Screening Breakdown on the Route toward the Metal-Insulator Transition in Modulation Doped Si<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>/</mml:mi></mml:math>SiGe Quantum Wells

Abstract

Exploiting the spin resonance of a two-dimensional (2D) electron in SiGe/Si quantum wells, we determine the carrier density dependence of the magnetic susceptibility. Assuming weak interaction, we evaluate the density of states at the Fermi level, $D({E}_{F})$, and the screening wave vector, ${q}_{\mathrm{TF}}$. Instead of the constant values of an ideal 2D system, we observe a gradual decrease towards the band edge. Calculating the mobility from ${q}_{\mathrm{TF}}$ yields good agreement with experimental values justifying the approach. The decrease in $D({E}_{F})$ is explained by potential fluctuations which lead to tail states that make screening less efficient and, in a positive feedback, cause an increase of the potential fluctuations.