1. Vibrational Properties and Optical Functions of
Epitaxial and Polycrystalline CZTSe
Levent Gütay1, Özden Demircioğlu1, Christiane Stroth1, Ingo Riedel1,
Rabie Djemour2, Alex Redinger2, Susanne Siebentritt2
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 2014
Abstract
Single crystalline CZTSe layers are grown epitaxially on GaAs(001) by molecular beam epitaxy (MBE). Growth is possible in
two orientations and in different crystal structures/polymorphs, which are not discernible by XRD. Polarization dependent
Raman spectroscopy is used to determine the orientation and the polymorph present. The analysis reveals further details
about the vibrational properties of kesterites. The dielectric function of the epitaxial kesterite is shown and compared to an
epitaxial sample containing disorder effects, and conventional polycrystalline samples.
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”.
Lille, France, 26 to 30 May 2014
Conclusions
• Confirmation of kesterite structure by polarization dependent Raman
• Assignment of B and E modes possible (some tentative)
• Mix of out-of-plane and in-plane crystal orientation (confirmed by TEM)
• Variations of dielectric function to be further investigated
Polarization orientation Raman
Raman intensity of an oriented crystal:
𝐼 𝜃 ‖/⟘
~|𝑒 𝑥 · 𝑄 𝑇 · 𝑅 · 𝑄 · 𝑒 𝑥 ̸ 𝑦|2
𝑅: Raman tensor, 𝑄(𝜃): rotation matrix,
𝑒 𝑥 ̸ 𝑦: incident and scattered light
Raman tensors:
“Out-of-plane” oriented kesterite
“In-plane” oriented kesterite
Polarized Raman: parallel and perpendicular
Angle of measurement θ = 75°
All peaks fitted by Lorentz; main peak at 195 cm-1 is asymmetric
Dielectric functions from ellipsometry
First band gap energy at (1.0 ± 0.2) 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
No clear trend for low energy region (< 2 eV).
Possible origins might be: secondary phases, interface to
substrate, crystal disorder, inhomogeneity.
[Bilbao crystallographic server]
Available information from Raman spectra
• Peak energies / peak widths
• Count of Raman active modes
• Mode symmetries (access via polarized Raman)
kesterite (No 84: 𝐼4):
stannite (No 121: 𝐼42𝑚):
 Raman allows to discern kesterite from other polymorphs
ki ks
c // z
a // y
a // x
͞z ( x , y ) z
θ
Polarized Raman results
A mode
(195 cm-1)
B mode
(80 cm-1)
E mode
(233 cm-1)
Interpretations
tensor entries: a ≈ b
3 A modes found at:
168, 172, 195 cm-1
( kesterite)
tensor entries: c/d ≈ 0.2
(mix of cos and sin)
mainly “out-of-plane”
B modes found at:
80, 211, 220 cm-1
*(106, 117, 204 cm-1)
contains “in-plane”
(“out-of-plane”  E=0)
E modes found at:
139, 155, 187, 233 cm-1
*(93, 127, 180, 241, 249 cm-1)
*(tentative assignment)
⃦: parallel ⟘:perpendicular
confirms second
band gap in CZTSe
⃦:
⟘:
⃦:
⟘:
—measured
⋯ fitted
Substrate (GaAs or Mo)
Intermix/Interface
CZTSe
Surface-layer
( Δ, Ψ )
(ħω)
Contact: levent.guetay@uni-oldenburg.de