Large-area compositional mapping of Cu(In_1−xGa_x)Se₂ materials and devices with spectroscopic ellipsometry

Abstract

A spectroscopic ellipsometry (SE) capability having the potential to scan production-scale areas at high speed has been developed and successfully applied to map the alloy composition of copper-indium-gallium-diselenide (CuIn 1 −x Ga x Se 2 : CIGS) thin films. This technique not only generates a compositional map but simultaneously provides maps of the more typical SE-determined properties as well, including bulk layer and surface roughness layer thicknesses. As a result, the methodology is suitable for characterization in online production-scale applications. In order to develop the mapping capability, CIGS films having different molar Ga contents x and fixed copper stoichiometry were deposited and measured in situ by SE in order to extract the complex dielectric functions (ε = ε 1 +iε 2 ) of these films. For mathematical interpolation between the available alloy contents, the (ε 1 , ε 2 ) spectra were parameterized using an oscillator sum. Best-fitting equations were obtained that relate each oscillator parameter to the Ga content x, as determined by energy dispersive X-ray analysis. This approach reduces the number of fitting parameters for (ε 1 , ε 2 ) from several to just one: the Ga content x. Because (ε 1 , ε 2 ) is now represented by this single parameter, the chances of parameter correlations during fitting are reduced, enabling production-scale compositional mapping of chalcopyrite films by SE.Index Terms-Ellipsometry, gallium-based semiconductor materials, optical variables measurement, photovoltaic (PV) cells, semiconductor film, thickness measurement.