Thermal double donors

We are faced with two interconnected problems related to the intelligibility of the presentation. The first one concerns the nomenclature of the centres other than isolated atoms and the second the labelling of the optical transitions. These problems are not trivial, [5], but not as severe for H-like centres as for deep centres. The different notations for the shallow thermal donor complexes in silicon, discussed in Sect. 6.4.2, are however, a counter-example of this statement. In this book, on the basis of the present knowledge, names of centres, in direct relation with their atomic structure, have been privileged, but the usual label has however been indicated. When the exact structure is not simple and when there exist an acronym, like TDD for thermal double donor , it has been used. The labelling by their excited states of the transitions of the shallow donor centres and of similar species, whose spectra... 

The positions of the lines of the known EM donor spectra reported in germanium for different centres (except the so-called thermal double donors, discussed separately, and those where the position of only one line is known) are given in Table 6.7. 

Table 6.23. Ionization energies (meV) of the thermal double donors TDDi in silicon, denoted here i, and indexed 0 and + in the neutral and singly-ionized states...

The electronic absorption of the O-related thermal double donors (TDDs) in silicon has been discussed in Sect. 6.4.1.1. The absorption spectra under stress of the first TDD°s (2po and 2p i lines) are rather puzzling at first sight [137]. They are characterized by the absence of splitting of the lines for F//... 

Data were obtained by using Fourier-transform infra-red spectroscopy. A model which assumed the sequential generation of thermal double-donors, and a fast-diffiising O dimer, was found to reproduce the experimental data. The diffusivity of the O dimer was estimated to be some 10 times that of interstitial O at 400C, and could be described by ... 

Sulphur, selenium and tellurium can be incorporated into Si in a variety of forms (Grimmeiss et al., 1981 Wagner et al., 1984). As isolated ions, they are all double donors, with levels around 260 and 550 meV from the conduction band. These impurities may also be introduced as pairs, which also act as a double donors (Pensl et al., 1986). Depending on the thermal history of the Si during diffusion of S, Se and Te, they may also be incorporated as higher-order impurity complexes (Grimmeiss et al., 1981 Wagner et al., 1984). 

Shallow donors (or acceptors) add new electrons to the CB (or new holes to the VB), resulting in a net increase in the number of a particular type of charge carrier. The implantation of shallow donors or acceptors is performed for this purpose. But this process can also occur unintentionally. For example, the precipitation aroimd 450°C of interstitial oxygen in Si generates a series of shallow double donors called thermal donors. As-grown GaN crystal are always heavily n type, because of some intrinsic shallow-level defect. The presence and type of new charge carriers can be detected by Hall effect measm ements. 

Bauld and coworkers, especially, developed the analogous Diels-Alder (4 + 2) cycloaddition reactions. These reactions are conveniently catalyzed by tris(4-bromophenyl)aminium hexachloroantimonate (78) or by photosensitization with aromatic nitriles. The radical cation-catalyzed Diels-Alder reaction is far faster than the uncatalyzed one, and leads to some selectivity for attack at the least substituted double bond for the monoene component (Scheme 18, 79 —> 80), but only modest endo selectivity (e- and x-80) [105]. Cross reactions with two dienes proved to be notably less sensitive to inhibition by steric hindrance of alkyl groups substituted on the double bonds than the uncatalyzed reactions, as cyclohexadiene adds detectably even to the trisubstituted double bond of 2-methylhexadiene (82), producing both 83 and 84. Dienes such as 85 react with donor-substituted olefins (86) to principally give the vinylcyclobutene products 87, but they may be thermally rearranged to the cyclohexene product 88 in good yield [105]. Schmittel and coworkers have studied the cation radical catalyzed Diels-Alder addition of both... 

Silenes (compounds with an Si=C double bond) are known to be extremely reactive compounds. Till now, only a few silenes, stabilized kinetically by bulky substituents, could be isolated under normal conditions and were characterized by X-ray structural analyses [1]. An effective stabilization of Si=C systems is also achieved by coordination of a base, such as amines, THF, or fluoride ions, to the electrophilic silene silicon atom [2], and recendy we succeeded in synthesizing thermally unexpectedly stable intramolecularly donor-stabilized silenes [3]. In this paper, the synthesis and, in particular, the behavior of the l-[2,6-bis(dimethylaminomethyl)phenyl]silenes 2a-d are described. 

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