Crystallization Of GeTe Research Articles

The Effect of Sputtering Sequence Engineering in Superlattice-like Sb-Rich-Based Phase Change Materials.

This paper presents a comprehensive investigation into the thermal stability of superlattice-like (SLL) thin films fabricated by varying the sputtering sequences of the SLL [Ge8Sb92(25nm)/GeTe(25nm)]1 and SLL [GeTe(25nm)/Ge8Sb92(25nm)]1 configurations. Our results reveal significantly enhanced ten-year data retention (Tten) for both thin films measured at 124.3 °C and 151.9 °C, respectively. These values surpass the Tten of Ge2Sb2Te5 (85 °C), clearly demonstrating the superior thermal stability of the studied SLL configurations. Interestingly, we also observe a distinct difference in the thermal stability between the two SLL configurations. The superior thermal stability of SLL [GeTe(25nm)/Ge8Sb92(25nm)]1 is attributed to the diffusion of the Sb precipitated phase from Ge8Sb92 to GeTe. This diffusion process effectively reduces the impact of the Sb phase on the thermal stability of the thin film. In contrast, in the case of SLL [Ge8Sb92(25nm)/GeTe(25nm)]1, the presence of the Sb precipitated phase in Ge8Sb92 facilitates the crystallization of GeTe, leading to reduced thermal stability. These findings underscore the significant influence of the sputtering sequence on the atomic behavior and thermal properties of superlattice-like phase change materials. Such insights provide a robust foundation for the design and exploration of novel phase change materials with improved thermal performance.

Thermal conductivity of GeTe crystals based on machine learning potentials

GeTe has attracted extensive research interest for thermoelectric applications. In this paper, we first train a neuro-evolution potential (NEP) based on a dataset constructed by ab initio molecular dynamics, with the Gaussian approximation potential (GAP) as a reference. The phonon density of states is then calculated by two machine learning potentials and compared with density functional theory results, with the GAP potential having higher accuracy. Next, the thermal conductivity of a GeTe crystal at 300 K is calculated by the equilibrium molecular dynamics method using both machine learning potentials, and both of them are in good agreement with the experimental results; however, the calculation speed when using the NEP potential is about 500 times faster than when using the GAP potential. Finally, the lattice thermal conductivity in the range of 300 K–600 K is calculated using the NEP potential. The lattice thermal conductivity decreases as the temperature increases due to the phonon anharmonic effect. This study provides a theoretical tool for the study of the thermal conductivity of GeTe.

Electron Diffraction Study of the Structural Changes in a Thin GeTe Crystal Exposed to High-Power Femtosecond Laser Radiation

Electron Diffraction Study of the Structural Changes in a Thin GeTe Crystal Exposed to High-Power Femtosecond Laser Radiation

Ultrathin GeTe Crystal in a Strong Femtosecond Laser Field: Manifestation of a Quantum Size Effect

The behavior of a thin-film GeTe crystal induced by intense femtosecond laser pulses (μm) has been studied using a pulsed electron diffractometer. The sample is an annealed 20-nm GeTe film on a copper grid with a carbon coating. It has been found that laser ablation results in the formation of an ultrathin GeTe crystal (assumingly, GeTe monolayer) with a high radiation resistance. Possible reasons for the detected nanosize effect are discussed.

Ultrathin GeTe Crystal in a Strong Femtosecond Laser Field: Manifestation of a Quantum Size Effect

The behavior of a thin-film GeTe crystal induced by intense femtosecond laser pulses (lambda = 0.8{kern 1pt} μm) has been studied using a pulsed electron diffractometer. The sample is an annealed 20-nm GeTe film on a copper grid with a carbon coating. It has been found that laser ablation results in the formation of an ultrathin GeTe crystal (assumingly, GeTe monolayer) with a high radiation resistance. Possible reasons for the detected nanosize effect are discussed.

Understanding crystallization in undoped and nitrogen doped GeTe thin films using substrate curvature measurements

Herein, we study the crystallization of undoped and nitrogen doped amorphous GeTe thin films (slightly rich in Ge) obtained by sputtering using substrate curvature measurements to understand the underlying mechanisms controlling stress evolution in the film throughout the phase transformation. At temperatures below crystallization temperature, amorphous films showed stress relaxation and the stress gradually became tensile with annealing time. The GeTe samples show a two-step crystallization wherein amorphous GeTe crystallized first (at the crystallization temperature Tx) followed by crystallization of excess Ge (Ge precipitation) at -Tx+50 °C. Upon GeTe crystallization, a sharp increase in the tensile stress is explained using a coalescence mechanism. This interpretation resolves the issue of the discrepancy between the measured stress buildup reported by several authors and the predicted stress jump from elastic accommodation of density change. The precipitation of excess Ge (from amorphous to crystalline) along grain boundaries in GeTe leads to compressive stress build-up. Nitrogen doping affects both the GeTe and Ge crystallization events leading to lesser tensile and compressive stress. The models for stress relaxation in the amorphous phase, stress build-up due to GeTe, and excess Ge crystallization are discussed.

Multistep Crystallization of Ge‐Rich GST Unveiled by In Situ synchrotron X‐ray diffraction and (scanning) transmission electron microscopy

Among many phase change materials, Ge‐rich GeSbTe (GST) alloys are of considerable interest due to their high thermal stability, a specification required for the next generation of embedded digital memories. This stability results from the fact that these alloys do not crystallize congruently but experience phase separation forming Ge and GST‐225 nanocrystals upon crystallization. However, the details of the crystallization process remain unclear. Combining in situ X‐ray diffraction studies during isothermal annealing and ex situ (scanning) transmission electron microscopy ((S)TEM) observations, the successive phases through which these alloys crystallize are identified. At low temperature (310 °C), the homogeneous amorphous material undergoes phase separation during wherein small regions of different Ge contents are formed. After a long incubation time, Pnma GeTe embryos first crystallize and trigger the heterogeneous crystallization of the Ge cubic phase. While the Ge phase progressively builds up through the addition of new small Ge crystals, cubic GeTe forms. At this point, the microstructure ceases to evolve, and Sb is still dispersed and contained within some remaining amorphous matrix surrounding Ge and GeTe crystals. Higher annealing temperatures (typically 400 °C) are needed to force Sb to diffuse and get incorporated into the GeTe grains to form cubic Ge2Sb2Te5.

Anisotropy and thermal properties in GeTe semiconductor by Raman analysis.

Low-symmetric GeTe semiconductors have attracted wide-ranging attention due to their excellent optical and thermal properties, but only a few research studies are available on their in-plane optical anisotropic nature that is crucial for their applications in optoelectronic and thermoelectric devices. Here, we investigate the optical interactions of anisotropy in GeTe using polarization-resolved Raman spectroscopy and first-principles calculations. After determining both armchair and zigzag directions in GeTe crystals by transmission electron microscopy, we found that the Raman intensity of the two main vibrational modes had a strong in-plane anisotropic nature; the one at ∼88.1 cm-1 can be used to determine the crystal orientation, and the other at ∼124.6 cm-1 can reveal a series of temperature-dependent phase transitions. These results provide a general approach for the investigation of the anisotropy of light-matter interactions in low-symmetric layered materials, benefiting the design and application of optoelectronic, anisotropic thermoelectric, and phase-transition memory devices based on bulk GeTe.

New insights in GeTe growth mechanisms

The compound GeTe, despite its simple stoichiometry, is rather unconventional and has been investigated both from a fundamental and technological perspective: it is of high interest for several technologies such as data-storage (phase change memories) and thermoelectricity. The understanding of GeTe growth is thus a key issue for technological applications and fundamental understanding. In this work, GeTe crystallization kinetics is compared to Ge/Te reactive diffusion kinetics using in situ X-ray diffraction measurements, as well as in situ transmission electron microscopy. GeTe crystallization from an amorphous solid solution exhibiting the stoichiometry of the compound GeTe is found to occur at the same temperature as for the reactive diffusion of an amorphous Ge layer on top of a polycrystalline Te layer. Furthermore, GeTe growth is of tridimensional type in the two cases, and can be modeled by the Johnson-Mehl-Avrami-Kolmogorov model. Using this model, the activation energies of nucleation and growth were determined for both crystallization and reactive diffusion. The results suggest that GeTe exhibits nucleation/reaction kinetics unusually low compared to atomic transport kinetics, contrasting with other germanides.

Temperature dependence of capacitance–voltage characteristics of germanium telluride thin films

Temperature-dependent capacitance–voltage and conductance–voltage measurements of 200-nm-thick germanium telluride (GeTe) thin films deposited via direct current magnetron sputtering were investigated. Measurements were performed across a temperature range of 27–357 °C, with a sweep of voltage values from −20 to +20 V at an operating frequency of 1 MHz. An amorphous–crystalline transition temperature (Tc) of 197 °C was found. Below Tc, the GeTe films were highly resistive, and their capacitance was insensitive to the applied bias voltage and temperature. Above Tc, the capacitance showed strong dependence on temperature, owing to the triggering of GeTe crystallization incorporating nucleation and growth processes. Applied bias voltage did not show a profound impact on the GeTe capacitance even after crystallization, but it had an observable impact on the film conductance beyond 250 °C. Moreover, the GeTe thin film capacitance increased by a factor of 13 upon crystallization. Hall-effect measurements revealed that the film conductivity increased upon crystallization, as did the hole concentration and mobility. Cross-sectional transmission electron microscopy demonstrated that the GeTe thin films crystallize via a nucleation-dominated crystallization mechanism.

Synthesis mechanism and magnetoresistance effect of millimeter-sized GeTe faceted crystals

GeTe materials have a wide range of applications in the fields of thermoelectrics, phase-change switching or storage, ferroelectrics, spintronics, and so on. In this paper, millimeter-sized GeTe faceted crystals are synthesized through heterogeneous chemical reactions without metal catalysts or transport agents. Most of the crystals have hexagonal surfaces, while a few crystals have rectangular surfaces, which are dominated by the closest packed (111) planes and the low-energy (100) planes, respectively. The 180° and 141°/39° ferroelectric domain walls are found on the (111) plane of hexagonal GeTe crystals by the chemical etching. By studying the products under different conditions, it's inferred that the GeTe faceted crystals are grown below the melting point by the reactions of preferentially deposited Ge motes and Te2/GeTe2 vapor phase, which are caused by the partial dissociation of polycrystalline GeTe. Hexagonal GeTe crystals with (111) plane exhibit non-saturating linear magnetoresistance (MR) of ∼25.9% under 9 T at 4.2 K, but the MR of rectangular crystals with (100) plane is just ∼9.5%. The MR effect of GeTe is likely related to the spin splitting on band structure due to the Rashba effect and the intrinsic inhomogeneity of telluride because of the heavily self-doped characteristic.

Ferroelectric engineering: Enhanced thermoelectric performance by local structural heterogeneity

Although traditional ferroelectric materials are usually dielectric and nonconductive, GeTe is a typical ferroelectric semiconductor, possessing both ferroelectric and semiconducting properties. GeTe is also a widely studied thermoelectric material, whose performance has been optimized by doping with various elements. However, the impact of the ferroelectric domains on the thermoelectric properties remains unclear due to the difficulty to directly observe the ferroelectric domains and their evolutions under actual working conditions where the material is exposed to high temperatures and electric currents. Herein, based on in-situ investigations of the ferroelectric domains and domain walls in both pure and Sb-doped GeTe crystals, we have been able to analyze the dynamic evolution of the ferroelectric domains and domain walls, exposed to an electric field and temperature. Local structural heterogeneities and nano-sized ferroelectric domains are generated due to the interplay of the Sb3+ dopant and the Ge-vacancies, leading to the increased number of charged domain walls and a much improved thermoelectric performance. This work reveals the fundamental mechanism of ferroelectric thermoelectrics and provides insights into the decoupling of previously interdependent properties such as thermo-power and electrical conductivity.

Intrinsic vacancy suppression and band convergence to enhance thermoelectric performance of (Ge, Bi, Sb)Te crystals

GeTe-based alloys have drawn increasing attention because of the excellent thermoelectric (TE) performance and potential applications in mid-temperature range. Although reducing carrier concentration by aliovalent doping plays an important role in maximizing the TE performance of GeTe alloys, modulating the intrinsic Ge vacancies is also indispensable. In this work, the intrinsic Ge vacancies of GeTe crystals were suppressed by the synthesis process of temperature gradient cooling technique (TGCT) and Bi-Sb co-doping. The later aliovalent doping also derives a facilitated band convergence, enhanced density-of-state effective mass, and maintained carrier mobility with optimized carrier concentration, along with enhanced phonon scattering from the fluctuations of mass and strain. Benefiting from enhanced electrical properties and reduced thermal conductivity, Ge0.91Bi0.06Sb0.03Te achieved a maximum ZT of 1.9 at 753 K, together with a comparable average ZT of 1.14 at 300–773 K. The results reveal the potential of the TGCT fabrication and Bi-Sb co-doping on enhancing TE performance of GeTe-based alloys with a strengthened suppression of Ge vacancies.

Cr7Ge33Te60/Hf16Ge6Sb78 Superlattice‐Like Thin Film with Triple‐Phase Transitions for Multilevel Phase‐Change Memory

A multilevel phase‐change memory cell that is based on a CrGeTe(CrGT)/HfGeSb(HfGS) superlattice‐like (SLL) structure is proposed. With increasing temperature in resistance–temperature tests, the SLL thin films exhibit a three‐step phase‐change process that corresponds to the crystallization of Sb, GeTe, and CrGeTe. A stable and reversible three‐step phase change is identified as the key characteristic for realizing multilevel storage. This so‐designed multiple‐crystallization SLL structure is a feasible way to increase the storage density.

High refractive index IR lenses based on chalcogenide glasses molded by spark plasma sintering

In this work, spark plasma sintering is used to mold non conventional chalcogenide glasses of high refractive index at low temperature (<400°C). This equipment, usually used for sintering refractory materials, is presented as efficient for both densification and high precision molding of IR transparent bulks and lenses of telluride glasses. Thermo-mechanical and optical characteristics of the selected Ge25Se10Te65 glass composition were investigated showing a refractive index of 3,12@10 µm and with however a limited resistance to crystallization. Mechanical milling of raw Ge, Se, Te elements leads to a major amorphous phase with the formation of a small proportion of GeTe crystals. Remaining GeTe crystals induce a fast crystallization rate during the sintering process leading to the opacity of the material. SPS flash moldings were then performed using melt quenched glass powders to produce complex lenses. It has been found that the critical parameter to reach optimal IR transparency is mainly the powder granulometry, which should be superior to 100 µm to prevent from MIE scatterings. The possibility of producing high refractive index infrared lenses has been demonstrated even with unstable glasses against crystallization.

Dielectric Spectra and d-bands Structure of Germanium, Tin, and Lead Tellurides

The known experimental absorption spectra were used to calculate the dielectric spectra of GeTe, SnTe, and PbTe crystals in the region of electron transitions from the core d-bands of cations. Their characteristic features and general regularities are revealed. Using the combined Argand diagram method, the dielectric spectra of each crystal were decomposed into six elementary oscillators and their main parameters were determined. The nature of formation of these oscillators is proposed on the basis of the theory of interband and exciton transitions.

Temperature‐Dependent Local Structural Changes of Amorphous Thin Ge20Te80 Film Revealed by In Situ Resistance, X‐Ray Diffraction, and Raman Spectroscopy Studies

Although numerous chalcogenide glass systems including GeTe6 and GexSe100−x have been explored for Ovonic Threshold Switch (OTS) selectors in vertically stackable cross‐point memory applications, yet an improved thermal and structural stability at elevated temperatures remains a key challenge. Herein, a systematic temperature‐dependent experimental investigation on the thermal stability and local structural changes of as‐deposited amorphous thin Ge20Te80 film is conducted for a wide range of temperatures from 25 to 260 °C. The coherent experimental studies reveal that the amorphous phase is stable up to 180 °C, and the crystallization process is initiated above 180 °C, by simultaneous crystallization of Te and GeTe until ≈238 °C as substantiated by temperature‐dependent sheet resistance, X‐ray diffraction, and Raman spectroscopic measurements. Furthermore, the local structural changes over the crystallization process of Ge20Te80 thin film are elucidated by Raman spectroscopy. These experimental findings provide a decisive understanding of the thermal stability and structural evolution of Ge20Te80 thin films toward designing stable OTS selector materials.

Crystallization of amorphous GeTe simulated by neural network potential addressing medium-range order

For the last decade, crystallization kinetics of phase change materials has been intensively investigated by molecular dynamics simulations. In particular, recent machine-learning potentials have advanced microscopic understanding of crystallization behavior of phase change materials by overcoming the computational limit of the density functional theory (DFT). Here we develop neural network potentials (NNP) for GeTe, an archetypal phase change material, and study the crystallization of amorphous GeTe. Consistently with the previous literature, we find that NNP results in a very short incubation time and the crystallization completes within a few nanoseconds, which is at variance with experimental measurement as well as DFT simulations. We show that such deficiencies of NNP originate from overly flat fourfold rings in the amorphous structure. By including explicitly relaxation paths from flat to puckered fourfold rings, we generate a modified NNP, which produces medium-range orders that are more consistent with DFT. Using the modified NNP, crystallization simulations are performed at two densities that represent partially or fully amorphized devices, and temperatures ranging from 500 to 650 K. At both densities, finite incubation times are clearly observed. In particular, the incubation time under the partially amorphized device condition is found to be 7 or 17 ns, which is consistent with experiments. By proposing a method to develop NNPs with correct medium-range order, this work will contribute to simulating phase change materials more accurately and realistically.

Спектры диэлектрической проницаемости и структура d-зон теллуридов германия, олова и свинца

Based on the experimental absorption spectra, the permittivity spectra of GeTe, SnTe, and PbTe crystals were obtained in the region of electron transitions from core d -bands of cations. Their specific features and general behavior are revealed. The permittivity spectra of each crystal were decomposed into six elementary components and their main parameters were determined by using of combined Argand diagrams method. Based on the theory of interband and exciton transitions, the nature of the formation of these oscillators was proposed.

(Invited) Mie-like Scattering of Phonons by a Single Vacancy in IV-VI Compounds

Defects engineering has been a widely used approach for developing low thermal conductivity materials. Also, for some materials (e.g., IV-VI compound), considerably large amount of vacancies was often observed in single crystalline samples. The atomic vacancy in dilute case was assumed as a point defect that can cause the Rayleigh scattering with its rate proportional to the fourth power of phonon frequency. Due to the large dependence of scattering rate on phonon frequency, a vacancy was considered to only weakly suppress the mode thermal conductivity of low frequency acoustic phonons. We show that this does not hold for IV-VI materials. We compare the phonon scattering by a single vacancy in group IV (diamond and Si) and IV-VI (PbTe and GeTe) crystals using an exact ab initio Green’s function approach. The phonon-vacancy scattering rates in diamond and Si follow the well-known fourth-power dependence on phonon frequency. However, PbTe and GeTe exhibit phonon scattering rates that have much weaker frequency dependence. As a result, a vacancy scatters low frequency acoustic phonons more strongly in PbTe and GeTe than in Si and diamond. This unusual behavior in PbTe and GeTe originates from the significant changes of interatomic force constants that extend up to the distance of 10 angstrom from the vacancy site. Therefore, a vacancy in IV-VI materials cannot be assumed as a point defect but its finite size should be considered for phonon scattering. In contrast, a vacancy in Si and diamond changes interatomic force constants only up to 3 angstrom range. Our study provides new insights on the phonon spectral contribution to thermal transport when vacancies exist in IV-VI materials and will help to develop low thermal conductivity materials by engineering vacancies.