Paper EN+TF+SS-WeA11
Thin Film Epitaxial Growth of CuInSe₂ Bicrystals for Grain-Boundary Studies

Wednesday, October 17, 2007, 5:00 pm, Room 602/603

Photovoltaics based on CuInSe₂ and related materials have the highest performance of any thin film devices. The nature and role of grain-boundaries in CuInSe₂ devices is currently poorly understood. Previous studies on the influence of grain-boundaries in Cu(In,Ga)Se₂ devices have shown that they exhibit little to no detrimental effect on photovoltaic device performance. Polycrystalline CuInSe₂ devices currently outperform single crystal CuInSe₂ devices. Past studies have been focused on average electrical properties or local surface electrical measurements for observation of the influence of boundaries on film properties. Since polycrystal devices have crystal sizes on the order of 1 micron, optoelectrical measurements are difficult. In order to alleviate these difficulties, a single high-angle grain-boundary in epitaxial CuInSe₂ has been grown on a GaAs bicrystal substrate using a hybrid sputtering and evaporation technique. Electron backscatter diffraction and x-ray diffraction show that film growth is epitaxial across all boundaries (high-angle and twin) in the bicrystal substrates. Atomic force microscopy, profilometry, and focused ion beam cross-sectioning results indicate that surface polarity and termination have a strong influence on film thickness and morphology. Near grain boundary morphology suggests mass transport leading to boundary migration is occurring. It is proposed that high surface energy anisotropy (on the opposing {111}_A,B surface facets) motivates mass transport during the growth of CuInSe₂ at crystal boundaries.