Semiconductor/semiconductivity electronic properties

As outlined above, electron transfer through the passive film can also be cmcial for passivation and thus for the corrosion behaviour of a metal. Therefore, interest has grown in studies of the electronic properties of passive films. Many passive films are of a semiconductive nature [92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102 and 1031 and therefore can be investigated with teclmiques borrowed from semiconductor electrochemistry—most typically photoelectrochemistry and capacitance measurements of the Mott-Schottky type [104]. Generally it is found that many passive films cannot be described as ideal but rather as amorjDhous or highly defective semiconductors which often exlribit doping levels close to degeneracy [105]. 

While considering trends in further investigations, one has to pay special attention to the effect of electroreflection. So far, this effect has been used to obtain information on the structure of the near-the-surface region of a semiconductor, but the electroreflection method makes it possible, in principle, to study electrode reactions, adsorption, and the properties of thin surface layers. Let us note in this respect an important role of objects with semiconducting properties for electrochemistry and photoelectrochemistry as a whole. Here we mean oxide and other films, polylayers of adsorbed organic substances, and other materials on the surface of metallic electrodes. Anomalies in the electrochemical behavior of such systems are frequently explained by their semiconductor nature. Yet, there is a barrier between electrochemistry and photoelectrochemistry of crystalline semiconductors with electronic conductivity, on the one hand, and electrochemistry of oxide films, which usually are amorphous and have appreciable ionic conductivity, on the other hand. To overcome this barrier is the task of further investigations. 

To implement this strategy, multilayered semiconductor structures were grown by MOCVD and then processed using lithographic techniques to create trenches of 10-20 p-m. Trenches of 10 pm were used to create arrays of 34 interdigitated LED/photodiode pairs, as shown in Fig. 8. As molecules adsorb onto the surfaces of these semiconducting materials, the electronic properties of the surfaces can be altered and thus changes in current can be observed when molecules such as ammonia and sulfur dioxide adsorb onto the surfaces of the diodes. 

The structure of semiconducting solids provides a convenient basis for understanding the important electronic properties of these materials. The important optical and electrical characteristics of semiconducting solids arise from the delocalized electronic properties of these materials. To understand the origin of this electronic delocalization, we must consider the nature of the bonding within semiconductor crystals. The basic model that has been successfully used to describe the electronic structure of semiconductors is derived from the Band Theory of solids. Our treatment of band theory will be qualitative, and the interested reader is encouraged to supplement our discussion with the excellent reviews by... 

To this point, we have seen how the structural, electronic, optical, and electrical properties of semiconductors can be treated within a common framework. The bonding in the lattice determines the structure of the solid, and the structure of the lattice in turn affects the band structure. This band structure then can be used to describe the chemical, optical, and electrical properties of the semiconducting solid. Thus, chemical control over the electronic properties of semiconductors is an important component of modem research in solid-state chemistry and solid-state physics. The concepts described above enable this process to be understood from a relatively qualitative, chemically based viewpoint. Further... 

The catalytic activities of metals and semiconductors would be expected to differ due to their different electronic properties. However, under conditions of oxidation catalysis many metals become coated with a more or less thin semiconducting film of the given metal oxide, and this might be the reason why the mechanism of hydrocarbon oxidation on metals and semiconductors has much in common (59). 

Thermal properties most often follow the electronic properties quite closely. For example, thermoelectric power (TEP) studies have revealed the unique semiconductor properties of carbons. In a pioneering and largely neglected study [73,74], Walker and Tietjen had already documented the recently rediscovered [75-79] TEP changes in both flat and curved sp carbons [80]. These can lead to both p- and n-type semiconducting behavior of carbons and thus make possible the fabrication of inter- and intramolecular logic gates, the basic units of... 

It was Ioffe (1951) who first pointed out that the basic electronic properties of a solid are determined primarily by the character of the bonds between nearest neighbors rather than by the long-range order. The chemical bond approach enabled Welker (1952) to predict many of the semiconducting properties of the III—V compounds. Mooser and Pearson (1956, 1960) expanded the chemical approach to semiconductivity into a systematic survey of different compounds and crystal classes. Although in the case of crystals this approach has been replaced by band structure calculations, several properties common to large classes of amorphous semiconductors become plausible if one understands their chemical bonding. 

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