XPS and XPD investigation of (1 1 2) CuInSe2 and Cu(InGa)Se2 surfaces

The (1 1 2) surfaces of CuInSe2 and Cu(InGa)Se2 single crystals were studied using X-ray photoelectron spectroscopy (XPS) and X-ray photoelectron diffraction (XPD) techniques after ion-beam cleaning and annealing at 660 deg. C. Analysis of the experimental data suggested the presence of a structure crystallographically relative to sphalerite on the surface. The three possible choices of such a structure are discussed: (1) a sphalerite-based structure with the composition Cu(InGa)3Se5, (2) a mixture of In2Se3 and Cu2Se and (3) Cu(InGa)Se2 mixed with In2Se3. Theoretical simulations of the XPD experimental data supported the presence on the surface either of a Cu(InGa)3Se5 structure with hexagonal stacking faults or a mixture of the phases Cu(InGa)Se2 and γ-In2Se3