The High Doping Regime

Abstract

The properties of quantum well structures are strongly influenced by the presence of dopant impurities either in the well or in the barrier. As long as the impurity concentration is low, the wave functions of the impurities are spatially separated and the energy levels of the associated states are discrete. However, when the impurity concentration increases, an overlap of the impurity wave functions will take place. Initialized by the higher states a broadening of the allowed energy levels will result. This gives rise to the formation of an impurity band split off from the lowest conduction/valence band. The dependences of the impurity bands on several quantum well parameters have been calculated by J. Serre et al. [178], for the n-type case. At high doping levels, the impurity band will overlap with the free carrier continuum to form a band tail. At this point, a plasma of free carriers is formed, which can move in the potential of the fixed ions of opposite charge. This level is ususally refered to as the metallic limit or the Mott transition [179], i.e., the electronic phase change from an insulating to a metallic behavior.