Janos Hajto

Theory of room temperature quantized resistance effects in metal-a-Si:H-metal thin film structures

Abstract A theoretical model of room temperature quantized resistance steps is described in terms of movement of a free electron Fermi surface into a quantized k -space (associated with electron confinement in real space) under the influence of applied electric field. The model is in good accordance with experimental observations such as the non-periodicity in quanta and voltage space and gives very realistic values for the parameters of electron lifetime, Fermi wavevector and geometry. Room temperature quantization is observed when the value of free electron lifetime (∼ 10 −14 s) is matched with a small size for electron confinement (∼ 7 nm or less).

Capacitance anomaly near the metal-non-metal transition in Cr-hydrogenated amorphous Si-V thin-film devices

Abstract We present experimental results that show a metal—non-metal (MNM) transition occurring in hydrogenated amorphous Si (a-Si: H) analogue memory devices as a function of temperature. The dc resistance of the devices undergoes a continuous change in the range 65–100 K from semiconductor-like behaviour to metallic behaviour, as the temperature increases. The ac conductivity, measured over the frequency range 1–3·1 × 107 Hz, shows an anomalous change as the temperature is varied over the MNM transition. Ac characteristics were modelled using multicomponent RC and RL equivalent circuits below and above the MNM transition region respectively. It is found that the capacitance increases markedly when the temperature approaches the MNM transition from the semiconductor side. Near the transition temperature this capacitance disappears, and the equivalent circuit now requires an inductive component together with a resistance which has a positive temperature coefficient of resistance equivalent to that of the ...

DC and ac measurements on metal/a-Si:H/metal room temperature quantised resistance devices

Abstract Direct-current (dc) and alternating-current (ac) conductivities of room temperature electroformed Cr/p + -a-Si:H/V thin film quantised resistance devices have been measured as a function of temperature, applied field and frequency. The quantised resistance does not change over a temperature range. This invariance is in accordance with the theoretical model suggested for high temperature quantised resistance phenomena. The onset of quantised resistance jumps is associated with the formation of an inductive component within the structure indicating a temporary formation of a metallic conduction channel under the effect of an applied electric field.

Current-induced instability in Cr-p+ a-Si:H-V thin film devices

Abstract Experimental results on the constant current stressing in hydrogenated amorphous silicon (a-Si:H) Cr–p+–V thin film devices are presented. With increasing injection of charge via either increasing bias or time, the current-voltage characteristics of devices exhibit instability, as shown by a decrease in the reverse current. This is interpreted in terms of the creation of defects in the a-Si:H. The defect generation rate. as measured by the voltage shift ΔV at a constant reverse current in the J-V curve, is found to follow a square-root time dependent law. In addition, a decrease in device conductance after stressing is also observed, which is described by a mechanism of dopant equilibrium during and after stressing.

Whole cell immobilised biosensors for toxicity assessment of a wastewater treatment plant treating phenolics-containing waste.

Wastewater treatment plants dealing with industrial wastes are often susceptible to overload of toxic influent that can partially or completely destroy treatment for extended periods. An obvious candidate for monitoring toxicity in such wastewater systems is bioluminescent bacteria. However, the natural bioluminescent bacteria can be particularly sensitive to some industrial wastes and therefore their response to normal operational conditions does not reflect the status of the microbial community responsible for treatment. Moreover, the salt dependence of the marine bioluminescent bacteria, and the temperature sensitivity of some strains, further complicate their use. Here we describe the construction of whole cell genetically modified bioluminescent biosensors and their immobilisation for use in monitoring the toxicity of a complex industrial wastewater containing phenolic materials. A hand-held luminometer was designed for laboratory or field use, and the immobilisation system designed with several things in mind: the geometry of the instrument; the need for containment of GM bacteria; the maximisation of the bioavailability of the wastewater to the biosensor. The performance of a candidate GM sensor was compared with Vibrio fischeri in liquid culture and after immobilisation in thin films of poly(vinyl alcohol) (PVA) cryogels. The biosensors were tested against pure phenol and 3-chlorophenol as a reference toxic chemical known to be much more toxic to bacteria than phenol. The biosensors were then tested with the phenolics-containing industrial wastewater. The immobilisation system proved to operate predictably with pure toxicants, and was able to discriminate toxicity of various zones within the wastewater treatment plant.

Phase diagrams and electro-optical properties of novel ferroelectric mixtures

Abstract Two binary mixtures were prepared, based on three members of the homologous series of 4-n-alkoxyphenyl esters of the 4′-n-alkoxybenzoic acids. Three structurally different chiral compounds were used to make novel ternary and four-component mixtures with chiral smectic C phase. The phase diagrams of two-component, the ternary and four-component mixtures are presented and the corresponding physical and electro-optical properties such as spontaneous polarization, tilt angle and switching time are discussed. Spontaneous polarization values up to 200nC/cm2 were found and two distinct modes of electro-optical switching, associated with different stages of helix distortion, were observed.

Field-induced anomalous changes in Cr/a-SI:H/V thin film structures.

Abstract Experimental results on the electronic properties of conditioned Cr/hydrogenated amorphous silicon (a-Si:H)/V thin film devices are presented. The devices under test were electro-formed, and had resistances in the range from several hundred Ohms to several kiloOhms. The current of conditioned devices varied non-linearly with bias at low voltages, but exhibited ‘jumps’ at a threshold voltage (Vth) (typically 2–3 V), leading to a resistance change of one to three orders of magnitude. Above Vth the current increased almost linearly with bias, and the carrier transport changed from a semiconducting behaviour to a more conducting (metallic) state. This was confirmed by a.c. characteristics of the conditioned devices, which showed a transition from a capacitive to an inductive behaviour around Vth. The threshold voltage Vth, was found to decrease with increasing temperature and disappeared at 340–350 K, but recovered when the temperature was reduced. The transition at Vth has been analysed in terms of an electrothermal mechanism. The calculated turnover temperature is approximately 346 K, close to that for the disappearance of Vth. We suggest that the observed transition could involve vanadium oxides such as VO2. The phase transition could also facilitate the underlying ‘quantisation’ effect.

Theory of room temperature quantized resistance steps in electroformed metal-a-Si:H-metal structures

Abstract Previous work by Hajto et al. [J. Non-Cryst. Solids 137/138 (1991) 499; Phil. Mag. 66 (1991) 349], Yun et al. [Appl. Phys. Lett. 63 (1993) 2493] and Jafar and Haneman [Phys. Rev. B 47 (1993) 10911] has suggested quantized electron transport at high temperature in amorphous silicon V-p+-Cr devices. A feature of transport in these devices is a sequence of steps in the resistance-voltage characteristic which seem to be of integer or half integer multiples of h 2e 2 Ω , the voltage at which such jumps occur is non-periodic. Such observations conflict with other forms of ballistic channels [J. Phys. C 60 (1998) 848] which show single jumps of h 2e 2 Ω at periodic voltages. We present a simple model that shows that this difference arises as a natural consequence of having a two terminal geometry in which the applied electric field along the channel influences the number of conducting sub-bands instead of the electric field controlling the width of the channel.

Synthesis and Optical Properties of Nonlinear Optical Polyarylates

Abstract Several polyarylates based on bisbenzylidenoketones have been synthesized by interfacial polycondensation. Two distinct types of polymers have been obtained: guest-host systems and polyarylates with NLO (nonlinear optical) side chains. The dispersion of the linear refractive index (n 0) is measured over the wavelength range from 800 to 2000 nm. The Holing formula, based on a semiclassical model of a simple harmonic oscillator, is used to calculate the NLO susceptibility X(3).

Aligned Ferroelectric Liquid Crystal/Polymer Network Devices

Abstract In this paper the preparation of fast switching ferroelectric liquid crystal/polymer network systems for display devices is reported. A composite mixture was prepared using a dye doped ferroelectric liquid crystal mixture mixed with a UV curable adhesive. The substrates of the cell do not need surface treatment and the alignment layer can be omitted. The alignment of the composite film was achieved using an external magnetic field. Liquid crystal molecules were forced to orient parallel to the direction of the external field. After UV curing of the material a planar aligned composite layer was obtained. The aligned composite films possess good electro-optic properties.