Radiation-Produced Absorption Bands in Silicon: Piezospectroscopic Study of a Group-V Atom-Defect Complex

A series of sharp new infrared-absorption bands produced by high-energy-electron irradiation in phosphorus- and arsenic-doped float-zone-refined silicon is reported. In P-doped silicon, the dominant band is at 1150 cm^-1, with associated weaker bands at 1310, 970, 1000, 1025, 1040, and 1450 cm^-1. In As-doped silicon, the corresponding bands are at 1260 cm^-1 and 1480, 1080, 1110, 1135, 1160, and 1650 cm^-1. The spectra recover in both materials at {~}140^∘{}C. Uniaxial-stress studies of these bands in oriented single crystals with polarized light are described. The results cannot be fit to the usual "piezospectrospic" classification of Kaplyanskii and Runciman. A model is presented which identifies the spectra as electronic transitions from the orbital singlet ground state of a single anistropic donor defect to a manifold of bound large-orbit effective-mass excited states. Extension of the Kaplyanskii classification to include the accidental degeneracy inherent in the effective-mass excited states in silicon explains the stress results. The symmetry of the defect is deduced to be monoclinic II. It is deduced that one or more group-V atoms are involved in the defect, but a microscopic identification of the defect is not possible.