Studies of acoustic waves, noise and charge pumping using single-electron devices

Doktorsavhandling, 2012

This thesis covers a range of experiments on single-electron devices, electrical circuits that utilize the discreetness of the electron charge. The Single-Electron Transistor (SET) is of special importance among these, being the most sensitive electrometer demonstrated, and sufficiently mature to serve as a building block in sensors and detectors for physical phenomena that can be transformed into charge signals. Such an example is the use of the SET as a local probe for Surface Acoustic Waves (SAW), demonstrated here for the first time. The SAW is a mechanical wave propagating on the surface of a solid, in this case a GaAs substrate. Due to the piezoelectricity of GaAs, the SAW is accompanied by a wave of electrical polarization which couples efficiently to an SET deposited on the same substrate. The experiments demonstrate a sensitivity to surface displacement of 30 am/√Hz, which is 2–3 orders of magnitude better than in previous SAW experiments. The SET is located between two on-chip acoustic reflectors separated by 2.9 mm, and has a sufficient measurement bandwidth to resolve the echoes of acoustic pulses between the reflectors. The resolution when averaging over many pulses is sufficient to resolve an average energy of h f per pulse reaching the SET. These experiments, along with associated theoretical treatments, indicate that studies on propagating acoustic waves in the quantum mechanical regime are feasible. The high charge sensitivity of the SET was also applied in prototypes for detection of single optical photons, where photo-generated electron-hole pairs near the surface of a semiconductor heterostructure are separated and transported to within the detection range of SETs deposited on the same chip. Another part of the thesis concerns the characterization of charge noise, which limits performance in SETs and related devices such as qubits and electron pumps. The dependence of the noise level on temperature and SET bias conditions is investigated, as well as its connection to the long-term relaxation observed after sudden application of a strong electric field. The results show that the sources of charge noise are in thermal equilibrium also at comparatively low temperature, and are in strong thermal contact with the electrons residing in the SET. This gives an indication about the microscopic nature and possible locations of the noise sources. The final part of the thesis is devoted to the study of quantized current sources, devices which can transport electrons one by one through a circuit at a controlled rate. To be com- parable with other metrological standards, such a current source must be accurate to around one part in 10^8, which requires careful study and elimination of all possible error sources. Errors due to Photon-Assisted Tunneling were studied in a resistively terminated multi-junction pump, and errors due to Andreev tunneling were demonstrated in a hybrid SINIS electron turnstile. The latter experiment shows that errors due to Andreev tunneling is reduced sub- stantially in turnstiles with high charging energy.