Influence of oxygen partial pressure on the structural and dielectric properties of Ba(Zr0.3Ti0.7)O3 thin films grown on (LaAlO3)0.3(Sr2AlTaO6)0.35 (001) using pulsed laser deposition

Epitaxial Ba(Zr_0.3Ti_0.7)O₃ thin films were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (001) single-crystal substrates by pulsed laser deposition at 700 °C in different oxygen partial pressures ranging from 6.7 Pa to 40.0 Pa. A strong correlation is observed between the structure and dielectric properties for the Ba(Zr_0.3Ti_0.7)O₃ thin films. The tetragonal distortion (ratio of in-plane and out-of-plane lattice parameter, a/c) of the films depends on the oxygen partial pressures. a/c varies from 0.989 at 6.7 Pa to 1.010 at 40.0 Pa, indicating the in-plain strain changes from compressive to tensile. The in-plain strain (either compressive or tensile) shifts the Curie temperature of the Ba(Zr_0.3Ti_0.7)O₃ thin films dramatically. Surface morphology and dielectric properties of Ba(Zr_0.3Ti_0.7)O₃ thin films have a strong dependence of the oxygen partial pressure. The film grown 26.7 Pa, which corresponds to a moderate in-plain tensile strain and a Curie temperature of ~ 30 °C, shows the largest relative permittivity, tunability and the best figure of merit in a broad frequency range (1 kHz-500 MHz), which may be a promising candidate for room-temperature microwave device applications.     

Electron energy-loss spectroscopy investigation of Y2O3 films on Si (001) substrate

Electron energy -loss spectroscopy has been used to investigate the interface between a Y₂O₃ film and the silicon substrate. The chemical composition of the interface layer is revealed to be nearly pure amorphous SiO₂. Yttrium silicates are found at the Y₂O₃/SiO₂ interface region. The formation of the interfacial yttrium silicates has been interpreted by the direct chemical reaction between the deposited Y₂O₃ film and the SiO₂ interface layer. The Si L₂₃ and O K edges of yttrium silicates (Y₂SiO₅ and Y₂Si₂O₇) have been calculated by the first-principle full multiple - scattering method. The theoretical results are consistent with the experimental spectra, which confirms the formation of yttrium silicates.     

Changes in Gd2O3 films grown on Si(100) as a function of nitridation temperature and Zr incorporation

We investigated Gd₂O₃ and Zr-incorporated Gd₂O₃ films grown on Si (100) as a function of nitridation temperature under an NH₃ ambient and the incorporation of Zr into the film. The formation of a silicide layer at the interfacial region was suppressed in cases of Zr-incorporated or NH₃-nitried Gd₂O₃ films. The crystalline structure was affected when zirconium, with a relatively small ionic radius, was substituted with gadolinium. When the concentration of Zr atoms in a Gd₂O₃ film reaches a specific level (Gd_0.6Zr_1.9O_4.3), phase transition occurred from cubic Gd₂O₃ to monoclinic ZrO₂. However, the monoclinic phase disappeared after nitridation at 900 °C in an NH₃ ambient. The majority of the nitrogen atoms accumulated near the interface in the films and the concentration of incorporated N increased with increasing Zr content and NH₃ annealing temperature. Moreover, nitrogen atoms bonded to Zr-silicate at the interface, in preference to ZrO₂ in the film. These incorporation characteristics of nitrogen into Zr-incorporated Gd₂O₃ film have an effect on the thermal stability and crystalline structure of a film.     

Plasma enhanced chemical vapor deposition of low dielectric constant SiOC(-H) films using MTES/O2 precursor

Low dielectric constant SiOC(-H) thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) using methyltriethoxysilane (MTES) and oxygen as precursors. The SiOC(-H) films were prepared with MTES/O₂ flow rate ratio of 80%, rf power of 700 W and the working pressure was varied from 110 to 150 mTorr. Then the films were annealed at different temperatures in an Ar ambient for 30 min in order to study their thermal stability. Film thickness and refractive index were measured by SEM and ellipsometry, respectively. Bonding characteristics of the films were investigated by Fourier transform infrared (FTIR) spectroscopy. The dielectric constant of SiOC(-H) film was evaluated by C-V measurements using Al/SiOC(-H)/p-Si structure. The dielectric constants as low as 2.4 have been obtained for the film annealed at 500 °C with the working pressure of 150 mTorr. The annealing treatment was found to reduce dielectric constant significantly due to abundant incorporation of methyl group into the Si-O network. These results demonstrated the promising characteristics of SiOC(-H) thin films deposited by using oxygen and MTES precursor.     

The synthesis of lead-free ferroelectric Bi0.5Na0.5TiO3-Bi0.5K0.5TiO3 thin films by sol-gel method

(1 − x)Bi_0.5Na_0.5TiO₃-xBi_0.5K_0.5TiO₃ [BNT-BKT-100x] thin films have been successfully deposited on Pt/Ti/SiO₂/Si substrates by a sol-gel process together with rapid thermal annealing. A morphotropic phase boundary (MPB) between Bi_0.5Na_0.5TiO₃ and Bi_0.5K_0.5TiO₃ was determined around x ∼ 0.15. Near the MPB, the film exhibits the largest grain size, the highest ε value (360) and the largest P_r value (13.8 μC/cm²). The BNT-BKT thin film system is expected to be a new and promising candidate for lead-free piezoelectric applications.     

Laser molecular beam epitaxy growth and properties of SrTiO3 thin films for microelectronic applications

Dielectric SrTiO₃ thin films were deposited on LaAlO₃ and Si substrates using laser molecular beam epitaxy. The correlations between the deposition parameters of SrTiO₃ thin films, their structural characteristics, and dielectric properties were studied. The conditions for achieving epitaxial SrTiO₃ thin films were found to be limited to deposition conditions such as deposition temperature. We show that the SrTiO₃ films with single (110) orientation can be grown directly on Si substrates. The nature of epitaxial growth and interfacial structures of the grown films were examined by various techniques, such as Laue diffraction and X-ray photoelectron spectroscopy. The SrTiO₃/Si interface was found to be epitaxially crystallized without any SiO₂ layer. Furthermore, we have measured dielectric properties of the grown SrTiO₃ multilayer suitable for tunable microwave device. A large tunability of 74.7%, comparable to that of SrTiO₃ single-crystal, was observed at cryogenic temperatures. Such STO thin films will be very promising for the development of microelectronic device applications.     

Ion-induced secondary electron emission from MgO and Y2O3 thin films

We report a detailed study of the electron emission from MgO and Y₂O₃ induced by the impact of 0.1-1 keV Ar⁺ ions. The mechanisms of ion-induced secondary electron emission from oxides are far less understood because charging of the target surface during ion irradiation prohibits the precise measurement of electron yield. For this study, targets were prepared by depositing 20 nm thick films of MgO and Y₂O₃ on the semi-conducting SnO₂ substrate, which helps in charge neutralization. Additionally, a pulsed ion beam was used to further reduce the surface charging. It was found that the electron yield of both targets increases with energy of the ion. However, at a given ion energy the electron yield of Y₂O₃ was larger than MgO. Another important result of this study is that the electron emission from these large band gap insulators did not show any threshold effect, in contrast to the metal targets. It may be due to local reduction of the band gap through electron promotion processes. In addition, a Monte Carlo program was used to calculate the yield of secondary electrons excited by projectile ions, recoiling target atoms and electron cascades, and average escape depth of the secondary electrons emitted from the MgO and Y₂O₃ thin films.     

Effect of processing on the properties of Bi3.15Nd0.85Ti3O12 thin films

Various crystallization parameters were studied during the fabrication of Bi_3.15Nd_0.85Ti₃O₁₂ (BNdT) thin films on Pt/Ti/SiO₂/Si (100) by metal organic solution decomposition method. The effect of crystallization processes, crystallization ambients on the properties of BNdT thin films such as orientation, ferroelectric properties were examined. By adopting different fabrication processes, it is possible to get both highly c-axis oriented as well as randomly oriented thin films. Highly c-axis oriented BNdT thin film showed a large remnant polarization (2P_r) of 70 μC/cm² at an applied voltage of 10 V and exhibited a fatigue free behavior unto 2 × 10⁹ switching cycles. The improved ferroelectric properties of BNdT thin films suggest their suitability for high density ferroelectric random access memory applications.     

Investigation of electronic structure of Si nanocrystals and their interface with host matrix in P-doped SiO2:Si and Al2O3:Si nanocomposites

The system of the nanoinclusions of Si in the SiO₂ and Al₂O₃ matrix (SiO₂:Si, Al₂O₃:Si) attracts great attention due to its ability of the luminescence in visible and near-IR range of spectrum. The influence of the P ion alloying on the electronic structure of nanocomposites was investigated. The P ion doping and post-annealed at T = 1000 °C (2 h) results in the enhancement of the photoluminescence (PL) peak connected with the Si nanocrystals. The electronic structure was investigated by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy losses spectroscopy (HREELS) methods. Ion surface modification and annealing forms the special nanostructure with Si nanocrystals in SiO₂ and Al₂O₃ matrix having high density of interfaces with special atomic structure and various degree of oxidation of Si atoms on the boundaries. HREELS investigations show that the P ion doping increases the probability of interband transitions in SiO₂:Si and Al₂O₃:Si composites.     

Optical properties of Sm doped Mg:LiNbO3 and Zn:LiNbO3 single crystals

Sm³⁺ doped Mg:LiNbO₃ and Zn:LiNbO₃ are grown by Czochralski method. Optical transmittance and emission spectra are measured and Judd-Ofelt theory is applied to determine phenomenological intensity parameters, oscillator strengths, radiant transition rates, total radiant lifetimes, and branching ratios. The calculations show that Judd-Ofelt parameters with the relation of Ω₄ > Ω₂ > Ω₆ exist, and ΣΩ_ξ (ξ = 2, 4 and 6) in Sm:Zn:LiNbO₃ decreases. Fluorescence spectra indicate that visible fluorescence of Sm³⁺ is made up of 570, 606, 613 and 654 nm emission bands in these crystals under 409 nm excitation.     

Effect of tartaric acid on the microstructure and photoluminescence of SrTiO3:Pr phosphors prepared by a sol–gel method

We reported in this study that a chelating agent, tartaric acid, can contribute to the formation of pure phase during the preparation of the SrTiO₃:Pr³⁺ nanoparticles by a simple sol–gel method. The effects of the tartaric acid concentration, annealing temperature and Al³⁺ ion doping concentration on the microstructure and photoluminescence of the SrTiO₃:Pr³⁺ nanoparticles were investigated in detail. It was found that even a small amount of tartaric acid could lead to the formation of pure phase SrTiO₃, while the highest red emission of Pr³⁺ ions happened at a molar ratio 2:1 of tartaric acid to the metal ions Sr²⁺ and Ti⁴⁺, an Al³⁺ ion doping concentration of 2%, and an annealing temperature of 1000 °C.     

The first bulk synthesis of ReO3-type tungsten trioxide, WO3, from nanometric precursors

A perovskite form of WO₃ has been synthesized in bulk for the first time at 0.66 GPa and 973 K with a=3.7823(4) Å [a₀=3.7719(4) Å, at ambient conditions] from nanometric powder of WO₃ with an average crystallite size of 35 nm. Data collected during tests to determine both the likelihood of retaining the structure at room temperature and the effect of high pressure on distortion have afforded analysis of thermal expansivity and compressibility of this phase. These result in V_T=53.407(5)exp(−3.9(12)×10⁻⁶(T−298)+1.91(9)×10⁻⁸(T²−298²)) Å³ and equation of state parameters of V₀=53.67(4) Å³, K₀=41.8(19) GPa with ∂K/∂P=K′=5.6(12).     

Large-scale synthesis of ear-like Si3N4 dendrites from SiO2/Fe composites and Si powders

Large-scale ear-like Si₃N₄ dendrites were prepared by the reaction of SiO₂/Fe composites and Si powders in N₂ atmosphere. The product was characterized by field emission scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The results reveal that the product mainly consists of ear-like Si₃N₄ dendrites with crystal structures, which have a length of several microns and a diameter of 100-200 nm. Nanosized ladder-like Si₃N₄ was also obtained when changing the Fe content in the SiO₂/Fe composites. The Si₃N₄ nanoladders have a length of hundreds nanometers to several microns and a width of 100-300 nm. The ear-like Si₃N₄ dendrites are formed from a two-step growth process, the formation of inner stem structures followed by the epitaxial growth of secondary branches.     

Optical spectroscopy of Dy ions in LiNbO3

A systematic spectroscopic study of Dy³⁺ doped LiNbO₃ is presented. The energy position of the Stark levels and their symmetry character is given for most of the multiplets. Luminescence of this system has been investigated in the visible and infrared. The only emitting state in this region is the metastable 4F_9/2 multiplet whose life time is temperature independent and with a value of 186 μs. Evidence about Dy³⁺ multicentres is also discussed.     

Electrical properties of LiNbO3 (electrolyte)/Cu (anode) bi-layers

In this work specific film structures of Li-Nb-O/Li/Li-Nb-O are investigated by AC Impedance Spectroscopy measurements at different temperatures. This gives the opportunity to investigate properties of the material itself and, at the same time, to consider the influence of the grain boundaries on the ionic behavior of the polycrystalline Lithium Niobate. On the other hand, LiNbO₃/Li/Cu multi-layers are studied as electrolyte/anode bi-layers and potential parts of “Li-free” microbatteries. The Li deficiency in the as deposited Li-Nb-O films is cured by forming a “sandwich” of Li-Nb-O/Li/Li-Nb-O, which after annealing becomes ionic conductor. The electrical behavior of an annealed film depends on two sources. The first is due to properties of the material itself and the second is based on the network of the grain boundaries. The average size of the grains is strongly influenced by the structure of the ohmic-contact/substrate. The electrical behavior of the electrolyte/anode interface of the “Li-free” structure LiNbO₃/Li/Cu/Au is very similar to the impedance measurements of the single LiNbO₃ single films. The whole multilayer structure, though, presents a third relaxation time which is consistent of a small resistance. This resistance is independent of temperature and it seems that is due to the metallic interface Li/Cu/Au.     

Review of proposed defect structures in LiNbO3

The problem of native defect structure in LiNbO₃ is discussed in the light of experimental data published till now. An attempt of uniform interpretation of these data has been undertaken.     

Crystallinity and surface roughness dependent photoluminescence of Y1 − xGdxVO4:Eu thin films grown on Si (100) substrate

Gd-substitution dependency on the photoluminescence in YVO₄:Eu³⁺ films grown on Si (100) substrates have been investigated by analyzing the crystalline phase and surface morphology of the films. The substitution of Gd induced not only the change of crystallinity but also the surface roughness of the films. The change of the preferred orientation in the films can be explained on the basis of the lattice mismatch between the film and Si (100) substrate. Also, the surface roughness of the films shows the similar behavior to the grain size as a function of Gd amounts. The photoluminescence (PL) intensity obtained from the Y_1 − xGd_xVO₄:Eu³⁺ films grown under optimized conditions have indicated that the PL intensity is more dependent on the surface roughness than the crystallinity of films. In particular, the incorporation of Gd into the YVO₄ lattice remarkably enhanced the intensity of PL and the highest emission intensity of Y_0.57Gd_0.40Eu_0.03VO₄ film was 3.3 times higher than that of YVO₄:Eu³⁺ film.     

Synthesis and characterization of porous silsesquioxane dielectric films

The silsesquioxane (SSQ) films with low dielectric constant have been successfully synthesized by covalently binding a thermally decomposable porogen [poly(amidoamine), PAMAM] to a host polymer (hydrogen methyl silsesquioxane, HMSQ) via a coupling agent. The decomposition behavior of the porogen as well as the thermal and dielectric properties of the host polymer heat-treated in different atmospheres have been studied and compared. The dielectric properties of the HMSQ-PAMAM porous films have been investigated as a function of porogen concentration. An average dielectric constant about 2.06 could be obtained with leakage current density on the order of 10^-7 A/cm² for a film with 20-wt.% loading of the PAMAM polymer.     

Electrical transport and magnetoresistance in La0.67Ca0.33MnO3/BaTiO3 composites

The results of the structure, electrical transport and magnetoresistance of a ferromagnet-ferroelectric-type La_0.67Ca_0.33MnO₃ (LCMO)/BaTiO₃ composites fabricated by the sol-gel method are presented. The structure and morphology characterization indicates no apparent variations in morphology and particle size in spite of the existence of BaTiO₃. The insulator-metal transition temperature (T_IM) is shifted to a higher temperature and resistivity decreases with the increase of low content BaTiO₃. Magnetoresistance (MR) of the composites is enhanced over the whole temperature range as a result of the introduction of BaTiO₃. By calculating in terms of a ferromagnetic grain coupling model, we attribute these transport properties to the enhancement of the ferromagnetic coupling between the neighboring grains, which could be explained by the increase of the carrier concentration at the grain boundary due to the introduction of BaTiO₃ and the associated magnetoelectric coupling effect.     

Grain boundary effects on the electrical and magnetic properties of Pr2/3Ba1/3MnO3 and La2/3Ca1/3MnO3 manganites

Electrical and magnetic properties of orthorhombic Pr_2/3Ba_1/3MnO₃ (PBMO) and La_2/3Ca_1/3MnO₃ (LCMO) manganites with considerable difference in variance factors (σ²) are reported here. PBMO with higher variance exhibits distinct intrinsic (due to grains) and extrinsic (due to grain boundaries) transitions in the resistivity behaviour. Extrinsic effects, however, are not observed in the lower σ² LCMO system. Low field magnetoresistivity (LFMR) data also substantiate these results. Increase in the density of states obtained through Mott's 3-D variable range hopping mechanism in the paramagnetic insulating regime indicates the suppression of magnetic domain scattering with applied magnetic field. Ferromagnetic metallic regime below the extrinsic transition in PBMO seems to emanate from the electron-magnon scattering process. LFMR at 77 K also points towards the higher canting of spins in the vicinity of grain boundary regions in PBMO compared to that in LCMO.