  Silicon can be isotopically enriched, allowing for the fabrication of highly
coherent semiconductor spin qubits. However, the conduction band of bulk Si
exhibits a six-fold valley degeneracy, which may adversely impact the
performance of silicon quantum devices. To date, the spatial characterization
of valley states in Si remains limited. Moreover, techniques for probing valley
states in functional electronic devices are needed. We describe here a
cryogen-free scanning gate microscope for the characterization of
Si/Si$_{0.7}$Ge$_{0.3}$ quantum devices at mK temperatures. The microscope is
based on the Pan-walker design, with coarse positioning piezo stacks and a fine
scanning piezo tube. A tungsten microscope tip is attached to a tuning fork for
active control of the tip-to-sample distance. To reduce vibration noise from
the pulse tube cooler, we utilize both active and passive vibration isolation
mechanisms, and achieve a root-mean-square noise in $z$ of $\sim$ 2 nm. Our
microscope is designed to characterize fully functioning
Si/Si$_{0.7}$Ge$_{0.3}$ quantum devices. As a proof of concept, we use the
microscope to manipulate the charge occupation of a Si quantum dot, opening up
a range of possibilities for the exploration of quantum devices and materials.
