Far-infrared and electrical transport studies of oxide-charge-induced localized states in a quasi-two-dimensional system

Far-infrared measurements of intersubband absorption spectra and dc electrical transport studies of n-type inversion layers in (100) Si metal-oxide-semiconductor field-effect transistors with mobile positive ions in the oxide have been performed at temperatures between 1.7 and 80 K. The properties of the electronic states in the quasi-two-dimensional conducting layer of the MOS devices were probed in detail by conductance, capacitance, and transconductance measurements, and by intersubband infrared optical-absorption measurements. Data were obtained with positive oxide charge density as a parameter, varied by drifting controlled amounts of positive ions ({Δ}Nₒₓ) to the oxide-semiconductor interface (1.3×{}10¹¹{≤}{Δ}Nₒₓ{≤}7.0×{}10¹¹ cm^-2). These results were compared with similar measurements on high-mobility devices in which no positive impurity ions have been purposely introduced. Studies were carried out over a wide range of net interfacial oxide charge densities (2×{}10¹⁰{≤}N_ox≤1×10¹² cm^-2) and substrate-source bias voltages (-9{≤}VS{≤}1 V) with the goal of attaining a better understanding of the nature of localization effects (e.g., two-dimensional carrier localization), interface scattering, and many-body Coulombic interactions (e.g., screening effects) in these structures. Results are compared with recent experimental investigations of this and similar systems and with predictions of available theoretical models. The present measurements provide evidence for the existence of impurity bands and long band tails at low electron densities (nₛ{≤}Nₒₓ) associated with subbands due to both the inequivalent conduction-band valleys and for screening of these localized states at high electron densities (nₛ{≥}2Nₒₓ).