Formation of Resonant Bonding during Growth of Ultrathin GeTe Films
Ruining Wang1, Wei Zhang2, Jamo Momand3, Ider Ronneberger4, 5, Jos E. Boschker1, Riccardo Mazzarello4, 5, Bart J. Kooi3, Henning Riechert1, Matthias Wuttig5, 6, Raffaella Calarco1,*
1 Paul-Drude-Institut für Festkörperelektronik, Hausvogteiplatz 5-7, 10117 Berlin, Germany
2Center for Advancing Materials Performance from the Nanoscale, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, PR China.
3Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands
4Institute for Theoretical Solid State Physics, RWTH Aachen University, 52056 Aachen, Germany
5JARA-FIT and JARA-HPC, RWTH Aachen University, 52056 Aachen, Germany
6 I. Physikalisches Institut, RWTH Aachen University, 52056 Aachen, Germany
KEYWORDS: molecular beam epitaxy, phase change materials, amorphous phase, growth onset, resonant bonding.
SUPPLEMENTARY INFORMATION
Extended simulation details:
# BLs / ahex [Å] / Bond Length [Å] / Ediff [meV/atom] / / Born Effective Charge Z* / TO/LO Frequencies [cm-1]Free-standing GeTe / 1 / 3.91 / 2.76 / 249.2 / (16.7, 16.7) / Ge (1.6, 1.6)
Te (-1.6, -1.6) / (167.7, 228.5)
2 / 4.02 / 2.77/3.31 / 170 / (24.9, 24.9) / Ge (1.8, 1.8)
Te (-1.8, -1.8) / (160.3, 212.6)
6 / 4.11 / 2.79/3.23 / 79.2 / / / /
12 / 4.13 / 2.81/3.19 / 44.6 / / / /
bulk / 4.17 / 2.83/3.14 / 0 / (57.6, 57.6, 47.9) / Ge (6.9, 6.9, 4.6)
Te (-6.8, -6.8, -4.6) / (74.5, 150.9)
GeTe on Sb(111) / 1 / 4.31 / 2.84 / / / / / /
2 / 4.31 / 2.84/3.25 / / / / / /
Free-standing GaSb / 1 / 4.31 / 2.60 / 584.6 / (22.5, 22.5) / Ga(2.7, 2.7)
Sb(-2.7, -2.7)
2 / 4.31 / 2.67 / 420.9 / (34.9, 34,9) / Ga(2.2, 2.2)
Sb(-2.2, -2.2)
6 / 4.30 / 2.64 / 186.1 / / / /
12 / 4.30 / 2.64 / 93.7 / / / /
bulk / 4.29 / 2.63 / 0 / (18.2, 18.2, 18.2) / Ga(1.7, 1.7, 1.7)
Sb(-1.7, -1.7, -1.7)
Table SI-1: DFT calculations of GeTe and GaSb thin films of different thickness. Both the volume and atomic positions are relaxed with the inclusion of van der Waals corrections.1 GeTe shows short-long bond patterns in both the thin-film and bulk models, whereas GaSb has only one type of bond. Z* values are averaged over each type of element for all the models. In the case of GeTe on Sb(111), and Z* could not be calculated because of the conductive nature of the substrate.
The calculations reveal that a free-standing film of 1 or 2 BLs of GeTe differs tremendously from the corresponding bulk phase both in terms of atomic arrangement as well as the dielectric function, see table 1. In particular, the in-plane lattice parameter ahex of 1BL GeTe reduces by more than 6% with respect to the bulk value, and ahex gradually increases to the bulk value as the film gets thicker. The change in bond lengths with increasing film thickness indicates that the out-of-plane lattice gradually approaches the GeTe bulk value too.
Remarkably, a 2 BL GeTe film deposited on the Sb(111) surface instead exhibits bulk-like bond lengths, which clearly indicates that this substrate promotes resonant bonding even in ultrathin GeTe films. Although resonant bonding is demonstrated to be unfavorable in a free-standing ultrathin film, any interfaces with other surfaces will have a strong influence on the stability of either the amorphous or crystalline phase.
The dielectric constant and Born effective charges were not calculated for 6BLs and 12BLs models, because the systems get metallic due to the presence of surface states 2 and to (small) charge transfer effects from one surface to the other.3
References
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3.Kaxiras, E., Bar-Yam, Y., Joannopoulos, J. D. & Pandey, K. C. Ab initio theory of polar semiconductor surfaces. I. Methodology and the (2x2) reconstructions of GaAs (111). Phys. Rev. B35, 9625–9635 (1987).
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