% IMPORTANT: The following is UTF-8 encoded. This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.
@PHDTHESIS{Chen:723103,
author = {Chen, Chao},
othercontributors = {Wuttig, Matthias and Lobo, Ricardo P. S. M.},
title = {{D}ielectric properties of amorphous phase-change
materials},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2018-223963},
pages = {1 Online-Ressource (x, 174 Seiten) : Illustrationen},
year = {2018},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2018},
abstract = {The AC conductivities and dielectric properties of five
amorphous phase-change materials (PCMs) and three ordinary
chalcogenides have been determined by employing a
combination of the AC electrical measurement (0.5 Hz –
186.2 Hz), the impedance spectroscopy (9 kHz – 3 GHz) and
the optical spectroscopy (20 cm-1 – 12000 cm-1, i.e., 0.6
THz – 360 THz). Those measurements almost range from the
DC limit to the first interband transition. In addition, the
temperature dependence of the low-frequency dielectric
permittivity and the AC conductivities of amorphous PCMs
were also investigated by the AC electrical measurement in
the range of 4 K – 170 K and by the impedance spectroscopy
in the range of 220 K – 350 K. Moreover, the aging effect
on these properties of amorphous GeTe thin films annealed
for one hour at successively higher temperatures, i.e. 333
K, 353 K, 373 K, 393 K, 403 K was studied by the AC
electrical measurement. This work mainly focuses on
amorphous PCMs. Firstly, measurements of AC conductivities
of amorphous PCMs have been extensively used to understand
the conduction process in these materials. No frequency
dependence of AC conductivities is discernible in the
impedance spectroscopy measurements, which is in line with
charge transport via extended states. Secondly, the
permittivities of amorphous PCMs are frequency independent
among the impedance measurement frequency range.
Consequently, there are no dielectric relaxations in this
range. Thirdly, the static dielectric constants of amorphous
PCMs significantly exceed their optical dielectric
constants. This observation is corroborated by transmittance
measurements in the far-infrared, which show optical
phonons. Particular attention is also paid to the
correlation between the dielectric constant and Born
effective charge of the amorphous phase-change materials.
From the intensity of these phonon modes, a large Born
effective charge is derived. Nevertheless, it is known that
crystalline PCMs such as GeTe possess even significantly
larger Born effective charges. Crystallization is hence
accompanied by a huge increase in the Born effective charge,
which reveals a significant change of bonding upon
crystallization. Interestingly, a clear stoichiometry trend
in the static dielectric constant along the pseudo-binary
line between GeTe and Sb2Te3 has been identified. On the
other hand, there is a comparison of dielectric properties
between the PCMs and non-PCMs. The optical dielectric
constants of amorphous PCMs increase a lot after
crystallization, while there is no difference between the
optical dielectric constants of the amorphous and
crystalline chalcogenide AgInTe2. This illustrates that the
PCMs undergo a change from covalent bonding to resonant
bonding on crystallization, but the amorphous and
crystalline phases of ordinary chalcogenides are both
governed by virtually the same covalent bonds. In addition,
the static dielectric constants obtained for PCMs on the
pseudo-binary line between GeTe and Sb2Te3 are compared with
those obtained for ordinary covalently-bonded chalcogenide
semiconductors. The static dielectric constants of both PCMs
and non-PCMs significantly enhance from amorphous to
crystalline, which hints that the contribution of infrared
active phonons is remarkably strengthened in the crystalline
states of both PCMs and non-phase-change materials.
Moreover, the temperature dependence of dielectric constants
of amorphous chalcogenides shows the contribution
enhancement of infrared active phonons with temperature.
Lastly, the aging effect on the dielectric property of
amorphous GeTe thin films derived from the experimental
results is in good agreement with the results of density
functional theory (DFT) calculations, which at the same time
reveal the bonding mechanisms and atomic structures in the
representative amorphous phase.},
cin = {131110 / 130000},
ddc = {530},
cid = {$I:(DE-82)131110_20140620$ / $I:(DE-82)130000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2018-223963},
url = {https://publications.rwth-aachen.de/record/723103},
}
Gast ::