Room temperature quantum dot transistors in silicon
Creators: name: Abualnaja, Faris
Publisher: name: Imperial College London
Year: 2021

description: Single-electron transistors (SETs) in silicon (Si), where charge can be controlled at the one electron level on a nanoscale charging island or quantum dot (QD), have great promise for the development of future quantum electronic circuits. Fabrication techniques have now been developed, where phosphorous (P) dopant atoms in Si can form QDs, reducing the charging island size to the atomic scale. This work extends the electrical operation of dopant atom QD SETs from cryogenic to room temperature (RT), by embedding the dopants in SiO2 tunnel barriers. Field-emission scanning-probe lithography (FE-SPL) and electron beam lithography (EBL) have both been used to define nanoscale point-contacts (PCs), followed by a 'geometric oxidation' process to completely oxidise these PCs and form tunnel barriers. Electrical characterisation at RT has demonstrated 'Coulomb diamond' charge stability current-voltage characteristics, indicating deep QD potential wells with electron addition energy ~0.3 eV and size ~2 nm. The latter is similar to the dopant separation, suggesting that segregated or clustered P dopant atoms form the QDs. Simulation results predict an underlying asymmetric, anharmonic, potential well, which can be modelled by a Morse-like potential of depth ~2 - 3 eV, sufficient for RT operation. Dual-gate RT electrical measurements of similar devices have been measured to investigate the electrostatic coupling properties of neighbouring dopant atoms. Double-QD (DQD) hexagonal patterns are seen in the charge stability diagram. These patterns are analysed further using an image processing program based on Otsu's Algorithm and the Hough Transform to reduce human observational bias. Complete and partial hexagonal features have been used to extract effective capacitance and resistance parameters for the device. Single-electron master equation simulations show the evolution of an ideal DQD hexagonal pattern with respect to variations in the effective circuit parameters. A qualitative reproduction of the experimental features is obtained, extending the understanding of atomic scale DQDs and their morphology, and electronic transport properties through impurities in insulators. Finally, a Szilard-Zurek one-electron gas 'Maxwell Demon' cycle has been analysed using these DQD systems. The density matrix has been solved to provide a detuning parameter that determines the occupational probability of a delocalised electron. Monte Carlo simulations and modelling of entropy exchanges between the QDs have shown that energy transfers cannot be accounted for by the gates or the thermal bath alone, and must occur in the 'Demon' memory. The stochastic electron occupation probability has been used to extract the entropy, which is then compared to theoretical fits. Entropy changes with a quasi-static gate voltage sweep shows a minimum, -kB ln(2), which corresponds to the entropy change for one bit of information. Finally, the effect of gate cycle trajectory, device capacitance and temperature are characterised to determine conditions to maximise the entropy changes.