High temperature characteristics of ZnO-based MOS-FETs with a photochemical vapor deposition SiO2 gate dielectric

The authors report the fabrication of ZnO-based metal-oxide-semiconductor field effect transistors (MOSFETs) with a high quality SiO₂ gate dielectric by photochemical vapor deposition (photo-CVD) on a sapphire substrate. Compared with ZnO-based metal-semiconductor FETs (MESFETs), it was found that the gate leakage current was decreased to more than two orders of magnitude by inserting the photo-CVD SiO₂ gate dielectric between ZnO and gate metal. Besides, it was also found that the fabricated ZnO MOSFETs can achieve normal operation of FET, even operated at 150 °C. This could be attributed to the high quality of photo-CVD SiO₂ layer. With a 2 μm gate length, the saturated I_ds and maximum transconductance (G_m) were 61.1 mA/mm and 10.2 mS/mm for ZnO-based MOSFETs measured at room temperature, while 45.7 mA/mm and 7.67 mS/mm for that measured at 150 °C, respectively.     

Dielectric properties of Cu0.5Tl0.5Ba2Ca3Cu4O12−δ bulk superconductor

The dielectric properties of Cu_0.5Tl_0.5Ba₂Ca₃Cu₄O_12−δ superconductor samples were studied at 79 and 290 K by means of capacitance (C) and conductance (G) measurements with the test frequency (f) in the range of 10 kHz to 10 MHz. A negative capacitance (NC) phenomenon has been observed, which is most likely arising due to higher Fermi level of ceramic superconductor samples than metal electrodes. Also the NC may be due to the space charge located at the multiple insulator–superconductor interfaces (grain boundaries) in the materials. The negative dielectric constant (ε′) and loss factor (tan δ) show strong dispersion at low frequencies. The lower thermal agitation at 79 K may enhance the polarizability and hence the dielectric constants (ε′ and ε″).     

Broadband Er-Yb co-doped superfluorescent fiber source

In this paper, extensive experimental results on broad-band double cladding Er³⁺-Yb³⁺ co-doped superfluorescent fiber sources (SFSs), characterizing their output power, mean wavelength, and bandwidth (BW) stability with variations of pump power, pump wavelength, and fiber temperature, have been reported. For a 55-cm fiber, SFS power from 3.7755 (maximum BW condition of more than 80 nm) to 9.1837 mW (maximum power condition, BW is about 34 nm) has been achieved. The SFS mean wavelength dependence on pump wavelength is highly pump temperature sensitive, and can be reduced to zero in a chosen pump temperature field. The intrinsic variation of the SFS mean wavelength λ_m with fiber temperature is also measured, and a linear variation from 15 to 45 °C with a slop of −0.053 nm/°C for L_f = 100 cm and −0.04 nm/°C for L_f = 55 cm is found.     

Characterization of optical waveguide in Nd: GdVO4 by triple-energy oxygen ion implantation

We report on the optical planar waveguide formation and modal characterization in Nd: GdVO₄ crystals by triple oxygen ion implantation at energies of (2.4, 3.0, and 3.6 MeV) and fluences of (1.4, 1.4, and 3.1)  × 10¹⁴ions/cm². The prism-coupling method is used to investigate the dark-mode property at wavelength of 632.8 nm. The refractive index profiles of the waveguide are reconstructed by an effective refractive index, n_eff method. The modal analysis shows that the fields of TE modes are well restricted in the guiding region, which means the formation of nonleaky waveguide in the crystal.     

The influence of the dielectric on the properties of dielectromagnetic soft magnetic composites. Investigations with silica and silica hybrid sol–gel derived model dielectric

Dielectromagnetics made from organic–inorganic hybrid silica-coated iron powders were characterised by determination of their physical, mechanical and magnetic properties. The influence of three main factors, dielectric composition, addition level and heat treatment conditions were investigated. Results showed that these factors have significant effects on the performance of the dielectromagnetics. Increase in the organic phase content in these dielectric coatings tends to increase both the electrical resistivity and magnetic permeability of dielectromagnetics, although the strength and density are slightly impaired. Increasing the coating thickness leads to improvements in resistivity and thus reduced eddy current losses, but these are offset by reductions in density, strength and particularly magnetic permeability. A hybrid organic–inorganic coating formulation based on 40 mol% MTMS and 60 mol% TEOS precursors was found to be the optimum composition investigated. Addition levels between 0.1% and 0.3% were found to offer a good compromise between maximum permeability (μ_max>400) and minimum loss (typically <8 W/kg) for operation at 50 Hz/1 T, and the system can be optimised within this range for the desired performance.     

Study of relaxor-like behaviour of laser ablated (Pb, La)Tio3 thin films

Lanthanum modified lead titanate (PLT) thin films are one of the potential candidates for the pyroelectric and memory applications due to their excellent dielectric, pyroelectric and ferroelectric properties. PLT thin films with 25 at.% of La were deposited on platinum coated Si substrates by the laser ablation technique. The phase transition studies were done in the temperature range of −40 to 150 °C as a function of frequency and ac field. A diffused phase transition with the shifting of the maximum dielectric permittivity (?_max) to higher temperatures with the increase of frequency and dielectric dispersion with frequency at the lower temperatures were observed. The variation of the temperature corresponding to maximum dielectric constant T_m, with frequency follows the Vogel-Fulcher relation, which is the characteristic of the relaxor-like behavior of the material. With the increase of ac drive, the T_max was shifted to lower value.     

Bandwidth tuning of hybrid liquid-crystal Šolc filters based on an optical cancelling technique

We demonstrate that in addition to their role in tuning the wavelength of an N-stage hybrid liquid-crystal Šolc filter, liquid-crystal cells can also be used to vary the transmission bandwidth of such filter around any of the tuned wavelength. This bandwidth tuning is based on the variation of the number of stages by what we call here an “optical cancelling technique”. This is achieved by varying the birefringence of the liquid-crystal cells whose optical path difference switches between two particular values. We show that for a 10-stage filter and at λ_i = 1.532 μm, the calculated 3-dB bandwidth varies from 2.6 to 11.8 nm when the number of “optically-cancelled” hybrid plates increases from 0 to 8. During the tuning process, the contrast ratio remains equal to that of the equivalent classical Šolc filter.     

Post-sintering annealing effect on the diffuse phase transition in (Pb1−xBax)(Yb0.5Ta0.5)O3 ceramics

The effect of post sintering annealing on the dielectric response of (Pb_1−xBa_x)(Yb_0.5Ta_0.5)O₃ ceramics in the diffuse phase transition range (x=0.2) has been investigated. The samples are prepared by conventional solid-state reaction method. The samples are sintered at 1300 °C for 2 h and annealed at different temperatures (800, 900 and 1000 °C) for 8 h and at 800 °C for different time durations (8, 12 and 24 h). A significant change in the dielectric response has been observed in all the samples. The dielectric constant increases remarkably and the dielectric loss tangent decreases. The dielectric peaks of the annealed samples are observed to be more diffused with noticeable frequency dispersion compared to the as sintered sample.     

Structural,optical and dielectric properties of Ni substituted NdFeO3

Present study reports the structural, optical and dielectric properties of Ni substituted NdFe_1−xNi_xO₃ (0 ≤ x ≤ 0.5) compounds prepared through the ceramic method. X-ray diffraction (XRD) confirmed an orthorhombic crystal structure of all the samples. Both unit cell volume and grain size were found to decrease with an increase in Ni concentration. Morphological study by Scanning electron microscope (SEM) shows less porosity with Ni substitution in present system. From UV–vis spectroscopy, the optical band gap was found to increase with Ni doping. This observed behavior was explained on the basis of reduction in crystallite size, unit cell volume and its impact on the crystal field potential of the system after Ni substitution. The dielectric properties (?′ and tanδ) as a function of frequency or temperature, and the ac electrical conductivity (σ_ac) as a function of frequency have been studied. Hopping of charge carriers between Fe²⁺ → Fe³⁺ ions and Ni²⁺ → Ni³⁺ ions are held responsible for both electrical and dielectric dispersion in the system. Wide optical band gap and a very high dielectric constant of these materials promote them to be a suitable candidate for memory based devices in electronic industry.     

Design of a wavelength demultiplexer based on a parabolically tapered multimode interference coupler

A compact wavelength demultiplexer is designed for the operation at 1.30 and 1.55 μm wavelengths using the three-dimensional semi-vectorial beam propagation method. The parabolically tapered multimode interference (MMI) coupler based on the deep-etched SiO₂/SiON rib waveguide is introduced into the present demultiplexer for reducing the length. The numerical results show that a MMI section of 330.0 μm in length, which is only 76% length of a straight MMI coupler, is achieved with the contrasts of 42.3 and 39.2 dB in quasi-TE mode, and 38.4 and 37.8 dB in quasi-TM mode at wavelengths 1.30 and 1.55 μm, respectively and the insertion losses below 0.2 dB. The modified finite difference scheme is applied to approximate the resulting equations along the transverse directions, in which the discontinuities of the derivatives of magnetic field components H_y and H_x along the vertical and horizontal interfaces, respectively, are involved.     

Enhanced photo-response of thin-film structures with nanocrystals

Thin-film multilayers with dielectric and semiconductor nanolayers of 200-10 nm thicknesses have been deposited by thermal evaporation onto irradiation-resistive substrates using pure crystals as evaporated targets. Some multilayers were γ-irradiated in air at room temperature with dose of 83 kGy. X-ray diffraction and microscopy studies reveal that the multilayers consist of nanometer-sized crystals with cubic structure and defined size. Film structures were oriented along the (1 1 1) plane. Absorption spectra of non-irradiated LiF nanocrystals of 100 nm size and those of initial crystals give evidence of metal colloids presence. Photoluminescence spectra of γ-irradiated nanostructures with various LiF content show the enhancement of F₃⁺-colour centres excitation in the region of metal colloids absorption and the increase is observed between emission intensities of F₃⁺ and F₂ centers with respect to initial crystals γ-coloured in identical conditions. Emission intensities of both centers under excitation in the M band correlate with LiF content. These effects, which are related to high-quality nanocrystals, but at the same time depend strongly on the defect content, especially as far as their 1-2 ps nonlinearities are concerned, could depend on nanocrystal purity and metal excess collection in their boundaries regions.     

Influence of porous morphology on optical dispersion properties of template free mesoporous titanium dioxide (TiO2) films

This paper focuses the influence of porous morphology on the microstructure and optical properties of TiO₂ films prepared by different sol concentration and calcination temperatures. Mesoporous TiO₂ thin films were prepared on the glass substrates by sol-gel dip coating technique using titanium (IV) isopropoxide. Porous morphology of the films can be regulated by chemical kinetics and is studied by scanning electron microscopy. The optical dispersion parameters such as refractive index (n), oscillator energy (E_d), and particle co-ordination number (N_c) of the mesoporous TiO₂ films were studied using Swanepoel and Wemple-DiDomenico single oscillator models. The higher precursor concentration (0.06 M), films exhibit high porosity and refractive index, which are modified under calcination treatment. Calcinated films of low metal precursor concentration (0.03 M) possess higher particle co-ordination number (N_c = 5.05) than that of 0.06 M films (N_c = 4.90) due to calcination at 400 °C. The lattice dielectric constant (E_∞) of mesoporous TiO₂ films was determined by using Spintzer model. Urbach energy of the mesoporous films has been estimated for both concentration and the analysis revealed the strong dependence of Urbach energy on porous morphology. The influence of porous morphology on the optical dispersion properties also has been explained briefly in this paper.     

Room temperature fabrication Oxide TFT with Y2O3 as a gate oxide and Mo contact

In this investigation, an operating voltage as low as 5 V has been achieved for Oxide TFT with Y₂O₃ as a gate oxide and a-IGZO as an active layer. The OTFT has been fabricated at room temperature using RF sputter. The mobility and threshold voltages are 11.3 cm²/V s and 3.4 V for the device with W/L = 0.8, respectively. The annealing at 400 °C in N₂ containing 5% H₂ ambient has been utilized to improve the electrical performance of TFT. The on-off current which is determined by gate dielectric has been observed to be 10⁴. It has also been observed that the dielectric properties of gate oxide deteriorate on annealing. The dielectric constant of Y₂O₃ is observed in the range between 5.1 and 5.4 measured on various devices.     

Multifractal analysis of ITO thin films prepared by electron beam deposition method

In this work, we developed the multifractality and its formalism to investigate the surface topographies of ITO thin films prepared by electron beam deposition method for various annealing temperatures from their atomic force microscopy (AFM) images. Multifractal analysis shows that the spectrum width, Δα (Δα = α_max − α_min), of the multifractal spectra, f(α), can be used to characterize the surface roughness of the ITO films quantitatively. Also, it is found that the f(α) shapes of the as-deposited and annealed films remained left hooked (that is Δf = f(α_min) − f(α_max) > 0), and falls within the range 0.149-0.677 depending upon the annealing temperatures.     

Spectroscopic ellipsometry study of the dielectric response of Au-In and Ag-Sn thin-film couples

Optical properties and phase composition of In-Au and Sn-Ag ultra-thin films grown by sequential evaporating and co-depositing of metals in a vacuum were investigated combining X-ray diffraction and spectroscopic ellipsometry methods. The atomic concentration ratios of bilayer and co-deposited samples were the same, i.e. In(Sn):Au(Ag) = 1:2. The XRD patterns indicated creation of AuIn, AuIn₂, Au₃In₂, Au₉In₄ and Ag₃Sn intermetallic compounds at room temperature. The effective complex dielectric functions of the composite layers, , were determined from ellipsometric quantities Ψ and Δ measured in a photon energy range of 0.6-6.5 eV. The free-carrier parameters (unscreened plasma frequency and free-carrier damping) and optical resistivity were evaluated using a semiclassical Drude-Lorentz model of the effective dielectric function. There was noticed a distinct influence of phase composition and surface morphology on the optical constants and conductivity of the samples: ρ_op changed from approximately 15 μΩ cm to 37 μΩ cm for Ag-Sn structures, composed of β-Sn and Ag₃Sn phases, and from 21 μΩ cm to 83 μΩ cm for Au-In multiphase system. Lower resistivity demonstrated diffusive layers formed after deposition of an In(Sn) thin film on the noble metal underlayer.     

Structural, electrical and piezoelectric properties of LiNbO3 thin films for surface acoustic wave resonators applications

In this work, 0.30 μm thick LiNbO₃ layers have been deposited by sputtering on nanocrystalline diamond/Si and platinised Si substrates. The films were then analyzed in terms of their structural and optical properties. Crystalline orientations along the (0 1 2), (1 0 4) and (1 1 0) axes have been detected after thermal treatment at 500 °C in air. The films were near-stoichiometric and did not reveal strong losses or diffusion in lithium during deposition or after thermal annealing. Pronounced decrease of the roughness on top of the LiNbO₃ layer and at the interface between LiNbO₃ and diamond was also observed after annealing, compared to the bare nanocrystalline diamond on Si substrate. Furthermore, ellipsometry analysis showed a better density and a reduced thickness of the surface layer after post-deposition annealing. The dielectric constant and losses have been measured to 50 and less than 3.5%, respectively, for metal/insulator/metal structures with 0.30 μm thick LiNbO₃ layer. The piezoelectric coefficient d₃₃ was found to be 7.1 pm/V. Finally, we succeeded in switching local domain under various positive and negative voltages.     

Effect of modifying a methyl siloxane-based dielectric by a polymer thin film for pentacene thin-film transistors

We report on the fabrication of pentacene thin-film transistors (TFTs) utilizing a spun methyl siloxane-based spin-on-glass (SOG) dielectric and show that these devices can give a similar electrical performance as achieved by using pentacene TFTs with a silicon dioxide (SiO₂) dielectric. To improve the electrical performance of pentacene TFTs with the SOG dielectric, we employed a hybrid dielectric of an SOG/cross-linked poly-4-vinylphenol (PVP) polymer. The PVP film was deposited onto the spun SOG dielectric prior to pentacene evaporation, resulting in an improvement of the saturation field effect mobility (μ_sat) from 0.01 cm²/(V s) to 0.76 cm²/(V s). The good surface morphology and the matching surface energy of the SOG dielectric that was modified with the polymer thin film allow the optimized growth of crystalline pentacene domains whose nuclei are embedded in an amorphous phase.     

Defect structure and dielectric properties of Bi-based pyrochlores probed by positron annihilation

Positron annihilation lifetime (PAL) and Doppler broadening (DB) techniques have been performed to identify structural defects of the bismuth based pyrochlore systems with generic formula (Bi_1.5Zn_0.5)(Zn_0.5−x/3Ti_xNb_1.5−2x/3)O₇ (x = 0, 0.25, 0.5,1.0, 1.5). We found that all studied compounds contain substantial amount of the lattice vacancy defects, the variation of the annihilation lifetime suggests that the defects structure undergoes significant changes. The complex defects could be produced with increasing content of Ti, resulting in a drop in the intensity I₂ in the Ti-rich sample. At 1 MHz their dielectric constant (?′) varies from 150 for Ti-poor system to 210 for Ti-rich system and loss tangent (tan δ) remains rather low level. The high dielectric constant response of the BZTN ceramics is attributed to loosening state of cations located in the center of octahedral, so favor off-center displacement. The occurrence of complex defects help to enhance the dielectric constant.     

Effect of sintering temperature on structure and nonlinear dielectric properties of Ba0.6Sr0.4TiO3 ceramics prepared by the citrate method

Ba_0.6Sr_0.4TiO₃ ceramics were prepared by a citrate precursor method. The structure and nonlinear dielectric properties of the resulting ceramics were investigated within the sintering temperature range 1200-1300 °C. Adopting fine Ba_0.6Sr_0.4TiO₃ powder derived from the citrate method was confirmed to be effective in reducing the sintering temperatures required for densification. The ceramic specimens sintered at 1230-1280 °C presented relative densities of around 95%. A significant influence of sintering temperature on the microstructure and nonlinear dielectric properties was detected. The discrepancy in nonlinear dielectric behavior among the specimens sintered at different temperatures was qualitatively interpreted in terms of the dielectric response of polar micro-regions under bias electric field. The specimens sintered at 1230 and 1250 °C attained superior nonlinear dielectric properties, showing relatively low dielectric losses (tan δ) of 0.24% and 0.22% at 10 kHz together with comparatively large figure of merits (FOM) of 121 and 142 at 10 kHz and 20 kV/cm, respectively.     

Electrical properties of SrBi2(Nb0.5Ta0.5)2O9 ceramics

Aurivillius SrBi₂(Nb_0.5Ta_0.5)₂O₉ (SBNT 50/50) ceramics were prepared using the conventional solid-state reaction method. Scanning electron microscopy was applied to investigate the grain structure. The XRD studies revealed an orthorhombic structure in the SBNT 50/50 with lattice parameters a=5.522 Å, b=5.511 Å and c=25.114 Å. The dielectric properties were determined by impedance spectroscopy measurements. A strong low frequency dielectric dispersion was found to exist in this material. Its occurrence was ascribed to the presence of ionized space charge carriers such as oxygen vacancies. The dielectric relaxation was defined on the basis of an equivalent circuit. The temperature dependence of various electrical properties was determined and discussed. The thermal activation energy for the grain electric conductivity was lower in the high temperature region (T>303.6 °C, E_a−ht=0.47 eV) and higher in the low temperature region (T<303.6 °C, E_a−lt=1.18 eV).