We present a new Scanning Probe Microscopy (SPM) technique, which is based upon force detection of electron tunneling events between a specially fabricated probe tip and sample at room temperature. The probe tip has an electrically isolated 100 nm metallic dot at its apex. The dot is created by field evaporation of platinum onto the end of the oxidized SPM tip. Electrostatic Force Microscopy (EFM) methods are used to measure the charge on this dot. The EFM has adequate sensitivity to detect a change in charge corresponding to a single electron[1,2]. To charge the dot, a small dc voltage is applied to the sample while the tip-sample distance is kept constant. For tunneling to occur, the probe is brought to a distance where the tunneling probability has a finite value (one electron/sec). When the charged probe is brought into this range discrete tunneling events are observed in both the amplitude and phase of the electrostatic force signal.(Fig 1) Single electron tunneling events are detected as an approximate 0.3 nm abrupt change of the probe oscillation amplitude. The amplitude and phase of the EFM signal is changed due to the abrupt decrease of the electrostatic force and force gradients acting on the metallic dot. The 0.3 nm reduction of the oscillation amplitude takes the tip outside of tunneling range due to the exponential dependence of the tunneling probability as function of tip-sample distance.
The electrostatic interaction between the probe and sample has been carefully modeled, including the effects of a tunneling event. The measured EFM response to a tunneling event has been directly compared with the predictions of the model with good agreement.
Force detected tunneling provides a way to measure ultra low currents (one electron/sec) between the specially fabricated tip and a conductive surface. It also provides a way to electrically characterize surface states on insulator surfaces.
L. J. Klein, C. C. Williams, and J. Kim, ”Electron tunneling detected by electrostatic force”, Appl. Phys. Lett. 77, 3615 (2000).
L.J. Klein and C.C. Williams, ”Single electron tunneling detected by electrostatic force”, App. Phys. Lett. (accepted)
Clayton C. Williams
Department of Physics, University of Utah
115 S 1400 E Rm 201, Salt Lake City, Utah 84112 USA
Phone: 801 5853226
Fax: 801 5814801