Tunneling and Scanning Tunnel Microscopy: a Critical Review

Abstract

Electron tunneling across the surface potential barrier has been applied for many years to probe surface atomic and electronic structure. This had led, for example, to the development of the field emission microscope, and more recently, of electron tunneling spectroscopy of insulating barriers in MIM structures. Some four years ago, Binning, Rohrer and coworkers developed the first vacuum tunneling microscope with atomic resolution. In this device, electrons tunnel across a vacuum gap between a sample surface and a scanning tip. Since then, a growing number of research groups have constructed such devices. The STM has provided detailed insight into the atomic structure of surfaces. The feasibility of scanning tunnel spectroscopy (STS) has been demonstrated, though fewer results have been reported. However, before the full potential of the twin techniques of STM and STS can be realized, two problems have to be resolved: (1) A more quantitative, but tractable theory, has to be developed to permit an unambiguous interpretation of the data. (2) A more reliable procedure for the preparation of the tip has to be developed to assure the quantitative reproducibility of the data. A critical review of tunneling theory and the transfer Hamiltonian formalism is presented. Inelastic and elastic resonant tunneling are discussed. The construction and operation of the STM are briefly described. The transfer Hamiltonian theory of the device is reviewed. The concept of surface corrugation is also discussed. Recent results by Kuk on the dependence of the observed corrugation amplitude on the atomic structure of the scanning tip are analyzed. Lastly, image potential states and their significance in the STS study of the electronic structure of the surface barrier are considered.