Space charge layer formation

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

Fig. 4.12 Diagram illustrating space charge layer formation in microcrystalline and nanocrystalline particles in equilibrium in a semiconductor-electrolyte interface. The nanoparticles are almost completely depleted of charge carriers with negligibly small band bending.

Scheme 8. Space-charge layer formation at a semiconductor-electrolyte interface...

Fig. 1.3. Space charge layer formation in a bulk and nanocrystalline semiconductor particle with band gap Eg and flat band potential of Uft in equilibrium with a solution redox system for which Fermi level is Efl. Vb is the band banding caused by depletion of charge carriers.

Fig. 9.3. Band bending and space charge layer formation at an n-type semiconductor-electrolyte interface (a) accumulation layer,...

Fig. 9.4. Comparison of the band bending, space charge layer formation and Fermi levels (E,r) for a large particle when r = r throughout the depletion layer and equation (9.18) applies, and for a small particle when r = tv and equation (9.19) applies. The semiconductor particles are considered to be in thermodynamic equilibrium with a redox pair of Nernst...

Although the observations for PPV photodiodes of different groups are quite similar, there are still discussions on the nature of the polymer-metal contacts and especially on the formation of space charge layers on the Al interface. According to Nguyen et al. [70, 711 band bending in melal/PPV interfaces is either caused by surface states or by chemical reactions between the polymer and the metal and... 

Because of the different potential distributions for different sets of conditions the apparent value of Tafel slope, about 60 mV, may have contributions from the various processes. The exact value may vary due to several factors which have different effects on the current-potential relationship 1) relative potential drops in the space charge layer and the Helmholtz layer 2) increase in surface area during the course of anodization due to formation of PS 3) change of the dissolution valence with potential 4) electron injection into the conduction band and 5) potential drops in the bulk semiconductor and electrolyte. 

The formation of two-layer PS on p-Si involves two different physical layers in which the potential-current relations are sensitive to the radius of curvature. The space charge layer of p-Si under an anodic potential is thin, which is responsible for the formation of the micro PS. The non-linear resistive effect of the highly resistive substrate is responsible for that of macro PS. The effect of high substrate resistivity should also occur for lowly doped n-Si. However, under normal conditions, the thickness of the space charge layer under an anodic potential, at which macro PS is formed, is on the same order of magnitude as the dimension... 

As shown in Eig. 3, the measured resistance R c in the depletion regime can reach several Mf2cm, but Eq. (12) indicates that it becomes equal to the effective resistance of the substrate which is less than 1 cm at the fh potential. Then the term l// sc is almost equal to zero under the depletion regime, and suddenly rises very sharply to almost infinity when the potential approaches the fb value (Fig. 4). This effect is extremely useful for a simple determination of the fb potential, which is found near OVvs. SCE, for p-type Si in a 5% HF aqueous solution. The procedure seems more rigorous, because it does not deal with an extrapolation method, and, owing to the sharp variation of the conductance, it needs just a few impedance measurements in the immediate vicinity of the fb potential. This method of investigation of the space charge layer, which reveals the formation of a depletion layer that is several Mf2 resistant, leads... 

The formation of space-charge layers at contacts or at the surface of a semiconductor may lead to the generation of photovoltages. This type of photo-emf results from the separation of electron-hole pairs under the influence of the electric field in the contact or surface space-charge region. 

When a semiconductor electrode is in contact with an electrolyte solution, thermodynamic equilibration takes place at the interface. This may result in the formation of a space charge layer within a thin surface region of the semiconductor, in which the electronic energy bands are generally bent upwards or downwards, respectively, in the cases of n- and p-type semiconductors. Fig. 2.3... 

The reaction may take place on the oxide grains only. The electron concentration of the grains is changed by the formation of a space charge layer. 

Fig. 46. Band structure model of Cu/Cu20/electrolyte with the space charge layer SCL, the valence band VB, the conduction band CB, and the sub-band SB formed by interband states, mediating electron transfer between the metal to the redox states of the Co(III) complex within the electrolyte. The formation of electron hole pairs by photoexcitation and the transfer of electrons to the empty states of the redox system via surface states SS is also indicated.

The electrochemistry of silicon is highly important as a tool for surface treatment and the formation of porous silicon. Under the reverse bias (anodic for n-type, cathodic for p-type) of silicon immersed in an electrolyte, a space charge layer is formed near the electrode surface, in which the concentration of the charge carriers differs from that of the bulk material. The width of this space charge layer depends on the type and density of dopants in the material and the potential bias. 

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