Tunneling Spectroscopy and Inverse Photoemission: Image and Field States

Abstract

Tunneling spectroscopy performed with the scanning tunneling microscope is used to study image-type surface states. The tunneling tip causes a Stark shift and expansion of the hydrogenic image-state spectrum, permitting a clear resolution of the individual states. A simple theoretical model provides a quantitative connection between the tunneling data and both previous and new inverse-photoemission data.Image states are an interesting set of surface states, which have attracted a great deal of attention in recent years. 1 " 6 These hydrogenlike states are bound to a surface by the response of the substrate to the presence of the electron, and kept outside the surface by the reflective properties of the substrate. A theoretical model in which electronic motion along and perpendicular to the surface is taken to be independent yields the following relation for the binding energy of each member of the hydrogenic series:where n is the principal quantum number, e n is the purely hydrogenic component of the binding energy, k ( | is the wave vector parallel to the surface, m* is the effective mass, and E ncorT is the shift of the bottom of the band caused by deviations from perfectly freeelectron motion along the surface. These states have been previously observed by LEED 2 and by /c-resolved inverse-photoemission spectroscopy (KRIPES) . 3 " 7 Also, the important role played by the image potential in the vacuum tunnel current has recently been pointed out. 7 For image states, the electric field characterizing a tunnel junction has the effect of causing a Stark shift in the states of the hydrogenic spectrum. The evolu-tion of the spectrum due first to corrugation effects and then to the electric field is shown in Fig. 1. The Stark shift has the effect of continuously shifting and expanding the image-state spectrum, with its accumulation of states at the vacuum level E VSiC , into the geometric resonance spectrum associated with the Vshaped potential created by the substrate and the field. Such geometric resonances were considered by Jason 8 to explain oscillations in field ionization, and by Gundlach, 9 who predicted oscillating tunneling I-V characteristics, which were observed experimentally in semiconductor planar tunnel junctions 10 and metalmetal interfaces 11 as well as on Au tips in vacuum tunneling. 12 The tunneling measurements we describe add the scanning capability of the scanning tunneling microscope (STM) 1314 to the well-established capabilities of traditional tunneling spectroscopy. The benefits of this addition are numerious, e.g., bare surfaces become accessible to the full arsenal of UHV surface treatments and analysis, and it becomes possible to select and probe a suitable small portion of the surface. Figure 2 illustrates representative tunneling spectra for several surfaces. They were taken with a voltage modulation of A V = 0.2 V at v = 200 to 400 Hz, a frequency above the response of the feedback system Ni (100) i i > O) H>--te ILU 1Z Mxi \ (a) n = co 10 ^ n=2 yn = 1 TUNNEL _ 1D BARRIER T 'P -10HFIG...

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Properties of oxide thin films and their adsorption behavior studied by scanning tunneling microscopy and conductance spectroscopy

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