Rhombohedral GeTe Research Articles

Efficient rhombohedral GeTe thermoelectrics for low-grade heat recovery

The existence of Ge vacancy induces an inherently hole concentration up to 1021 cm−3 in pristine thermoelectric GeTe, which results in a need of ∼10 % aliovalent dopants for optimization. However, the extra carrier scattering by the dopants would notably deteriorate the carrier mobility. In this work, CuSbTe2-alloying is revealed to increase the formation energy of Ge-vacancy, leading to a rapid reduction in hole concentration down to 1.3 × 1020 cm−3 but with a mobility above 100 cm2/V-s. Along with a further Pb-substitution at Ge sites for lattice thermal conductivity reduction and band alignment, a peak figure of merit zT of ∼2.4 at 625 K is eventually realized in the rhombohedral structure, leading to a specific power density of as high as 70 W/m and a conversion efficiency of ∼10 % under a temperature difference of 302 K. This illustrates r-GeTe as truly efficient thermoelectrics particularly for low-grade heat recovery applications.

Achieving high carrier mobility and thermoelectric performance in nearly twin-free rhombohedral GeTe (00l) films

GeTe-based thermoelectric (TE) films have garnered significant attentions due to their promising TE performance near room temperature. However, it is challenging to further optimizing the TE performance due to the inferior carrier mobility (μ) and the excessively high hole density (p). Herein, we developed a novel method based on molecular beam epitaxy technique to successfully fabricate nearly twin-free GeTe (00l) films via incorporating Bi2Te3 buffer layers to alleviate epitaxial strain. Consequently, μ was significantly enhanced. Additionally, through comprehensively investigating the processing conditions, we found that substrate temperature and Te/GeTe flux ratio can shape intrinsic atomic defects and further decrease p. With the optimal synthesis and processing conditions, the GeTe film achieves optimized p of 3.44 × 1020 cm−3 and a high μ of 73.31 cm2/V/s, which lead to the highest room-temperature power factor of 2.67 mWm/K2, outperforming the values of other GeTe films. This work provides important guidance on fabricating twin-free GeTe films and on further improving their TE performance.

Tuning the electronic structure of rhombohedral and cubic GeTe for thermoelectric application: Influence of molybdenum doping

Substitutional doping to engineer the electronic structure of materials has received high prominence in developing high performance thermoelectric materials. Herein, we study the effect of molybdenum doping on the electronic structure of GeTe and provide insights into the observed enhancement of thermoelectric performance. We discover that Mo doping has a huge impact on the band structure of both rhombohedral and cubic phase of GeTe. In addition to increasing the band gap, Mo doping causes huge increase in the density of states near the Fermi level. We also notice convergence of valence sub-bands and hyperconvergence of conduction sub-bands besides introduction of multiple carrier valleys promoting transport in both p and n-type materials. These features undoubtedly make Mo doping a beneficial approach in the development of lead free GeTe thermoelectrics.

Anomalous enhancement of thermoelectric performance in GeTe with specific interaxial angle and atomic displacement synergy

Discovery and engineering of novel phase structures pave new avenues in the study of phonon-electron decoupling in thermoelectric materials, with the final aim of enhancing their figure of merit ZT . Here, we report a metastable GeTe phase that shows coexisting cubic and rhombohedral phases and enhances the ZT of GeTe-based alloys to ∼0.8 at 300 K and ∼1.3 at 373 K. This is comparable with the state-of-the-art Bi 2 Te 3 -based alloys in the low-temperature range. The ZT enhancement shows a positive correlation with an increased contribution of a special rhombohedral GeTe phase with synergetic specific interaxial angle and atomic displacement, which triggers a multiple-band convergence for enhancing carrier mobility. Moreover, quenching-induced extra phase boundaries and enhanced anharmonicity may cause extra phonon scattering that reduces the lattice thermal conductivity. This work highlights the importance of precision designing phase structures for solving the phonon-electron coupling problem in thermoelectric materials. • A metastable GeTe with coexisting cubic and rhombohedral phases is revealed • An r-GeTe with a specific interaxial angle and atomic displacement is extracted • Weight carrier mobility can be dramatically increased by engineering this phase • The obtained thermoelectric performance is comparable with Bi 2 Te 3 Phase engineering provides an avenue to realize phonon-electron decoupling in thermoelectric materials. Wang et al. reveal a metastable GeTe with coexisting cubic and rhombohedral phases where the rhombohedral GeTe with synergetic specific interaxial angle and atomic displacement triggers multiple-band convergence for sharply increasing the carrier mobility.

Synergistically Optimized Thermal Conductivity and Carrier Concentration in GeTe by Bi-Se Codoping.

The GeTe compound has been revealed to be an outstanding thermoelectric compound, while its inherent high thermal conductivity restricts further improvement in its performance. Herein, we report a study on the synergistic optimization of the thermoelectric performance of GeTe by Bi-Se codoping. It is shown that the introduction of Bi decreases the carrier concentration and increases the structural parameter of the interaxial angle. With Se doping in the Te site, the lattice thermal conductivity is markedly reduced, while the carrier mobility is slightly influenced. Compared with the singly Se-doped GeTe, the Ge1-xBixTe1-ySey samples are more closed to a cubic phase, as indicated by the larger interaxial angle. On account of the reduction of carrier concentration and thermal conductivity, a ZTmax of 1.80 at 665 K and a high ZTave of 1.39 (400-800 K) are obtained in Ge0.94Bi0.06Te0.85Se0.15. This work reveals that the interaxial angle is vital to the performance optimization of rhombohedral GeTe.

Localized crystal imperfections coupled with phase diagram engineering yield high-performance rhombohedral GeTe thermoelectrics

Cubic β-GeTe alloys are apt to hoist the conversion efficiency for mid-temperature thermoelectric (TE) generators with their remarkably high peak figure-of-merit (zT) values at T > 700 K. Nevertheless, the TE device that consolidated by β-GeTe alloys have existed concerns as the rhombohedral α-GeTe to β-GeTe phase-transition leverage over the workability and thermal stability. Therefore, research efforts are redirected towards the high-performance α-GeTe, which stabilizes at a rhombohedral lattice below 700 K. By incorporating the maximal Bi solubility, the α-GeTe shows severe lattice distortion without forming the undesired impurities that affect the transport properties and deteriorate the thermal stability. The α-GeTe with high-dose Bi fulfills the counterbalance between low thermal conductivity κ and elevated power factor PF = S2ρ−1. Herein, the Bi0.1Ge0.9Te crystal attains the peak zT of 1.5 at 625 K and 1.9 at 713 K for its rhombohedral and cubic state, respectively. The Bi0.1Ge0.9Te features hierarchical twinning accompanied with dense stacking faults which explains its ultra-low lattice thermal conductivity κL in the temperature range of 300 K–700 K. The trade-off between incompatible low-κ and high-PF could be optimized via the phase diagram and defect engineering, which synergistically open a new category for TE performance advancement.

Achieving High Thermoelectric Performance by NaSbTe2 Alloying in GeTe for Simultaneous Suppression of Ge Vacancies and Band Tailoring

AbstractGeTe alloys have attracted wide attention due to their high conversion efficiency. However, pristine GeTe possesses intrinsically massive Ge vacancies, leading to a very high hole concentration (1021 cm−3). Herein, a decreased carrier concentration is realized by alloying NaSbTe2 in GeTe due to the increased formation energy of Ge vacancies. This alloying also lowers energy separation between the valence bands in the rhombohedral GeTe and induces two extra valence band pockets around the Fermi surface along Γ‐L and L‐W in the cubic GeTe, all of which contributes to the higher power factors over a wide temperature range. Combined with the low lattice thermal conductivities due to plenty of dislocations and strains as a result of the crystallographic disorder of Na, Ge, and Sb, a maximum zT ≈ 2.35 at 773 K and a zTave of 1.33 from 300 to 773 K are achieved in (GeTe)90(NaSbTe2)10.

Electronic, Optical, and Thermoelectric Properties of Bulk and Monolayer Germanium Tellurides

Electronic, optical, and thermoelectric properties of germanium tellurides (GeTe) were investigated through a series of first-principles calculations of band structures, absorption coefficients, and thermoelectric transport coefficients. We consider bulk GeTe to consist of cubic and rhombohedral phases, while the two-dimensional (2D) GeTe monolayers can form as a 2D puckered or buckled honeycomb crystals. All of the GeTe variants in the bulk and monolayer shapes are excellent light absorbers in a wide frequency range: (1) bulk cubic GeTe in the near-infrared regime, (2) bulk rhombohedral GeTe and puckered monolayer GeTe in the visible-light regime, and (3) buckled monolayer GeTe in the ultraviolet regime. We also found specifically that the buckled monolayer GeTe exhibits remarkable thermoelectric performance compared to the other GeTe phases due to a combination of electronic band convergence, a moderately wide band gap, and unique 2D density of states from the quantum confinement effect.

Unusual Thermoelectric Anisotropy in Rhombohedral Germanium Telluride

Developing high performance thermoelectrics to convert waste heat into useful electrical power is an important challenge that has the potential to impact our energy future. This requires exploring and discovering new materials with enhanced electronic and thermal (phonon) transport properties. Advances in computational modeling now allow the thermoelectric characteristics of materials to be calculated fully predictively, and thus represents a powerful tool to be partnered with experimental efforts.In this talk, I will present our recent findings on rhombohedral GeTe, a layered quasi-2D material predicted to have unusual anisotropic transport properties [1]. Our approach combines density functional theory with the Boltzmann transport equation, to extract the detailed electron and phonon dispersions along with the electron-phonon scattering rates, in order to predict the thermoelectric transport coefficients. Our results show that the electrons preferentially conduct along the cross-plane direction, while the holes favor the in-plane direction. Interestingly, this feature enhances the thermoelectric figure-of-merit ZT for n-type GeTe, since the conductivity (and power factor) is maximized perpendicular to the atomic layers while the lattice (phonon) thermal conductivity is minimized - leading to a three-fold increase in ZT. The cross-plane power factor and mobility in n-GeTe reach roughly 32 μW/cm-K2 and 500 cm2/V-s, respectively. An analysis of the band structure reveals that the large cross-plane transport originates from high-velocity conduction states, formed by the Ge p-orbitals that span across the interstitial region. These findings illustrate how the dominant electron and phonon transport directions are effectively decoupled in n-GeTe; a feature that can potentially be found in other materials to achieve enhancements in thermoelectric performance.[1] V. Askarpour and J. Maassen, “Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe”, Phys. Rev. B 100, 075201 (2019). Figure 1

GeTe-Based Alloy Films Prepared by a Room Temperature Solution Process.

This article describes the preparation of GeTe-based alloy films using a solution-based technique. The dissolution behavior of GeTe was initially examined by comparing the weight loss of GeTe powder in different solvents, and it was found that, unlike in the cases of n-butylamine and NH₄OH, KOH fully dissolved GeTe to form an agglomerate-free solution. X-ray diffraction analysis revealed that the reaction between GeTe and KOH resulted in the formation of rhombohedral GeTe, cubic GeTe₄, and hexagonal Te structures after drying. GeTe-based alloy films were then prepared by the spin coating of the GeTe-containing solutions on a silicon substrate. The surface morphology and reflectance properties of the prepared films were found to be highly dependent on the spin speed, with optimization of the spin coating parameter resulting in the deposition of a continuous and smooth film.

High‐Performance GeTe‐Based Thermoelectrics: from Materials to Devices

AbstractHigh‐performance GeTe‐based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to low‐temperature rhombohedral GeTe, the high‐symmetry and high‐temperature cubic GeTe has a low energy offset betweenLandΣpoints of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit,ZTvalues, of GeTe‐based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe‐based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe‐based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe‐based thermoelectrics.

Electronic and elastic properties of rhombohedral GeTe: Exfoliation energy of a monolayer from (111) surface

In the present study, the electronic and elastic properties of rhombohedral germanium telluride (GeTe) - a low temperature phase (space group R3m), have been reported using density functional theory with Rabe Rappe Kaxiras Joannopoulos ultrasoft pseudopotentials. The calculated results are in good agreement with the experimentally reported results. The exfoliation energy for a GeTe monolayer from the (111) surface of bulk GeTe has been calculated with a much lesser computation cost and the results show that ∼0.66 J/m2 energy is required to lift off a GeTe monolayer and also suggests the study to minimize the exfoliation energy to enhance the feasibility of the formation of GeTe monolayer.

Band and Phonon Engineering for Thermoelectric Enhancements of Rhombohedral GeTe.

Rhombohedral GeTe can be approximated as the directional distortion of the cubic GeTe along [111]. Such a symmetry-breaking of the crystal structure results in an opposite arrangement in energy of the L and Σ valence bands, and a split of them into 3L+1Z and 6Σ+6η, respectively. This enables a manipulation of the overall band degeneracy for thermoelectric enhancements through a precise control of the degree of crystal structure deviating from a cubic structure for the alignment of the split bands. Here, we show the effect of AgBiSe2-alloying on the crystal structure as well as thermoelectric transport properties of rhombohedral GeTe. AgBiSe2-alloying is found to not only finely manipulate the crystal structure for band convergence and thereby an increased band degeneracy, but also flatten the valence band for an increased band effective mass. Both of them result in an increased density of state effective mass and therefore an enhanced Seebeck coefficient along with a decreased mobility. Moreover, a remarkably reduced lattice thermal conductivity of ∼0.4 W/m-K is obtained due to the introduced additional point defect phonon scattering and bond softening by the alloying. With the help of Bi-doping at the Ge site for further optimizing the carrier concentration, thermoelectric figure of merit, zT, of ∼1.7 and average zTave of ∼0.9 are achieved in 5% AgBiSe2-alloyed rhombohedral GeTe, which demonstrates this material as a promising candidate for low-temperature thermoelectric applications.

Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe

In this study, we calculate the $T$=300 K scattering and thermoelectric transport properties of rhombohedral GeTe using first-principles modeling. The room-temperature phase of GeTe has a layered structure, with cross-plane and in-plane directions oriented parallel and perpendicular to [111], respectively. Based on rigorous electron-phonon scattering, our transport calculations reveal unusual anisotropic properties; n-type GeTe has a cross-plane electrical conductivity that is roughly 3$\times$ larger than in-plane. p-type GeTe, however, displays opposite anisotropy with in-plane conducting roughly 2$\times$ more than cross-plane, as is expected in quasi-2D materials. The power factor shows the same anisotropy as the electrical conductivity, since the Seebeck coefficient is relatively isotropic. Interestingly, cross-plane n-GeTe shows the largest mobility and power factor approaching 500 cm$^2$/V-s and 32 $\mu$W/cm-K$^2$, respectively. The thermoelectric figure-of-merit, $zT$, is enhanced as a result of this unusual anisotropy in n-GeTe since the lattice thermal conductivity is minimized along cross-plane. This decouples the preferred transport directions of electrons and phonons, leading to a threefold increase in $zT$ along cross-plane compared to in-plane. The n-type anisotropy results from high-velocity electron states formed by Ge p-orbitals that span across the interstitial region. This surprising behavior, that would allow the preferential conduction direction to be controlled by doping, could be observed in other quasi-2D materials and exploited to achieve higher-performance thermoelectrics.

Enhanced thermoelectric performance through crystal field engineering in transition metal–doped GeTe

The change of multivalence band structure configuration between rhombohedral and cubic phase in GeTe offers additional dimension to modify its thermoelectric properties. Here, we report the synergetic optimization of electronic and thermal transport properties in rhombohedral GeTe doped with transition metal Ti. The Seebeck coefficient of Ge1-xTixTe is significantly increased, and the corresponding thermal conductivity is decreased. The structure refinement shows that Ti doping could reduce the lattice constant c/a ratio. Density functional theory calculations demonstrate that the affected crystal field rather than Ti orbitals is contributing to the valence band convergence and a Seebeck coefficient enhancement. Further optimization incorporates the effects of Bi substitution for reducing the carrier concentrations and introducing more point defects. This work not only confirms the transition metal elements as promising dopants for GeTe-based materials but also indicates that the strategy of manipulating the crystal field effect can be exploited as a direct but effective route for improving thermoelectric performance.

Persistence of the R3m Phase in Powder GeTe at High Pressure and High Temperature

As a phase-change material (PCM), rhombohedral GeTe (space group R3m) was believed to transform to the cubic rock-salt phase (B1) at 3–4 GPa, associated with the disappearance of a Peierls distortion. However, using a combination of synchrotron X-ray diffraction and theoretical calculations, we found that the R3m phase persists from ambient pressure up to pressures of about 15.8 GPa, in contrast to previous reports. Neither was the B1 phase observed in a heating X-ray powder diffraction experiment. The spurious transformation from R3m to B1 is caused by changes to the compression ratio of lattice parameters in the R3m phase under high pressure/temperature. These findings provide insight into transitions of PCMs, relevant to other materials undergoing displacive transitions under high pressure/temperature.

High-Performance GeTe Thermoelectrics in Both Rhombohedral and Cubic Phases.

GeTe experiences phase transition between cubic and rhombohedral through distortion along the [111] direction. Cubic GeTe shares the similarity of a two-valence-band structure (high-energy L and low-energy Σ bands) with other cubic IV-VI semiconductors such as PbTe, SnTe, and PbSe, and all show a high thermoelectric performance due to a high band degeneracy. Very recently, the two valence bands were found to switch in energy in rhombohedral GeTe and to be split due to symmetry-breaking of the crystal structure. This enables the overall band degeneracy to be manipulated either by the control of symmetry-induced degeneracy or by the design of energy-aligned orbital degeneracy. Here, we show Sb-doping for optimizing carrier concentration and manipulating the degree of rhombohedral lattice distortion to maximize the band degeneracy and then electronic performance. In addition, Sb-doping significantly promotes the solubility of PbTe, enhancing the scattering of phonons by Ge/Pb substitutional defects for minimizing the lattice thermal conductivity. This successfully realizes a superior thermoelectric figure of merit, zT of >2 in both rhombohedral and cubic GeTe, demonstrating these alloys as top candidates for thermoelectric applications at T < 800 K. This work further sheds light on the importance of crystal structure symmetry manipulation for advancing thermoelectrics.

Crystallization behavior of (GeTe4)x(GaTe3)100-x glasses for far-infrared optics applications

Differential scanning calorimetry (DSC), X-ray diffraction (XRD), infrared microscopy and Raman spectroscopy were used to study the crystallization behavior of the (GeTe4)x(GaTe3)100-x glasses for far-infrared optics. Two independent overlapping crystallization processes were found – the initial surface-located precipitation of hexagonal Te and Ga2Te5 phases, followed by formation of the rhombohedral GeTe phase. The initial precipitation process, and in particular the formation of the Ga2Te5 phase, was found to be catalyzed by presence of mechanically induced defects. Finely powdered materials with higher GaTe3 content also exhibited more pronounced separation of the two crystallization sub-processes. Glass stability of the prepared glasses was evaluated in terms of the Hrubý criterion - the (GeTe4)86(GaTe3)14 composition was found to be the most stable and most resilient to the negative crystallization-enhancing influence of structure defects. Pros and cons of the compositional evolution of the crystallization behavior (determined via full kinetic description of the involved crystallization sub-processes and kinetic prediction of the crystallization behavior) were discussed with regard to the ceramics and glass-ceramics applications. Glasses with low GaTe3 content appear to be most suitable for preparation of fully ceramic materials, whereas glasses with high GaTe3 content seem to be most suitable for the glass-ceramics applications.

Coupling between acoustic and soft transverse optical phonons leads to negative thermal expansion of GeTe near the ferroelectric phase transition

GeTe is a well-known ferroelectric and thermoelectric material that undergoes a structural phase transition from a rhombohedral to the rocksalt structure at $\sim 600-700$ K. We present a first principles approach to calculate the thermal expansion of GeTe in the rhombohedral phase up to the Curie temperature. We find the minimum of the Helmholtz free energy with respect to each structural parameter in a manner similar to the traditional Gr{\"u}neisen theory, and account for the temperature dependence of elastic constants. We obtain the temperature variation of the structural parameters of rhombohedral GeTe in very good agreement with experiments. In particular, we correctly reproduce a negative volumetric thermal expansion of GeTe near the phase transition. We show that the negative thermal expansion is induced by the coupling between acoustic and soft transverse optical phonons, which is also responsible for the low lattice thermal conductivity of GeTe.

Slight Symmetry Reduction in Thermoelectrics

Thermoelectric research on germanium telluride (GeTe) has been mainly focused on the enhancement of its performance in the high-temperature cubic phase since the 1960s. Recently in Joule, Pei and co-workers achieved an unprecedented thermoelectric figure of merit in rhombohedral-phase GeTe by exploiting slight symmetry breaking in the structure, which simultaneously improved the electronic properties and reduced the lattice thermal conductivity.