Coaxial-tip piezoresistive cantilever probes for high-resolution scanning gate microscopy [electronic resource]

Scanning probe techniques provide a wealth of information about the nanoscale properties of materials and devices. In scanning gate microscopy (SGM), the current through a sample is recorded as a sharp, conductive tip that modifies the local electrostatic potential is scanned above the surface. SGM has been used to map current flow, carrier density and potential barriers. Existing, unshielded SGM probes have significant stray capacitance, resulting in poor lateral resolution when they are used to image nanostructures. Thus, there is a need for a probe that minimizes stray capacitance to produce highly-localized electric fields. This probe must also self-sense topography for tip-sample alignment, as the conventional laser-based detection methods can disturb photosensitive samples. In this thesis, we present a new scanning probe that integrates a coaxial tip on a piezoresistive cantilever. The coaxial tip is comprised of a heavily-doped silicon inner conductor and an aluminum outer shield, separated by a silicon dioxide insulator. By shielding the inner conductor up to the tip apex, this tip configuration minimizes stray capacitance to produce narrow electrostatic potential profiles. A piezoresistor is embedded at the root of the cantilever and enables electrical measurement of deflection at the free end. Scanning gate microscopy is commonly performed at room temperature (room-T) and low temperature (low-T). We discuss the design of piezoresistive cantilevers for atomic force microscopy (AFM) under both temperature regimes. We introduce a numerical optimizer that we used to identify 12 cantilever designs for use at room-T and low-T for hard, semiconductor samples and soft, biological samples. We show the results of finite-element analysis used to predict the electrostatic potential profiles produced by unshielded and coaxial tips. We investigate how the full-width at half-maximum (FWHM) of the coaxial tip perturbation varies with lift height and tip geometry. We discuss the development of a 7-mask process to fabricate scanning probes with both a coaxial tip and a piezoresistor. We compare two methods to create sub-micron tip apertures with focused ion beam milling, and provide a recipe that can repeatably produce openings with a radius of 30 nm. We describe the characterization of the piezoresistive cantilevers at room-T on a commercial AFM and at low-T on a home-built cryogenic scanning system. Finally, we provide images of the potential profile from the coaxial tip, obtained using a quantum point contact at low-T. In a measurement bandwidth from 1 Hz to 10 kHz, our scanning probes achieve a vertical displacement resolution of 2.8 A at 293 K and 82 A at 2 K, where the low temperature performance is limited by amplifier noise. When the coaxial tip is 100 nm above a sample, the FWHM of the electrostatic potential profile it produces at the surface is less than 240 nm, representing a 2.3x improvement in the lateral resolution of SGM over unshielded tips.

Keywords: Electrical Engineering
Advisor: Howe Roger Thomas primary advisor. Thesis advisor
Pruitt Beth primary advisor. Thesis advisor
Goldhaber-Gordon David