Investigation on the effects of PbO content and seeding layers of TiO2 and ZrO2 on the orientation and microstructure of Pb(Zr0.52Ti0.48)O3 ferroelectric films grown by reverse dip-coating method of sol–gel

Lead zirconate titanate Pb(Zr_0.52Ti_0.48)O₃ (PZT) thin films were grown on Si (100) and Pt(111)/Ti/SiO₂/Si(100) substrates by a new reverse dip-coating method of sol–gel process. The method was first proposed and applied to coat films. It has several advantages over the conventional sol–gel coating method, including: no consideration of the mechanical transmission that is difficult to manipulate with costly exact apparatus in classical dip-coating procession, convenient processing control, simplicity, low cost, less pollution, and easy fabrication films on large areas and irregular shaped devices etc. This paper studied the factors including PbO content of precursor, TiO₂ and ZrO₂ layers, which are related to raw materials of PZT precursor and influence greatly the crystal orientation of the final thin films. We find that the PZT films deposited by precursor with 20% mole excess Pb displayed strong (111) preferred orientation, with 5% mole excess Pb showed a little (100) orientation and pyrochlore phase. The precursor with 10% mole excess Pb was found prompting the PZT films phase transformation with (110) preferred orientation. In addition, the results show that the TiO₂ and ZrO₂ seeding layers had totally different effects on the preferred orientation of PZT films. The films with TiO₂ seeding layer were highly (111) oriented and exhibited better ferroelectric properties (remnant polarization Pr = 14.2 μC cm^− 2, coercive field Ec = 59.1 Kv cm^− 1) than those of the films with ZrO₂ seeding layer shown (100) orientation (Pr = 7.4 μC cm^− 2, Ec = 42.9 Kv cm^− 1).     

Oxygen partial pressure dependence of the structural properties of CdO thin films deposited by a modified reactive vacuum evaporation process

Thin films of cadmium oxide were thermally deposited on glass substrates at partial pressures of oxygen, pO₂ in the range 1.33×10⁻² to 0.133 Pa at a substrate temperature of 160 °C. Energy dispersive analysis of X-ray fluorescence (EDAX) revealed that the CdO films deposited at pO₂ value of 4.00×10⁻² Pa were nearly stoichiometric. X-ray diffractometry (XRD) confirmed the polycrystalline nature of the film structure. All the films showed an fcc structure of the NaCl-type, as the dominant phase. The films exhibited preferred orientation along the (1 1 1) diffraction plane. The texture coefficients calculated for the various planes at different oxygen partial pressures (pO₂) indicated that the maximum preferred orientation of the films occurred along the (1 1 1) plane at an oxygen partial pressure of 4.00×10⁻² Pa. This was interpreted in terms of oxygen chemisorption and desorption processes. The lattice parameters determined from the diffraction peaks were in the range 4.655–4.686 Å. The average lattice parameter a₀ found by extrapolation using the Nelson–Riley function was 4.696 Å. Both the lattice parameter and the crystallite size were found to increase with increased partial pressure of oxygen. On the other hand, the strain and dislocation density were found to decrease as the partial pressure of oxygen was raised. A maximum (80%) in the optical transmittance at λ=600 nm and minimum in the electrical resistivity (9.1×10⁻⁴ Ω cm) of the films occurred at an optimum partial pressure of oxygen of 4.00×10⁻² Pa. The results are discussed.     

Synthesis of TiO2 thin films using single molecular precursors by MOCVD method for dye-sensitized solar cells application and study on film growth mechanism

For dye-sensitized solar cells application, in this study, we have synthesized TiO₂ thin films at deposition temperature in the range of 300–750 °C by metalorganic chemical vapor deposition (MOCVD) method. Titanium(IV) isopropoxide, {TIP, Ti(OⁱPr)₄} and Bis(dimethylamido)titanium diisopropoxide, {BTDIP, (Me₂N)₂Ti(OⁱPr)₂} were used as single source precursors that contain Ti and O atoms in the same molecule, respectively. Crack-free, highly oriented TiO₂ polycrystalline thin films with anatase phase were deposited on Si(1 0 0) with TIP at temperature as low as 450 °C. XRD and TED data showed that below 500 °C, the TiO₂ thin films were dominantly grown in the [2 1 1] direction on Si(1 0 0), whereas with increasing the deposition temperature to 700 °C, the main film growth direction was changed to [2 0 0]. Above 700 °C, however, rutile phase TiO₂ thin films have only been obtained. In the case of BTDIP, on the other hand, only amorphous film was grown on Si(1 0 0) below 450 °C while a highly oriented anatase TiO₂ film in the [2 0 0] direction was obtained at 500 °C. With further increasing deposition temperatures over 600 °C, the main film growth direction shows a sequential change from rutile [1 0 1] to rutile [4 0 0], indicating a possibility of getting single crystalline TiO₂ film with rutile phase. This means that the precursor together with deposition temperature can be one of important parameters to influence film growth direction, crystallinity as well as crystal structure. To investigate the CVD mechanism of both precursors in detail, temperature dependence of growth rate was also carried out, and we then obtained different activation energy of deposition to be 77.9 and 55.4 kJ/mol for TIP and BTDIP, respectively. Also, we are tested some TiO₂ film synthesized with BTDIP precursor to apply dye-sensitized solar cell.     

Substrate effect on in-plane dielectric and microwave properties of Ba(Sn0.15Ti0.85)O3 thin films

The microstructure and in-plane dielectric and microwave properties of Barium tin titanate Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films grown on (1 0 0) LaAlO₃ and (1 0 0) MgO single-crystal substrates through sol–gel process were investigated. X-ray diffraction and field emission scanning electron microscopy were used to characterize crystal structure of phases and microstructure of the thin films, respectively. Microwave properties of the films were measured from 1 to 10 GHz by the interdigital capacitor configuration. The obvious differences in the dielectric and microwave properties are attributed to the stress in the films, which result from the lattice mismatch and difference in the thermal expansion coefficients between the film and substrates. This work clearly reveals the highly promising potential of BTS films for application in tunable microwave devices.     

The epitaxial growth of (1 1 1) oriented monocrystalline Si film based on a 4:5 Si-to-SiC atomic lattice matching interface

Due to a huge lattice mismatch of about 20% theoretically existing between SiC and Si, it is difficult for growing monocrystalline Si/SiC heterojunction to realize the light control of SiC devices. However, based on a 4:5 Si-to-SiC atomic lattice matching interface structure, the monocrystalline Si films were epitaxially prepared on the 6H-SiC (0 0 0 1) substrate by hot-wall chemical vapor deposition in our work. The film was characterized by X-ray diffraction analysis with only (1 1 1) orientation occurring. The X-ray rocking curves illustrated good symmetry with a full width at half maximum of 0.4339° omega. A 4:5 Si-to-SiC atomic matching structure of the Si/6H-SiC interface clearly observed by the transmission electron microscope revealed the essence of growing the monocrystalline Si film on the SiC substrate.     

Growth of the Zn1−xMnxO alloy by the MOCVD technique

Single-phase thin films of the diluted magnetic semiconductor Zn_1−xMn_xO have been grown by the MOCVD technique. Depositions have been done at T = 450 °C on fused silica and (0 0 0 1) sapphire substrates. Layers on silica exhibit polycrystalline structure with [0 0 1] preferential orientation while Zn_1−xMn_xO films are (0 0 0 1) epitaxially grown on c-sapphire with the epitaxy relation: 30° rotation of the Zn_1−xMn_xO [1 0 0] direction with respect to the [1 0 0] of the substrate. The manganese content varies in the (0–30%) range and is always higher in samples grown on sapphire substrates under the same conditions. Variations of a and c lattice parameters, assessed by X-ray diffraction, follow Vegard's law and attest to the incorporation of substitutional Mn²⁺ ions. Hall effect measurements show a decrease of the mobility with the incorporation of manganese in ZnO, and optical transmission results present the shift of the absorption edge towards higher energies.     

Photoluminescence properties of TiO2: Eu3 + thin films deposited on different substrates

Strong photoluminescence of Eu^3 + due to intra 4f transitions are obtained from amorphous xerogel TiO₂: Eu^3 + films prepared by sol–gel method and treated at a low temperature of 100 °C. The films are deposited on four different substrates: Si, Al, AAO (anodic alumina oxide) and porous silicon. We find that the luminescence intensity on AAO substrate increased 4 times comparing with that of Si or Al, and luminescence intensity decreases obviously on porous silicon substrate. Energy transfer mechanism from TiO₂ host to Eu^3 + is deduced through analysis of photoluminescence and photoluminescence excitation spectrum. Concentration quenching of Eu^3 + does not appear even at high atomic concentration of 7.69%.     

Structural and optical properties of BaTi2O5 thin films prepared by pulsed laser deposition at different substrate temperatures

BaTi₂O₅ thin films were prepared on MgO (1 0 0) substrates by pulsed laser deposition. The effect of substrate temperature (T_sub) on the structural and optical properties of the films, such as crystal phase, preferred orientation, crystallinity, surface morphology, optical transmittance and bandgap energy, was investigated. The preferred orientation of the films changed form (7 1 0) to (0 2 0) depending on T_sub, and the b-axis oriented BaTi₂O₅ thin film could be obtained at T_sub = 973–1023 K. The surface morphology of the films was different with changing T_sub, which showed a dense surface with an elongated granular texture at T_sub = 973–1023 K. The crystallinity and surface roughness increased at the elevated substrate temperatures. The as-deposited BaTi₂O₅ thin films were highly transparent with an optical transmittance of ～70%. The bandgap energy was found to decrease with increasing substrate temperature, from 3.76 eV for T_sub = 923 K to 3.56 eV for T_sub = 1023 K.     

RBS,XRR and optical reflectivity measurements of Ti–TiO2 thin films deposited by magnetron sputtering

Single-, bi- and tri-layered films of Ti–TiO₂ system were deposited by d.c. pulsed magnetron sputtering from metallic Ti target in an inert Ar or reactive Ar + O₂ atmosphere. The nominal thickness of each layer was 50 nm. The chemical composition and its depth profile were determined by Rutherford backscattering spectroscopy (RBS). Crystallographic structure was analysed by means of X-ray diffraction (XRD) at glancing incidence. X-ray reflectometry (XRR) was used as a complementary method for the film thickness and density evaluation. Modelling of the optical reflectivity spectra of Ti–TiO₂ thin films deposited onto Si(1 1 1) substrates provided an independent estimate of the layer thickness. The combined analysis of RBS, XRR and reflectivity spectra indicated the real thickness of each layer less than 50 nm with TiO₂ film density slightly lower than the corresponding bulk value. Scanning Electron Microscopy (SEM) cross-sectional images revealed the columnar growth of TiO₂ layers. Thickness estimated directly from SEM studies was found to be in a good agreement with the results of RBS, XRR and reflectivity spectra.     

Evidence of morphotropic phase boundary displacement in lead-free (Bi0.5Na0.5)1 − xBaxTiO3 polycrystalline thin films

Lead-free (Bi_0.5Na_0.5)_1 ? xBa_xTiO₃ (BNBT) thin films with compositions at x = 0.055, 0.100, and 0.150 were prepared by chemical solution deposition on Pt/TiO₂/SiO₂/(100)Si substrates. The dielectric behavior of the films was studied, and the ferroelectric-antiferroelectric phase transition observed was used to situate the morphotropic phase boundary (MPB) for compositions with x ~ 0.100 (BNBT-10), a value that differs from that reported for bulk materials (BNBT-5.5). Extrinsic effects derived from the thin-film configuration (e.g., microstrains, residual stresses) may be responsible for the shift of the MPB. Consequently, the dielectric permittivity is significantly improved for this composition, showing the best ferroelectric response obtained up to now for films of the BNBT system (P_r = 13.0 μC/cm², E_c = 70 kV/cm).     

Optical constants and band gap determination of Pb0.95La0.05Zr0.54Ti0.46O3 thin films using spectroscopic ellipsometry and UV–visible spectroscopy

We report the structural evolution and optical properties of lanthanum doped lead zirconate titanate (PLZT) thin films prepared on Pt/TiO₂/SiO₂/Si substrates by chemical solution deposition. X-ray diffraction demonstrates the post-deposition annealing induced crystallization for PLZT films annealed in a temperature (T_a) range of 550–750 °C. PLZT films annealed at higher temperature exhibit polycrystalline structure along with larger grain size. Optical band gap (E_g) values determined from UV–visible spectroscopy and spectroscopic ellipsometry (SE) for PLZT films were found to be in the range of 3.5–3.8 eV. E_g decreases with increasing T_a. The optical constants and their dispersion profiles for PLZT films were also determined from SE analyses. PLZT films show an index of refraction in the range of 2.46–2.50 (λ = 632.8 nm) with increase in T_a. The increase in refractive index at higher T_a is attributed to the improved packing density and crystallinity with the temperature.     

Nanocrystalline sol–gel TiO2–SnO2 coatings: Preparation,characterization and photo-catalytic performance

In this study, preparation of SnO₂ (0–30 mol% SnO₂)–TiO₂ dip-coated thin films on glazed porcelain substrates via sol–gel process has been investigated. The effects of SnO₂ on the structural, optical, and photo-catalytic properties of applied thin films have been studied by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Surface topography and surface chemical state of thin films were examined by atomic force microscopy and X-ray photoelectron spectroscopy. XRD patterns showed an increase in peak intensities of the rutile crystalline phase by increasing the SnO₂ content. The prepared Sn doped TiO₂ photo-catalyst films showed optical absorption in the visible light area exhibited excellent photo-catalytic ability for the degradation of methylene blue under visible light irradiation. Best photo-catalytic activity of Sn doped TiO₂ thin films was measured in the TiO₂–15 mol% SnO₂ sample by the Sn⁴⁺ dopants presented substitution Ti⁴⁺ into the lattice of TiO₂ increasing the surface oxygen vacancies and the surface hydroxyl groups.     

Properties of highly oriented K(Sr,Ba)2Nb5O15 thin films derived from a metal-alkoxide precursor solution

Highly oriented tungsten–bronze K(Sr,Ba)₂Nb₅O₁₅ (KSBN) thin films have been fabricated by a chemical solution deposition method. Alkoxy-derived K(Sr_0.5Ba_0.5)₂Nb₅O₁₅ (KSBN50) thin films directly crystallized into a tetragonal tungsten–bronze phase on fused silica, MgO(1 0 0), and Pt(1 0 0)/MgO(1 0 0) substrates with c-axis preferred orientation at 700 °C by optimizing the processing conditions. Ferroelectric KSBN50 thin films on Pt(1 0 0)/MgO(1 0 0) exhibited the diffuse-phase transition and typical relaxor-type dielectric behavior, which are characteristic properties along the c-axis of the tungsten–bronze (Sr,Ba)Nb₂O₆ crystal. The KSBN thin films synthesized on fused silica and MgO(1 0 0) showed high transparency over a wide wavelength range. The propagation modes of the synthesized KSBN thin films were characterized by the prism coupling method. The values of their refractive indices in TE and TM modes were 2.27 and 2.25, respectively.     

Al2O3–TiO2 thin films on glass substrate by sol–gel technique

In the present research, self-cleaning Al₂O₃–TiO₂ thin films were successfully prepared on glass substrate using a sol–gel technique for photocatalytic applications. We investigated the phase structure, microstructure, adhesion and optical properties of the coatings by using XRD, SEM, scratch tester and UV/Vis spectrophotometer. Four different solutions were prepared by changing Al/Ti molar ratios such as 0, 0.07, 0.18 and 0.73. Glass substrates were coated by solutions of Ti-alkoxide, Al-chloride, glacial acetic acid and isopropanol. The obtained gel films were dried at 300 °C for 10 min and subsequently heat-treated at 500 °C for 5 min in air. The oxide thin films were annealed at 600 °C for 60 min in air. TiO₂, Ti₃O₅, TiO, Ti₂O, α-Al₂O₃ and AlTi phases were determined in the coatings. The microstructural observations demonstrated that Al₂O₃ content improved surface morphology of the films and the thickness of film and surface defects increased in accordance with number of dipping. It was found that the critical load values of the films with 0, 0.07, 0.18 and 0.73 Al/Ti molar ratios were found to be 11, 15, 22 and 28 mN, respectively. For the optical property, the absorption band of synthesized powders shifted from the UV region to the visible region according to the increase of the amount of Al dopant. The oxide films were found to be active for photocatalytic decomposition of methylene blue.     

Growth and characterization of (Pb,La)TiO3 films with and without a special buffer layer prepared by RF magnetron sputtering

The (Pb, La)TiO₃ (PLT) ferroelectric thin films with and without a special buffer layer of PbO_x have been deposited on Pt/Ti/SiO₂/Si(100) substrates by RF magnetron sputtering technique at room temperature. The microstructure and the surface morphology of the films annealed at 600 °C for 1 h have been investigated by X-ray diffraction (XRD) and atomic force microscope (AFM). The surface roughness of the PLT thin film with a special buffer layer was 4.45 nm (5 μm × 5 μm) in comparison to that of 31.6 nm (5 μm × 5 μm) of the PLT thin film without a special buffer layer. Ferroelectric properties such as polarization hysteresis loop (P–V loop) and capacitance–voltage curve (C–V curve) of the films were investigated. The remanent polarization (P_r) and the coercive field (E_c) are 21 μC/cm² and 130 kV/cm respectively, and the pyroelectric coefficient is 2.75 × 10^− 8 C/cm² K for the PLT film with a special buffer layer. The results indicate that the (Pb, La)TiO₃ ferroelectric thin films with excellent ferroelectric properties can be deposited by RF magnetron sputtering with a special buffer layer.     

Effect of fluorine doping and SiO2 under-layer on the optical properties of TiO2 thin films

Fluorine-doped TiO₂ thin films were prepared via sol–gel method. Optical constants were obtained by fitting theoretical UV–Vis transmittance spectra to experimental ones within Forouhi–Bloomer dispersion model. The results show that doping with fluorine does not affect the absorption edge of TiO₂ films. The linear part of the absorption edge for the films on SiO₂/soda lime substrate is red shifted in comparison with that on bare soda lime and a broadened ‘tail’ up to ∼ 2.7 eV is observed. It is attributed to additional localized states within the TiO₂ band gap introduced by structural defects in titania lattice originated from TiO₂/SiO₂ interface.     

Evolution of different structural phases of TiO2 films with oxygen partial pressure and Fe doping and their electrical properties

We have studied the influence of oxygen partial pressure (OPP; 250 mTorr–1 × 10^?5 Torr) and Fe doping (2 and 4 at.%) on structural and electrical properties of TiO₂ thin films on LaAlO₃ substrates. X-ray photoelectron spectroscopy suggests that Fe is not in metal cluster form. It is found that the evolution of the three phases; anatase, rutile and brookite of TiO₂ as well as the magneli phase (Ti_nO_2n?1) strongly depends on the OPP and Fe doping concentration. All the films grown at 250 mTorr show insulating behavior, whereas films grown at 1 × 10^?2 and 1 × 10^?4 Torr reveal high temperature metallic to low temperature semiconducting transition. Interestingly, films deposited at 1 × 10^?5 Torr reveal charge ordering, which is contributed to the magneli phase of TiO₂. The present study suggests that functionality of TiO₂ thin film based devices can be tuned by properly selecting the OPP and dopant concentration.     

Two-step growth of Cd0.96Zn0.04Te/Si(111) epilayers by HWE

High quality Cd_1−xZn_xTe, x = 0.04 epilayers are successfully grown directly on hydrogen-terminated Si(111) substrates by hot wall epitaxy method. Growth conditions are optimized in order to grow single crystal films with desired composition. It is found that surface morphology of the epilayers is dramatically affected by the growth temperature and the growth rate at the early stage of the crystal growth. Applying limited high substrate temperature of T_sub = 440 °C and low growth rate of 0.04 μm/h, the crystallinity is significantly improved and for the first time a pseudomorphic 2D growth is observed notwithstanding of the large lattice mismatch. Designing a suitable two-step growth process makes it possible that Cd_1−xZn_xTe/Si(111), x = 0.04 epilayers are fabricated with good surface morphology, which could be used as lattice matched substrates for MCT and MCZT epitaxy.     

Effect of substrate on microstructure and mechanical properties of sol–gel prepared (K,Na)NbO3 thin films

Lead-free ferroelectric (K, Na)NbO₃ (KNN) thin films (～200 nm thickness) were prepared using a modified sol–gel method by mixing K and Na acetates with the Nb–tartarate complex, deposited by spin-coating method on Pt/Al₂O₃ and Pt/SiO₂/Si substrates and sintered at 650 °C. Pure perovskite phase of K_0.65Na_0.35NbO₃ in film on silicon were revealed, while film on alumina contained also small amount of secondary pyrochlore Na₂Nb₈O₂₁ phase. Homogenous microstructure of film on Si substrate was smoother with the lower roughness (～7.4 nm) and contained spherical (～50 nm) particles. The mechanical properties of films were characterized by nanoindentation. The modulus and hardness of KNN films were calculated from their composite values of film/substrate systems using discontinuous and modified Bhattacharya model, respectively. The KNN film modulus was higher on alumina substrate (91 GPa) in comparison with silicon substrate (71 GPa) and values of film hardness were the same (4.5 GPa) on both substrates.     

Structural and optical analyses of polycrystalline Zn1−xSbxSe thin films prepared by resistive heating technique

Here we report the influence of Sb doping on the structural and optical properties of Zn_1−xSb_xSe (0 ⩽ x ⩾ 0.15) thin films prepared by thermal evaporation technique on glass substrate. Various characterization techniques such as X-ray diffraction (XRD), EDS, Raman spectroscopy and spectroscopic ellipsometer are employed to assess the structural and optical properties of the deposited films. XRD analysis reveals the formation of polycrystalline cubic structure having preferred growth orientation along (1 1 1) plane without any evidence of secondary phases. Crystallographic parameters like grain size, micro strain, dislocation density, number of crystallites per unit area and texture coefficient point out the structural modification in ZnSe films with Sb inclusion. Raman analysis shows the existence of three 1LO, 2LO and 3LO phonon modes at 251, 511 and 745 cm⁻¹ in pure ZnSe while 3LO mode disappears by the incorporation of Sb atoms in ZnSe matrix. Increase in FWHM of Raman peaks with Sb concentration also indicates the change in crystalline quality of ZnSe films which is in accordance with our XRD results. Spectroscopic ellipsometry results demonstrate a decreasing trend for the optical band gap energy (from 2.61 eV to 1.81 eV) with increasing Sb content.