Porous silicon condition

Nickel-mesoporous silicon structures are of considerable industrial interest for various applications. Anisotropy of magnetic properties of the nickel nanowires inside porous silicon conditioned by their high aspect ratio is applicable for the magnetic store production [1], Moreover, these structures offer much promise for the rectenna (a special type of antenna that is used to directly convert microwave energy into DC electricity) fabrication. So, it is of value to study in detail the process of the nickel electrodeposition into pores of porous silicon and elaborate control methods for pore filling with metal. 

Figure 4. Formation condition for porous silicon solid line - peak current density, dotted line - current density at the maximum slope (see Figure.2).18...

Porous silicon was discovered over 35 years ago by Uhlir.28 The porous material is created by electrochemical dissolution in HF-based electrolytes. Hydrofluoric acid, on its own, etches single-crystal Si extremely slowly, at a rate of only nanometers per hour. However, passing an electric current between the acid electrolyte and the Si sample speeds up the process considerably, leaving an array of deep narrow pores that generally run perpendicular to the Si surface. Pores measuring only nanometers across, but micrometers deep, have been achieved under specific etching conditions. 

This chapter will focus on organic/silicon interfaces formed via solution phase reactions using hydrogen-terminated crystalline silicon surfaces as a starting point. While some of the surface chemistry issues have been reviewed previously [7,8], more recent developments will be emphasized here. We will not discuss the considerable literature of reactions with porous silicon [8], or studies of molecules reacting with clean silicon surfaces under ultrahigh vacuum (UHV) conditions [9-11] which have been reviewed elsewhere. 

Historically, the first reports of porous silicon layers were by Uhlir [59] and Turner [60]. These authors reported on the electropolishing of silicon and noted that under certain conditions a porous layer was formed at the silicon surface. The first models for porous layer formation assumed that the layer was formed on the silicon substrate by a deposition process thought to involve the reduction of divalent silicon to amorphous Si via a disproportionation reaction in solution [61]. Subsequently, Theunissen [62] showed that the porous structure was the result of a selective etching process within the silicon, contradicting the silicon deposition model. 

Deviation of 60 mV/decade can be seen in Table 5.3 under different conditions. In addition to the potential distribution in the two double layers, there are two other possible causes for the deviations. The first is possible potential drops in other parts of the electrical circuit, e.g., in the electrolyte and semiconductor. The second possibility is the change of effective surface area due to the formation of a porous silicon layer during the course of i-V curve measurement. In addition, if the reaction is controlled by a process involving the Helmholtz layer, the apparent Tafel slope may be smaller than the 60 mV/decade as would be expected from the formula, B = kTI23anq, because the effective dissolution valence n is not a constant with respect to potential but varies from 2 to 3 in the exponential region. 

Silicon exhibits a diverse range of electrochemical phenomena, such as current oscillation, anisotropic etching, formation of porous silicon, etc. Each of these phenomena has extremely rich details that are governed by complex relationships between structures and properties of silicon electrodes on the one hand and between properties and experimental conditions on the other. The silicon/electrolyte interface is a complex system in which a great many variables are interacting with each other in a great many ways." ... 

I. Ronga, A. Bsiesy, F. Gaspard, F. Herino, M. Ligeon, and F. Muller, Electrical characterization of the silicon-electrolyte interface in the conditions of porous silicon formation, J. Electrochem. Soc. 138, 1403, 1991. 

The mechanism of electrochemical etching to produce porous silicon has been studied by a number of researchers [11-13]. Although it is certain that several different reactions are occurring simultaneously, anodic etching of crystalline silicon ultimately leads to oxidation and dissolution of the surface to silicon hexafluoride (Scheme 16.1). Under these conditions, Si-Si bonds are electrochemically activated and react with fluoride ions to form soluble, molecular perfluoro species solvation of these silicon fluorides by the etching medium yields a physically irregular, high area porous silicon matrix. Visual indicators for the anodization are the appearance... 

Ongoing investigations into the chemistry of porous silicon surfaces seek to develop methods for the preparation of chemically functional interfaces that protect the underlying silicon nanocrystallites from degradation without changing or annihilating their intrinsic behavior. The native, hydride-terminated surface is only metastable under ambient conditions and oxidation of freshly prepared porous silicon commences within minutes when exposed to air. While surface oxide can suitably passivate the nanocrystalline silicon and stabilize its photoluminescence, the electrically insulating and structurally defective character of this oxide layer... 

Luminescence may also arise as a consequence of electrode reactions at semiconductors that result in injection of minority carriers under accumulation conditions (see references in Kelly et al., 1999). An interesting example is the efficient red electroluminescence observed during the reduction of persulphate ions at a porous silicon layer on n-Si (Meulenkamp et al, 1995). Here the luminescence arises from electron/hole recombination, and the holes are injected by the persulphate radical ion according to the scheme... 

Platinum ions reduce to metallic Pt by injecting holes into the Si valence band. Thus Pt ions act as an oxidizing agent for silicon, and result in the simultaneous formation of photoluminescent porous silicon under certain conditions. Nickel ions may exchange charge with both the conduction and the valence band. The reduction of Ni ions competes with hydrogen evolution, and the deposition of Ni can only be achieved at high pH where it is kinetically faster. The role of silicon surface states as reaction intermediates is discussed. 

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