Absorption spectra of impurities in silicon—I: Group-III acceptors

Abstract

The low-temperature absorption spectra of group-III impurities in silicon yield information about the ionization energies and excited states of the impurity centers. The optical values for the ionization energies of boron (0.046 eV) and indium (0.154 eV) are in good agreement with the thermal values, but the optical values for aluminum (0.67 eV) and gallium (0.71 eV) are appreciably larger than the thermal values. Variations in ionization energy among the group-III impurities are found to be accompanied by variations in the character of the excitation and photoionization spectra. The term schemes for the impurities are derived from the position of the excitation bands and the assumption that the 4 p level is the same for all of the group-III impurities. Aside from the appearance of fine structure, the p-type states appear to fit a simple hydrogen model. From the positions of the p levels, an effective mass of 0.045 is calculated for holes using the expression for the energy levels of a hydrogen model. Variations in the oscillator strengths of the excitation bands among the group-III impurities are attributed to variations in the position and character of the 1 s levels which arise from effects at the impurity atom that cause the simple effective-mass formalism to break down. The factors which should be included in a more complete theoretical treatment of the impurity centers are discussed.