Semiconductors, characteristic oxide films

An interesting example of the kinetic effect in semiconductor photocorrosion is photopassivation and photoactivation of silicon (Izidinov et al., 1962). Silicon is an electronegative element, so it should be dissolved spontaneously and intensively in water with hydrogen evolution. But in most of aqueous solutions the surface of silicon is covered with a nonporous passivating oxide film, which protects it from corrosion. The anodic polarization curve of silicon (dashed line in Fig. 20) is of the form characteristic of electrodes liable to passivation as the potential increases, the anodic current first grows (the... 

An interesting avenue for investigation is to examine the adsorption characteristics on single crystals concurrently with electrical measurements. Thus, any relationship which possibly exists between the slow states and the chemisorption might be positively revealed. Examination of the adsorption characteristics of reduced germanium crystals and the effect of the fast states would also be of interest. These studies have been initiated. It remains clear at this time, however, that the semiconductor properties of the germanium influence the surface properties of the thin oxide films supported thereon. The influence is clear in the case of propanol adsorption and the differences are even more dramatic in the case of water adsorption. 

Thns, Ch acts as a capacitor in series with the parallel combination of Csc and Css- hi many sitnations becanse of the presence of an oxide film on the surface, particularly for sihcon in non-HF electrolytes, an extra RC component is involved due to the charges and states in the oxide and at the semiconductor/oxide interface. The characteristics of these charges and states are discnssed in Chapter 3 on sihcon oxide. 

B. Agius, M. Froment, and S. R. Rochet, Oxygen transport studied by labeling in thin thermal silicon oxide films in connection with their structural characteristics, in Passivity of Metals and Semiconductors, M. Froment (ed.), p. 453, Elsevier, Amsterdam, 1983. 

Since protection of electrodes against corrosion in the photoelectrolysis cells is a question of vital importance, many attempts have been made to use protective films of different nature (metals, conductive polymers, or stable semiconductors, eg., oxides). Of these, semiconductive films are less effective since they often cause deterioration in the characteristics of the electrode to be protected (laying aside heterojunction photoelectrodes specially formed with semiconducting layers of different nature [42]). When metals are used as continuous protecting film (and not catalytical "islands" discussed above), a Schottky barrier is formed at the metal/semiconductor interface. The other interface, i.e., metal/electrolyte solution is as if connected in series to the former and is feeded with photocurrent produced in the Schottky diode upon illuminating the semiconductor (through the metal film). So, the structure under discussion is but a combination of the "solar cell" and "electrolyzer" within the photoelectrode Unfortunately, light is partly lost due to absorption by the metal film. 

In this section, the basic characteristics of time-resolved THz spectroscopy (TRTS) are discussed. Generation and detection of THz radiation is briefly introduced and the layout and operation of a common THz setup is described. Also, theoretical models suited to describe the response of carriers in the THz frequency window are presented for three different semiconductor geometries that are relevant for conventional and novel PV applications (Section 11.2.2.1) bulk semiconductors, (Section 11.2.2.2) polycrystals and mesoporous oxide films and (Section 11.2.2.3) semiconductor quantum dots (QDs). 

The above mechanistic aspect of electron transport in electroactive polymer films has been an active and chemically rich research topic (13-18) in polymer coated electrodes. We have called (19) the process "redox conduction", since it is a non-ohmic form of electrical conductivity that is intrinsically different from that in metals or semiconductors. Some of the special characteristics of redox conductivity are non-linear current-voltage relations and a narrow band of conductivity centered around electrode potentials that yield the necessary mixture of oxidized and reduced states of the redox sites in the polymer (mixed valent form). Electron hopping in redox conductivity is obviously also peculiar to polymers whose sites comprise spatially localized electronic states. 

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