Real-time studies of phase transformations in Cu–In–Se–S thin films—3: Selenization of Cu–In precursors

Phase transformations during selenization of thin films of elemental Cu, elemental In, and intermetallic CuxIn with x=(1.8, 1.0, 0.8) are investigated using real-time energy dispersive X-ray diffraction. From the temporal development of the X-ray peak intensities, phase predominance sequence diagrams are constructed including the intensity ratio of the CuKα/InKα fluorescence lines. It was found that vapor selenization of Cu layers results in the formation of Cu2−xSe. At elevated temperature, linear growth of Cu2−xSe is observed which is interpreted as a surface controlled reaction mechanism. Indium selenization proceeds via the phase sequence In4Se3→InSe→In2Se3. During CuxIn selenization, the transformation sequence towards more Cu-rich intermetallic phases CuIn2→Cu11In9 and Cu11In9→Cu16In9 precedes the selenization process. During selenization of precursors with x>1.4, this sequence continues bringing up the intermetallic phases Cu7In3 and α-CuIn. The latter are formed at the bottom of the films. Thus, the bottom of the film becomes depleted from Indium by In diffusion. In general, the selenization process is slower than the sulfurization processes investigated in Part 2 of this study. A model is proposed which includes Cu2−xSe as a preliminary precursor for CuInSe2 growth. This Cu2−xSe formation at the surface of the films is proposed to be surface reaction limited and is proposed to be the limiting step for the CuInSe2 formation. Precursors with x⩽1 transform into CuInSe2 with the participation of In–Se phases.