Amorphous GeO2 Research Articles

Unveiling the cation ratio mediated structural modifications in TiO2:GeO2 mixtures for gravitational-wave detectors

Amorphous thin films of Ti doped GeO2 are of interest for coatings of the mirrors in gravitational wave detectors (GWDs) due to their low internal friction (Vajente et al 2021 Phys. Rev. Lett. 127 071101). The addition of Ti to amorphous GeO2 (a-GeO2) enables tailoring of the optical and structural properties of the mixtures. However, the specific modifications that occur in the amorphous network with the addition of Ti are not known. In this work, x-ray photoelectron spectroscopy is used to identify modifications to the bonding of Ge and Ti atoms in mixtures of Ti doped a-GeO2 with different Ti cation content. The formation of (Ti–O–Ge) bonds is evidenced from: (1) the presence of a peak which intensity increases with Ti content and causes a shift to lower binding energy (BE) of the core level O 1s peak; (2) the shift to higher BE of the Ti 2p 3/2 peak and a decrease in the energy split; and (3) the shift to lower BE of the Ge 3d 5/2 peak and increase in the energy split. These changes reflect modifications to the bonding when Ge replaces Ti in Ti–O–Ti bonds and Ti replaces Ge in Ge–O–Ge bonds due to their difference in electronegativity. A decrease in the O–O nearest-neighbour distance due to the incorporation of Ti atom is also observed from the broadening of the valence band spectra. The results show the 0.44 Ti doped a-GeO2 mixture has a balance between the (Ti–O–Ge) and the (Ge–O–Ge) networks, not observed in Ti poor and Ti rich mixtures. This finding could have important consequences in the optimisation of amorphous Ti doped a-GeO2 mixtures for low internal friction coatings of GWDs.

Nuanced dilute doping strategy enables high-performance GeTe thermoelectrics

In thermoelectrics, doping is essential to augment the figure of merit. Traditional strategy, predominantly heavy doping, aims to optimize carrier concentration and restrain lattice thermal conductivity. However, this tactic can severely hamper carrier transport due to pronounced point defect scattering, particularly in materials with inherently low carrier mean-free-path. Conversely, dilute doping, although minimally affecting carrier mobility, frequently fails to optimize other vital thermoelectric parameters. Herein, we present a more nuanced dilute doping strategy in GeTe, leveraging the multifaceted roles of small-size metal atoms. A mere 4% CuPbSbTe3 introduction into GeTe swiftly suppresses rhombohedral distortion and optimizes carrier concentration through the aid of Cu interstitials. Additionally, the formation of multiscale microstructures, including zero-dimensional Cu interstitials, one-dimensional dislocations, two-dimensional planar defects, and three-dimensional nanoscale amorphous GeO2 and Cu2GeTe3 precipitates, along with the ensuing lattice softening, contributes to an ultralow lattice thermal conductivity. Intriguingly, dilute CuPbSbTe3 doping incurs only a marginal decrease in carrier mobility. Subsequent trace Cd doping, employed to alleviate the bipolar effect and align the valence bands, yields an impressive figure-of-merit of 2.03 at 623 K in (Ge0.97Cd0.03Te)0.96(CuPbSbTe3)0.04. This leads to a high energy-conversion efficiency of 7.9% and a significant power density of 3.44 W cm−2 at a temperature difference of 500 K. These results underscore the invaluable insights gained into the constructive role of nuanced dilute doping in the concurrent tuning of carrier and phonon transport in GeTe and other thermoelectric materials.

Amorphous GeO2@C composite anodes for long cycling stability and performance in Li-ion batteries

Amorphous GeO2@C composite anodes for long cycling stability and performance in Li-ion batteries

GeO2 Nanoparticles Decorated in Amorphous Carbon Nanofiber Framework as Highly Reversible Lithium Storage Anode.

Germanium oxide (GeO2) is a high theoretical capacity electrode material due to its alloying and conversion reaction. However, the actual cycling capacity is rather poor on account of suffering low electron/ion conductivity, enormous volume change and agglomeration in the repeated lithiation/delithiation process, which renders quite a low reversible electrochemical lithium storage reaction. In this work, highly amorphous GeO2 particles are uniformly distributed in the carbon nanofiber framework, and the amorphous carbon nanofiber not only improves the conduction and buffers the volume changes but also prevents active material agglomeration. As a result, the present GeO2 and carbon composite electrode exhibits highly reversible alloying and conversion processes during the whole cycling process. The two reversible electrochemical reactions are verified by differential capacity curves and cyclic voltammetry measurements during the whole cycling process. The corresponding reversible capacity is 747 mAh g-1 after 300 cycles at a current density of 0.3 A g-1. The related reversible capacities are 933, 672, 487 and 302 mAh g-1 at current densities of 0.2, 0.4, 0.8 and 1.6 A g-1, respectively. The simple strategy for the design of amorphous GeO2/carbon composites enables potential application for high-performance LIBs.

Electroluminescence from Er-doped GeO2 nanofilms fabricated by atomic layer deposition on silicon: Effect of annealing temperature on film properties

Er-doped GeO2 (GeO2:Er) nanofilms are fabricated by atomic layer deposition on silicon, from which the 1530 nm electroluminescence (EL) is achieved. The GeO2:Er nanofilms maintain the amorphous structure after annealing at 600–1000 °C, while the annealing above 800 °C results in the loss of Ge due to the volatile GeOx (x < 2) species. These GeO2:Er nanofilms could withstand the wet lithography process and the prototype devices could operate under the constant current. The 650 °C annealed GeO2:Er devices manifest the threshold of ∼40 V and 10−7 A, while the maximum injection current is 0.28 A/cm2. The EL power density from the device doped with 3.09 mol% Er reaches to 2.2 mW/cm2, with the external quantum efficiency of 7.3% and power efficiency of 0.17%. The conduction mechanism for the excitation of Er emission confirms to the trap-assisted tunneling mode within the amorphous GeO2:Er nanolaminates. This work demonstrates the potential of GeO2:Er nanofilms in the utilization of Si-based optoelectronics.

Thermoelectric properties and microstructure of nanocomposite Sb-GeO2 and Sb–TiO2 thin films

This work reports on fabrication and thermoelectric properties of Sb-GeO2 and Sb–TiO2 nanocomposite thin films. The formation of nanocomposite structure includes the precipitation of Sb phase and the amorphous GeO2 or TiO2 phase. In the Sb-GeO2 nanocomposite, the formation of large rhombohedral Sb grains is observed, where monoclinic Sb are formed as intermediate phase. This contributes remarkably to the elevation of conductivity, while results in the reduction of Seebeck coefficient and power factor. Contrary, the effective inhibition of Sb crystallization caused by TiO2 leads to the smaller monoclinic and rhombohedral Sb grains with homogeneous distribution in annealed Sb–TiO2 thin film. The existence of monoclinic Sb facilitates to acquire excellent thermoelectric properties. Tiny grains within the films introduce more grain boundaries, making it possible to scatter and filter low energy carriers. This leads to high conductivity, remarkable rise of the Seebeck coefficient, and excellent power factor of 765.5 μW/mK2 at 650 K. The results demonstrate that the Sb–TiO2 nanocomposite materials with high power factor can be a candidate for thermoelectric devices.

Sensitivity of the Kβ″ X-ray Emission Line to Coordination Changes in GeO2 and TiO2.

The hard X-ray Kβ″ emission line shows sensitivity with respect to a wide range of cation-ligand coordination, which we investigate in the cases of GeO2 and TiO2 on the basis of ab initio spectral calculations on amorphous and crystalline structures. In compressed amorphous GeO2, the sampling of a large number of instantaneous coordination polyhedra from an ab initio molecular dynamics trajectory reveals that the functional relation between the Kβ″ shift and coordination is close to linear between 4-fold and 7-fold coordination. A similar sensitivity of the Kβ″ emission line exists in the coordination range between six and nine of crystalline high-pressure TiO2 polymorphs. Our results demonstrate the potential of the Kβ″ emission line in research on the structure of amorphous oxide material.

Towards structural reconstruction from X-ray spectra.

We report a statistical analysis of Ge K-edge X-ray emission spectra simulated for amorphous GeO2 at elevated pressures. We find that employing machine learning approaches we can reliably predict the statistical moments of the Kβ'' and Kβ2 peaks in the spectrum from the Coulomb matrix descriptor with a training set of ∼ 104 samples. Spectral-significance-guided dimensionality reduction techniques allow us to construct an approximate inverse mapping from spectral moments to pseudo-Coulomb matrices. When applying this to the moments of the ensemble-mean spectrum, we obtain distances from the active site that match closely to those of the ensemble mean and which moreover reproduce the pressure-induced coordination change in amorphous GeO2. With this approach utilizing emulator-based component analysis, we are able to filter out the artificially complete structural information available from simulated snapshots, and quantitatively analyse structural changes that can be inferred from the changes in the Kβ emission spectrum alone.

Amorphous carbon nanofibers incorporated with ultrafine GeO2 nanoparticles for enhanced lithium storage performance

As a potential anode material with high theoretical specific capacity, the practical application of GeO 2 is largely hindered by huge volume change and low reactivity, Herein, we proposed the engineering of dual amorphous GeO 2 @C nanofibers via facile electrospinning and thermal annealing process. In this design, numerous ultrafine GeO 2 particles are evenly anchored in carbon nanofibers, which could relief the stress in the carbon material during the lithium/delithiation process. Moreover, the low binding energy of Ge-O bond in the dual amorphous GeO 2 /C nanofibers render for convenient charge transfer. As a result, the electrode delivers highly reversible conversion (GeO 2 + 4Li ↔ Ge + 2Li 2 O) and alloying (Ge + 4.4Li ↔ GeLi 4.4 ) reaction. Furthermore, the GeO 2 @C nanofibers with large specific surface area also supply more active sites for pseudocapacitive lithium storage. Benefitting from the typical structure, the dual amorphous GeO 2 @C nanofibers electrode exhibits high reversible capacity of 1053 mAh g −1 at 0.3 A g −1 after 1000 cycle, and superior rate capacity of 476 mAh g −1 at 5 A g −1 . The excellent electrochemical performance endows dual amorphous GeO 2 @C nanofibers competitive electrode material for next-generation lithium-ion batteries. • Dual amorphous GeO 2 @C nanofibers were prepared by a facile and efficient approach. • Dual amorphous GeO 2 @C nanofibers display outstanding lithium storage properties. • Dual amorphous structure benefits for superior lithium storage.

Formation of nanogratings driven by ultrafast laser irradiation in mid-IR heavy oxide glasses

This work demonstrates for the first time the formation of nanogratings inside Barium Gallo-Germanate (BGG) glasses using femtosecond laser direct writing. These heavy oxide glasses, with a characterized transparency window up to 6 μm, along with the polarization sensitive birefringent nanostructures induced by femtosecond laser, offer an excellent platform for the development of mid-Infrared photonic applications. The fabricated glasses, containing both rare earth elements (Gd, Y) and transition metals (Zn, Ta), are directly compared to amorphous GeO2 and commercial alumino-germanate (Corning-9754) glasses commonly used in this field. The nanogratings processing window in each glass was investigated by varying laser parameters (pulse duration, energy, repetition rate), and electron microscopy imaging directly evidenced the porous nanostructures. Among the results, a strong form birefringence in 8% ZnO containing BGG glass, e.g., up to −0.029 ± 0.001, was reported. Finally, birefringent embedded Fresnel GRIN plates were directly written in a BGG glass, to demonstrate the functionalization potential of these materials.

Solution-based coating and printing of polycrystalline Ge films using GeO2 solution by moderate-pressure hydrogen plasma reduction

A printable-formation method for Ge films using a GeO2 solution was developed in this study. The GeO2 film was prepared by applying GeO2 ink on a glass substrate and reducing it using high-pressure H2-based plasma. The crystallinity, electrical conductivity, and photosensitivity of the films were examined before and after the plasma treatment. The film transformed from amorphous GeO2 to crystalline Ge after treatment with the H2-based plasma. The Ge film obtained after H2 plasma reduction exhibited an electrical resistivity of 4000 Ω cm and a photosensitivity of 1.9 under AM 1.5G illumination of 100 mW cm−2. In addition, the dependence of the reduction behavior of the GeO2 film on the process parameters, such as the input power, substrate temperature, treatment time, and hydrogen concentration, were investigated systematically. The reduction of GeO2 by H2 plasma proceeded with an increase in the input power. The thickness of the reduced Ge film depended strongly on the substrate temperature, and the plasma reduction process required a low reduction temperature and short reaction time as compared to those of the thermal reduction process. The presence of only 4% H2 gas in the process atmosphere played an important role in the reduction and crystallization of the Ge film, although pure He plasma could also reduce the GeO2 film. Thus, we demonstrated the position-controlled preparation of Ge films using a commercial inkjet printer loaded with a GeO2 ink solution.

Crystallization of GeO2 thin films into α-quartz: from spherulites to single crystals

Piezoelectric quartz (SiO2) crystals are widely used in industry as oscillators. As a natural mineral, quartz and its relevant silicates are also of interest in geoscience and mineralogy. However, the nucleation and growth of quartz crystals are difficult to control and not fully understood. Here we report successful solid-state crystallization of thin film of amorphous GeO2 into quartz on various substrates, including Al2O3, MgAl2O4, MgO, LaAlO3 and SrTiO3. At relatively low annealing temperatures, the crystallization process is spherulitic: with fibers growing radially from the nucleation centers and the crystal lattice rotating along the growth direction with a linear dependence between the rotation angle and the distance to the core. For increasingly higher annealing temperatures, quartz crystals begin to form. The edges of the sample play an important role in facilitating nucleation followed by growth sweeping inward until the whole film is crystallized. Control of the growth allows single crystalline quartz to be synthesized, with crystal sizes of hundreds of microns achieved on sapphire substrates, which is promising for further piezoelectric applications. Our study reveals the complexity of the nucleation and growth process of quartz and provides insight for further studies.

Investigation of physically transient resistive switching memory based on GeO2 thin films

Physically transient resistive switching memory has attracted much attention in recent years for potential applications in security information storage and environmental protection. Herein, we demonstrated that resistive switching memory composed of Ag/GeO2/fluorine-doped tin oxide structures exhibited remarkable electrical properties and could be rapidly dissolved by de-ionized water. The robust capabilities of the as-fabricated memory with more than 100 enduring cycles, a comparable resistance window larger than the OFF/ON ratio of 102, and a long retention time of more than 10 h was achieved. In addition, the degradable characteristics were demonstrated by the dissolution of amorphous GeO2 thin films in de-ionized water with the disappearance of resistive switching properties. Furthermore, the switching mechanism has been explained by the filament model, and the degradable mechanism of GeO2 films has been investigated by the change of chemical bonds and resistive switching properties in different humidity environments. Our results suggest that GeO2-based resistive switching memory could find opportunities for applications in physically transient electronics.

Ge coordination inNaAlGe3O8glass upon compression to 131 GPa

Structural transformations at high pressure in NaAlGe3O8 glass were investigated by means of x-ray absorption spectroscopy at the Ge K edge in combination with a diamond anvil cell. The obtained results provide a detailed picture of the local structural behavior of Ge in a chemically complex glass under compression. First and second shell bond distances (RGe-O and RGe...Ge) were extracted assuming contributions of two scattering paths (Ge-O and Ge…Ge). We observed a significant extension of the Ge-O distance from 1.73 to 1.82 A between 3 and ∼26GPa, accompanied by an increase of the fitted number of nearest neighbors from ∼4 to ∼6. These observations can be attributed to the change from tetrahedral to octahedral Ge coordination. Second shell bond distances Ge…Ge are also consistent with this structural transformation. Between 34 and 131 GPa, the evolution of the fitted Ge-O distance implies a gradual volume reduction of the Ge octahedra. At the highest probed pressure of 131 GPa a Ge-O distance of 1.73 A was found, which is similar to the one obtained at ambient conditions for Ge in fourfold coordination. The compressibility of the Ge-O octahedron in NaAlGe3O8 beyond 34 GPa is considerably higher than the one reported for amorphous GeO2 from x-ray diffraction analysis but it is similar to the one reported for the Ge octahedron in crystalline rutile-type GeO2. We attribute the high compressibility of the Ge-O bond in NaAlGe3O8 glass to the presence of Al and Na that increase the system's complexity and therefore its degrees of freedom. Beyond 110 GPa the data on NaAlGe3O8 glass indicate the onset of polyhedral distortion. The performed study provides insights into the structural changes of complex and polymerized germanate glasses or melts at extreme pressure conditions.

Role of Oxygen on Chemical Segregation in Uncapped Ge2Sb2Te5 Thin Films on Silicon Nitride

Germanium antimony telluride has been the most used and studied phase-change material for electronic memory due to its suitable crystallization temperature, amorphous to crystalline resistance contrast, and stability of the amorphous phase. In this work, the segregation of Ge in a Ge2Sb2Te5 film of 30 nm thickness during heating inside the transmission electron microscope was observed and characterized. The Ge2Sb2Te5 film was deposited using sputtering on a Protochips Fusion holder and left uncapped in atmosphere for about four months. Oxygen incorporated within the film played a significant role in the chemical segregation observed which resulted in amorphous Ge-O island boundaries and Sb and Te rich crystalline domains. Such composition changes can occur when the phase-change material interfaces insulating oxide layers in an integrated device and can significantly impact its electrical and thermal properties.

The study of oxygen-deficient centers in Al-doped amorphous germanium oxides

Electronic and optical properties of Germanium oxygen-deficient center (GeODC) defect and Al-doped GeODC defect in 96 atoms amorphous GeO2 (a-GeO2) supercells have been investigated. All the 64 oxygen sites in the a-GeO2 configuration have been orchestrated as possible candidates for the construction of 64 different GeODCs. A Ge atom has been substituted by an Al atom in GeODCs to form the Al-GeODCs. It is discovered that the refractive index of a-GeO2 is reduced by GeODC defect, contrary to the situation when Al is doped in GeODC defect. The absorption peak at 5.15 eV is related to GeODC defect. After Al-doping, a new defect state is generated near the top of valence band (TVB), resulting in an absorption peak at 4 eV. The variations of refractive index and optical properties exhibited offer a significant and profound guidance of Al and Ge co-doping in silica optical fiber.

A First-Principles Study on the Oxygen Adsorption and Interface Characteristics with a-GeO2 of Ge[001] Nanowire

First-principles calculations were performed to study the atomic and the electronic structures of the oxygen adsorption on the sidewalls of Ge[001] nanowires. For several possible adsorption sites, the adsorption energy of atomic oxygen, as well as the atomic configuration and electronic properties of the adsorbed structure, were examined. The adsorption stability is not much affected by the nanowire diameter for diameters ranging from 0.8 nm to 2 nm while the electronic structures like band gap and gap states, vary with the nanowire diameter. The interface between the Ge nanowire and amorphous GeO2 (a-GeO2) was also investigated. The interface region is composed of Ge suboxides like those between Ge bulk slab and a-GeO2. The valence band offsets between Ge nanowire and a-GeO2 decreases considerably as the diameter of nanowire shrinks while the conduction band offsets show less dependence on the diameter of the nanowire.

Orientation-dependent structural and electronic properties of Ge/a-GeO2 interfaces: first-principles study

While the properties of the non-(1 0 0) Ge surface become more important with the scaling down of Ge-based transistor devices, the stability and electronic properties of the interface between non-(1 0 0) Ge and amorphous GeO2 (a-GeO2) compared to those of Ge(1 0 0) and a-GeO2 are still not well known. In this study, first-principles calculations were performed to systematically study the atomic and electronic structures of Ge/a-GeO2 interfaces with various surface orientations of Ge. The study shows that the Ge(1 1 1)/a-GeO2 and Ge(1 0 0)/a-GeO2 interfaces have the lowest and highest interface energies, respectively. The stability of the Ge/a-GeO2 interface is governed by the interfacial bond density and the minimization of the dangling bonds. We find that the interface region, composed of the Ge suboxides, dominates the electronic structures of the Ge/a-GeO2. The Ge atoms with uncompensated dangling bonds result in various trap states within the band gap of Ge, which is related to the charge neutrality level of the Ge defect. The band offsets between Ge and a-GeO2 show little dependence on the original Ge orientation.

Phase Transformation of GeO2 Glass to Nanocrystals under Ambient Conditions.

Theoretically, the accomplishment of phase transformation requires sufficient energy to overcome the barriers of structure rearrangements. The transition of an amorphous structure to a crystalline structure is implemented traditionally by heating at high temperatures. However, phase transformation under ambient condition without involving external energy has not been reported. Here, we demonstrate that the phase transformation of GeO2 glass to nanocrystals can be triggered at ambient conditions when subjected to aqueous environments. In this case, continuous chemical reactions between amorphous GeO2 and water are responsible for the amorphous-to-crystalline transition. The dynamic evolution process is monitored by using in situ liquid-cell transmission electron microscopy, clearly revealing this phase transformation. It is the hydrolysis of amorphous GeO2 that leads to the formation of clusters with a size of ∼0.4 nm, followed by the development of dense liquid clusters, which subsequently aggregate to facilitate the nucleation and growth of GeO2 nanocrystals. Our finding breaks the traditional understanding of phase transformation and will bring about a significant revolution and contribution to the classical glass-crystallization theories.

Rigidity Enhancement of GeO2 by Y-Doping for Reliable Ge Gate Stacks

The poor reliability of the GeO2/Ge stack is improved by appropriate cation doping [e.g., Yttrium (Y)-doping] into amorphous GeO2 as a result of the enhancement of the GeO2 network structure stability. In this paper, we discuss the impact of Y-doping on structural modulation of GeO2 on a Ge substrate in thermal treatment. By doping a small amount of Y into amorphous GeO2, the crystallization of GeO2 to $\alpha $ -quartz and $\alpha $ -cristobalite structures is efficiently suppressed without toughening the local Ge-O bond. This is direct evidence of rigidity enhancement of the GeO2 tetrahedral network structure by cation doping.