Semiconducting materials, characterization

Free-electron lasers have long enabled the generation of extremely intense, sub-picosecond TFlz pulses that have been used to characterize a wide variety of materials and ultrafast processes [43]. Due to their massive size and great expense, however, only a few research groups have been able to operate them. Other approaches to the generation of sub-picosecond TFlz pulses have therefore been sought, and one of the earliest and most successfid involved semiconducting materials. In a photoconductive semiconductor, carriers (for n-type material, electrons)... 

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

A certain relationship, which exists between the bulk and surface properties of semiconducting materials and their electrochemical behavior, enables, in principle, electrochemical measurements to be used to characterize these materials. Since 1960, when Dewald was the first to determine the donor concentration in a zinc oxide electrode using Mott-Schottky plots, differential capacity measurements have frequently been used for this purpose in several materials. If possible sources of errors that were discussed in Section III.3 are taken into account correctly, the capacity method enables one to determine the distribution of the doping impurity concentration over the surface" and, in combination with the layer-by-layer etching method, also into the specimen depth. The impurity concentration profile can be constructed by this method. It has recently been developed in greatest detail as applied to gallium arsenide crystals and multilayer structures. 

Electrodes of two-dimensional sheet polymers of phthalocyanines prepared by in situ reaction of tetracyanobenzene with thin metal films were prepared and characterized in their photoelectrochemical characteristics. Anodic photocurrents were detected, characterizing these films as n-type semiconducting materials. It was also found that such films could be reduced at more positive potentials than unsubstituted phthalocyanines. This change when compared to films of divalent unsubstituted phthalocyanines is caused by unreacted CN groups that were found in the films " . Such films therefore can be looked at as substituted phthalocyanines with electron-withdrawing CN groups which explains their photoelectrochemical characteristics. 

Also in the field of reagents for materials characterization fluorinated 1,3,4-thiadiazoles have found some applications. In fact, the couple 5-trifluoromethyl-2-mercapto-1,3,4-thiadiazolate/5,5 -bis(2-trifluoromethyl-1,3,4-thiadiazole) disulfide 265 was employed as organic redox couple in nonaqueous media to perform capacitance measurements through Electrochemical Impedance Spectroscopy (EIS) on semiconductive materials (Fig. 17) [148]. 

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