Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ thin films were deposited on LaNiO₃(LNO)/SiO₂/Si(1 0 0) and Pt/Ti/SiO₂/Si(1 0 0) substrates by metal-organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2P_r are about 22.6 and 8.8 μC/cm² under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d_33eff values 95 and 81 pm/V. The improved piezoelectric properties could make (Na_1−xK_x)_0.5Bi_0.5TiO₃ thin film as a promising candidate for piezoelectric thin film devices.     

Exchange bias in (1 1 0)-orientated Bi0.9La0.1FeO3/La0.5Ca0.5MnO3 films

We performed a systematic study on the exchange bias in (1 1 0)-orientated Bi_0.9La_0.1FeO₃/La_0.5Ca_0.5MnO₃ (BLFO/LCMO) heterostructure with a fixed BLFO film thickness of 600 nm and different LCMO layers ranging from t=0 to 30 nm. The LCMO is found to be weakly ferromagnetic, with the Curie temperature descending from ∼225 K to 0 as the layer thickness decreases from 30 nm to 3 nm. The main magnetic contributions come from the BLFO film, and the areal magnetization ratio is 1:0.07 for t=5 nm and 1:0.82 for t=30 nm for BLFO to LCMO at the temperature of 5 K. Further experiments show the presence of significant exchange bias, and it is, at the temperature of 10 K, ∼40 Oe for t=0 and ∼260 Oe for t=30 nm. The exchange bias reduces dramatically upon warming and disappears above the blocking temperature of the spin-glasslike behavior observed in the samples. The possible origin for exchange bias is discussed.     

Structural characterization of thermally evaporated Bi2Te3 thin films

Thermally evaporated Bi₂Te₃ thin films were deposited on glass substrates. X-ray diffraction study confirmed that the growned films are polycrystalline in nature having hexagonal structure. The film exhibits preferential orientation along the [0 1 5] direction for the films of all thickness together with other abundant planes [0 1 1 1] and [1 1 0]. Various structural parameters such as lattice constants, crystallite size, strain, and dislocation density have been calculated and they are found to be thickness dependent. The lattice parameters are found to be a=4.38 Å and c=30.40 Å. The grain size of the films increases with thickness as the dislocation density and the microstrain decreases with thickness. The mean bond energy and the average coordination number of Bi₂Te₃ thin film are found to be 1.72 eV and 2.4, respectively.     

Low-frequency alternative-current magnetic susceptibility of amorphous and nanocrystalline Co60Fe20B20 films

This investigation experimentally studies the low-frequency alternating-current magnetic susceptibility (χ_ac) of amorphous and nanocrystalline CoFeB films by measuring the magnetic field established by passing currents of various frequencies through such films of various thicknesses (t_f). A CoFeB film is sputtered onto a glass substrate with t_f from 100 Å to 500 Å under the following conditions: (a) As-deposited films were maintained at room temperature (RT) and (b) films were post-annealed at T_A=150 °C for 1 h. The samples thus obtained are analyzed in a magnetic field that was generated by an alternating current (AC) at various frequencies from 10 Hz to 25,000 Hz. The experimental results demonstrate that the χ_ac declines as the thickness of the as-deposited sample and the post-annealed sample (T_A=150 °C) increases because the lower coercivity (H_c) of thinner CoFeB films is similar to a soft magnetic characteristic and is associated with a higher χ_ac value. The best χ_ac value is obtained at a thickness of 100 Å under both conditions. The χ_ac value of the post-annealed sample exceeds that of the RT sample at thicknesses from 100 Å to 500 Å because the magneto crystalline anisotropy of the post-annealed sample yields the highest χ_ac value at the optimal resonance frequency (f_res), at which the spin sensitivity is maximal. The X-ray diffraction patterns (XRD) of as-deposited CoFeB films reveal their amorphous structure. The XRD results for the post-annealed films include a main peak at 2θ=44.7° from the body-centered cubic (BCC) nanocrystalline CoFe that indicated a (110) textured structure. Post-annealing treatment caused that the amorphous structure to become more crystalline by a thermally driven process, such that the χ_ac value of the post-annealed sample exceeded that of the RT sample. This experimental result demonstrates that the χ_ac value decreased as the thickness of the thin film increased. Finally, the CoFeB thin films had the best χ_ac at low frequency (<50 Hz) following post-annealing treatment. The results obtained under the two conditions indicate that the maximum χ_ac value and the optimal f_res of a 100 Å-thick CoFeB thin film were 1.6 and 30 Hz, respectively, following post-annealing at T_A=150 °C for 1 h, suggesting that a 100 Å-thick CoFeB thin film that has been post-annealed at T_A=150 °C can be utilized as a gage sensor and in transformer applications at low frequencies.     

Correlation between local hysteresis and crystallite orientation in PZT thin films deposited on Si and MgO substrates

(0 0 1)-Oriented tetragonal ferroelectric PbZr_0.53Ti_0.47O₃ (PZT) thin films (90 nm of thickness) have been grown on TiO_x/Pt/TiO₂/SiO₂/Si and TiO_x/Pt/MgO substrates. The existence of (1 0 0)-oriented crystallites in the c-axis matrix of the (0 0 1)-oriented films has been evidenced by using four circles X-ray diffraction. Depending on the substrate, the ratio of the lattice parameters c/a was found to be 1.02 (Si) and 1.07 (MgO) and this was correlated with the coercive field values. Local piezoelectric hysteresis loops produced by atomic force microscopy have been taken with profit to characterize the switching properties of the ferroelectric domains at the scale of individual crystallites. In each case, (1 0 0)-oriented crystallites require much higher voltage than (0 0 1)-oriented crystallites for switching. These results are explained by taking into account the strain imposed by the substrate in the film. We conclude that piezoelectric hysteresis loops produced by atomic force microscopy provide very rich information for addressing the local switching property of individual crystallites in PZT thin films.     

Preparation and characterization of transparent NiO thin films deposited by spray pyrolysis technique

Nickel oxide thin films were successfully fabricated with various deposition time (t_d = 5, 10, and 15 min) on glass substrates using spray pyrolysis technique. The deposited films undergo thermal treatment at 350 °C for various annealing time (t_a = 0, 15, 30 and 60 min). In this study, the effect of t_d and t_a on film thickness was observed and their influence on structural, morphological and optical properties were investigated. The films deposited with t_d = 5 min showed amorphous structure while the films grown at higher deposition time became partially crystallized with preferred growth along (1 1 1) direction. Heat treatment carried out in air allowed us to tune the polycrystalline structure and the diffraction intensity at preferred peak increases with the increase in t_a which is a consequence of better crystallinity. This was reflected in the AFM micrographs of the films which suggested that the thermal annealing (or increasing t_a) facilitates the process of grain-growth, and improves the crystalline microstructure. The optical transmission of the films was found to vary with t_d and t_a and thus film thickness. The thinner films show higher transparency in the UV–vis spectral region. The optical band gap was blue-shifted from 3.35 eV to 3.51 eV depending on t_a. The effect of t_a on the various optical constants of the NiO films has also been discussed.     

Effect of thickness on the structure, morphology and optical properties of sputter deposited Nb2O5 films

Nb₂O₅ films with the thickness (d) ranging from 55 to 2900 nm were deposited on BK-7 substrates at room temperature by a low frequency reactive magnetron sputtering system. The structure, morphology and optical properties of the films were investigated by X-ray diffraction, atomic force microscopy and spectrophotometer, respectively. The experimental results indicated that the thickness affects drastically the structure, morphology and optical properties of the film. There exists a critical thickness of the film, d_cri =2010 nm. The structure of the film remains amorphous as d < d_cri. However, it becomes crystallized as d > d_cri. The root mean square of surface roughness increases with increasing thickness as d > 1080 nm. Widths and depths of the holes on film surface increase monotonously with increasing thickness, and widths of the holes are larger than 1000 nm for the crystalline films. Refractive index increases with increasing thickness as d < d_cri, while it decreases with increasing thickness as d > d_cri. In addition, the extinction coefficient increases with increasing thickness as d > d_cri.     

Non-hysteretic metal-insulator transition of VO2 films grown by excimer-laser-assisted metal organic deposition process

We examined the correlation between thickness of an epitaxial VO₂ phase grown on a TiO₂ (0 0 1) substrate by the excimer-laser-assisted metal organic deposition (ELAMOD) process and the metal-insulator transition (MIT) property of it. The abrupt and hysteretic MIT was observed for the epitaxial films (thickness: t ≥ 6 nm), and the epitaxial film (t ≤ 4 nm) showed semiconductor behavior. When an amorphous VO_x layer was prepared on the ultrathin epitaxial phase (t ≤ 4 nm) by the ELAMOD, a non-hysteretic MIT was successfully observed. The non-hysteretic MIT was found to be owing to roughened interface between the epitaxial phase and the amorphous phase, where there would be a number of structural defects.     

The impact of annealing temperature and time on the electrical performance of Ti/Pt thin films

In this study, we focus on the influence of annealing time t_PDA (i.e. 30 min and 630 min) on the room-temperature resistivity of electron-beam-evaporated titanium/platinum thin films when exposed to thermal loads up to temperatures T_PDA of 700 °C. The titanium has a fixed thickness of 5 nm and serves as an adhesion layer. The thickness d_f,Pt of the platinum top layer is varied between 21 and 97 nm. Up to annealing temperatures of 450 °C, the film resistivity of the bi-layer system is linearly correlated with the reciprocal platinum film thickness independent of t_PDA, as expected from the size effect. At t_PDA = 30 min, the change in intrinsic film stress dominates the electrical behavior in this annealing regime, predominantly at large d_f,Pt values. Compared to t_PDA = 630 min, however, the increase in resistivity especially at low platinum film thickness is substantially larger demonstrating that titanium starts to diffuse at these long annealing times even at moderate temperatures. At T_PDA = 600 °C, the diffusion of titanium into the top layer leads to an enhanced increase in film resistivity ρ_f, especially at low platinum thicknesses and low annealing times, as the mean penetration depth of diffused titanium is under these conditions in order of d_f,Pt. Above T_PDA = 600 °C, ρ_f is slightly increased at t_PDA = 30 min. At t_PDA = 630 min, however, the film resistivity is decreased at d_f,Pt < 58 nm. This is attributed to grain growth and re-crystallization effects. Furthermore, the mean penetration depths of titanium substantially exceed d_f,Pt resulting predominantly in Ti_xO_y formation on the top film surface and hence, having low impact on ρ_f.     

Planar quarter wave stacks prepared from chalcogenide Ge-Se and polymer polystyrene thin films

Planar quarter wave stacks based on amorphous chalcogenide Ge-Se alternating with polymer polystyrene (PS) thin films are reported as Bragg reflectors for near-infrared region. Chalcogenide films were prepared using a thermal evaporation (TE) while polymer films were deposited using a spin-coating technique. The film thicknesses, d∼165 nm for Ge₂₅Se₇₅ (n=2.35) and d∼250 nm for polymer film (n=1.53), were calculated to center the reflection band round 1550 nm, whose wavelengths are used in telecommunication. Optical properties of prepared multilayer stacks were determined in the range 400-2200 nm using spectral ellipsometry, optical transmission and reflection measurements. Total reflection for normal incidence of unpolarized light was observed from 1530 to 1740 nm for 8 Ge-Se+7 PS thin film stacks prepared on silicon wafer. In addition to total reflection of light with normal incidence, the omnidirectional total reflection of TE-polarized light from 8 Ge-Se+7 PS thin film stacks was observed. Reflection band maxima shifted with varying incident angles, i.e., 1420-1680 nm for 45° deflection from the normal and 1300-1630 nm for 70° deflection from the normal.     

Theoretical investigation of the intrinsic piezoelectric properties for tetragonal BaTiO3 epitaxial films

The orientation dependences of the converse longitudinal piezoelectric constant d_33,f, and the in-plane converse piezoelectric constant e_31,f, are calculated for tetragonal barium titanate epitaxial films. The calculations demonstrate that both e_31,f and d_33,f have their maximum values along an axis close to the (1 1 1) direction of the pseudo-cubic system, which are similar to the orientation dependence results for a tetragonal BaTiO₃ single crystal. The calculated piezoelectric constants for a (1 1 1) oriented BaTiO₃ epitaxial film (e_31,f = −23 C/m², d_33,f = 124 pm/V) suggest that it is a good candidate material for lead-free MEMS applications.     

Fabrication and characterization of La-doped HfO2 gate dielectrics by metal-organic chemical vapor deposition

La-doped HfO₂ gate dielectric thin films have been deposited on Si substrates using La(acac)₃ and Hf(acac)₄ (acac = 2,4-pentanedionate) mixing sources by low-pressure metal-organic chemical vapor deposition (MOCVD). The structure, thermal stability, and electrical properties of La-doped HfO₂ films have been investigated. Inductive coupled plasma analyses confirm that the La content ranging from 1 to 5 mol% is involved in the films. The films show smaller roughness of ∼0.5 nm and improved thermal stability up to 750 °C. The La-doped HfO₂ films on Pt-coated Si and fused quartz substrates have an intrinsic dielectric constant of ∼28 at 1 MHz and a band gap of 5.6 eV, respectively. X-ray photoelectron spectroscopy analyses reveal that the interfacial layer is Hf-based silicate. The reliable value of equivalent oxide thickness (EOT) around 1.2 nm has been obtained, but with a large leakage current density of 3 A/cm² at V_g = 1V + V_fb. MOCVD-derived La-doped HfO₂ is demonstrated to be a potential high-k gate dielectric film for next generation metal oxide semiconductor field effect transistor applications.     

Thickness-dependent magnetization reversal in CoZrNb amorphous films

Structure and magnetization of CoZrNb amorphous films prepared by DC magnetron sputtering have been studied as a function of film thickness (t), from 35 to 840 nm. Using comprehensive characterization, we show that the CoZrNb amorphous films possess a single phase and no nanocrystalline can be detected. The magnetic measurements indicate that the magnetization reversal of CoZrNb films is strongly dependent on t. That is, the coercivity is abruptly reduced to be lower than 4 Oe with t increasing from 35 to 105 nm, and then gradually decreases to ∼0.2 Oe as t increases. This coercivity transition versus t is accompanied by the strong magnetization reversal when t is larger than 105 nm. The results reveal that CoZrNb amorphous films with comparatively large film thickness (>100 nm) are suitable for sensors and anti-faked materials.     

Effect of film morphology on the magnetic properties for Nd-Fe-B thin films

The microstructure and magnetic properties of Nd-Fe-B thin films with a particulate structure were investigated. The nominal thickness of the Nd-Fe-B layer (t_N) was varied from 2 to 50 nm on a (0 0 1) Mo buffer layer. The films were grown with their c-axis perpendicular to the plane, and the morphology of the film with t_N=2 nm was shown to be particulate from an atomic force microscope image. The slope of the initial magnetization curve became steeper by increasing the t_N value in the initial magnetization curve, indicating that the film morphology composed of single domain particles changed to that of multi-domain particles with growth. The film with t_N=8 nm, which had a structure consisting of a mixture of single and multiple domain particles, showed the maximum value of the coercivity measured in the direction perpendicular to the film plane (H_c) as 19.5 kOe.     

Electromechanical properties of lanthanum-doped lead hafnate titanate thin films for integrated piezoelectric MEMS applications

This paper focuses on the deposition and electromechanical characterization of lanthanum-doped lead hafnate titanate (PLHT) thin films as key material in piezoelectric microelectromechanical systems (pMEMS). PLHT (x/30/70) and PLHT(x/45/55) films with a thickness between 150 nm and 250 nm were deposited by chemical solution deposition (CSD). Thereby x varies between 0 and 10% La content. The electrical characterization shows that undoped (x=0) PLHT exhibit ferroelectric behavior similar to PZT of the same composition. La doping results in reduced ferroelectric properties and also affects the electromechanical properties. Measurements using a double beam laser interferometer yield a piezoelectric coefficient d ₃₃ of 60 pm/V, which stays constant with an increasing electric field. This leads to a linear displacement compared to undoped PLHT or conventional PZT films used for MEMS applications.     

An investigation of structural phase transformation and electrical resistivity in Ta films

In this work, we report the effect of substrate, film thickness and sputter pressure on the phase transformation and electrical resistivity in tantalum (Ta) films. The films were grown on Si(1 0 0) substrates with native oxides in place and glass substrates by varying the film thickness (t) and pressure of the working gas (p_Ar). X-ray diffraction (XRD) analysis showed that the formation of α and β phases in Ta films strongly depend on the choice of substrate, film thickness t and sputter pressure p_Ar. A stable α-phase was observed on Si(1 0 0) substrates for t ≤ 200 nm. Both α and β phases were found to grow on glass substrates at all thicknesses except t = 100 nm. All the films grown on Si(1 0 0) substrates for p_Ar ≤ 6.5 mTorr had α-phase with strong (1 1 0) texture normal to the film plane. The glass substrates promoted the formation of β-phase in all p_Ar except p_Ar = 5.5 mTorr. The resistivity ρ was observed to decrease with t, whereas ρ was increased with p_Ar on Si(1 0 0) substrates. In all films, the measured resistivity ρ was greater than the bulk resistivity. The resistivity ρ was influenced by the effects of surface roughness and grain size.     

Piezoelectric measurements on BiFeO3 single crystal by Raman scattering

The piezoelectric response of BiFeO₃ at low temperature has been investigated by Raman scattering measurements. The application of an external electric field at T=10 K induces frequency shifts of the lowest frequency mode related to the Bi-O bonds and corresponding to the soft mode of the ferroelectric transition. The piezoelectric effect is responsible for the softening of this mode via the tensile stress leading to the expansion of the crystal. The phonon deformation potential associated with the soft mode has been estimated around −200 cm⁻¹/strain units using the linear piezoelectric coefficient d₃₃=16 pm/V. It found in the range of the ones obtained for typical piezoelectrics.     

Magnetic properties of strontium hexaferrite films prepared by pulsed laser deposition

The magnetic properties of strontium hexaferrite (SrFe₁₂O₁₉) films fabricated by pulsed laser deposition on the Si(100) substrate with Pt(111) underlayer have been studied as a function of film thickness (50–700 nm). X-ray diffraction patterns confirm that the films have c-axis perpendicular orientation. The coercivities in perpendicular direction are higher than those for in-plane direction which indicates the films have perpendicular magnetic anisotropy. The coercivity was found to decrease with increasing of thickness, due to the increasing of the grain size and relaxation in lattice strain. The 200 nm thick film exhibits hexagonal shape grains of 150 nm and optimum magnetic properties of M_s=298 emu/cm³ and H_c=2540 Oe.     

Influence of annealing temperature on microstructure and optical properties of sol-gel derived tungsten oxide films

Tungsten oxide (WO₃) thin films have been extensively studied for their interesting physical properties and a variety of potential applications in electrochromic devices. In order to explore the possibility of using these in electrochromic devices, a preliminary and thorough study of the optical properties of the host materials is an important step. Based on this, the influence of annealing temperature on the structural, surface morphological, optical and electrochromic properties has been investigated in the present work. The host material, WO₃ films, has been prepared from an ethanolic acetylated peroxotungstic acid sol containing 5 wt.% oxalic acid dehydrate (OAD) by sol-gel technique. The monoclinic structure and textured nature change of the films with the temperature increasing have been investigated by X-ray diffraction analysis. The surface morphology evolution of the films has been characterized by SEM. The shift in absorption edge towards the higher wavelength region observed from optical studies may be due to the electron scattering effects and the optical band filling effect that reveals the crystallization of the film. The amorphous film shows better optical modulation (ΔT = 76.9% at λ = 610 nm), fast color-bleach kinetics (t_c ∼ 4 s and t_b ∼ 9 s) and good reversibility (Q_b/Q_c = 90%), thereby rendering it suitable for smart window applications.     

Metal organic chemical vapor deposition of ultrathin ZrO2 films on Si(1 0 0) and Si(1 1 1) studied by electron spectroscopy

The growth of ultrathin ZrO₂ films on Si(1 0 0)-(2 × 1) and Si(1 1 1)-(7 × 7) has been studied with core level photoelectron spectroscopy and X-ray absorption spectroscopy. The films were deposited sequentially by chemical vapor deposition in ultra-high vacuum using zirconium tetra-tert-butoxide as precursor. Deposition of a > 50 Å thick film leads in both cases to tetragonal ZrO₂ (t-ZrO₂), whereas significant differences are found for thinner films. On Si(1 1 1)-(7 × 7) the local structure of t-ZrO₂ is not observed until a film thickness of 51 Å is reached. On Si(1 0 0)-(2 × 1) the local geometric structure of t-ZrO₂ is formed already at a film thickness of 11 Å. The higher tendency for the formation of t-ZrO₂ on Si(1 0 0) is discussed in terms of Zr-O valence electron matching to the number of dangling bonds per surface Si atom. The Zr-O hybridization within the ZrO₂ unit depends furthermore on the chemical composition of the surrounding. The precursor t-butoxy ligands undergo efficient C-O scission on Si(1 0 0), leaving carbonaceous fragments embedded in the interfacial layer. In contrast, after small deposits on Si(1 1 1) stable t-butoxy groups are found. These are consumed upon further deposition. Stable methyl and, possibly, also hydroxyl groups are found on both surfaces within a wide film thickness range.