"Imaging Electrons in Low Dimensional Nanostructures"

Ania C. Bleszynski
Harvard University

Abstract: A liquid He cooled scanning probe microscope (SPM) with a conducting tip has been used to image electrons in low dimensional nanostructures. I will talk about three systems we have imaged: two-dimensional electron gases, quantum dots, and semiconducting nanowires. We have directly imaged coherent electron wave flow from a quantum point contact (QPC) formed in a GaAs/AlGaAs two-dimensional electron gas. A negative voltage on the SPM tip depletes a small divot in the electron gas, backscattering electron waves. The tip is raster scanned above the surface and a spatial map of the electron flow is obtained by recording changes in QPC conductance as a function of tip position. An imaging interferometer was created by introducing a circular mirror to reflect electron waves emitted from the (QPC). Interference fringes are present in the images and are shown to move as the mirror is moved via its gate voltage, thus forming an interferometric spectrometer. Spatial images of a single-electron quantum dot formed in a 2DEG by surface gates are also presented. The images were obtained by fixing the tip voltage and recording the dot conductance while scanning the SPM tip above the quantum dot. The images show a ring of increased conductance centered on the dot, where the dot conductance is on the Coulomb blockade conductance peak between 0 and 1 electrons. Lastly, I will present images of InAs and InP nanowires, grown using the vapor-liquid-solid (VLS) technique. The charged SPM tip is scanned in an area above the nanowire; the resulting change in nanowire conductance is recorded to form the image. Concentric rings of increased conductance, centered on the wire, are present in the images and are seen to grow in size with more negative backgate voltages.