Fabrication and Characterization of Aluminum Single Electron Transistors for Scanning Probes
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
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.
radio frequency single electron transistor