Ge Te System Research Articles

Boosting the Thermoelectric Properties of Ge0.94Sb0.06Te via Trojan Doping for High Output Power.

GeTe stands as a promising lead-free medium-temperature thermoelectric material that has garnered considerable attention in recent years. Suppressing carrier concentration by aliovalent doping in GeTe-based thermoelectrics is the most common optimization strategy due to the intrinsically high Ge vacancy concentration. However, it inevitably results in a significant deterioration of carrier mobility, which limits further improvement of the zT value. Thus, an effective Trojan doping strategy via CuScTe2 alloying is utilized to optimize carrier concentration without intensifying charge carrier scattering by increasing the solubility of Sc in the GeTe system. Because of the high doping efficiency of the Trojan doping strategy, optimized hole concentration and high mobility are obtained. Furthermore, CuScTe2 alloying leads to band convergence in GeTe, increasing the effective mass m* in (Ge0.84Sb0.06Te0.9)(CuScTe2)0.05 and thus significantly improving the Seebeck coefficient throughout the measured temperature range. Meanwhile, the achievement of the ultralow lattice thermal conductivity (κL ∼ 0.34 W m-1 K-1) at 623 K is attributed to dense point defects with mass/strain-field fluctuations. Ultimately, the (Ge0.84Sb0.06Te0.9)(CuScTe2)0.05 sample exhibits a desirable thermoelectric performance of zTmax ∼ 1.81 at 623 K and zTave ∼ 1.01 between 300 and 723 K. This study showcases an effective doping strategy for enhancing the thermoelectric properties of GeTe-based materials by decoupling phonon and carrier scattering.

Enhancement of thermoelectric performance by stacking fault control in (GeTe)1-x(Bi2Te3)x compounds, synthesized by hot press sintering method

We investigated the thermoelectric (TE) properties of GeTe by incorporating Bi2Te3 in the GeTe system. The addition of Bi2Te3 into the GeTe matrix leads to phonon scattering caused by stacking fault or structural defect, resulting in a substantial reduction in lattice thermal conductivity κlat. We observed a significant decrease in total thermal conductivities in the vertical and horizontal pressing directions, respectively. The optimized carrier properties and low thermal conductivity in the composites have led to a remarkable improvement in the dimensionless figure of merit (zT) 1.26 at 650 K. This study highlights the potential for creating high-performance thermoelectric materials through GeTe-Bi2Te3 compounds, emphasizing the importance of defect engineering and stacking faults in reducing thermal conductivity and improving thermoelectric efficiency.

Failure mechanism investigation and endurance improvement in Te-rich Ge–Te based ovonic threshold switching selectors

The endurance of ovonic threshold switching (OTS) selectors is a key element for memory application. However, multi-element system for OTS in recent studies will induce element or phase segregation and lead to device failure. Since pure Te based device characterizes relatively high off current, in this work, we studied a Te-rich Ge–Te based OTS selector. We first conducted a failure analysis on Ge–Te based OTS selector. Through first-principles calculations, we found that a relatively larger Ge concentration in the Ge–Te system may lead to a worse device endurance after continuous operation due to the migration of Ge atoms. Experiments further proved that device endurance can be improved more than two orders of magnitude through decreasing Ge concentration and the element segregation is greatly weakened by the composition close to elemental. Finally, a significantly improved endurance of 2 × 1010 was realized in Ge10Te90 based OTS selectors. In the meantime, the Ge10Te90 based OTS selectors show good switching performance and potential for use in memory applications.

Tunable quantum gaps to decouple carrier and phonon transport leading to high-performance thermoelectrics

Thermoelectrics enable direct heat-to-electricity transformation, but their performance has so far been restricted by the closely coupled carrier and phonon transport. Here, we demonstrate that the quantum gaps, a class of planar defects characterized by nano-sized potential wells, can decouple carrier and phonon transport by selectively scattering phonons while allowing carriers to pass effectively. We choose the van der Waals gap in GeTe-based materials as a representative example of the quantum gap to illustrate the decoupling mechanism. The nano-sized potential well of the quantum gap in GeTe-based materials is directly visualized by in situ electron holography. Moreover, a more diffused distribution of quantum gaps results in further reduction of lattice thermal conductivity, which leads to a peak ZT of 2.6 at 673 K and an average ZT of 1.6 (323–723 K) in a GeTe system. The quantum gap can also be engineered into other thermoelectrics, which provides a general method for boosting their thermoelectric performance.

High-performance lead-free cubic GeTe-based thermoelectric alloy

The ferroelectric phase transition of GeTe thermoelectric materials caused by the 4s2 lone-pair electrons of Ge2+ leads to the sudden change of thermal expansion coefficient, which severely restrains its practical applications. Herein, we demonstrate a successful way to suppress the ferroelectric phase-transition temperature down to below room temperature in GeTe. Our approach involves LiSbTe2 alloying, where the charge distribution is manipulated to reduce the influence of the stereochemical activity of the cation lone-pair electrons on the crystal structure. LiSbTe2 alloying can also promote multi-band convergence and enhance phonon scattering. Consequently, a superior ZTave of 1.38 within 300—773 K is achieved in cubic (GeTe)0.70(LiSbTe2)0.30 alloys, which is significantly higher than the reported results for room temperature or near-room temperature cubic GeTe systems so far. Moreover, the calculated conversion efficiency of 17% is also the highest among all nontoxic p-type polycrystalline thermoelectric materials. This work takes a solid step forward in the application of GeTe.

Achieving synergistic performance through highly compacted microcrystalline rods induced in Mo doped GeTe based compounds

Among the lead-free thermoelectric material, germanium telluride (GeTe) has been extensively investigated due to its high thermoelectric performance (ZT) in mid-temperature; however, high p-type carrier density (∼1021 cm−3) hinder its suitability for higher ZT. To enhance the thermoelectric performance of the environmentally favorable GeTe, we explored the Mo doping significantly optimizes the carrier concentration along with uniquely unveiled microcrystalline rods accompanying compact grain boundaries, high-density planar defects, and point defects effectuating all-frequency phonon scattering yields to lower down the thermal conductivity. Furthermore, Sb/Bi co-doping with Mo at the Ge sites predominantly reduces the carrier concentration and thermal conductivity to attain a higher ZT. The co-doping of Bi manifested a more prominent role in achieving the highest ZT of ∼2.3 at 673 K for the sample composition with Ge0.89Mo0·01Bi0.1Te. This study demonstrates an exciting hidden aspect of microstructural modification by forming highly dense microcrystalline rods through Mo doping to achieve high performance in the GeTe system.

Metadynamics sampling in atomic environment space for collecting training data for machine learning potentials

The universal mathematical form of machine-learning potentials (MLPs) shifts the core of development of interatomic potentials to collecting proper training data. Ideally, the training set should encompass diverse local atomic environments but conventional approaches are prone to sampling similar configurations repeatedly, mainly due to the Boltzmann statistics. As such, practitioners handpick a large pool of distinct configurations manually, stretching the development period significantly. To overcome this hurdle, methods are being proposed that automatically generate training data. Herein, we suggest a sampling method optimized for gathering diverse yet relevant configurations semi-automatically. This is achieved by applying the metadynamics with the descriptor for the local atomic environment as a collective variable. As a result, the simulation is automatically steered toward unvisited local environment space such that each atom experiences diverse chemical environments without redundancy. We apply the proposed metadynamics sampling to H:Pt(111), GeTe, and Si systems. Throughout these examples, a small number of metadynamics trajectories can provide reference structures necessary for training high-fidelity MLPs. By proposing a semi-automatic sampling method tuned for MLPs, the present work paves the way to wider applications of MLPs to many challenging applications.

Superior room-temperature power factor in GeTe systems via multiple valence band convergence to a narrow energy range

GeTe systems are excellent p-type thermoelectric materials at medium temperatures (500–800 K). Recently, their electronic properties were enhanced by band convergence of L and Σ valence bands, which increased the power factor at 500–800 K. However, there are only a few thermoelectric materials suitable for lower-temperature waste-heat recovery applications, and their demand is rapidly increasing. Herein, we demonstrated that the well-known L and Σ bands of GeTe and the additional Δ band converge at a desirable energy level to a very narrow energy range as a result of fine-tuning the crystal symmetry and composition by forming a GeTe-Sb2Te3 alloy. Alloying additionally facilitates a decrease in the bandgap, thereby increasing the near-room-temperature power factor. The GeTe-Sb2Te3 alloy achieved very high room-temperature and maximum power factors of 2.6 mW m−1 K−2 at 300 K and 3.3 mW m−1 K−2 at 423 K, respectively, despite GeTe being a medium-temperature thermoelectric material. Thus, we confirmed the applicability of GeTe as a thermoelectric material at room temperature. This study facilitates the development of high-performance near-room-temperature thermoelectric materials based on existing medium-temperature systems.

Quantum Boltzmann equation for non-reversible transient transport in Rashba-Landau coupled low-dimensional GeTe systems

We investigate non-reversible transient electron transport and non-adiabatic decay of resistance in non-ferromagnetic GeTe wires/thin films and find both directional hysteresis and offset of its minimum away from B=0 as a magnetic field B(t) is swept. Our study demonstrates interplay among Rashba spin-orbit coupling, Landau quantization and Zeeman splitting by employing quantum-Boltzmann equation and a non-adiabatic current. We further find non-reciprocal chemical potential fluctuations from quantum-capacitance coupling and show these extremely long-lived fluctuations decayed on the time scale of tens of milliseconds in the 1D case and over hundred seconds in the 2D case.

The Equilibrium Phase Formation and Thermodynamic Properties of Functional Tellurides in the Ag–Fe–Ge–Te System

Equilibrium phase formations below 600 K in the parts Ag2Te–FeTe2–F1.12Te–Ag2Te and Ag8GeTe6–GeTe–FeTe2–AgFeTe2–Ag8GeTe6 of the Fe–Ag–Ge–Te system were established by the electromotive force (EMF) method. The positions of 3- and 4-phase regions relative to the composition of silver were applied to express the potential reactions involving the AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 compounds. The equilibrium synthesis of the set of phases was performed inside positive electrodes (PE) of the electrochemical cells: (−)Graphite ‖LE‖ Fast Ag+ conducting solid-electrolyte ‖R[Ag+]‖PE‖ Graphite(+), where LE is the left (negative) electrode, and R[Ag+] is the buffer region for the diffusion of Ag+ ions into the PE. From the observed results, thermodynamic quantities of AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 were experimentally determined for the first time. The reliability of the division of the Ag2Te–FeTe2–F1.12Te–Ag2Te and Ag8GeTe6–GeTe–FeTe2–AgFeTe2–Ag8GeTe6 phase regions was confirmed by the calculated thermodynamic quantities of AgFeTe2, Ag2FeTe2, and Ag2FeGeTe4 in equilibrium with phases in the adjacent phase regions. Particularly, the calculated Gibbs energies of Ag2FeGeTe4 in two different adjacent 4-phase regions are consistent, which also indicates that it has stoichiometric composition.

Improved thermoelectric performance of GeTe via efficient yttrium doping

Optimization of the carrier concentration by doping/alloying is a key prerequisite for improving thermoelectric performance. In reality, it is, however, challenging to identify effective dopants in some specific systems. Herein, we discovered that Y doping significantly reduced the carrier concentration, enhanced the power factor, and, therefore, boosted thermoelectric performance. The high Y-doping efficiency had origin in the fact that the compensating effect due to Ge vacancies was weaker in comparison to other dopants in GeTe because the slightly increased rhombohedral interaxial angle suggested that the Ge-vacancy concentration would not be altered dramatically. In addition, first-principles calculations revealed that Y doping did not induce significant modifications on the band structure, consistent with electrical transport measurement results. Moreover, Bi doping was found to be more powerful than Sb doping to further increase electrical resistivity and to enhance performance in which the highest peak ZT ∼ 1.8 together with a high average ZT ∼ 1.1 from 300 K to 723 K was achieved. Our work not only highlighted the efficacy of Y doping in enhancing the thermoelectric performance of the GeTe system but also developed advanced GeTe-based materials for power generation applications.

Overview of the Role of Alloying Modifiers on the Performance of Phase Change Memory Materials

Phase change memory (PCM) based on chalcogenide compounds is considered to be an excellent candidate for next-generation memory because of its high speed, low energy consumption, high-density information storage, and durable stability. This has become a topic of general interest during the last two decades. Various systems have been proposed to explore more suitable compositions for PCM applications such as Ge-Te and Sb-Te binary alloys, GeTe-Sb2Te3 pseudo-binary alloys, and novel Te-free Sb-based alloys. In spite of this, the comprehensive performance of these pure systems still cannot fully satisfy the need for commercialization. Modification of alloys is an effective approach to enhance performance; this has been investigated significantly. Nevertheless, there are relatively few reports that directly provide integrated data on the effect of various alloying modifiers on the properties of PCM materials to obtain the relevant information conveniently, understand the role of chemical tailors on the properties of PCM materials deeply, and select modifiers matching the matrix systems efficiently. To achieve these goals and facilitate studies on PCM materials, this overview begins by summarizing and analyzing the role of alloying tailors on the performance of representative GeTe, Sb2Te3, Ge2Sb2Te5, and Sb-based systems containing Te (Sb2Te, Sb3Te, Sb4Te) and without Te (Sb-M), followed by comparing the comprehensive performance of optimized systems.

Realizing high figure of merit plateau in Ge1-xBixTe via enhanced Bi solution and Ge precipitation

We reported in this work that both a broad plateau (623–773 K) of high figure of merit ZT (over 1.8) and a decent peak ZT∼2.0 at 773 K can be simultaneously achieved in Bi-doped Ge1-xBixTe sample as x = 0.07 when it experienced a water quench process. Distinct from Bi2Te3 alloying, Bi-doping in GeTe not only produces Ge “neutral vacancies” which eventually evolve into planar van der Waals gaps, but also generate residual Ge atoms which precipitate intrinsically as secondary phase. Additional water quench process is concluded to be of two main functions: (i) increasing the solution of Bi in GeTe matrix, and (ii) suppressing the Ge phase aggregation. The former function further modulates the matrix structure and electronic band, resulting in an improved Seebeck coefficient; while the latter one results in increased Ge/GeTe phase boundaries hence enhanced phonon scattering. Besides, the structure of intrinsic Ge precipitation and enhanced Bi solution is thermally stable; the water quenched Ge0.93Bi0.07Te sample can retain the high thermoelectric performance after 2-days annealing at 773 K. Our finding in this work might shed light on future researches in GeTe and related thermoelectric material systems.

Thermal stability and microstructure of germanium antimony telluride thin films under interdiffusion conditions

This paper investigates the interdiffusion behaviors of alloy components of the Ge–Sb–Te system and its influence on the thermal and electrical instability of Ge–Sb–Te alloy films. When SbxTe1−x/Ge bilayer films are annealed at 400 °C, the Ge–Sb–Te intermediate layers are formed at the interface between SbxTe1−x and Ge. This is caused by the interdiffusion of Ge, Sb, and Te. The highest degree of interdiffusion is observed in the Ge–Te system, while the existence of Sb retards the interdiffusion of Ge and Te in the SbxTe1−x/Ge bilayer films. The outdiffusion and sublimation of low-melting-point Sb and Te occur at the annealing temperature of 500 °C, which leads to the instability of the Ge–Sb–Te intermediate layers. The phase-change memory (PCM) device employing a Ge2Sb2Te5 intermediate layer exhibit stable endurance, because of the robustness of Ge2Sb2Te5 compound with respect to the phase separation during the repetitive set–reset cycles.

Thermoelectric properties of GeTe and Sb2Te3 calculated by density functional theory

The GeTe and Sb2Te3 system materials are compound very narrow band gaps semiconductors in the energy range 0.20-2.80 eV and highly regarded as good performance TE materials. We calculated thermoelelctric properties of GeTe and Sb2Te3 system by using the density functional therory (DFT) and Boltzmann transport therory calculations based on QUANTUM ESPRESSO and BoltzTraP package. The Seebeck coefficient, electrical conductivity, thermal conductivity and power factor dependent on Fermi energy were predicted for future experimental of Ge-Sb-Te materials.

Atomic-scale study of the amorphous-to-crystalline phase transition mechanism in GeTe thin films

The underlying mechanism driving the structural amorphous-to-crystalline transition in Group VI chalcogenides is still a matter of debate even in the simplest GeTe system. We exploit the extreme sensitivity of 57Fe emission Mössbauer spectroscopy, following dilute implantation of 57Mn (T½ = 1.5 min) at ISOLDE/CERN, to study the electronic charge distribution in the immediate vicinity of the 57Fe probe substituting Ge (FeGe), and to interrogate the local environment of FeGe over the amorphous-crystalline phase transition in GeTe thin films. Our results show that the local structure of as-sputtered amorphous GeTe is a combination of tetrahedral and defect-octahedral sites. The main effect of the crystallization is the conversion from tetrahedral to defect-free octahedral sites. We discover that only the tetrahedral fraction in amorphous GeTe participates to the change of the FeGe-Te chemical bonds, with a net electronic charge density transfer of ~ 1.6 e/a0 between FeGe and neighboring Te atoms. This charge transfer accounts for a lowering of the covalent character during crystallization. The results are corroborated by theoretical calculations within the framework of density functional theory. The observed atomic-scale chemical-structural changes are directly connected to the macroscopic phase transition and resistivity switch of GeTe thin films.

Te-based chalcogenide materials for selector applications

The implementation of dense, one-selector one-resistor (1S1R), resistive switching memory arrays, can be achieved with an appropriate selector for correct information storage and retrieval. Ovonic threshold switches (OTS) based on chalcogenide materials are a strong candidate, but their low thermal stability is one of the key factors that prevents rapid adoption by emerging resistive switching memory technologies. A previously developed map for phase change materials is expanded and improved for OTS materials. Selected materials from different areas of the map, belonging to binary Ge-Te and Si-Te systems, are explored. Several routes, including Si doping and reduction of Te amount, are used to increase the crystallization temperature. Selector devices, with areas as small as 55 × 55 nm2, were electrically assessed. Sub-threshold conduction models, based on Poole-Frenkel conduction mechanism, are applied to fresh samples in order to extract as-processed material parameters, such as trap height and density of defects, tailoring of which could be an important element for designing a suitable OTS material. Finally, a glass transition temperature estimation model is applied to Te-based materials in order to predict materials that might have the required thermal stability. A lower average number of p-electrons is correlated with a good thermal stability.

Gamma radiation induced effects in floppy and rigid Ge-containing chalcogenide thin films

We explore the radiation induced effects in thin films from the Ge-Se to Ge-Te systems accompanied with silver radiation induced diffusion within these films, emphasizing two distinctive compositional representatives from both systems containing a high concentration of chalcogen or high concentration of Ge. The studies are conducted on blanket chalcogenide films or on device structures containing also a silver source. Data about the electrical conductivity as a function of the radiation dose were collected and discussed based on material characterization analysis. Raman Spectroscopy, X-ray Diffraction Spectroscopy, and Energy Dispersive X-ray Spectroscopy provided us with data about the structure, structural changes occurring as a result of radiation, molecular formations after Ag diffusion into the chalcogenide films, Ag lateral diffusion as a function of radiation and the level of oxidation of the studied films. Analysis of the electrical testing suggests application possibilities of the studied devices for radiation sensing for various conditions.

Phase separation in chalcogenide semiconductors of the Ge-Te system upon thermal cycling

Close-to-eutectic compositions in the Ge-Te system are analyzed and their behavior with repeated heat treatments is studied by means of differential scanning calorimetry and X-ray diffraction. It is shown that repeated heat treatment of the compositions under study leads to phase separation in the material. Assumptions are made about the nature of processes occurring in the materials subjected to heat treatment.

Thermoelectric properties and microstructures of AgSbTe2‐added p‐type Pb0.16Ge0.84Te

AbstractThe alloys in the AgSbTe2GeTe system and the AgSbTe2PbTe system are known as excellent thermoelectric (TE) materials, i.e. TAGS and LAST, respectively. Although the TE properties of the AgSbTe2(Pb,Ge)Te system have not been reported. Here, we show the high‐temperature TE properties of the alloys in the AgSbTe2(Pb,Ge)Te system. The samples of (Ag0.5Sb0.5Te)100−x(Pb0.16Ge0.84Te)x (x = 75, 80, 85, and 90) were prepared and the electrical resistivity (ρ), Seebeck coefficient (S), and thermal conductivity (κ) were examined in the temperature range from room temperature to 720 K. The samples prepared by hot‐press were composed of two phases: the GeTe‐based matrix phase and the PbTe‐based precipitate. These samples exhibited the moderate ρ and S as well as relatively low κ, leading to high thermoelectric figure of merit ZT (= S2T/ρκ, where T is the absolute temperature). The maximum ZT value was 0.84 at 720 K obtained in the sample of x = 80.