Absorption spectra of impurities in silicon—II: Group-V donors

Abstract

The infra-red absorption spectra due to neutral, n-type, impurity centers (P, As, Sb) in silicon have been measured at liquid helium temperature, and ionization energies and term schemes for these materials have been determined from the data. The ionization energies found are P: 0.0503 eV; As: 0.0533 eV; and Sb: 0.0426 eV. These values are approximately 10 per cent greater than the thermal ionization energies. The differences can be accounted for partly by (1) a FranckCondon effect and (2) the presence of low-lying excited states. The positions of the excited p states of the impurity centers are all found to be roughly in the same positions relative to the conduction band, in good agreement with the results of the effective mass treatment of the impurity centers by K ohn and L uttinger. The variations observed between the various impurities are attributed to differences in the effective potentials in the immediate vicinity of the impurity ion cores. These differences are not taken into account in the effective mass theory.