1. Fitting with MoSe2 layer, thickness: ~220 nm
Experimental Data and Fit
Photon Energy (eV)
0.6 1.4 2.2 2.9 3.7 4.5
indegrees
0
5
10
15
20
25
Model Fit
Exp E 65°
Exp E 70°
Exp E 75°
Sample Modeling and Results
Spectroscopic Ellipsometry on Cu2ZnSnSe4 Thin Films
Ö. Demircioglu 1; L. Gütay 1; I. Riedel 1; M. Mousel 2; A. Redinger 2; S. Siebentritt 2
(1) Laboratory for Chalcogenide Photovoltaics, University of Oldenburg, Oldenburg, Germany;
(2) Laboratory for Photovoltaics, University of Luxembourg, Belvaux, Luxembourg
5th European Kesterite Workshop
Abstract
We have analyzed various Cu2ZnSnSe4 (CZTSe) thin films by spectroscopic ellipsometry (SE) for determination of their
dielectric functions. The studied samples were single-crystalline epitaxial layers grown by molecular-beam epitaxy and
polycrystalline thin films from physical vapor deposition. The surface of the samples was etched to reduce roughness effects
and to remove secondary phases in the surface region. The rear surface of the CZTSe films was revealed by a lift-off from the
substrate. By this approach the backside of the absorber could be analyzed and the present MoSe2 and its effect on the SE
measurements could be investigated. By this approach a refined modeling of the measured SE data could be achieved which
allows to extract a more reliable dielectric function from the SE experiments which exclusively relates to the CZTSe phase.
Further, it is demonstrated that SE can be applied for non-destructive detection of the secondary phase MoSe2 at the back
contact and its amount/thickness. The behavior of the extracted dielectric functions strongly indicate the presence of a second
band gap in CZTSe.
Tallinn, Estonia, 13-14 November 2014
Substrate ( Mo)
MoSe2
( Δ, Ψ )
(ħω)
Substrate (GaAs or Mo)
Intermix/Interface
CZTSe
Surface-layer
(ħω)
( Δ, Ψ )
Comparison of Results for different samples
Conclusions
Raman results:
• Absorber topview: CZTSe
• Substrate after lift-off: only MoSe2
• Absorber backside:Minor fraction of
further secondary phases
• More reliable result for dielectric function of CZTSe by implementation of MoSe2 layer in
modeled sample structure
• Detection of the existence and amount (thickness) of secondary phase MoSe2 layer at the
back contact by non-destructive spectral Ellipsometry method
• Variation of dielectric function for different samples might be caused by variations of the
composition and the presence of minor fractions of further secondary phases, disorder etc.
• Capability of SE for identification of further secondary phases to be further investigated.
CZTSe
Surface-layer
(ħω) ( Δ, Ψ )
Glue and lift-off
Ellipsometry and Raman
measurement
Ellipsometry and
Raman measurement
MoSe2
Substrate (GaAs or Mo)
insert ε of MoSe2
Ellipsometry results:
• Better fit quality with MoSe2-layer
• Extraction of amount of MoSe2
• More reliable dielectric function
of CZTSe with MoSe2-layer Fitting of CZTSe model without MoSe2 layer
First band gap energy at (0.95 ± 0.1) eV
Critical points for
− shoulder at (1.3 ± 0.1)eV
− maximum at (2.3 ± 0.2)eV
− minimum at (3.2 ± 0.2) eV
second band gap in CZTSe
Epsilon of MoSe2 layer
Dielectric Functions
References:
1. S. G. Choi. et al, JOURNAL OF
APPLIED PHYSICS 111, 033506 (2012)
2. Clas Persson, JOURNAL OF APPLIED
PHYSICS 107, 053710 (2010)
Extraction of dielectric functions
ε1 and ε2 by a fitting procedure
within an optical modelling
of the sample system.
Acknowledgements
The LCP research group (Oldenburg) is funded by the EWE
AG, Oldenburg, Germany.
The work at the LPV group (Luxembourg) was supported by
the National Research Fund (Luxembourg), and the Marie
Curie project “Kestcell”.
Experimental Data and Fit
Photon Energy (eV)
0.7 1.5 2.2 3.0 3.7 4.5
indegrees
0
5
10
15
20
25
Model Fit
Exp E 65°
Exp E 70°
Exp E 75°
(from [2])
Contact: oezden.demircioglu@uni-oldenburg.de
Ellipsometry
Change in polarization of light after reflection
on sample surface is measured.
Measured values:
• amplitude ratio Ψ
• phase difference Δ
Data Analysis