  Silicon-germanium heterostructures have successfully hosted quantum dot
qubits, but the intrinsic near-degeneracy of the two lowest valley states poses
an obstacle to high fidelity quantum computing. We present a modification to
the Si/SiGe heterostructure by the inclusion of a spike in germanium
concentration within the quantum well in order to increase the valley
splitting. The heterostructure is grown by chemical vapor deposition and
magnetospectroscopy is performed on gate-defined quantum dots to measure the
excited state spectrum. We demonstrate a large and widely tunable valley
splitting as a function of applied vertical electric field and lateral dot
confinement. We further investigate the role of the germanium spike by means of
tight-binding simulations in single-electron dots and show a robust doubling of
the valley splitting when the spike is present, as compared to a standard
(spike-free) heterostructure. This doubling effect is nearly independent of the
electric field, germanium content of the spike, and spike location. This
experimental evidence of a stable, tunable quantum dot, despite a drastic
change to the heterostructure, provides a foundation for future heterostructure
modifications.
