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We report the optical determination of exciton binding energies in small-period GaAs/Ga0.7Al0.3As superlattices by means of low-temperature photoluminescence excitation spectroscopy and photoluminescence spectroscopy as a function of temperature. The heavy-hole exciton binding energy decreases with decreasing superlattice period. Our experimental findings are in reasonable agreement with a variational calculation.

A theoretical description of substitutional transition-metal ions in semiconductors is presented. It is based on a defect-molecule approach with renormalized parameters in a manner similar to what has been done for vacancies in silicon. The self-consistent calculation allows conciliation of two apparently opposite behaviors a quasiatomic spectroscopy classically described by crystal-field theory and the stability of several charge states related to the presence of strongly polarizable bonds. The need for going beyond unrestricted Hartree-Fock theory is emphasized and a method for incorporating the effect of Coulomb correlations in the lower configurations is proposed. Multiplet splitting is then calculated for chromium in GaAs and Si, where the excitation spectrum is shown to resemble that predicted by the Tanabe-Sugano diagrams, but with additional excitations from bonding to antibonding states.