Evidence for resonant bonding in phase-change materials studied by IR spectroscopy
Phase-change materials (PCM) attract attention due to their unique properties. This remarkable portfolio also makes them promising for applications in novel data storage devices. In this study, we discuss differences in the optical properties of PCM and non-PCM in the IR caused by the presence or ab...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2017
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| Цитувати: | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy / K. Shportko, H. Volker, M. Wuttig // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 1. — С. 69-73. — Бібліогр.: 22 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860292432421191680 |
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| author | Shportko, K. Volker, H. Wuttig, M. |
| author_facet | Shportko, K. Volker, H. Wuttig, M. |
| citation_txt | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy / K. Shportko, H. Volker, M. Wuttig // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 1. — С. 69-73. — Бібліогр.: 22 назв. — англ. |
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| container_title | Semiconductor Physics Quantum Electronics & Optoelectronics |
| description | Phase-change materials (PCM) attract attention due to their unique properties. This remarkable portfolio also makes them promising for applications in novel data storage devices. In this study, we discuss differences in the optical properties of PCM and non-PCM in the IR caused by the presence or absence of resonant bonding.
|
| first_indexed | 2026-03-19T23:31:37Z |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 1. P. 69-73.
doi: https://doi.org/10.15407/spqeo20.01.069
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
69
PACS 68.55.Ng, 78.20.-e, 78.30.Ly, 82.80.Gk
Evidence for resonant bonding in phase-change materials
studied by IR spectroscopy
K. Shportko1, H. Volker2, M. Wuttig2
1V. Lashkaryov Institute for Semiconductor Physics, NAS of Ukraine,
41, prospect Nauky, 03680 Kyiv, Ukraine, e-mail: konstantin@shportko.com
2I. Institute of Physics (IA), RWTH University of Technology Aachen,
Sommerfeld str. 14, Aachen 52056, Germany
Abstract. Phase-change materials (PCM) attract attention due to their unique properties.
This remarkable portfolio also makes them promising for applications in novel data
storage devices. In this study, we discuss differences in the optical properties of PCM
and non-PCM in the IR caused by presence or absence of resonant bonding.
Keywords: phase-change materials, resonant bonding, optical properties.
Manuscript received 20.01.17; revised version received 21.02.17; accepted for
publication 01.03.17; published online 05.04.17.
1. Introduction
Electronic memories utilize a pronounced change in
resistance to store information. At present, dynamic
random access and flash memories are the two
implementations of choice in the IT industry. Both face
scaling problems and have rather different attributes.
This has motivated the search for alternatives. Already
in the 1960s, S. Ovshinsky suggested to use rapid phase
transitions from amorphous to crystalline states in some
chalcogenide alloys (later called “phase-change
materials” (PCM)) for memory applications [1]. These
materials have recently been introduced in the memory
market by companies like Samsung
(www.samsung.com) and Micron (www.micron.com).
PCM, which include Ge-Sb-Te (GST) alloys,
exhibit a remarkable difference of their electrical and
optical properties in the amorphous and crystalline
states. Their application potential originates from this
property contrast and the ability to rapidly re-switch
between these states. Currently, PCM are widely used in
optical data storage and considered to be promising for
non-volatile memory technology [2]. Thus, the
fundamental understanding of properties of the phase
change alloys might hold significant promise for
material optimization in next generation storage devices.
In modern chemistry, the idea of resonance
between equivalent valence-bond configurations, as put
forward by Pauling [3], is a generally accepted concept
crucial for understanding many molecules such as
benzene. While resonance has been considered in the
description of solids as early as 1957 [4], it has become
more widely recognized after 1973 especially through
the works of Lucovsky and White [5] and of Littlewood
[6] on IV-VI compounds, in which 6-fold coordination
accompanies bonding by three p-electrons per atom.
In our previous study [7], we have reported the
difference of 70–200% of the optical dielectric constant
between the crystalline and the amorphous phases of
PCM, which is attributed to a significant change in
bonding type between the two phases. We suggested that
PCM in the crystalline phase utilize the resonant
bonding, which explains the following contrast of
properties. The optical dielectric constant of the
amorphous phases is that expected of a covalent
semiconductor, whereas that of the crystalline phases is
strongly enhanced by resonant bonding effects. In
addition, the sub-gap (Drude-like) absorption by free
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 1. P. 69-73.
doi: https://doi.org/10.15407/spqeo20.01.069
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
70
charge carriers in the crystalline phase and the larger
bandgap of the amorphous phase were reported [7].
Meta-stable GSTs crystallize in the distorted rock-
salt structure with the anion sub-lattice occupied by Te
atoms, whereas the cation sub-lattice is randomly
occupied by Ge atoms, Sb atoms and empty lattice sites
(for example, 12,5% and 10% for Ge1Sb2Te4 and
Ge2Sb2Te5, respectively) [8, 9]. In this work, we have
chosen the following PCM: Ge1Sb2Te4 (GST124),
Ge2Sb1Te4 (GST214), Ge2Sb2Te4 (GST224), Ge2Sb2Te5
(GST225), Ge3Sb2Te6 (GST326), Ge3Sb4Te8 (GST348),
GeTe with resonant bonding and compare their
properties with covalently bonded non-PCM: InSb,
AgInTe2 (AIT), GeSe and SnSe. To show the difference
between these two classes of materials, we applied the
Moss rule that combines the optical dielectric constant
ε∞ and the band gap Eg in the following way [7]:
const.2 ≈ε ∞ gE (1)
The data of the amorphous and the crystalline
covalently bonded non-PCM, as well as that of the both
states of the PCM, exhibits the significant contrast
between these two classes of materials. The difference in
the ( ) ( )
amgcrg EE 22
∞∞ εε ratio presented in Fig. 1 is
caused by the fact that amorphous PCM possess
systematically lower values of Moss “constants” than
those in the corresponding crystalline state, which is not
the case for non-PCM. This evidence can be explained
by the change of bond type upon crystallization of PCM.
This behaviour separates PCM from other covalently
bonded materials, which do not reveal such a significant
property contrast between both states. This is in line with
Lucovsky and White, who have reported that materials
with resonant bonding in the crystalline phase exhibit a
property contrast with the corresponding amorphous
phase [5].
0
1
2
3
4
5
M
os
s
ra
tio
Fig. 1. The ratio ( ) ( )
amgcrg EE 22
∞∞ εε of non-PCM (brown
squares) and PCM (blue circles).
This work is aimed to examine evidences of the
resonant bonding in GSTs, which can be found beyond
the spectral range reported in [7], namely, in the range of
the phonon modes (below 0.025 eV) and interband
electron transitions (around 3 eV).
2. Experimental details
In this work, we have studied samples of PCM and non-
PCM in the amorphous and crystalline phases. The
investigated samples were prepared in the form of thin
films in the following way: a 150-nm Al layer was
deposited onto a glass substrate. After that, the PCM or
non-PCM film (1,000 nm) was d.c. sputter deposited
onto it. To obtain the polycrystalline samples, the as-
deposited amorphous films were annealed in Ar
atmosphere for 30 min at temperatures about 10 °C
above their corresponding crystallization temperatures.
In our experiments, we used meta-stable crystalline GST
samples with distorted rock-salt structure. The structure
of the studied samples was checked by X-ray diffraction.
The layer thickness was determined on reference
samples prepared in the same sputter session using a
Bruker Dektak profilometer.
To study the dielectric function of selected
materials, we applied FTIR spectroscopy. IR reflectance
spectra were measured within the range 4 to 40 meV and
from 0.5 to 2 eV, using a Bruker IFS 66v/s spectrometer
with a Hg lamp and a globar as the radiation sources as
well as DTGS detector. A gold mirror (300 nm thick
layer of gold, deposited on glass) with a reflectance
index of 0.99 in the IR was used as a reference for
reflectance measurements. The angle of incidence of
radiation is about 10 and taken into account in
calculations of the reflectance spectra.
FTIR reflectance spectra were fitted using SCOUT
software. To model the phonons’ contribution to the
dielectric function, we used the Kim model, which is a
mixture of Lorentz and Gauss profiles [10]. To fit the
spectra in the range of the electron interband transitions
the Tauc–Lorentz oscillator was used in the model of the
dielectric function [11].
3. Results and discussion
We start with the analysis of the influence of the
resonant bonds on the dielectric function in the range of
the electron interband transitions. When discussing the
consequences of application of the Moss rule to the non-
PCM and PCM, we took the position of the optical
absorption edge for the input parameter Eg. In what
follows, we particularly focus on the behaviour of the
imaginary part of their dielectric function around this
range.
Fig. 2 displays ε2 around the electron interband
transitions, which cause absorption above 0.5 eV. As
mentioned in the Experimental section, we used a Tauc–
Lorentz oscillator to model this increase of the
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 1. P. 69-73.
doi: https://doi.org/10.15407/spqeo20.01.069
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
71
imaginary part of their dielectric function around this
range. It is interesting that this simple model is
sufficient, except for the position of the absorption edge,
which has been discussed in [7]. The peak value of ε2
exhibits significant differences between the amorphous
and crystalline state of PCM. This is in contrast to the
case of non-PCM, where the position of the absorption
edge and peak value of ε2 are of the same order for both
crystalline and amorphous state. This feature can also be
seen in the data of GST225 presented in [12], however,
it was not explained there.
According to the concepts of quantum mechanics,
ε2, the imaginary part of the dielectric function in the
range of the interband transition is related with the
matrix element [13] that can be calculated employing
Fermi’s golden rule [14]:
fifif M ρ
π
=λ
22
h
, (2)
where λif is the transition probability, ifM – matrix
element for the transition and ρf – joint density of states.
The joint density of states hardly changes upon
crystallization. It is confirmed by DFT calculations and
XPS/UPS experiments [15]. Therefore, the significant
contrast in ε2 for PCM between the amorphous and
crystalline state (~2 times) can be only explained by the
increase of the transition matrix element in PCM upon
crystallization. This statement is supported by data for
non-PCMs, which show no change in bonding upon
crystallization and, therefore, possess values of the ratio
( ) ( )
amcr max2max2 εε around 1, as shown in Fig. 3.
0
1
2
3
4
ε 2
ra
tio
Fig. 3. The ratio ( ) ( )
amcr max2max2 εε for the non-PCM
(brown squares) and PCM (blue circles).
Further, we will focus on the next aspect: the
consequences of the presence of resonant bonding in the
crystalline PCM on their dielectric function in the range
of phonon modes. To facilitate the analysis of these
consequences, an influence of crystallization on the
dielectric function of the conventional semiconductor
material AIT in this specific spectral range was first
considered.
The imaginary part of the dielectric function ε2 of
non-PCM AIT in the 0.004-0.04 eV range is shown in
Fig. 4 (Inset A). Being compared with the amorphous
phase, the crystalline one in AIT displays: i) a similar
profile of ε2, which means identical phonon absorption
peaks arrangement, due to the similar short-range order
and similar bonding type in both phases, ii) sharper and
0
20
40
0.5 1 1.5 2 2.5 3
ε2
E, eV
2
1
5
4
3
6
7
8
9
10
11
12
13
14
15
16
17
1819
20
21
22
Fig. 2. Imaginary part of the dielectric function ε2 of PCM and non-PCM in the range of the electron interband transitions.
PCM: 1 – GST124 am, 2 – GST124 cr, 3 – GST214 am, 4 – GST214 cr, 5 – GST 224 am, 6 – GST224 cr, 7 – GST225 am,
8 – GST225 cr, 9 – GST326 am, 10 – GST326 cr, 11 – GST348 am, 12 – GST348 cr, 13 – GeTe am, 14 – GeTe cr; non-
PCM: 15 – InSb am, 16 – InSb cr, 17 – AIT am, 18 – AIT cr, 19 – GeSe am, 20 – GeSe cr, 21 – SnSe am, 22 – SnSe cr.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 1. P. 69-73.
doi: https://doi.org/10.15407/spqeo20.01.069
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
72
higher phonon absorption peaks due to the presence of
long-range order, absent in the amorphous phase.
In contrast to AIT, we observe three significant
differences between ε2 of amorphous and crystalline
phases in GST225 shown in Fig. 4: i) a different pattern
of ε2, ii) phonon absorption peaks of the crystalline
GST225 are located at lower energies than those of the
corresponding amorphous state, and iii) huge difference
(10 times) in the intensities of phonon absorption peaks,
which cannot be only explained by acquisition of the
long-range order upon crystallization. Furthermore, the
shift of the phonon absorption peaks and the difference
in their intensities between amorphous and crystalline
states correlate with the stoichiometry (Fig. 4, Inset B):
both effects are more pronounced in GST326 and less
pronounced in GST124, respectively.
The observed phenomena can be explained in the
terms of resonant bonding. The difference in the profile
of ε2 is shown in Fig. 4 can be attributed to different
local atomic arrangement in the amorphous and
crystalline states of PCM [16]; larger coordination
number, longer bond length and larger coefficient of
thermal expansion explain softening the phonon modes
in the crystalline states of PCM [17]. These two
phenomena provide evidence that the bonding type in
the crystalline state of PCM is not the same as that
inherent to the amorphous state, i.e., covalent bonding.
An important feature of the resonant bonding is that the
electron density distribution is highly delocalized [5]. As
a result, the materials of this group possess high Born
effective charges [18], as well as a large electronic
polarizability and dielectric constants [7]. In [19–22], it
has been shown that the intensities of the IR active
modes (and corresponding oscillator strengths) are
related with the Born effective charge.
Thus, the difference in the intensities of the IR
active phonon peaks in PCM is an experimental
evidence of the much higher Born effective charge in the
crystalline phase. Systematic variation of the
stoichiometry of the studied GST alloys (increase of the
Ge content) lowers the concentration of the vacancies in
studied materials and, therefore, is responsible for the
stoichiometry-related dependence in Fig. 4 (Inset B):
materials with lower vacancy concentration exhibit
higher intensities of the IR active modes and,
consequently, higher Born charges.
4. Conclusions
Full utilization of the potential inherent to these PCM
requires the comprehensive understanding of their
properties. Crystalline PCM can be separated from other
semiconductor materials due to their unique properties
portfolio. In this study, we have shown that resonant
bonding in crystalline PCM influences their dielectric
function in the range of the IR active phonons and the
interband electron transitions.
Acknowledgements
K. Shportko gratefully acknowledges the funding from
the DAAD (German Academic Exchange Service) and
the SFB 917 ‘Resistively Switching Chalcogenides for
Future Electronics – Structure, Kinetics and Device
0
40
80
120
4.0E-03 1.4E-02 2.4E-02 3.4E-02
2
E, eV
AIT am.
AIT cr.
GST225 cr.
GST225 am.
0
10
20
30
4.0E-03 2.4E-02
2
E, eV
2
6
10
14
18
0.008 0.010 0.012
Pe
ak
in
te
ns
ity
ra
tio
Peak position, eV
B GST124
GST225
GST326
Aε ε
Fig. 4. Imaginary part of the dielectric function ε2 of non-PCM (AIT) and PCM (GST225) in the range of phonon modes.
(Inset A. Magnification of ε2 for amorphous and crystalline AIT as well as amorphous GST225; Inset B. Crystalline GST:
the ratio ( ) ( )
amcr max2max2 εε vs. corresponding peak positions.)
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2017. V. 20, N 1. P. 69-73.
doi: https://doi.org/10.15407/spqeo20.01.069
© 2017, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
73
Scalability’.
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| id | nasplib_isofts_kiev_ua-123456789-214910 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1560-8034 |
| language | English |
| last_indexed | 2026-03-19T23:31:37Z |
| publishDate | 2017 |
| publisher | Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| record_format | dspace |
| spelling | Shportko, K. Volker, H. Wuttig, M. 2026-03-03T11:06:37Z 2017 Evidence for resonant bonding in phase-change materials studied by IR spectroscopy / K. Shportko, H. Volker, M. Wuttig // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2017. — Т. 20, № 1. — С. 69-73. — Бібліогр.: 22 назв. — англ. 1560-8034 PACS: 68.55.Ng, 78.20.-e, 78.30.Ly, 82.80.Gk https://nasplib.isofts.kiev.ua/handle/123456789/214910 https://doi.org/10.15407/spqeo20.01.069 Phase-change materials (PCM) attract attention due to their unique properties. This remarkable portfolio also makes them promising for applications in novel data storage devices. In this study, we discuss differences in the optical properties of PCM and non-PCM in the IR caused by the presence or absence of resonant bonding. K. Shportko gratefully acknowledges the funding from the DAAD (German Academic Exchange Service) and the SFB 917 ‘Resistively Switching Chalcogenides for Future Electronics – Structure, Kinetics and Device Scalability’. en Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України Semiconductor Physics Quantum Electronics & Optoelectronics Evidence for resonant bonding in phase-change materials studied by IR spectroscopy Article published earlier |
| spellingShingle | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy Shportko, K. Volker, H. Wuttig, M. |
| title | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy |
| title_full | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy |
| title_fullStr | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy |
| title_full_unstemmed | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy |
| title_short | Evidence for resonant bonding in phase-change materials studied by IR spectroscopy |
| title_sort | evidence for resonant bonding in phase-change materials studied by ir spectroscopy |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/214910 |
| work_keys_str_mv | AT shportkok evidenceforresonantbondinginphasechangematerialsstudiedbyirspectroscopy AT volkerh evidenceforresonantbondinginphasechangematerialsstudiedbyirspectroscopy AT wuttigm evidenceforresonantbondinginphasechangematerialsstudiedbyirspectroscopy |