Compound semiconductors 110 surfaces

Flamers R. J., Coulter S. K., Ellison M. D., Flovis J. S., Padowitz D. F., Schwartz M. P., Greenlief C. M., Russell J. N. Jr Cycloaddition Chemistry of Organic Molecules With Semiconductor Surfaces Acc. Chem. Res. 2000 33 617 624 Keywords carbonyi group, semiconductor materiais, surface reaction, aikenes, aromatic compounds, azo compounds, cycioaikadienes, isothiocyanates, unsaturated compounds... 

Figure 9.60 Many different thiol-containing linkers can be used to prepare water-soluble QDs. The monothiol compounds suffer from the deficiency of being easily oxidized or displaced off the surface, thus creating holes for potential nonspecific binding. The dithiol linkers are superior in this regard, as they form highly stable dative bonds with the semiconductor metal surface that do not get displaced. The PEG-based linkers are especially effective at creating a biocompatible surface for conjugation with biomolecules.

MALDI, which is LDI utilizing a particular sample preparation). Although the performance of MALDI is superior to LDI in the analysis of many groups of compounds, LDI is still the perferred choice in some important applications, including cmde oil analysis [155], fullerene detection in rocks [156], atmospheric aerosol analysis [157], semiconductors, and surface analysis [158]. Reference 21 is a comprehensive review of the use of LDI (and several other ion sources) in analysis of inorganics. 

Schmidt, W. G. III-V compound semiconductor (001) surfaces. Applied Physics A Materials Science and Processing 75, 89 (2002). 

Pourbaix (16) has prepared theoretical stability diagrams of potential vs. pH for many common metals and nonmetalloids. A review of these results indicates that semiconductor compounds of Au, Ir, Pt, Rd, Ru, Zr, Si, Pd, Fe, Sn, W, Ta, Nb, or Ti should serve as relatively acid-stable photoanodes for the electrolysis of water. Indeed, all of the stable photo-assisted anode materials reported in the literature, as of March, 1980 (see Table III) contain at least one element from this stability list, with the exception of CdO. Rung and co-workers (18) observed that the CdO photoanode was stable at a bulk pH of 13.3. The Pourbaix diagram for Cd (16) shows that an oxide film passivates Cd over the concentration range 10.0 < pH < 13.5. Hence the desorption of the product H+ ion for the particular case of CdO must be exceptionally facile without producing an effective surface pH lower than 10.0. This anamolous behavior for CdO is not well understood. 

In molecular beam epitaxy (MBE) [317], molecular beams are used to deposit epitaxial layers onto the surface of a heated crystalline substrate (typically at 500-600° C). Epitaxial means that the crystal structure of the grown layer matches the crystal structure of the substrate. This is possible only if the two materials are the same (homoepitaxy) or if the crystalline structure of the two materials is very similar (heteroepitaxy). In MBE, a high purity of the substrates and the ion beams must be ensured. Effusion cells are used as beam sources and fast shutters allow one to quickly disrupt the deposition process and create layers with very sharply defined interfaces. Molecular beam epitaxy is of high technical importance in the production of III-V semiconductor compounds for sophisticated electronic and optoelectronic devices. Overviews are Refs. [318,319],... 

When the compound formation step is performed in the gas phase, as in ILGAR, the end product is less mobile and therefore more homogeneously distributed over the surface. Very thin continuous layers can therefore be prepared in this way, as shown by Muffler et al. (2002). In their work one or more metallic components are deposited from solution on the substrate and then converted to the semiconductor compound by exposure to a reactant gas. The method is able to produce extremely thin coatings of chalcopyrite, chalcogenide and oxide semiconductors on nearly arbitrarily shaped substrates, including very deep nanoporous structures. A number of binary and ternary compounds, such as CdS, ZnS, CulnSa, In2Sc3, ZnO, ZrOa, 263 and others, have been prepared. 

As can be seen, II-VI semiconductor compounds can be used as sensing materials in all types of gas sensors, including chemiresistors, SAW, heterojunction based, and optical. They can be applied to surface functionalizing and composites forming as well. The application of II-VI saniconductor compounds in quantum dots-based gas sensors will be discussed in Chap. 5 (Vol. 2). Operating characteristics of several Il-VI-based gas sensors are shown in Figs. 5.18 and 5.19. 

In chemical vapour deposition a mixture of the gaseous reactants is passed over a solid surface, under such conditions that the reaction will take place at the surface only. Well known examples are reactions for the formation of epitaxial silicon, e.g., by reduction of silicon tetrachloride with hydrogen, or by the decomposition of silane (silicium hydride). Also semiconductor compounds such as gallium arsenide and indium phosphide are produced in a similar manner. This is a very specialized area, a description of which is outside the scope of this book. Nevertheless, chemical vapour deposition is an interesting principle for manufacturing solid materials that might find broader application, so that a brief introduction may be useful here. 

In summary, on nonpolar compound surfaces, the surface energy is minimized by transferring electronic charge from the cation to the anion, thus yielding empty dangling bonds at the cation versus occupied dangling bonds at the anion. As we will see in Section 13.4, this is a general mechanism of the compound semiconductors, which holds also for the nonpolar surfaces of wurtzite crystals. 

However, metallic behavior could be avoided if electron correlation effects occur. Strong electron correlation effects are believed to play an important role in the electronic and structural properties of some semiconductor surfaces [38-40]. However, clear experimental evidence for electron correlation effects in order to explain the electronic properties of compound surface reconstruction has not yet been demonstrated. 

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