  Nanofabricated metal gate electrodes are commonly used to confine and control
electrons in electrostatically defined quantum dots. However, these same gates
impart a complicated strain geometry that affects the confinement potential and
potentially impairs device functionality. Here we investigate strain-induced
fluctuations of the potential energy in Si/SiGe heterostructures, caused by (i)
lattice mismatch, (ii) materials-dependent thermal contraction, and (iii)
deposition stress in the metal gates. By simulating different gate geometries,
ranging from simple to realistically complicated, and including features like
overlapping metal and oxide layers, we can explain most observed strain
features. In particular, we show that strain-induced potential fluctuations can
be suppressed by employing overlapping gates that cover the whole active
region, when the oxide layers are thin. These results suggest that strain
effects should not present a serious challenge to qubit uniformity when
following simple design rules.
