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Materials 2018, 11(1), 145; https://doi.org/10.3390/ma11010145

Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1−xGaxSe2 Growth: Indium-Gallium Selenide Co-Evaporation

1
Center for Photovoltaics Innovation and Commercialization & Department of Physics and Astronomy, University of Toledo, Toledo, OH 43606, USA
2
Virginia Institute of Photovoltaics, Old Dominion University, Norfolk, VA 23529, USA
*
Author to whom correspondence should be addressed.
Received: 10 December 2017 / Revised: 10 January 2018 / Accepted: 10 January 2018 / Published: 16 January 2018
(This article belongs to the Section Thin Films)
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Abstract

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1−xGax)2Se3 (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function ε = ε1 − iε2, spectra. Here, RTSE has been used to obtain the (ε1, ε2) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (ε1, ε2) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (ε1, ε2) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (ε1, ε2) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control. View Full-Text
Keywords: spectroscopic ellipsometry; III2-VI3 semiconductor materials; real time analysis; complex dielectric function; photovoltaic cells; thickness measurement; compositional analysis; CuIn1−xGaxSe2 spectroscopic ellipsometry; III2-VI3 semiconductor materials; real time analysis; complex dielectric function; photovoltaic cells; thickness measurement; compositional analysis; CuIn1−xGaxSe2
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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Pradhan, P.; Aryal, P.; Attygalle, D.; Ibdah, A.-R.; Koirala, P.; Li, J.; Bhandari, K.P.; Liyanage, G.K.; Ellingson, R.J.; Heben, M.J.; Marsillac, S.; Collins, R.W.; Podraza, N.J. Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1−xGaxSe2 Growth: Indium-Gallium Selenide Co-Evaporation. Materials 2018, 11, 145.

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