Electrical and physical properties of high-Ge-content Si/SiGe p-type quantum wells

The electrical and physical properties of high-Ge-content Si/SiGe p-type quantum wells grown by chemical-vapor deposition on (001) Si are reported. Modulation-doped Si/SiGe square wells with thicknesses in the range of 40--100 \AA{} and Ge fractions in the range of 10--45 % are examined by a variety of techniques to understand the compositional dependence of the hole transport properties. Sensitivities to spacer thickness, well width, compositional grading, and growth temperature are also examined. The hole mobilities are found to be substantially below the lattice-limited mobility of holes in Si and the hole mobilities in similar heterostructures in III-V materials. Moreover, the mobility decreases rapidly with increasing Ge content for contents greater than 25%, in contrast with what is expected from the modification of the band structure with increased Ge. Shubnikov--de Haas and magnetoresistance measurements also show anomalous features that would not be expected for ideal quantum wells. Modulated optical reflectance imaging and atomic force microscopy reveal increases in nonuniformity with increasing Ge content on \ensuremath{\sim}1-\ensuremath{\mu}m and \ensuremath{\sim}1000-\AA{} scales, respectively. Evidence of compositional nonuniformity, or clustering, is seen on 20--50-\AA{} scales by cross-sectional transmission electron microscopy. Transmission electron energy loss spectroscopy shows that the layers contain moderate compositional nonuniformities (\ifmmode\pm\else\textpm\fi{}5% Ge) on the scale of several hundred \AA{} or stronger nonuniformities on scales \ensuremath{\lesssim}100 \AA{}, which is also consistent with results from Raman scattering. It is concluded that the low mobilities and other features of the hole transport in the present samples are associated with compositional nonuniformities or Ge clustering and with increased thickness and compositional nonuniformities for higher Ge content.