Table of contents

Sandwich structured (Bi, La)₄Ti₃O₁₂/Pb(Zr, Ti)O₃/(Bi, La)₄Ti₃O₁₂ thin films were fabricated on Pt/Ti/SiO₂/Si substrates, with the intention of simultaneously utilizing the advantages of both (Bi, La)₄Ti₃O₁₂ (BLT) and Pb(Zr, Ti)O₃ (PZT) thin films such as non-fatigue behaviours of BLT and good ferroelectric properties of PZT. Both BLT and PZT layers were prepared by a chemical solution deposition technique. The experiments demonstrated that the sandwich structure showed fatigue-free characteristics at least up to 10¹⁰ switching bipolar pulse cycles under 8 V and excellent retention properties. The sandwich structured thin films also exhibited well-defined hysteresis loops with a remanent polarization (2P_r) of 8.8 µC cm^-2 and a coercive field (E_c) of 47 kV cm^-1. The room-temperature dielectric constant and dissipation factor were 210 and 0.031, respectively, at a frequency of 100 kHz. These results suggest that this sandwich structure is a promising material combination for ferroelectric memory applications.

Widely tunable terahertz (THz) wave generation by optical parametric processes based on laser light scattering from the polariton mode of nonlinear crystals is reviewed. Using parametric oscillation of LiNbO₃ or MgO-doped LiNbO₃ crystal pumped by a nanosecond Q-switched Nd : YAG laser, we have realized widely tunable coherent THz-wave sources in the range between 0.7 and 3 THz, with a simple configuration. For the efficient coupling of the THz wave, a monolithic grating coupler or an Si-prism array coupler was used. We report the detailed characteristics of the oscillation and the radiation, including tunability, spatial and temporal coherency, uni-directivity, and efficiency. Further, Fourier transform limited THz-wave spectrum narrowing was achieved by introducing the injection seeding method. A linewidth of about 100 MHz (0.0033 cm^-1) was assured by the absorption spectrum measurement of low-pressure water vapour. At the same time, the THz-wave output was increased hundreds of times higher than that of a conventional generator which has no injection seeder. In addition, a wider tunability was observed using a tunable diode laser as the injection seeder. This room-temperature-operated, tabletop system promises to be a new widely tunable THz-wave source that is suited to a variety of applications.

Formation of interfacial phase in the La_0.82Ca_0.18MnO₃/La_0.18Ca_0.82MnO₃ composite and associated effects have been experimentally studied. Interfacial phase appears when the composite is sintered at temperatures above 950°C, and its composition experiences a considerable change during the sintering according to the x-ray diffraction analysis. Compared with magnetization, the transport property is relatively insensitive to the phase constitution of the composite. The appearance of the third phase and the resulted extra grain boundaries produce only a weak enhancement of magnetoresistance at the percolation point (~26% in volume fraction) of the new phase.

The grain-boundary structure and the temperature dependence of resistivity were investigated for (1-x)LCMO + xYSZ, where LCMO and YSZ represent La_2/3Ca_1/3MnO₃ and yttria-stabilized zirconia, respectively. It is shown that the YSZ doped samples for x<2% show transport and low-field magnetoresistance (LFMR) behaviours different from that for x>2%. The metal-insulator transition temperature (T_p) decreases for x<2% but increases for x>2% as x is increased. LFMR increases with x for x<2% but decreases for x>2%. The experimental observations are discussed on the basis of scanning electron microscopy (SEM) analysis, which reveals YSZ appearing at the grain boundaries of LCMO for x<2%, while presenting itself within the sample in the form of clusters for x>2%.

A method of metallic thin-film surface conductivity monitoring is proposed and realized. The method is based on the dependency of the single-frequency laser output on the position of a thin absorbing film in the linear cavity. The real and imaginary parts of the surface conductivity of cobalt and chromium thin (~10 nm) films were measured for radiation with wavelength of about 1 µm.

We present an analysis of spin injection efficiency that is of general application and relevant to a wide range of spin-electronic devices. By applying simple band structure ideas to a single interface between a metallic ferromagnet and a three-dimensional semiconductor, two conflicting figures of merit are identified - spin accumulation and polarization of injected current - and their validity to the analysis of different device types is discussed. The injected spin accumulation is smaller than the all-metal injection case by a factor (m*/m_e)^3/2(kT/E_F)^1/2e^ε_D/kT. Moreover, the injected spin current at the interface is reduced by the factor (m*/m_e)^3/2(kT/E_F)(γ_S/γ_F) e^ε_D/kT, where γ for a particular material is the square root of its momentum scattering to spin-flip scattering ratio, m* and m_e are the effective and free masses, respectively, and ε_D is the donor binding energy. These results are consequent on the boundary condition that the spin channel electrochemical potentials are continuous at the interface. By inserting an insulating tunnel barrier between the ferromagnet and the semiconductor, not only is this boundary condition removed and the spin polarization of the injected current restored to the all-metal magnitude, but also the spin accumulations in the metal and the semiconductor even have opposite signs. This implies that thin or discontinuous tunnel barriers have the worst spin injection efficiency of any configuration. We finally note that for injected spin current into metals with polarization approaching 100%, the Fermi surface is polarized to a depth which exceeds the equilibrium carrier depth by a factor l_SD/λ, hence >>1.

Photoirradiation effects on polymer light-emitting devices (PLEDs) based on poly(3-dodecylthiophene) and poly(3-octadecylthiophene) have been studied. The PLEDs used have a semitransparent Al cathode to make the photodegradation rate of the polymer film in PLED comparable to that of a naked polymer film. Upon photoirradiation in air using an incandescent lamp (150 W), the electroluminescence from the devices rapidly diminished and was weakened so as to be undetected within several minutes. The quantum efficiency of electroluminescence as well as that of photoluminescence also dropped rapidly due to photoirradiation. These results imply that the photoinduced creation of defects acting as luminescence quenchers plays a crucial role in the photodegradation of PLEDs based on poly(3-alkylthiophene) (PAT), rather than the degraded carrier-transport nature of the polymer due to scission of the main chain. On the other hand, the optical absorption due to π-π* transition of the polymer film shows little change under the same photoirradiation condition. This indicates that most π-conjugation systems of polymer main chains survived photodegradation, which suggests a long diffusion length of excitons or a strong interchain interaction in PAT.

We report on the observation of superluminescence and high spectral current tunability (181 nm) of InAs light emitting diodes operating at 3.0 µm. The source is based on an optical whispering gallery mode which is generated near the edges of the mesa and which is responsible for the superluminescence.

A hydrodynamic supersonic plasma jet near a point charge is analysed. The plasma jet is continuous in the field of a negative charge and discontinuous in the field of a positive charge. For a copper plasma with an initial concentration of n_i⁰ = 10²⁰ m^-3 and a jet velocity of 1.5×10⁴ m s^-1, the thickness of the charge separation layer is about 50D, where D is the Debye length. A shock front forms whose location depends on the point charge eZ_pc; for Z_pc = + 5 and +10, the shock is located 0.43×10^-2 and 0.71×10^-2 cm, respectively, upstream of the point charge.

Empirical data on streamer formation and propagation in near-atmospheric pressure N₂ and N₂/O₂ mixtures are presented. The data were obtained primarily from high-speed, high-sensitivity shutter and streak photography of streamers produced in a ≈13 cm gap. The streamer propagation velocity as a function of applied voltage, polarity, total pressure, and O₂ concentration are provided. In addition, information on streamer bifurcation, and streamer shape and size is included.

The breakdown process has two phases: a fast phase featuring the propagation of streamers, and much slower phase involving the heating of the gas to form the spark channel. Also we found that the addition of O₂ significantly alters the streamer characteristics and behaviour, indicating that photoionization processes play an important role.

Kr and Xe plasmas are very intensive emitters in the spectral range of 100-150 Å, which is relevant for a number of applications (for example microlithography). We present investigations of the extreme utraviolet (EUV) emission from a slow capillary discharge with Kr and Xe fillings. The emission of Kr ions (Kr VIII to Kr XI) within the range of 70-150 Å consists of three bands of lines of about 10 Å width with maxima at 116, 103 and 86 Å. Xe emission bands of about 15 Å width have their maxima at 136 and 115 Å (Xe IX to Xe XII). The radiation duration in this spectral range is ~150 ns for both elements. At the optimum conditions, the Kr emission at 103 Å is 2-3 times more intense than the Xe emission at 136 Å. The measured spectral energy of Kr radiation is about 0.1 J sr^-1 Å^-1. Experimental results are compared with numerical modellings of the dynamics and emission of the capillary discharge plasma, which enables the determination of plasma parameters and the future use of the codes as additional instruments for plasma diagnostics.

Molybdenum oxide thin films were obtained by r.f. reactive sputtering of a molybdenum target. Optical and electrical characterization was performed in order to determine the energy gap and to investigate the electrical transport mechanism. In particular, electrical characterization showed that the transport properties of our thin films were governed by a scattering mechanism associated with the inter-grain barrier height. Finally, the analysis of the electrical behaviour of these films in controlled atmosphere allowed us to test the gas sensing properties, and to show that the conduction energy increased in the presence of electroactive gases.

Microcrystallites of hexagonal silicon (Si) have been deposited during the elaboration of SiO₂ films. A good quality crystallographic structure is clearly identified in this uncommon material through microdiffraction, Raman scattering and photoluminescence techniques. Raman spectra exhibit two lines, one in crossed polarizations and the other in parallel polarizations. The temperature effect on phonon frequencies is discussed. The optical gap, estimated until now only through calculations, is found to be 1.45 eV at a temperature of -140°C. We propose that a grain silicon oxide coating plays a crucial role in the stabilization of this metastable phase of Si.

Polyaniline thin films were prepared by ac plasma polymerization technique. Capacitance, dielectric loss, dielectric constant and ac conductivity of these films were investigated in the frequency range from 100 Hz to 1 MHz and in the temperature range from 300 to 373 K. Capacitance and dielectric loss decreased with frequency and increased with temperature. This type of behaviour was found to be in good agreement with an existing model. The ac conductivity σ(ω) was found to vary as ω^s with the index s⩽1. Annealing of polyaniline thin films in high vacuum at 373 K for 1 h was found to reduce the dielectric loss. FTIR studies reveal that the aromatic ring is retained in the polyaniline thin films, which enhances the thermal stability of the polymer films.

Infrared optical properties of lead titanate (PbTiO₃) thin films grown on Pt/Ti/SiO₂/Si substrates are studied by infrared spectroscopic ellipsometry (IRSE) in the spectral range of 2.5-12.6 µm. The refractive index and extinction coefficient of PbTiO₃ thin films are determined by fitting the IRSE data using a derived classical dielectric function formula. The refractive index decreases; on the contrary, the extinction coefficient increases as the wavelength increases. The absorption coefficient is greater than 1000 cm^-1 for wavelengths over 11.7 µm, but very small and less than 100 cm^-1 for wavelengths under 4.8 µm. The effective static ionic charge has also been derived, which is smaller than it would be in a purely ionic material for PbTiO₃ thin films.

Both cryoporometry and relaxometry probe the surface-to-volume ratio of a porous material. Nuclear magnetic resonance (NMR) relaxometry uses the random motion of molecules, whereas cryoporometry uses the melting-point depression of a confined liquid. An NMR setup has been built to simultaneously perform cryoporometry and relaxometry measurements. Using materials with a well-defined pore size, i.e. silica gels, both methods are compared with the standard N₂-adsorption technique, and a good correlation is found. The methods can be used in the pore size range between 1 and 100 nm. By performing NMR relaxometry during cryoporometry, more information about the pore-size distribution can be obtained. Besides for silica gels, this is demonstrated for mortar, which has a complicated pore structure.

Contrary to some theoretical considerations, we show that electric field favours the formation of both vapour bubble in superheated liquid and liquid drop in supersaturated gas. In the latter case the field must exceed some threshold value. The work required for nucleus formation decreases significantly near the electrode. The conclusions are based on the expression for this work that we use in our analysis in the presence of electric field, and are in agreement with experimental observations.

The effective dielectric constant of a two-component material is investigated. By considering the shape distribution of the components, we derive Maxwell-Garnett-type approximation, which has been established based on the reciprocity theorem (del Rio J A et al 1998 Solid State Commun.106 183). Based on a self-consistent condition on the electric field (Bruggeman-type effective medium approximation), we obtain the well-known Lichtenecker's mixture formulae with α = 1/2. Moreover, we derive the differential effective medium approximation by taking into account the shape distribution. All these formulae are checked through the spectral density function and compared with previous bounds.

The pressure induced by vapour flow during keyhole wall evaporation in deep penetration laser welding could have a more significant role in stabilizing keyhole walls, compared to that due to ablation pressure induced by local wall evaporation. In this paper, vapour flow modelling in a blind keyhole is presented by considering both simple geometries such as straight or inclined cylinders and more realistic profiles deduced from a self-consistent equilibrium calculation. The numerical approach used in the discretization is based on the finite element method that allows us to solve a two-dimensional Navier-Stokes set of equations assuming incompressible flow. In our model, the laser beam aperture and multiple reflections effects are taken into account. A ray-tracing procedure allows one to obtain absorbed intensities and thus the local surface temperature on the keyhole walls. The deduced local gas ejection velocities on the edge of the Knudsen layer are thus the boundary conditions of the Navier-Stokes problem.

We have studied emission characteristics of a PZT ferroelectric cathode under the influence of a strong accelerating field by varying the triggering conditions. The beam current pulse reveals a rising and a steady phase. In the rising phase, the time variation of the beam current is found to be linearly dependent on both the trigger voltage and the diode voltage at the time when the current starts. In the steady phase, field emission characteristics are observed. The results show that the diode voltage is not only accelerating the emitted electrons but also assisting the electron emission from the ferroelectric cathode. An empirical model is proposed and is found to yield a reasonable beam current pulse when the electric field on the surface of the cathode is uniformly distributed. It also provides us with a new possibility to diagnose the emission process of a ferroelectric electron gun.