For epitaxial growth, as with all other thin-film growth processes, it is essential to have real-time information about the actual growth rate and film thickness. Optical reflectance is usually the method of choice because it gives direct access to the growing layers and can be implemented through a single normal incidence viewport. However, the information gained by reflectance measurements is not limited to growth rate alone.
Reflectance makes it possible to monitor multiple essential properties of the growing layers. Usually, these three wavelengths are measured: