Demonstration of the tunnel-diode effect on an atomic scale

THE tunnel diode 1 , which is widely used in high-speed electronics applications 2 , depends on the property of negative differential conductivity, that is, a negative slope in the current–voltage curve. The mechanism underlying the tunnel diode's behaviour, namely the existence of a range of biases for which tunnelling is forbidden or suppressed following a bias for which tunnelling is strongly favoured, has been employed subsequently in the design of new devices that also display the conductance anomaly, such as the double-barrier resonant-tunnelling device 3 . It has been predicted 4 that the conductance anomaly could result from a similar mechanism at the tunnel junction of the scanning tunnelling microscope (STM), where localized states on adsorbate and tip atoms give rise to allowed and suppressed energies for tunnelling. The STM has the capability to image regions of negative differential conductivity induced by individual atoms on a surface. Here we report the observation of negative differential conductivity on particular binding sites of a Si (111) surface doped with boron. Specific current–voltage characteristics are shown to be related to the presence or absence of the dopant at individual atomic sites, and negative differential conductivity is observed at –1.4 V tip bias at a specific type of site. Tunnelling spectroscopy indicates that the effect results from a tunnel-diode mechanism.