| AVS 58th Annual International Symposium and Exhibition | |
| Spectroscopic Ellipsometry Focus Topic | Thursday Sessions |
| Session EL+AS+EM+MS+PS+TF-ThA |
| Session: | Spectroscopic Ellipsometry for Photovoltaics, Metals and Oxide Thin Films |
| Presenter: | Dinesh Attygalle, University of Toledo |
| Authors: | D. Attygalle, University of Toledo V. Ranjan, Old Dominion University P. Aryal, University of Toledo S. Marsillac, Old Dominion University R.W. Collins, University of Toledo |
| Correspondent: | Click to Email |
With record efficiencies above 20%, Cu(In,Ga)Se2 (CIGS) based solar cells have shown the greatest potential for success among the thin film photovoltaics technologies. Thermal co-evaporation of individual elements has proven to produce extremely high quality CIGS materials, provides a high level of flexibility, but also generates greater challenges in process optimization. The limitations of existing process monitoring capabilities, hence the challenge of correcting process fluctuations in real time, has led the industrial community toward more controllable CIGS deposition processes. Real time spectroscopic ellipsometry (RTSE) can be used successfully in the monitoring of complicated processes -- including CIGS film preparation by co-evaporation using precursor films of (Inx,Ga1-x)2Se3. Information extracted from RTSE includes the evolution of bulk layer and surface roughness layer thicknesses, the composition and phase, as well as the layer dielectric functions, all of which can assist in understanding the fabrication process and in optimizing solar cells. In this study, the focus is on the transitions of Cu-poor to Cu-rich CIGS and vice versa by observing the changes in (ψ, Δ) spectra obtained by RTSE. The commonly used monitoring method, which involves observing the changes in emissivity of the film, largely depends on the apparatus design, the substrate, and the bulk layer thickness. When a CIGS film is prepared by exposing a precursor film of (Inx,Ga1-x)2Se3 to Cu and Se fluxes, thereby becoming Cu-rich, a semi-liquid Cu2-xSe phase is believed to form on top of a bulk layer consisting of mixed phases of Cu(In,Ga)Se2 and Cu2-xSe [1]. A multilayer optical model, with appropriate effective medium approximation layers to represent this scenario, has shown good agreement with the observed (ψ, Δ) spectra. Since RTSE is highly sensitive to monolayer-level changes in the top-most layer, RTSE gives superior sensitivity in Cu-rich to Cu-poor end point detection, which occurs when the top Cu2-xSe phase drops below detectable limits. Furthermore this method is less affected by the substrate and bulk layer thickness. Although careful analysis of RTSE can give a wealth of information about CIGS material properties and their evolution, this type of end point detection can be successful simply by monitoring the real time changes in the (ψ, Δ) spectra.
[1] J. AbuShama, R. Noufi, Y. Yan, K. Jones, B. Keyes, P. Dippo, M. Romero, M. Al-Jassim, J. Alleman, and D.L. Williamson, "Cu(In,Ga)Se2 Thin-film evolution during growth from (In,Ga)2Se3 precursors", Mat. Res. Soc. Symp. Proc. paper H7.2.1, (2001).