Effects of higher sub-band occupation in (100) Si inversion layers

At low temperatures and for moderate surface charge densities, electrons in inversion layers on (100) p-type Si are two-dimensional in character; i.e., they occupy only one electric quantum level, or sub-band. For electron densities greater than about 6 × 1012 per cm2, depending upon substrate doping, we have observed deviations from two-dimensional behavior in oscillatory magnetoconductance (Shubnikov-de Haas) measurements, which are interpreted as evidence of the onset of occupation of a higher sub-band. The effects of such occupation of higher sub-bands have been calculated for the four different resistivities used in the experiments. The observed change in effective density of states [(10-20)%] and its dependence upon substrate resistivity are in good agreement with theory. The experimental values for the threshold charge densities for higher sub-band occupation are higher than the theoretical values by a factor of about 1.5. © 1976 The American Physical Society.