Effects of annealing process and Mn substitution on structure and ferroelectric properties of BiFeO3 films

The BiFe_1 − xMn_xO₃ (BFMO) (x = 0.03, 0.05 and 0.07) thin films were deposited on indium tin oxide/glass substrates using a metal organic decomposition method. X-ray diffraction analysis reveals that the structure of BiFeO₃ films is distorted somewhat by Mn substitution. The leakage measurements indicate that Mn doping content is a dominant factor affecting the leakage current of BFMO films. Due to the poor crystallinity, a small remanent polarization (P_r) is observed in the BFMO_x = 0.05 thin film annealed using the conventional method. The P_r values for the sequential-layer annealed BFMO films are found to be related to the intensity of (012) peak and the content of defect complexes between the oxygen vacancies and acceptors.     

Effects of growth temperature and film thickness on the electrical properties of Ba0.7Sr0.3TiO3 thin films grown on platinized silicon substrates by pulsed laser deposition

Barium strontium titanate Ba_0.7Sr_0.3TiO₃ (BST) thin films, with different growth temperatures (T_g) as well as different film thicknesses, have been prepared on Pt/Ti/SiO₂/Si substrates by a reactive pulsed laser deposition method. We observed strong dependences of dielectric properties, such as the Curie-Weiss temperature, dielectric constant, loss tangent, dielectric tunability and leakage current, on the T_g and the BST film thickness. With increase of T_g from 630 to 750 °C, the dielectric constant gradually increases due to the increase in the crystallinity and the grain size. However, the dielectric tunability, loss tangent and leakage current characteristics drastically degrade when the T_g increases up to 750 °C, due to the diffuse and rough interface. The BST film grown at 690 °C shows the best overall dielectric properties with a figure-of-merit of 33 (at 400 kV/cm). These results suggest that film growth process could be optimized by systematically investigating the structure-property relationships. Furthermore, as the BST film thickness increases from 250 to 560 nm, the dielectric properties are remarkably enhanced. The film thickness effect is attributed to the interfacial low-dielectric layers (the so-called “dead layer”) between the BST film and both metal electrodes, which is well explained in terms of a series capacitor model. The thickness and the average dielectric constant for the dead layer are experimentally estimated to be 1.9 nm and 20.3, respectively, in Pt/BST/Pt capacitors.     

Electric and magnetic properties of Pb(Zr0.52Ti0.48)O3-CoFe2O4 particle composite thin film on the SrTiO3 substrate

We deposited a thin epitaxial Pb(Zr_0.52Ti_0.48)O₃ (PZT) layer on the (0 0 1) SrTiO₃ (STO) substrate doped with Nb (0.5 wt.%), then grew composite thin film of CoFe₂O₄ (CFO) and PZT phases on it. X-ray diffraction and high resolution transmission electron microscopy showed that the PZT and CFO phases in the film had perfect epitaxial structures. CFO nanoparticles were embedded in PZT matrix randomly, which was useful to enhance the insulativity of the composite film. The composite thin film exhibited good ferromagnetic and ferroelectric properties. The dielectric constants of the composite thin film kept unchangeable in a wide bias electric field, but increased in a magnetic field, namely, magnetodielectric effect. The possible reasons for the magnetodielectric effect were discussed.     

Deposition of ZnO thin films on SiO2/Si substrate with Al2O3 buffer layer by radio frequency magnetron sputtering for high frequency surface acoustic wave devices

ZnO films with c-axis (0002) orientation have been grown on SiO₂/Si substrates with an Al₂O₃ buffer layer by radio frequency magnetron sputtering. Crystalline structures of the films were investigated by X-ray diffraction, atomic force microscopy and scanning electron microscopy. The center frequency of the surface acoustic wave (SAW) device with a 4.8 μm thick Al₂O₃ buffer layer was measured to be about 408 MHz, which was much higher than that (265 MHz) of ZnO/SiO₂/Si structure and approaches that (435 MHz) of ZnO/sapphire. It is a possible way as an alternative for the sapphire substrate for the high frequency SAW device applications, and is also useful to integrate the semiconductor and high frequency SAW devices on the same Si substrate.     

The growth of BaTiO3 films on (001) MgAl2O4 substrates by pulsed laser deposition technique

The microstructural evolution of the BaTiO₃ films grown on (001) MgAl₂O₄ spinel substrates at different temperatures by means of pulsed laser deposition technique is studied via transmission electron microscopy (TEM). The BaTiO₃ film grown at 850 °C consists of columnar grains of random orientations. Once the substrate temperature is over 900 °C, the BaTiO₃ films grow on (001) MgAl₂O₄ substrates epitaxially. The cross-sectional TEM study reveals that the boundaries and interfaces act as the sources to emit stacking faults and twins which are detrimental to the film quality. The quality of epitaxial films increases with the growth temperature, and is optimized at the growth temperature of 1050 °C. The evolution of film microstructures with the growth temperature is discussed in view of the growth temperature, the surface structure of MgAl₂O₄ substrates, and the phase transition of BaTiO₃.     

Structural, electrical and optical properties of Bi2Se3 and Bi2Se(3−x)Tex thin films

Thin films of Bi₂Se₃, Bi₂Se_2.9Te_0.1, Bi₂Se_2.7Te_0.3 and Bi₂Se_2.6Te_0.4 are prepared by compound evaporation. Micro structural, optical and electrical measurements are carried out on these films. X-ray diffraction pattern indicates that the as-prepared films are polycrystalline in nature with exact matching of standard pattern. The composition and morphology are determined using energy dispersive X-ray analysis and scanning electron microscopy (SEM). The optical band gap, which is direct allowed, is 0.67 eV for Bi₂Se₃ thin films and the activation energy is 53 meV. Tellurium doped thin films also show strong optical absorption corresponding to a band gap of 0.70-0.78 eV. Absolute value of electrical conductivity in the case of tellurium doped thin film shows a decreasing trend with respect to parent structure.     

Chemical vapor deposition of SiO2 films by TEOS/O2 supermagnetron plasma

A TEOS/O₂ supermagnetron double electrode plasma system was used to deposit SiO₂ films. Deposition rates were measured as a function of rf power and substrate stage temperature. With an increase of rf power on both electrodes from 40 to 80W, the deposition rate increased; however, with a further increase of rf power from 80 to 120W, the deposition rate ceased to increase or decreased only a small amount. The presence of O-H bonds from bonded water in the film was evaluated using buffered HF (BHF) etching solution. With an increase of rf power from 40 to 120W, the BHF etch rate decreased; i.e., the number of O-H bonds were reduced. A minimum BHF etch rate was observed at a rf phase difference of 180° between the two rf power sources. A SiO₂ film was deposited on a trench-patterned quartz substrate. A flat surface SiO₂ layer with air gaps (voids) was formed on the high-aspect ratio (depth/width=1.5-2) trench area.     

Formation of β-FeSi2 thin films on non-silicon substrates

β-FeSi₂ films were prepared on non-silicon substrates by sputtering. The crystalline growth, stress induced cracks and adhesive ability to the substrate were investigated on substrate temperature and thermal expansion coefficient of substrate materials. It was found that crack formation in β-FeSi₂ films was dependent on the thermal expansion coefficients of CaF₂, MgO and quartz glass insulating materials. High-density cracks were observed from β-FeSi₂ films on CaF₂ and quartz glass substrates with large difference of the thermal expansion coefficient between β-FeSi₂ film and substrate materials, and it was crack-free on MgO substrate with a thermal expansion coefficient close to that of β-FeSi₂ films. Polycrystalline β-FeSi₂ films grew on Mo, Ta, W, Fe and stainless steel (SS) substrates at low substrate temperature around 400 °C. There was no α-FeSi₂ phase confirmed in the films. All the films had continuous structures without noticeable cracks even though they have different thermal expansion coefficients. Capacity-voltage measurements showed that β-FeSi₂ films formed on SS substrates has n-type conductivity, with residual carrier concentrations of about 1.3∼6.4 × 10¹⁸ cm^− 3. Auger electron spectroscopy depth profile measurements identified homogeneous distribution of Fe and Si atoms in the film region, but with a large interface region between the film and the substrate.     

Mechanism of biaxial alignment in thin films, deposited by magnetron sputtering

Biaxially aligned TiN layers have been deposited by reactive unbalanced magnetron sputtering. In this work, a mechanism for the resulting microstructure and biaxial alignment of the deposited TiN layers will be discussed. According to the described model, the resulting biaxial alignment is caused by an overgrowth mechanism (zone T) due to an anisotropy in growth rate of the different oriented grains towards the incoming material flux. Hence, the in-plane alignment will mainly depend on two parameters: the mobility during the growth (zone T condition) and the spread on the incoming material flux. This spread on the incoming material flux has been calculated by an earlier published Monte Carlo simulation program of the transport of sputtered particles towards the substrate. The model for the mechanism of biaxial alignment is validated by comparing the experimental and theoretical influence of target-substrate distance and working pressure on the resulting in-plane alignment.     

The influence of structure changes in the properties of TiCxOy decorative thin films

The main purpose of this work consists in the preparation of titanium oxycarbide, TiC_xO_y, thin films, in which the presence of oxygen changed the film properties between those of titanium carbide and those of titanium oxide. Varying the oxide/carbide ratio allowed to tune the structure of the films between titanium oxide and carbide and consequently electronic, mechanical and optical properties of the films. The depositions were carried out from a TiC target by direct current, dc, reactive magnetron sputtering, varying the oxygen flow rate. The obtained results showed that the film's properties can be divided into 3 different regimes — i) carbide, ii) a transition zone and iii) an oxide one. X-ray diffraction results revealed the occurrence of a face-centered cubic phase (TiC-type) for low oxygen content, also obtained in the TiC_1.6(O) film, with a clear tendency towards amorphization with the increase of the oxygen flow rate. For the highest oxygen contents, the results revealed the development of a mixture of poorly crystallized TiO₂ phases. The colour results indicated a strong dependence on the O/Ti ratio. A progressive reduction of hardness and residual stresses with the increase of the O/Ti ratio was also observed. The residual stresses, as well as the film structure, seem to play an important role on the adhesion of the coatings. The static friction coefficient revealed also some correlation with the mechanical properties, but mainly with the surface roughness.     

Irreversible photoinduced changes in As48S52 amorphous thin films prepared by pulsed laser deposition

Thin amorphous As-S films were prepared using pulsed laser deposition. Raman scattering spectroscopy, variable angle spectroscopic ellipsometry, and optical transmittance spectra revealed irreversible photostructural effects, significant photoinduced changes of refractive index, and optical band gap energy in the films. Observed effects are discussed in terms of structural transformations of basic structural units.     

Structural properties of films grown by magnetron sputtering of a BiFeO3 target

Thin films have been prepared on silicon substrates by pulsed DC magnetron sputtering of a BiFeO₃ target under an Ar/O₂ mixture with various oxygen flow (0-50 sccm) at room temperature. Due to the low deposition-temperature, the as-grown films are amorphous. As a consequence, we have investigated the effect of the annealing temperature (400-900 °C) and atmosphere (100% N₂ or 90% N₂/10% O₂) on the crystallisation. Microstructural investigations (XRD, HREM, Raman and FTIR spectroscopy) have been performed in order to determine the structural properties of the films and to establish the experimental parameters leading to BiFeO₃ films. A temperature-dependent Raman study points to a spin-phonon coupling around the Néel temperature.     

Effects of substrate temperature on Bi0.8La0.2FeO3 thin films prepared by pulsed laser deposition

Bi_0.8La_0.2FeO₃ thin films on Pt/TiO₂/SiO₂/Si substrates at various substrate temperatures from 500 °C to 750 °C are prepared by pulsed laser deposition, and their microstructures and ferroelectric/magnetic properties are carefully investigated using various techniques. It is observed that the crystallographic orientation and Fe-ion valence state depend significantly on the substrate temperature, which consequently influences considerably on the ferroelectric and magnetic properties of the thin films. A considerable improvement of the ferroelectric and magnetic properties of the thin films can be achieved by optimizing the substrate temperature for deposition.     

Spectroellipsometric assessment of HfO2 thin films

Present paper looks into the possibilities and limitations of near ultraviolet-visible range spectroscopic ellipsometry in investigating HfO₂ thin films (thickness < 7 nm). The “high k” dielectric films were produced by Atomic Layer Deposition—ALD, sputtering, and Metalo-Organic Chemical Vapour Deposition—MOCVD, on silicon and on silicon/silicon dioxide structures. Using a simple optical model (Cauchy dispersion, with an Urbach absorption tail), suitable for the optical range investigated, we extract the thickness of the layers and their optical constants. Results related to the optical properties show the important impact made by the initial surface and the growth/deposition procedure. It is also shown that for the case of ALD HfO₂ films grown on RTO oxides a significant increase in the absorption coefficient is recorded in the 4.7-5.15 eV range; this can be linked with the formation of defects related to oxygen vacancies. Subsequent anneal cycles performed in oxygen reveal that changes do occur both at the transition layer level, and in the structure of the HfO₂ film, for which an increase in the absorption is recorded.     

YSZ thin films deposited by e-beam technique

YSZ thin films were grown evaporating cubic and tetragonal phase ZrO₂ stabilized by 8 wt.% of Y₂O₃ (8% of YSZ) ceramic powders by using e-beam deposition technique. Operating technical parameters that influence thin film properties were studied. The influence of substrate crystalline structure on growth of deposited YSZ thin film was analyzed there. The YSZ thin films (1.5-2 μm of thickness) were deposited on three different types of substrates: Al₂O₃, optical quartz (SiO₂), and Alloy 600 (Fe-Ni-Cr). The dependence of substrate temperature, electron gun power, and phase of ceramic powder on thin film structure and surface morphology was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The substrate temperature was changed in the range of 20-600° C (during the YSZ thin film deposition) and its influence on the crystallinity of deposited YSZ thin films was analyzed. It was found that electron gun power and substrate temperature has the influence on the crystallite size, and texture of YSZ thin films. Also, the substrate has no influence on the crystal orientation. The crystallite size varied between 20 and 40 nm and increased linearly changing the substrate temperature. The crystal phase of evaporated YSZ powder has the influence on the structure of the deposited YSZ thin films.     

Z-scan measurement for the nonlinear absorption of Bi2.55La0.45TiNbO9 thin films

Bi_2.55La_0.45TiNbO₉ (BLTN-0.45) thin films with layered aurivillius structure were fabricated on fused silica substrates by pulsed laser deposition technique. Their structure, fundamental optical constants, and nonlinear absorption characteristics have been studied. The film exhibits a high transmittance (> 60%) in visible-infrared region. The optical band gap energy was found to be 3.44 eV. The optical constant and thickness of the films were characterized using spectroscopic ellipsometric (SE) method. The nonlinear optical absorption properties of the films were investigated by the single-beam Z-scan method at a wavelength of 800 nm laser with a duration of 80 fs. We obtained the nonlinear absorption coefficient β = 4.64 × 10^− 8 m/W. The results show that the BLTN-0.45 thin film is a promising material for applications in absorbing-type optical device.     

TiO2 anatase thin films deposited by spray pyrolysis of an aerosol of titanium diisopropoxide

Titanium dioxide thin films were deposited on crystalline silicon (100) and fused quartz substrates by spray pyrolysis (SP) of an aerosol, generated ultrasonically, of titanium diisopropoxide. The evolution of the crystallization, studied by X-ray diffraction (XRD), atomic force (AFM) and scanning electron microscopy (SEM), reflection and transmission spectroscopies, shows that the deposition process is nearly close to the classical chemical vapor deposition (CVD) technique, producing films with smooth surface and good crystalline properties. At deposition temperatures below 400 °C, the films grow in amorphous phase with a flat surface (roughness∼0.5 nm); while for equal or higher values to this temperature, the films develop a crystalline phase corresponding to the TiO₂ anatase phase and the surface roughness is increased. After annealing at 750 °C, the samples deposited on Si show a transition to the rutile phase oriented in (111) direction, while for those films deposited on fused quartz no phase transition is observed.     

Crystallized TiO2 film growth on unheated substrates by pulse-powered magnetron sputtering

Crystallized TiO₂ films were successfully grown in situ without heating by bipolar-pulse sputtering method at high deposition rates (∼40 nm/min). The optical emission study of the sputtering plasma during growth revealed that the “pulse geometry” had a great influence on the electron/ion temperature of the plasma. It was revealed that the crystallization and the accompanying enhancement in the photocatalytic activity were not caused by the “temperature effect” but caused by the “plasma effect”.     

Pulsed laser deposition optical waveguiding Bi3TiNbO9 thin films on fused silica

The layered-perovskite ferroelectric Bi₃TiNbO₉ (BTN) optical waveguiding thin films have been prepared on fused silica substrates by pulsed laser deposition (PLD). X-ray θ-2θ scans revealed that the films are single-phase perovskite and highly (00l) textured. The wavelength dependence of the transmittance of the films was determined. We obtained an average transmittance of 75% in the wavelength range of 400-1100 nm and the band gap E_g=3.55 eV. The optical waveguiding properties of the films were characterized by using prism-coupling method. The distinct m lines of the guided transverse magnetic (TM) and transverse electric (TE) modes of the BTN films waveguide have been observed. The cross sectional morphology of the film was studied by scanning electron microscopy (SEM).     

Fabrication of biaxially textured magnesium oxide thin films by ion-beam-assisted deposition

Biaxially textured MgO thin films were grown by ion-beam-assisted deposition. The film growth parameters of film thickness, ion-to-atom arrival ratio (r-value), ion beam angle, and ion beam voltage were studied. Film characterization was performed by X-ray diffraction, pole figure analysis, and atomic force microscopy (AFM). Full-width half-maximum (FWHM) of MgO (220) ?-scans and MgO (002) ω-scans, respectively, were used to evaluate in-plane and out-of-plane film texture. MgO (220) ?-scan FWHM of 3.2° and MgO (002) ω-scan FWHM of 1.2° was achieved on amorphous Si₃N₄-coated Si substrates using a 1500-V ion source oriented at 45° to the substrate normal and an r-value of 0.90. Depositions on metallic substrates yielded MgO (220) ?-scan FWHM values of 5.2° and MgO (002) ω-scan FWHM of 2.5°. Root-mean-square surface roughness of these films as measured by AFM was ≈2.3 nm.