Clustering in the approach to the metal-insulator transition

A phenomenological model of donor clusters is found to describe the optical and magnetic properties well at donor densities below the Mott density. The effects of clustering on the single-particle density of states is discussed. Microscopic calculations lead to the conclusion that in multi-valley semiconductors large clusters are sufficiently electronegative to attract an electron from isolated donors or donor pairs and as a result, there is no Mott-Hubbard gap due to correlation. The insulating property is ascribed to Anderson localization at the Fermi level.