Showing papers in "Journal of The Electrochemical Society in 1982"

The Oxidation of Shaped Silicon Surfaces

Abstract: Nonplanar silicon surfaces were prepared and oxidized at 900°–1100°C and the oxide morphology was studied by transmission electron microscopy of thin sections. A 30% decrease in oxide thickness at silicon step edges following 900° and 950°C wet oxidation is attributed to the effect of locally compressive intrinsic stress within the oxide on the solubility of oxygen. Oxidation inhibition becomes less at higher temperatures due to the relief of stress (during growth) by viscous flow of the oxide.

Reaction of Oxygen with Si(111) and (100): Critical Conditions for the Growth of SiO2

Abstract: The reactions of oxygen (O/sub 2/) with the clean Si(111) and (100) surfaces have been studied at high temperatures (890/degree/-1150/degree/C) for oxygen pressures between 5*10/sup -5/ and 5*10/sup -2/ Torr. The critical conditions involving oxygen pressure and substrate temperature which are necessary for the growth of SiO/sub 2/ to occur on these Si surfaces have been determined, and are found to be independent of substrate orientation and doping type under the conditions studied. The observed critical conditions for growth are consistent with a theoretical model which focuses on the kinetics of SiO/sub 2/ cluster growth and on the thermodynamics of the competing etching reaction leading to production of SiO. Evidence is presetnted for epitaxial growth on Si(100) of cubic SiO/sub 2/, /beta/-cristobalite. 16 refs.

Enhanced Stability of Photoelectrodes with Electrogenerated Polyaniline Films

Abstract: Polyaniline was electrodeposited on platinum and semiconducting (Cd-chalcogenides, Si, GaAs, GaP) electrodes from aqueous solution (pH = 1) containing aniline. The electrochemical behavior of such films was examined in aqueous and nonaqueous solutions by cyclic voltammetry. Results show that the polyaniline film is conducting in both cathodic and anodic regions at pH < 3 as evidenced by the electrochemical redox reactions of various redox couples on polyanilinecoated electrodes. In general, all polyaniline-coated semiconductor electrodes examined in this work exhibited enhanced stability of the photocurrent when compared to that of the naked electrodes.

Electrochemistry of Prussian Blue Modified Electrodes: An Electrochemical Preparation Method

Abstract: A new type of Prussian blue modified electrode is described. The Prussian blue modified electrode is electrochemically prepared in a solution of ferric‐ferricyanide. The amount of Prussian blue on electrodes such as platinum, glassy carbon, and is easily controlled by changing the current density, the electrode potential, and the time of the electrolysis. The waves observed at +0.2 and +1.0V vs. SCE are due to the reduction and the oxidation of the ferric part and of the ferrous part in the Prussian blue crystal, or , respectively. This electrode exhibits excellent stability in aqueous solution. A spectroelectrochemical property of the modified electrode is also described.

Vibrational Spectroscopic Determination of Structure and Ion Pairing in Complexes of Poly(ethylene oxide) with Lithium Salts

Abstract: A structural model for crystalline complexes of poly(ethylene oxide) (PEO) with various lithium salts is presented, based on vibrational spectroscopic studies; these complexes are known to exhibit ionic conductivities of > 10 -/sup 5/ (/OMEGA/-cm)-/sup 1/ at 100/sup 0/C. Cation-dependent vibrational bands observed in the Raman indicate that significant cation-oxygen atom interactions occur and suggest that the polyether chain may wrap around the lithium cations. Spectroscopic studies indicate extensive contact ion pairing occurs in the PEO*LiNO/sub 3/ complex, and this may contribute to the somewhat lower ionic conductivity of this complex as compared to other lithium salt complexes with similar structures but weaker cation-anion interactions. 27 refs.

A Viscous Flow Model to Explain the Appearance of High Density Thermal SiO2 at Low Oxidation Temperatures

Abstract: Higher density results from the thermal oxidation of Si in dry at lower oxidation temperatures. More than 3% higher density is observed for grown at 600°C as compared with 1150°C. A consistent model for the formation of this material is deduced based on the following: the temperature dependence of the density, the annealing behavior of the higher density , and on the literature and new measurements of the intrinsic stress in films. The model considers viscous flow of a Maxwell solid and hinges on the attainment of the necessary free volume for oxidation at lower oxidation temperatures.

The Chemical Nature of Aluminum Corrosion V . Energy Transfer in Aluminum Dissolution

Abstract: The transfer of energy during the dissolution of aluminum is represented by a potential energy surface. In chloride solution, the high energy aluminum metal ionizes rapidly to the A1 +§ ion which also hydrolyzes rapidly. At the start of the reaction, in the microsecond range, there are two species available for reaction, A1 +++ and AI(OH) § The reaction follows two alternative paths; the energy levels of the alternative species, A1C1 +* and AI(OH)C1 + have been measured. These two species react further to form a reasonable stable basic aluminum chloride that is transformed slowly to AI(OH)3 and finally to AI~_O~ �9 H20. The energy levels of these species have been measured. Reaction in sulfate solution differs because the intermediate species, AI(OHh + and AI(OH)SO~ occupy low energy levels, and the basic aluminum sulfate has a lower energy than AI(OH)3. In most practical cases of aluminum corrosion, the reaction proceeds in the reverse direction. The reactants are the oxide-covered aluminum surface and the anion combining to form the basic aluminum salt.

Conformation and Ion‐Transport Models for the Structure and Ionic Conductivity in Complexes of Polyethers with Alkali Metal Salts

Abstract: Two ion‐transport mechanisms are described for ion transport in polyether‐alkali metal salt complexes: an intrahelical jumping process along crystalline (helical) regions of the polymer, and a transport process in the amorphous regions which is dependent on formation of fourfold coordination sites via mutual motion of ether oxygens from two or more polymer chains. The intrahelical jumping process may exhibit Arrhenius behavior, while transport in the amorphous regions should behave like a configurational entropy dominated process, showing a temperature dependence like , where is the equilibrium glass transition temperature. For the highly crystalline poly(ethylene oxide) complexes, an Arrhenius behavior is observed to dominate, whereas for the amorphous polyether salt complexes the configurational entropy behavior is observed. Even for the highly crystalline complexes, however, amorphous regions separate the crystalline regions of the polymer, and segmental motion of the polymer chains is postulated to be crucial here as well. The models are consistent with the observed frequency‐dependent ionic conductivity, as well as spectroscopic, x‐ray, thermal, and physical characterization measurements. They provide a reasonable microscopic picture for ion motion and yield testable predictions concerning the dependence of the ionic conductivity on pressure, temperature, and crystallinity.

The Influence of Surface Recombination and Trapping on the Cathodic Photocurrent at p‐Type III‐V Electrodes

Abstract: The factors contributing to the large overpotential with respect to flatband potential required for hydrogen evolution and certain redox reactions at illuminated p-type electrodes of various III-V compounds have been investigated. Results of photoelectrochemical studies, involving minority carrier transfer to redox systems in the electrolyte, indicate that surface sites acting as recombination centers are important. Results of impedance measurements made with illuminated electrodes emphasize the importance of charge localization in surface states. A model based on surface recombination and electron trapping can explain the photocurrent-potential and impedance characteristics of the p-type electrodes. 35 refs.

Efficient p ‐ InP ( Rh ‐ H alloy ) and p ‐ InP ( Re ‐ H alloy ) Hydrogen Evolving Photocathodes

Abstract: The solar to chemical energy conversion efficiencies of hydrogen generating photoelectrolytic cells with (Rh‐H alloy) and (Re‐H alloy) photocathodes have been increased, respectively, to 16.2% and 14.4%, as calculated from the electrical energy conserved in the photoassisted electrolysis of . Based on the electrical energy produced from the photoelectrochemically generated hydrogen in an ideal fuel cell operating at the thermodynamic potential, the solar to chemical conversion efficiences are, respectively, 13.3% and 11.4%. The cells were improved by increasing the barrier at the junction by hydrogen saturation, while maintaining the recombination passivating layer of hydrated indium oxide at pores in the thin, light transmitting metallic catalyst layer.

Gas Flow Patterns in Horizontal Epitaxial Reactor Cells Observed by Interference Holography

Abstract: Interference holography is used to visualize gas flow patterns and temperature profiles in epitaxial systems. It is demonstrated that in water‐cooled horizontal reactor cells the carrier gases and He give dynamically stable laminar flow profiles throughout the reactor. There is no indication of a stagnant or boundary layer for flow velocities up to 80 cm/sec in this type of cell. In air‐cooled cells, and He also give stable laminar flow profiles, but beyond velocities of 40 cm/sec a cold gas finger appears in these flows due to undeveloped flow and temperature profiles. In contrast to the stable flow characteristics of and He, the flows of and Ar always are unstable due to convective motions. Besides this intrinsic instability, these flows are accompanied by severe entrance effects (especially undeveloped flow profiles), which dominate the flows for flow rates higher than 4 cm/sec. This is observed for both air‐ and water‐cooled cells. Another phenomenom which is observed for and Ar is that beyond 4 cm/sec the convective gas breaks up into a thin (8 mm) laminar layer close to the susceptor across which the entire temperature gradient is present and a highly turbulent/convective part above this laminar layer. Analysis shows that for Ar and about eight times longer entrance lengths in the reactor cell are needed to achieve fully developed velocity and temperature profiles as compared with and He. This explains the dominant influence of this effect on the flows of Ar and . When the influence of the entrance effect on the profiles is taken into account, all the observed flow patterns and temperature gradients are in agreement with the theoretical flows which can be predicted on basis of the respective Reynolds and Rayleigh (or Grashof) numbers.

The Nature of the Passive Film on Iron II . A‐C Impedance Studies

Abstract: The passive film on iron in borate buffer has been studied with wide frequency range impedance techniques. Two well‐defined potential domains are observable, i.e., above and below 1.0V vs. the reversible hydrogen electrode in the same solution (RHE). A discussion of the interpretation of electrochemical impedances and of interfering artifacts is presented. An analysis of the impedance curves raises serious questions about the interpretation of the data in terms of a semiconductor model.

The Anodic Evolution of Ozone

Abstract: Trends of the anodic ozone evolution reaction are characterized for several different anode material and electrolyte combinations. The effect of electrolyte temperature and concentration, current density, and additions of small quantities of the fluoride ion are explored. The general behavior of ozone current efficiencies are rationalized in terms of coverage of the anode surface by adsorbed anionic material and the electronegativity of the electrolyte anions. At 0°C ozone current efficiencies of over 50% are reported for hexafluorophosphoric acid electrolyte which represent substantially higher yields than any previously achieved.

Nitridation of Silicon and Oxidized‐Silicon

Abstract: The nitridation of silicon and oxidized‐silicon has been studied. The nitrided films were prepared at 900°–1150°C under ammonia partial pressures of 10−3 to 5 kg/cm2 in nitrogen and were analyzed by ellipsometry and Auger electron spectroscopy. For films formed by nitridation of silicon, we found that the growth kinetics and properties such as chemical composition, etching rate, and oxidation resistance were independent of the ammonia partial pressure. The nitridation of silicon can be explained by a modified Ritchie‐Hunt theory, which assumes that a very slow surface reaction at the ammonia‐nitride interface is the rate‐determining factor, using the logarithmic rate law. According to this modified Ritchie‐Hunt theory, the nitridation of silicon proceeds mainly by cation migration under a constant electric field. On the other hand, it was found that the nitridation of oxidized‐silicon depended strongly on the ammonia parital pressure. This dependence may be caused by diffusion of ammonia or its derivatives through the oxide. The conversion of silicon dioxide to silicon oxynitride occurred throughout the oxide.

Temperature Dependence of InP and GaAs Etching in a Chlorine Plasma

Abstract: and etching in chlorine plasmas at 0.3 Torr follows an Arrhenius dependence on substrate temperature. Apparent activation energies, , of and , respectively, were determined from both optical emission of product species, and step height or weight change measurements. For , equals the heat of vaporization of , and the absolute etch rate (7 μm/min at 250°C) is in reasonable agreement with the predicted vaporization rate of . Hence, volatilization of is most likely the rate‐controlling step for etching . Sputter Auger analysis shows that etching in a chlorine plasma leaves multiple layer coverage of on (removable by washing with deionized water), and submonolayer levels of chlorine on . Both surfaces are rich in the group III element. The etched surface morphologies of and are strongly dependent on temperature, exhibiting a rough‐to‐smooth texture transition above ~250° and ~120°C, respectively.

Semiconductor liquid junction solar cell based on chemically deposited Bi/sub 2/S/sub 3/ thin film and some semiconducting properties of bismuth chalcogenides

Abstract: The photoelectrochemical behavior of chemically deposited Bi/sub 2/S/sub 3/ thin film in aqueous solution is studied. This paper also describes the dependence of the deposition of bismuth chalcogenides on the pH of the solution. The electrical resistance, mobility, and carrier concentration of bismuth chalcogenides have been measured. Optical absorption spectra reveal the bandedges which are 1.47 and 0.35 ev for Bi/sub 2/S/sub 3/ and Bi/sub 2/Se/sub 3/, respectively. The activation energy of electrical conduction of Bi/sub 2/S/sub 3/ obtained is approximately 0.9 ev. 12 refs.

Resistivities of Thin Film Transition Metal Silicides

Abstract: Resistivities of the transition metal silicides have been determined. The silicides were formed either by reacting thin metal films with silicon or polysilicon substrates or by sintering cosputtered (metal + silicon) films on silicon, polysilicon, or oxide substrates. The sheet resistance of the film was determined as a function of the sintering temperature, in the range of 300°–1100°C. At the same time, the intermetallic compound formation due to sintering at different temperatures was investigated by the use of x‐ray diffraction technique. It was found that the initial interaction between the metallic film and the substrate was slower for polysilicon substrates compared to silicon substrates; the end product, however, was identical. The resistivity of the refactory metal silicides was lowest for the disilicide and increased with increasing atomic number in a given period of the periodic table. The spread in the resistivities of the 4th period silicides was found to be larger than that of the 6th period silicides. Similarities with the borides, carbides, and nitrides of the same metals are pointed out. On the other hand, for VIII group metal silicides, resistivities were low and no correlation was observed.

High Pressure Oxidation of Silicon in Dry Oxygen

Abstract: The effects of oxidant pressure on the kinetics of thermal oxidation of (100) and (111) oriented silicon wafers in dry ambients were investigated over the temperature range of 800°–1000°C and the pressure range of 1–20 atm. The parabolic and linear rate constants were found to be proportional to oxygen pressure to the power of 1 and 0.7, respectively. A change in activation energy similar to that observed for pyrogenic steam oxidation occurs around 900°C. Oxide charges in oxides prepared in high pressure dry ambients were characterized and found to be compatible with present technological requirements for VLSI device fabrication, although increases in electron trapping due to neutral traps have been observed.

Equivalent Circuit Analysis of the Impedance Response of Semiconductor/Electrolyte/Counterelectrode Cells

Abstract: A method to determine the equivalent circuit of a semiconductor/electrolyte/counterelectrode cell is presented. The electrical analog of the phenomenon leading to frequency dependent behavior has been taken to be a resistor in series with a capacitor. The case where a large number of RC elements are present has been determined in the following way. The values of the capacitances and time constants of these elements have been assumed to be distributed exponentially. For this distribution it follows that the equivalent circuit can be represented as a frequency dependent resistor in parallel with a frequency dependent capacitor. Expressions for the frequency dependent resistance and capacitance have been derived. The impedance and admittance responses of a large number of circuits have been simulated. The circuit which best represented an n-Fe/sub 2/O/sub 3/ (1% TiO/sub 2/)/1M NaOH/Pt cell was determined by comparing the simulated responses with the measured responses. 18 refs.

Small‐Signal A‐C Response Theory for Electrochromic Thin Films

Abstract: The identification and character izat ion of the processes responsible for the electrochromic propert ies of thin transi t ion metal oxide films are mat ters of high current interest. Several authors (1-3) have applied small-s ignal a-c techniques in this area. Ho et al. (2) have analyzed their a-c data on WO3 with injected Li using the standard Randles (4) equivalent circuit, but with a modified (finite length) Warburg element. Glarum and Marshall (3) have devised a slightly different circuit from their data on IrO2 with injected protons. Both sets of authors have given some discussion of the theory underlying the use of these circuits. In a somewhat ear l ier paper (5) the present authors der ived an equivalent circuit for an electrochemical system characterized by an electrode adsorpt ion-react ion-diffusion sequence that yields the circuits ment ioned above, or parts of them, as l imit ing cases. Much of this analysis has been recent ly republished independent ly by Braunshtein et al. (6). In the present paper we discuss our earl ier t r ea tment as it might be applied to an elect rochromic system. Our t rea tment leads to an equivalent circuit which, we believe, may_ be useful in the analysis of impedance or admit tance data on electrochromic thin films, par t icular ly if the injection of atoms into the film involves an adsorbed intermediate . We consider an electrochemical cell consisting of an inert electronic conductor, a thin layer of e lectrochromic mater ia l AyB, a liquid electrolyte with mobile A + ions, and an electrode of solid A metal, or if A represents hydrogen, a hydrogen electrode. We shall assume that current flow through the system is effect ively one-dimensional , at least over the region in which a significant potential drop occurs. We also assume that AyB is a sufficiently good electronic conductor that the t ransport of A within the layer of A~B occurs purely by diffusion. We assume that the system has been al lowed to come to equi l ibr ium under a steady applied potent ial difference. Then the AyB layer has a spatial ly uniform composition and the potential drop falls essentially between the surface of the A~B layer in contact with the electrolyte and the A electrode. We ass~,me that an A-+ion combines wi th an electron f rom the conduction band to form an adsorbed in termedia te before en te r ing the AyB film. Adopting the notation of our ear l ier work (5), we let PR denote the concentrat ion of the A + ions at the point of closest approach to the AyB film, let r denote the concentrat ion of the adsorbed intermediate , and let bL denote the concentrat ion of A just inside the surface of the AuB film. Then for any deviat ion f rom the equi l ibr ium potential difference the equations governing the behavior of the reactant species at the A~B/l iquid interface may be wri t ten (5, 7)

Modeling of Porous Insertion Electrodes with Liquid Electrolyte

Abstract: The dynamics of porous insertion electrodes during charge or discharge is described by a simplified mathematical model, accounting for the coupled transport in electrode and electrolyte phases. A numerical method to evaluate the response of this model to either controlled potential or controlled current is outlined, and numerical results for the discharge of a porous TiS/sub 2/ electrode in an idealized organic electrolyte are presented. It is demonstrated how electrolyte depletion is the principal limiting factor in the capacity obtained during discharge of this electrode system. This depletion is a consequence of the mobility of the ions not inserted, therefore the performance or this type of electrode is optimized by choosing electrolytes with transport number as close to unity as possible for the inserted ion. 23 refs.

Electrochemical Studies of Coal Slurry Oxidation Mechanisms

Abstract: Oxidation of a coal slurry solution at platinum electrodes in strong acidic media has been studied in detail. The results indicate that the iron ions leached out from coal are responsible for major anodic currents. The Fe(II) is electrolytically oxidized to Fe(III) at the anode, which oxidizes coal to various products including CO/sub 2/. The rate constants for the catalytic reaction are determined to be 3.0*10/sup -5/ and 1.1*10/sup -4/ sec/sup -1/ for Fe(III) and Ce(IV), respectively. The activation energy for the processes are 13.3 and 7.0 kcal/mol for Fe(III) and Ce(IV). Current efficiencies for CO/sub 2/ production are approximately 15-30%. 6 refs.

A Proposed Hydrogenation/Nitridization Passivation Mechanism for GaAs and Other III–V Semiconductor Devices, Including InGaAs Long Wavelength Photodetectors

Abstract: A surface passivation method for , with possible extensions to and , is proposed. For , a correlation between recent device work (on solar cells, field effect transistors, MOS devices, and photodiodes) and the experimental Ga‐As‐O phase diagram provides strong evidence of the role of elemental surface or interface arsenic or arsenic oxide in device performance degradation. On this basis, a hydrogenation/nitridization passivation technique is proposed. The reactions to remove surface As and are The surfaces may be plasma coated in the same chamber with a wide bandgap nitride (e.g., ) for passivation and to tie up any elemental Ga. The hydrogenation and nitridization steps may be simultaneous if an ammonia plasma is used. A final layer for long‐term surface protection is recommended. Recent experimental data on surface treatments support this passivation mechanism. The extension to (for long wavelength optical detectors) is by thermochemical calculations supported by recent parallel measurements of the In‐Ga‐As‐O phase diagram. Note the conductive indium oxide must also be removed, as by the reaction