Silicon/Electrolyte interface

A. Valance. Porous silicon formation Stability analysis of the silicon-electrolyte interface. Phys Rev B 52 8323, 1995. 

The fundamental and applied electrochemistry of the silicon/electrolyte interface is presented in an authoritative review by Dr. Gregory Zhang, with emphasis in the preparation of porous silicon, a material of significant technological interest, via anodic dissolution of monocrystalline Si. The chapter shows eloquently how fundamental electrokinetic principles can be utilized to obtain the desired product morphology. 

A detailed and comprehensive review on all aspects of the fundamental and applied electrochemistry of silicon/electrolyte interface was provided in a recently published book.1 The objective of this paper is to provide a conceptual analysis of the mechanisms for the morphology and formation of porous silicon using the large body of the information assembled in the book and to provide an integrated view of the formation mechanisms that can be coherent with the various morphological features on the... 

Distribution of the Applied Potential in the Electric Layers at the Silicon/Electrolyte Interface in HF Solutions ... 

The overall formation mechanism of PS must involve the fundamental electrochemical reactions in three essential aspects 1. nature of reactions, reactants, products, intermediates, number of steps, and their sequences, 2. nature and rate of charge transport in the different phases at silicon/electrolyte interface, 3. spatial and temporal distributions of reactions and the cause of such distributions. The first and second aspects, which governs the properties of a uniform and flat surface and do not involve geometric factors, have been characterized in previous Sections and the major characteristics are summarized in Table 5. This Section deals with the third aspect, that is, spatial and temporal... 

Surface lattice structure Density of active surface atoms and reactivity of the surface determined by the crystalline orientation of silicon/electrolyte interface... 

Figure 21. The energy band diagram (only the conduction band is shown) calculated for the silicon/electrolyte interface with a potential drop of 5 V and different radii of curvature. Ec is the conduction bandedge in the bulk and Ecs is the conduction bandedge at the surface. AE AEj, AE1/2, and AE1/5 are the possible tunneling energy ranges for different radii of curvature. The distribution of occupied states at the interface, Dred, is also schematically indicated. After Zhang.24...

A Schottky diode is always operated under depletion conditions flat-band condition would involve giant currents. A Schottky diode, therefore, models the silicon electrolyte interface only accurately as long as the charge transfer is limited by the electrode. If the charge transfer becomes reaction-limited or diffusion-limited, the electrode may as well be under accumulation or inversion. The solid-state equivalent would now be a metal-insulator-semiconductor (MIS) structure. However, the I-V characteristic of a real silicon-electrolyte interface may exhibit features unlike any solid-state device, as... 

Divalent dissolution is initiated by a hole from the bulk approaching the silicon-electrolyte interface which allows for nucleophilic attack of the Si atom (step 1 in Fig. 4.3). This is the rate-limiting step of the reaction and thereby the origin of pore formation, as discussed in Chapter 6. The active species in the electrolyte is HF, its dimer (HF)2, or bifluoride (HF2), which dissociates into HF monomers and l ions near the surface [Okl]. The F ions in the solution seem to be inactive in the dissolution kinetics [Se2], Because holes are only available at a certain anodic bias, the Si dissolution rate becomes virtually zero at OCP and the surface remains Si-H covered in this case, which produces a hydrophobic silicon surface. 

Fig. 6.1 Mechanisms that could produce a passivation of the silicon-electrolyte interface (top) and the corresponding band diagrams (bottom). Note that passivation by an anodic...

Silicon has been and will most probably continue to be the dominant material in semiconductor technology. Although the defect-free silicon single crystal is one of the best understood systems in materials science, its electrochemistry to many people is still a matter of alchemy. This view is partly a result of the interdisciplinary aspects of the topic Physics meets chemistry at the silicon-electrolyte interface. 

Wunsch F, Nakato Y, Tributsch (2002) Minority carrier accumulation and interfacial kinetics in nano-sized Pt-dotted silicon electrolyte interfaces studied by microwaves techniques. J Phys Chem B 106 11526-11530... 

Although surface reactions at the silicon/electrolyte interface have been studied for many years, the nature of the interactions between the silicon surface and fluoride containing electrolytes are only recently becoming understood. Recent characterizations using various spectroscopic and microscopic techniques have resulted in new insight into the properties of silicon surfaces on an atomistic scale. Significant issues remain to be resolved, however, such as the reaction mechanisms, the processes leading to pore formation, and the nature of surface oxides. 

It can be expected from the nature of silicon/electrolyte interfaces described in the previous sections that the surface states on silicon electrodes may have different physical and chemical characteristics such as type, quantity, distribution, transfer kinetics, and so on, depending on the surface condition. Table 2.12 shows examples of measurements of surface states reported in the literature. Thus, while the energy levels in bulk silicon and electrolyte can be described by a general theory, those of surface states can only be dealt with by specific theories applicable to the specific situations. 

TABLE 2.12. Observation of Surface States at Silicon/Electrolyte Interfaces"... 

Surface states at a silicon/electrolyte interface are determined by the preparation prior to entering the electrolyte and by the changes that occur in the electrolyte. Thus, in general, an experimentally determined flatband potential is specific to the particular silicon/electrolyte interface at a given time and may significantly vary even with the same material and electrolyte. 

FIGURE 2.34. Examples of the energetic positions of the band edges determined for a number of silicon/electrolyte interfaces. The energy levels of the redox couples related to Si and H2O are also plotted. 

The band diagram of silicon electrode in an electrolyte can be drawn when the flatband potential of the interface is determined. Figure 2.34 shows the band diagrams for various silicon/electrolyte interfaces. As described above, the flatband potential depends on many factors specific to the silicon/electrolyte interface under a given set of experimental conditions, as does the band diagram. 

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