Piezospectroscopy of nitrogen-oxygen shallow donor complexes in silicon

Electronic transitions of N-O related shallow donors in silicon doped with nitrogen and oxygen have been studied under uniaxial stress by Fourier transform spectroscopy in the far infrared. In first approximation, the stress dependence of the hydrogeniclike transitions is described by ground and excited state wave functions belonging to the ${T}_{2}$ representation of the tetrahedral group. Small splittings of the lines originate from the lifting of orientational degeneracy of low-symmetry centers. Nonlinear stress responses of some of the transitions are attributed to interactions with higher-lying $1s$ states. Full analysis of the two most prominent shallow donors N-O-3 and N-O-5 in terms of selection rules and line intensities under polarized light prove that the electronic states are perturbed by a defect potential of apparent ${C}_{2\text{v}}$ symmetry, related to centers with an alignment along the $⟨110⟩$ and a twofold symmetry axis along the $⟨001⟩$ lattice direction. The donor wave function itself can be constructed from Bloch states belonging to a single pair of conduction band minima. It is concluded that present theoretical models for the microstructure of these complexes must be reconsidered, taking into account that any deviations from ${C}_{2\text{v}}$ symmetry must be small and have no measurable effect on the stress pattern.