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-ty pe quantum wells grown by chemical-vapor deposition on (001) Si are re ported. Modulation-doped Si/SiGe square wells with thicknesses in the range of 40-100 angstrom and Ge fractions in the range of 10-45 % are examined by a variety of techniques to understand the compositional de pendence of the hole transport properties. Sensitivities to spacer thi ckness, 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 i n 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 b and structure with increased Ge. Shubnikov-de Haas and magnetoresistan ce measurements also show anomalous features that would not be expecte d for ideal quantum wells. Modulated optical reflectance imaging and a tomic force microscopy reveal increases in nonuniformity with increasi ng Ge content on approximately 1-mum and approximately 1000-angstrom s cales, respectively. Evidence of compositional nonuniformity, or clust ering, is seen on 20-50-angstrom scales by cross-sectional transmissio n electron microscopy. Transmission electron energy loss spectroscopy shows that the layers contain moderate compositional nonuniformities ( +/- 5% Ge) on the scale of several hundred angstrom or stronger nonuni formities on scales less than or similar to 100 angstrom, which is als o consistent with results from Raman scattering. It is concluded that the low mobilities and other features of the hole transport in the pre sent samples are associated with compositional nonuniformities or Ge c lustering and with increased thickness and compositional nonuniformiti es for higher Ge content.