Development of Low-Temperature Scanning Capacitance Microscopy for Measurement of Single Quantum Dots

Self-assembled single quantum dots are widely considered to be leading candidates for spin-based quantum bits. The characterization of such systems requires local information about both charge and spin degrees of freedom as a function of temperature and magnetic field. We describe an extension of a working low-temperature AFM/optical microscope to enable scanning capacitance measurements of quantum dots. Our system relies on the sensitivity of a microwave resonator to perturbations from the scanning probe (similar to RCA's CED technology), using a quartz tuning fork with an etched tungsten tip. The expected sensitivity of the instrument (10$^{-21}$ F) is much below the capacitance of a single self-assembled quantum dot (10$^{-18}$ F). To measure the capacitance, we first use the AFM to locate a single quantum dot, and then collect local C-V information using the measured frequency shift of the resonator.