Properties of Impurity States In n-i-p-i Superlattice Structures

The primary effect of the static potential of the impurity atoms in doping superlattices is the formation of space charge induced quantum wells. In most of our previous studies of n-i-p-i doping superlattices we have focussed our interest to the remarkable features which result from the spatial separation between electrons and holes, such as tunability of the electronic structure, electron-hole recombination lifetimes, increased by many orders of magnitude compared with those of bulk semiconductors, or huge optical nonlinearities. In this lecture we will concentrate on the point defect aspects of impurities in doping superlattices. Topics to be discussed will include the impurity band formation by shallow (donor) and less shallow (acceptor) impurities over the whole concentration range from very low to high concentrations. We propose various n-i-p-i and hetero n-i-p-i structures which should be idealy suite for optical studies of impurity- and Hubbard bands, and for electrical investigations of the density of states of the conductivity in these bands as a function of (tunable) carrier and dopant density. In particular it is expected that these structures represent unique systems for investigations of the Mott-Hubbard transition in two dimensions.