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  • (2010) Willems van Beveren, Laurens Henry; Tan, Kuan; Lai, Nai Shyan; Dzurak, Andrew; Hamilton, Alex
    Journal Article
    We report the fabrication and study of Hall bar field-effect transistors in which an overlapping-gate architecture allows four-terminal measurements of low-density two-dimensional electron systems while maintaining a high density at the Ohmic contacts. Comparison with devices made using a standard single gate show that measurements can be performed at much lower densities and higher channel resistances, despite a reduced peak mobility. We also observe a voltage threshold shift which we attribute to negative oxide charge, injected during electron-beam lithography processing.

  • (2010) Tan, Kuan-Yen
    This thesis demonstrates the fabrication and the measurement of single-electron tunnelling through deliberately implanted individual phosphorus (P) donors in double-gated silicon nanoscale field-effect-transistors (nanoFET). These structures conveniently allow the control of the number, depth, and tunnel coupling of the donors. For each P donor, three possible charge states have been observed, in which two are successively occupied by spin-down and spin-up electrons in a magnetic field. These states are separated by a charging energy consistent with those between the D0 and D− charge states of a P donor coupled to the surrounding electrodes. These experiments provide important understanding of the electric and magnetic properties of individual phosphorus donors needed for the realization of Si:P nanoelectronics. The demonstrated ability to resolve spin states of donors in these devices is the first step towards the measurement of electrically-detected magnetic resonance for a single-donor electron spin. Due to the tunability of the Fermi levels of the reservoirs of the nanoFET, the density of states (DOS) in the source and drain reservoirs are found to be quasi-1-dimensional. These reservoir DOS modulate the transport current through the nanoFET, consequently manifesting themselves as conductance peaks in bias stability diagrams. Further investigations showed that in a externally applied magnetic field, these DOS peaks shift in magnetic field at precisely half the Zeeman-splitting energy compared with peaks related to P-donor bound electrons, in good agreement with theoretical predictions. This result enables convenient discrimination between spin and orbital excited states from other features attributed to DOS modulations in the leads. Another important feature observed in the bias stability diagrams is the mutual charging effect of adjacent parallel donors in the conducting channel of the nanoFET. Charging a single donor in parallel with other active donors gives rise to a potential shift which is significant compared with the D0 and D− charging energy. In addition, the mutual charging energy also provides a good indication of the proximity of the active donors in the conducting channel. The successful demonstration of resonant tunnelling through locally-doped P donors in silicon in these nanoFET structures indicates that they can provide a convenient platform for the investigation of single-atom nanoelectronics and spintronics in silicon.