Surfaces - Our Window onto the World of Materials

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Si (111) surface of silicon

Silicon, used in computer applications, must be virtually defect-free and cannot contain more than part-per-billion levels of certain impurities. To control the purity of materials with this level of precision, each new layer of atoms must be added in the correct manner as the material grows. The new surface layer formed becomes buried by additional atoms as growth continues. In this sense, understanding materials growth boils down to understanding the behavior of atoms (or molecules) on surfaces.

We are concerned with the growth of materials on pre-existing substrates. This technique, called thin-film growth, is used in many applications, the most important being the fabrication of integrated circuits for computer memory and logic chips.

Thin-film growth usually involves exposing a hot substrate to reactive gas phase atoms or molecules. The types of events that are important become obvious once you think about what might happen to these atoms. Atoms come raining down from a source of some kind and land on the surface. To find the right location for attachment to the existing surface these atoms must be able to move about. This is why the growth temperature is so important. At low temperatures movement is restricted and many atoms fail to attach correctly and a poorly ordered polycrystalline or even an unordered amorphous material, results. The most favorable attachment locations are steps or ledges that are present on the surface. No surface is perfectly flat -- all are characterized by flat regions called terraces, that are separated from each other by steps. You can imagine that atoms cannot burrow their way into the flat terrace regions but can easily attach at steps, and in doing so they extend the size of the terrace above the step. For this reason, its not surprising that growth is best accomplished on substrates that start off having small terraces with lots of steps in between.

Visualizing Atoms on Surfaces

How then can we learn what happens on a surface during growth. The solution of course is to simply watch the surface as growth occurs. It is possible to follow individual atoms and 'watch' what happens as they move about on the surface by using a Scanning Tunneling Microscope (STM). Although STM is a rather complicated technique it is easy to understand how it works. STM visualizes a surface in the same way you can "visualize" a crate of oranges by simply running your finger back and forth over the oranges and the hollows between them. Instead of a finger STM uses a very sharp tip (usually made of tungsten wire) which is brought close to a surface. See diagram.