Dissolution of semiconductors

From a chemical point of view a hole at the surface of a semiconductor entails a missing electron and hence a partially broken bond. Consequently semiconductors tend to dissolve when holes accumulate at the surface. In particular this is true for enrichment layers of p-type material. At the depletion layers of n-type materials the holes required for the dissolution can also be produced by photoexcitation. 

Such dissolution reactions usually contain several steps and are complicated. An important example is silicon. In aqueous solutions this is generally covered by an oxide film that inhibits currents and hence corrosion. However, in HF solutions it remains oxide free, and p-type silicon dissolves readily under accumulation conditions. This reaction involves two holes and two protons, the final product is Si(IV), but the details are not understood. A simpler example is the photodissolution of n-type CdS, which follows the overall reaction  

On polar semiconductors the dissolution may also involve electrons from the conduction band, leading to the production of soluble anions. For example, under accumulation conditions the dissolution of n-type CdS takes place according to the reaction scheme  

The dissolution of semiconductors is usually an undesirable process since it diminishes the stability of the electrode and limits their use 

See for example, N. W. Ashcroft and N. D. Mermin, Solid State Physics, Holst, Rinehart and Winston, 1976, p.221 ff or any other textbook on solid state physics. 

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The same disciission may apply to the anodic dissolution of semiconductor electrodes of covalently bonded compounds such as gallium arsenide. In general, covalent compoimd semiconductors contain varying ionic polarity, in which the component atoms of positive polarity re likely to become surface cations and the component atoms of negative polarity are likely to become surface radicals. For such compound semiconductors in anodic dissolution, the valence band mechanism predominates over the conduction band mechanism with increasing band gap and increasing polarity of the compounds. 

Equation 9—49 is the anodic transfer of surface cation into aqueous solution (cation dissolution) and Eqn. 9-60 is the anodic oxidation (hole capture) of surface anion producing molecules ofX2, i (e.g. gaseous oxygen molecules irom oxide ions). Electric neutrality requires that the rate of cation dissolution equals the rate of anion oxidation hence, the rate of the oxidative dissolution of semiconductor electrode can be represented by the anodic hole current for the oxidation of surface anions. 

Fig. 9-16. Polarization curves of anodic oxidative dissolution and cathodic reductive dissolution of semiconductor electrodes of an ionic compound MX iiixcp) (iMxh )== anodic oxidative (cathodic reductive) dissolution current solid curve = band edge level pinning at the electrode interface, dashed curve = Fermi level pinning.

The study of the anodic dissolution of semiconductors has played an important role in clarifying the nature of faradaic processes occurring at semiconductor surfaces [112, 113]. In principle, anodic dissolution on such semiconductors as Ge, GaAs, and GaP might proceed either by hole capture from the VB or electron injection into the CB. For Ge, for example... 

The chapters in this volume address challenging problems associated with the observation and interpretation of anodic dissolution of semiconductors, electrode reactions in nonaqueous solvents, and charge-transfer across the interface between two immiscible electrolytes. In-situ FTIR spectroscopy of surface reactions, and a review of electrochemical methods of pollution abatement complete the range of timely topics included. 

The tetravalent state is the stable fomi of a silicon when oxidized. The mechanism of oxidation and dissolution of silicon and geimanium was first studied by Turner et al. in an interest to understand the etching and cleaning of silicon and germanium surfaces [2], The mechanism of the oxidation and dissolution of semiconductor is like that of a metal except that (1) two types of charge carriers can be involved, valence band holes and conduction band electrons, and (2) the density of charge carriers at the solid-liquid interface is much smaller for the semiconductor than for a metal. The electrochemical reaction of silicon in an aqueous solution is given by Eq. 1 ... 

Dissolution of Semiconductors. The mechanism of dissolution of semiconductors is essentially similar to that of metals, described above. The basic difference in the dissolution behavior of metals and semiconductors lies in the concentration and type of charges responsible for surface reactions. In semiconductors, the concentration of charge carriers is much smaller than in metals because of the predominantly covalent nature of bonding. The electron transfer process may involve either valence band or conduction band electrons at the semiconductor electrode while only conduction band electrons take part at metal electrodes. Furthermore, the kinetics of dissolution of metals is determined by electrochemical reactions occurring in the soiution or at the solution-metal interface, whereas the rate-determining process in the dissolution of semiconductors may also involve phenomena taking place inside the surface. 

In contrast with metals, there are two types of carriers that can take part in the anodic and cathodic partial reactions involved in the dissolution of semiconductors electrons in the conduction band and holes in the... 

From Eqs. 47a and 47b, we note that the rate of production of electrons and holes is proportional to (c - a), and is equal to (c - a) times the dissolution rate. For c > a, there is a net generation of electrons and holes, implying, thereby, that the dissolution process is not controlled by charge carriers (carrier control). For c < a, on the other hand, there Is a net consumption of electrons and holes by the dissolution reaction. This means that, in this case, the supply of carriers controls the dissolution process (diffusion control). It should be noted that electrons and holes are produced in equai numbers during the electrochemical dissolution of semiconductors (Eqs 47a, 47b). 

The mechanism of dissolution of semiconductors usually involves a number of discrete steps in which intermediate reactions corresponding to surface states take place (17-25). For example, to describe the dissolution kinetics of GaAs in Cr03-HF solutions, the following oxidation and reduction steps have been proposed (19)(25). Oxidation of GaAs takes place in a series of consecutive steps ... 

Information aboutthe mechanism of dissolution of semiconductors may be obtained from investigations of current-potential characteristics of a semiconductor-electrolyte system (16)( 18-23), and from photoluminescence and electroluminescence spectra of semiconductor-electrolyte interface (26-28). For details of surface reaction mechanisms, the reader is referred to the above cited literature. 

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