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Ground state and excited state calculations of
GeTe
Wojciech We̷lnic a,b , Silvana Botti b , Matthias Wuttig a and Lucia Reining b
a I. Physikalisches Institut, RWTH Aachen, 52056 Aachen, Germany
b Laboratoire des Solides Irradiés, École Polytechnique, 91128 Palaiseau cedex - France
Chalcogenide alloys, which show a significant change of optical reflectivity and electric conductivity
under phase transformation from the amorphous to the crystalline structure are widely used
for optical data storage today (e.g. DVD-RW). As they can also be used as non volatile memories
(Phase-Change-RAM), they are furthermore promising candidates for future electronic data storage
techniques. In this work ground state and excited state calculations are presented for GeTe, the
basic phase-change-material. It is reported to crystallize in a cubic rocksalt phase and in a trigonal
phase, which can be described as a distorted cubic structure. From literature relatively little is
known about the amorphous structure, but the available data hints towards a tetragonal, diamondlike
local coordination. The focus of the project is on the relation between optical properties and
geometrical structure in different phases of GeTe, ranging from the two crystalline phases, defect
structures and the amorphous phase. The origin for the difference of the optical and electronic
properties in these different states is examined. The problem of obtaining a realistic amorphous
structure is approached in two different ways: on one side we employ ab initio molecular dynamics
within a 64-atom supercell and on the other side we use a simple model structure which reproduces
the local configuration reported in earlier experimental work. The electronic structure is presented
in the GW-correction and the theoretical spectra are calculated within TDDFT and GW-RPA.
They are compared with experimental data of thin film GeTe-samples. Differences between theory
and experiment are discussed as well as the changes in the optical and electronic properties upon
phase transition from the crystalline to the amorphous state.
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