A Hard-Sphere Model to Simulate Alloy Thin Films

A model is developed for the simulation of alloy thin films by forming a layer consisting of spheres of two different sizes mixed randomly in any desired proportion. Included in the model is the possibility for vibration to simulate low-temperature annealing, as well as the use of a “substrate” containing periodic grooves to simulate epitaxial growth. Optical (Fraunhofer) diffraction patterns are taken from photographs of the model for comparison with the structural features observed directly. The model serves to show strikingly the role of the size factor in determining film structures. In particular, it shows that for size differences near 25% amorphous structures are obtained over a wide range of compositions. It also shows that such amorphous structures are essentially unchanged by vibration “annealing.” On the periodically grooved substrate, the existence of an “epitaxial” deposition rate is demonstrated. These and other features of the model are compared with experience concerning evaporated thin films and found to be in excellent qualitative agreement.