EM Acceptors

Since the first energy level calculations of the EM centres in silicon and germanium [28,34], many calculations have been undertaken to explain quantitatively the absorption and photoluminescence (PL) spectra associated with these centres in many semiconductors. The first part of this chapter is devoted to the presentation of the energy level calculations of EM donors and it is followed by the results of the calculations for EM acceptors. The modification to EMA, which is independent of the chemical nature of the centres, is also discussed. The chapter concludes with results of calculations of the oscillator strength (OS) for transitions between the ground states and the acceptor or donor states. 

The levels structure of the EM acceptor centres is determined by the characteristics of the VB of their host crystal near from its absolute extremum. As mentioned before, this extremum is located at k = 0 in most semiconductors. The contribution of the atomic p states of the constituent semiconductor atoms is predominant in the VB (for the compound crystals, it is related to the most electronegative atom). When spin-orbit (s-o) coupling is included, the pseudo-angular momentum J associated with the upper VB is L + S where L = 1 corresponds to the p electrons of the host crystal. For this reason and since they correspond to the pseudo-angular momenta J = 3/2 and 1/2, in the description of the acceptor states in diamond-type semiconductors, the T8 and r7- VBs are often labelled the p3/2 and j> /2 bands, respectively. 

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... 

The behaviour of group-II FAs in silicon is interesting. Mg behaves as an interstitial double donor and Zn as a substitutional double acceptor. Be shows an acceptor behaviour, but it has been stated that only T0% of the Be concentration in silicon is electrically active (quoted by Crouch et al. [45]). Discrete acceptor spectra have been reported at LHeT in Be- and Zn-doped silicon and they include many complexes. In the case of Be, EM acceptor-like spectra associated with four different centres have been reported [45,99]. They are sometimes denoted in the literature Be-I, Be-II, Be-III, and Be-IV, with ionization energies of 192, 146, 200, and 93meV, respectively. Spectra due to (Be, Lij) pairs have also been reported [45,137]. Only the first low-energy lines of these spectra are observed, but the line spacings are comparable to those observed for the group-III p3/2 spectra. Some of the p1/2 spectra associated with these centres have also been observed [99,137]. 

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...

The EM acceptor levels are calculated from the VB parameters and, by analogy with the situation for EM donors, we consider first the splitting of the VB under a uniaxial stress. A stress along a <100> or < 111 > direction reduces the point group symmetry of the diamond structure to D4h or Dslinear regime, the strain-induced part H of the VB Hamiltonian corresponding to (8.1) can be written as [57] ... 

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