AVS 59th Annual International Symposium and Exhibition
    Spectroscopic Ellipsometry Focus Topic Monday Sessions
       Session EL+TF+AS+EM+SS+PS+EN+NM-MoM

Invited Paper EL+TF+AS+EM+SS+PS+EN+NM-MoM1
Multichannel Spectroscopic Ellipsometry: Applications in I-III-VI2 Thin Film Photovoltaics

Monday, October 29, 2012, 8:20 am, Room 19

Session: Spectroscopic Ellipsometry for Photovoltaics and Semiconductor Manufacturing
Presenter: R.W. Collins, University of Toledo
Authors: R.W. Collins, University of Toledo
D. Attygalle, University of Toledo
P. Aryal, University of Toledo
P. Pradhan, University of Toledo
N.J. Podraza, University of Toledo
V. Ranjan, Old Dominion University
S. Marsillac, Old Dominion University
Correspondent: Click to Email
Multichannel spectroscopic ellipsometry (SE) has been applied successfully as an in situ, real time tool for optimizing, monitoring, and controlling multi-stage deposition processes in various thin film photovoltaics (PV) technologies. A particularly challenging process optimization problem involves the thermal co-evaporation of individual elements of Cu, In, Ga, and Se in a three-stage process, which has proven to produce high quality Cu(In1-xGax)Se2 (CIGS) materials and high performance PV devices. This three-stage process provides a high level of flexibility in determining the phase, composition, and microstructure of the film, but also generates greater challenges in run-to-run reproducibility of the optimized process. Information extracted from real time SE measurements includes the evolution of the bulk layer and one or more surface layer thicknesses, as well as layer dielectric functions. The layer dielectric functions can be analyzed further to extract the phase and alloy compositions and the defect density or grain size, which can assist in understanding the fabrication process, in optimizing solar cells, and ultimately in monitoring and controlling the optimized process for improved reproducibility. In this study, the focus is on analysis of ellipsometric (ψ, Δ) spectra acquired by real time SE in order to characterize (i) the structural and compositional evolution in (In,Ga)2Se3 film growth from In, Ga, and Se fluxes in the first stage, (ii) the transition from Cu-poor to Cu-rich CIGS at the end of the second stage, which occurs under Cu and Se fluxes, and (iii) the transition from Cu-rich to the desired Cu-poor CIGS, which defines the end of the third and final stage, and occurs under a second application of In, Ga, and Se fluxes. After the transition from Cu-poor to Cu-rich material in the second stage, a Cu2-xSe phase near the surface of the bulk layer is tracked. In the Cu-rich to Cu-poor transition, this Cu2-xSe phase has fully reacted with In, Ga, and Se to form CIGS. Studies using a standard Mo substrate and 2 μm thick CIGS for solar cells have also revealed features in the (ψ, Δ) spectra characteristic of the anticipated changes in the near surface phase composition as established by detailed modeling on thinner and smoother films. Although careful analysis of real time SE is expected to provide quantitative information on the surface properties and their evolution in this case of solar cells, control of the deposition has been successful simply by monitoring real time changes in the ellipsometric (ψ,Δ) spectra.