Silicon anodic films

Friction The coefficient of friction of the sealed anodic film is 0 76, falling to 0-19 after impregnation with silicone oil . These results were obtained with anodised wire. 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

Anodic bonding can be modified to bond glass plates (i.e., Pyrex to Pyrex). To achieve this, a silicon nitride film (200 nm) was used as an adhesive layer, and the anodic bonding conditions of 1500 V, 450°C, 10 min, were used [144]. In another example, a 160- im Si3N4 film was used and the bonding conditions were 100 V, 400°C for 1 h [143],... 

A p-type HgCdTe layer 3 is attached to a silicon read-out substrate 1. N-type regions 6 are formed on the side-walls of holes which go through the HgCdTe layer. A protective film 5 is formed in the vicinity of the regions 6. An anodized film 8 is formed on the layer 3 and an inverted layer (n-type) will be formed underneath the film. Electron-hole pairs generated between the detector elements will be absorbed by the inverted layer thereby suppressing cross-talk between the detector elements. 

Baranchugov V, Markevich E, Poliak E, Salitra G, Aurbach D. Amorphous silicon thin films as a high capacity anodes for Li-ion batteries in ionic liquid electrolytes. Electrochem Commun 2007 9 796-800. 

If the hole concent ration in the semiconductor is relatively low, as in low resistivity n-type germanium or silicon, the available holes in the surface region are used up at low current densities and the etch rate is slow. The anodic current under these conditions can be increased by providing additional holes at the surface. Holes produced as a result of illuminating the semiconductor give uniform electrolytic etching on n-type semiconductors. Germanium is electro-lytically etched in several electrolytes while silicon can only be dissolved anodically in fluoride solutions. A thick film of amorphous silicon forms on silicon anodes in acid fluoride solutions below a critical current density. 

This will be called the thick anode film throughout this paper to distinguish it from the thin film which is present during electropolishing silicon. The thick film can grow to a thickness of as much as several tenths of a millimeter, whereas the thin film is probably less than 100 A in thickness. 

Uhlir (7) suggested that die thick anode film which forms on silicon in aqueous HP solutions below the critical current density is a suboxide of silicon. Faust (27) and Wang (30) also assumed it to be some kind of oxide layer. On the basis of experimental results which indicated that die critical current density was determined by the rate of mass transfer of HP to the silicon, Turner (29) presumed the anode film to be (SiF 2)x ... 

Uhlir (7) found an effective valence of 2.0 + 0.2 for silicon dissolution during thick film formation. He also observed that the him reacted with water, alcohol, and even toluene with gas evolution after being dried and stored in air for as long as one year. The gas evolved has been identified as hydrogen. Turner (29) observed that pieces of the film react with explosive violence when put In contact with a strong oxidizing agent such as concentrated nitric acid. These results all indicate that the silicon in the anode film exists in some reduced form. 

The following series of electrochemical and chemical equations are proposed to explain all the observed facts concerning the thick anode film formed on silicon in HF solutions. The initial process is silicon dissolution ... 

J. SPRAGUE (Sprague Electric) I have a comment to make on your mechanism for the formation of thick anodic films on silicon. We obtained similar results except that in certain cases the silicon layer was found by x-ray diffraction, to be polycrystalline rather than amorphous. 

D. R. TURNER The difference between the two materials lies in the formation of a thick anode film on silicon. Once this film is removed and electropolishing occurs, the mechanism of dissolution is, I believe, quite similar. The amount of material etched electrolytically per coulomb of electricity is related to electrochemical equivalent/density. The electrochemical equivalent for Ge is about twice that of Si, but the density of Ge is about 2 times that of Si so etch rates are about the same. 

The electrical properties of silicon oxide play a critical role in many phenomena on sihcon electrodes, particularly in the growth of anodic films. Anodic oxides can... 

Conductive anodic silicon oxide films can be produced by doping a submonolayer of platinum in the oxide. ° The platinum is deposited on the silicon surface gal-vanostatically from 5% H2PtCl6 for a thickness from 0.002 to 2.5 monolayers. The silicon is then anodized in 0.2 M H2SO4 under illumination followed by a heat treatment. The Pt is present in the film at 0.25 (at Si/Si02) to 0.03 (SiOa/solution) atom % and either as Pt or as PtO with an energy level close to the n-Si valence band edge. 

Chemical etching is a process for removal of silicon dioxide films through dissolution in solutions. Dissolution of silicon oxides, in the context of this book, is related to the anodic behavior of silicon electrodes. However, the dissolution of anodic oxides is not well studied. In contrast, there is a wealth of information on the dissolution of other types of oxides. Much of this information must also be applicable, at least the qualitative and mechanistic nature, to that of anodic oxides. Also, because oxides of different types are commonly used in device fabrication, compiling the etch rate data of these oxides and those of silicon (presented in Chapter 7) in the same volume would be useful in practice. Additionally, because silica-water interaction, which has been extensively investigated in the geological field, is fundamental to the etching of silicon oxides, some of the results from the investigations on the dissolution of rocks and sands are also included. 

Passive films formed on a silicon surface in aqueous solutions are in general oxide films. There is rather limited systematic information on the structure and properties of thin silicon anodic oxide films, particularly those formed in solutions of high silicon solubilities. On the other hand, the thicker oxide films formed at large potentials have been better characterized (see Chapter 3) and the information associated can be used for understanding the thin oxide films formed at relatively low potentials. 

The electroless deposition of metals on a silicon surface in solutions is a corrosion process with a simultaneous metal deposition and oxidation/dissolution of silicon. The rate of deposition is determined by the reduction kinetics of the metals and by the anodic dissolution kinetics of silicon. The deposition process is complicated not only by the coupled anodic and cathodic reactions but also by the fact that as deposition proceeds, the effective surface areas for the anodic and cathodic reactions change. This is due to the gradual coverage of the metal deposits on the surface and may also be due to the formation of a silicon oxide film which passivates the surface. In addition, the metal deposits can act as either a catalyst or an inhibitor for hydrogen evolution. Furthermore, the dissolution of silicon may significantly change the surface morphology. 

Silicon is highly unstable in aqueous electrolytes due to the formation of an insulating oxide film which prevents the use of n-Si as photoanode. On the other hand, the silicon electrode has poor kinetics for hydrogen evolution which is not desirable for its use as a photocathode. Many methods have been explored to stabilize Si electrodes in aqueous solutions for possible applications as photochemical cells. They include coating the surface with noble metals, metal oxides, metal silicides, or organic materials as shown in Table 6.6. Also, some redox species, the reduction of which can favorably compete with the oxidation of silicon, can be used to stabilize silicon anodes... 

Based on a systematic study of the charge transfer kinetics of the silicon/elec-trolyte in HF solutions and the understanding that the anodic film was of amorphous... 

P. Schmuki, H. Bohni, and J. A. Bardwell, In-situ characterization of anodic silicon oxide films by AC impedance measurements, J. Electrochem. Soc. 142, 1705, 1995. 

G. Mende, H. Flietner, and M. Deutscher, Optimization of anodic silicon oxide films for low temperature passivation of silicon surfaces, J. Electrochem. Soc. 140, 188, 1993. 

G. Mende and E. Hensel, The electrophysical properties of anodically grown silicon oxide film. Thin Solid Films 168, 51, 1989. 

Another example of fibre-like deposits is given by the electrodeposition of [Per][Au(mnt)2] (Per = perylene. Figure 4.1) on a silicon anode which leads to a uniform black film. Scanning electron micrographs reveal that the film is made of nanowires (Figure 4.39). The diameter of an individual nanowire is in the 35-55 nm range, smaller than the [TTF][Ni(dmit)2]2 fibres previously described. The conductivity of... 

As with metals, semiconductors are also subject to passivation. Figure 22.9 shows the anodic dissolution and the passivation of n-type and p-type silicon electrodes in sodium hydroxide solution [13]. Silicon dissolves in basic solution in the form of soluble divalent silicon, Si(OH),iq or Si(OH)2jaq, and passivates forming a silicon dioxide film. 

Considering the influence of applied conditions on stoichiometry deviation in SiC under electrochemical treatment, let us also present some data related to silicon carbide anodization in the potentiostatic regime. The treatment was performed using HF-based electrolyte under conditions where anodic current density values are 4-10 mA cm-2. In spite of the value of the current being comparable with that used for the formation of nanoporous PSC structures, SiC anodization under potentiostatic conditions results in built-in adherent film ( anodic film in the... 

By analogy with photosensitive anodic films appearing over the surface of silicon during its treatment in fluorine-containing electrolytes, it can be suggested that the built-in film represents a mixed phase of electrochemical reaction products or results from the anodized surface amorphization [27,28]. In its turn, this indicates that under potentiostatic conditions the chemical processes at the SiC/HF junction are predominant. The results obtained emphasize that not every porous-like phase formed as a result of SiC anodization can be considered as PSC. 

Dimethylformamide stands out from the other solvents in the group. The AOS films synthesized in an electrolyte based on it demonstrate a relatively uniform dielectric properties over the anode surface. The molecules of this solvent have the largest donor number suggesting their enhanced capabilities to take part in the electron transfer to the groups with uncompensated bonds which form on the silicon anode surface. The adsorption of such molecules on the solid surface is likely to be assigned to chemisorption. 

Anodizing is an electrolytic passivation process that increases the thickness of natural oxide layers on the surface of metals [13]. It basically forms an anodic oxide finish on a metal s surface to increase corrosion resistance. For the anodizing process, the metal to be treated serves as the anode (positive electrode, where electrons are lost) of an electrical circuit. Anodized films are most often applied to protect aluminum alloys. An aluminum alloy is seen on the front bicycle wheel in Fig. 2 [14]. For these alloys, aluminum is the predominant metal. It typically forms an alloy with the following elements copper, magnesium, manganese, silicon, tin, and zinc [15]. Two main classifications for these alloys are casting alloys and wrought alloys, both of which can be either heat treatable or non-heat treatable. 

Abel, P. R. Lin, Y.-M. CeUo, H. Heller, A. Mullins, C. B. Improving the stability of nanostructured silicon thin film lithium-ion battery anodes through their controlled oxidation, ACS Nano 2012, 6, 2506-2516. 

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