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| Dark center. In these scanning tunneling microscope images of current flow through a single atom on a silicon surface, the dimming of the center shows current decreasing as the voltage increases (from right to left). The halo shows that current flowing around the atom behaves normally. [Credit: M. Rashidi et al./Univ. of Alberta] |
Negative differential resistance (NDR) refers to current decreasing as voltage increases, contrary to a normal resistor. The phenomenon is useful in electronics, and now a research team has demonstrated a reliable form of single-atom NDR and has explained in detail how it works. To verify their model, the team used a scanning tunneling microscope in a new way—they measured the time it takes for electrons to hop onto a single atom and showed that this time is critical for the NDR effect. The work opens the door to integration of NDR into microelectronic devices.
NDR was first observed in the tunnel diode 50 years ago [1]. Tunnel diodes are used in switching devices, oscillators, and other applications. However, it has proven difficult to incorporate them into integrated circuits, limiting their wider use in microelectronics. Researchers have found cases of nanoscale NDR, but they have been either unreliable or hard to control.
Robert Wolkow of the University of Alberta in Edmonton, Canada, and his colleagues created a robust, single-atom NDR device by baking a silicon wafer to remove surface-attached oxygen and then immersing it briefly in atomic hydrogen at very low pressure. Hydrogen atoms bonded to almost every surface silicon atom, leaving only a few atoms exposed. These atoms provided so-called dangling bonds, each of which hosted two electrons, one with higher energy than the other.
APS Focus: Negative Resistance with a Single Atom, David Lindley
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