Defect Structure of Heteroepitaxial Zn2-2xCuxInxS2 Layers Grown by Pulsed Laser Deposition on (111) Si, (001) Si and (001) GaP Substrates

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Zn2-2xCuxInxS2 (ZCIS) films might be attractive for absorbers in solar cells (band gap variation from Eg(ZnS) = 3.67 eV to Eg(CuInS2) = 1.53 eV at room temperature). Therefore, we have grown ZCIS on (001)GaP, (001)Si and (111)Si substrates at 500°C by pulsed laser deposition (PLD). Epitaxial growth on (001) GaP substrates could be proved for all compositions between ZnS and CuInS2. However, there is a striking asymmetry in twin nucleation that occurs for cubic ZCIS (CuInS2 content less than 22 mol-%) preferably on the (1̄1̄1) and (111) planes of the [1̄10] zone. In addition, beside the dislocations lying in-clined to the interface, perfect 60° misfit dislocations coexist that align parallel to [110] and [1̄10] , having b=1/2<011> and {111} slip planes inclined to the interface. Additional Cu- and/or In-sulfides could not be detected in the monocrystalline films. CuAu-I type ordering was observed for CuInS2-rich compositions as Zn0.22Cu0.46In0.32S, Zn0.044Cu0.544In0.412S and pure CuInS2 only. On (001) Si the films appear to be polycrystalline for all compositions between Zn0.2Cu0.4In0.4S and Zn0.62Cu0.19In0.19S. At Zn0.7Cu0.15In0.15S there is the transition between poly- and monocrystalline growth, i.e. the range of monocrystalline growth is strongly restricted to the lattice matched position. In these layers twins have been nucleated on all four inclined {111} planes. On chemically treated (111) Si the films grew epitaxially for compositions 0.1 ≤ x ≤ 0.7. They are monocrystalline, but consist of polysynthetically twinned column-like grains which are precisely oriented to the (111) Si sub-strate. Coherent Σ=3{111} twin boundaries exist parallel to the interface and incoherent Σ=3{112} grain boundaries are for-med perpendicular to it. Perfect dislocations that lie along <110> directions and parallel to the interface envelop the colums to form a dislocation network like a honeycomb. The films become polycrystalline for compositions x > 0.7. On non-treated (111) Si substrates, similar to the deposition on molybdenum-coated glass, the ZCIS films are polycrystalline, but a distinct <111> texture exists. Extended defects have been produced inside both the GaP and chemically treated Si substrates due to the preferred Cu out-diffusion from the layer. In the GaP substrates a Cu3P phase is formed but within the Si substrates amor-phisized regions occur exclusively. With increasing ZnS content the propensity to form these defects decreases appreciably.