Development of gallium gradients in three-stage Cu(In,Ga)Se2 co-evaporation processes

ABSTRACT We use secondary‐ion mass spectrometry, X‐ray diffraction and scanning electron microscopy to investigate the development over time of compositional gradients in Cu(In,Ga)Se2 thin films grown in three‐stage co‐evaporation processes and suggest a comprehensive model for the formation of the well‐known ‘notch’ structure. The model takes into account the need for compensating Cu diffusion by movement of group‐III ions in order to remain on the quasi‐binary tie line and indicates that the mobilities of In and Ga ions differ. Cu diffuses towards the back in the second stage and towards the front in the third, and this is the driving force for the movement of In and Ga. The [Ga]/[In + Ga] ratio then increases in the direction of the respective Cu movement because In has a higher mobility at process conditions than has Ga. Interdiffusion of In and Ga can be considerable in the (In,Ga)2Se3 film of the first stage, but seems largely to cease in Cu(In,Ga)Se2 and shows no signs of being boosted by the presence of a Cu2Se layer. Copyright © 2011 John Wiley & Sons, Ltd. We evaluate the development of gradients during the three‐stage co‐evaporation growth of Cu(In,Ga)Se2 (CIGS) and offer a model that explains the formation of the “notch” profile as well as the survival of intentionally added gradients. Cu in ‐ and out‐diffusion is compensated for by movement of In or Ga, and a lower mobility of Ga causes preferential movement of In. In‐Ga interdiffusion that erodes gradients is at its strongest when the film is very Cu‐poor.