Selenium flux effects on Cu(In, Ga)Se2 growth rate, and control by in-line X-ray fluorescence

In-line X-ray fluorescence (XRF) is demonstrated for Cu(In, Ga)Se₂ (CIGS) co-evaporation source control. XRF is found to accurately measure film composition for direct metals source control, as well as an indirect monitor for selenium rate using the nature of the self-limiting diselenide. It was found that below an optimum Se flux value, the film growth rate is limited by the Se atom availability, while a higher Se overpressure reduces the growth rate by gas phase scattering. This correlation allows the growth rate to act as a monitor and possible control mechanism for production scale CIGS thermal evaporation. It is also possible to use the thickness measured by XRF as a feedback control for the Se overpressure.