We propose and demonstrate a relaxed-SiGe/strained-Si (SiGe/s-Si) enhancement-mode gate stack for quantum dots. The enhancement-mode SiGe/s-Si structure is pursued because it spaces the quantum dot away from charge and spin defect rich dielectric interfaces and minimizes background dopants. A mobility of 1.6 × 10 5 cm 2 /Vs at 5.8 × 10 11 /cm 2 is measured in Hall bars that witness the same device process flow as the quantum dot. Periodic Coulomb blockade (CB) is measured in a double-top-gated lateral quantum dot nanostructure. The CB terminates with open diamonds up to ±10 mV of DC voltage across the device. The devices were fabricated within a 150 mm Si foundry setting that uses implanted ohmics and chemical-vapor-deposited dielectrics, in contrast to previously demonstrated enhancement-mode SiGe/s-Si structures made with AuSb alloyed ohmics and atomic-layer-deposited dielectric. A modified implant, polysilicon formation and annealing conditions were utilized to minimize the thermal budget so that the buried s-Si layer would not be washed out by Ge/Si interdiffusion.

Articles you may be interested inMobility and metal-insulator transition of the two-dimensional electron gas in SiGe/Si/SiGe quantum wells

We report the magneto-transport, scattering mechanisms, and effective mass analysis of an ultra-low density two-dimensional hole gas capacitively induced in an undoped strained Ge/Si0.2Ge0.8 heterostructure. This fabrication technique allows hole densities as low as p ∼ 1.1 × 1010 cm−2 to be achieved, more than one order of magnitude lower than previously reported in doped Ge/SiGe heterostructures. The power-law exponent of the electron mobility versus density curve, μ ∝ nα, is found to be α ∼ 0.29 over most of the density range, implying that background impurity scattering is the dominant scattering mechanism at intermediate densities in such devices. A charge migration model is used to explain the mobility decrease at the highest achievable densities. The hole effective mass is deduced from the temperature dependence of Shubnikov-de Haas oscillations. At p ∼ 1.0 × 1011 cm−2, the effective mass m* is ∼0.105 m0, which is significantly larger than masses obtained from modulation-doped Ge/SiGe two-dimensional hole gases.

We have investigated the valley splitting of two-dimensional electrons in high quality Si/Si1−xGex heterostructures under tilted magnetic fields. For all the samples in our study, the valley splitting at filling factor ν = 3 (∆3) is significantly different before and after the coincidence angle, at which energy levels cross at the Fermi level. On both sides of the coincidence, a linear density dependence of ∆3 on the electron density was observed, while the slope of these two configurations differs by more than a factor of two. We argue that screening of the Coulomb interaction from the low-lying filled levels, which also explains the observed spin-dependent resistivity, is responsible for the large difference of ∆3 before and after the coincidence.

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