Phase Transition In GeTe Research Articles

Realizing a Superior Conversion Efficiency of ≈11.3% in the Group IV-VI Thermoelectric Module.

GeTe-based materials exhibit superior thermoelectric performance, while the development of power generation devices has mainly been limited by the challenge of designing the interface due to the phase transition in GeTe. In this work, via utilizing the low-temperature nano-Ag sintering technique and screening suitable Ti-Al alloys, a reliable interface with excellent connection performance has been realized. The Ti-Al intermetallic compounds effectively inhibit the diffusion process at Ti-34Al/Ge0.9Sb0.1Te interface. Thus, the thickness of the interfacial reaction layer only increases by ≈2.08µm, and the interfacial electrical contact resistivity remains as low as ≈15.2 µΩ cm2 even after 30 days of isothermal aging at 773 K. A high conversion efficiency of ≈11.3% has been achieved in the GeTe/PbTe module at a hot-side temperature of 773 K and a cold-side temperature of 300 K. More importantly, the module's performance and the reliability of the interface remain consistently stable throughout 50 thermal cycles and long-term aging. This work promotes the application of high-performance GeTe materials for thermoelectric power generation.

Molecular dynamics simulation of the ferroelectric phase transition in GeTe: Displacive or order-disorder character

Experimental investigations of the phase transition in GeTe provide contradictory conclusions regarding the nature of the phase transition. Considering growing interest in technological applications of GeTe, settling these disputes is of great importance. To that end, we present a molecular dynamics study of the structural phase transition in GeTe using a machine-learned interatomic potential with ab initio accuracy. First, we calculate the asymmetric shape of the radial distribution function of the nearest-neighbor bonds above the critical temperature, in agreement with previous studies. However, we show that this effect is not necessarily linked to the order-disorder phase transition and can occur as a result of large anharmonicity. Next, we study in detail the static and dynamic properties of the order parameter in the vicinity of the phase transition and find fingerprints of both order-disorder and displacive phase transition.

The origin of the lattice thermal conductivity enhancement at the ferroelectric phase transition in GeTe

The proximity to structural phase transitions in IV-VI thermoelectric materials is one of the main reasons for their large phonon anharmonicity and intrinsically low lattice thermal conductivity κ. However, the κ of GeTe increases at the ferroelectric phase transition near 700 K. Using first-principles calculations with the temperature dependent effective potential method, we show that this rise in κ is the consequence of negative thermal expansion in the rhombohedral phase and increase in the phonon lifetimes in the high-symmetry phase. Strong anharmonicity near the phase transition induces non-Lorentzian shapes of the phonon power spectra. To account for these effects, we implement a method of calculating κ based on the Green-Kubo approach and find that the Boltzmann transport equation underestimates κ near the phase transition. Our findings elucidate the influence of structural phase transitions on κ and provide guidance for design of better thermoelectric materials.

Phase-transition temperature suppression to achieve cubic GeTe and high thermoelectric performance by Bi and Mn codoping

Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit (ZT) of ∼1.5 at 773 K and an average ZT of ∼1.1 from 300 to 773 K in cubic Ge0.81Mn0.15Bi0.04Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications.

Role of vacancies in the high-temperature pseudodisplacive phase transition in GeTe

To date, the high-temperature phase transition R3m$\text{\ensuremath{-}}Fm\phantom{\rule{0.83328pt}{0ex}}\overline{3}\phantom{\rule{0.83328pt}{0ex}}m$ in GeTe is commonly believed to be second-order displacive with an anomalous volume contraction at the phase transition temperature, ${T}_{PT}$, from diffraction measurements. Three main results are here reported: (i) the phase transition is accompanied by latent heat absorption, (ii) in the high-resolution x-ray-diffraction powder pattern the cubic phase appears before the merging of the rhombohedral peaks, and (iii) the cubic phase possesses a larger amount of germanium vacancies than the rhombohedral phase. From results (i)--(iii) we conclude that the phase transition is not purely second-order displacive and that the volume contraction observed at ${T}_{PT}$ can be explained with the formation of vacancies in the cubic phase.

Reversible Phase Transition in Gete through Resonance Bond Swiching

Phase change materials, especially those along the GeTe-Sb2Te3 tie-line (GST), have been widely used in applications of memory devices. Using these materials, information can be stored as optical and electrical change contrast arising from thermally induced structural transitions between the crystalline and amorphous phases. Amorphous phase is obtained by heating the material of crystalline phase upon melting temperature and cooling it fast enough so that it solidify in the amorphous phase. On the other hand, crystalline phase is obtained by heating the material of amorphous phase during relative longer time (a few ns) than amorphization time within appropriate temperature range between crystallization temperature and melting temperature. Contrary to this relatively small difference of heating condition, their physical properties are extremely different from each other. Besides, their fast switching properties and rather good reversibility show promising characteristics for phase change materials as a candidate material for universal memory as well as a most intriguing topic in material society. Despite of these specific material properties, the fundamental reasons for this reversible and fast transition are still in a debate and needs to be understood more considerably as a view point of local structure and orbital environment. Many of the previous studies for understanding phase change mechanism concentrated in comparison of orbital structure between the crystalline phase and amorphous phase. However, the investigation on the modified crystalline phases which is weakly perturbed by thermal variation or external stress could show possible phase transition path. In this study, using uni-axial stress (bending stress), the crystalline GeTe is modulated to have reversible structure deformation which gives rise to drastic change on optical property with resonance bond breaking. Modulated GeTe also shows reversible electrical change caused by orbital rearrangement and irreversible electrical change caused by defect formation. A stress which is relatively smaller thermodynamic perturbation than heat also can be an important variable in phase change mechanism and it might support the reason why both of set and reset state in phase change memory contain various of chemical states.

Anomalous temperature-induced volume contraction in GeTe

The recent surge of interest in phase change materials GeTe, Ge$_2$Sb$_2$Te$_5$, and related compounds motivated us to revisit the structural phase transition in GeTe in more details than was done before. Rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied by high resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. Results of our studies clearly show an anomalous volume contraction of 0.6\% at the phase transition from the rhombohedral to cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.

Soft-phonon mediated structural phase transition in GeTe

Inelastic neutron scattering experiments on powder samples of GeTe together with density functional theory investigations of the phonon dynamics in the low- and high-temperature phases of GeTe crystal are reported. The dispersion of phonons in the high-temperature rocksalt phase show soft branches with the lowest one at the $\ensuremath{\Gamma}$ high-symmetry point. The structural phase transition in GeTe is reconsidered and shown to be driven by the con-densation of exactly three components of the triply degenerate optical transverse soft-phonon mode at the Brillouin zone center. The mechanism proposed allows us to explain the formation of structural distortions in the low-temperature ferroelectric phase of GeTe revealed by various experiments. A displacive nature of the phase change in crystalline GeTe is supported by the results of the present theoretical studies.

Peierls distortion mediated reversible phase transition in GeTe under pressure

With the advent of big synchrotron facilities around the world, pressure is now routinely placed to design a new material or manipulate the properties of materials. In GeTe, an important phase-change material that utilizes the property contrast between the crystalline and amorphous states for data storage, we observed a reversible phase transition of rhombohedral ↔ rocksalt ↔ orthorhombic ↔ monoclinic coupled with a semiconductor ↔ metal interconversion under pressure on the basis of ab initio molecular dynamics simulations. This interesting reversible phase transition under pressure is believed to be mediated by Peierls distortion in GeTe. Our results suggest a unique way to understand the reversible phase transition and hence the resistance switching that is crucial to the applications of phase-change materials in nonvolatile memory. The present finding can also be expanded to other IV-VI semiconductors.

Influence of Sn Migration on phase transition in GeTe and Ge2Se3 thin films

Phase transitions in GeTe/SnSe and Ge2Se3/SnTe are investigated using time resolved x-ray diffraction. GeTe exhibits a structural transition from rhombohedral to the cubic phase at 300 °C, which is ∼100 °C lower than that of pure GeTe. This is facilitated by incorporation of Sn from SnSe. Sn migration is observed explicitly in Ge2Se3/SnTe by separation of SnSe phase. Amorphous Ge2Se3 is also found to crystallize at a lower temperature of 300 °C resulting in orthorhombic GeSe and monoclinic GeSe2. Thus, inclusion of a Sn containing layer may offer a means to tailor phase transition in Ge-chalcogenide thin films for phase change memory applications.

EXAFS Study of the Influence of Impurities on the Phase Transition in GeTe

EXAFS technique was used to study the local environment of Se, Pb, Sn, Mn and In impurity atoms in germanium telluride. These data were used to clarify the mechanisms by which the impurity influence the phase transition temperature T c . It was shown that the rate of decrease of T c in doped GeTe is determined primarily by the charge state of impurity, the type of the chemical bond between impurity and main atoms of the crystal and by the size of impurity atoms.

Influence of Se, Pb and Mn impurities on the ferroelectric phase transition in gete studied by exafs

EXAFS was used to study the local environment of Se, Pb and Mn impurity atoms in germanium telluride. It is found that the three impurity atoms do not occupy fixed on-center positions in GeTe and that the origin and magnitude of the displacements are different. It is shown that the rate of decrease of Tc when doping GeTe with these impurities is determined primarily by the size of impurity atoms and the type of chemical bond between impurity and main atoms of the crystal.

Ab initio determination of a structural phase transition temperature.

The temperature of the structural phase transition in GeTe is calculated complete ab initio from a model lattice-dynamical Hamiltonian that is constructed from microscopic quantum-mechanical total-energy calculations. The phase transition in the model system is studied through a renormalization-group-theory approach, leading to the prediction of a fluctuation-driven first-order transition at 657\ifmmode\pm\else\textpm\fi{}100 K.

Neutron diffraction study on the structural phase transition in GeTe

The structural phase transition in GeTe has been investigated by neutron diffraction in the temperature range 295-716 K. Accurate positional and thermal parameters have been obtained as a function of temperature both in the rhombohedral hR2 phase(binary analogue of grey arsenic structure A7) and in the cubic CF8 phase (NaCl-type, B1). The temperature variation of the rhombohedral distortion and the relative shifts of the Ge and Te substructures show that the structural phase transition in GeTe is a displacive type.