Very Low Electron Density in Undoped Enhancement-Mode Si/SiGe Two-Dimensional Electron Gases with Thin SiGe Cap Layers

We report the lowest electron density (4.9×10 cm and 1.1×10 cm) and high mobility (~400,000 cm/Vs and ~200,000 cm/Vs) of undoped enhancement-mode Si/SiGe two-dimensional electron gases comparing to samples previously reported with similar thin SiGe cap thickness (55nm and 27nm). The dominant scattering mechanism over a wide range of two-dimensional electron density in both samples is the scattering from remote charges at oxide/silicon interface. In addition, a clear metal-insulator transition is observed in the sample with a 27-nm SiGe cap.