Electronic structure of deep-lying sulfur centers in Si

A theoretical analysis has been made of infrared absorption spectra measured earlier for four deep-lying sulfur donor centers in Si. The analysis is based on a group-theoretic treatment for the behavior of electronic levels under uniaxial stress. The data consist of absorption frequencies and polarization selection rules determined for calibrated uniaxial stress applied along the [001], [111], and [110] axes. Both 1s→np transitions and 1s→1s transitions are observed. The A and B centers (binding energies 0.1095 and 0.1877 eV, respectively) are found to be He-like, while the C and D centers (binding energies 0.3705 and 0.6136 eV, respectively) are found to be ${\mathrm{He}}^{+}$-like. The D-center spectra are consistent with $T_d$ symmetry, while the A-, B-, and C-center spectra are consistent in most respects with $C_{3v}$ or $D_{3d}$ symmetry. The analysis of the 1s→np spectra yields values of ∼7.9 eV for the conduction-band pure shear deformation potential, somewhat lower than the value of ∼10 eV determined from spin-resonance experiments. A well-defined 1s($A_1$ ${)}_{\mathrm{ground}}$ →1s($T_2$) spectrum is observed for the D center; 1s($A_1$ ${)}_{\mathrm{ground}\to 1\mathrm{s}\left(\mathrm{E}\right)}$ and 1s($A_1$ ${)}_{\mathrm{ground}\to 1\mathrm{s}(A_1}$) spectra are observed for the B center. From comparison of the data with theory, parameters of the ground and excited levels are determined. One noteworthy result is the finding that the 1s→np spectral frequencies observed under stress for the A center are consistent with a twofold-degenerate 1s(E) ground state.