Two-level growth of potassium niobate KNbO3 single crystals: a new growth method for ABO3-type materials

Based on our recent study on the directional growth of KNbO₃ single crystals, a new growth scheme called the two-level growth (TLG) has been developed. By using the TLG method, high quality transparent KNbO₃ single crystals can be prepared with high reproducibility. The technique includes two growth stages. In level-one growth, a dimensionally defined (shaped) KNbO₃ seed with high crystal perfection is fabricated. The target crystal with desired dimensions and an architectural structure can be obtained in the second level of the growth. A process flow chart for the TLG method has been presented and six representative types of the growths have been introduced. Generally, crystals grown by TLG have much less crystal imperfections than those obtained from the conventional way. Technological merits of the TLG scheme have been studied, indicating that this growth scheme can also be employed to other oxide perovskite ABO₃ single crystal growths.     

Synthesis,characterization and cathodoluminescence of high‐purity ZnS microbelts

ZnS microbelts with flat morphology have been synthesized through a physical vapor transportation method inside an induction furnace. The structure, morphology and cathodoluminescence properties of ZnS microbelts have been studied using various characterization methods. The width of the ZnS microbelts can be up to several micrometers and the length can reach to tens of µm. Optical measurements indicate that the ZnS microbelts show a sharp UV emission at 337 nm, a weak band at 377 nm, a strong visible luminescence band at 464 nm, and a shoulder at 665 nm. Finally, possible explanations for the optical emissions of ZnS microbelts are discussed.     

Dependence of the order and crack density of polystyrene opals on volume fraction,humidity and temperature

We have fabricated polystyrene opals by vertical deposition with colloidal spheres of 419 nm in diameter. Different parameters such as the concentration, temperature and relative humidity have been systematically varied in order to study the dependence of the crystalline quality of the opals on these parameters. The opals have been optically and structurally characterized, paying particular attention to the size and distribution of the domains for each fabrication condition. We have noticed a dependence of the size of the domains on the thickness which corroborates a previous study. From these results we can conclude that the characterization of the homogeneity of the thickness of the opals can be done just by using microscopy. We also report a dependence of the order of the opals on relative humidity and a selective adhesion of the opals to the substrate depending on concentration and surface chemistry. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structural Perfection of ZnSe Crystals Grown by Travelling Heater Method

ZnSe crystals have been grown by the travelling heater method (THM) with PbCl₂ as solvent. Our investigations have shown that ZnSe crystals could be obtained with high crystalline perfection referred to a full width at the half maximum (FWHM) down to 20 arcsec of double crystal rocking curves measured at chemically polished (100)-oriented surfaces. Solvent growth techniques like THM should have a remarkable potential for ZnSe bulk growth if the capture of solute as inclusion and precipitation can be avoided.     

The role of surface and interface structure in crystal growth

Crystal growth occurs at the interface of a crystal and its growth medium. Due to the abrupt termination at the surface, at the interface the properties of the crystal will typically deviate from the bulk and this can affect the growth behaviour. Also the properties of the growth medium at the interface will typically differ from the bulk. In growth from solution, for example, the liquid will show ordering induced by the crystal surface or have a different composition. Here techniques to study such growth interfaces will be discussed together with examples of the effect that the properties of the interface can have on the growth.     

The migration of gas-filled brine inclusions in rock salt under a temperature gradient

In natural halite crystals two sorts of inclusions will move if there exists a temperature gradient. The direction of moving brine inclusions is orientated towards the heat source. Brine inclusions which additionally contain more than 10% gas move towards the cold end of a specimen. This model is well known but detailed information about the migration mechanism of these so-called SORBY inclusions have been lacking in literature so far. In this paper it will be demonstrated that SORBY inclusions having a diameter of up to 20 micrometers can move undisturbed through the crystal. SORBY inclusions with a diameter of more than 20 micrometers are fixed and form droplets with their tips directed towards the cold side of the specimen. Later when the tips have grown the inclusions will emit a new daughter inclusion which can move through the crystal. The velocity of migration depends on the generation of daughters. Extended kinds of SORBY inclusions are stable for a long time. However, these can decay into many individuals which will move through the crystal like the others.     

Non-display bio-optic applications of liquid crystals

Liquid crystal (LC) devices have been largely developed for LC displays and in the last decade for optical telecommunications; however, their application in the field of optical imaging is just starting to emerge. They can be miniaturised and thus have a great potential to be used with miniature optical imaging systems for biomedical applications. LC devices specifically designed for integration into biomedical optical imaging systems are reviewed. Using a combination of a few LC retarders, spectropolarimetric imaging of tissue can be obtained. LC tunable filters with high dynamic range and large throughput are designed for hyperspectral imaging and for spectral domain optical coherence tomography. The designs are based on several concepts, both using the classical stack of retarders and using more modern designs based on single layer in a Fabry–Perot cavity, in a resonant waveguide structure or in a photonic crystal.     

Glasses and glass-ceramics based on GeSe2–Sb2Se3 and halides for far infrared transmission

New glass compositions have been studied by introducing metallic halides into the well known GeSe₂–Sb₂Se₃ system. Glass forming region and thermo-mechanical properties have been investigated for each added constituent. Nanocrystals with controllable size were reproducibly and homogeneously generated in different glassy matrix with different annealing times and temperatures. We have demonstrated that a controlled crystal growth and a molding process can be combined together. The shaped glass-ceramics are still transparent and show improved mechanical properties.     

Algorithm and computer-program search for variants of polyomino packings in plane

An algorithm and a computer program that can be used in the search for all possible periodic packings of a specified set of polyominoes with a specified packing factor have been developed. The algorithm is based on the method of discrete modeling of molecular packings.     

Ferroelectric liquid crystal over silicon devices

Over the past 20 years, ferroelectric liquid crystal over silicon (FLCOS) devices have made a wide impact on applications as diverse as optical correlation and holographic projection. To cover the entire gamut of this technology would be difficult and long winded; hence, this paper describes the significant developments of FLCOS within the Engineering Department at the University of Cambridge.

The purpose of this paper is to highlight the key issues in fabricating silicon backplane spatial light modulators (SLMs) and to indicate ways in which the technology can be fabricated using cheap, low-density production and manufacturability. Three main devices have been fabricated as part of several research programmes and are documented in this paper. The fast bitplane SLM and the reconfigurable optical switches for aerospace and telecommunications systems (ROSES) SLM will form the basis of a case study to outline the overall processes involved. There is a great deal of commonality in the fabrication processes for all three devices, which indicates their potential strength and demonstrates that these processes can be made independent of the SLMs that are being assembled. What is described is a generic process that can be applied to any silicon backplane SLM on a die-by-die basis. There are hundreds of factors that can affect the yield in a manufacturing process and the purpose of a good process design procedure is to minimise these factors. One of the most important features in designing a process is fabrication experience, as so many of the lessons in this business can only be learned this way. We are working with the advantage of knowing the mistakes already made in the flat panel display industry, but we are also faced with the fact that those mistakes took many years and many millions of dollars to make.

The fabrication process developed here originates and adapts earlier processes from various groups around the world. There are also a few totally new processes that have now been adopted by others in the field. Many, such as the gluing process, are still on-going and have to be worked on more before they will fully suit ‘manufacturability’.     

Dynamics of Laponite solutions: An interpretation within the coupling model scheme

The basic predictions of the coupling model (1) have been tested on Laponite suspensions at different clay and salt concentrations. In spite of parameters obtained through a fit analysis slightly different from that predicted by the coupling model, several interesting results consistent with the model have been found. These findings can open a new route to the interpretation of the intriguing dynamical behavior of this widely studied and discussed charged colloidal system.     

AlInP benchmarks for growth of AlGaInP compounds by organometallic vapor-phase epitaxy

We have demonstrated that source material and growth system purity can be successfully evaluated by characterizing AlInP samples grown by organometallic vapor-phase epitaxy with photocurrent versus voltage measurements in an electrochemical cell. The samples can be grown and characterized in about 1 h, making them well-suited for system benchmarks. Zn-doped AlInP has the greatest sensitivity for O contamination, a recurring problem in the growth of AlGaInP alloys. High O concentrations in the Zn-doped benchmarks cause the photoresponse to fall dramatically. Secondary ion mass spectrometry data are consistent with compensation of Zn acceptor states by O donor-like trap states. Photocurrents of Si-doped and Se-doped AlInP are less sensitive to the O contamination, and the behavior of these n-type samples suggests that multiple energy states can be associated with the O impurities and dopant atoms. The benchmarks have been used to identify O contamination in trimethyl indium and phosphine and to evaluate new growth systems. Application of the benchmark to growth of GaInP solar cells with AlInP window layers is also discussed.     

Fabrication of moth-eye structure on photoresist film by laser control of reaction time constant

We have proposed the fabrication of high-contrast-moth-eye structure using continuous wave (CW) laser. Nonlinear-optical reaction time constant of the photoresist can be controlled by manipulating the exposure intensity and scanning speed of the laser. As a result, shortening the time constant in the intense beam area and lengthening that in the weak beam area compared with the scanning speed, we can limit the exposed area.     

Globally anisotropic high porosity silica aerogels

We discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain (i.e. axial compression). In addition, intrinsic anisotropy can result during growth and drying stages and, suitably controlled, it can be correlated with preferential radial shrinkage in cylindrical samples. We have performed small angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We show that global anisotropy originating from either strain or shrinkage leads to optical birefringence and that optical cross-polarization studies are a useful characterization of the uniformity of the imposed global anisotropy.     

Facile synthesis of silver‐nanobeadwire transparent conductive film by organic‐precursor paint reduction

A new facile process has been developed to synthesize silver nanowire transparent conductive films by reducing high‐aspect‐ratio, needle‐shaped silver carboxylates, prepared as the precursor, while retaining their morphology. Nanowire precursors are simply obtained by ultrasound irradiation when silver (I) oxide and carboxylic acids are used as starting materials. The aspect ratio of the precursors is increased when the ultrasound irradiation is followed by microwave irradiation. Silver nanowires can be easily obtained by reducing these precursors with hydrazine gas or irradiating them with Ar plasma for a short time. The morphology of the resulting nanowires is different from that of typical nanowires that have been reported. The nanowires have porous morphology, where small nanoparticles are connected to form nanobeadwire. The nanobeadwire is considered to increase the transparency of the film because of its rough morphology. This new method can synthesize silver nanobeadwires and fabricate the transparent conductive film simultaneously, in one‐step, so that it can significantly simplify manufacturing processes.     

Transient Current Dependent on Nematic Director Orientation

Transient currents induced by the polarity reversal of an applied DC voltage in nematic liquid crystals have been studied experimentally and theoretically. The experimental results of transient currents in the cells with various treated substrate surfaces have been presented. In the theoretical studies, the presence of adsorbed charge layers on the substrate surfaces and a spatial distribution of carrier mobility dependent on a director orientation have been assumed. By one dimensional mobility distribution of drift sheet carrier across the cell, the carrier transport process and the generation mechanism of the transient current with or without peaks can be well understood. The experimental results on transient currents have been successfully explained from the interpretation of the different adsorbed charge ratio which is the ratio of adsorbed charges to the total amount of the charges in the cell.     

Computational studies of the transient behavior of horizontal MOVPE reactors

Dynamic models of the mass transfer in horizontal reactors used for metalorganic vapor phase epitaxy (MOVPE) of compound semiconductors have been developed and used to study the duration of transients on the substrate during the growth of heterostructures. Dispersion of reactants during gas switching can be detrimental to the abruptness of the interfaces. A solution to this problem is the operation of MOVPE reactors at very high flow rates and low pressures, but this leads to low conversions of the expensive precursors. Our simulations indicate that critical Péclet numbers can be identified, beyond which no significant reduction in the duration of transients on the substrate occurs when increasing the gas velocity. The substrate always reached a new steady state much faster than the entire reactor. Symmetry of filling and purging with respect to transients was also observed. Performance diagrams connecting optimal operating conditions with reactor geometry have been constructed. Time-dependent models of the MOVPE process can help identify optimal operating conditions and reactor shapes leading to abrupt interfaces with maximum precursor utilization.     

Electron Transport in Configurational Model of Ferromagnetic Fulleride with Triple Orbital Degeneracy

Abstract

Phenomenon of ferromagnetic ordering was for a long time associated exclusively with transition metal and rare-earth compounds. Nowadays this view is challenged by growing evidence that in molecular carbon-based systems the ferromagnetic alignment of spins can be observed as well. We have developed a microscopical model of a fulleride electronic subsystem taking into account triple orbital degeneracy of energy states within the configurational-operator approach. Using the Green function method the energy spectrum of the model has been calculated. Conditions for the ferromagnetic state stabilization have been determined. Static electrical conductivity and effective masses of current carriers in the system with orbitally degenerated energy band have been obtained. In the ground state and for low temperatures at different forms of unperturbed density of electronic states the concentration dependences of transport characteristics for less-then-half-filled lower quasiparticle subband have been calculated.     

Color Gamut of Liquid Crystal Polysiloxanest

Polysiloxanes with suitable side-chains form a cholesteric liquid crystal phase that can be applied to a substrate to produce a durable coating which exhibits strong colours below the glass transition temperature. The peak reflectance is almost 0.5 and a nearly 1.0 peak reflectance can be obtained using a half-wave retardation plate coated on both sides with the material and viewed from one direction against a black background. Values below 0.5 and between 0.5 and 1.0 have been also obtained using suitable solvents and alignment techniques. Many desired spectral reflectance distributions can be then obtained because of the additive color properties of such cholesteric coatings. As a result it is possible to produce a larger colour gamut than with real surface colours such as pigments and dyes. Employing commercially available material experimental work was carried out demonstrating a greatly expanded colour gamut on the chro-maticity diagram. Also it is possible to modify the spectral reflectance distribution of coloured substrates by coating them with a material having a reflectance peak at the desired band of wavelength.     

High-rate deposition of nanocrystalline silicon using the expanding thermal plasma technique

Nanocrystalline silicon films have been deposited at very high deposition rates using the expanding thermal plasma technique and their structural properties have been analyzed. The crystallinity and crystallite size and orientation have been determined for various hydrogen-to-silane dilution ratios and it is shown that films with a crystalline fraction of 60–80% can be deposited at deposition rates within the range 1.5–3.0 nm/s. The hydrogen concentration and atomic densities in the film have been investigated by infrared spectroscopy and elastic recoil detection/Rutherford backscattering revealing underestimation of the hydrogen content by infrared spectroscopy as well as a reduced atomic film density for the nanocrystalline silicon films.