Development of Electronic and Electrical Materials from Indian Ilmenite

Some new complex electronic materials have been prepared by mixing bismuth oxide (Bi₂O₃) and ilmenite in different proportions by weight, using a mixed-oxide technique. Room-temperature x-ray diffraction analysis confirms the formation of a new compound with trigonal (rhombohedral) crystal structure with some secondary phases. Studies of dielectric parameters (ε _r and tan δ) of these compounds as a function of temperature at different frequencies show that they are almost temperature independent in the low-temperature range. They possess high dielectric constant and relatively small tangent loss even in the high-temperature range. Detailed studies of impedance and related parameters show that the electrical properties of these materials are strongly dependent on temperature, showing good correlation with their microstructures. The bulk resistance, evaluated from complex impedance spectra, is found to decrease with increasing temperature. Thus, these materials show negative temperature coefficient of resistance (NTCR)-type behavior similar to that of semiconductors. The same has also been observed from their I–V characteristics. Complex electric modulus analysis indicates the possibility of a hopping conduction mechanism in these systems with nonexponential-type conductivity relaxation. The nature of the variation of the direct-current (dc) conductivity with temperature confirms the Arrhenius behavior of these materials. The alternating-current (ac) conductivity spectra show a typical signature of an ionic conducting system, and are found to obey Jonscher’s universal power law.     

Open Tip Glass Microelectrodes: Conduction Through the Wall at the Tip

A theoretical and experimental study of the electrical resistance and the tip potential of open tip glass microelectrodes is presented. The main physico-chemica1l factors considered are the diffusion process of ions through the open tip, the ionic conduction in the bulk solutions, and the ionic conduction in the electrochemical double layers at the glass-electrolyte interface. In order to take into account possible conduction through the glass at the tip, a small part of the microtip is considered to be highly hydrated and its length is considered as an upper limit for the extent of hydration. Its longitudinal conduction is assumed to be much greater than in the remaining truncated cone. This lumped model, like a previous one which assumed a uniform distributed hydration, can be used to explain the electrical properties of open tip glass microelectrodes. It is concluded that end effects at the tip of a glass microelectrode are important and could explain in part their electrical properties.     

On the Influence of Diffusion, Double Layer, and Glass Conduction on the Electrical Resistance of Open Tip Glass Microelectrodes

A theoretical study of the electrical resistance of open tip glass microelectrodes based on the main physicochemical phenomena occurring at the microtip is described. An analytical expression for the total resistance of a microelectrode is derived from a model which assumes an electrically insulated glass wail. The model takes into account the diffusion process through the open tip, the ionic conduction in the bulk solutions, and the ionic conduction in the electrochemical double layers. These latter elements embody to some extent the conduction in a hydrated glass layer at the glass surface. A detailed study of the ionic flow through the microtip is also presented. The validity of this model is supported by experimental results presented at the end of this paper. Comparisons between these experimental results and their theoretical predictions are shown to be of electrophysiological interest. Practical comments involving an improved utilization of glass microelectrodes are also discussed throughout this paper.     

New Insights into Tunable Volatility of Ionic Materials through Counter‐Ion Control

Recent development in the field of small molecular materials has led to great advances in the performance of vacuum‐evaporated organic light‐emitting diodes. However, as a significant class of phosphorescent emitters, ionic transition metal complexes are seldom sublimable due to the inherent ionic nature and low vapor pressure, restricting their applications in state‐of‐the‐art devices fabricated by vacuum evaporation deposition. Here a facile, feasible and versatile strategy is shown to tune the volatility of ionic transition metal complexes through counter‐ion control. By introducing counter‐ions with large steric hindrance and well‐dispersed charges, a series of evaporable ionic iridium complexes are developed, and efficient vapor‐processed devices with a high brightness, small efficiency roll‐off, and polychromic emission ranging from deep‐blue to red‐orange are achieved. Our findings unlock the utilization of ionic functional materials in vacuum‐evaporated devices, and may open new doors for modern electronic materials technology.     

Dielectric and Impedance Spectroscopy of Barium Orthoniobate Ceramic

Barium orthoniobate (Ba₃Nb₂O₈), a derivative of the perovskite family, was prepared using a high-temperature solid-state reaction technique (calcination temperature = 1425°C and sintering temperature = 1450°C for 4 h). Preliminary x-ray structural analysis with room-temperature x-ray diffraction data confirmed the formation of a single-phase compound with hexagonal crystal structure. Study of the microstructure of a gold-coated pellet by scanning electron microscopy (SEM) showed that the sample has well-defined grains that are distributed uniformly throughout the surface of the sample. Detailed studies showed that the dielectric parameters (ε _r and tan δ) of the compound at three different frequencies (10 kHz, 100 kHz, and 1000 kHz) are almost constant in the low-temperature region (from room temperature to about 200°C). An anomaly in the relative permittivity (ε _r) (～357°C) suggests the possible existence of a ferroelectric–paraelectric phase transition of diffuse type in the material. Detailed studies of impedance and related parameters show that the electrical properties of the material are strongly dependent on temperature, showing good correlation with its microstructure. The bulk resistance (evaluated from impedance studies) is found to decrease with increasing temperature. This shows that the material has negative temperature coefficient of resistance (NTCR), similar to that of semiconductors. Studies of electric modulus indicate the presence of a hopping conduction mechanism in the system with nonexponential-type conductivity relaxation. The nature of the variation of the direct-current (dc) conductivity with temperature confirms the Arrhenius and NTCR behavior in the material. The alternating-current (ac) conductivity spectra show a typical signature of an ionic conducting system and are found to obey Jonscher’s universal power law.     

Atmospheric adsorption effects in hot-wire chemical-vapor-deposition microcrystalline silicon films with different electrode configurations

Hot-wire chemical-vapor-disposition (CVD) thin silicon films are studied by means of dark conductivity, FTIR, hydrogen evolution, and SEM surface characterization. Three types of metastability are observed: (1) long term irreversible degradation due to oxidization processes on the film surface, (2) reversible degradation determined by uncontrolled water and/or oxygen adsorption, and (3) a fast field-switching effect in the film bulk. We propose that this effect is associated with the morphology changes during film growth and an electrical field induced by adsorbed atmospheric components on the film surface. It is found that metastable processes close to the film surface are stronger than in the bulk.     

Bake stability of CdTe and ZnS on HgCdTe: An x-ray photoelectron spectroscopy study

Mercury cadmium telluride (Hg_1?xCd_xTe or MCT) has been commonly used in devices for infrared (IR) detection. For the optimum performance of the device, a compatible surface-passivation technology that provides long-term stability is required. Using x-ray photoelectron spectroscopy (XPS), the present study examines the effects on Hg_0.8Cd_0.2Te passivated with CdTe and ZnS undergoing baking in vacuum at temperatures typically used for dewar bakeout. Spectra recorded as a function of depth in both cases clearly show out-diffusion of Hg from the substrate toward the surface, even before the bakeout. On baking in vacuum, dramatic changes are observed in the ZnS/MCT case with complete loss of Hg from the sample up to the tested depth of more than 1,000 Å. Compositions of the HgCdTe matrix, formed after Hg out-diffusion, before and after the bakeout are also calculated at selected depths (from 250 Å to 700 Å), which is vital information from a device point of view, as it affects the bandgap of this narrow-band semiconductor.     

Resistive Switching in Bulk Silver Nanowire–Polystyrene Composites

Traditionally, bulk nanocomposites of electrically conducting particles and insulating polymers have been categorized as either insulating or conducting when the nanoparticle concentration is below or above the percolation threshold, respectively. Meanwhile, thin‐film polymer nanocomposites can exhibit resistive switching behavior appropriate for digital memory applications. Here, we present the first report of reversible resistive switching in bulk, glassy polymer nanocomposites. At compositions close to the electrical percolation threshold measured at low voltage, silver nanowire‐polystyrene nanocomposites demonstrate reversible resistive switching with increasing voltage at room temperature. Nanocomposites with compositions outside of this range exhibit either irreversible switching, or no switching at all. We propose that resistive switching in these materials is the result of the field‐induced formation of silver filaments that bridge adjacent nanowire clusters, extending the percolation network and decreasing the sample’s bulk resistivity. These findings break from the usual dichotomy of insulating or conducting properties in polymer nanocomposites and could inspire new devices that capitalize on this responsive behavior in these versatile materials.     

The influence of addition elements on the early resistance changes observed during electromigration testing of Al metal lines

This paper provides a critical review on early resistance changes observed during electromigration testing of Al, AlSi and AlSiCu metal lines. At present, high resolution in situ electrical resistance measurements are widely accepted as a valuable tool for the study of electromigration. It will be shown however that the results of these measurements should be interpreted with care. It will indeed be shown that, particularly for Si and/or Cu alloyed metallizations, an early resistance change measurement (during electromigration) can contain information that has no link with the damage induced by the electromigration process. A number of disturbing factors will be identified, which are all induced by temperature driven processes. The first type of disturbance is well known: the immediate change of the measured resistance with temperature steps and fluctuations (thermometer effect). The second type of disturbance is not so widely recognised. It is induced by time dependent changes that are observed over an extended period of time, following a preceding temperature step. Two types of disturbing contributions to resistance changes of this second type are identified, which will be denoted as irreversible changes and reversible changes. The irreversible resistance changes are usually observed during the first annealing of the metal line. The reversible changes are typically detected at the start of an electromigration measurement, when the current stress is switched on. It is shown that both the reversible and irreversible changes are caused by precipitation/dissolution reactions of addition elements. It is also shown that the often observed parabolic initial resistance increase that is detected at the start of electromigration experiments should be attributed to the time dependent, reversible dissolution of the addition element(s). Comparable experiments, executed however at a much reduced current level so that no Joule heating takes place, and hence no reversible processes are initiated, show that the kinetics of the purely electron-wind induced resistance changes are completely different: instead of a parabolic initial increase, an incubation time can be observed during the first stage of the measurement.     

Bistable resistive switching of a sputter-deposited Cr-doped SrZrO/sub 3/ memory film

Sputter-deposited Cr-doped SrZrO/sub 3/-based metal-insulator-metal structures exhibited bistable resistive reversible switching as observed under bias voltage and voltage pulse. The ratio of resistance of the two leakage states (high-H, low-L) was about five orders of magnitude. The conduction of the L-state satisfied Frenkel-Poole emission and that of the H-state followed ohmic mechanism, causing the resistance ratio to decrease with increasing bias voltage. The transition time of H- to L-state was five orders of magnitude higher than that of L- to H-state. The transition from H- to L-state was the restricted part for reversible switching operation. The difference in transition time of the two states should be related to the respective conduction mechanisms.     

Effect of the Structure and Valence State on the Properties of VO2 Thin Films

VO2 thin films with good switching properties were prepared by controlling the annealing time and the annealing temperature in a vacuum system.The structural,optical and electrical properties of the samples were cahracterized by using XRD,XPS,UV-VIS and electrical measurements.The witching parameters of VO2 thin film were investigated too.The results indicate that before and after phase transition the resistance of VO2 thin films changes aobut three orders of magnitude,the variation of film transmittance of 40% has been carried out with the absorptivity switching velocity of about 0.2607/min at 900 nm.The structural property of samples has been improved but the phase-transition properties have been decreased by increasing the annealing time and annealing temperature.The valence of V ions and the structure of samples have great effect on phase transition properties of VO2 thin films.Discussion on the effecs of annealing time and annealing temperature on the phase-transition temperature and hysteresis width shows that the best reasonable annealing tiem and annealing temperatre can be achieved.     

Enhanced Ionic Conductivity in Nanostructured,Heavily Doped Ceria Ceramics

The electrical properties of nanostructured, heavily yttria‐ or samaria‐doped ceria ceramics are studied as a function of grain size using electrochemical impedance spectroscopy (EIS). A remarkable enhancement in the total ionic conductivity of about one order of magnitude is found in nanostructured samples, compared with the intrinsic bulk conductivity of conventional microcrystalline ceramics. This effect is attributed to the predominance of grain‐boundary conduction in the nanostructured materials, coupled with an increase in the grain‐boundary ionic diffusivity with decreasing grain size.     

An operational definition for breakdown of thin thermal oxides of silicon

As MOS devices with thinner gate oxides are put into production, the high-field oxide breakdown definition used for process monitoring must be revised to account for noncatastrophic electrical conduction. This conduction is due to electron injection by Fowler-Nordheim tunneling into the oxide conduction band, and it can be as large as 0.1 A/cm²without causing irreversible breakdown of thin oxides. We propose that breakdown should be defined as the passage of a large current at a low value of applied electric field, after stressing of the oxide at a high field. We show that this definition represents a truly irreversible catastrophic breakdown, that it can be adapted easily for automated testing, and that it yields reliable results for breakdown of thin (less than 500 Å) gate oxides.     

Application of eddy current technique to vertical bridgman growth of CdZnTe

The eddy current technique was used to reveal the interface shape during vertical Bridgman growth of CdZnTe and to follow changes in the properties of the solidified ingot as it was cooled to room temperature after growth. Experiments were performed where partially solidified charges were decanted to show the interface shape. Eddy current analysis of the partially solidified charge indicated a concave interface shape in qualitative agreement with the shape of the decanted ingot. However, due to noise, interference, and possibly the inhomogeneous nature of the melt, only some of the eddy current signals could be analyzed empirically for interface shape; absolute values of conductivity could not be calculated from the eddy current data. Eddy current measurements made to follow changes in conductivity during post-growth cooling showed a minimum in the data during an 800°C annealing step indicative of a transition in the electrical properties of the ingot. On further cooling, a dramatic increase in the bulk conductivity of the ingot was noted. Such a transition can probably be described as a Mott transition.     

Bake stability of long-wavelength infrared HgCdTe photodiodes

The bake stability was examined for HgCdTe wafers and photodiodes with CdTe surface passivation deposited by thermal evaporation. Electrical and electrooptical measurements were performed on various long-wavelength infrared HgCdTe photodiodes prior to and after a ten-day vacuum bakeout at 80°C, similar to conditions used for preparation of tactical dewar assemblies. It was found that the bakeout process generated additional defects at the CdTe/ HgCdTe interface and degraded photodiode parameters such as zero bias impedance, dark current, and photocurrent. Annealing at 220°C under a Hg vapor pressure following the CdTe deposition suppressed the interface defect generation process during bakeout and stabilized HgCdTe photodiode performance.     

Preparation and electrical properties of inas thin films

Thin films of InAs have been deposited on mica substrates through a vacuum evaporation technique by means of controlling the substrate and source temperatures. The films with large crystal grain were found to have the best electrical properties. The maximum electron mobility of 12, 400 cm²/V·sec at room temperature was obtained in an undoped film of 3 Μm thickness at a donor concentration of 3.5 × 10¹⁶ cm⁻³. The temperature dependence of both electron mobility and resistivity of these films was slightly lower than those reported for bulk crystal type InAs.     

Low‐Temperature Superionic Conductivity in Strained Yttria‐Stabilized Zirconia

Very high lateral ionic conductivities in epitaxial cubic yttria‐stabilized zirconia (YSZ) synthesized on single‐crystal SrTiO₃ and MgO substrates by reactive direct current magnetron sputtering are reported. Superionic conductivities (i.e., ionic conductivities of the order ～1 Ω^?1cm^?1) are observed at 500 °C for 58‐nm‐thick films on MgO. The results indicate a superposition of two parallel contributions – one due to bulk conductivity and one attributable to conduction along the film–substrate interface. Interfacial effects dominate the conductivity at low temperatures (<350 °C), showing more than three orders of magnitude enhancement compared to bulk YSZ. At higher temperatures, a more bulk‐like conductivity is observed. The films have a negligible grain‐boundary network, thus ruling out grain boundaries as a pathway for ionic conduction. The observed enhancement in lateral ionic conductivity is caused by a combination of misfit dislocation density and elastic strain in the interface. These very high ionic conductivities in the temperature range 150–500 °C are of great fundamental importance but may also be technologically relevant for low‐temperature applications.     

氧化锆热障陶瓷铁弹增韧行为的原位电镜研究

氧化钇稳定氧化锆陶瓷(YSZ)具有基于铁弹性的高温增韧行为,它应用于发动机叶片表面的热障涂层表现出优异的热循环耐久性和高温韧性.然而,随着对发动机性能进一步提升的需求,当前的YSZ陶瓷难以满足更为严苛的使役环境.因此对铁弹性能微观机理开展相关的研究,并提升其高温稳定性就显得十分重要.本文采用原位透射电镜技术,获得对单晶...     

Electrical Properties of Ca<Subscript>9</Subscript>ZnLi(PO<Subscript>4</Subscript>)<Subscript>7</Subscript> Ceramics Prepared by Reactive Pressureless Sintering

Well-crystallized Ca₉ZnLi(PO₄)₇ ceramics were prepared by reactive pressureless sintering at atmospheric pressure. The single-phase Ca₉ZnLi(PO₄)₇ ceramics were confirmed by x-ray diffraction (XRD). The dielectric and electrical properties were investigated over a wide frequency range (1 Hz to 1 MHz) by complex impedance spectroscopy at different temperatures between 25°C and 600°C. A dielectric anomaly was observed at 440°C, which might be related to the phase transition. The impedance Cole–Cole plot was used to analyze the results of complex impedance measurements, revealing that the electrical properties depend strongly on frequency and temperature. Two relaxation dispersions of the electrical parameters were found and analyzed in terms of bulk and grain-boundary ionic transfer processes. The slope of the alternating-current (AC) conductivity over a wide range of temperatures provides activation energies from 0.48 eV to 1.69 eV. These results suggest that the conduction process is of the mixed type.     

Electrical conduction mechanisms in the temperature-dependent current-voltage characteristics of poly(3-hexylthiophene)/n-type Si devices

The electrical conduction mechanisms in the temperature-dependent current-voltage characteristics of poly(3-hexylthiophene) (P3HT)/n-type Si devices were investigated. Carrier transport in the low forward-voltage region at room temperature is dominated by thermionic emission (TE). However, at high voltages the current is limited by series resistance and space charge limited current (SCLC) mechanisms. It is shown that the ideality factor increases as temperature decreases, because of a TE-to-SCLC transition. In order to obtain a greater understanding of the transition from TE to SCLC behavior, few-layer black phosphorus (BP) was incorporated into P3HT (i.e., P3HT:BP) and the P3HT:BP/n-type Si device was fabricated. It is suggested that the rectifying behavior is affected by the bulk effects of the P3HT layer. However, the incorporation of BP into P3HT leads to a significant increase in the hole mobility, suppressing the bulk effects of the P3HT layer.