Tailoring of Reactive Magnetron Sputtering for the Deposition of Active Semiconducting Films

    Magnetron sputtering, invented by the renowned plasma physicist Penning already in 1939, is since more than 30 years a widely used large-area, plasma-assisted deposition method for many industrial applications, mostly used in fields like architectural and low emissivity glass coatings, mirrors and absorbers for solar concentrators, magnetic films for hard disks or hard coatings for tools, i.e., mainly for metals, oxides and nitrides. In the rising thin film photovoltaics industry, magnetron sputtering is only used for the deposition of metallic back contacts (Ag, Mo) and transparent, conductive window layers (ITO, ZnO) or for metallic precursor films. However, it is not yet applied on a technical scale for the absorber layers in thin film solar cells, i.e., for the active semiconductors.
    In this talk, obstacles are outlined, which delayed the use of magnetron sputtering for active semiconducting layers. The energies of species (sputtered atoms, positive and negative ions, energetic neutrals) are discussed and its influence on the film growth, especially of reflected neutral argon atoms (Ar0) and negative ions (O-, S-, Se-). Due to the low defect formation energies of semiconductors, tailoring of the discharge conditions (low particle energies) is mandatory for the preparation of semiconducting films of high electronic quality. The possibilities of reactive magnetron sputtering (RMS) are demonstrated for the deposition of active chalcopyrite absorber films for efficient thin film solar cells. The effects of the plasma excitation energy on the electronic properties of the active layers in thin film solar cells and photoelectrodes are demonstrated for the materials Cu(In,Ga)Se2 and BiVO4.