Fabrication and characterization of ultra-small tunnel junctions for single electron devices
Work on the fabrication processes has shown that traditional tunnel junction formation techniques result in structure sizes which are too large to provide the high temperature effects required. Where lithographic techniques alone are used to shrink pattern dimensions, the processes become unreliable. In the case of the suspended mask shadow evaporation process used here, a limiting reliable overlap width of 40nm is expected and experienced. Attempts to fabricate structures below this size resulted in unreliable tunnel junction formation. The second technique investigated, the crossed track technique, suffered from serious problems arising from the angled evaporation process and from step coverage difficulties. The third fabrication technique attempts to control the placement of grains within the aluminium film. This technique has the advantages of simplicity and ability to form the smallest tunnel junctions with the material system used here. This system was chosen as the main fabrication process for investigation of high temperature single electron devices in this work. Measurements of resistivity and resistivity temperature dependence of the aluminium films were used to characterize the film types. The temperature dependence and magnitude of the resistivity have shown the films to be very conductive, or metallic. By virtue of this high conductivity, the structure behaviour should be dominated by the device, or tunnel junction, properties. The results obtained from the devices at 4.2K do not show the presence of single electron effects. However, the fabricated structures did demonstrate tunnelling behaviour. The absence of single electron effects has been attributed to the structure sizes. Despite being among the smallest possible in aluminium metallizations, these granular structures are apparently too large. The explanation for this is derived from the presence of stray capacitances between the grains forming the tunnel junctions. This raises the junction capacitance and therefore reduces the charging energy of the junction and the temperature of operation.