Scanning Single-Electron Transistor Microscopy
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The devices of nanotechnology will require fabrication and characterization tools capable of near-atomic resolution, such as scanning probe devices. These employ a small, ultrasharp tip placed upon the surface of a solid. The tip measures an electrical, mechanical, or thermal property of the surface on a scale that ranges from 100 nm down to the size of a single atom. The most familiar and widely used devices of this class are the scanning tunneling microscope and the atomic-force microscope. The scanning device being discussed here detects the electric fields arising from charge on a surface. It is capable of seeing individual electrons and even much smaller amounts of charge. The device is a marriage of the sharp-tip scanning probe geometry with the single-electron transistor (SET). The SET is based on the quantum-mechanical phenomenon of electron tunneling and the principle of Coulomb blockade. It has a conductance that is unusually sensitive to an external electric field, such as provided by the charge e of a nearby electron. A probe with this capability having atomic-scale spatial resolution would potentially be a powerful tool for surface characterization in both technology and surface science. The present-date SET probes have not reached this status. So far the demonstrated resolution for charge on the surface of a solid is of order 0.001e with a spatial resolution of ∼ 30 nm. The operation has been restricted to liquid He temperature (< 4 K). The performance of SET probes can be expected to improve in the future as new nanoscale fabrication techniques become available.