Imaging Electron Flow in Graphene

Abstract

Scanning probe techniques can be used to probe electronic properties at the nanoscale, to shed light on the physics of nanoscale devices: Graphene is of great interest for its promise in both applied (e.g. spintronics and valleytronics) and fundamental research (e.g. quantum Hall and Dirac fermions). We successfully used a cooled scanning gate microscope to image the motion of electrons along cyclotron orbits for magnetic focusing in graphene. Part of my time at Harvard was also spent incorporating a low temperature scanning capacitance setup into the existing microscope as well as building a low temperature coarse positioning system. To image magnetic focusing of electrons in graphene, a conducting tip of a scanned probe microscope is held just above the sample surface, and an applied tip-to-sample voltage creates an image charge that is moved while the transresistance between two leads is measured. The sample is a high mobility hBN-graphene-hBN sandwich etched into hall bar geometry with two point contacts along each side. By tuning the transverse magnetic field B and electron density n in the graphene layer, we observe the first few magnetic focusing peaks. For values of B and n that correspond to the first magnetic focusing peak, we observe an image of the cyclotron orbit that extends from one point contact to the other. We also study the effects of B and n on the spatial distribution of electron trajectories as we move away from the magnetic focusing peak. We also present the design and implementation of a cooled scanning capacitance probe that operates at liquid He temperatures to image electrons in nanodevices. In this setup, an applied sample-to-tip voltage creates an image charge that is measured by a cooled charge amplifier adjacent to the tip. The circuit is based on a low-capacitance, high-electron-mobility transistor(HEMT) (Fujitsu FHX35X). The input is a capacitance bridge formed by a low capacitance pinched-off HEMT transistor and the tip-sample capacitance. We have achieved a low noise level (0.13 e/ Hz^0.5) and high spatial resolution (100 nm) for this technique, which promises to be a useful tool to study electronic behavior in nanoscale devices.