We report the observation of a two-dimensional electron gas (2DEG) in a Si quantum well with mobility 1.6×106 cm2/Vs at carrier densities n≥1.5×1011/cm2. The 2DEG, which resides in an undoped Si/SiGe heterostructure, is capacitively induced using an insulated-gate field-effect transistor (IGFET) device structure; its mobility is determined from transport and quantum Hall effect measurements at 0.3 K. Our IGFET device makes it now possible to access by transport experiments the low electron density regime down to n∼1×1010/cm2.

Recent interest in topological quantum computing has driven research into topological nanowires, one-dimensional quantum wires that support topological modes including Majorana fermions. Most topological nanowire designs rely on materials with strong spin-orbit coupling, such as InAs or InSb, used in combination with superconductors. It would be advantageous to fabricate topological nanowires using Si owing to its mature technology. However, the intrinsic spin-orbit coupling in Si is weak. One approach that could circumvent this material deficiency is to rotate the electron spins using nanomagnets. Here, we perform detailed simulations of realistic Si/SiGe systems with an artificial spin-orbit gap induced by a nanomagnet array. Most of our results are also generalizable to other nanomagnet-based topological nanowire designs. By studying several concrete examples, we gain insight into the effects of nanomagnet arrays, leading to design rules and guidelines. Finally, we present an experimentally realizable design using magnets with a single polarization.

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We report the magneto-transport study and scattering mechanism analysis of a series of increasingly shallow Si/SiGe quantum wells with depth ranging from ∼ 100 nm to ∼ 10 nm away from the heterostructure surface. The peak mobility increases with depth, suggesting that charge centers near the oxide/semiconductor interface are the dominant scattering source. The power-law exponent of the electron mobility versus density curve, μ ∝ nα, is extracted as a function of the depth of the Si quantum well. At intermediate densities, the power-law dependence is characterized by α ∼ 2.3. At the highest achievable densities in the quantum wells buried at intermediate depth, an exponent α ∼ 5 is observed. We propose and show by simulations that this increase in the mobility dependence on the density can be explained by a non-equilibrium model where trapped electrons smooth out the potential landscape seen by the two-dimensional electron gas.

In this paper, we present our study of the maximum electron density, nmax, accessible via low-temperature transport experiments in enhancement-mode Si/Si1−xGex heterostructure field-effect transistors. Experimentally, we find that nmax is much higher than the value obtained from self-consistent Schrödinger-Poisson simulations and that nmax can be changed only by changing the Ge concentration in the Si1−xGex barrier layer, not by varying the barrier layer thickness. The discrepancy between experiments and simulations is explained by a non-thermal-equilibrium tunneling-limited model.