Far-infrared studies of the cyclotron-resonance line shape of two-dimensional electrons in silicon in the quantum limit

Systematic far-infrared magneto-optical measurements of cyclotron-resonance spectra of quasi-two-dimensional electrons have been carried out on a series of n-channel Si metal-oxide-semiconductor devices with a wide range of mobilities (4500 to >10 000 ${\mathrm{cm}}^2$/V s) at very low electron densities (1.0×${10}^{10}$ to 8.8×${10}^{11}$ ${\mathrm{cm}}^{\mathrm{-}2}$), low temperatures (2–40 K), and high magnetic fields (up to 15 T). A multiple-line structure with at least three distinct components in a crossover region of intermediate Landau-level filling factors (the Landau-level occupancy) has been observed. The ‘‘anomalous’’ behavior in the line shapes and an observed apparent upward shift in resonance frequency at the lowest filling factors result from the relative intensity variation of individual components as a function of filling factor. The observed systematic correlation of the magnitude of the anomalies with the inverse of the sample mobility demonstrates the importance of localization. The effects of magnetic field, temperature, substrate bias, and surface band structure on these characteristics have been investigated. Results are compared with theoretical models and recent experimental work on an alternative system (GaAs heterostructures). It is argued that both localization and electron-electron correlations play important roles and must be considered on an equal footing in explaining the anomalous behavior in the low-density regime.