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Optical Properties of CuIn 1-x Ga x Se 2 Quaternary Alloys
S. Theodoropoulou 1 , D. Papadimitriou 1 *, K. Anestou 1 , Ch. Cobet 2 , and N. Esser 2
1
National Technical University of Athens, Department of Physics, GR-15780 Athens, Greece.
2 Institute for Analytical Sciences (ISAS), Albert-Einstein Strasse 9, D-12489 Berlin, Germany.
*dimitra@central.ntua.gr
The optical properties of CuIn 1-x Ga x Se 2 epitaxial layers suitable for thin film photovoltaic applications were determined by
Spectroscopic Ellipsometry. Layers with Ga-content varying between x=0.08 and 0.82 were measured in the energy range
2.0-4.5 eV (Fig. 1). For CuIn 1-x Ga x Se 2 with x=0.08, variable angle ellipsometric measurements were performed for angles of
incidence between 55 o and 75 o (Fig. 2). Accurate values of refractive index n and extinction coefficient k were determined for
six different Ga-contents. Moreover, the higher band-gap energies of quaternary selenides were obtained. In particular, four
transitions assigned to critical points at the Brillouin-zone center (Γ) and edge N point were observed. These are: E(ΓΧ),
Ε(ΧΓ), E 1 (A), and E 1 (B). Among them, the E 1 (A) dominated all spectra. Transition energies were obtained from the second
derivative of the ε 2 spectra (Fig. 3) and are discussed together with the E o (A), Ε o (B), and E o (C) fundamental gap energies
measured by Photoreflectance (PR) Spectroscopy. Ellipsometric and PR data complement one another.
Keywords: CuIn 1-x Ga x Se 2 , optical properties, ellipsometry, photoreflectance.
ε 2
9
8
7
6
5
4
x=0.08
x=0.19
x=0.33
x=0.50
x=0.60
x=0.82
E(ΓX)
E 1
(A)
E(XΓ)
E 1
(B)
transition energy (eV)
5,0
4,5
4,0
3,5
3,0
2,5
E (XΓ)
Ε 1
(Α)
E (ΓΧ)
Ε 1
(Β)
3
1,5 2,0 2,5 3,0 3,5 4,0 4,5
energy (eV)
Figure 1. Complex dielectric function ε1+iε2 of
CuIn 1-x Ga x Se 2 epitaxial layers at 65 o angle of incidence.
2,0
1,5
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9
energy (eV)
Figure 3. Transition energies of CuIn 1-x Ga x Se 2 alloys
determined from the fits of d 2 /dE 2 .
8
55 o
60 o
65 o
70 o
7,5
7,0
6,5
6
6,0
1
ε
5,5
ε 2
4
5,0
4,5
75 o energy (eV)
2
1,5 2,0 2,5 3,0 3,5 4,0 4,5
4,0
Figure 2. Variable angle measurements of the complex
dielectric function ε1+iε2 of CuIn 1-x Ga x Se 2 epitaxial
layers with x=0.08.
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