34th International Field Emission Symposium / 34ème Symposium International d'Emission de Champ

J. Phys. Colloques 48 (1987) C6-101-C6-106
DOI: 10.1051/jphyscol:1987617

USE OF AN STM TO DEFINE AND MEASURE AN OPERATIONAL TUNNELING TIME

P.H. Cutler¹, T.E. Feuchtwang¹, Z. Huang¹, T.T. Tsong¹, H. Nguyen², A.A. Lucas³ et T.E. Sullivan<sup>4

1</sup>  Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A
²  AT&T Bell Laboratories, Murray Hill, NJ 07974, U.S.A. and Department of Physics, the Pennsylvania State University, University Park, Pennsylvania 16802, U.S.A.
³  IBM Almaden Research Center, San Jose, CA 95120, U.S.A. and Facultés Universitaires Notre-Dame de la Paix, B-5000 Namur, Belgium
⁴  RCA Solid State Technology Center, Summerville, NJ 08540, U.S.A.

Abstract
Recent measurements of dc current-volcage characteristics of Scanning Tunneling Microscope junctions have confirmed their expected high rectification property. We have exploited this property to (i) study rectification at infrared frequencies and to (ii) arrive at a procedural definition of an electron tunneling time. A laser beam of linearly polarized light is focused on an STM junction and the resulting dc bias induced across the junction by the alternating, asymmetrical tunnel current is detected. This dc bias should vanish at sufficiently high laser frequency for fixed tip-to-surface distance or when withdrawing the tip away from the surface for fixed laser frequency. The average electron tunneling time is then the inverse cut-off frequency observed in such a laser rectification experiment. Results have been obtained that exhibit rectification for W-Si, W-Ni, and W-Au STM junctions. The rectification experiment with a W-Si junction and a YAG laser at 1.06 microns shows strong responsivity and a near linear decay of rectified signal versus increasing tip-surface distance. The observed cut-off occurs around 25 Å tip-surface separation and is consistent with an electron traversal time of the barrier at the tungsten Fermi velocity. These results are in agreement with tunneling times obtained from numerical solutions of wave packet transmission through model potential barriers.