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 Si1-x Gex heterostructure with mobility μ=1.9× 105 cm2 V s at density n=1.45× 1011 cm-2. The critical density, at which the thermal coefficient of low T resistivity changes sign, is ∼0.32× 1011 cm-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× 1011 cm-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× 1011 cm-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.