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Optical properties of Cu2ZnSnSe4 thin films by
spectroscopic ellipsometry and photoluminescence
Özden Demircioğlu1, Jose Fabio Lopez Salas1, Germain Rey2, Thomas Weiss2,
Susanne Siebentritt2, Jürgen Parisi1, Levent Gütay1 1Laboratory for Chalcogenide Photovoltaics, Institute of Physics, University of Oldenburg, Oldenburg, Germany
2Laboratory for Photovoltaics, Physics and Materials Science Research Unit, University of Luxembourg, Luxembourg
EMRS – Spring Meeting 2016
Lille, France, 2 to 6 May 2016
Spectroscopic Ellipsometry (SE)
Data analysis and Model
Results
ZnSe and MoSe2 with different amounts at back side of all samples
ZnSe fraction on front side only in sample C
Confirmation of MoSe2 layer thickness by SEM (not shown)
Presence of ZnSe determined, quantities not confirmed
Extracted band gaps: SE measurement : Surface & bulk sensitive
Sample A: 0.95 ± 0.01 eV
Sample B: 0.92 ± 0.01 eV
Sample C: 0.93 ± 0.01 eV
R&T measurement : Bulk sensitive
Sample A: 0.97 ± 0.01 eV
Sample B: 0.94 ± 0.01 eV
Sample C: 0.95 ± 0.01 eV
Relative differences of extracted band gaps identical for SE and R&T
Differences in absolute band gaps from SE and R&T due to different
sensitivities for bulk and surface
Extraction of dielectric functions ε1 and ε2 by optical modeling of sample
Excitation wavelength at 532 nm and 457.9 nm
ZnSe at back side of all samples
Different amounts of MoSe2 on remaining substrates after absorber lift-off
ZnSe at the front side only in sample C confirmation of SE results
Conclusion
Application of SE as a non-destructive technique for detection of secondary
phases in layer and/or at the back sides
- Determination of presence and amount of MoSe2 at back side by SE
- Determination of presence of ZnSe for front and back side by SE
( confirmed by Raman spectroscopy and photoluminescence)
Reliable band gap determination by SE, band gap variations between
samples are identical for both methods
Selective probing of ZnSe defect peak PL by choice of excitation wavelength
Acknowledgements The work at the LCP research group (Oldenburg) is funded by EWE AG, Oldenburg, Germany, and the
BMBF (German ministry of Education and Science), funding Nr. 03SF0530A (project “Free-Inca”). The
work at the LPV group (Luxembourg) was supported by the FNR (National Research Fund) (Luxembourg).
Contact: [email protected]
Raman spectroscopy
Photoluminescence (PL)
(ħω) ( Δ, Ψ ) surface roughness
Absorber
(CZTSe)
Intermix layer
Substrate (Mo)
MoSe2
Intermix layer
ZnSe
Intermix layer
EMA- (ZnSe:CZTSe)
Energy fit
range (eV)
0.75-3.0
Roughness
(nm)
EMA
(nm)
CZTSe
(nm)
Interface layer (nm)
MSE
(ZnSe:CZTSe)
mix. ZnSe mix. MoSe2 mix.
Sample A 14.0 0.0
(0.0:100)
1221 0.5 2.0 0.0 4.0 4.0 2
Sample B 0.5 0.0
(0.5:99.5)
1079 2.5 8.0 2.0 3.0 0.0 3
Sample C 12.0 7.0
(40:60)
1323 0.0 5.0 0.0 200 15.0 26
PL from CZTSe absorber at 0.83-0.86 eV
ZnSe defect transition at 1.2 eV at back side of each sample
Intensity dependence of ZnSe defect peak on excitation wavelength
No observation of ZnSe defect transition at front sides of samples
SE and Raman are more sensitive to detect ZnSe (compare sample C)
References [1] R. Djemour, M. Mousel, A. Redinger, L. Gütay, A. Crossay, D. Colombara, P. J. Dale, S.
Siebentritt, Appl. Phys. Lett. 102,222108 (2013)
[2] R.Kondrotas, R. Juskenas, A. Naujokaitis, G. Niaura, Z. Mockus, S. Kanapeckaite, B. Cechavicus,
K. Juskevicius, E. Saucedo, Y. Sánchez, Thin Solid Films 589 (2015) 165-172
α=𝐴(ℎ𝑣−𝐸𝑔)
ℎ𝑣
1/2
α:absorption coefficient
A:constant
hν:photon energy
α =−1
𝑡ln(
𝑇
(1−𝑅)2)
t:thickness
T:transmission
R:reflectance
*
*
*
*
* Surface and intermix layers modelled by
Effective Medium Approximation (EMA),
contain 50:50 mixture of adjacent layers.
Variable mixture of ZnSe:CZTSe layer.
Overview
Investigation of Cu2ZnSnSe4 (CZTSe) thin films by spectroscopic ellipsometry
(SE), Raman, photoluminescence (PL) and optical reflection & transmission
(R&T)
SE analysis for non-destructive detection of MoSe2 layer at the back and
ZnSe secondary phase at the front and back interface region of absorber
Complex optical modeling including multiple layers for evaluation of SE data
Confirmation of SE results by PL and resonant Raman spectroscopy,
Comparison of band gaps from SE and R&T
Samples:
-Samples A and B from a high temperature co-evaporation process
-Sample C from co-evaporation followed by an annealing step
-Back side of absorber accessed by lift-off from substrate
Black markers: CZTSe Red markers: ZnSe Orange markers : MoSe2