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This study conducts a comprehensive investigation into the physical properties of germanium telluride (GeTe, GT) thin films and their associated electronic devices to establish fundamental relationships that are essential for achieving reliable threshold switching (TS) characteristics. Chemical composition variations in GT thin films significantly influence their crystallinity, atomic vibration modes, chemical binding status, bandgap, and distribution of electronic traps. The increase in the tellurium (Te) content results in elevated Urbach energy, subsequently reducing the effective bandgap energy. Excluding GT 8 , trap energy increases proportionally with increasing Te content. Correlating electrical properties with physical characteristics reveals key conditions necessary for stable TS behavior: maintaining amorphous crystallinity, optimizing the chemical composition of the GT layer, and ensuring an appropriate bandgap and trap energy. These findings underscore the importance of precise chemical composition control for reliable TS performance. Insights from this study can guide nanoscale analyses of bonding structures in Te-based binary TS systems and present opportunities for material optimization across various applications.

Using a monochromator in transmission electron microscopy, a low-energy-loss spectrum can provide inter- and intra-band transition information for nanoscale devices with high energy and spatial resolutions. However, some losses, such as Cherenkov radiation, phonon scattering, and surface plasmon resonance superimposed at zero-loss peak, make it asymmetric. These pose limitations to the direct interpretation of optical properties, such as complex dielectric function and bandgap onset in the raw electron energy-loss spectra. This study demonstrates measuring the dielectric function of germanium telluride using an off-axis electron energy-loss spectroscopy method. The interband transition from the measured complex dielectric function agrees with the calculated band structure of germanium telluride. In addition, we compare the zero-loss subtraction models and propose a reliable routine for bandgap measurement from raw valence electron energy-loss spectra. Using the proposed method, the direct bandgap of germanium telluride thin film was measured from the low-energy-loss spectrum in transmission electron microscopy. The result is in good agreement with the bandgap energy measured using an optical method., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 Elsevier Ltd. All rights reserved.)

Germanium telluride (GeTe) compounds exhibit excellent thermoelectric performance. In this study, copper selenide (Cu 2 Se) was used to tune the crystal structure and carrier concentration ( n H ) of GeTe materials. The zT of the 1% Cu 2 Se-doped GeTe sample reaches 1.32, which is 52% higher than that of the pure phase. The results show that Cu 2 Se tunes the GeTe crystal structure and carrier concentration to achieve promising enhancements to the thermoelectric performance. Meanwhile, a herringbone-like crystal structure that reduces the lattice thermal conductivity was observed. However, because the directional movement of Cu ions at high temperatures leads to an increase in electrical conductivity, the electronic thermal conductivity also increased. This study focuses on crystal engineering strategies for the study of nontoxic thermoelectric materials.

GeTe is an emerging lead-free thermoelectric material, but its excessive carrier concentration and high thermal conductivity severely restrict the enhancement of thermoelectric properties. In this study, the synergistically optimized thermoelectric properties of p-type GeTe through Bi-Cu 2 S coalloying are reported. It can be found that the donor behavior of Bi and the substitution-interstitial defect pairs of Cu + ions effectively reduce the hole concentration to an optimal level with carrier mobility less affected. At the same time, Bi-Cu 2 S coalloying induces many phonon scattering centers involving stacking faults, nanoprecipitations, grain boundaries and tetrahedral dislocations and suppresses the lattice thermal conductivity to 0.64 W m -1 K -1 . Consequently, all effects synergistically yield a peak ZT of 1.9 at 770 K with a theoretical conversion efficiency of 14.5% (300-770 K) in the (Ge 0.94 Bi 0.06 Te) 0.988 (Cu 2 S) 0.012 sample, which is very promising for mid-low temperature range waste heat harvest.

Chalcogenide phase-change materials (PCMs) based random access memory (PCRAM) enter the global memory market as storage-class memory (SCM), holding great promise for future neuro-inspired computing and non-volatile photonic applications. The thermal stability of the amorphous phase of PCMs is a demanding property requiring further improvement. In this work, we focus on indium, an alloying ingredient extensively exploited in PCMs. Starting from the prototype GeTe alloy, we incorporated indium to form three typical compositions along the InTe-GeTe tie line: InGe 3 Te 4 , InGeTe 2 and In 3 GeTe 4 . The evolution of structural details, and the optical properties of the three In-Ge-Te alloys in amorphous and crystalline form, was thoroughly analyzed via ab initio calculations. This study proposes a chemical composition possessing both improved thermal stability and sizable optical contrast for PCM-based non-volatile photonic applications.

The data presented in this article relate to the research article entitled "Superior room-temperature power factor in GeTe systems via multiple valence band convergence to a narrow energy range" [T. Oku et al., Mater. Today Phys. 20 (2021) 100484 (10.1016/j.mtphys.2021.100484)]. Polycrystalline (GeTe) n Sb 2 Te 3 ( n  = 10, 12, 16, 20, and 24) bulk samples were prepared by melting and annealing. The Ge defect concentration of each composition was estimated from Rietveld refinement of the synchrotron X-ray powder diffraction patterns. Electrical properties, such as the electrical resistivity and Seebeck coefficient, were measured from three specimens of each composition to confirm reproducibility. Electronic-band-structure parameters and electronic density-of-states of each composition were obtained by first-principles calculations., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2021 The Author(s). Published by Elsevier Inc.)

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb 2 Te 3 significantly suppresses the thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6. These results not only attest to the efficacy of using phase transition in manipulating the microstructures of GeTe-based materials but also open up a new thermodynamic route to develop higher performance TE materials in general., (© 2020 Wiley-VCH GmbH.)

Renewed interest in the ferroelectric semiconductor germanium telluride was recently triggered by the direct observation of a giant Rashba effect and a 30-year-old dream about a functional spin field-effect transistor. In this respect, all-electrical control of the spin texture in this material in combination with ferroelectric properties at the nanoscale would create advanced functionalities in spintronics and data information processing. Here, we investigate the atomic and electronic properties of GeTe bulk single crystals and their (111) surfaces. We succeeded in growing crystals possessing solely inversion domains of ∼10 nm thickness parallel to each other. Using HAADF-TEM we observe two types of domain boundaries, one of them being similar in structure to the van der Waals gap in layered materials. This structure is responsible for the formation of surface domains with preferential Te-termination (∼68%) as we determined using photoelectron diffraction and XPS. The lateral dimensions of the surface domains are in the range of ∼10-100 nm, and both Ge- and Te-terminations reveal no reconstruction. Using spin-ARPES we establish an intrinsic quantitative relationship between the spin polarization of pure bulk states and the relative contribution of different terminations, a result that is consistent with a reversal of the spin texture of the bulk Rashba bands for opposite configurations of the ferroelectric polarization within individual nanodomains. Our findings are important for potential applications of ferroelectric Rashba semiconductors in nonvolatile spintronic devices with advanced memory and computing capabilities at the nanoscale.

Germanium boasts a high charge capacity, but it has detrimental effects on battery cycling life, owing to the significant volume expansion that it incurs after repeated recharging. Therefore, the fabrication of Ge composites including other elements is essential to overcome this hurdle. Herein, highly conductive Te is employed to prepare an alloy of germanium telluride (GeTe) with the addition of a highly conductive matrix comprising titanium carbide (TiC) and carbon (C) via high-energy ball milling (HEBM). The final alloy composite, GeTe-TiC-C, is used as a potential anode for lithium-ion cells. The GeTe-TiC-C composites having different combinations of TiC are characterized by electron microscopies and X-ray powder diffraction for structural and morphological analyses, which indicate that GeTe and TiC are evenly spread out in the carbon matrix. The GeTe electrode exhibits an unstable cycling life; however, the addition of higher amounts of TiC in GeTe offers much better electrochemical performance. Specifically, the GeTe-TiC (20%)-C and GeTe-TiC (30%)-C electrodes exhibited excellent reversible cyclability equivalent to 847 and 614 mAh g -1 after 400 cycles, respectively. Moreover, at 10 A g -1 , stable capacity retentions of 78% for GeTe-TiC (20%)-C and 82% for GeTe-TiC (30%)-C were demonstrated. This proves that the developed GeTe-TiC-C anodes are promising for potential applications as anode candidates for high-performance lithium-ion batteries.

As a key method to convert solar into chemical energy, photocatalytic water decomposition has garnered attention. Moreover, the development of graphene and graphene-like two-dimensional (2D) materials has brought fresh vitality in the field of photocatalysis. Here, we prepared two to four layers of GeTe nanosheets by ultrasonic-assisted liquid-phase exfoliation in argon and air, which we referred to as Ar-GeTe and O-GeTe, respectively. The photocatalytic hydrogen production potential of 2D GeTe was experimentally investigated for the first time. The results indicated that minimally layered GeTe samples are indirect-gap semiconductors with the GeTe band gap increasing after oxidation. All samples have suitable band positions that can drive photocatalytic water splitting into H 2 under mild conditions, providing maximum hydrogen evolution rates of 1.13 mmol g -1 h -1 (Ar-GeTe) and 0.54 mmol g -1 h -1 (O-GeTe). With density functional theory computations, the structural stability of GeTe in air was discussed, revealing that oxygen atoms could easily combine with Ge to form a more stable structure, thus impacting the photocatalytic performance of 2D GeTe. Therefore, the light requirement and oxygen deficiency of the material give an advantage in the field of energy supply in space.

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 Ge 0.81 Mn 0.15 Bi 0.04 Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications., Competing Interests: The authors declare no conflict of interest.

Germanium telluride (GeTe) is a phase change material (PCM) that has gained recent attention because of its incorporation as an active material for radio frequency (RF) switches, as well as memory and novel optoelectronic devices. Considering PCM-based RF switches, parasitic resistances from Ohmic contacts can be a limiting factor in device performance. Reduction of the contact resistance ( R c ) is therefore critical for reducing the on-state resistance to meet the requirements of high-frequency RF applications. To engineer the Schottky barrier between the metal contact and GeTe, Sn was tested as an interesting candidate to alter the composition of the semiconductor near its surface, potentially forming a narrow band gap (0.2 eV) SnTe or a graded alloy with SnTe in GeTe. For this purpose, a novel contact stack of Sn/Fe/Au was employed and compared to a conventional Ti/Pt/Au stack. Two different premetallization surface treatments of HCl and deionized (DI) H 2 O were employed to make a Te-rich and Ge-rich interface, respectively. Contact resistance values were extracted using the refined transfer length method. The best results were obtained with DI H 2 O for the Sn-based contacts but HCl treatment for the Ti/Pt/Au contacts. The as-deposited contacts had the R c (ρ c ) of 0.006 Ω·mm (8 × 10 -9 Ω·cm 2 ) for Sn/Fe/Au and 0.010 Ω·mm (3 × 10 -8 Ω·cm 2 ) for Ti/Pt/Au. However, the Sn/Fe/Au contacts were thermally stable, and their resistance decreased further to 0.004 Ω·mm (4 × 10 -9 Ω·cm 2 ) after annealing at 200 °C. In contrast, the contact resistance of the Ti/Pt/Au stack increased to 0.012 Ω·mm (4 × 10 -8 Ω·cm 2 ). Transmission electron microscopy was used to characterize the interfacial reactions between the metals and GeTe. It was found that formation of SnTe at the interface, in addition to Fe diffusion (doping) into GeTe, is likely responsible for the superior performance of Sn/Fe/Au contacts, resulting in one of the lowest reported contact resistances on GeTe.

The electric and nonvolatile control of the spin texture in semiconductors would represent a fundamental step toward novel electronic devices combining memory and computing functionalities. Recently, GeTe has been theoretically proposed as the father compound of a new class of materials, namely ferroelectric Rashba semiconductors. They display bulk bands with giant Rashba-like splitting due to the inversion symmetry breaking arising from the ferroelectric polarization, thus allowing for the ferroelectric control of the spin. Here, we provide the experimental demonstration of the correlation between ferroelectricity and spin texture. A surface-engineering strategy is used to set two opposite predefined uniform ferroelectric polarizations, inward and outward, as monitored by piezoresponse force microscopy. Spin and angular resolved photoemission experiments show that these GeTe(111) surfaces display opposite sense of circulation of spin in bulk Rashba bands. Furthermore, we demonstrate the crafting of nonvolatile ferroelectric patterns in GeTe films at the nanoscale by using the conductive tip of an atomic force microscope. Based on the intimate link between ferroelectric polarization and spin in GeTe, ferroelectric patterning paves the way to the investigation of devices with engineered spin configurations.

Phase-change materials (PCMs) represent the leading candidates for universal data storage devices, which exploit the large difference in the physical properties of their transitional lattice structures. On a nanoscale, it is fundamental to determine their performance, which is ultimately controlled by the speed limit of transformation among the different structures involved. Here, we report observation with atomic-scale resolution of transient structures of nanofilms of crystalline germanium telluride, a prototypical PCM, using ultrafast electron crystallography. A nonthermal transformation from the initial rhombohedral phase to the cubic structure was found to occur in 12 ps. On a much longer time scale, hundreds of picoseconds, equilibrium heating of the nanofilm is reached, driving the system toward amorphization, provided that high excitation energy is invoked. These results elucidate the elementary steps defining the structural pathway in the transformation of crystalline-to-amorphous phase transitions and describe the essential atomic motions involved when driven by an ultrafast excitation. The establishment of the time scales of the different transient structures, as reported here, permits determination of the possible limit of performance, which is crucial for high-speed recording applications of PCMs.