Illumination dissolution valence

Fig. 4.5 Dissolution valence nv as a function of anodic current density for low doped p-type and strongly illuminated, low doped n-type samples (<1017cnT3, 2.5% HF, at RT). For current densities belowJPS the samples were measured with and without the microporous layer. This produces a minor difference in indicated by two data points.

The efficiency of hydrogen evolution and effective dissolution valence are directly correlated, and their relation varies with potential, illumination, and doping of the silicon. The overall relation among these two parameters and the factors are summarized in Fig. 5.23. 

FIGURE 5.23. Summary of effective dissolution valence and hydrogen evolution efficiency as a function of potential for different materials and illumination conditions. L is illumination intensity. 

Reaction paths (1) and (11) in Fig. 5.70 account for the anodic reactions onp-Si and illuminated n-Si in HF solutions at high light intensities. Path (1) is involved in the exponential region at an anodic potential much lower than Vp responsible for direct dissolution of silicon and dissolution valence of 2, while path (11) is involved at a potential above Vp responsible for the indirect dissolution of silicon through formation and dissolution of oxide and for the dissolution valence of 4. At a potential that is lower... 

As discussed in Chapter 5, the effective dissolution valence of silicon, n, can vary between 2 and 4 depending on the type of silicon, potential, and illumination intensity. In general, n increases with increasing anodic current density for all types of silicon substrates as shown in Fig. 5.21. The value of n sharply changes at the peak current... 

For p-Si in the PS formation region, n tends to increase with doping concentration particularly at high doping concentrations as shown in Fig. 8.8 for n-Si the effect of doping concentration on dissolution valence is seen to depend on current density. It decreases with increasing illumination intensity as shown in Fig. 8.9. Substrate orientation has little effect on n as shown in Fig. 8.10. ° ... 

FIGURE 8.8. Dissolution valence versus dopant concentration. Illumination intensity is 26mW/cm. After Arita. ... 

Hydrogen evolution on silicon may proceed chemically or electrochemically depending on the conditions. Hydrogen evolution near OCP and at anodic potentials can proceed completely chemically, that is, without involving the carriers from the electrode. The chemical nature of hydrogen evolution is responsible for less than 4 of the silicon effective dissolution valence as shown in Fig. 6. A change from a chemical process to an electrochemical process occurs when the potential varies from anodic values to cathodic values as schematically illustrated in Fig. 10. Hydrogen evolution at cathodic potentials is predominantly electrochemical due to the lack of silicon dissolution and abundance of electrons on the surface on ra-Si or illuminated p-Si. 

The effective valence for dissolution increases from a value of about 2 at low current density to a value of 4 in the electropolishing domain [59-63, 67-69], as shown in Fig. 10a [63] and Fig. 10b [67]. While gravimetric determinations may be subject to a number of errors derived from oxide formation and hydrogen adsorption, these data show that the dissolution process associated with pore formation, in the absence of illumination, proceeds through interfacial charge transfer reaction involving two... 

Deposition of metals on a silicon surface can be either a conduction band process or a valence band process depending on the redox potential of the metal and solution composition. Deposition of Au on p-Si in alkaline solution occurs only under illumination indicating that it is a conduction band process due to the unfavorable position of the redox couple for hole injection. " On the other hand, deposition of platinum on p-Si can occur in the dark by hole injection into the valence band. For Cu, although the deposition proceeds via the conduction band as shown in Fig. 6.9, it can also proceed via the valence band because a large anodic current of n-Si occurs in the dark in copper-containing HF solution as shown in Fig. 6.10. The reduction of copper under this condition is via hole injection. The holes are consumed by silicon dissolution and the silicon reaction intermediates then inject electrons into the conduction band, resulting in the anodic current on n-Si in the dark. 

For n-Si, anodic reactions in the dark at current densities higher than the limiting current is a conduction band process but can involve both the conduction band and the valence band under illumination. The relative participation of conduction band versus valence band is a function of light intensity the conduction band dominates at a low light intensity and the valence band at a high light intensity [34-36]. Participation of holes and electrons in the dissolution reactions varies with the type of silicon, illumination conditions, and electrode potential. 

When photoelectrochemical solar cells became popular in the 1970s, many reports appeared concerning the stability, dissolution, and flat-band potential of semiconductors in solutions. These papers investigated parameters such as the energy level of the band edges, which is critical for the thermodynamic stability of the semiconductor and how to determine the potential for the onset of the (photo) electrochemical etching [38-40]. The criterion for thermodynamic stability of a semiconductor electrode in an electrolyte solution is determined by the position of the Fermi level with respect to the decomposition potential of the electrode with either the conduction band electrons or valence band holes E. Under illumination, the quasi-Fermi level replaces the Fermi level. The Fermi level is usually found within the band gap of the semiconductor and its position is not easily evaluated (especially the quasi-Fermi level of minority carriers). Therefore it was found more practical to use the conduction band minimum (Eq) and valence band maximum (Ey) as criteria for electrode corrosion. Thus, a semiconductor will be corroded in a certain electrolyte by the conduction band electrons if its... 

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