Oxide-Charge-Induced Localized States and Screening in a Model Two-Dimensional system

Far-infrared measurements of intersubband absorption spectra of $n$-inversion layers in (100) Si metal-oxide-semiconductor field-effect transistors with mobile positive ions in the oxide have been carried out between 4.2 and 70 K. Results provide direct evidence for screening of localized states in this quasi two-dimensional electronic system as well as the existence of long band tails and impurity bands at low electron densities associated with subbands due to both the inequivalent conduction-band valleys.