In-plane magnetoresistivity of high-mobility two-dimensional electrons in an undoped Si/SiGe quantum well at 20 mK

Two-dimensional electron gas (2DEG) in a Si quantum well with mobility over 1$\times $10$^{6}$ cm$^{2}$/Vs has been realized using an undoped Si/SiGe heterostructure. In this high quality 2DEG, the 2D metal-insulator transition occurs at a characteristic density n$_{c}$=1.35$\times $10$^{10}$/cm$^{2}$, the lowest reported in the Si systems. We have further measured the in-plane magnetoresistivity at 20mK. It is observed that, in sharp contrast to modulation-doped heterostructures with lower electron mobility where the ratio of the saturation resistance in high in-plane magnetic field to the zero-magnetic-field resistance is strongly enhanced near n$_{c}$, no such enhancement is observed in this high-mobility undoped field-effect transistor. The characteristic magnetic field, B$_{S}$, at which the in-plane magnetoresistivity saturates, follows the linear electron density dependence previously observed in modulation-doped structures at high densities. However, B$_{S}$ deviates considerably from this linear dependence at low densities and does not extrapolates to a finite density at B$_{S}$ =0.