Structure, dielectric, ferroelectric, and optical properties of (1 − x)Ba(Zr0.2Ti0.8)O3 − x(Ba0.7Ca0.3)TiO3 thin films prepared by sol-gel method

We report high dielectric tunabilities of (1 − x)Ba(Zr_0.2Ti_0.8)O₃ − x(Ba_0.7Ca_0.3)TiO₃ (BZT-xBCT) (x = 0.15, 0.30, 0.40, 0.45, 0.50, and 0.55) thin films prepared by a sol-gel method. The films show a pure perovskite structure with random orientation. They have moderate dielectric constant ranging from 350 to 500 and low dielectric loss near 3.0% at 1 kHz with 0 V bias at room temperature. The dielectric tunability of the BZT-0.55BCT thin films is up to 65% at 400 kV/cm and 100 kHz. The films exhibit a high optical transmission in the range of 420 nm-1500 nm. Their optical band gap energies are about 3.90 eV.     

Synthesis of epitaxial BaZrO3 thin films by chemical solution deposition

This work reports on the low temperature preparation and characterization of BaZrO₃ (BZO) epitaxial thin films by chemical solution deposition (CSD). The X-ray θ-2θ scan and φ-scan measurements have demonstrated that the BZO films exhibit cube-on-cube epitaxy on (100) MgO substrates, with the full width at half maximum (FWHM) for the ω-scan and φ-scan of 0.35° and 0.46°, respectively. The SEM and AFM analyses revealed that the morphology of the films is strongly correlated with annealing temperature. The root mean square roughness for the film annealed at 600 °C is 3.63 nm, while for the film grown at 1000 °C is 5.25 nm.     

Structural and optical properties of heteroepitaxial beta Ga2O3 films grown on MgO (100) substrates

Gallium oxide (Ga₂O₃) films were deposited on MgO (100) substrates by metalorganic vapor phase epitaxy. Structure analyses showed that the films deposited at 550-700 °C were epitaxial β-Ga₂O₃ films with an out of plane relationship of β-Ga₂O₃(100)||MgO(100). The film deposited at 650 °C showed the best crystallinity and the microstructure of the film was investigated by high resolution transmission electron microscopy. A theoretical model of the growth mechanism was proposed and the in-plane epitaxial relationship was given to be β-Ga₂O₃[001]||MgO<011>. A four-domain structure inside the epitaxial film was clarified. The β-Ga₂O₃ film deposited at 650 °C showed an absolute average transmittance of 95.9% in the ultraviolet and visible range, which had an optical band gap of 4.87 eV.     

Structural, dielectric and optical properties of Ba(Ti, Zr)O3 thin films prepared by chemical solution deposition

Ba(Ti_0.95Zr_0.05)O₃ (BTZ) thin films grown on Pt/Ti/SiO₂/Si(100) substrates were prepared by chemical solution deposition. The structure and surface morphology of BTZ thin films has been studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). At 100 kHz, the dielectric constant and dissipation factor of the BTZ film are 121 and 0.016, respectively. The ellipsometric spectrum of the BTZ thin film annealed at 730 °C was measured in the range of wavelength from 355 to 1700 nm. Assuming a five-layer model (air/surface roughness layer/BTZ/interface layer/Pt) for the BTZ thin films on platinized silicon substrates, the optical constant spectra (refractive index n and the extinction coefficient k) of the BTZ thin films were obtained.     

Effects of Mn doping on structural and dielectric properties of sol-gel-derived (Ba0.835Ca0.165)(Zr0.09Ti0.91)O3 thin films

We reported the effects of Mn doping on the structure and dielectric properties of (Ba_0.835Ca_0.165)(Zr_0.09Ti_0.91)O₃ (BCZT) thin films prepared by sol-gel method. The (Ba_0.835Ca_0.165)Mn_x(Zr_0.09Ti_0.91)_1 − xO₃ (x = 0, 0.002, 0.005, and 0.01) thin films exhibited a pure pseudo-cubic perovskite structure with random orientation. Scanning electron microscopy and atomic force microscopy observation showed that increasing Mn-doping amount caused a decrease in particle size and a cluster of the particles, while the film surface remained smooth and crack-free. Compared with the undoped film, Mn doped BCZT thin films exhibited smaller dielectric constant and lower dielectric loss. The figure of merit reached the maximum value of 16.7 with a tunability of 53.6% for the film with 0.5 mol % Mn doping, when a bias electric field of 400 kV/cm was applied at 100 kHz. The results indicated that the Mn doped BCZT thin films are suitable for tunable microwave device applications.     

Enhanced ferroelectric properties of Mg doped (Ba,Sr)TiO3 thick films grown on (001) SrTiO3 substrates

Highly (001)-oriented 1 mol% Mg doped (Ba_0.67,Sr_0.33)TiO₃ (BST) films with a thickness of 1.25 μm were grown on (110) SrRuO₃/(001) SrTiO₃ substrates by pulsed laser deposition. X-ray diffraction measurements reveal that the BST thick films have very high crystalline quality, and have a distorted lattice with a large tetragonality a/c = 1.012. The BST thick films have a remanent polarization (P_r) value as large as 10.1 μC/cm² and a coercive electric field (E_c) value of 65.0 kV/cm. The films possess dielectric constant and loss values of ε_r = 385.36 and tgδ = 0.038 at 1 kHz and room temperature. The leakage currents of the films are on the order of 10^− 5 A/cm² at ± 150 kV/cm. The mechanism for enhancing electric properties of the Mg doped BST films was also discussed.     

Effect of process parameters and post deposition annealing on the optical, structural and microwave dielectric properties of RF magnetron sputtered (Ba0.5,Sr0.5)TiO3 thin films

Thin films of (Ba_0.5,Sr_0.5)TiO₃ (BST5) in the thickness range 400-800 nm have been deposited by RF magnetron sputtering on to quartz substrates at ambient temperature. All the properties investigated, i.e. structure, microstructure, optical and microwave dielectric, show a critical dependence on the processing and post processing parameters. The surface morphology as studied by atomic force microscopy reveals ultra fine grains in the case of as deposited films and coarse grain morphology on annealing. The as-deposited films are X-ray amorphous and exhibit refractive index in the range 1.9-2.04 with an optical absorption edge value between 3.8 and 4.2 eV and a maximum dielectric constant of 35 at 12 GHz. The dispersion in refractive index fits into the single effective oscillator model while the variation in the optical parameters with oxygen percentage in the sputtering gas can be explained on the basis of packing fraction changes. On annealing the films at 900 °C they crystallize in to the perovskite structure accompanied by a decrease in optical band gap, increase in refractive index and increase in the microwave dielectric constant. At 12 GHz the highest dielectric constant achieved in the annealed films is 175. It is demonstrated that with increasing oxygen-mixing percentage in the sputtering gas, the microwave dielectric loss decreases while the dielectric constant increases.     

Dielectric and ferroelectric properties of Ba(Sn0.15Ti0.85)O3 thin films grown by a sol-gel process

Ferroelectric Ba(Sn_0.15Ti_0.85)O₃ (BTS) thin films were deposited on LaNiO₃-coated silicon substrates via a sol-gel process. Films showed a strong (1 0 0) preferred orientation depending upon annealing temperature and concentration of the precursor solution. The dependence of dielectric and ferroelectric properties on film orientation has been studied. The leakage current density of thin films at 100 kV/cm was 7 × 10⁻⁷ A/cm² and 5 × 10⁻⁵ A/cm² and their capacitor tunability was 54 and 25% at an applied field of 200 kV/cm (measurement frequency of 1 MHz) for the thin films deposited with 0.1 and 0.4 M spin-on solution, respectively. This work clearly reveals the highly promising potential of BTS compared with BST films for application in tunable microwave devices.     

Structural and optical analysis of nanocrystalline thin films of mixed rare earth oxides (Y1-xErx)2O3

Nanocrystalline thin films of mixed rare earth oxides (Y_1-xEr_x)₂O₃(0.1 ≤ x ≤ 1) were deposited by electron beam evaporation technique on polished fused silica glass at different substrate temperatures (200-500 °C). The effect of the substrate temperature as well as the mixing parameter (x) on the structural and optical properties of these films has been investigated by using X-ray diffraction (XRD), energy dispersive x-ray analysis and optical spectrophotometry. XRD investigation shows that mixed rare earth oxides film (Y_1-xEr_x)₂O₃ grown at lower substrate temperature (T_s ≤ 300 °C) are poorly crystalline, whereas films grown at higher substrate temperatures (T_s ≥ 400 °C) tend to have better crystallinity. Furthermore, the mixing parameter (x) was found to stabilize the cubic phase over the entire of 0.1 ≤ x ≤ 1. The crystallite size of the films was found to vary in the range from 25 to 39 nm. Optical band gap of the films was deterimined by analysis of the absoprtion coeffifcient. For films deposited at different substrate temperatures direct and indirect transitions occur with energies varied from 5.29 to 5.94 eV and from 4.23 to 4.51 eV, respectively. However, films of different composition x, give optical band gap varied from 6.14 to 5.86 eV for direct transition and from 5.23 to 4.22 eV for indirect transitions. Consequently, one may conclude that it is possible to tune the energy band gap by relative fraction of constituent oxides. It was found that optical constants increase with increasing the substrate temperature. Nevertheless, the values of n and k decrease with increasing the mixing parameter, x.     

Effect of thermal annealing time on optical and structural properties of TeO2 thin films

TeO₂ thin films were deposited on quartz substrates by rf reactive sputtering technique from a Te metal target. The obtained samples were annealed in an argon atmosphere at 450 °C for different annealing times up to 90 min. X-ray diffraction studies revealed that the as-grown samples were amorphous and there was no appreciable change in structure for a short annealing time. Thin films became polycrystalline with the tetragonal (α-phase) structure of tellurium dioxide crystal with the increase of the thermal annealing time. The refractive index and optical energy gap of the films were calculated by modelling transmittance spectra. The optical energy gap decreased continuously from 3.83 eV to 3.71 eV with increasing thermal annealing time.     

Composition controlling of KNbO3 thin films prepared by pulsed laser deposition

The composition of KNbO₃ thin films prepared by pulsed laser deposition is crucially influenced by the deposition configuration. In the present study, the composition of KNbO₃ thin films grown on Si (100) substrates by pulsed laser ablation was tried to be controlled by adjusting the target-substrate distance and the oblique angle of substrate from the plume axial direction. It was found that the K deficiency in the films can be effectively avoided by setting the substrate at an appropriate oblique angle from the plume axial direction. The stoichiometric KNbO₃ thin films with a K/Nb molar ratio of 0.98 were successfully obtained, where the substrates were set at an oblique angle of 3-12° from the plume axis while the target-substrate distance was kept at 40 mm.     

Large electrocaloric effect of highly (100)-oriented 0.68PbMg1/3Nb2/3O3-0.32PbTiO3 thin films with a Pb(Zr0.3Ti0.7)O3/PbOx buffer layer

0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ (PMN-PT) thin films with a lead zirconate titanate Pb(Zr_0.3Ti_0.7)O₃ (PZT)/PbO_x buffer layer were deposited on Pt/TiO₂/SiO₂/Si substrates by radio frequency magnetron sputtering technique, and pure perovskite crystalline phase with highly (100)-preferred orientation was formed in the ferroelectric films. We found that the highly (100)-oriented thin films possess not only excellent dielectric and ferroelectric properties but also a large electrocaloric effect (13.4 K at 15 V, i.e., 0.89 K/V) which is attributed to the large electric field-induced polarization and entropy change during the ferroelectric-paraelectric phase transition. The experimental results indicate that the use of PZT/PbO_x buffer layer can induce the crystal orientation and phase purity of the PMN-PT thin films, and consequently enhance their electrical properties.     

Physical properties of (La,Sr)Ti(O,N)3 thin films grown by pulsed laser deposition

Thin films of La_xSr_1−xTiO_3+x/2 (x = 0, 0.25, 0.5, 0.75, 1) were grown by laser ablation on two different kinds of substrates (SrTiO₃ (STO) and MgO) and were subsequently ammonolysed to yield the corresponding oxynitrides La_xSr_1−xTi(O,N)₃. For both substrates all films were found to grow epitaxially to the (1 0 0) direction of the cubic perovskite structure, except for x = 0.5 that grew parallel to the (1 1 0) direction. For some of the films TiN was detected as impurity phase. Scanning electron microscopy revealed that the films are dense and homogeneous with thicknesses around 350 nm. Atomic force microscopy showed that the surface roughness of the films varied between 4.2 and 14.1 nm. The employed substrate had a strong influence on the electrical properties. Films grown on STO exhibited a metallic behaviour, in contrast to the films grown on MgO, which were insulating.     

Line defects, planar defects and voids in SrTiO3 films grown on MgO by pulsed laser and pulsed laser interval deposition

Two and three dimensional growth of SrTiO₃ films on (001) MgO substrate was achieved by pulsed laser interval and pulsed laser deposition respectively. The growth mode was monitored by in-situ reflection high energy electron diffraction. Interval deposition forces layer-by-layer growth of materials even with such a large lattice misfit (~ 7.9%). A titanium dioxide buffer monolayer was deposited to allow the film to wet the substrate to encourage two dimensional growth of the strontium titanate. A variety of defects was investigated using transmission electron microscopy and high resolution scanning transmission electron microscopy. Misfit dislocations, steps at the interface, Ti-rich defects and regularly shaped nano-holes connected by anti-phase boundaries were found to be the dominant defects in these films grown layer by layer. The edges of the nano-holes were mainly along [010] and [100] for a [001] growth direction. The large strain between the two crystal systems with large lattice mismatch leads to in-plane tensile stress during the layer-by-layer growth. The stress is relieved in part by the holes. The films with a three dimensional growth mode possess a uniform surface with dislocations as the dominant defects. The individual densities of the various defects, including a Ti-rich phase and misfit and threading dislocations, are determined by the kinetics of the deposition method.     

Structural and optical properties of CdS thin films grown by chemical bath deposition

Cubic cadmium sulphide (CdS) thin films with (111) preferential orientation were prepared by chemical bath deposition (CBD) technique, using the reaction between NH₄OH, CdSO₄ and CS(NH₂)₂. The films properties have been investigated as a function of bath temperature and deposition time. Structural properties of the obtained films were studied by X-ray diffraction analysis. The structural parameters such as crystallite size have been evaluated. The transmission spectra, recorded in the UV visible range reveal a relatively high transmission coefficient (70%) in the obtained films. The transmittance data analysis indicates that the optical band gap is closely related to the deposition conditions, a direct band gap ranging from 2.0 eV to 2.34 eV was deduced. The electrical characterization shows that CdS films' dark conductivities can be controlled either by the deposition time or the bath temperature.     

Enhanced ferroelectric properties in Bi-doped K0.5Na0.5NbO3 thin films prepared by pulsed laser deposition

Lead-free ferroelectric Bi-doped K_0.5Na_0.5NbO₃ (KNN) and undoped KNN films were prepared by pulsed laser deposition. Bi-doped film exhibited good crystallization and improved ferroelectric properties. The dielectric constant and loss tangent were 1038 and 0.138 at 1 kHz, respectively. The remanent polarization (P_r = 28 μC/cm²) of Bi-doped film was about four times larger than that of the undoped film, which attributed to the decrease of oxygen vacancies concentration. The coercive field (E_c = 24 kV/cm) of Bi-doped films was half of the undoped film. The conduction mechanisms of Bi-doped film determined to be Space-Charge-Limited-Current and Poole–Frenkle emission at low and high electric field, respectively.     

Enhanced ferro- and piezoelectric properties in (100)-textured Nb-doped Pb(ZrxTi1 − x)O3 films with compositions at morphotropic phase boundary

To develop high-performance piezoelectric films on conventional Pt(111)/Ti/SiO₂/Si(100) substrates, sol-gel-derived highly [100]-textured Nb-doped Pb(Zr_xTi_1 − x)O₃ (PNZT) thin films with different Zr/Ti ratios ranging from 20/80 to 80/20 were prepared and characterized. The phase structure, ferroelectric and piezoelectric properties of the PZNT films were investigated as a function of Zr/Ti ratios, and it was confirmed that there was distinct phase transition of the PNZT system from tetragonal to rhombohedral when the Zr/Ti ratio varied across the morphotropic phase boundary (MPB). The Nb-doped PZT films showed enhanced remanent polarization but reduced coercive field, whose best values reached 75 μC/cm² and 82 kV/cm, respectively at the composition close to MPB. In addition, the [100]-textured PNZT film at MPB also shows a high piezoelectric coefficient up to 161 pm/V. All these properties are superior to those for undoped PZT films.     

VO2 thin films deposited on silicon substrates from V2O5 target: Limits in optical switching properties and modeling

Thermochromic VO₂ thin films presenting a phase change at T_c = 68 °C and having variable thickness were deposited on silicon substrates (Si-001) by radio-frequency sputtering. These thin films were obtained from optimized reduction of low cost V₂O₅ targets. Depending on deposition conditions, a non-thermochromic metastable VO₂ phase might also be obtained. The thermochromic thin films were characterized by X-ray diffraction, atomic force microscopy, ellipsometry techniques, Fourier transform infrared spectrometry and optical emissivity analyses. In the wavelength range 0.3 to 25 μm, the optical transmittance of the thermochromic films exhibited a large variation between 25 and 100 °C due to the phase transition at T_c: the contrast in transmittance (difference between the transmittance values to 25 °C and 100 °C) first increased with film thickness, then reached a maximum value. A model taking into account the optical properties of both types of VO₂ film fully justified such a maximum value. The n and k optical indexes were calculated from transmittance and reflectance spectra. A significant contrast in emissivity due to the phase transition was also observed between 25 and 100 °C.     

Thickness dependence of strain and in-plane dielectric properties of highly (001) oriented (Ba,Sr)TiO3 thin films

Highly (001) oriented (Ba,Sr)TiO₃ thin films, grown on (001) LaAlO₃ substrates by pulsed laser deposition, exhibit strong variation of strain over the thickness range of 20-800 nm. The tensile elastic residual strain reaches a minimum value at a thickness of 250 nm, while the inhomogeneous strain decreases gradually with increasing film thickness. The 250-nm-thick film has the largest in-plane dielectric constant due to a smallest tensile elastic strain in the film and the largest in-plane tunability of 40% is achieved in the thickest film.     

Engineering of luminescence from Gd2O3:Eu nanophosphors by pulsed laser deposition

Nanostructured Gd₂O₃:Eu³⁺ thin films were prepared by pulsed laser ablation technique. The dependence of structural, morphological and optical properties of these films on photoluminescence was systematically studied by varying the annealing temperature, Eu³⁺ incorporation concentration and laser fluence. The intensity of the XRD peak from (2 2 2) crystal plane was found to increase with annealing temperature in the range 973–1173 K. Films annealed at 1173 K show a preferential growth along (2 2 2) crystal plane of the cubic Gd₂O₃ and enhanced photoluminescence at 612 nm. XRD and Micro-Raman spectra and lattice strain investigations suggest that Eu³⁺ incorporation introduce a strong lattice distortion in Gd₂O₃ matrix. Morphological investigations using atomic force microscopy indicate a strong influence of the annealing process on the surface roughness and particle size. This kind of transparent thin film phosphors may promise for applications in flat-panel displays and X-ray imaging systems.