Metal–insulator transition of spatially separated electrons and holes in mixed type I–type II GaAs/AlAs quantum wells

Low temperature photo-induced far-infrared and microwave absorption studies of mixed type I–type II GaAs/AlAs multiple quantum wells have revealed features related to a metal–insulator transition. The far infrared experiments show an absorption feature at fields below electron cyclotron resonance (eCR) that shifts up with increasing electron–hole density and eventually pins to the eCR field position. In microwave experiments a broad, photoinduced absorption line is observed at fields above that of eCR; the position and strength of this feature depend on excitation intensity, temperature and microwave power. This disparate behavior can be understood qualitatively in terms of internal transitions of spatially separated electrons bound to holes localized laterally in short-range well-width fluctuations and clustered in long-range potential maxima in the narrow wells. These transitions evolve with increasing excitation intensity via a metal-insulator transition into dimensional magnetoplasma resonances of “puddles” of an electron liquid confined laterally by the potential of the clustered holes.