Fabrication and Characterization of Aluminum Single Electron Transistors for Scanning Probes

Doktorsavhandling, 2006

The main objective of the work presented in this thesis was to fabricate scanning single electron transistor probes that operate at pumped liquid helium temperatures. This required single electron transistors (SETs) with low resistance (R_) and relatively high charging energy (E_C), and the technology to integrate SETs onto probes. By reducing the oxygen pressure during the in situ oxidation of a 2-angle evaporation, the specific tunnel resistance of aluminum tunnel junctions was reduced. SETs with charging energies up to 20K*kB and with a typical resistance of 100 kOhm were fabricated. These SETs were DC- measured both at 4.2K and at 90 mK. The inverse resistance-capacitance (RC) product was as high as 42 GHz, and this was obtained without increasing the noise above that, typical for aluminum SETs. SETs with even lower RC product, corresponding to a cut of frequency of 120 GHz, were produced and measured with RF methods. These measurements yielded the highest reported charge sensitivity of any SETs, At 40mK we ob- tained, 0.9 e/Sqrt(Hz) in the superconducting state and 1.0 e/Sqrt(Hz) in the normal state. RF operation at 4.2K was demonstrated with this sample, also with a very good charge sensitivity: 1:9 ¹e=pHz. To further characterize this SET, we measured the 1/f noise up to 10 MHz, and showed that one strong resistance °uctuator was situated in one of the tunnel junctions. In a second noise characterization, we measured the shot noise of this device and extracted the Fano factor at small and large voltage biases. In the last part of this work, we developed a general method to fabricate sharp cantilevers and perform e-beam lithography, with dimensions down to 30nm at the tip of these cantilevers. The alignment precision was better than 30 nm between the cantilever structures and the lithographic pattern. With this method, we were able to mass produce SETs at the corners of cantilevers. These SETs were DC characterized at 100mK and at pumped liquid helium temperatures (T = 1.3 and 2 K), and had typical parameters of R = 100 kOhm and E_C = 3.5K/k_B. We furthermore integrated one of these cantilevers on a tuning fork AFM, and performed a proof of concept AFM scan. The RMS height noise of this scan was ~ 1 nm, which indicates that very small scanning heights, and hence high lateral resolution, should be possible with this combined probe.