  We report the fabrication and characterization of a gate-defined double
quantum dot formed in a Si/SiGe nanomembrane. In the past, all gate-defined
quantum dots in Si/SiGe heterostructures were formed on top of strain-graded
virtual substrates. The strain grading process necessarily introduces misfit
dislocations into a heterostructure, and these defects introduce lateral strain
inhomogeneities, mosaic tilt, and threading dislocations. The use of a SiGe
nanomembrane as the virtual substrate enables the strain relaxation to be
entirely elastic, eliminating the need for misfit dislocations. However, in
this approach the formation of the heterostructure is more complicated,
involving two separate epitaxial growth procedures separated by a wet-transfer
process that results in a buried non-epitaxial interface 625 nm from the
quantum dot. We demonstrate that in spite of this buried interface in close
proximity to the device, a double quantum dot can be formed that is
controllable enough to enable tuning of the inter-dot tunnel coupling, the
identification of spin states, and the measurement of a singlet-to-triplet
transition as a function of an applied magnetic field.
