  Silicon quantum dot qubits must contend with low-lying valley excited states
which are sensitive functions of the quantum well heterostructure and disorder;
quantifying and maximizing the energies of these states are critical to
improving device performance. We describe a spectroscopic method for probing
excited states in isolated Si/SiGe double quantum dots using standard baseband
pulsing techniques, easing the extraction of energy spectra in multiple-dot
devices. We use this method to measure dozens of valley excited state energies
spanning multiple wafers, quantum dots, and orbital states, crucial for
evaluating the dependence of valley splitting on quantum well width and other
epitaxial conditions. Our results suggest that narrower wells can be beneficial
for improving valley splittings, but this effect can be confounded by
variations in growth and fabrication conditions. These results underscore the
importance of valley splitting measurements for guiding the development of Si
qubits.
