Semiconductors with CB Degeneracy

After several reports between 1965 and 1980, no new information has been published on the spectroscopy of donors in GaP. Odd-parity transitions from the ground to excited states associated with the lowest X band for the Si, S and Te donors have been reported in the 55-100 meV ( 440-810cm-1) spectral domain [10,39,196,223]. The spectra are superimposed on the two-phonon spectrum of GaP and the FWHMs of the absorption lines at LHeT are - ()Ai meV. LHeT photoconductivity measurements in the photoionization region of shallow impurities in GaP revealed dips due to electronic transitions accompanied by the emission of LA(X) and LO (r) phonons with energies of 404 and 254 cm 1, respectively, and they have contributed to the understanding of the donor spectra [222]. LHeT transmission spectra of GaP Si samples at LHeT showing Si donor transitions are displayed in Fig. 6.45. 

The number of donor transitions observed in GaP is limited and some attributions can differ, but line 2p i is observed for all the donors and its position is taken as a reference. The energies of these transitions are given in Table 6.38. 

Some allowed and forbidden transitions are observed together with phonon emission (see text). For SiGa, the ground state is Is (T2). For the other donors, it is Is (Ai). When available, the values in cm-1 are indicated in parentheses 

The critical 3p i-2p i spacing is expected to be independent of the nature of the chemical donor, but it differs significantly between Sica and Op. This reflects the fact that the CB minimum for the donors on P site is associated with the Xi camel s back structure. Variational calculations based on k.p perturbation theory have been performed for P-site donor and compared self-consistently with spectroscopic data [40]. The calculations are performed as a function of the ratio of a non-parabolicity parameter9 Q to the separation A between CBs Xi and X3. The authors use an anisotropy parameter p equal 

Some two-electron PL lines have been observed at 1.6 K in GaP S and GaP Se samples, where the donor electron is left in an excited (usually even-parity) state [52]. Absorption dips due to transitions to even-parity states with phonon emission have also been reported for the S, Se, and Te donors by Scott [222] and they are included in Table 6.38. The experimental values of the energy levels of the corresponding states, not given in the original reference are for the S donor, 26.0, 17.1, 12.4 and 9.7 meV for 2s (Ai), 3s (Ai), 3do (Ai), and 4s (Ai), respectively [41]. 

---