Crystalline GeTe Research Articles

Optical properties of Sn-substituted GeTe phase-change materials under high pressure

The emerging phase change materials (PCMs) have attracted much attention due to their unique properties and applications. However, the optical properties of PCM at high pressures have not been well explored. In this work, the microstructure and optical properties of GeTe and Sn-substituted GeTe at high pressures up to 30 GPa have been systematically investigated. We find that the substitution of small amounts of Sn for Ge combined with the introduction of high pressure can induce an optical contrast of 117% in crystalline GeTe, which is even higher than the highest optical contrast reported so far (90%) caused by amorphous GeTe to crystalline GeTe. The pressure-induced contrast does not originate from any phase transition as previously, but from the combined effect of Peierls distortion elimination and Ge vacancy formation, which leads to an increase in carrier concentration. The relationship between pressure/atomic composition, Peierls distortion/vacancy formation, and optical contrast can be well explained by a combination of experiments and theoretical calculations. Therefore, this study reveals the effect of high pressure on the optical properties of PCMs and provides new ideas for the design of high-optical-contrast PCMs.

Optimization of electrical properties and radio frequency applications of GeTe thin film

GeTe belongs to a chalcogenide phase change material, which can dynamically achieve reversible switching between the crystalline state of low resistivity and the amorphous state of high resistivity by utilizing the thermally induced phase change characteristics. The GeTe is an important functional material in the fields of memristors and nonvolatile radio frequency (RF) switches. For RF switch applications, this paper focuses on optimizing the electrical performance of GeTe thin films prepared by magnetron sputtering. By comprehensively analyzing the effects of substrate materials, sputtering conditions, and annealing conditions on the resistivity of crystalline GeTe films, effective conditions for preparing low resistivity GeTe films are explored. Fig. (a) shows that compared with the GeTe film on a SiO<sub>2</sub> substrate, the film on an Al<sub>2</sub>O<sub>3</sub> substrate can obtain higher crystallinity and lower resistivity. For the deposition power and pressure shown in Fig. (b), the combination of medium power (50–80 W) and low pressure (2–3 mTorr) is beneficial for low crystalline resistivity of GeTe film. Additionally, Fig. (c) shows that higher annealing temperature (350–400 ℃) can realize lower film resistivity. Finally, the experimental results show that the lowest crystalline resistivity of the prepared GeTe thin film reaches 3.6×10<sup>–6</sup> Ω·m, and the resistance ratio is more than 10<sup>6</sup>. Based on rectangular chips of GeTe film, a parallel millimeter-wave switch with zero static power is also constructed. As shown in Fig. (d), the insertion loss is less than 2.4 dB, and the isolation is greater than 19 dB in a 1–40 GHz frequency band, demonstrating the potential application of GeTe thin films in the field of broadband high-performance discrete nonvolatile RF switches.

Nitrogen tuned crystal structure, optical band gap, localized states on amorphous and crystalline GeTe films

The nitrogen (N) doping effect on crystal structure, optical band gap, and localized states for amorphous/crystalline GeTe films was studied. The crystallization temperature and sheet resistance were improved by N doping. The local bonding structure of amorphous and crystalline N x ( GeTe ) 1 - x was elucidated by Raman spectra. The changes in optical band gap and localized states density for the films were evaluated by transmittance spectra. The band gap broadening after N doping is crucial to reducing the threshold current for phase change memory.

Structure and optical properties of GeTe film controlled by amorphous to crystalline phase transition

GeTe films have been showed great applications in the field of optical devices, especially in infrared absorption due to their narrow band gap. In this paper, amorphous GeTe (a-GeTe) film was prepared by pulsed laser deposition. Then, crystalline GeTe (c-GeTe) film was obtained by annealed in a tubular furnace at 200 °C. The X-ray diffraction indicates that GeTe film is transferred from amorphous to crystalline phase after annealing. The X-ray photoelectron spectroscopy shows that the presence of Ge and Te elements corresponding to the structure of GeTe. The high resolution transmission electron microscope also indicates that the disordered structure of GeTe film is transferred into ordered. Compared with a-GeTe film, the optical property of c-GeTe film has higher absorbance at 0.94 on 1100 nm measured by UV–Vis–NIR absorption spectroscopy. According to the calculation results, the optical band gaps of c-GeTe film and a-GeTe film are 0.82 eV and 0.87 eV respectively, indicating that the c-GeTe films is more favorable for photon transition. Fourier transform infrared spectroscopy also confirms that the infrared absorption properties of c-GeTe films is superior to those of the a-GeTe film. These results provide a fundamental research for the application of c-GeTe film in the field of infrared absorption.

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Disorder-induced Anderson-like localization for bidimensional thermoelectrics optimization

Nonthermal Transport of Energy Driven by Photoexcited Carriers in Switchable Solid States of GeTe

Phase change alloys have seen widespread use from rewritable optical discs to the present day interest in their use in emerging neuromorphic computing architectures. In spite of this enormous commercial interest, the physics of carriers in these materials is still not fully understood. Here, we describe the time and space dependence of the coupling between photoexcited carriers and the lattice in both the amorphous and crystalline states of one phase change material, GeTe. We study this using a time-resolved optical technique called picosecond acoustic method to investigate the \textit{in situ} thermally assisted amorphous to crystalline phase transformation in GeTe. Our work reveals a clear evolution of the electron-phonon coupling during the phase transformation as the spectra of photoexcited acoustic phonons in the amorphous ($a$-GeTe) and crystalline ($\alpha$-GeTe) phases are different. In particular and surprisingly, our analysis of the photoinduced acoustic pulse duration in crystalline GeTe suggests that a part of the energy deposited during the photoexcitation process takes place over a distance that clearly exceeds that defined by the pump light skin depth. In the opposite, the lattice photoexcitation process remains localized within that skin depth in the amorphous state. We then demonstrate that this is due to supersonic diffusion of photoexcited electron-hole plasma in the crystalline state. Consequently these findings prove the existence of a non-thermal transport of energy which is much faster than lattice heat diffusion.

Effect of characteristic size on the collective phonon transport in crystalline GeTe

We study the effect of characteristic size variation on the phonon thermal transport in crystalline GeTe for a wide range of temperatures using the first-principles density-functional method coupled with the kinetic collective model approach. The characteristic size dependence of phonon thermal transport reveals an intriguing collective phonon transport regime, located in between the ballistic and the diffusive transport regimes. Therefore, systematic investigations have been carried out to describe the signatures of phonon hydrodynamics via the competitive effects between grain size and temperature. A characteristic nonlocal length associated with phonon hydrodynamics and a heat wave propagation length has been extracted. The connections between phonon hydrodynamics and these length scales are discussed in terms of the Knudsen number. Further, the scaling relation of thermal conductivity as a function of characteristic size in the intermediate size range emerges as a crucial indicator of the strength of the hydrodynamic behavior. A ratio concerning normal and resistive scattering rates has been employed to understand these different scaling relations, which seems to control the strength and prominent visibility of the collective phonon transport in GeTe. This systematic investigation emphasizes the importance of the competitive effects between temperature and characteristic size on phonon hydrodynamics in GeTe, which can lead to a better understanding of the generic behavior and consequences of the phonon hydrodynamics and its controlling parameters in low-thermal conductivity materials.

Unusual Force Constants Guided Distortion-Triggered Loss of Long-Range Order in Phase Change Materials.

Unusual force constants originating from the local charge distribution in crystalline GeTe and Sb2Te3 are observed by using the first-principles calculations. The calculated stretching force constants of the second nearest-neighbor Sb-Te and Ge-Te bonds are 0.372 and −0.085 eV/Å2, respectively, which are much lower than 1.933 eV/Å2 of the first nearest-neighbor bonds although their lengths are only 0.17 Å and 0.33 Å longer as compared to the corresponding first nearest-neighbor bonds. Moreover, the bending force constants of the first and second nearest-neighbor Ge-Ge and Sb-Sb bonds exhibit large negative values. Our first-principles molecular dynamic simulations also reveal the possible amorphization of Sb2Te3 through local distortions of the bonds with weak and strong force constants, while the crystalline structure remains by the X-ray diffraction simulation. By identifying the low or negative force constants, these weak atomic interactions are found to be responsible for triggering the collapse of the long-range order. This finding can be utilized to guide the design of functional components and devices based on phase change materials with lower energy consumption.

MOCVD Growth of GeTe/Sb2Te3 Core–Shell Nanowires

We report the self-assembly of core–shell GeTe/Sb2Te3 nanowires (NWs) on Si (100), and SiO2/Si substrates by metalorganic chemical vapour deposition, coupled to the vapour–liquid–solid mechanism, catalyzed by Au nanoparticles. Scanning electron microscopy, X-ray diffraction, micro-Raman mapping, high-resolution transmission electron microscopy, and electron energy loss spectroscopy were employed to investigate the morphology, structure, and composition of the obtained core and core–shell NWs. A single crystalline GeTe core and a polycrystalline Sb2Te3 shell formed the NWs, having core and core–shell diameters in the range of 50–130 nm and an average length up to 7 µm.

Structural transformation and phase change properties of Se substituted GeTe

GeTe1−xSex (0 ≤ x ≤ 1.0) alloys have been prepared both in bulk and thin film forms to study the effect of selenium (Se) substitution for tellurium (Te) on the phase change properties. It is observed that with increasing Se substitution in GeTe, the structure transforms from rhombohdral structure to orthorhombic structure. Rietveld Refinement analysis support the phase transformation and show that the short and long bond lengths in crystalline GeTe decrease with increasing Se substitution but the rate of reduction of shorter bond length is more than the longer bond length. The GeTe1−xSex thin films undergo amorphous to crystalline phase change when annealed at high temperatures. The transition temperature shows an increasing trend with the Se substitution. The contrast in electrical resistivity between the amorphous and crystalline states is 104 for GeTe, and with the Se substitution, the contrast increases considerably to 106 for GeTe0.5Se0.5. Devices fabricated with thin films show that the threshold current decreases with the Se substitution indicating a reduction in the power required for WRITE operation. The present study shows that the crystalline structure, resistance, bandgap, transition temperature and threshold voltage of GeTe can be effectively controlled and tuned by the substitution of Te by Se, which is conducive for phase change memory applications.

Phase transitions in germanium telluride nanoparticle phase-change materials studied by temperature-resolved x-ray diffraction

Germanium telluride (GeTe), a phase-change material, is known to exhibit four different structural states: three at room-temperature (one amorphous and two crystalline, α and γ) and one at high temperature (crystalline, β). Because transitions between the amorphous and crystalline states lead to significant changes in material properties (e.g., refractive index and resistivity), GeTe has been investigated as a phase-change material for photonics, thermoelectrics, ferroelectrics, and spintronics. Consequently, the temperature-dependent phase transitions in GeTe have been studied for bulk and thin-film GeTe, both fabricated by sputtering. Colloidal synthesis of nanoparticles offers a more flexible fabrication approach for amorphous and crystalline GeTe. These nanoparticles are known to exhibit size-dependent properties, such as an increased crystallization temperature for the amorphous-to-α transition in sub-10 nm GeTe particles. The α-to-β phase transition is also expected to vary with size, but this effect has not yet been investigated for GeTe. Here, we report time-resolved x-ray diffraction of GeTe nanoparticles with different diameters and from different synthetic protocols. We observe a non-volatile amorphous-to-α transition between 210 °C and 240 °C and a volatile α-to-β transition between 370 °C and 420 °C. The latter transition was reversible and repeatable. While the transition temperatures are shifted relative to the values known for bulk GeTe, the nanoparticle-based samples still exhibit the same structural phases reported for sputtered GeTe. Thus, colloidal GeTe maintains the same general phase behavior as bulk GeTe while allowing for more flexible and accessible fabrication. Therefore, nanoparticle-based GeTe films show great potential for applications such as in active photonics.

Metavalent Bonding in Solids: Characteristic Representatives, Their Properties, and Design Options

Heavier chalcogenides display a surprisingly wide range of applications enabled by their unconventional properties. Herein, recent studies of three groups of chalcogenides from a chemical bonding perspective are reviewed to reveal the underlying reason for their wide range of applications. For IV–VI materials (GeTe, SnTe, PbTe, PbSe, and PbS), the unique property portfolio and bond‐breaking behavior are related to a novel chemical bonding mechanism termed “metavalent bonding” (MVB). The same phenomena are also found for several V2VI3 solids (Bi2Te3, Bi2Se3, Sb2Te3, and β‐As2Te3) and some ternary chalcogenides including crystalline (GeTe)1–x(Sb2Te3)x alloys. This provides evidence for the prevalence of MVB in these compounds. Subsequently, a quantum‐chemistry‐based map is presented. Using the transfer and sharing of electrons between adjacent atoms as its two coordinates, materials using MVB are all found in a well‐defined region of the map, characterized by sharing about one electron between adjacent atoms and only small charge transfer. This also implies that the degree of MVB is tailored either via Peierls distortions (electron sharing) or charge transfer (electron transfer), leading to the transition toward covalent bonding and ionic bonding, respectively. The tailoring of MVB provides a new approach for materials design.

Phonon hydrodynamics in crystalline GeTe at low temperature

A first-principles density functional method along with the direct solution of linearized Boltzmann transport equations are employed to systematically analyze the low-temperature thermal transport in crystalline GeTe. The extensive thermal transport simulations, ranging from room temperature to cryogenic temperatures, reveal the emergence of a phonon hydrodynamic regime in GeTe at low temperature. The reduction of grain boundary scattering is found to play a crucial role along with the divergent trend of umklapp and normal scattering at low temperatures in accommodating the hydrodynamic regime. Average scattering rates for normal, umklapp, and other resistive processes are distinguished for a wide range (4-300 K) of temperatures and used for identifying various phonon transport regimes. Therefore, the variations of lattice thermal conductivity, phonon propagation length, and thermal diffusivity with temperature, related to these transport regimes (ballistic, hydrodynamic, and kinetic), have been thoroughly investigated. The modewise decomposition of lattice thermal conductivity and the distinction of thermal diffusivity according to different scattering processes reveal rich information on the dominant phonon modes and phonon scattering processes in GeTe at low temperature. Further, the kinetic-collective model is used to elucidate the hydrodynamic behavior of phonon scattering through the relative study of collective and kinetic contributions to the thermal transport properties. In this context, the Knudsen number is estimated through the characteristic nonlocal length and the grain size, which further quantifies the consistent hydrodynamic behavior of phonon thermal transport for GeTe. Finally, phonon-vacancy scattering for GeTe is realized, and vacancies are found strongly to influence the hydrodynamic window while incorporating the other resistive scattering mechanisms.

Thermal conductivity of amorphous and crystalline GeTe thin film at high temperature: Experimental and theoretical study

Thermal transport properties bear a pivotal role in influencing the performance of phase change memory (PCM) devices, in which the PCM operation involves fast and reversible phase change between amorphous and crystalline phases. In this paper, we present a systematic experimental and theoretical study on the thermal conductivity of GeTe at high temperatures involving fast change from amorphous to crystalline phase upon heating. Modulated photothermal radiometry (MPTR) is used to experimentally determine thermal conductivity of GeTe at high temperatures in both amorphous and crystalline phases. Thermal boundary resistances are accurately taken into account for experimental consideration. To develop a concrete understanding of the underlying physical mechanism, rigorous and in-depth theoretical exercises are carried out. For this, first-principles density functional methods and linearized Boltzmann transport equations (LBTE) are employed using both direct and relaxation time based approach (RTA) and compared with that of the phenomenological Slack model. The amorphous phase experimental data has been described using the minimal thermal conductivity model with sufficient precision. The theoretical estimation involving direct solution and RTA method are found to retrieve well the trend of the experimental thermal conductivity for crystalline GeTe at high temperatures despite being slightly overestimated and underestimated, respectively, compared to the experimental data. A rough estimate of vacancy contribution has been found to modify the direct solution in such a way that it agrees excellently with the experiment. Umklapp scattering has been determined as the significant phonon-phonon scattering process. Umklapp scattering parameter has been identified for GeTe for the whole temperature range which can uniquely determine and compare Umklapp scattering processes for different materials

The interplay between Peierls distortions and metavalent bonding in IV–VI compounds: comparing GeTe with related monochalcogenides

In this article, we revisit bonding in crystalline GeTe, a simple binary alloy that is also a popular phase change material, and use an ab initio approach that goes beyond the usual one electron description obtained with density functional theory. By considering the electron pair density, we obtain a measure of the number of pairs of electrons that are shared between neighbors. Employing the charge transfer between adjacent atoms as the second quantifier of chemical bonding, we obtain a map which separates ionic, covalent and metallic bonding. Interestingly, GeTe is not located in any of these regions, but instead is located in a region where materials with a peculiar set of properties prevails. The corresponding materials have been coined incipient metals and their bonding ‘metavalent bonding’ (MVB). They often possess a Peierls distortion, which stabilizes the rhombohedral crystal structure by breaking the cubic symmetry. For these materials, the electron population of longer and shorter bonds is close to one-half, and charge transfer between adjacent atoms is quasi-independent of the degree of distortion. The energy gained by the Peierls distortion is much smaller than the energy gained by creating the cubic structure, delocalizing one electron over two bonds. Such Peierls distortions are not observed for aromatic compounds which utilize resonant bonding and have properties which differ significantly from the property portfolio of metavalently bonded materials. This stresses the difference between metavalent bonding and the resonant valence bond view of aromatic compounds and molecules. MVB is also responsible for the anomalies in dielectric properties and the anharmonicity of the solids. The comparison between PbTe, GeTe and GeS is particularly instructive, showing that bonding in these materials shows interesting differences, where metavalent bonds govern the behavior of PbTe and GeTe, while GeS is dominated by the Peierls distortion.

Impact of atomic vacancy on phase change and structure in GexTe1−x films

Binary compound Ge–Te, which displays intriguing functionalities, has been intensively studied from both fundamental and technological perspectives. In Ge–Te compound, a deviation from the Ge–Te stoichiometry will lead it thermodynamically unstable as well as to the change in the phase change characters. In this study, a series of non-stoichiometric Ge–Te films were prepared and a detailed study on the impact of Ge vacancy was carried out. The phase change characters can be tuned by adjusting the composition. Although the Ge vacancy does not lead to phase separation and Te precipitation in these films, Raman spectroscopy analysis reveals a dramatic change in the bonding environment. The microstructure has been modified by the induced Ge vacancy, especially the threefold Te unit and Ge–Ge bond in the crystalline GeTe.

RF Power-Handling Performance for Direct Actuation of Germanium Telluride Switches

The study presented in this article concerns germanium telluride (GeTe) phase-change material-based switches, actuated via direct heating and arranged through two configurations: series or shunt. It is concluded that direct heating provides a performing solution for GeTe amorphization, preventing heater aging. The two configurations are compared in terms of RF performance, power handling, and linearity. Some design rules are derived from empirical data, consolidated with thermal simulations. In either configuration, a large, thick, and short GeTe switch geometry is preferable. Higher isolation can be obtained in shunt configuration, while lower insertion loss can be reached in a series configuration. The Figure-of-Merit, as cutoff frequency, is 11 and 21 THz for shunt and series configurations, respectively. In terms of power handling, for amorphous GeTe, results confirm the existence of a threshold voltage leading to better handling for longer switches in both configurations. For crystalline GeTe, design rules that link the maximum current through the switch before failure to the material geometry are derived for the very first time. It is shown that current is proportional to the GeTe switch width, to the square root of its thickness, and inversely proportional to its length. The shunt configuration presented here holds 31 dBm at ON-state and more than 35 dBm at OFF-state while the series configuration holds 27 dBm at ON-state and 32 dBm at OFF-state. For power handling, a balance exists between series and shunt configurations with a ratio of 4 in the crystalline phase and of 0.25 in the amorphous phase.

Investigation of the oxidation process in GeTe-based phase change alloy using Ge K-edge XANES spectroscopy

Abstract In this work, we clearly demonstrate the efficacy of using XANES spectroscopy in conjunction with a Pilatus detector as a sensitive tool to allow the study of the oxidation process in GeTe alloys via depth profile analysis. On the basis of Ge K-edge XANES spectra, it was found that GeTe alloys do not oxidize readily after an initial native surface oxidation that occurs upon exposure to oxygen in the air at the elevated temperatures, 100 °C and 330 °C. We demonstrate that amorphous GeTe possesses a higher predisposition to oxidation than crystalline GeTe when exposed to the air at temperature of 100 °C. When the temperature is set to 330 °C in an air ambient, we show that the amorphous to crystal phase transition affects the oxidation process more significantly than the simple annealing of crystalline GeTe. We suggest that the higher tendency of GeTe films to oxidize during the phase transition is a consequence of the breaking of Ge–Ge bonds in the presence of oxygen atoms which subsequently leads to the extra formation of Ge–O bonds during crystallization.

Intermediate crystallization kinetics in Germanium-Tellurides

Germanium-Telluride has been widely studied as a phase-change material due to its fast crystallization speed. The understanding of the crystallization kinetics is important to evaluate the potential applications of the material, but this is limited by the conventional calorimetry with low heating rate and narrow temperature range. We here employed an ultrafast calorimetry method, named flash differential scanning calorimetry, to investigate the crystallization kinetics of GexTe100-x in a wide compositional range (15 ≤ x ≤ 55). By means of the X-ray diffraction, we found the complicated competition between crystalline GeTe and Te (or Ge) phases in these binary alloys. The crystallization kinetics of first crystalline phase were estimated and it was found that, GexTe100-x generally has intermediate crystal growth speed and fragility, which is ascribed to the border between covalent and metallic properties. Component dependences of maximum crystal growth rate (Umax) and fragility were investigated, revealing the component in x = 20.4 has the lowest Umax of 1.22 × 10−3 m s−1 with the smallest fragility of 42.2, and the component in x ≈ 50 possesses the largest Umax of 3.5 m s−1. It confirms that, GeTe is the most suitable phase-change material for information storage and GeTe4 is the best media for information transparency in Ge-Te binary. Moreover, a tri-counter pattern was carried out for obtaining the crystal growth rate directly in studied supercooled GexTe100-x liquids (15 ≤ x ≤ 55). In addition, we first found a peculiar component Ge22Te78 with terrible thermal properties, i.e., phase separation, low crystallization temperature, ultrahigh fragility and anomalous crystallization kinetics. More importantly, together with the crystallization kinetics parameters of other glass formers, it was found a specific relation between reduced glass temperature (Trg) and Umax for which can be benefit to simplify material screenings and performance optimizations.

Atomic disordering processes in crystalline GeTe induced by ion irradiation

The damaging process of GeTe up to amorphization has been studied by introducing controlled levels of disorder by irradiation with 150 keV Ar+ ions. In situ reflectivity measurements and ex-situ resistance and Raman spectroscopy analysis have been employed to study the impact of ion bombardment on the electrical conduction properties and on the bonding. The results obtained are indicative for three different stages of film damage. The first step appears to be dominated by point defects, affecting the temperature coefficient of resistance (TCR) and inducing a transition from positive (metallic conduction) to negative TCR values (conduction dominated by localized states), whilst the material still remains crystalline. The second step is characterized by the annealing of the defects induced, presumably, by the formation of complex defects that act as sinks for point defect recombination. This process is facilitated by the high atomic mobility. The third phase of damage starts at a fluence of 3.5 × 1014 cm−2 and finally converts the material to the amorphous state, characterized by higher resistance and decreased optical reflectivity. The modifications observed upon ion irradiation provide important insights into the possible states that can be achieved in crystalline GeTe through different local atomic arrangements towards amorphization.