Alteration of structural, electronic, and vibrational properties of amorphous GeTe by selenium substitution: An experimentally constrained density functional study

Abstract

The structural, vibrational and electronic properties of several compositions of amorphous Ge-Se-Te are studied from a combination of X-ray diffraction and density functional-based molecular dynamics. Different structural properties are considered such as structure factors, pair distribution functions, angular distributions, coordination numbers and neighbor distributions. We compare results with experimental findings and a satisfying agreement is found for the structure functions in real and reciprocal spaces. The short range order is found to be more complex than in related binaries that result in mixed geometries ( 65-75 % tetrahedral, and remaining defect octahedral) for a dominant four-fold Ge (80 %). The chalcogen atoms are dominantly 2-fold, the former having furthermore an important fraction of 3-fold coordinated atoms (30-40 %). The obtained model structures indicate that Ge-Ge, Ge-Se, and Ge-Te bonds dominate with small fractions of Te-Te bonds remaining from the base system GeTe. The investigation of electronic properties indicates that the addition of Se atoms will lead to Te-related bands that are much more localized so that Ge-Te-Se can be regarded as having an increased covalent character with respect to GeTe.

Impact of structural short‐range order and tetrahedral germanium on electronic and dielectric properties in phase‐change materials

“…One, furthermore, notes an increased localization for the s‐band in the region −13 eV$$\le E-E_F\le$$ −6 eV (Figure 6A). Such a result mimics what has been recently observed in Se‐enriched amorphous GeTe 40 . Here, the progressive transformation from GeTe into a tetrahedral GeSe network induces indeed, a more pronounced localization of the bonding for the latter which is consistent with its increased covalent character.…”

“…The projected EDOS furthermore shows that the low energy bands relate to s‐contributions from Te, centered at −11.6 eV (Figure 3C), the conduction band being dominated by contributions from p‐orbitals of all species (Ge, Sb, and Te in GST225) but also from s‐Ge orbitals. Similar features are obtained for GeTe 40 …”

“…Such a result mimics what has been recently observed in Se-enriched amorphous GeTe. 40 Here, the progressive transformation from GeTe into a tetrahedral GeSe network induces indeed, a more pronounced localization of the bonding for the latter which is consistent with its increased covalent character. It manifests by larger IPR close to the edges of the s-band, and hence s-Se bands.…”

“…If six low standard (i.e., rigid) deviations $$\sigma _\theta$$ around a Ge atom are detected, a tetrahedron is identified because this geometry is defined by six rigid angles that give rise to corresponding low angular standard deviations 42 . On system average, such typical “ tetrahedral ” angles are well defined, and lead to a corresponding angle that is, in fact, very close to the tetrahedral angle as analyzed in detail for GeTe 25 and different GST alloys 40 …”

“…42 On system average, such typical "tetrahedral" angles are well defined, and lead to a corresponding angle that is, in fact, very close to the tetrahedral angle as analyzed in detail for GeTe 25 and different GST alloys. 40 The validity of the method has been checked on a variety of systems including the archetypal Ge-S and Ge-Se chalcogenides. Here, the topological constraint count from MD trajectories 43,44 was able to reproduce exactly the Maxwell mean-field constraint count, 42 providing confidence that it can be extended to the description of the topology of more complex amorphous systems such as the present one.…”