  Using an ultra-low temperature, high magnetic field scanning probe
microscope, we have measured electric potentials of a deeply buried two
dimensional electron gas (2DEG). Relying on the capacitive coupling between the
2DEG and a resonant tip/cantilever structure, we can extract electrostatic
potential information of the 2DEG from the dynamics of the oscillator. We
present measurements using a quartz tuning fork oscillator and a 2DEG with a
cleaved edge overgrowth structure. The sensitivity of the quartz tuning fork as
force sensor is demonstrated by observation of Shubnikov de Haas oscillations
at a large tip-2DEG separation distance of more than 500 nm.
