Electronic structure of bound excitons in semiconductors

Some recent advances in the understanding of the electronic structure of bond excitons (BE:s) in semiconductors are discussed, with the emphasis on properties of complex defects or deep level defects, which have not previously been reviewed in any detail. The scope is limited to our most common semiconductor materials, and no complete coverage of the field is attempted. Rather we emphasize a number of general simple models, that are useful in the classification of BE properties, according to type of defect potential, host material band structure, etc. The simple model adopted by Hopfield Thomas and Lynch appears to be useful in most cases of neutral (“isoelectronic”) defects; these may be classified as either dominantly hole-attractive or electron-attractive, even for complex defects. The primary particle bound at a neutral defect typically has a very localized wavefunction which is not possible to describe in an effective-mass like model. The localized nature of the bound hole at hole-attractive neutral complex defects makes it very sensitive to the symmetry of the local defect potential, which may change the magnetic properties of such bound holes from p-like to spin-like. The influence of spin-orbit coupling is also discussed for such bound hole states, as it governs the balance between p-like and spin-like properties of bound holes. Electrons bound as primary particles at electron-attractive neutral defects are usually much simpler, since they are generally found to be spin-like. Their g-value is strongly dependent on the degree of localization in the defect potential, though. The second bound particle at neutral defects is usually described quite well in an effective-mass like model, modified for a repulsive central cell potential. Donors and acceptors have one particle of the same kind to create a correlated two-particle state. The structure of this state is discussed for different defect symmetries. It is pointed out that splittings due to a local strain field lower than tetrahedral are usually quite large, often only one two-particle state remains in the band gap. The third particle for donor bound excitons (DBE:s) or acceptor bound excitons (ABE:s) essentially experiences a distributed Coulomb potential, and can again typically be described as effective-mass like (pseudo-acceptor and pseudo-donor, respectively). A number of different defects in different materials are used to illustrate the general trends in BE electronic structure, the data were mainly published during the last two years, or still unpublished. In addition, some recent developments in special areas such as phonon coupling to BE:s, satellite spectra for BE:s and BE transfer between different defects are briefly discussed.