surfactant (rus. сурфактант) — in the thin-film technology — active impurity, which, being applied on the substrate surface, normally as a monolayer or fractions of a monolayer, changes the thin film growth modes on the substrate in the required direction (as a rule, it changes island nucleation into layer-by-layer growth). It is worth noting, in other fields of knowledge this term is used in the meaning of a surfactant.

Description

In order to be a surfactant, an impurity shall meet the following requirements:

– to stimulate two-dimensional growth in an environment where three-dimensional growth usually takes place;

– to be insoluble in the film so that in the film there is a negligible amount of surfactant left.

The second requirement can be met in two ways. First, the surfactant can segregate on the film surface due to continuous exchange with growing film atoms, adsorbed on the surface.

Second, the surfactant atoms may remain fixed at the film – substrate interface under a film layer. Such types of surfactants are sometimes called interfactants.

A classic example of growth with a segregating surfactant is the epitaxy of Ge films on Si (111) with surfactant Sb present (see fig.). The growth of Ge on a clean DAS model goes on by the Stranski-Krastanov growth mode, where three-dimensional islands are formed on top of a pseudomorphic Ge-Si layer 3 monolayers thick (ML). When using ~1 ML of Sb as a surfactant, three-dimensional islands are suppressed and do not form, a continuous Ge film grows layer by layer. And Ge (111)2×1-Sb surface reconstruction takes place with 1 ML of Sb on the surface of the film.

None of the known examples of surfactant effect on thin film growth modes is associated with the classical definition of the surfactant as a substance that lowers the free energy of the surface (similar to surface-active substances). In fact, the surfactant affects the growth of the film by changing the kinetic growth parameters (e.g., affecting the mobility of adatoms on the surface).

Illustrations

SEM image of Ge film consisting of 50 monolayers (ML) grown on Si(111) surface: а — without sur
SEM image of Ge film consisting of 50 monolayers (ML) grown on Si(111) surface: а — without surfactants; b — with 1 ML of Sb as surfactant. Authors: P. Zahl, P. Kury, M. Horn-von Hoegen, Solid State Physics Institute, Hannover, Germany [2].

Authors

  • Zotov Andrey V.
  • Saranin Alexander A.

Source

  1. Zahl P., Kury P., Horn-von Hoegen M. Interplay of surface morphology, strain relief, and surface stress during surfactant mediated epitaxy of Ge on Si // Appl. Phys. A. 1999. V. 69, №5. P. 481–488.

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