Showing papers in "Journal of The Electrochemical Society in 1990"

Anisotropic Etching of Crystalline Silicon in Alkaline Solutions I . Orientation Dependence and Behavior of Passivation Layers

Abstract: The anisotropic etching behavior of single‐crystal silicon and the behavior of and in an ethylenediaminebased solution as well as in aqueous , , and were studied. The crystal planes bounding the etch front and their etch rates were determined as a function of temperature, crystal orientation, and etchant composition. A correlation was found between the etch rates and their activation energies, with slowly etching crystal surfaces exhibiting higher activation energies and vice versa. For highly concentrated solutions, a decrease of the etch rate with the fourth power of the water concentration was observed. Based on these results, an electrochemical model is proposed, describing the anisotropic etching behavior of silicon in all alkaline solutions. In an oxidation step, four hydroxide ions react with one surface silicon atom, leading to the injection of four electrons into the conduction band. These electrons stay localized near the crystal surface due to the presence of a space charge layer. The reaction is accompanied by the breaking of the backbonds, which requires the thermal excitation of the respective surface state electrons into the conduction band. This step is considered to be rate limiting. In a reduction step, the injected electrons react with water molecules to form new hydroxide ions and hydrogen. It is assumed that these hydroxide ions generated at the silicon surface are consumed in the oxidation reaction rather than those from the bulk electrolyte, since the latter are kept away from the crystal by the repellent force of the negative surface charge. According to this model, monosilicic acid is formed as the primary dissolution product in all anisotropic silicon etchants. The anisotropic behavior is due to small differences of the energy levels of the backbond surface states as a function of the crystal orientation.

The Evolution of Silicon Wafer Cleaning Technology

Abstract: The purity of wafer surfaces is an essential requisite for the successful fabrication of VLSI and ULSI silicon circuits. Wafer cleaning chemistry has remained essentially unchanged in the past 25 years and is based on hot alkaline and acidic hydrogen peroxide solutions, a process known as "RCA Standard Clean." This is still the primary method used in the industry. What has changed is its implementation with optimized equipment: from simple immersion to centrifugal spraying, megasonic techniques, and enclosed system processing that allow simultaneous removal of both contaminant films and particles. Improvements in wafer drying by use of isopropanol vapor or by "slow‐pull" out of hot deionized water are being investigated. Several alternative cleaning methods are also being tested, including choline solutions, chemical vapor etching, and UV/ozone treatments. The evolution of silicon wafer cleaning processes and technology is traced and reviewed from the 1950s to August 1989.

Studies of Lithium Intercalation into Carbons Using Nonaqueous Electrochemical Cells

Abstract: Li/graphite and Li/petroleum coke cells using a in a 50:50 mixture of propylene carbonate (PC) and ethylene carbonate (EC) electrolyte exhibit irreversible reactions only on the first discharge. These irreversible reactions are associated with electrolyte decomposition and cause the formation of a passivating film or solid electrolyte interphase on the surface of the carbon. The amount of electrolyte decomposition is proportional to the specific surface area of the carbon electrode. When all the available surface area is coated with the film of decomposition products, further decomposition reactions stop. In subsequent cycles, these cells exhibit excellent reversibility and can be cycled without capacity loss.

Particle Size Effects for Oxygen Reduction on Highly Dispersed Platinum in Acid Electrolytes

Abstract: The particle size effect for oxygen reduction kinetics on highly dispersed Pt particles in acid electrolytes are discussed. It is suggested that the change in the fraction of surface atoms on the (100) and (111) crystal faces of Pt particles, which are assumed to be cubo‐octahedral structures, can be correlated to the mass activity (A/g Pt) and specific activity (μA/cm2 Pt) of highly dispersed Pt electrocatalysts. The maximum in mass activity that is observed at ~3.5 nm in several studies is attributed to the maximum in the surface fraction of Pt atoms on the (100) and (111) crystal faces, which results from the change in surface coordination number with a change in the average particle size. The reduction of oxygen on supported Pt particles in acid electrolytes is classified as a demanding or structure‐sensitive reaction; the specific activity increases with an increase in particle size.

Formation Mechanism and Properties of Electrochemically Etched Trenches in n‐Type Silicon

Abstract: Three‐dimensional structures in silicon are increasingly coming into use for the fabrication of mechanical and electrical devices. The fabrication of deep trenches is one of the most important problems in VLSI (very large scale integration) technology. In this study the spontaneous trench formation in n‐type silicon immersed in hydrofluoric acid under anodic bias is demonstrated and the resulting microstructures are characterized. Trenches with arbitrary cross sections and high aspect ratios for microelectronic (e.g., 42 μm depth and 0.6 μm diam) and for power component application (e.g., 60 μm depth and 10 μm diam) have been produced by a standard masking technique. The trench formation is explained by a model which takes into account the conditions of the space charge region the minority carrier current and the crystal orientation. A passivating sidewall layer is not needed in this model. The dimensions and the shape of anodically etched trenches can be varied over a wide range by adjusting the critical parameters.

Ionic Conductivity of Solid Electrolytes Based on Lithium Titanium Phosphate

Abstract: Solid electrolytes based on lithium titanium phosphate were prepared, and their phase, porosity of the sintered pellets, and electrical conductivity were studied. The conductivity was increased and the porosity decreased greatly by partially replacing Ti4+ and P5+ in with M3+ and Si4+ions, respectively. The maximum conductivity at 298 K is for . The conductivity was considerably increased by the mixing of binders such as or with . The main reason for the conductivity enhancement of these electrolytes seems to be attributable to the increase of the sintered pellet density with the enhancement of the lithium concentration at the grain boundaries.

Electrochemistry of manganese dioxide in lithium nonaqueous cell. I: X-ray diffractional study on the reduction of electrolytic manganese dioxide

Abstract: X-ray diffractional (XRD) studies were carried out for the electrochemical reduction of heat-treated electolytic MnO{sub 2} (250{degree} and 400{degrees}C for 7 days) A series of XRD examinations indicated that the reaction proceed without the destruction of the core structure of electrolytic manganese dioxide (EMD) The structural changes during the electrochemical reductions of both EMDs were described , assuming a tetragonal sublattice Both heat-treated electolytic MnO{sub 2}s (HEMDs) behaved similarly in the tetragonal sublattice parameter vs the reduction degree plots During the first half of the reduction, HEMD phase having a tetragonal sublattice (a = 439 {minus} 440 {Angstrom}, c = 286 {minus} 290 {Angstrom}) was converted into a new Li{sub x} MnO{sub 2} phase having an expanded tetragonal sublattice a = 49 {minus} 50 {Angstrom}, c = 282 {minus} 286 {Angstrom}, ie, in a two-phase reaction In the 30-90% reduction, the a-axis increased continuously as a function of reduction degree, ie, in a homogeneous phase reaction The possible crystal structure of the deep discharge product Li{sub x} MnO{sub 2} (x{gt}08) is discussed, assuming a tetragonal unit cell (a = ca 5 {Angstrom}, c = ca 285 {Angstrom}) having the NiAs-type structure by analogy with Li{sub x} RuO{sub 2}, and anmore » orthorhombic unit cell, ie, a = 2 {times} b {Angstrom}, b = ca 5 {Angstrom}, c = ca 285 {Angstrom}« less

Nonaqueous Electrochemistry of Magnesium Applications to Energy Storage

Abstract: Research leading to the construction of an ambient temperature rechargeable magnesium battery based on organic electrolytes and positive electrodes capable of reversible intercalation of Mg+2 ions is discussed. The number of combinations of solvent, solute, and intercalation cathode which give reasonable battery performance is much more limited for Mg than for alkali metals. The only electrolytes which allowed Mg dissolution and deposition were solutions of organomagnesium compounds in ethers or tertiary amines; many of these were unstable in the presence of transition metal oxides or sulfides which were found to function acceptably as intercalation electrodes. Possible directions for future research which could solve these problems are discussed, as well as theoretical aspects of magnesium compound behavior in nonaqueous solvents.

Anisotropic Etching of Crystalline Silicon in Alkaline Solutions II . Influence of Dopants

Abstract: The etching behavior of highly boron doped silicon in aqueous solutions based of ethylenediamine, KOH, NaOH, and LiOH was studied. For all etchants, a strong reduction of the etch rate for boron concentrations exceeding approximately 2 �9 10 ~9 cm -3 was observed. This value is in good agreement with published data for the onset of degeneracy of p-type silicon. The reduction of the etch rate was found to be inversely proportional to the fourth power of the boron concentration. For a given high boron concentration, the etch stop effect was found to be most effective for ethylenediamine-based solutions and low concentration KOH and least effective for highly concentrated KOH. On the basis of these results, a model is proposed attributing the etch stop phenomenon to electrical effects of holes rather than chemical effects of boron. Due to the high dopant concentration the width of the space charge layer on the silicon surface shrinks drastically. Therefore, electrons injected into the conduction band by an oxidation reaction cannot be confined to the surface and rapidly recombine with holes from the valence band. The lack of these electrons impedes the reduction of water and thereby the formation of new hydroxide ions at the silicon surface. Since the transfer of four electrons is required for the dissolution of one silicon atom the observed fourth power law for the decrease of the etch rate can be explained. The reduction of the etch rate on silicon doped with germanium or phosphorus is much smaller and follows a different mechanism.

Electrogenerated Chemiluminescence: An Oxidative‐Reduction Type ECL Reaction Sequence Using Tripropyl Amine

Abstract: A new electrogenerated chemiluminescence (ECL) reaction which utilizes tripropyl amine and is presented. The mechanism of light generation appears general enough to include a range of amines and luminophores. An oxidative‐reduction mechanism is proposed. Upon electrochemical oxidation of both the luminophore and amine, a strong emission is observed. Voltammetric analysis reveals the potential for greatest light emission at the tripropyl amine oxidation. The emission is from the excited state of . An electron transfer reaction from the deprotonated tripropyl amine radical and is the central reaction for excited state production. An estimate of the ECL efficiency cannot be made, due to the complex nature of the reaction.

Electrochemical Reduction of Carbon Dioxide on Various Metal Electrodes in Low‐Temperature Aqueous KHCO 3 Media

Abstract: Electrochemical reduction was investigated on 32 metal electrodes in aqueous medium. The current efficiency of reduction on Ni, Ag, Pb, and Pd increases significantly with lowering the temperature. The ratio of reduction products are also changed by lowering temperature. Potential dependence of and on an Hg electrode supports the electron transfer mechanism for production. Formation of methane and ethylene is observed on almost all metal electrodes used, although the efficiency is mainly very low except for Cu. A periodic table for reduction, which is drawn based on the dependence of reduction products on various metals, suggests the existence of a systematic rule for the electrocatalytic reduction of on metal surfaces.

Corrosion Studies of Rapidly Solidified Magnesium Alloys

Abstract: Effects of alloying elements on the corrosion resistance of mostly binary rapidly solidified (RS) magnesium alloys were examined using electrochemical techniques. Uniform corrosion rates were measured at room temperature in a pH 9.2 sodium borate buffer using electrochemical impedance spectroscopy (EIS). Comparisons of electrochemically and gravimetrically determined corrosion rates of bulk magnesium and magnesium alloys showed that the charge transfer resistance could be used to accurately measure the corrosion rates of these materials provided the attack was indeed uniform. The correlation broke down for pure magnesium, which suffered a nonuniform attack due to the undermining of small particles. Corrosion rate measurements of binary alloys showed that aluminum was the only element which caused a decrease in the corrosion rate of magnesium; the corrosion rate decreased with increasing aluminum content. Low concentrations of zinc and lithium resulted in alloys with corrosion rates slightly higher than that of the pure magnesium. Anodic polarization scans were used to compare as‐cast and rapidly solidified commercial ternary alloy AZ61 in borate (pH 9.2) and carbonate/bicarbonate (pH 10.0) buffer solutions containing 100 ppm, 1000 ppm, and 3.5 weight percent sodium chloride. The RS AZ61 formed a more protective film than the cast material under these conditions.

Theory of Steady‐State Passive Films

Abstract: A theory is developed for the steady-state properties of passive films that form on metals and alloys in aqueous environments. This theory is based on the point defect model developed earlier, and predicts that they steady-state thickness of the barrier film and the log of the steady-state current will vary linearly with applied voltage. These relationships may be used to estimate empirical parameters that describe the dependencies of the potential drop across the barrier film/environment interface on the applied voltage and pH and to estimate kinetic parameters for dissolution of the film. If dissolution at the film-solution interface occurs very slowly, the primary passive film is envisaged to consist of a rigid oxide sublattice that transmit cations from the metal to a gel-like, precipitated upper layer. If dissolution at the barrier film/environment interface occurs rapidly, then a steady-state thickness is achieved by a balance between the rate of dissolution of the film at the film-solution interface and the rate of growth of the film into the underlying metal phase. Due to the outward movement of oxygen vacancies (i.e., inward movement of oxygen ions) through the barrier layer. The model is found to account for many experimental data that have beenmore » published in the literature on the quasi steady-state properties of passive films.« less

Electrochemical Bonding of Amines to Carbon Fiber Surfaces Toward Improved Carbon‐Epoxy Composites

Abstract: Electrochemical oxidation of ω‐diamines on carbon fibers allows the bonding of these molecules to the carbon fibers. The conditions leading to this surface modification are described, and the influence of the different parameters is discussed. The structure of the bonded layer is studied by voltammetry, XPS, and SIMS. A mechanism is proposed which involves the coupling of a radical cation to the fiber. Toughness of carbon‐epoxy composites is improved when prepared from these modified fibers.

The Structure and Activity of Pt‐Co Alloys as Oxygen Reduction Electrocatalysts

Abstract: This paper reports on carbon-supported Pt-Co (3:1 atom ratio) catalysts prepared in both acid and alkaline aqueous media, followed by heat-treatments to promote alloy formation. both preparations began with a commercial 10% Pt-on-carbon catalyst, with Pt particle sizes of 15--30 {Angstrom}. Significantly greater alloying was observed in the catalyst prepared in the acid medium. X-ray diffraction studies of the acid-prepared catalyst demonstrated lattice parameters tending away from Pt (3.927 {Angstrom}) and forward that for Pt{sub 3}Co (3.831 {Angstrom}), greatly increased particle sizes, and significant ordering evidenced by the presence of super-lattice reflections. In all cases, catalysts prepared in the alkaline medium were alloyed to a lesser extent, were of moderately increased particle size, and gave no indication of alloy ordering. Activity testing under phosphoric acid fuel cell conditions demonstrated that the most highly alloyed catalysts were not significantly more active than pure Pt catalyst of comparable crystallite size. Loss of cobalt in the phosphoric acid environment was the lowest in catalysts which were the most alloyed, and were the Pt{sub 3}Co ordered phase was present.

Study of Passive Films Formed on AISI 304 Stainless Steel by Impedance Measurements and Photoelectrochemistry

Abstract: Moss-Schottky plots and photoelectrochemical measurements were made on films formed at different potentials on AISI 304 stainless steel in a borate/boric acid solution, pH 9.2. The results allowed the determination of the semiconductive properties and band structure of the films, which account for the existence of two kinds of films depending on the formation potential. For potentials below 0 V (SCE), the results point out for a film with an inverse spinel structure constituted by Cr-substituted magnetite with two donor levels. Above 0 V only one donor level is detected, which should be Fe{sup 2 +} on tetrahedral sites.

Ion and Solvent Transport in Ion‐Exchange Membranes I . A Macrohomogeneous Mathematical Model

Abstract: A mathematical model for the simulation of ion and solvent transport within an ion‐exchange membrane is developed and analyzed. A Nernst‐Planck equation is employed for the description of ion transport by diffusion, migration, and convection. Solvent transport driven by pressure and electric‐potential gradients is described by an equation of motion. The physicochemical parameters used in this analysis are experimentally easy to obtain. The set of equations used in the simulation are presented in a dimensionless form so that important dimensionless parameters can be evaluated. In particular, an electrokinetic Peclet number is identified; large values of this dimensionless quantity, for example, would indicate that convection is the dominant mode of mass transport.

Electrochemistry of Conductive Polymers VIII: In Situ Spectroelectrochemical Studies of Polyaniline Growth Mechanisms

Abstract: The earlier stage of the polymerization reaction of polyaniline has been studied employing in situ spectroelectrochemical techniques. The results indicate that the nitrene cation appears to be a key intermediate species, which leads to all three possible dimers including a head‐to‐tail dimer (N‐phenyl‐p‐phenylenediamine), a tail‐to‐tail dimer (benzidine), and a head‐to‐head dimer (hydrazobenzene). The oxidized forms of these dimers were all shown to be capable of growing polyaniline in the presence of aniline, even though aniline was not oxidized. The cyclic voltammetric peaks observed during the PA synthesis in the middle potential regions were shown to arise from the redox reactions of these dimers, oligomers, and degradation products of polyaniline including quinoneimines and p‐benzoquinone.

Mechanism of SiN x H y Deposition from NH 3 ‐ SiH4 Plasma

Abstract: The plasma‐enhanced chemical vapor deposition process for films has been in use for over two decades, but the chemistry of the process has yet to be explained. In the present work, the composition of a 13 MHz parallel plate glow discharge plasma was analyzed by line‐of‐sight sampling from the film deposition plane into a triple‐quadrupole mass spectrometer, which can resolve compositional ambiguities at a given mass number by utilizing collision‐assisted secondary cracking. At low RF power, disilane was the main plasma product even when was 25/1, whereas at higher power (0.1 W/cm2 of cross section) disilane was eliminated and tetra‐aminosilane, , and the triaminosilane radical, , became dominant. The concentration of these aminosilanes closely tracked deposition rate, and they are believed to be the principal film precursors. Films deposited with maximized and disilane suppressed in the plasma were excess in N and contained no Si‒H bonding, consistent with the precursor composition. Silane utilization was near unity. The composition and properties of films deposited under these "amino‐saturated" plasma conditions were examined vs. substrate temperature, . With increasing , there occurred a densification, a loss of H and excess N in a ratio, and an increase in tensile stress, suggesting surface and subsurface chemical condensation of the adsorbed precursors via . Postdeposition flash desorption showed , not , to be the main volatile product of condensation. These results demonstrate that plasma chemistry can be manipulated to control film properties in a predictable manner.

Electrochemical Studies on Protective Thin Co3 O 4 and NiCo2 O 4 Films Prepared on Titanium by Spray Pyrolysis for Oxygen Evolution

Abstract: Electrochemical studies were done on thin and films on Ti by cyclic voltammetric and potentiostatic polarization techniques. The films were quite conductive and prevented the underlying surface from playing a harmful role during measurements. The results indicated the formation of a surface quasi‐reversible redox couple, CoIV/CoIII, in solution. The reaction order with respect to OH− ion concentration was found to be nearly 2. The Tafel slopes decreased with increasing concentrations. was observed to evolve oxygen gas at a lower overpotential as compared to that of . Electrode kinetic parameters suggest similar mechanisms for the oxygen evolution for both the oxides.

Water‐Balance Calculations for Solid‐Polymer‐Electrolyte Fuel Cells

Abstract: This paper reports on the proper balance between water production and removal which is particularly important for successfully operating solid, polymer-electrolyte fuel cells. Imbalance between production and evaporation rates can result in either flooding of the electrodes or membrane dehydration, both of which severely limit performance. We present a mathematical model of the solid-polymer-electrolyte fuel cell that identifies operating conditions that result in a water balance. The model is one dimensional and is derived from basic principles of gas-phase transport. The mechanisms of membrane water transport are not included explicitly in the model, and the membrane is taken as uniformly wetted, which renders the model most applicable to thin membranes. We suggest how humidification of reactant gases can be adjusted as current density is varied during an experiment (at constant temperature, pressure, and reactant feed rates) in order to accommodate the fuel cell's changing demands for water. Humidification requirements of inlet reactant gases for a wide range of practical operating temperatures, pressures, and gas feed rates have been identified. The analysis also identifies conditions in which reactant transport limitations govern the behavior of the fuel cell.

Growth and Luminescence Properties of Mg‐Doped GaN Prepared by MOVPE

Abstract: Growth and luminescence properties of Mg‐doped prepared by metal‐organic vapor phase epitaxy, in which or is used as the Mg source gas, are reported for the first time. It was found that the Mg concentration in is proportional to the flow rate of the Mg source gas; the doping efficiency of Mg into is independent of the substrate temperature from 850° to 1040°C; and Mg in acts as an acceptor and forms blue luminescence centers. By using, an efficient near‐UV and blue LED can be fabricated.

Metal Oxide Cathode Materials for Electrochemical Energy Storage: A Review

Abstract: Due to their rather low molecular weight and their favorable electrochemical and solid‐state properties, first row transition metal oxides seem to be specially attractive as cathode materials in electrochemical energy storage systems. Therefore, we undertook a detailed overview, covering electrochemical, conductivity, ion diffusivity, spectroscopic, and other physico‐chemical data on metal oxides in relation to their behavior in batteries. Metal oxide‐based primary batteries have achieved a high technological level and yield energy densities of up to 300 Wh kg−1 or 880 Wh l−1. Oxide‐based secondary batteries, on the other hand, typically yield less than 100 Wh kg−1. Based on the present review, V, Cr, Mn, and Co oxides seem to be the most promising solid‐state cathode materials for future high performance secondary batteries.

Magnetic Field Effects on Mass Transport

Abstract: It has been shown that the stationary limiting diffusion current on a steady electrode is proportional to {ital B}{sup 1/3}{ital C}{sup 4/3} where {ital C} is the electroactive species concentration and {ital B} the magnetic field intensity. A new impedance technique is developed which consists of the frequency response analysis of the limiting diffusion current to a sinusoidal magnetic field perturbation. In the low frequency range, all the impedance diagrams can be reduced, in Bode coordinates, by {omega}{ital B}{sup {minus}2/3}{ital C}{sup {minus}2/3}. This response is due to convective mass transport and is similar to the electrohydrodynamical impedance obtained through the modulation of the rotation speed of a rotating disk electrode.

Oxidation of Single‐Crystal Silicon Carbide Part I . Experimental Studies

Abstract: The oxidation of single crystal in dry oxygen (10−3–1 atm and 1200°–1500°C) followed parabolic kinetics. Two different apparent activation energies were calculated for oxidation of the (0001) C faces of , approximately 120 kJ/mol below 1350°C and 260 kJ/mol above 1350°C. Two regimes were not apparent for oxidation of the (0001) Si faces, and apparent activation energies lay between 223 and 298 kJ/mol. Double oxidation experiments using and indicated that the process is dominated by the transport of molecular oxygen at lower temperatures (<1300°C) with a substantial contribution from diffusion of ionic oxygen at higher temperatures. Epitaxially grown Si 13C films on alpha‐Si 12C substrates via CVD were used to study carbon transport behavior during oxidation of . Depth profiles for carbonaceous species using SIMS showed that carbon can transport quickly through the oxide layer, which eliminates the possibility that transport of carbonaceous species is rate controlling in the oxidation of . Oxidation mechanisms of are discussed on the basis of these results.

Electrochemical Behavior of Aluminum‐Base Intermetallics Containing Iron

Abstract: The effect of components Fe, Mn, and Si on the electrochemical behavior of phases Al 3 Fe, αAl(Fe, Mn)Si, and δAlFeSi were investigated in NaOH solutions. Anodic polarization of the phases leads, under certain conditions, to an enrichment of the surface with the more noble components. Enrichment with Fe is detrimental to cathodic behavior. The added presence of Mn or Si in the phase reduced the effect of Fe on both the anodic and cathodic reaction rates. The mechanisms are discussed briefly along with the significance of these phenomena on the localized corrosion of aluminum alloys