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Published online by Cambridge University Press: 05 June 2012
In the previous chapter we considered perfect crystals which are periodic and thus infinite in extent. There is one deviation from perfect periodicity that is always present: all real crystals have surfaces. We argued above that the surface has a small effect on bulk properties. This may be true, but the growth of crystals is the process of adding surface layers. We will discuss a number of aspects of surfaces and growth here.
Observing surfaces: scanning tunneling microscopy
In the previous chapter we showed how that structure of a surface could be revealed by using LEED; this technique is useful for periodic surfaces. As we will see, crystal surfaces have interesting deviations from periodicity such as steps. An enormous advance in studying this aspect of surfaces was the development of direct atomic resolution microscopy.
A remarkable instrument of this type is the scanning tunneling microscope (STM) invented by G. Binnig and H. Rohrer (Binnig, Rohrer, Gerber & Weibel 1982). The idea of this device is that quantum mechanical tunneling is very sensitive to the distance through which the particles tunnel.
Suppose electrons tunnel through a classically forbidden region of length x. The current will be of order j ∼ e−x/a where a is the attenuation length of the electron wave function in the forbidden region. The STM consists of a sharp metal tip which is brought into near contact with a surface by manipulating piezo-electric drivers.
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