Alloys evaporation

The pressure at which materials evaporate depends on the type of material. Therefore also the vapor pressure will be different for each component in the alloy. The deposition rate depends on the vapor pressure (see equation), therefore the components will evaporate independently with different rates. Consequently, the composition of the deposited film will differ from the composition of the evaporated source.

Solutions:

• deposit materials with similar vapor pressure (GaAs is an example)
• use two sources with the two materials and heat them at different temperature. By setting the right temperatures, equal evaporation rates can be achieved (see picture below). However it is difficult to control the temperature very precisely, and the composition may not be constant.
• Alternate deposition of the two materials by using shutters. A sandwich with alternating films will be obtained. By heating the film, interdiffusion will occur in in the multilayered film and a uniform composition can be obtained.