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Summary Thermoelectric materials could play an important role in global sustainable energy. However, improving thermoelectric efficiency has proved difficult, largely due to the complex interdependence of electronic properties of solids. Early work by Ioffe has developed into the standard thermoelectric optimization paradigm of tuning the electronic carrier concentration in semiconductors. Although the localization theory of electrons by Anderson and Mott has developed in parallel, its potential for thermoelectrics optimization has not been explored. Here, we show that structural-disorder-induced electron localization also provides an effective optimization strategy for thermoelectric materials. By using a transport model that includes the relevant physics of localization, it is shown that the maximum thermoelectric figure of merit can be increased ∼20% by tuning both carrier concentration and disorder. The benefit of slight disorder is confirmed in two model Ge-Sb-Te material systems. Particularly for highly degenerate semiconductors, this bidimensional optimization strategy provides a new methodology to attain high thermoelectric performance.

GeTe-Sb2Te3 superlattices have attracted major interest in the field of phase-change memories due to their improved properties compared with their mixed counterparts. However, their crystal structure and resistance-switching mechanism are currently not clearly understood. In this work epitaxial GeTe-Sb2Te3 superlattices have been grown with different techniques and were thoroughly investigated to unravel the structure of their crystalline state with particular focus on atomic stacking and van-der-Waals bonding. It is found that, due to the bonding anisotropy of GeTe and Sb2Te3, the materials intermix to form van-der-Waals heterostructures of Sb2Te3 and stable GeSbTe. Moreover, it is found through annealing experiments that intermixing is stronger for higher temperatures. The resulting ground state structure contradicts the dominant ab-initio results in the literature, requiring revisions of the proposed switching mechanisms. Overall, these findings shed light on the bonding nature of GeTe-Sb2Te3 superlattices and open a way to the understanding of their functionality.

In this paper we explore the potential of stoichiometry determination for chalcogenide superlattices, promising candidates for next-generation phase-change memory, via X-ray diffraction. To this end, a set of epitaxial GeTe/Sb2Te3 superlattice samples with varying layer thicknesses is sputter-deposited. Kinematical scattering theory is employed to link the average composition with the diffraction features. The observed lattice constants of the superlattice reference unit cell follow Vegard's law, enabling a straight-forward and non-destructive stoichiometry determination., physica status solidi (RRL) - Rapid Research Letters (2019)

The intensity distribution of a laser beam has a high impact on laser processing applications. In many applications, the emitted light of a laser beam source is transformed from a Gaussian intensity distribution into other intensity distributions with the intent to improve the process quality. Numerous approaches have been pursued for this purpose in the past. The vast majority of these optical systems is static. As a result the laser material processing process is limited to a specific intensity distribution. Different systems like membrane deformable mirrors can be used for shaping multiple intensity distributions. However, the control of such systems is complex and requires a deep understanding of the underlying operating principle of the specific mirror system. In this paper a new approach for active beam shapers made of catalog components, like spherical and cylindrical lenses, is introduced. Two optical systems for active beam shaping are designed which can change between two different intensity distributions by moving an individual spherical/ cylindrical lens along the beam path. One system forms a laser spot with Gaussian like intensity distribution and a TopHat shaped intensity distribution respectively. The second optical system is capable of forming a laser spot with Gaussian intensity distribution and a homogenous line shaped intensity distribution respectively. Also the mechanical housing for these optical systems is presented.

A route to realize strain engineering in weakly bonded heterostructures is presented. Such heterostructures, consisting of layered materials with a pronounced bond hierarchy of strong and weak bonds within and across their building blocks respectively, are anticipated to grow decoupled from each other. Hence, they are expected to be unsuitable for strain engineering as utilized for conventional materials which are strongly bonded isotropically. Here, it is shown for the first time that superlattices of layered chalcogenides (Sb2Te3/GeTe) behave neither as fully decoupled two-dimensional (2D) materials nor as covalently bonded three-dimensional (3D) materials. Instead, they form a novel class of 3D solids with an unparalleled atomic arrangement, featuring a distribution of lattice constants, which is tunable. A map to identify further material combinations with similar characteristic is given. It opens the way for the design of a novel class of artificial solids with unexplored properties.

Disorder has a tremendous impact on charge transport in crystalline compounds on the pseudo-binary line between Sb2Te3 and GeTe. Directly after crystallization, the pronounced disorder on the cation sublattice renders crystalline Ge1Sb2Te4—a composition with a carrier density of the order of 1020 cm−3—an Anderson insulator. Annealing, however, induces the reduction of disorder and eventually triggers an insulator-to-metal transition. This study presents data on the electrical properties, the optical conductivity, and structural properties of the pseudo-binary compositions between Ge3Sb2Te6 and GeTe. In contrast to the preceding investigations, which rely on the annealing temperature for tuning the electrical properties, this study elucidates the impact of stoichiometry and demonstrates that the stoichiometry may be employed as an alternative control parameter for the metal-to-insulator transition. The combination of annealing temperature and stoichiometry, therefore, provides a rich playground for tailoring disorder and, as a consequence, the transport of charge.

The extraordinary electronic and optical properties of the crystal-to-amorphous transition in phase-change materials have led to important developments in memory applications. A promising outlook is offered by nanoscaling such phase-change structures. Following this research line, we study the interband optical transmission spectra of nanoscaled $\text{GeTe}/{\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ chalcogenide superlattice films. We determine, for films with varying stacking sequence and growth methods, the density and scattering time of the free carriers, and the characteristics of the valence-to-conduction transition. It is found that the free carrier density decreases with increasing GeTe content, for sublayer thicknesses below $\ensuremath{\sim}3$ nm. A simple band model analysis suggests that GeTe and ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ layers mix, forming a standard GeSbTe alloy buffer layer. We show that it is possible to control the electronic transport properties of the films by properly choosing the deposition layer thickness, and we derive a model for arbitrary film stacks.

Phase-change materials form a unique material class, characterized by a rather unusual combination of physical properties. Exhibiting fast crystallization and a large contrast in optical reflectivity and electrical conductivity between the amorphous and the crystalline state, they are ideally suited for non-volatile data storage. Here, we present the thermoelectric properties of Ge3Sb2Te6 and Ge8Sb2Te11 phase-change alloys measured between room temperature and 120 °C. Both systems display intrinsically high Seebeck coefficients and low thermal conductivities. While low electrical conductivities preclude the employment of Ge3Sb2Te6 in thermoelectric applications, the GeTe rich Ge8Sb2Te11 exhibits high ZTs of up to 0.7 in the temperature range investigated, which renders this alloy a potential p-type thermoelectric material.

Controlling thermal transport in solids is of paramount importance for many applications. Often thermal management is crucial for a device's performance, as it affects both reliability and power consumption. A number of intricate concepts have been developed to address this challenge, such as diamond-like coatings to enhance the thermal conductivity or low symmetry complex super-structures to reduce it. Here, a different approach is pursued, where we explore the potential of solids with a high yet controllable degree of disorder. Recently, it has been demonstrated that an unconventionally high degree of structural disorder characterizes a number of crystalline phase-change materials (PCMs). This disorder strongly impacts electronic transport and even leads to disorder induced localization (Anderson localization). This raises the question how thermal transport is affected by such conditions. Here thermal transport in highly disordered crystalline Ge-Sb-Te (GST) based PCMs is investigated. Glass-like thermal properties are observed for several crystalline PCMs, which are attributed to strong scattering by disordered point defects. A systematic study of different compounds along the pseudo-binary line between GeTe and Sb2Te3 reveals that disordered vacancies act as point defects responsible for pronounced phonon scattering. Annealing causes a gradual ordering of the vacancies and leads to a more 'crystal-like' thermal conductivity. While both vibrational and electronic degrees of freedom are affected by disorder, the consequences differ for different stoichiometries. This opens up a pathway to tune electrical and thermal transport by controlling the degree of disorder. Materials with tailored transport properties may not only help to improve power efficiency and scaling in upcoming phase-change memories but are also of fundamental interest in the field of thermoelectric materials.