Other Semiconductors

Other Semiconductors.—Kennedy et alf have continued to study FcjOj photoelectrodes, and their most recent work shows that high efficiencies are obtained with Si-doped sintered electrodes. Dare-Edwards et alf have characterized lithium-doped NiO in some detail but, as expected, the very low carrier mobility in this material makes it quite unsuitable for solar energy conversion. Gissler has investigated trigonal Se films, and Davidson and Willsher have given further details of the properties of HgS powder photoanodes. Derivatized tin-oxide electrodes have been prepared by Fox et al.f and Janzen et al. have successfully attached the photosynthetic reaction centre molecule isolated from Rhodopseudomones sphaeroides to tin oxide (see also Section 2). 

Photosensitization.—The photosensitization of semiconductor electrodes is still the subject of a number of papers every year. Although the chance is small that single-crystal systems will benefit appreciably from photosensitization, the possibilities for high surface area systems, such as dispersions, are more exciting. Key references on photosensitization have been included in this review, although they do little to change the currently accepted view of the mechanism or efficiency of photosensitization. 

Other Semiconductors. Gerischer,41 in a very helpful paper on electrochemical photo and solar cells, has explained the mode of action of the semiconductor-electrolyte interface (when the semiconductor is in its depletion mode) as a Schottky barrier, and how this can lead to separation of hole-electron pairs 

The white light used has spectral distribution resembling that of sunlight (Reproduced by permission from/. Electroanalyt. Chem. Interfacial Electrochem., 1975, 58, 263) 

Anodic photocurrents characteristic of -type semiconductors have been observed for CdSe in the presence of ferrocyanide,42 and for CdS sensitized by rhodamine or quinocyanine, in the presence of hydroquinone as a supersensitizer.43 

THE ISOTROPIC MUON HYPERFINE PARAMETER FOR Mu IN SEMICONDUCTORS. THE s DENSITY (rj ) IS EQUAL TO THE REDUCED HYPERFINE PARAMETER A/Afree where Afrcc = 4463.302 mhz. the data marked T — 0 WERE EXTRAPOLATED TO T = 0 K 

The work on diamond is important both from an experimental and a theoretical viewpoint. Since the carbon atoms that make up diamond are simpler to deal with theoretically, some calculations on hydrogen and muonium in diamond are considered to be more reliable than similar calculations on higher-Z materials. Thus diamond can be used as a testbed for new ideas on simple defects such as muonium or hydrogen and the associated theoretical methods. For example, the first theoretical confirmation of the BC model of Mu and the metastablility of Mu was made for diamond (Claxton et al., 1986 Estle et al., 1986 Estle et al., 1987). 

To put this in better perspective, although it is true that the hyperfine values for Mu in GaAs and GaP are closer than for any other pair of similar crystals (they differ by 30 MHz or 1.0% see Table II), there are several other cases in which A values are close but just not that close. For example, Table II shows that the hyperfine parameters for ZnS and ZnSe differ by 91 MHz or 2.6% and those for Mu in CuCl and CuBr differ by 39 MHz or 3.1%. All of these could be explained if they corresponded to muonium in a tetrahedral interstitial surrounded by four cations to which they more strongly bond than to the anions, a suggestion similar to that of Souiri et al. (1987) and Cox (1987). Whether this could also be consistent with the closeness of the A values for Mu1 and Mu11 in CuCl and in CuBr, with the pLCR observation of appreciable anion bonding for Mu in CuCl (see Section IV.4) and with the cluster of hyperfine parameters in SiC near the average of the diamond and silicon values (see Section IV.5), will probably require further experimentation and especially theoretical study to determine. 

The most convincing evidence for the BC model of Mu in III-V materials comes from the nuclear hyperfine structure in GaAs. The hyperfine parameters for the nearest-neighbor Ga and As on the Mu symmetry axis and the corresponding s and p densities are given in Table I. One finds a total spin density on the As(Ga) of 0.45 (0.38) with the ratio of p to 5 density of 23 (4) respectively. The fact that 83% of the spin density is on the two nearest-neighbor nuclei on the Mu symmetry axis agrees with the expectations of the BC model. From the ratios of p to s one can estimate that the As and Ga are displaced 0.65 (17) A and 0.14(6) A, respectively, away from the bond center. The uncertainties of these estimates were calculated from spin polarization effects, which are not known accurately, and they do not reflect any systematic uncertainties in the approximation. These displacements imply an increase in the Ga—As bond of about 32 (7)%, which is similar to calculated lattice distortions for Mu in diamond (Claxton et al., 1986 Estle et al., 1986 Estle et al., 1987) and Si (Estreicher, 1987). 

The /u-LCR data also show structure from nuclei that are more distant and therefore have smaller nuclear hyperfine parameters (A 100 MHz). This structure is observed for fields applied along (110) and (111) directions for both GaAs (Kiefl, 1986) and GaP. Since these data are not yet under- 

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Small metal clusters are also of interest because of their importance in catalysis. Despite the fact that small clusters should consist of mostly surface atoms, measurement of the photon ionization threshold for Hg clusters suggest that a transition from van der Waals to metallic properties occurs in the range of 20-70 atoms per cluster [88] and near-bulk magnetic properties are expected for Ni, Pd, and Pt clusters of only 13 atoms [89] Theoretical calculations on Sin and other semiconductors predict that the stmcture reflects the bulk lattice for 1000 atoms but the bulk electronic wave functions are not obtained [90]. Bartell and co-workers [91] study beams of molecular clusters with electron dirfraction and molecular dynamics simulations and find new phases not observed in the bulk. Bulk models appear to be valid for their clusters of several thousand atoms (see Section IX-3). 

Some aspects of adsorption on oxides and other semiconductors can be treated in terms of the electrical properties of the solid, and these are reviewed briefly here. More details can be found in Refs. 84 and 182. 

StiU another method used to produce PV cells is provided by thin-fiLm technologies. Thin films ate made by depositing semiconductor materials on a sohd substrate such as glass or metal sheet. Among the wide variety of thin-fiLm materials under development ate amorphous siUcon, polycrystaUine sUicon, copper indium diselenide, and cadmium teUuride. Additionally, development of multijunction thin-film PV cells is being explored. These cells use multiple layers of thin-film sUicon alloys or other semiconductors tailored to respond to specific portions of the light spectmm. 

The excellence of a properly formed Si02—Si interface and the difficulty of passivating other semiconductor surfaces has been one of the most important factors in the development of the worldwide market for siUcon-based semiconductors. MOSFETs are typically produced on (100) siUcon surfaces. Fewer surface states appear at this Si—Si02 interface, which has the fewest broken bonds. A widely used model for the thermal oxidation of sihcon has been developed (31). Nevertheless, despite many years of extensive research, the Si—Si02 interface is not yet fully understood. 

A useful classification of sensitizing dyes is the one adopted to describe patents in image technology. In Table 1, the Image Technology Patent Information System (ITPAIS), dye classes and representative patent citations from the ITPAIS file are Hsted as a function of significant dye class. From these citations it is clear that preferred sensitizers for silver haUdes are polymethine dyes (cyanine, merocyanine, etc), whereas other semiconductors have more evenly distributed citations. Zinc oxide, for example, is frequendy sensitized by xanthene dyes (qv) or triarylmethane dyes (see Triphenylmethane and related dyes) as well as cyanines and merocyanines (see Cyanine dyes). 

In the UV most of the materials of interest, e.g. Si, polysilicon, SiGe, GaAs, and other semiconductor materials, are strongly absorbing this enables surface-sensitive measurements. Surface roughness, native oxide covering, material composition, and structural properties can be analyzed. 

The present chapter deals with the CVD of metals and some metal alloys and intermetallics. The metals are listed alphabetically. The range of applications is extensive as many of these materials play an important part in the fabrication of integrated circuits and other semiconductor devices in optoelectronic and optical applications, in corrosion protection, and in the design of structural parts. These applications are reviewed in greater depth in Chs. 13 to 19. 

Many applications of silicon are found in integrated circuits and other semiconductor devices and include the following (see Chs. 13-16 on applications of CVD). 

Thin films in the fabrication of transistors (FET) and other semiconductor applications. PI... 

The electronic properties of silicon and other semiconductor materials are shown in Table 13.2. 

Although silicon cannot compete with other semiconductor materials in specific areas, it is overall an excellent material, a fact amply shown by its dominance of the market for the last forty years. It is readily available with a high degree of purity and relatively low cost. Most IC s are made from silicon and this is likely to remain so for some time. 1 1... 

It has a high-temperature potential (>1000°C), which far surpasses that of other semiconductors. 

In 1976, Hodes et al. [85] reported the stabilization of polycrystalline CdSe photoelectrodes (prepared by electrodeposition and subsequent heat treatment) in de-aerated alkaline aqueous or organic (not specified) solution of S , in which some elemental sulfur was dissolved. It was claimed that besides CdSe, other semiconductors such as CdS, CdTe (n- and p-types), ZnSe, or Bi2S3 are also stable as photoelectrodes in a polycrystalline form in the sulfide solution, and that such cells under AMI sunlight are stable over periods of months, a rather exaggerated claim. It was described in addition how a part or all of the converted energy could be stored in a controlled way in the system by the introduction of an electrode of porous silver. 

On the other hand, the nonlinear optical properties of nanometer-sized materials are also known to be different from the bulk, and such properties are strongly dependent on size and shape [11]. In 1992, Wang and Herron reported that the third-order nonlinear susceptibility, of silicon nanocrystals increased with decreasing size [12]. In contrast to silicon nanocrystals, of CdS nanocrystals decreased with decreasing size [ 13 ]. These results stimulated the investigation of the nonlinear optical properties of other semiconductor QDs. For the CdTe QDs that we are concentrating on, there have been few studies of nonresonant third-order nonlinear parameters. 

Semiconductors. In Sections 2.4.1, 4.5 and 5.10.4 basic physical and electrochemical properties of semiconductors are discussed so that the present paragraph only deals with practically important electrode materials. The most common semiconductors are Si, Ge, CdS, and GaAs. They can be doped to p- or n-state, and used as electrodes for various electrochemical and photoelectrochemical studies. Germanium has also found application as an infrared transparent electrode for the in situ infrared spectroelectrochemistry, where it is used either pure or coated with thin transparent films of Au or C (Section 5.5.6). The common disadvantage of Ge and other semiconductors mentioned is their relatively high chemical reactivity, which causes the practical electrodes to be almost always covered with an oxide (hydrated oxide) film. 

Various other semiconductor materials, such as CdSe, MoSe, WSe, and InP were also used in electrochemistry, mainly as n-type photoanodes. Stability against photoanodic corrosion is, naturally, much higher with semiconducting oxides (Ti02, ZnO, SrTi03, BaTi03, W03, etc.). For this reason, they are the most important n-type semiconductors for photoanodes. The semiconducting metal oxide electrodes are discussed in more detail below. 

The partial oxidation of conjugated polymers is generally referred to as p-doping, again in analogy to other semiconductor materials, but the basic process is the removal of electrons as in any other branch of chemistry, i.e.,... 

Other semiconductor crystals for which photoplasticity has been observed during hardness measurements include III-V compounds (such as GaAs— Koubaiti et al., 1997), and II-VI compounds (such as ZnS and ZnO—Klopfs-tein et al., 2003). Flowever, since the effect declined in these studies with the depth of indentation, it is likely that the observations are artifacts associated with changes of the indenter/specimen friction coefficients. An extensive review of photoplastic effects in II-VI compounds is given by Osip yan et al. (1986). 

There are at least a dozen semiconductor manufacturers making the popular 384x series controllers. All of them behave slightly differently. The same applies to any other semiconductor device made by several vendors. Be cautious of so-called equivalents. So if your company s smart-alecky purchase officer has just cooked up a new deal to procure your 1N5408 diodes at half price (from some hitherto unknown manufacturer on the Mainland, for example), replace it and confirm that it is not causing the problem. 

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