Symmetry and structure of N-O shallow donor complexes in silicon

Shallow donors in silicon related to nitrogen-oxygen complexes have been investigated by piezospectroscopy of their hydrogenic transitions in the far infrared. Complete stress dependences up to 0.25 GPa were obtained for the 1s→2p0 and 1s→2p± transitions of the most prominent members of the (N, O)-family, N-O-3 and N-O-5. Very unusual for shallow donors in silicon, the symmetry of the ground state wave function is T2-like. The lifting of orientational degeneracy for stress in the 〈1 0 0〉, 〈1 1 1〉, and 〈1 1 0〉 directions is compatible with a C2v defect symmetry. Data from the other species of the (N, O)-family are indicative for the same symmetry. The microscopic structure of these centers, in part contradictory to present theoretical models, is discussed.