The Thermal Double Donors

The n-type doping observed in germanium, accidentally polluted by air admission during zone refining, led to assume that, like chalcogens, the O element was a n-type dopant in germanium [57]. This assumption was clarified by the work of Bloem et al. [27], which proved the existence of O-related donor complexes containing several O atoms. 

A centre, called the a trap in connection with the study by Haynes and Hornbeck [98], has been reported to be the first TDD species, but without giving data on its ionization energies [159]. The electronic properties of this centre, labelled BTD-a for bistable thermal donor, or alternatively TDDO, have remained elusive for some time and they will be discussed with the metastability properties of the TDDs. To avoid any confusion in the following, the ionization energies of the different TDDs in the neutral and singly-ionized states are listed in Table 6.23. Apparently, no data have been reported for TDDi+ above TDD9. 

A thermal donor denoted TDD7, with an ionization energy of 55.8 meV when neutral, has been added to this list by Emtsev et al. [58]. The ratio E +/E o varies from 2.25 for % = 1 and stabilizes to 2.19 for i = 7, close to the EM ratio 2.21 for He-like donors. As the energy spans of the parity-allowed 

Line TDD00a TDD10 TDD20 TDD30 TDD40 

Except for TDDO, the uncertainty varies between 0.1 and 0.2 cm-1 (13 and 25peV). The ionization energy Ei0 is obtained by adding to the 3p i line position the calculated EM value (3.120 meV) of the 3p i state Elbow or near-coincidence, a [176], b [263] 

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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...

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