Silicon anodes

Fig. 7-2 Anode head and dimensions of high-silicon anode.

XPS spectra were obtained using a Perkin-Elmer Physical Electronics (PHI) 555 electron spectrometer equipped with a double pass cylindrical mirror analyzer (CMA) and 04-500 dual anode x-ray source. The x-ray source used a combination magnesium-silicon anode, with collimation by a shotgun-type collimator (1.). AES/SAM spectra and photomicrographs were obtained with a Perkin-Elmer PHI 610 Scanning Auger Microprobe, which uses a single pass CMA with coaxial lanthanum hexaboride (LaBe) electron gun. 

Application of XPS Using Silicon Anode X-Ray Source, Scanning... 

Figure 1. Capacity vs. time plot for uncoated silicon anode material.

Fig. 4.9 Cyclic voltammograms of silicon anodes of different crystal orientation (1015 crrf3, boron doped, versus a Pt pseudoreference electrode in 5% HF), with the characteristic...

Green M, Fielder E, Scrosati B, Wachtler M, Moreno JS. Structured silicon anodes for lithium battery applications. Electrochem Solid-State Lett 2003 6 A75-A79. 

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. 

Fig. 1 shows the porous silicon structures formed on different silicon wafers. Porous silicon on p-type wafer is characteri d by a sponge-like structure with pore wall thickness of 2-4 nm and 5-6 rnn for wafers with resistivity of 12 and 0.03 Q cm, respectively (Fig. la,b). Porous silicon on n-type silicon (0.01 Q cm) shows a branch-like structure (Fig. lc,d). In this case mother pores branch out and form the daughter pores. The pore wall thickness is 7-10 nm for porous silicon anodized with the light exposition (Fig. Ic) and 15-20 nm for porous silicon anodized without the light exposition (Fig. Id). 

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. 

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... 

O. Tescheke and D. M. Soares, Isolated submicrometer filaments formed by silicon anodization in HE solutions, J. Electrochem. Soc. 143, LlOO, 1996. 

V. P. Parkhutik, L. K. Glinenko, and V. A. Labunov, Kinetics and mechanism of porous layer growth during n-type silicon anodization in HF solution. Surf. Technol, 20, 265, 1983. 

V. Uritsky, Role of electron/hole processes in the initial stage of silicon anodization. Mater. Sci. Forum 185-188, 115, 1995. 

Y. Matsumoto, T. Shimada, and M. Ishida, Novel prevention method of stiction using silicon anodization for SOI structure, Yensors Acfitaiors A72, 153, 1999. 

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... 

In this paper we examine the conditions of 4-15 nm thick oxide layers synthesis against the physicochemical properties of the electrochemical system (solvents, current-conducting additives of the electrolyte, silicon anode) and their roles in chemisorption processes. 

The anodic treatment process was carried out in a combined regime first, galvanostatically at a current density of 1 mA/cm up to the formation voltages (Uform) of 10-50 V, and then, when the given Uform was reached, potentiostatically, up to the current of 0.01-0.02 mA. To measure characteristics of the anodically oxidized silicon (AOS) (thickness, dielectric properties, and their surface distribution) the anodic treatment of the silicon wafers was done in a cell with the anodization area of 5 cm. Single-crystal boron-doped (100) silicon wafers of resistivity 0.3 Ohm cm, phosphorus-doped (100) and (111) silicon substrates of resistivities 0.1 and 4.5 Ohm-cm, respectively, and boron-doped (100) and( 111) silicon wafers of resistivity 4.5 Ohm-cm were used as silicon anodes. 

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. 

Front-side illumination The condition at the pore tip of an illuminated n-type electrode is different from that in the dark, because the presence of a breakdown field is not necessary to generate charge carriers. Every depression or pit in the surface of the n-type silicon anode bends the electric field in the SCR in a way that the concave surface regions become more efficient in collecting holes than the convex ones. Concave regions are etched preferentially and the pores start to grow, consequently enhancing the local current density [68]. 

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