Dielectric Relaxation and Photo-electromotive Force in Ge-Sb-Te/Si
Structures
R. A. Castro-Arata
1
, M. A. Goryaev
1
, A. A. Kononov
1
, Y. Saito
2
, P. Fons
2
, J. Tominaga
2
,
N. I. Anisimova
1
and A. V. Kolobov
1,2
1
Herzen State Pedagogical University of Russia, 191186, St. Petersburg, Russia
2
National Institute of Advanced Industrial Science & Technology, 305-8565, Tsukuba Central 5, 1-1-1 Higashi, Japan
Keywords: Structures Ge-Sb-Te/Si, Dielectric Properties, Photovoltaic Effect.
Abstract: The dielectric properties and photovoltaic effect spectra in the compositions of amorphous layers GeSb
2
Te
4
(GST 124), Ge
2
Sb
2
Te
5
(GST 225) и GeSb
4
Te
7
(GST 147) applied on the monocrystallic silicon surface are
investigated. It is shown that with a change in the GST composition, both the dielectric capacitivity and the
frequency at which the maximum dielectric loss is observed change. It was found that the value of the change
in photo-electromotive force is different for different layers: on samples with GST 124, the influence of
amorphous layers is by an order of magnitude greater than for GST 225, and by 3 orders of magnitude greater
than for GST 147.
1 INTRODUCTION
Complex blend chalcogenide glassy semiconductors
(CGSs) attract the attention of researchers in
connection with their use in numerous devices of
micro- and optoelectronics. For example,
chalcogenide glassy semiconductors are currently
used in the manufacture of thermal imaging systems
(Cha, et al, 2012), fibers and transparent flat
waveguides in the IR range (Snopatin, et al, 2009), in
optical sensors (Charrier, et al, 2012) and nonlinear
optics (Zhang, et al, 2015).
Electrophysical and structural properties of
chalcogenide semiconductors have been intensively
studied recently (Siegrist, et al, 2011, Zhang, et al,
2012, Gabardi, et al, 2015), which is associated with
their successful application in non-volatile memory
devices (fiscal memory, phase-change memory). The
principle of operation of such devices is based on a
sharp change in the electrophysical properties of the
material during a reversible phase transition between
crystalline and amorphous states. Complex
chalcogenides of the Ge-Sb-Te (GST) system are
some of the most popular materials of fiscal memory
(Kozyukhin, et al, 2014).
The aim of this work is to establish the features of
the dielectric relaxation spectra and photo-
electromotive (photo-EMF) force in Ge-Sb-Te/Si
structures based on GST layers obtained by HF
magnetron sputtering.
The study of the features of polarization processes
in structures based on silver halides and chalcogenide
glassy semiconductors, including photostimulated
ones, allows to determine at what energy level the
transport of charge carriers is carried out,
distinguishing between zone and hopping
mechanisms, and what is the nature of charge carriers,
and also to evaluate a number of microscopic
parameters of the studied compounds
(Goryaev,
1997, 1998, Bordovskii, et al, 2001, Castro, et al,
2006, Anisimova, et al, 2010).
2 EXPERIMENTAL
Thin films of the Ge-Sb-Te system (GeSb
2
Te
4
(GST
124), Ge
2
Sb
2
Te
5
(GST 225) и GeSb
4
Te
7
(GST 147))
with a thickness of the order of 50 nanometers were
obtained by HF magnetron sputtering at room
temperature on silicon substrates. The structural
features of the samples were studied on a DRON-7 X-
ray diffractometer. The obtained diffractograms
(figure 1) measured at large 2θ scattering angles of X-
rays in the range from 10° to 80° indicate the
amorphous nature of the films under study. The
elemental composition of the samples was studied
146
Castro-Arata, R., Goryaev, M., Kononov, A., Saito, Y., Fons, P., Tominaga, J., Anisimova, N. and Kolobov, A.
Dielectric Relaxation and Photo-electromotive Force in Ge-Sb-Te/Si Structures.
DOI: 10.5220/0009154201460150
In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 146-150
ISBN: 978-989-758-401-5; ISSN: 2184-4364
Copyright
c
 2022 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved

using a Carl Zeiss EVO 40 scanning electron
microscope (SEM).
10 20 30 40 50 60 70 80 90
0
5
10
15
20
25
30
35
40
Ge
2
Sb
2
Te
5
Si
69
27
Intensity, quanta per second
2θ, deg
Figure 1: Diffractogram of the samples of Ge-Sb-Te/Si
structure with 2θ angles indication.
The dielectric spectra of the studied layers were
measured on a Concept-81 spectrometer
(Novocontrol Technologies GmbH) designed to
study the dielectric and conductive properties of a
wide variety of materials. The measurements were
carried out in the frequency range f = 10
2
Hz…10
7
Hz
at room temperature. The voltage applied to the
samples was U = 10
-1
В. The relative experimental
error did not exceed ± 3%.
The experimental data were the values of the
imaginary and real part of the impedance of the cell
with the measured sample:
)(
1
)(
*
0
*
ωω
ω
I
U
ZiZ
Ci
RZ =
′′
+
′
=+=
.
The spectra of complex dielectric constant and
conductivity were calculated from the impedance
spectra according to the following formulas:
0
*
*
1
)( CZ
i
i
ωω
εεε
−
=
′′
−
′
=
,
where
d
S
C
0
0
ε
=
is empty cell capacity.
The photo-EMF of the samples was measured by the
capacitor method (Akimov, 1966) under modulated
illumination on the installation, the block diagram of
which is shown in figure 2. To evaluate the efficiency
of the internal photoelectric effect, the measured
signals ΔU were normalized to the same number of
light quanta incident on the sample.
Figure 2: Installation diagram for measuring the spectra of
the photo-EMF by the capacitor method. S – the light
source, LM – the light modulator, M – the monochromator,
CC – the capacitor cell, E – the sample, SD – the
synchronous detector, NA – the narrow-band amplifier.
3 RESULTS AND DISCUSSION
3.1 Dielectric Properties
The frequency dependence of the dielectric constant
ε ′ at room temperature of the layers of the Ge-Sb-Te
system is shown in figure. 3. It is clear from the figure
that
ε ′ decreases with increasing frequency,
approaching a constant value at high frequencies, due
to the contribution of only electron polarization and
space charge polarization (Anisimova, et al, 2010). In
the low-frequency region, the manifestation of
dipole-relaxation and interfacial polarization can be
assumed.
10
3
10
4
10
5
10
6
0,6
0,7
0,8
0,9
1,0
GST 147
GST 124
GST 225
ε
' [a.u.]
f [Hz]
Figure 3: The frequency dependence of the dielectric
capacitivity of the layers of the Ge-Sb-Te system.
Measurement of quantity of the dielectric loss
tgδ=
ε ′′/ ε ′ in the studied layers (Fig. 4) revealed the
existence of a maximum of losses in the medium-
frequency region. The presence of a maximum on the
curves tgδ=f(
ω ) at room temperature indicates the
existence of relaxation processes that cause relaxation
losses in the studied samples. With a change in the
GST composition, both the dielectric capacitivity and
the frequency at which the maximum dielectric loss
is observed (see table 1) change.
Dielectric Relaxation and Photo-electromotive Force in Ge-Sb-Te/Si Structures
147

10
3
10
4
10
5
40
80
120
160
200
240
GST 147
GST 124
GST 225
tgδ [10
-3
]
f [Hz]
Figure 4: The frequency dependence of the dielectric loss
factor of the layers of the Ge-Sb-Te system.
Table 1: The value of structural and relaxation HN
parameters of samples of the glassy Ge-Sb-Te system.
Состав 2θ
гало
о
S (Å) τ
max
(с) ∆ε α
HN
β
HN
GeSb
4
Te
7
27.00 4.05 1.34*10
-5
6.03*10
-1
0.91 0.67
GeSb
2
Te
4
28.38 3.86 1.76*10
-5
6.18*10
-1
0.88 0.90
Ge
2
Sb
2
Te
5
27.75 3.95 1.62*10
-5
4.20*10
-1
0.95 0.68
An analysis of the nature of the distribution of
relaxators by relaxation times in the Ge-Sb-Te system
within the framework of the Havriliak-Negami (HN)
function model (Kremer and Schonhals, 2003)
revealed the
existence of a non-Debye oscillatory
process with a distribution of relaxation times
according to the Cole-Davidson model for the case of
an asymmetric distribution of relaxators by relaxation
times (β≠1.00) (see table 1):
[]
HN
HN
i
β
α
ωτ
ε
εωε
)(1
)(
+
Δ
+=
∞
∗
,
where ε
∞
is the high frequency limit of the real part of
the dielectric capacitivity, Δε is the dielectric
increment (the difference between the low-frequency
and high-frequency limits), ω=2πf, α
HN
and β
HN
are
shape parameters describing respectively the
symmetric (β=1.00 is the Cole-Cole distribution) and
asymmetric (α=1.00 is the Cole-Davidson
distribution) expansion of the relaxation function.
3.2 Photovoltaic Effect
Figure 5 shows the spectra of the capacitor of the
photo-electromotive force of the initial silicon
samples and samples with amorphous layers of the
Ge-Sb-Te system deposited on the semiconductor
surface.
The above-cite results show that the value of
photo-EMF of the initial samples (curves 1) in the
entire studied region of the spectrum decreases with
increasing wavelength in accordance with a change in
Figure 5: Spectra of the photo-EMF of initial silicon (1) and
samples with amorphous layers GST 124 (2), GST 225 (3),
and GST 147 (4) deposited on the semiconductor surface.
the absorption coefficient of the silicon. When
amorphous GST layers are deposited on the
semiconductor surface, the sign of the recorded signal
changes (curves 2–4). This may be due to the fact that
the photo-electromotive force has diffusion and drift
components.
The diffusion component (Dember electromotive
force) is the result of diffusion of photocurrent
carriers in a semiconductor due to their concentration
gradient created by absorbed light (Akimov, 1966):
𝑉=
𝑘𝑇
𝑒
𝜇

−𝜇

𝜇

+𝜇

𝑙𝑛
𝜎

𝜎

,
where μ
n
and μ
p
are mobility of electrons and holes,
σ
i
and σ
d
are conductivity at the front (illuminated)
and rear surfaces of the sample. The sign of Dember
electromotive force should be opposite to the sign of
the main carriers of the semiconductor. This is fully
confirmed by the sign of the photo-EMF of the initial
silicon samples having hole conductivity (curves 1).
The drift component is determined by the drift of
light-generated carriers in the field of a surface
charge. In case of locking bending of zones, when
diffusion and drift currents add up, photo-
electromotive force increases. In the case of anti-
locking bending, the direction of the drift current is
opposite to the diffusion, therefore, the resulting
photo-electromotive force can not only decrease, but
also change its sign.
The results of the effect of GST layers deposited on
the silicon surface indicate that in this case an anti-
locking bending of zones is formed in the
semiconductor. In such heterojunctions, a
PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology
148

predominance of the drift component of the photo-
EMF with a corresponding change in polarity is
observed (figure 2, curves 2-4). At the same time, a
change in the photoelectric effect spectra occurs,
which may be due to the spectral dependence of the
magnitude of the resulting band bending upon
excitation. So, on the deposition of dyes on a silicon
surface, the effect of a change in the sign of the
photopotential was revealed under illumination in
different spectral regions (Komolov, et al, 2006), as
well as a sensitized photovoltaic effect (Goryaev,
2017, 2019, Goryaev and Castro, 2018).
Figure 5 also shows that the magnitude of the
change in photoelectromotive force is different for
different layers: for GST 124 samples, the influence
of amorphous layers is by an order of magnitude
greater than for GST 225, and by 3 orders of
magnitude greater than for GST 147.
The difference in the nature of the change in
photo-electromotive force for different GST
compositions and the specific features of the
dielectric spectra can be explained by the specific
features of the energy spectrum of the amorphous
phase of the Ge-Sb-Te system. Fluctuations in the
composition should lead to the fluctuations in the
electrophysical properties of materials, and,
accordingly, to fluctuations in the edges of the bands
and energy levels of localized states in accordance
with the model proposed in (Voronklov, et al, 1974).
Changes in the energy spectrum reflect changes in the
structure that the amorphous system undergoes with
an increase in the concentration fraction of
germanium and a decrease in the concentration
fraction of antimony. According to the presented data
of X-ray spectral analysis (figure 1), with a change in
composition, a change in the distance S between the
most frequently encountered pairs of atoms is
observed. Decrease or increase in S causes a change
in polarization due to a change in the resulting dipole
moment of the system. A change in the polarization
of the system, in turn, is expressed in a change in the
dielectric capacitivity. These changes determine the
different contribution to the development of the
observed relaxation and photostimulated processes in
the Ge-Sb-Te/Si structures.
4 CONCLUSIONS
The dielectric relaxation and photo-electromotive
force spectra in Ge-Sb-Te/Si structures were
experimentally studied. It was found that the value of
the change in photo-EMF is different for different
layers: on samples with GST 124, the influence of
amorphous layers is by an order of magnitude greater
than for GST 225, and by 3 orders of magnitude
greater than for GST 147. This difference in the
nature of the change in photoelectromotive force and
the features of the dielectric spectra for different GST
compositions can be explained by the structural
features of the amorphous phase of the Ge-Sb-Te
system.
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