Semiconductor/semiconductivity 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)... 

Semiconductors may also be made from a maferial which is normally an insulator by infroducing an impurify, a process known as doping. Figure 9.9 shows fwo ways in which an impurify may promote semiconducting properties. In Figure 9.9(a) fhe dopanf has one more valence election per atom fhan fhe hosf and confribufes a band of filled impurify levels 1 close to fhe conduction band of fhe hosf. This characterizes an n-fype semiconductor. An example is silicon (KL3s 3p ) doped wifh phosphoms (KL3s 3p ), which reduces fhe band gap to abouf 0.05 eY Since kT af room femperafure is abouf 0.025 eY the phosphoms... 

Since the first synthesis of mesoporous materials MCM-41 at Mobile Coporation,1 most work carried out in this area has focused on the preparation, characterization and applications of silica-based compounds. Recently, the synthesis of metal oxide-based mesostructured materials has attracted research attention due to their catalytic, electric, magnetic and optical properties.2 5 Although metal sulfides have found widespread applications as semiconductors, electro-optical materials and catalysts, to just name a few, only a few attempts have been reported on the synthesis of metal sulfide-based mesostructured materials. Thus far, mesostructured tin sulfides have proven to be most synthetically accessible in aqueous solution at ambient temperatures.6-7 Physical property studies showed that such materials may have potential to be used as semiconducting liquid crystals in electro-optical displays and chemical sensing applications. In addition, mesostructured thiogermanates8-10 and zinc sulfide with textured mesoporosity after surfactant removal11 have been prepared under hydrothermal conditions. 

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. 

The electronic paper idea and the work we ve done in organic semiconductors have had components including work by individual scientists, but there have also been components of a multidisciplinary team approach. For example, chemists have been associated with the design and characterization of the semiconducting organic materials, developing the understanding of what functionalities are needed and other factors. 

The surface concentration, size distribution and other properties of metal nanoparticles formed in a dark on the surface of the inert wide-band-gap semiconducting oxides under contact, photocatalytic, or photoelectrochemical deposition depend substantially on the concentration, bulk distribution, and energy characteristics of donor defects in the initial semiconductor substrate. As a rule, the necessary condition for the formation of the smallest-sized particles in the highest surface concentration is the maximum shift of the surface potential of semiconducting matrix from its equilibrium value during metal deposition. This is part of the reason for the experimentally observed fact that the particles formed in the condition of photocatalytic deposition are characterized by less average size and cover superior portion of surface than those obtained under cathodic deposition, all other factors being equal. 

As has been pointed out previously, ionic compounds are characterized by a Fermi level EF that is located within an s-p-state energy gap Ef. It is for this reason that ionic compounds are usually insulators. However, if the ionic compound contains transition element cations, electrical conductivity can take place via the d electrons. Two situations have been distinguished the case where Ru > Rc(n,d) and that where Rlt < Rc(n,d). Compounds corresponding to the first alternative have been discussed in Chapter III, Section I, where it was pointed out that the presence of similar atoms on similar lattice sites, but in different valence states, leads to low or intermediate mobility semiconduction via a hopping of d electrons over a lattice-polarization barrier from cations of lower valence to cations of higher valence. In this section it is shown how compounds that illustrate the second alternative, Rtt < 72c(n,d), may lead to intermediate mobility, metallic conduction and to martensitic semiconductor metallic phase transitions. 

As can be seen, the heart of the device is the semiconducting mesoporous or nanocrystalline oxide, composed of a network of nanoparticles that have been sintered together to establish electronic conduction, which is characterized by the total depletion of the semiconductor due to the small size of the nanoparticles and porous structure [65]. The Fermi level in the dark is therefore near the bandgap center, allowing for the generation of high photovoltages upon illumination [71]. 

On the semiconducting side, the interfacial layer has two zones. The first zone lies within the evanescent tail of the metal. The second zone is the remaining region where, due to interdiffusion, a composition or doping different from the original bulk semiconductor exists. A similar description can be characterized on the metal side and the new alloyed metal zone may be of sufficient width to become the metal forming the barrier. This new interfacial metal can have different characteristics from the originally deposited metal. 

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