University of Texas at El Paso
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An Apparatus for Measuring Quantum Capacitance at Cryogenic Temperature
In a capacitor whose plates are formed by metal and semiconductor films, the quantum capacitance is a contribution to the total capacitance that is a direct measure of the semiconductor’s density of states, a thermodynamic quantity fundamental to the transport characteristics of the semiconductor. Quantum capacitance is most interesting at cryogenic temperatures, where thermal broadening of features in the density of states is suppressed. In a cryostat, the quantum capacitance can be as many as six orders of magnitude smaller than the capacitance of the cables connecting the capacitor to the measurement electronics, making the design of the probe—the apparatus housing the sample platform and cabling—critical to the feasibility of the experiment. Finally, the thermal properties of the probe necessitate compromise with its electronic properties. Here we present the design and fabrication of such a capacitance probe to be used in the Quantum Design Physical Property Measurement System (PPMS) at temperatures down to 2 K. The probe’s stainless-steel tubing and cabling provide thermal anchoring to the cold isothermal region at the bottom of the cryostat without conducting a prohibitively large heat load from the top, while the configurability of the probe end allows for different experiments to be built without complete disassembly of the probe. We also present first experiments on capacitors fabricated from a Au/Ti metal top plate, Al2O3 dielectric, and Cd3As2 bottom plate. Such measurements will give new, quantitative understanding of the density of states in a complicated heterostructure system of current interest.