Semiconductors, thermodynamic

Although several single-crystal, wide-band gap semiconductors provide electrochemical and optical responses close to those expected from the ideal semiconductor-electrolyte model, most semiconducting electrodes do not behave in this manner. The principal and by far overriding deviation from the behavior described in the previous section is photodecomposition of the electrode. This occurs when the semiconductor thermodynamics are such that thermal or photogenerated valence band holes are sufficiently oxidizing to oxidize the semiconductor lattice [8,9]. In this case, kinetics routinely favor semiconductor oxidation over the oxidation of dissolved redox species. For example, irradiation of n-CdX (X = S, Se, or Te) in an aqueous electrolyte gives rise exclusively to semiconductor decomposition products as indicated by... 

Batsanov SS, Ovsyannikova lA (1966). X-ray spectroscopy and effective charges of atoms in compounds of Mn. In Chemical bond in semiconductors thermodynamics. Nauka, Minsk... 

There are difficulties in making such cells practical. High-band-gap semiconductors do not respond to visible light, while low-band-gap ones are prone to photocorrosion [182, 185]. In addition, both photochemical and entropy or thermodynamic factors limit the ideal efficiency with which sunlight can be converted to electrical energy [186]. 

Charge carriers in a semiconductor are always in random thermal motion with an average thermal speed, given by the equipartion relation of classical thermodynamics as m v /2 = 3KT/2. As a result of this random thermal motion, carriers diffuse from regions of higher concentration. Applying an electric field superposes a drift of carriers on this random thermal motion. Carriers are accelerated by the electric field but lose momentum to collisions with impurities or phonons, ie, quantized lattice vibrations. This results in a drift speed, which is proportional to the electric field = p E where E is the electric field in volts per cm and is the electron s mobility in units of cm /Vs. 

Heterostructures and Superlattices. Although useful devices can be made from binary compound semiconductors, such as GaAs, InP, or InSb, the explosive interest in techniques such as MOCVD and MBE came about from their growth of ternary or quaternary alloy heterostmctures and supedattices. Eor the successful growth of alloys and heterostmctures the composition and interfaces must be accurately controlled. The composition of alloys can be predicted from thermodynamics if the flow in the reactor is optimised. Otherwise, composition and growth rate variations are observed... 

The physical and thermodynamic properties of silane in the context of semiconductor appHcations have been reviewed in detail (8). Tabulations of properties of various silanes in the context of inorganic chemistry have also been pubHshed (9). Table 1 contains selected physical properties of inorganic silanes. 

The above data are correct to about 20 kJ mole but it will be seen that the general trend among these more covalent bonds does appear to be a decrease in stability from carbon to silicon, i.e. the same way as was found for more ionic bonds in the halides. Thermodynamic data for metallorganic methyl compounds used in the produchon of semiconductor systems are shown in Table 2.3. 

We now apply the thermodynamic and kinetic theory of Chapters 5-8 to four problems making rain getting fine-grained castings growing crystals for semiconductors and making amorphous metals. 

The reason for the exponential increase in the electron transfer rate with increasing electrode potential at the ZnO/electrolyte interface must be further explored. A possible explanation is provided in a recent study on water photoelectrolysis which describes the mechanism of water oxidation to molecular oxygen as one of strong molecular interaction with nonisoenergetic electron transfer subject to irreversible thermodynamics.48 Under such conditions, the rate of electron transfer will depend on the thermodynamic force in the semiconductor/electrolyte interface to... 

A smaller class of type II alloys of II-VI binaries also exists, including the (CdS) ,(ZnSe)i (CdS) ,(ZnTe)i (CdSe) ,(ZnSe)i (CdS) ,(CdTe)i-. (CdSe)x(CdTe)i i , and (CdS) c(ZnS)i i systems, which transform at some critical composition from the W to the ZB structure. Importantly, the transition temperatures are usually well below those required to attain a thermodynamically stable wurtzite form for the binary constituents (e.g., 700-800 °C for pure CdS and > 1,020 "C for pure ZnS). The type 11 pseudobinary CdxZni jcSe is of considerable interest in thin film form for the development of tandem solar cells as well as for the fabrication of superlattices and phosphor materials for monitors. The CdSe Tei-x alloy is one of the most investigated semiconductors in photoelectrochemical applications. 

Primarily connected to corrosion concepts, Pourbaix diagrams may be used within the scope of prediction and understanding of the thermodynamic stability of materials under various conditions. Park and Barber [25] have shown this relevance in examining the thermodynamic stabilities of semiconductor binary compounds such as CdS, CdSe, CdTe, and GaP, in relation to their flat band potentials and under conditions related to photoelectrochemical cell performance with different redox couples in solution. 

Park SM, Barber ME (1979) Thermodynamic stabilities of semiconductor electrodes. Electroanal Chem 99 67-75... 

In screening electrolyte redox systems for use in PEC the primary factor is redox kinetics, provided the thermodynamics is not prohibitive, while consideration of properties such as toxicity and optical transparency is important. Facile redox kinetics provided by fast one-electron outer-sphere redox systems might be well suited to regenerative applications and this is indeed the case for well-behaved couples that have yielded satisfactory results for a variety of semiconductors, especially with organic solvents (e.g., [21]). On the other hand, many efficient systems reported in the literature entail a more complicated behaviour, e.g., the above-mentioned polychalcogenide and polyiodide redox couples actually represent sluggish redox systems involving specific interactions with the semiconductor... 

Fig. 5.8 The energy levels of n-type M0S2 at the flat band potential relative to the positions of various redox couples in CH3CN/[n-Bu4N]C104 solution. The valence band edge of the semiconductor as revealed by accurate flat band potential measurement is at ca. +1.9 V vs. SCE implying that photooxrdations workable at Ti02 are thermodynamically possible at illuminated M0S2 as well. (Reproduced with permission from [137], Copyright 2010, American Chemical Society)...

The (photo)electrochemical behavior of p-InSe single-crystal vdW surface was studied in 0.5 M H2SO4 and 1.0 M NaOH solutions, in relation to the effect of surface steps on the crystal [183]. The pH-potential diagram was constructed, in order to examine the thermodynamic stability of the InSe crystals (Fig. 5.12). The mechanism of photoelectrochemical hydrogen evolution in 0.5 M H2SO4 and the effect of Pt modification were discussed. A several hundred mV anodic shift of the photocurrent onset potential was observed by depositing Pt on the semiconductor electrode. 

Bard AJ, Wrighton MS (1977) Thermodynamic potential forthe anodic dissolution of n-type semiconductors - A crucial factor controlling durability and efficiency in photoelectrochem-ical cells and an important criterion in the selection of new electrode/electrolyte systems. J Electrochem Soc 124 1706-1710... 

The nature and general pathway of the photocatalytic action of semiconductors are nowadays well established (see Fig. 5), though numerous structural, thermodynamic and mechanistic peculiarities often make the detailed pathways for the same PC in different reactions or different PCs in the same reaction, also somewhat different [3, 4, 6, 8]. 

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