Scope of electron transport studies by thermally stimulated discharge current measurement

A logical approach to electron transport studies for barrier conduction in layered structures was adopted by thermally stimulated discharge current (TSDC) measurement. The scope and applicability of this technique to the evaluation of the thermoelectric parameters of relaxation time, detrapping energy and depolarization rates are demonstrated here. These are characterized by the controlling factors of layer resistance and the resultant thermal and voltage gradients which apply to the drift of electrons arising from both dipolar and interfacial charges. The methodologies used in this study are suitable for parametric evaluation of structured electronic devices. This revised version was published online in July 2006 with corrections to the Cover Date.     

Recent Development of Advanced Functional Polymers for Semiconductor Encapsulants of Integrated Circuit Chips and High-temperature Photoresist for Electronic Applications

A new polymer system of semiconductor devices was studied in response to the multifunctional systems evolved. A variety of functional polymers have been developed in the manufacture of semiconductor and integrated circuit (IC) packaging devices by R&D of high-temperature polymers. With the increase in integration of electronic devices and the need to reduce overall size, market needs are moving to multilevel metallization. Toray's core polymer technologies for electronic devices in the past 35 years (1961–95) are reviewed. The new technology of IC encapsulants of biphenyl type epoxy compounds is described for the new generation 16 megabits dynamic randon accessory memory (DRAM) electronic memory device, with good heat dissipation characteristics and low stress with an anti-flammability UL V-0 property of halogen-free formulation. As core functions are built into devices, packaging and mount technologies become more important. A new photosensitive high-temperature polymer stable up to 500°C with photosensitivity and high resolution has been developed. The trend toward a high degree of integration in solid-state technology requires the use of new high-temperature photosensitive insulating materials. Toray's "Photoneece" system provides such versatile polyimide pattern-generation techniques, containing a unique photosensitive polyimide precursor which can be spun or coated on the substrate. The main components of polyimide consist of poly(amic acid), a tertiary amine having methacrloyl group and a sensitizer. Through analyses of visible, fluorescence and electron spin resonance (ESR) spectroscopy, and flash photolysis and quantitative analyses, a new reaction mechanism is proposed. By photo-irradiation, the stable ion radical is formed without vinyl radical polymerization. The polymer is excited to form an excited singlet state. An anion radical of pyromellitic diamide moiety in a polymer chain is generated after intersystem crossing to an excited triplet state. The resultant relief of the photosensitive polyimide precursor, after exposure to UV light with a mask, development and cure processing, is transformed into a cyclized aromatic polyimide. The new system has higher photosensitivity and resolution and eliminates three steps in the conventional pattern-making process for integrated circuits, resulting in a significant cost reduction. The characterization of pattern generation, the conversion to polyimide patterns, and the properties of both Photoneece and the patterns are discussed. Initial photoreaction of an ionic-bonded photosensitive polyimide was studied by fluorescence, ESR and flush photolysis. A charge transfer complex between a polyamic acid (polyimide precursor) and an aromatic amine (sensitizer) was formed by UV irradiation from fluorescence measurement. Photo-induced radical was observed by ESR measurement. The photo-induced radical was an anion radical of polyamic acid from flush photolysis. From these results, a new photo-induced charge separation in an ionic bonded photosensitive polyimide film was found. Photo-induced electron transfer from an aromatic amine (sensitizer) to acid part of the polyamic acid occurs. © 1997 John Wiley & Sons, Ltd.     

Organic electronics—Silicon device design without semiconductor band theory

Before the invention of the transistor in the 1940s, semiconductors were used as detectors in radios in a device called a “cat’s whisker”. At that time their operation was completely mysterious. Only after the introduction of semiconductor band theory did it become clear that the “cat’s whisker” is a primitive example of a metal-semiconductor Schottky diode. Today organic materials are being investigated for their electronic properties. Such materials are especially attractive for lightweight, flexible, and low-cost solar cells and light emitting devices, as well as transistors and electrophotographic photoreceptors. Yet, even after 40 years of work and a large database, the physics and chemistry that determines the electronic properties of organic materials are not well understood. Practicing organic electronics is like attempting to do silicon device design without semiconductor band theory. It is the purpose of this paper to briefly summarize what is known about the electronic properties of organic materials from charge transport data. It will be shown that our understanding of the charge transport mechanism and the electronic properties of organic materials is at a rudimentary phase which is a limiting factor in applying these materials to practical devices, very similar to the “cat’s whisker” phase of inorganic semiconductor research.     

Tip-enhanced Raman spectroscopy for nanoscale strain characterization

Tip-enhanced Raman spectroscopy (TERS), which utilizes the strong localized optical field generated at the apex of a metallic tip when illuminated, has been shown to successfully probe the vibrational spectrum of today’s and tomorrow’s state-of-the-art silicon and next-generation semiconductor devices, such as quantum dots. Collecting and analyzing the vibrational spectrum not only aids in material identification but also provides insight into strain distributions in semiconductors. Here, the potential of TERS for nanoscale characterization of strain in silicon devices is reviewed. Emphasis will be placed on the key challenges of obtaining spectroscopic images of strain in actual strained silicon devices. Figure Figure Concept of Tip Enhanced Raman Spectroscopy (TERS), which utilizes the strong localized optical field generated at the apex of a metallic tip when illuminated. TERS has been demonstrated to successfully probe the vibrational spectrum of today’s and tomorrow’s state-of-the-art silicon and next generation semiconductor devices     

Advanced surface modification of indium tin oxide for improved charge injection in organic devices

A new method is described for surface modification of ITO with an electroactive organic monolayer. This procedure was done to enhance hole injection in an electronic device and involves sequential formation of a monolayer of a pi-conjugated organic semiconductor on the indium tin oxide (ITO) surface followed by doping with a strong electron acceptor. The semiconductor monolayer is covalently bound to the ITO, which ensures strong adhesion and interface stability; reduction of the hole injection barrier in these devices is accomplished by formation of a charge-transfer complex by doping within the monolayer. This gives rise to very high current densities in simple single layer devices and double layer light emitting devices compared to those with untreated ITO anodes.     

Design Features of a High Resolution Microcalorimeter EDX System

 High resolution, superconducting detectors allow energy dispersive X-ray spectrometry (EDX) with energy resolution and energy threshold far beyond the levels obtained with semiconductor detectors. These cryogenic detectors are run at temperatures of less than 100 mK and combine the excellent energy resolution of wavelength dispersive X-ray spectrometry (WDX) with the fast, energy dispersive analysis of EDX. CSP cryogenic spectrometer’s microcalorimeter type EDX cryodetectors are equipped with a mechanical cooling system that runs vibration free and allows completely automated operations on scanning electron microscopes (SEMs), field emission guns (FEGs) and transmission electron microscopes (TEMs). This detector type offers new opportunities in material analysis, especially when low excitation energies are applied or light elements are to be determined.     

Surface transfer doping of diamond: A review

Ultra-wide bandgap materials show great promise as a solution to some of the limitations of current state of the art semiconductor technology. Among these, diamond has exhibited great potential for use in high-power, high-temperature electronics, as well as sensing and quantum applications. Yet, significant challenges associated with impurity doping of the constrained diamond lattice remain a primary impediment towards the development of diamond-based electronic devices. An alternative approach, used with continued success to unlock the use of diamond for semiconductor applications, has been that of ‘surface transfer doping’ - a process by which intrinsically insulating diamond surfaces can be made semiconducting without the need for traditional impurity doping. Here, we present a review of progress in surface transfer doping of diamond, both a history and current outlook of this highly exploitable attribute.     

Sol-Gel Processed TiO2 Films for Photovoltaic Applications

The dye sensitized solar cells (DYSC) provides a technically and economically credible alternative concept to present day p-n junction photovoltaic devices. In contrast to the conventional systems where the semiconductor assumes both the task of light absorption and charge carrier transport the two functions are separated here. Light is absorbed by a sensitizer which is anchored to the surface of a wide band gap semiconductor. Charge separation takes place at the interface via photo-induced electron injection from the dye into the conduction band of the solid. Carriers are transported in the conduction band of the semiconductor to the charge collector. The present concepts evolved in the context of research on mesoporous oxide semiconductor films prepared via a sol-gel process. The use of transition metal complexes having a broad absorption band in conjunction with oxide films of nanocrstalline morphology permits to harvest a large fraction of sunlight. Nearly quantitative conversion of incident photons into electric current is achieved over a large spectral range extending over the whole visible region. Overall solar (standard AM 1.5) to electric conversion efficiencies over 10% have been reached. There are good prospects to produce these cells at lower cost than conventional devices. The lecture will present the current state of the field. We shall discuss new concepts of the dye-sensitized nanocrystalline solar cell (DYSC) including solid heterojunction variants and analyze the perspectives for the future development of the technology into the next millennium.     

Electron transfer dynamics

This paper surveys the current ‘state-of-art’ of the theoretical understanding of electron transfer dynamics in donor-acceptor systems, which provide the conceptual and technical basis for solar energy conversion via optical and optoelectronic molecular devices and for the primary charge separation in photo-synthesis.     

负性光敏聚酰亚胺材料研究进展

芳香族聚酰亚胺具有优异的绝缘性、热稳定性、介电性能、机械性能以及化学稳定性,可在微电子工业中用作绝缘和保护材料。负性光敏聚酰亚胺具有芳香族聚酰亚胺的优点,同时也具有感光性,使得微电子加工过程中减少对光刻胶的依赖,简化图案形成的工艺,有力促进了生产效率。本文综述了负性光敏聚酰亚胺的发展,重点介绍聚酰亚胺光刻胶体系的原理及近期研究进展,并对负性光敏聚酰亚胺的发展给予展望。     

Device physics of polymer light-emitting diodes

This article reviews a device model for the current and light generation of polymer light-emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devices. The transport properties of holes in PPV have been investigated with indium tin oxide (ITO)/PPV/Au hole-only devices. The hole current is dominated by bulk conduction properties of the PPV, in contrast to previous reports. As the hole current is space-charge limited, the hole mobility as a function of electric field E and temperature T can be directly determined. The hole mobility exhibits a field dependence ln(μ) ∼ ✓E as also has been observed from time-of-flight experiments in many molecularly doped polymers and amorphous glasses. For the zero-field hole mobility an activation energy of 0.48 eV is obtained. The electron conduction in PPV has been studied by using Ca/PPV/Ca electron-only devices. It appears that the electron current is strongly reduced by the presence of traps with a total density of 10¹⁸ cm⁻³. Combining the results of electron- and hole-only devices a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. It is calculated that the unbalanced electron and hole transport gives rise to a bias-dependent efficiency. By comparison with experiment it is found that the recombination process in PPV is for 95% nonradiative. Furthermore, the experiments reveal that the bimolecular recombination process is thermally activated with an identical activation energy as measured for the charge carrier mobility. This demonstrates that the recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency (photon/carrier) of a PLED is temperature independent. © 1998 John Wiley & Sons, Ltd.     

Electric-Field-Induced Alignment of Functionalized Carbon Nanotubes Inside Thermally Conductive Liquid Crystalline Polyimide Composite Films

The positive liquid crystals, 4′-heptyl-4-biphenylcarbonitrile (7CB), are used to functionalize carbon nanotubes (LC-CNT), which can be aligned in the liquid crystalline polyimide (LC-PI) matrix under an alternating electric field to fabricate the thermally conductive LC-CNT/LC-PI composite films. The efficient establishment of thermal conduction pathways in thermally conductive LC-CNT/LC-PI composite films with a low amount of LC-CNT is achieved through the oriented alignment of LC-CNT within the LC-PI matrix. When the mass fraction of LC-CNT is 15 wt %, the in-plane thermal conductivity coefficient (λ_∥) and the through-plane thermal conductivity coefficient (λ_⊥) of the LC-CNT/LC-PI composite films reach 4.02 W/(m ⋅ K) and 0.55 W/(m⋅K), which are 90.5 % and 71.9 % higher than those of the intrinsically thermally conductive LC-PI films respectively, also 28.8 % and 5.8 % higher than those of the CNT/LC-PI composite films respectively. Meanwhile, the thermally conductive LC-CNT/LC-PI composite films also possess excellent mechanical and heat resistance properties. The Young's modulus and the heat resistance index are 2.3 GPa and 297.7 °C, respectively, which are higher than the intrinsically thermally conductive LC-PI films and the thermally conductive CNT/LC-PI composite films under the same amount of CNT.     

Phenomenological model for the interpretation of impedance/admittance spectroscopy results in polymer light-emitting electrochemical cells

A phenomenological model has been developed to account for the results of impedance/admittance spectroscopy measurements from light-emitting electrochemical cells (LECs) comprising a polymer electrolyte and two different conjugated polymers used as organic semiconductor. The application of a d.c. offset bias superimposed to the a.c. modulation voltage was used to observe the transition from the behavior prior to device operation and after the formation of the electrochemical p-i-n junction. The analysis of the whole device “conductivity” as a function of the applied bias and of the frequency was used to support the assumptions considered to develop the model. The results show that the device, after the p-i-n junction formation, can be considered as composed by two highly conductive electrochemically doped (n and p) regions and a thin (few tens nanometers), insulating layer, where the electrical current is dominated by electronic charge carrier injection via tunneling through a rectangular energy barrier. Before the p-i-n junction formation, there is no doping of semiconductor material, and the device electrical properties are dominated by the intrinsic electronic charge carriers in the organic semiconductor. Results from devices made of organic semiconductors with different band gap energy and different layer thicknesses are used to corroborate the proposed model.     

Influence of γ-irradiation in the molten state on the dynamic mechanical β relaxation and on the thermally stimulated depolarization currents of polyethylene

Dynamic mechanical and thermally stimulated depolarization current data have been obtained on polyethylene, γ-irradiated in the molten state, as a function of irradiation dose. The β peak which appears in mechanical experiments has been attributed to motions of the chain backbone. The branches, like the crosslinks coming from γ-irradiation, are fixed tie points for the chains.     

Are the critical exponents for Anderson localization due to disorder well understood?

Semiclassical theory has been used, with some success, to discuss Anderson localization due to disorder. Our attention is focused on the quantum–chemical network model via a Boltzmann–like equation, and García-García’s semiclassical approach, contacted with early work of Care and March on compensated semiconductor. This work is related with the recent semiclassical treatment on the effect of disorder on the nature of electron states in the quantum–chemical network model.     

Synthesis and characterization of high-barrier polyimide containing rigid planar moieties and amide groups

A high-performance polyimide was prepared by the dipolymerization of 4,4'-diaminobenzanilide (DABA) and pyromellitic dianhydride (PMDA). Due to the introduction of rigid planar moieties and amide groups, the polyimide shows outstanding properties, such as high glass transition temperatures (435 °C), excellent thermal stability (T_d5%, 542 °C, coefficient of thermal expansion, −3.2 ppm K⁻¹), and superior mechanical properties. Most importantly, the polyimide exhibits excellent barrier properties, with oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) low to 7.9 cm³ (m² day)⁻¹ and 5.1 g (m² day)⁻¹, respectively. Wide angle X-ray diffractograms (WAXD), positron annihilation lifetime spectroscopy (PALS) and molecular dynamics simulations reveal that the excellent barrier properties are mainly attributed to the high crystallinity, high extent of in-plane crystalline orientation, and low free volume, which are resulted from the rigid planar structure and strong interchain hydrogen bonding. The high-barrier and thermally stable polyimide has an attractive potential application prospect in the fields of micro-electronics encapsulation and high grade packaging industry.     

Structural characterization and dielectric properties of the solid solutions AgPb(Sb,Bi)S3

The new solid solutions AgPbSb_1 − x Bi _x S₃ were prepared by solid state reactions. The phases were characterized by powder X-ray diffractions (XRD), scanning electron microscopy, and thermal analysis. The XRD patterns of different members (x = 0.5, 0.7, 0.8, and 1.0) are consistent with pure phases crystallizing in the cubic PbS-type structure. The electrical characterization was carried out using ac impedance spectroscopy and dc methods. The temperature dependence of the dc conductivity shows typical semiconductor Arrhenius behavior. The impedance measurements were performed in the frequency range of 0.1 Hz to 10 MHz and at the temperature range of 15 °C to 350 °C. The ac conductivity conforms to Jonscher’s universal power law. The frequency dependence of the dielectric permittivity follows the normal dielectric material behavior, and the relaxation is thermally activated. The frequency and temperature dependences of the electrical data are found to follow Summerfield scaling formalism.     

Electrochemical dehalogenation of polytrifluorochloroethylene

The electrochemical reduction of polytrifluorochloroethylene suspended in an electrolyte using organic electron transfer mediators led to surface modification of the polymer. Pyrolysis of the reduction product was accompanied by the formation of a highly porous thermally stable polycarbon material with pore sizes in a very wide range, approximately from 1 to 10⁵ nm.     

Preparation, characterization and dielectric properties of 4,4-diphenylmethane diisocyanate (MDI) based cross-linked polyimide films

Four different types of cross-linked polyimides based on 4,4-diphenylmethane diisocyanate (MDI) were prepared by the reaction of different types of conventional poly(amic acid) intermediates with MDI as a cross-linking agent. Subsequently, they were thermally imidized in order to obtain corresponding cross-linked polyimide structure. The results of FTIR-ATR showed that MDI can effectively react with carboxylic acid groups of PAA to form cross-linked polyimide films. TGA, FTIR-ATR and SEM analyses were carried out for characterization of cross-linked polyimide (CPI) films. Moreover, the electrical properties such as dielectric breakdown strength, dielectric constant, I-V characteristics and loss factor of MDI based cross-linked polyimides have been checked. In addition, some physical properties such as water uptake, adhesion, hardness and solubility properties of the films were investigated.The results showed that all CPI films have good insulating properties such as high dielectric breakdown voltage, low loss factor (tan δ), leakage density and excellent physical properties.     

Oxygen and oil barrier properties of microfibrillated cellulose films and coatings

The preparation of carboxymethylated microfibrillated cellulose (MFC) films by dispersion-casting from aqueous dispersions and by surface coating on base papers is described. The oxygen permeability of MFC films were studied at different relative humidity (RH). At low RH (0%), the MFC films showed very low oxygen permeability as compared with films prepared from plasticized starch, whey protein and arabinoxylan and values in the same range as that of conventional synthetic films, e.g., ethylene vinyl alcohol. At higher RH’s, the oxygen permeability increased exponentially, presumably due to the plasticizing and swelling of the carboxymethylated nanofibers by water molecules. The effect of moisture on the barrier and mechanical properties of the films was further studied using water vapor sorption isotherms and by humidity scans in dynamic mechanical analysis. The influences of the degree of nanofibrillation/dispersion on the microstructure and optical properties of the films were evaluated by field-emission scanning electron microscopy (FE-SEM) and light transmittance measurements, respectively. FE-SEM micrographs showed that the MFC films consisted of randomly assembled nanofibers with a thickness of 5–10 nm, although some larger aggregates were also formed. The use of MFC as surface coating on various base papers considerably reduced the air permeability. Environmental scanning electron microscopy (E-SEM) micrographs indicated that the MFC layer reduced sheet porosity, i.e., the dense structure formed by the nanofibers resulted in superior oil barrier properties.