Modification of 1D Ballistic Transport using an Atomic Force Microscope

We have used the scanning charged tip of an Atomic Force Microscope (AFM) to produce images of the conductance variation of a quantised 1D ballistic channel. The channel was formed using electron beam defined 700 nm wide split gate surface electrodes over a high mobility GaAs/AlGaAs heterostructure with a two dimensional electron gas (2DEG) 98 nm beneath the surface. We operate the AFM at 1.5 K and 4.2 K in magnetic fields up to 2 T to observe several phenomena. With a dc voltage on the AFM tip we have produced conduction images of the tip potential perturbation, as the channel is a sensitive probe of the electrostatic potential. We have also performed gate sweeps with the tip at a series of points across the width of the channel. The observed structure in transconductance corresponds to the theoretical electron density for the first three sub-bands. When certain gates were biased near pinch off, stable two level switching was observed in the images. We were able to control the state of the switch with the gate bias and tip position, and so roughly locate the position of the switching source. One of the responsible defect systems was located beneath the 2DEG, and due to screening of the tip potential, the image reveals the 1D channel. By asymmetrically biasing the two gate electrodes we have produced images showing a total channel movement of 102 nm across the width of the channel, but accompanied by 201 nm of movement along the length of the channel which is due to imperfections in the surface electrodes and disorder in the doping layer.