Two-dimensional metal-insulator transition and in-plane magnetoresistance in a high-mobility strained Si quantum well

The apparent metal-insulator transition is observed in a high-quality two-dimensional electron system (2DES) in the strained Si quantum well of a Si∕Si_1-xGeₓ heterostructure with mobility μ=1.9×10⁵0.3em0excm²∕V0.2em0exs at density n=1.45×10¹¹0.3em0excm^-2. The critical density, at which the thermal coefficient of low T resistivity changes sign, is ~0.32×10¹¹0.3em0excm^-2, a very low value obtained in Si-based 2D systems. The in-plane magnetoresistivity ρ(Bᵢₚ) was measured in the density range, 0.35×10¹¹<n<1.45×10¹¹0.3em0excm^-2, where the 2DES shows the metalliclike behavior. It first increases and then saturates to a finite value ρ(Bc) for Bᵢₚ>Bc, with Bc the full spin-polarization field. Surprisingly, ρ(Bc)∕ρ(0)~1.8 for all the densities, even down to n=0.35×10¹¹0.3em0excm^-2, only 10% higher than nc. This is different from that in clean Si metal-oxide-semiconductor field-effect transistors, where the enhancement is strongly density dependent and ρ(Bc)∕ρ(0) appears to diverge as n→nc. Finally, we show that in the fully spin-polarized regime, dependent on the 2DES density, the temperature dependence of ρ(Bᵢₚ) can be either metalliclike or insulating.