Oxidation of the semiconductor

Etch Mechanisms. Most wet etches for the compound semiconductors employ oxidation of the semiconductor followed by dissolution of the oxide. For this reason, many wet etches contain the oxidant hydrogen peroxide, although nitric acid can also be used. One advantage of wet etching over dry is the absence of subsurface damage that is common with dry etching. Metal contacts placed on wet-etched surfaces exhibit more ideal characteristics than dry-etched surfaces. 

Annealed Films. For many years, the CdSe photoelectrode— polysulphide electrolyte PEC— was probably the most studied system in PEC research. The bandgap of (bulk—see later) CdSe is ca. 1.7 eV, which is close to the theoretical optimum of 1.5 eV for photovoltaic cells in general and in relative terms, that system was fairly stable in terms of self-oxidation of the semiconductor fihn in the electrolyte by the photogenerated holes. 

In the development of photoelectrochemical (PEC) solar cells, one of the most difficult problems is the corrosion problem. In any solvent, but particularly in solvents with water present, anodic currents flowing from the solid to the solution will usually lead to corrosion. Specifically the corrosion will take the form of anodic oxidation of the semiconductor, with the products remaining as a film, dissolving into the solution, or evolving as a gas. Any such action will degrade the solar cell. 

The multi-equivalent anodic oxidation of the semiconductor involves several consecutive electrochemical steps, the first of which can be symbolized as... 

In this section we use two types of complex charge-transfer reaction to illustrate the general approach to the elucidation of reaction mechanisms at single crystal electrodes. These reactions are photocurrent doubling at -type and p-type semiconductors and the (photo)anodic oxidation of the semiconductor itself. 

Kinetic studies in which oxidation of a reducing agent present in solution competes with oxidation of the semiconductor have yielded a wealth of information, allowing quite detailed dissolution mechanisms to be proposed [88]. Generally, in these studies an RRDE is used the products formed at the semiconductor disk are detected electrochemically at a noble metal ring. 

The reactions of Eqs. 50a and 50b correspond to complete oxidation of the semiconductor as SiOj. However, if the depletion of the oxidizing agent in the vicinity of the interfacial layer is fast, incomplete oxidation takes place in which the number of charges involved in the oxidation process is less than that predicted by these equations. For example, corresponding to Eq. 50b for the dissolution of Si we have (15) ... 

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