Scaling and Confinement in Ultrathin Chalcogenide Films as Exemplified by GeTe

Abstract

Chalcogenides such as GeTe, PbTe, Sb2Te3, and Bi2Se3 are characterized by an unconventional combination of properties enabling a plethora of applications ranging from thermo‐electrics to phase change materials, topological insulators, and photonic switches. Chalcogenides possess pronounced optical absorption, relatively low effective masses, reasonably high electron mobilities, soft bonds, large bond polarizabilities, and low thermal conductivities. These remarkable characteristics are linked to an unconventional bonding mechanism characterized by a competition between electron delocalization and electron localization. Confinement, that is, the reduction of the sample dimension as realized in thin films should alter this competition and modify chemical bonds and the resulting properties. Here, pronounced changes of optical and vibrational properties are demonstrated for crystalline films of GeTe, while amorphous films of GeTe show no similar thickness dependence. For crystalline films, this thickness dependence persists up to remarkably large thicknesses above 15 nm. X‐ray diffraction and accompanying simulations employing density functional theory relate these changes to thickness dependent structural (Peierls) distortions, due to an increased electron localization between adjacent atoms upon reducing the film thickness. A thickness dependence and hence potential to modify film properties for all chalcogenide films with a similar bonding mechanism is expected.

Metavalent Bonding in Layered Phase‐Change Memory Materials

“…[81] Our suggestion for such use is to grow high-quality thin films to minimize the presence of inverse blocks, allowing tuning of optical properties more effectively. The bonding mechanism and optical characters in layered chalcogenides could be further explored by evaluating the thickness-dependent properties in ultrathin films [82][83][84][85][86] or in the alternately grown heterostructure thin films, such as the TiTe 2 /Sb 2 Te 3 heterostructure. [87][88][89][90] At last, we note that presence of the inverse block defects should also affect the electronic and thermal conduction like other extended defects, such as swapped bilayers [91] and stacking faults with non-QL blocks, [92] which could be tailored for thermoelectric [93,94] and topological [92,95] applications of trigonal Sb 2 Te 3 and related chalcogenides.…”

Confinement‐Induced Phonon Softening and Hardening in Sb2Te3 Thin Films

“…A similar to the data processing in previous studies. [9] The contributions of ellipsometry spectra and reflectance spectra to the fitted dielectric function were weighted by a factor to make them contribute equally to the mean squared deviation. A Downhill simplex method was employed for the unconstrained optimization.…”

“…In a recent study, an unusual thickness‐dependence of film properties has already been reported for the monochalcogenide Germanium Telluride (GeTe). [ 9 ] Such monochalcogenides are characterized by a number of unconventional properties, too. They possess a moderate electrical conductivity (σ ≈ 10 2 − 10 4 S cm −1 ), a large optical dielectric constant (ε ∞ > 15), large Born effective charges ( Z * ≈ 5 − 8) and large Grüneisen parameters (γ > 3).…”

“…Indeed, pronounced changes in atomic arrangement as well as optical and vibrational properties were found for GeTe films as a function of film thickness. [ 9 ] Thus, discussing confinement effects from a bonding perspective could be a promising approach to understand application‐relevant property changes in sesquichalcogenides, too.…”

Photoexcitation-Induced Nonthermal Ultrafast Loss of Long-Range Order in GeTe

“…6 The destruction of the longer bonds is induced by photoexciting electrons from highenergy states in the valence band to low-energy states in the conduction band, enhancing the concentration of nonequilibrium charge carriers. These charge carriers populate antibonding states of the weaker metavalent bonds, 34 as shown in the COHP curves in Figure S6, leading to structural relaxation and bond rupture, causing the collapse of the long-range order. This mechanism effectively opens a path for lowering the energy barrier for bond breaking and triggering structural disorder at a temperature lower than the nominal melting point, similar to photoexcitation preferential bond rupture in As 2 S 3 35 and GST.…”