Valley-orbit

Optical absorption measurements give band-gap data for cubic silicon carbide as 2.2 eV and for the CC-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in silicon carbide is considered to be predominantly covalent in nature, but with some ionic character (55). In a Raman scattering study of valley-orbit transitions in 6H-silicon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the silicon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

Table 5.9. Comparison between the experimental values and spacings (meV) of the Is manifold energy levels showing the amplitude of the valley-orbit/chemical splittings of the group-V donors in silicon and germanium and the calculated values of [2], where no chemical effect is included. More accurate experimental values of the E (Is (Ai)) are given in Tables 6.3 and 6.7...

Table 6.6. Valley-orbit splitting ls(Ai) — Is (T2)(rneV(crri 1 in parentheses)) of the isolated single donors in germanium...

A valley-orbit splitting of the Is state of Nc is apparent from this figure as a temperature raise populates the ls(E) state (a normal ordering of the levels is assumed). The transitions from the ls(E) state are clearly broader than those from ls(Ai). Small sharp lines can also be observed in the two spectra of Fig. 6.10, showing no thermalization effect. They are attributed to an unidentified effective-mass donor with no detectable valley-orbit splitting, denoted EMD in the original reference [170]. 

The measured Is (Ai) - ls(E) valley-orbit splitting of the Nc donor is 8.36meV so that the ls(E) level energy is 45.83meV, slightly less than the one-valley EM value, but such a situation is also encountered for the Sb and Bi ls(E) levels in silicon (Table 6.5). For EMD, E10 is close to that calculated in the EMA and no valley-orbit splitting is detected. 

The Ch-related donor spectra differ on that point as several parity-forbidden transitions are observed. They start with symmetry-allowed transitions from the Is ground state to the valley-orbit split Is excited states, and are supplemented with 2s (T2) and 3s (T2) lines and Fano resonances within the photoionization spectrum. This is shown in Fig. 6.13 for Se°. Compared to group-V donors, this extends the energy span of the Ch°-related spectra to the ionization energy of the Is (T2) level (35-40 meV in isolated chalcogens) and it can even increase to 40-48 meV when singlet-triplet spin-forbidden transitions are observed. 

Fig. 6.17. Enlargement of the spectral regions of the 2p i and 3p i lines of Mg+ showing their splitting (236 and 74 peV, respectively) by valley-orbit and central-cell interactions. The split components are indexed l and h in Table 6.18 [249]. Copyright 1994 by the American Physical Society...

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