| AVS 56th International Symposium & Exhibition | |
| Electronic Materials and Processing | Thursday Sessions |
| Session EM-ThA |
| Session: | Quantum Structures and Nitrides Devices |
| Presenter: | L.P. Cockins, McGill University, Canada |
| Authors: | L.P. Cockins, McGill University, Canada Y. Miyahara, McGill University, Canada S.D. Bennett, McGill University, Canada A.A. Clerk, McGill University, Canada S. Studenikin, National Research Council, Canada P. Poole, National Research Council, Canada A. Sachrajda, National Research Council, Canada P. Grutter, McGill University, Canada |
| Correspondent: | Click to Email |
The ability of quantum dots to confine single charges at discrete energy levels makes them a promising platform for quantum computation where the intrinsic properties of single electrons, such as spin, act as the conventional 1 and 0 bit in a classical computer. In order to control initialization and to scale up the number of bits, an understanding of both the energy levels of single quantum dots and the variation between dots need to be characterized.
Self assembled quantum dots are of considerable interest in this field because their size, shape, and material can be controlled during the growth process. Controlling these properties is important as these influence the confinement potential, thereby controlling the energy levels of the dot. However, the method of growth makes positioning of the quantum dots difficult and usually they are randomly distributed over the sample surface. This, in addition to the small size of the dot, makes it challenging for lithography techniques to access the quantum dots to perform either charge transport or charge sensing measurements so that the dot properties can be measured.
An atomic force microscope can be used to spatially access the dots, and by applying a voltage between cantilever tip and back-electrode (beneath the dot), the energy levels of individual dots can be probed. At low temperatures the dots are in the Coulomb blockade regime and individual electrons can be controllably added by applying a sufficient bias voltage to overcome this electrostatic repulsive energy. The oscillating cantilever in these experiments is responsible for both loading/emptying the dots through electrical gating and also detecting tunneling events through a change in resonant frequency and/or the amount of energy required to maintain a constant oscillation amplitude. Electrical leads are not required in this experiment which not only leaves the surface electrostatically intact but also gives us the freedom to investigate any dot on the surface.
Using an atomic force microscope we demonstrate the ability to probe the energy levels in few electron self assembled InAs quantum dots. The charging energy, level spacing, and shell structure of single dots are extracted and supported theoretically. Multi-dot complexes are also investigated and pairs of dots which are either capacitively or tunnel coupled are observed. Increasing the oscillation amplitude of the cantilever allows for the additional electron to enter the dot at a higher energy level, in a way probing the excited states of the dot similar to excited state spectroscopy. These findings are also supported by theoretically.