Optical studies of as-deposited and annealed Cu₇GeS₅I thin films
Cu₇GeS₅I thin films were obtained by non-reactive radio frequency magnetron sputtering onto silicate glass substrates. Optical transmission spectra of as-deposited and annealed Cu₇GeS₅I thin films were measured in the temperature interval 77–300 K. The temperature behaviour of Urbach absorption edge...
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irk-123456789-1215632017-06-15T03:04:45Z Optical studies of as-deposited and annealed Cu₇GeS₅I thin films Studenyak, I.P. Bendak, A.V. Rybak, S.O. Izai, V.Yu. Kúš, P. Mikula, M. Cu₇GeS₅I thin films were obtained by non-reactive radio frequency magnetron sputtering onto silicate glass substrates. Optical transmission spectra of as-deposited and annealed Cu₇GeS₅I thin films were measured in the temperature interval 77–300 K. The temperature behaviour of Urbach absorption edge and dispersion of refractive index for as-deposited and annealed Cu₇GeS₅I thin films was analyzed. Influence of annealing on the optical parameters and disordering processes in Cu₇GeS₅I thin films was studied. 2016 Article Optical studies of as-deposited and annealed Cu₇GeS₅I thin films / I.P. Studenyak, A.V. Bendak, S.O. Rybak, V.Yu. Izai, P. Kúš, M. Mikula // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2016. — Т. 19, № 2. — С. 192-196. — Бібліогр.: 15 назв. — англ. 1560-8034 DOI: 10.15407/spqeo19.02.192 PACS 78.40.Ha, 77.80.Bh http://dspace.nbuv.gov.ua/handle/123456789/121563 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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Cu₇GeS₅I thin films were obtained by non-reactive radio frequency magnetron sputtering onto silicate glass substrates. Optical transmission spectra of as-deposited and annealed Cu₇GeS₅I thin films were measured in the temperature interval 77–300 K. The temperature behaviour of Urbach absorption edge and dispersion of refractive index for as-deposited and annealed Cu₇GeS₅I thin films was analyzed. Influence of annealing on the optical parameters and disordering processes in Cu₇GeS₅I thin films was studied. |
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Studenyak, I.P. Bendak, A.V. Rybak, S.O. Izai, V.Yu. Kúš, P. Mikula, M. |
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Studenyak, I.P. Bendak, A.V. Rybak, S.O. Izai, V.Yu. Kúš, P. Mikula, M. Optical studies of as-deposited and annealed Cu₇GeS₅I thin films Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Studenyak, I.P. Bendak, A.V. Rybak, S.O. Izai, V.Yu. Kúš, P. Mikula, M. |
| author_sort |
Studenyak, I.P. |
| title |
Optical studies of as-deposited and annealed Cu₇GeS₅I thin films |
| title_short |
Optical studies of as-deposited and annealed Cu₇GeS₅I thin films |
| title_full |
Optical studies of as-deposited and annealed Cu₇GeS₅I thin films |
| title_fullStr |
Optical studies of as-deposited and annealed Cu₇GeS₅I thin films |
| title_full_unstemmed |
Optical studies of as-deposited and annealed Cu₇GeS₅I thin films |
| title_sort |
optical studies of as-deposited and annealed cu₇ges₅i thin films |
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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2016 |
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Optical studies of as-deposited and annealed Cu₇GeS₅I thin films / I.P. Studenyak, A.V. Bendak, S.O. Rybak, V.Yu. Izai, P. Kúš, M. Mikula // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2016. — Т. 19, № 2. — С. 192-196. — Бібліогр.: 15 назв. — англ. |
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Semiconductor Physics Quantum Electronics & Optoelectronics |
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2025-07-08T20:07:44Z |
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| fulltext |
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 2. P. 192-196.
doi: 10.15407/spqeo19.02.192
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
192
PACS 78.40.Ha, 77.80.Bh
Optical studies of as-deposited and annealed Cu7GeS5I thin films
I.P. Studenyak1, A.V. Bendak1, S.O. Rybak1, V.Yu. Izai1, P. Kúš2, M. Mikula2
1Faculty of Physics, Uzhhorod National University,
3, Narodna Sq., 88000 Uzhhorod, Ukraine
2Faculty of Mathematics, Physics and Informatics, Comenius University,
Mlynska dolina, 84248 Bratislava, Slovakia,
E-mail: studenyak@dr.com
Abstract. Cu7GeS5I thin films were obtained by non-reactive radio frequency magnetron
sputtering onto silicate glass substrates. Optical transmission spectra of as-deposited and
annealed Cu7GeS5I thin films were measured in the temperature interval 77–300 K. The
temperature behaviour of Urbach absorption edge and dispersion of refractive index for
as-deposited and annealed Cu7GeS5I thin films was analyzed. Influence of annealing on
the optical parameters and disordering processes in Cu7GeS5I thin films was studied.
Keywords: thin film, magnetron sputtering, annealing, optical absorption, refractive
index, Urbach rule.
Manuscript received 14.12.15; revised version received 06.04.16; accepted for
publication 08.06.16; published online 06.07.16.
1. Introduction
Cu7GeS5I crystals belong to the argyrodite family of
tetrahedrally close-packed structures and are known
as superionic conductors [1]. Some electrochemical
properties of Cu7GeS5I crystals are reported in Ref.
[2]. They are characterized by high electrical
conductivity and low activation energy [3]. Optical
studies have shown that the absorption edge of
Cu7GeS5I crystals exhibits Urbach behaviour in a
wide temperature range [3].
The compositional dependence of the lattice
parameter of the alloys and single crystals of Cu7GeS5I–
Cu7GeSe5I system was shown to be linear, described by
the Vegard law, which is the evidence for formation of a
continuous row of substitutive solid solutions [4]. At
room temperature Cu7GeS(Se)5I-based solid solutions
crystallize in the cubic symmetry (space group mF 34 ).
The short-wavelength edge of the diffuse reflection
spectra of Cu7Ge(S1–xSex)5I solid solutions is shown to
shift towards longer wavelengths with the substitution of
S atoms by Se [4]. The compositional studies of
electrical conductivity in Cu7Ge(S1–xSex)5I solid
solutions revealed that S→Se anionic substitution results
in a nonlinear increase of the electrical conductivity by
more than an order of magnitude [5]. It should be noted
that the total electrical conductivity of Cu7GeSe5I
crystals at room temperature was found to be rather high
and typical for the advanced superionic conductors [6].
Due to the high ionic conductivity, they are the attractive
materials for applications in the different functional
elements of the solid state ionics.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 2. P. 192-196.
doi: 10.15407/spqeo19.02.192
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
193
The investigations of the thin films based on
Cu7GeS5I superionic conductors only begin. Thus, in this
paper the optical properties of Cu7GeS5I thin films are
studied. Besides, the comparative analysis of optical
parameters in single crystals and thin films as well as the
comparative analysis of optical parameters in as-
deposited and annealed thin films are performed.
2. Experimental
Cu7GeS5I compounds were synthesized from extra pure
Cu, Ge, S and CuI compounds, additionally purified by
distillation in vacuum. Thin films of Cu7GeS5I
compounds were deposited onto silicate glass substrates
by non-reactive radio frequency magnetron sputtering,
the film growth rate was 3 nm/min. The deposition was
carried out at room temperature in Ar atmosphere. The
structure of the deposited films was analyzed by X-ray
diffraction; the diffraction patterns show the films to be
amorphous. Annealing was performed for 24 h at 100 ºC
in vacuum.
Optical transmission spectra of Cu7GeS5I thin films
were studied in the interval of temperatures 77–300 K by
an MDR-3 grating monochromator, UTREX cryostat
was used for low-temperature studies. Spectral
dependences of absorption coefficient and dispersion
dependences of refractive index of thin films were
calculated using the well-known method [7].
3. Results and discussion
The optical transmission spectra at different
temperatures in the interval of temperatures 77–300 K in
as-deposited Cu7GeS5I thin film are shown in Fig. 1. A
long-wavelength shift of short-wavelength part of
absorption spectra and interference maxima with
increasing of temperature is observed. The same
temperature behaviour of transmission spectra was
revealed for annealed Cu7GeS5I thin film.
Fig. 1. Optical transmission spectra of Cu7GeS5I thin film at
various temperatures: (1) 77, (2) 150, (3) 200, (4) 250 and (5)
300 K.
Fig. 2. Spectral dependences of the absorption coefficient of
Cu7GeS5I thin film at various temperatures: 77 (1), 150 (2),
200 (3), 250 (4), and 300 K (5). The insets show the
temperature dependence of the steepness parameter σ as well
as the temperature dependences of the absorption edge energy
position Eg
α (α = 104 cm–1) (1) and Urbach energy EU (2).
Table. The parameters of Urbach absorption edge and EPI
for Cu7GeS5I crystal and thin films.
Material
As-
deposited
film
Annealed
film Crystal
α
gE (300 K), eV 2.162 2.090 2.125
UE (300 K), meV 163.8 131.9 35.0
0α , cm–1 5.18×104 5.31×104 1.1×106
0E , eV 2.431 2.310 2.371
0σ 0.223 0.265 0.81
pωh , meV 59.3 54.6 28.7
Eθ , K 688 634 333
( )0UE , meV 132.2 103.1 17.8
( )1UE , meV 281.4 210.0 35.1
)0(α
gE , eV 2.213 2.138 2.247
α
gS 7.67 6.36 8.5
Fig. 2 presents the spectral dependences of the
absorption coefficient at different temperatures in
interval 77–300 K for as-deposited Cu7GeS5I thin film. It
is shown that the optical absorption edge for both as-
deposited and annealed Cu7GeS5I thin films in the region
of its exponential behaviour are described by Urbach
rule [8]
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 2. P. 192-196.
doi: 10.15407/spqeo19.02.192
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
194
( ) ⎥
⎦
⎤
⎢
⎣
⎡ −ν
⋅α=να
)(
exp, 0
0 TE
EhTh
U
, (1)
where )(U TE is the Urbach energy, α0 and E0 are the
coordinates of the convergence point of the Urbach
bundle, hν and T are the photon energy and temperature,
respectively. Constants α0 and E0 for as-deposited and
annealed Cu7GeS5I thin films are given in Table. For
comparison the Table contains the corresponding
parameters for Cu7GeS5I crystal. The exponential form
of the long-wavelength side of the absorption edge is
usually associated with exciton (electron)-phonon
interaction (EPI) [9]. The Urbach energy )(TEU is
related to another parameter, the slope of the Urbach
edge )(Tσ as )()( TkTTEU σ= , k being the Boltzmann
constant. The inset in Fig. 2 shows that for Cu7GeS5I
thin film in whole investigated temperature interval the
σ(T) dependence described by the Mahr relation [9]:
( ) ⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ ω
⋅⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
ω
⋅σ=σ
kT
kTT p
p 2
tanh2
0
h
h
(2)
where σ0 is a constant independent of temperature and
related to the EPI constant g as σ0 =2/3g; pωh is the
effective average phonon energy in a single-oscillator
model, describing the EPI. For as-deposited and
annealed Cu7GeS5I thin films σ0 < 1, which is an
evidence for the strong EPI [10]. The values of effective
phonon energy pωh taking part in formation of the
absorption edge and σ0 parameter are given in Table. It
is revealed that annealing leads to the insignificant
weakening of EPI (increase of σ0 parameter) and
decrease of pωh value.
For the characterization of the absorption edge
spectral position, such parameter as α
gE ( α
gE is the
energy position of the exponential absorption edge) at a
fixed absorption coefficient value α was determined. We
used the α
gE values taken at α = 104 cm–1 for thin films
as well as α = 103 cm–1 for single crystal (Table). The
temperature dependences of the α
gE and Urbach energy
EU for Cu7GeS5I thin film are presented in the inset in
Fig. 2. It is shown that for as-deposited and annealed
Cu7GeS5I thin films it can be described in the Einstein
model by relations [11, 12]
( ) ⎥
⎦
⎤
⎢
⎣
⎡
−θ
θ−= ααα
1exp
1)0()(
E
E T
kSETE ggg , (3)
( ) ( ) ( ) ⎥
⎦
⎤
⎢
⎣
⎡
−θ
+=
1exp
1)( 1U0UU T
EETE
E
(4)
where )0(α
gE and α
gS are the energy position of
absorption edge at 0 K and a dimensionless constant,
respectively; Eθ is the Einstein temperature,
corresponding to the average frequency of phonon
excitations of a system of non-coupled oscillators,
( )0UE and ( )1UE are constants. The )0(α
gE , α
gS , Eθ ,
( )0UE and ( )1UE parameters obtained for the as-
deposited and annealed Cu7GeS5I thin films are listed in
Table. The temperature dependences of the α
gE and
Urbach energy EU for as-deposited Cu7GeS5I thin film
calculated from Eqs. (3) and (4) are shown in the inset in
Fig. 2 by solid and dashed lines, respectively. It should
be noted that the α
gE and EU values are seen to decrease
with annealing. The Urbach energy EU decrease being an
evidence for the ordering processes in the annealed film.
Besides, Table contains the above mentioned parameters
for Cu7GeS5I single crystal.
It is revealed that the optical absorption edge
spectra of the thin films under investigation is highly
smeared and characterize by the lengthy Urbach tail
which results in high values of the Urbach energy EU
(Table). Absorption edge smearing and appearance of its
Urbach behaviour are explained by the influence of
different types of disordering [13], i.e. the Urbach
energy EU is described by the equation
( ) ( ) ( )
( ) ( ) dynXstatX
TXT
EE
EEEE
,U,U
UUUU
++
+=+=
(5)
where ( )TEU and ( )XEU are the contributions of
temperature and structural disordering to EU,
respectively. In Ref. [14] it is shown that structural
disordering ( )XEU consists from the contributions of
static structural disordering ( ) statXE ,U and dynamic
structural disordering ( ) dynXE ,U . The static structural
disordering ( ) statXE ,U in Cu7GeS5I thin film is primarily
caused by structural imperfections due to the high
concentration of disordered copper vacancies and the
dynamic structural disordering ( ) dynXE ,U is related to
the intense motion of mobile copper ions, participating
in the ion transport, and is responsible for the ionic
conductivity. The first term in the right-hand side of
Eq. (4) represents the static structural disordering, and
the second one represents temperature-related types of
disordering: temperature disordering due to thermal
lattice vibrations and dynamic structural disordering due
to the presence of mobile ions in the superionic
conductor. Preliminary electrical studies have shown the
total electrical conductivity of the thin film at T = 300 K
to be tσ = 0.07 S/m at the frequency of 1 MHz. Thus,
the thin film prepared on the base of Cu7GeS5I
compound was shown to be characterized by a high
value of the electrical conductivity which can be used
for the creation of miniature solid electrolyte batteries
and supercapacitors of new generation.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 2. P. 192-196.
doi: 10.15407/spqeo19.02.192
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
195
Fig. 3. Refractive index dispersions of Cu7GeS5I thin film at
various temperatures: 77 (1), 150 (2), 200 (3), 250 (4) and
300 K (5). The inset shows the temperature dependence of
refractive index.
For the estimation of the contribution of the
different types of disordering into the Urbach energy
EU we used the procedure described in Ref. [15]. Thus,
the contribution of static structural disordering into the
as-deposited Cu7GeS5I film Urbach energy is shown to
be 80.7%. The high value of above mentioned
contribution for Cu7GeS5I thin film caused by the (1)
the absence of long-range order in the atomic
arrangement and chemical bond breakdown, (2) a lower
density of the atomic structure packing due to the
presence of pores, (3) transition from the three-
dimensional bulk structure to the two-dimensional
planar structure. It is revealed that annealing leads to
the decrease of absolute and relative contributions of
static structural disordering into EU what is the
evidence of structural ordering.
Dispersion dependences of the refractive index for
the as-deposited Cu7GeS5I thin film (Fig. 3) were
obtained from the interference transmission spectra. In
the transparency region a slight dispersion of the
refractive index is observed, increasing with
approaching the optical absorption edge. With
increasing temperature, a nonlinear increase of the
refractive index in the as-deposited and annealed
Cu7GeS5I thin films is revealed. Besides, the annealing
leads to the refractive index increase from 2.360 to
3.120 at λ = 1 µm.
Finally, it should be noted that the transition from
crystal to thin film caused the substantial increase of
Urbach energy EU, enhance the EPI (decrease of σ0
parameter) and increase of the effective phonon energy
pωh as well as the increase of the relative contribution
of static structural disordering into EU from 50.9 to
80.7%.
4. Conclusions
Cu7GeS5I thin films are deposited onto silicate glass
substrates by non-reactive radio frequency magnetron
sputtering. Temperature behaviour of the optical
transmission spectra for as-deposited and annealed
Cu7GeS5I thin films is similar in the interval 77–300 K.
With increasing temperature, a red shift of the optical
absorption edge was revealed, in the range of its
exponential behaviour is well described by the Urbach
rule. The temperature dependences of the energy
position of absorption edge, the Urbach energy, and the
refractive index of as-deposited and annealed Cu7GeS5I
thin films were analyzed. The temperature and structural
disorder affect the shape of the Urbach absorption edge,
the contribution of static structural disordering into the
Urbach energy for the as-deposited and annealed
Cu7GeS5I thin films was estimated.
Acknowledgements
Andrii Bendak (contract number 51501903) is strongly
grateful to the International Visegrad Fund scholarship
for the funding of the project.
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