Low energy ion scattering: Surface preparation and analysis of Cu(In,Ga)Se₂ for photovoltaic applications

Cu(In,Ga)Se₂ (CIGS) single-crystal epitaxial films have been analyzed by low energy ion scattering, which is sensitive to exactly the outermost surface atomic layer, to determine the surface chemistry as a function of preparation conditions. The samples were grown by a hybrid sputtering and evaporation method on cation (A) or anion (B) terminated (111) GaAs substrates and had smooth surfaces. The samples were exposed to excited atomic oxygen or hydrogen beams or were sputtered with 500 eV Ar⁺ ions. Atomic O∗ treatment resulted in an otherwise clean, oxidized surface including all film constituents. Atomic H∗ resulted in strong enhancement of the surface Ga population, probably due to a preexisting Ga native oxide in the outermost atomic layer. Sputtering produced a clean surface that was closest to the bulk composition of the film as measured by energy-dispersive spectroscopy.