Two sub-band conductivity of Si quantum well

We report on two sub-band/bi-layer transport in double gate SiO₂–Si–SiO₂ quantum well with 14 nm thick Si layer at 270 mK. At symmetric well potential the experimental sub-band spacing changes monotonically from 2.3 to 0.3 meV when the total electron density is adjusted by gate voltages between 0.7×10¹⁶–. The conductivity is mapped in large gate bias window and it shows strong non-monotonic features. At symmetric well potential and high density these features are addressed to sub-band wave function delocalization in the quantization direction and to different disorder of the top and bottom interfaces of the Si well. In the gate bias regimes close to second sub-band/bi-layer threshold the non-monotonic behavior is interpreted to arise from scattering from the other electron sub-system with localized or low mobility states.