Unveiling the role of copper content in the crystal structure and phase stability of epitaxial Cu(In,Ga)S2 films on GaP/Si(001)
Résumé
This study examines the growth condition to obtain a single-phase Cu(In,Ga)S2 (CIGS) chalcopyrite film epitaxially grown by coevaporation on a GaP/Si(001) pseudo-substrate. In particular, we report the structural differences between KCN-etched Cu-rich and Cu-poor CIGS films coevaporated on GaP/Si(001) by 1-stage process. The Cu-poor CIGS film consists of at least three phases; the main crystal is found to be chalcopyrite-ordered, coexisting with In-rich CuIn5S8, and CuAu-ordered CuInS2, all sharing epitaxial relationships with each other and the GaP/Si(001) pseudo-substrate. On the other hand, the Cu-rich CIGS film is single-phase chalcopyrite and displays sharper X-ray diffraction peaks and a lower density of microtwin defects. The elimination of the secondary CuAu-ordered phase with Cu excess is demonstrated. In both films, the chalcopyrite crystal exclusively grows with its c-axis aligned with the out-of-plane direction of Si[001]. This study confirms prior findings on the thermodynamics of Cu-In-Ga-S and the stability of secondary phases.
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