Irreversible capacity of the amorphous silicon thin-film electrodes

Irreversible processes accompanying the lithium incorporation into amorphous thin-film silicon are investigated. It is shown that the irreversible processes occurred during the cathodic polarization result in the formation of passive film at the silicon surface. The passive film at silicon is close, in its composition, to the passive film at carbonaceous materials. However, unlike the carbonaceous electrodes, no effect of electrolyte composition on the irreversible capacity of the silicon electrodes is observed.     

Simulation of the infrared spectra of amorphous silicon alloys

The infrared absorption spectra of hydrogenated amorphous silicon alloys have been simulated using MOPAC. The calculated spectra compare well with those obtained experimentally. Simulation studies have also been carried out on the effect of contamination of hydrogenated amorphous silicon by common atmospheric contaminants: oxygen and nitrogen. These studies provide a basis for the study of diffusion and reconstruction of amorphous silicon alloys during annealing and photodegradation. This article indicates that infrared spectroscopy, combined with molecular modeling, could be used to study the molecular mechanisms associated with the photodegradation of amorphous silicon alloys. © 1996 by John Wiley & Sons, Inc.     

On the nature of the free volume in amorphous polymers

In a previous publication¹ it was shown that the free volume in amorphous polymers cannot consist of empty sites according to the quasilattice model. In this paper it is demonstrated why theories which are based on the assumptions of this model are nevertheless partly successful. It is even possible to calculate all three jumplike alterations in the second derivatives of Gibbs free energy at the glass transition in accordance with the experimental results. As far as we know, this is the first time that, in addition to the steplike changes in the thermal expansion coefficient Δα and the specific heat Δc_p, the analogous change in the compressibility Δ_χ can also be described correctly by a theory. The parameters for accommodation however reveal, that the quasilattice model does not apply. Consequently better-founded equations are derived, which despite their simplicity, give excellent agreement with the experimental data. One conclusion is that if the free volume consists of cavities in the material, their mean size has to be at least ten times smaller than that of a segment of the chainlike polymer molecules.     

Analytics of hard amorphous silicon containing PECVD-coatings

The present communication deals with investigations of -SiC(N,O):H thin films produced by DC-plasma enhanced chemical vapor deposition under high power conditions. In contrast to traditional processes liquid precursors (hexamethyldisiloxane-HMDSO, hexamethyldisilazane-HMDSN), diluted in argon or nitrogen have been used for the decomposition in the glow discharge.     

Use of amorphous silicon films as sensors

Transport phenomena in an amorphous silicon film deposited by a glow discharge technique have been reviewed, and many interesting applications of this material in sensors are considered. A large Seebeck effect is observed in the highly-conductive amorphous silicon film and a thermopile power sensor has been developed using this film. The large elastoresistance effect observed in the film has been investigated for possible use in strain sensors, touch sensors and real-time correlators. The light-related phenomena and their sensor applications have also been briefly reviewed.     

Studies of the reaction between the amorphous carbon and silicon

The kinetic parameters and the mechanism of reaction carbon black-silicon was studied by DTA, QXRD and microscopic methods. The possibility of SiC micropowders production at temperatures below the melting point of silicon was proved.

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Zusammenfassung Mittels DTA, QXRD und mikroskopischen Methoden wurden die kinetischen Größen und der Mechanismus der Reaktion Ruß-Siliziumdioxid untersucht. Es wurde die Möglichkeit der Herstellung von mikropulverisiertem SiC bei Temperaturen unter dem Schmelzpunkt von Silizium bewiesen.

     

Vibrations in amorphous silicon and its alloys

Amorphous silicon (a-Si) is a material of considerable technological importance, especially for photovoltaic cells. It is usually “doped” with hydrogen and a number of other elements (B, P, F, C, N). Spectroscopy of the extended vibrations of the Si-network and the local modes induced by the dopants is widely used for the characterization of the material. In this paper data obtained with Raman, ir, and neutron spectroscopy are discussed with emphasis on the local modes of hydrogen and other impurities.     

Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.     

Elimination of irreversible capacity of amorphous silicon: direct contact of the silicon and lithium metal

A method of elimination of the amorphous silicon irreversible capacity is suggested, which is based on the direct contact of the silicon and lithium metal under electrolyte. It is shown that this contact yields a solid-electrolyte film over the electrode surface even prior to its initial cathodic polarization, which results in the elimination of the irreversible capacity of amorphous silicon.     

Physicochemical properties of amorphous silicon dioxide produced from nepheline

The properties of silicon dioxide precipitates produced by acid processing of nepheline have been investigated. Regardless of the production method, silicon dioxide takes an intermediate position between commercial silica gel and white soot in terms of the structure and some physicochemical properties, this position determining possible applications of this product.     

Application of accelerated molecular dynamics schemes to the production of amorphous silicon

The evolving nature of a Stillinger-Weber modeled silicon glass is studied using two accelerated molecular dynamics scheme, specifically, hyperdynamics and self-guided algorithms due to Voter and due to Wu and Wang, respectively. We obtain an acceleration of the dynamics, a "boost," on the order of 20 without incurring any significant computational overhead. The validity of the results using accelerated methods is provided by comparison to a conventional molecular dynamics (MD) algorithm simulated under constant temperature conditions for more than 100 ns. We found that performing a sensitivity analysis of the effect of the parameters lambda and t1 before applying the self-guided MD scheme was important. Values of lambda greater than 0.1 and t1 equal to 1 ps were found to give improved structural evolution as compared to a conventional MD scheme. The hyperdynamics approximation scheme was found to be effective in obtaining boosts in the range of 4-12 for a small system without changing the dynamics of the evolution. However, for a large system size such an approach introduces significant perturbations to the pertinent equations of motion.     

The mechanism of plasma-induced deposition of amorphous silicon from silane

Time-resolved mass spectrometric data show that the concentration of di- and trisilane, which are formed from monosilane under discharge conditions typical for the deposition of high electronic quality amorphous silicon, correlate with the measured deposition rate of a-Si. The data can be quantitatively and self-consistently described by a simple set of consecutive reactions:

1. SiH₄ →-SiH₂ + H₂
2. SiH₂ + SiH₄ → Si₂H₆
3. SiH₂ + Si₂H₆ → Si₃H₈
4. Si _n H_{2(n + 1)} →n ·a-Si:H+(n+1)H₂,n=2,3

The only fitting parameter necessary for an excellent fit of the measured data over a wide range of experimental parameters is the value of the reactive sticking coefficient .for the decomposition of di- and trisilane (reaction 3). The resultant value agrees well with the published data of other authors and with those calculated from the measured deposition rate and Si₂H₆, concentration. We did not find and physically meaningful way to lit the measured data with the various “SiH₃ models” proposed by other authors who assumed that the dominant species responsiblefor the deposition of a-Si: H is the SiH₃, radical. For this and some additional reasons mentioned in the present paper. the SiH₃ model is in disagreement with available experimental data.     

Atomic silicon in siloxanic networks: the nature of the oxo-oxygen-silicon bond

The existence of atomic silicon cryptates in siloxanic networks has been studied theoretically via density functional calculations. By modeling with model molecules the candidate sites to host atomic silicon, we found that metastable adducts can be formed only in regions where the siloxanic network is not subjected to steric constraints; stationary states are instead unstable in highly reticulated siloxanic networks. The nature of the oxo-oxygen-silicon bond at the SiO2 surface is analyzed in detail. It is concluded that silicon is kept at the surface in atomic-like configuration by (i) sigma charge donation from oxo-oxygen atoms into the empty silicon psigma orbital; (ii) pi charge back-donation from singly occupied silicon 3ppi orbitals into empty sigma* model molecule orbitals. Surprisingly, these results attribute to atomic silicon the character of bifunctional Lewis acid.     

Tensile deformation induced structural rearrangement in amorphous silicon nitride

Silicon nitride exhibits good mechanical properties and thermal stability at high temperatures. Since experiments have limitations in nanoscale characterization of the chemical structure and related properties, atomistic simulation is a proper way to investigate the mechanism of this unique feature. In this paper, the melt-quench method is used to generate the amorphous structure of silicon nitride; then the structural properties of silicon nitride under tensile deformation were studied by angular pair distribution functions. The corresponding mechanism of tensile stress induced structure rearrangement is explored.     

Hypoxanthine sensor based on an amorphous silicon field-effect transistor

The pH response of an a-ISFET with xanthine oxidase immobilized on a ca. 20-μm thick poly(vinyl butyral) membrane over the gate insulator, is used to detect the uric acid produced by enzyme-catalyzed oxidation of hypoxanthine. The pH sensitivity between pH 5.0 and 10.0 is ca 48 mV/pH at 32°C in 10 mM phosphate buffer. The change in the output gate voltage 1 min after sample injection, is linearly related to the logarithm of the hypoxanthine concentration in the range 0.02–0.1 mM. The optimum buffer pH is 7.5. The system can be used for 2 weeks with 30% loss of enzymatic activity.     

Electronic nature of carbonium ions and their silicon analogues

Nonclassical ions or carbonium ions have multi-center bonding from delocalized sigma or pi electrons. The 2-norbornyl cation, its derivative 6,6-difluoro-2-norbornyl cation, tris-homocyclopropenyl cation, 7-norbornenyl cation, and 4-cyclopentenyl cation and their corresponding silicon analogues were studied in this work. All carbocations have topologically different 3c-2e systems. The magnitude of all delocalization indexes between each atomic pair of the 3c-2e bond can be used to predict homoaromaticity. The silicon analogues have a topologically different 3c-2e bond from their corresponding carbocation.     

Effect of charge on bond strength in hydrogenated amorphous silicon

We have studied the effect of excess charge on the bond strength in the silanes SiH₄ and Si₂H₆ to assess whether charge trapping in a solid-state lattice might promote the technologically important photodegradation of amorphous silicon alloys (the Staebler-Wronski effect). The calculations indicate that both positive and negative charges reduce the strength of Si? H and Si? Si bonds considerably, to the point where they may be broken easily by visible or even infrared light. © 1994 by John Wiley & Sons, Inc.     

An EELS and EXELFS study of amorphous hydrogenated silicon carbide

A series of thin films of amorphous hydrogenated silicon carbide (a-SiCH) produced by RF plasma decomposition of propane and silane has been studied by electron energy-loss spectroscopy (EELS) and extended energy-loss fine structure (EXELFS) studies. The composition of the films has been determined by EELS and the nearest neighbour spacings have been determined by EXELFS. These results, along with the energy of the plasmon loss peaks, have been compared with the deposition conditions for each film. The results show that for a large gas ratio (C₃H₈/(C₃H₈+SiH₄)) the films have a high proportion of carbon and are similar to a-CH in structure, whereas those films prepared with Y = 0.4 or 0.5 have nearest neighbour spacings consistent with those for tetrahedrally bonded carbon. The films prepared with lowest Y have nearest neighbour spacings similar to those for amorphous silicon carbide. The results for a-SiCH have been compared with the results of EELS and EXELFS of CVD diamond films, amorphous carbon and amorphous silicon.     

Stress dependence of paramagnetic point defects in amorphous silicon oxide

Room-temperature red cathodoluminescence (CL) emission (R band) arising from the paramagnetic point-defect population present in amorphous silicon oxide (SiOx) has been characterized with respect to its shift upon applied stress, according to a piezo-spectroscopic (PS) approach. The R band (found at around 630 nm) originates from nonbridging oxygen hole centers (NBOHC; Si-O*) generated in the presence of oxygen-excess sites. It is shown that reliable stress assessments can be obtained in silica glass with a relatively high spatial resolution, provided that appropriate spectroscopic procedures are developed to precisely extract from the CL spectrum the shift upon stress of the R band, isolated from other partly overlapping bands. Macroscopic and microscopic PS calibration procedures are shown to lead to consistent results on silica materials with different chemical characteristics and, thus, with different intrinsic defect populations. In addition, quantitative calibrations of both electron probe size and luminescence emission distribution within the electron probe are given. As an application of the PS technique, the magnitude of the residual stress piled up (mainly due to a thermal expansion mismatch) at a sharp silica/silicon interface has been characterized by taking into account the gradient in defect population developed as a function of distance from the interface. In the Results and Discussion section, brief comments are offered regarding the possible impact of highly spatially resolved stress assessments in silica glass upon the development of new materials and advanced electronic devices.