factor as low as tan $\delta$ = 0.007. The corresponding maximum breakdown field reaches $\sim$1.6 MV/cm, yielding a maximum stored charge per unit area of up to 8 $\mu$C/cm². This work demonstrates the potential of amorphous Bi–Ti–O as a high-performance thin-film dielectric material that is compatible with high-performance integrated circuits.

SnO₂–ZnO thin films consisting of nanoscale crystallites were obtained on glass and silicon substrates by solid-phase low-temperature pyrolysis. The synthesized materials were studied by XRD and SEM methods, electrophysical and optical properties were evaluated, as well as the band gap was calculated. It was shown that regardless of the phase composition all films were optically transparent in the visible range (310–1000 nm). The nanocrystallites’ minimum size, the highest activation energy of the conductivity and the smallest band gap calculated for indirect transitions were shown for a thin film 50SnO₂–50ZnO. It was assumed that the band gap decreasing might be attributed to the existence of surface electric fields with a strength higher than 4 × 10⁵ V/cm.

The research findings of the phase composition, nanostructure and optical properties of strontium–barium niobate thin films are discussed. ${\text{Sr}}_x$Ba${{}_{1-}}_x$Nb₂O₆ nanosized films ($x$ = 0.5 and 0.61) were characterized by XRD, SEM and AFM studies. Reflective multi-angle ellipsometry and spectrophotometry were used to determine the optical parameters (refractive index, its dispersion, and thickness of the damaged surface layer) of thin films. It was shown that SBN-50 and SBN-61 thin films were grown $c$-oriented on Al₂O₃ (0001) and heteroepitaxial on MgO (001) substrates. The increase of refractive index, approaching its maximum value in the bulk material for a given composition as the film thickness increases, is observed.

This paper reports the impact of the laser pulse repetition frequency on growth processes, morphological and electro-physical parameters of nanocrystalline LiNbO₃ thin films obtained by the pulsed laser deposition technique. It was found that the nucleation process in LiNbO₃ films could controllably change by increasing the laser pulse repetition frequency. The film obtained at the repetition frequency of 4 Hz consists of local islands and clusters with a diameter of 118.1 ± 5.9 nm. Nanocrystalline films, grown at the repetition frequency of 10 Hz, possess a continuous granular structure with a grain diameter of 235 ± 11.75 nm. Achieved results can be used for the development of promising “green” energy devices based on lead-free piezoelectric energy harvesters.

Decreasing the scale of vanadium dioxide (VO₂) structures is one of the ways to enhance the switching speed of the material. We study the properties of VO₂ films of altered thicknesses in the range of 20–170nm prepared on c-sapphire substrates with a TiO₂ sublayer by pulsed laser deposition (PLD) method. The synthesis regime to design a TiO₂ film was preliminarily optimized based on XRD data. XRD patterns reveal an epitaxial growth of the VO₂ films with distortion of the monoclinic cell to hexagonal symmetry. The positions of the lattice vibration modes in Raman spectra are similar to those in bulk VO₂ when the film thickness is greater than $\sim 30$nm. For VO₂ films thicker that $\sim 20$nm, a lattice strain results in the modes’ positions and intensity change. However, the electrically triggered transition in a $\sim 50$nm thick VO₂ film reveals forward and reverse switching times as short as 20ns and 400ns, correspondingly.

Lead-free piezoelectric sodium bismuth titanate ((${\text{Bi}}_{0.5}$${\text{Na}}_{0.5}$)TiO₃, BNT) thin films were epitaxially grown onto (001)-, (110)-, and (111)-oriented Nb:SrTiO₃ (STO) single crystal substrates prepared by sol–gel processing. Highly oriented growth in (001), (110), and (111) BNT thin films was obtained in this work benefiting from the lattice match between the BNT film and the STO substrate. The different growth models in thin films with various orientations result in various surface morphologies dependent on the film orientation. The piezoresponse of the BNT thin films was represented exhibiting a strong orientation dependence that (110)>(001)>(111). This is contributed by the various domain switching contribution related to the crystal symmetry and polarization distribution in the three oriented thin films.

Lead-free (${\text{K}}_{0.5}$${\text{Na}}_{0.5}$)NbO₃ (KNN) and ${\text{Li}}_{0.06}$(${\text{K}}_{0.5}$${\text{Na}}_{0.5}$)${}_{0.94}$NbO₃ (LKNN) thin films were fabricated by a sol-gel method. The effects of Li substitution on crystal structure, microstructure and electrical properties of KNN film were systematically studied. Li doping can enhance the ferroelectric and piezoelectric properties of KNN film. Compared with pure KNN film, the LKNN film possesses larger remanent polarization ($P_r$$\sim$ 9.3 $\mu$C/${\mathrm{\text{cm}}}^2$) and saturated polarization ($P_s$$\sim$ 41.2 $\mu$C/${\mathrm{\text{cm}}}^2$) and lower leakage current density ($\sim {10}^{-5}$A/${\mathrm{\text{cm}}}^2$ at 200 kV/cm). Meanwhile, a typical butterfly shaped piezoelectric response curve is obtained in the LKNN film with a high piezoelectric coefficient ($d_{33}$$\sim$ 105 pm/V). Excellent fatigue resistance ($\sim$${10}^9$ switching cycles) and aging resistance ($\sim$ 180 days) demonstrate the long-term working stability of LKNN film. These findings indicate that KNN-based lead-free piezoelectric films have a broad application prospect in microelectromechanical systems (MEMS).

${\text{Bi}}_{3.25}$${\text{La}}_{0.75}$Ti₃${\text{O}}_{12}$(BLT) thin films are promising materials used in non-volatile memories. In this work, BLT films were deposited on Pt(111)/Ti/SiO₂/Si substrates by rf-magnetron sputtering method followed by annealing treatments. The microstructures of BLT thin films were investigated via X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). With the increase in annealing temperature, the grain size increased significantly and the preferred crystalline orientation changed. A well-saturated hysteresis loop with a superior remnant polarization of 15.4 $\mu$C/cm² was obtained for BLT thin films annealed at 700^∘C. The results show that the dielectric constant decreased with the increase in grain sizes.

BaM thin films have been synthesized by dispersing the dried gel nano powders prepared by Sol-Gel method. The solution was made by dissolving iron nitrate Fe(NO₃).9H₂O, barium nitrate Ba(NO₃)₂ and citric acid in deyonized water and methanol. This sol was slowly evaporated until a dried gel was formed. This dried gel was then added to ethylene glycol. The final solution was vigorously shaken and mixed in ultrasonic cleaner for 30 min to disperse particles sufficiently. Then the prepared solution spin coated on Si(110) substrate. The obtained thin films were dried at 120 °C and then calcined at 900 °C for 1 h. The films were characterized using X-ray diffraction (XRD) and vibrating sample magnetometer (VSM).

Amorphous germanium (a-Ge) films (thickness 25-450 nm) were obtained on the glass substrates at room temperature by the vacuum (3·10^-6mmHg) pulsed laser (Q-switched glass: Nd³⁺ laser: wavelength — 1.06 μm, pulse duration — 30 ns, intensity in the Ge target irradiation zone — ~10⁹ W/cm²) deposition method in non-equilibrium conditions of growth. The optical properties of fabricated films were studied in the range of 400-1000 nm using a spectrometer Filmetrics F20. It was shown that the optical properties of a-Ge films are explained by the Tauc model for amorphous semiconductors. It was also established that the optical gap of films depends on their thickness.

Thin film materials are the key elements of continued technological advances made in the fields of optoelectronic, photonic and magnetic devices. Thin film studies have directly or indirectly advanced many new areas of research in solid state physics and chemistry which are based on phenomena uniquely characteristic of the thickness, geometry and structure of the film. The processing of materials into thin films allows easy integration into various types of devices. Thin films are extremely thermally stable and reasonably hard, but they are fragile. On the other hand organic materials have reasonable thermal stability and are tough, but are soft. Thin film mechanical properties can be measured by tensile testing of freestanding films and by the micro beam cantilever deflection technique, but the easiest way is by means of nanoindentation. Optical experiments provide a good way of examining the properties of semiconductors. Particularly measuring the absorption coefficient for various energies gives information about the band gaps of the material. Thin film materials have been used in semiconductor devices, wireless communications, telecommunications, integrated circuits, rectifiers, transistors, solar cells, light-emitting diodes, photoconductors and light crystal displays, lithography, micro- electromechanical systems (MEMS) and multifunctional emerging coatings, as well as other emerging cutting technologies.

A magnetized sheet plasma ion source was developed for steady state high density plasma with strong density and high temperature gradients. This feature provides efficient formation of negative hydrogen (H^-) ions over a wide beam extraction area through the electron volume process. A hexapole confinement at the cathode, addition of argon and magnesium seeding led to the increase of H^- yield. The device configuration is suitable for plasma based materials processing namely, synthesis of TiN, SiH, SnO₂, and the formation of advanced MAX phase materials Ti₂AlC, Ti₂CdC and NbAlC.