Spin–orbit coupling, mass anisotropy, time-reversal symmetry, and spontaneous symmetry breaking in the spectroscopy of shallow centers in elemental semiconductors

The electronic states of shallow donors and acceptors, explored under high resolution with Fourier Transform infrared and Raman spectroscopy, underscore the intimate connection between the localized states of these impurities and the electronic band structure of the host as contemplated in the effective mass theory. Spin–orbit interaction, mass anisotropy of the conduction band minimum (donors) and of the valence band maximum (acceptors), the site symmetry of the impurity and the possible spontaneous departure from it when the electronic ground state is degenerate are the features highlighted in our discussion of the excitation spectra of donors in silicon and boron acceptors in diamond. As in atomic spectroscopy, the removal of degeneracy under external perturbation and the polarization and field dependence of the spectral components provide a wealth of insightful characterizations. We demonstrate them with the Zeeman effect of the Raman transition between the spin–orbit split counterparts of the 1s ground state of boron acceptors in diamond; g-factors and the extreme mass anisotropy of the hole emerge from these studies. They also provide beautiful examples of time-reversal symmetry, conservation laws and selection rules connecting the Stokes to the anti-Stokes Raman scattering.