We have developed ways to image the motion of electrons through nanoscale devices using a cooled scanning probe microscope (SPM). Nanoscale devices are promising for nanoelectronics, spintronics and quantum information processing. SPM images reveal how electrons travel through the structure by displaying the device conductance as the SPM tip is scanned above. The metallized tip acts as a movable gate that can backscatter or deflect electrons in an open system or pull an electron onto a quantum dot. Cooling to liquid helium temperatures allows us to see quantum phenomena, including interference fringes for electron waves and the Coulomb blockade for a quantum dot. For a two-dimensional electron gas, we have imaged magnetic focusing of electrons flowing along cyclotron orbits between two quantum point contacts, and observed fringes due to quantum interference. For quantum dots, we started by imaging a 1-electron GaAs dot. The images show a ring of high conductance about the dot as the first electron is added. The SPM can also measure changes in the energy of a quantum state, such as the diamagnetic shift in the ground state energy of a 1-electron dot. Semiconductor nanowires provide a very promising way to make tiny dots and dot circuits, so small that they are difficult to control using lithographic gates. We have used the SPM tip as a movable gate to image a few-electron InAs quantum dot confined in an InAs/InP nanowire by two InP barriers. High-conductance rings in the images locate the dot, and show how many electrons it contains. We plan to study tunnel-coupled double InAs dots defined by InP barriers in an InAs/InP nanowire, by using the tip to individually tune the charge on each dot. The SPM is a promising tool for the development of quantum dots circuits that can locate and move electrons, and measure their energy.
*With Kathy Aidala, Ania Bleszynski, Parisa Fallahi, Rob Parrot, Tobias Kramer, Eric Heller, M.P. Hanson, Art Gossard, Linus Fröberg, Lars Samuelson, Erik Bakker, and Leo Kouwenhoven. Supported at Harvard by the ARO and the NSF-funded NSEC, at Lund by the Swedish VR, SSF, KAW, the ONR, and a NODE EU program, and at Delft by NanoNed.
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