Transport and optical studies of the D--conduction band in doped GaAs/AlGaAs quantum wells

The properties of D- ions in quantum wells were studied. It is found that, with an intermediate concentration of D- ions, electrons in the quantum wel ls possess both band-like and impurity-like properties. The appearance of t he Quantum Hall effect makes it possible to rule out the existence of an im purity band that is separated from the conduction band. The results are int erpreted in terms of the formation of a D- conduction band, with the D- ban d becoming a tail of the conduction band. The implications of our experimen tal results on the metal-insulator transitions in doped semiconductors are discussed.