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Advancing Materials Research
tunneling. Electrons tunnel from a small metallic tip held above the surface, across a vacuum barrier, into the surface if the tip is biased negatively with respect to the surface or vice versa if the bias is changed. The vertical resolution is achieved by the exponential dependence of the tunneling current on the tip-to-surface separation. A typical variation in current is an order of magnitude for every angstrom of separation. An image in the lateral direction is achieved by scanning the tip across the surface. The lateral resolution is determined primarily by the size and shape of the “probe” or tip. Ultimately, experimentally realizable resolution in the vertical direction is dictated by the electronic and vibrational stability of the individual instrument, typically 0.1 angstrom. The horizontal or lateral resolution (2 to 3 angstroms) depends not only on the stability but upon the size and shape of the tip. An “image” is usually obtained by moving the tip across the surface with a piezoelectric x-y scan while maintaining a constant current (fixed height) with a third piezoelectric device. The many experimental difficulties associated with vibrations, electronic stability, and tip shape will not be discussed here.
A fundamental problem of this technique is one of interpretation: How does one translate tunneling images into pictures that reflect the identity and position of individual atoms of an unsolved structure? Solutions tend to be part science, part art, and the subject of continuous discussion. Both an advantage and a disadvantage of scanning tunneling microscopy is that the pictures obtained are in general a mixture of spectroscopy and microscopy. It is tempting to say that the tunneling microscope sees atoms, but, in fact, electrons tunneling from the Fermi energy of the tip see the spatial characteristics of the local density of states of the surface at an energy level equal to the bias voltage, which is higher than the Fermi energy. Therefore, the image is a function of bias voltage. This cross between microscopy and spectroscopy has tremendous advantages if we can learn to use it properly.
Keeping in mind the caveats listed above, the following few paragraphs describe experimental observations, beginning with structural studies where the reconstruction of clean metal and semiconductor surfaces has been observed. These observations include images of gold, silicon, germanium, and gallium arsenide. The steps on a Si(111) surface have been imaged and show how the surface defects causing the surface reconstruction are incorporated into the step edge. Images of germanium-silicon alloys indicate that at the surface there is an ordered GeSi alloy. These structural studies are just beginning to present a picture of the role of defects such as step edges and vacancies on such surface processes as epitaxial growth and catalysis. Preliminary steps are being taken to image foreign atoms on surfaces, including large molecules such as DNA or viruses.
The tunneling microscope can also provide images of the change in the electronic distribution at the surface. Two recent examples hint at future