  Conventional quantum transport methods can provide quantitative information
on spin, orbital, and valley states in quantum dots, but often lack spatial
resolution. Scanning tunneling microscopy, on the other hand, provides
exquisite spatial resolution of the local electronic density of states, but
often at the expense of speed. Working to combine the spatial resolution and
energy sensitivity of scanning probe microscopy with the speed of microwave
measurements, we couple a metallic probe tip to a Si/SiGe double quantum dot
that is integrated with a local charge detector. We first demonstrate that a
dc-biased tip can be used to change the charge occupancy of the double dot. We
then apply microwave excitation through the scanning tip to drive
photon-assisted tunneling transitions in the double dot. We infer the double
dot energy level diagram from the frequency and detuning dependence of the
photon-assisted tunneling resonance condition. These measurements allow us to
resolve $\sim$65 $\mu$eV excited states, an energy scale consistent with
typical valley splittings in Si/SiGe. Future extensions of this approach may
allow spatial mapping of the valley splitting in Si devices, which is of
fundamental importance for spin-based quantum processors.
