Chemical instability at chalcogenide surfaces impacts chalcopyrite devices well beyond the surface

摘要

Portion of the In Se -Cu Se pseudobinary diagram showing the stable (solid lines) and metastable (dotted lines) phase equilibria (reproduced with permission from Gödecke et al. © Carl Hanser Verlag GmbH & Co.KG, München). The thin-film growth regions commonly referred to as Cu-poor and Cu-rich appear at the top as red and cyan fading portions, respectively. Magnification of the phase diagram in the region of interest for CIS processing (and extension to room temperature ) with hollow circles indicating the compositions investigated: CIS-P (red, Cu-poor), CIS-S (black, nearly stoichiometric) and CIS-R (cyan, Cu-rich). Insets: corresponding cross-sectional scanning electron microscopy images (CIS-R subject to KCN etching to remove the excess Cu Se phase). Schematic illustration of Cu-poor (top) and Cu-rich (bottom) films after growth (left) and cool down (right). In the Cu-poor case, In antisites are dispersed within the CIS lattice and part of them accumulate at grain boundaries during cool down, leading to Cu-poor grain boundaries (GB). Typical compositions of grain interiors at room-temperature match with the metastable Cu-poor region of existence, red patterned region in . Similarly, Cu antisites are dispersed within the CIS lattice of Cu-rich material, but Cu interstitials may also be present, possibly accounting for the enhanced grain growth during processing and for the existence of a supersaturated metastable phase at room temperature, cyan patterned region in . Photoluminescence spectra of CIS-P and CIS-R films acquired at 10 K. Whereas CIS-R shows excitonic emissions (labelled EX, indicative of high crystal quality) and up to three donor–acceptor pair transitions (labelled DA), CIS-P shows no excitonic emissions but broader, more red-shifted and more asymmetric defect-related transitions, typical of a high degree of compensation .