Effects of donor-donor interaction on the absorption spectra of shallow donors in silicon

In uncompensated phosphorus or arsenic-doped silicon, observation of the shallow, donor absorption line spectrum for donor concentrations from 10(12) to 2 x 10(18)/cc, yields information about the bound states of the donor electrons and the behavior of the conduction band edge. With increasing concentration, the photon induced IS(A) → 2P(o) and IS(A) → y 2P(±) transition lines are observed to broaden in energy, and shift to slightly lower energies; the lineshapes, Lorentzian at low donor concentrations, become asymmetric with the formation of a tail to the low energy side. A superposed absorption background is observed for donor concentrations greater than 2 x 10(16)/cc. Above 2 x 10(17)/cc an absorption edge in the 10-to-30 meV region with an exponential energy dependence is observed. The change in the absorption line spectrum is due primarily to the final states. The halfwidths of the transitions are well explained by a donor-pair model (or effective hydrogen molecule (Macek, 1971)), with the donor distribution assumed random. The difference in broadening of the 2P(±) level compared to the 2P(o) level is due to the anisotropic effective mass of the donor electron. The transition lineshapes are quantitatively explained in terms of the convoluted Fano function (Bhatia, 1970; Fano, 1962). The Fano parameters Q and T, are interpreted in terms of the evidence for the conduction band tailing and the possible configuration interaction. The integrated absorption coefficient of the line spectrum and the decrease of the absorption cross section above the conduction band threshold with increasing concentration are also accounted for in terms of band tailing. The experimental results are in agreement with the infrared absorption data for antimony-doped germanium (Nisida and Horii, 1969).Science, Faculty ofPhysics and Astronomy, Department ofGraduat