EM Acceptor Spectra

The first report of the absorption spectrum of acceptors in semiconductors is probably the paper by Burstein et al. [26], showing the LHeT transmission spectra of two p-type silicon samples. In one of these spectra, broad electronic lines, attributed to boron, could be seen at 0.034, 0.040 and 0.043 eV while in the other, from a nominally undoped sample, lines near 0.055 and 0.06 eV were observed, now known to be due to the aluminium acceptor. Since then, many acceptors have been identified in silicon and other semiconductor crystals and with the same technological incentive as for donors, their optical spectroscopy has been widely used to characterize them, evaluate their concentrations, provide physical insight into the VB structures of the crystals and more recently evaluate the effect of impurity isotope broadening in quasi-monoisotopic crystals. 

The electrical and optical activity of acceptors as a function of their charge state and of the electrical compensation of the semiconductor can be derived in the same way as what has been described for donors. Also, in all semiconductors and insulators, many of the spectroscopic properties of the hydrogen-like acceptors are determined by the energy structure of the VB maximum, located at k = 0. There is no strict equivalent of the Fano resonances observed for donors in crystals with several equivalent CB minima in k space, but discrete 

In this chapter, the experimental results of the acceptor absorption spectra are presented. We follow the same sequence as in Chap. 6, beginning with acceptors in group-IV crystals and extending later to compound materials. For the donor spectra, a pseudo-atomic notation of the lines could be established relatively quickly. On the contrary, for the acceptor spectra, whose interpretation was essentially more difficult, notations varied from one material to the other, and I have taken some time, especially in the case of silicon, to try to establish a correlation between the lines and their spectroscopic attributions. 

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In B- and AZ-doped FZ silicon samples subjected to NTD, two new EM acceptor spectra are observed after annealing at 500-600° C, besides the above-discussed B-X and Al-X spectra [179]. These spectra, called Bntd and AZntd, are similar to those of the isolated acceptors, but their ionization energies are 28.24 and 43.25meV for Bntd and AZntd, respectively. The ionization energy of Bntd is smaller than the EM energy calculated for acceptors in silicon (31.6 meV) by Baldereschi and Lipari [13]. Such centres have also been... 

Fig. 7.16. Absorption between 1000 and 1200 cm-1 of EM acceptor spectra of the Be-II centre (Be2°) in silicon. The main spectrum is denoted a and the weaker one (3. The infinity symbol gives the ionization limit. Numbering 1 of Table 7.1 is used. The lines 4-4A-4B are not resolved in these spectra. The weak feature at 125.4 meV is not related to Be2° [77]. Reproduced with permission from Trans Tech Publications...

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