High-electron-mobility Si/SiGe heterostructures: Influence of the relaxed SiGe buffer layer

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 secondary-ion mass spectroscopy, X-ray rocking analysis, transmission electron microscopy, Rutherford backscattering and temperature-dependent Hall measurements. The highest Hall mobilities of 173000 cm² V^-1 s^-1 at 1.5 K were found in a sample grown on a thick, linearly graded SiGe buffer layer deposited at 750 degrees C. Such layer sequences reach room-temperature mobilities around 1800 cm ² V^-1 s^-1. Mainly at lower temperatures, a strong reduction of the Hall mobility is found if either a conventional buffer layer without Ge grading is used, or if the modulation-doped SiGe barrier of the active layers begins to relax with respect to the strained Si channel.