Faculté des sciences

Development of an aluminum single electron transistor scanning probe

Suter, Kaspar ; Staufer, Urs (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2007 ; Th.2028.

This thesis presents the development of a fabrication process for an Aluminum single electron transistor, experimental results verifying its functionality, and its integration on the tip of a scanning probe for scanning probe microscopy. When an electron passes through a tunnel barrier, it changes the barrier's capacitance, which in turn builds up a voltage across the barrier. If the thermal... Plus

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    Summary
    This thesis presents the development of a fabrication process for an Aluminum single electron transistor, experimental results verifying its functionality, and its integration on the tip of a scanning probe for scanning probe microscopy. When an electron passes through a tunnel barrier, it changes the barrier's capacitance, which in turn builds up a voltage across the barrier. If the thermal activation energy is smaller than the charging energy, and the barrier resistance sufficiently suppresses quantum fluctuations, then this voltage buildup prevents any further electrons from tunneling. In other words, to be able to observe single electron charging effects in a tunnel barrier, its capacitance C must so small that the charging energy e2/2C is larger than the thermal energy kBT and the quantum fluctuations h/T=h/RC. If those conditions are observed, then tunneling is blocked, the current is suppressed, the conductance drops, and the device is in Coulomb blockade. A single electron transistor is a single electron charging effect based device, where two tunnel barriers are connected in series, defining a small island between them. This suppresses the quantum-mechanical uncertainty of the electron location. A gate is capacitively coupling to the island as third electrode. If the energies allow for observation of single electron charging effects, and a bias voltage is applied across the two tunnel barriers, then electrons may tunnel through both of them, resulting in a current, given that the tunnel barriers are not in Coulomb blockade. This is determined by the voltage applied to the gate electrode. A small change in the external polarization charge on the gate electrode (by fractions of the elementary charge) may move the single electron transistor from a conductive state into Coulomb blockade and vice versa. The current voltage characteristic of a single electron transistor is e-periodic with the gate voltage, since increasing the gate voltage allows to increase the number of electrons on the island one by one. A process to fabricate single electron transistors was developed from scratch, employing the double angle Niemeyer-Dolan evaporation technique. A resist stack of poly(methyl-methacrylate) on top of copolymer was exposed with an electron beam lithography system. Resistively heated thermally evaporated Al was controlledly oxidized, followed by a second Al evaporation step at a different inclination angle. The overlap of the two evaporation steps defines the tunnel junction area. The device's functionality was successfully tested in a 4He and a 3He-4He dilution cryostat for normal conductive and superconductive states. Given that single electron transistor is very sensitive to change in charge, the scanning of a single electron transistor over a sample allows to map the change in charge, or charge distribution. The change in gate electrode polarization is measured by the change in current in the single electron transistor. In short, the current in the scanning single electron transistor changes because the capacitive coupling between the gate electrode and charges in the sample changes. Such a tool is intended to be used for local probe experiments, such as to probe the electric potentials and fields of device built in a two dimensional electron gas. A self-sensing and self-actuating quartz tuning fork based atomic force microscope probe was chosen as a platform for the implementation of the scanning single electron transistor. A monolithic Si handling chip with a notch where the proximal U-shaped end of the tuning fork can be lodged, is microfabricated with anisotropical potassium hydroxide etch. A cantilever extends from the chip body, and its end slightly exceeds the distal end of the tuning fork, and narrows down to form a tip shape, where the single electron transistor is patterned. A scanning single electron transistor probe was fabricated, and shows the same electrical room temperature behavior as the successfully a low temperature tested test structure single electron transistors. But only experimental verification will be able to show if the fabricated scanning single electron transistors probes are functional or not.