n-type modulation-doped Si/SiGe heterostructures were grown on different types of partly relaxed SiGe buffer layers, which are required in this material system to obtain a large enough conduction band offset. The samples were characterized by

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Exploiting the spin resonance of a two-dimensional (2D) electron in SiGe͞Si 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, D͑E F ͒, and the screening wave vector, q 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 TF yields good agreement with experimental values justifying the approach. The decrease in D͑E 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.

We have studied the low-temperature (T = 25 mK) ballistic transport of electrons in a split-gate device labricated from a Si/Sio7Geo3 heterostruture. In the absence of a magnet c field the conductance is quantized in Jnits of i x 4$/h as the gale voltage is tuned where i is the nJmber of occupied sLbbands. In this system. both the spin ana the val.ey degeneracies have to be laken into account. These can be lifted by applying a perpendicular magnet:c field. Magnetk using a simple square wel. potential model.