Small Ge Research Articles

Nuclear Data Sheets for A = 227

The evaluated spectroscopic data are presented for ten known nuclides of mass 227 (Po, At, Rn, Fr, Ra, Ac, Th, Pa, U, Np). For 227Po, 227At, 227Rn, 227Pa, 227U and 227Np nuclei, only the ground-state information is available. Their decay characteristics are mostly unknown. Levels in 227Fr are known only from the decay of 227Rn to 227Fr. This decay scheme at present cannot be normalized to deduce γ intensities per 100 decays due to lack of knowledge about multipolarities of many low-energy transitions. The levels in 227Ra, 227Ac and 227Th are known from several decays and reactions, including particle-transfer data for 227Ra and 227Ac. The decay scheme of 227Ra to 227Ac was last studied in 1971 using small Ge detectors. Improved γ-ray intensity data need to be obtained with a better γ-detection system. The datasets for 227Ac have undergone extensive revisions, including detailed data for 231Pa α decay from 1986BaYK report, and single-proton transfer data from 1986MaYU thesis. High-spin (J>13/2 or so) structures are known only for 227Th. Level lifetime data are quite scarce for all the nuclides in this mass chain, thus limiting the knowledge of reduced transition probabilities.Band structures for 227Fr, 227Ra, 227Ac and 227Th are known in detail, together with evidence of weak octupole deformation and consequent parity-doublet structures.This evaluation was carried out as part of a joint IAEA-ICTP workshop for Nuclear Structure and Decay Data, organized and hosted by the IAEA, Vienna and ICTP, Trieste, March 24–28, 2014. The evaluation work was coordinated by B. Singh (McMaster). This work supersedes previous A=227 evaluation (2001Br31) published by E. Browne which covered literature before May 2001.

Local structure of Ge quantum dots determined by combined numerical analysis of EXAFS and XANES data

The sensitivity of X-ray absorption near-edge structure (XANES) to the local symmetry has been investigated in small (∼4 nm) matrix-free Ge quantum dots. The FDMNES package was used to calculate the theoretical XANES spectra that were compared with the experimental data of as-prepared and annealed nanoparticles. It was found that XANES data for an as-prepared sample can only be adequately described if the second coordination shell of the diamond-type structural model is included in the FDMNES calculations. This is in contrast to the extended X-ray absorption fine-structure data that show only the first-shell signal. These results suggest that, despite the high degree of disorder and a large surface-to-volume ratio, as-prepared small Ge quantum dots retain the diamond-type symmetry beyond the first shell. Furthermore, we utilized this sensitivity of XANES to the local symmetry to study annealed Ge quantum dots and found evidence for significant structural distortion which we attribute to the existence of surface disorder in the annealed oxygen-free Ge quantum dots.

Laser-induced particle size tuning and structural transformations in germanium nanoparticles prepared by stain etching and colloidal synthesis route

In this study, with the aid of Raman measurements, we have observed transformations in small (∼3 nm and ∼10 nm) free-standing Ge nanoparticles under laser light exposure. The nanoparticles were obtained by the chemical stain etching of a monocrystalline Ge wafer and of Ge powder and by colloidal synthesis route. We found that the transformation path depends on laser power and exposure time. At relatively low values of the laser power (2 mW) over a period of 100 min, the Raman signal indicates transformation of the sample from a nanocrystaline to bulk-like state, followed by partial oxidation and finally a conversion of the entire sample into alpha-quartz type GeO2. However, when the laser power is set at 60 mW, we observed a heat release during an explosive crystallization of the nanocrystalline material into bulk Ge without noticeable signs of oxidation. Together with the transmission electron microscopy measurements, these results suggest that the chemical stain etching method for the preparation of porous Ge may not be a top-down process as has been widely considered, but a bottom up one. Systematic studies of the laser exposure on Ge nanoparticles prepared by colloidal synthesis results in the fact that the explosive crystallisation is common for H-terminated and partially disordered Ge nanoparticles regardless of its particle size. We suggest possible bio-medical applications for the observed phenomena.

Mechanisms of Si and Ge diffusion on surfactant terminated (111) silicon and germanium surfaces

Surfactant mediated growth of Ge layers and formation of small Ge clusters on Si(111) are promising assemblage processes with envisioned applications in areas such as nanoelectronics or photovoltaics. They critically depend on migration of Si and Ge adatoms on surfactant terminated Si(111) and Ge(111) surfaces. We address Si and Ge adsorption and migration on surfactant (Bi, Sb) terminated (111) surfaces of Si and strained Ge by comprehensively mapping potential energy surfaces using density functional calculations. The main migration paths are identified and corresponding energy barriers are reported. It is shown that the energy barrier for adatom migration through Bi or Sb surfactant trimers (by actually breaking the trimers) is virtually degenerate to the mechanism involving traveling of the adatom around the same surfactant structures. We also find a low-energy anchoring site that is suggested to act as a nucleation structure and to trigger the clustering process. These results suggest a fundamentally new picture for the whole Si(111)Bi(Sb) epitaxial process.

Pressure-Induced Amorphization and a New High Density Amorphous Metallic Phase in Matrix-Free Ge Nanoparticles.

Over the last two decades, it has been demonstrated that size effects have significant consequences for the atomic arrangements and phase behavior of matter under extreme pressure. Furthermore, it has been shown that an understanding of how size affects critical pressure-temperature conditions provides vital guidance in the search for materials with novel properties. Here, we report on the remarkable behavior of small (under ~5 nm) matrix-free Ge nanoparticles under hydrostatic compression that is drastically different from both larger nanoparticles and bulk Ge. We discover that the application of pressure drives surface-induced amorphization leading to Ge-Ge bond overcompression and eventually to a polyamorphic semiconductor-to-metal transformation. A combination of spectroscopic techniques together with ab initio simulations were employed to reveal the details of the transformation mechanism into a new high density phase-amorphous metallic Ge.

Improvement in the Electrical Performance of Ge-Doped InZnO Thin-Film Transistor.

We fabricated amorphous oxide semiconductor thin-film transistors (TFTs) using Ge-doped InZnO (Ge-IZO) thin films as active-channel layers. The Ge-IZO thin films were deposited at room temperature by radio-frequency (RF) magnetron co-sputtering system, and then annealed in air for 1 h at 300 °C. Some processing parameters such as sputtering oxygen partial pressure [O2/(Ar + O2)] and sputtering power for GeO2 target were changed to investigate what was the optimal amount of Ge in the Ge-IZO active layer. A small concentration of Ge added to IZO by co-sputtering enhanced the carrier concentration, mobility, and conductivity; but further increase in Ge concentration degraded the device performance. In order to optimize the electrical properties of Ge-IZO TFTs, we tried to adjust the processing parameters and the best Ge-IZO TFT was obtained at a co-sputtering oxygen partial pressure of 2% and GeO2 target power of 10 W. The fabricated Ge-IZO TFT exhibited an on/off ratio of 3.0 x 10(7), a saturation mobility of 13.05 cm2/V·s, a subthreshold swing of 0.95 V/dec, and a threshold voltage of 0 V. XPS and XRD analyses of Ge-IZO films were performed to investigate the binding energies of atoms in Ge-IZO films and the crystallinity of the films. 90% transmittance of visible light was achieved, which makes the technology useful for transparent devices.

Self-Assemble Formation of Ge Dots on Si(100) via C/Ge/C/Si Structure

Formation of Ge dots via C2/Ge/C1/Si structure through solid-phase epitaxy by subsequent annealing at 650C in a molecular beam epitaxy chamber was investigated. The dot density increased with C1 coverage similar to the case of Ge/C1/Si structure. This suggests that Si-C bonds at Ge/Si interface are effective to form small and dense Ge dots. The smallest and densest dots were formed for C1 = 0.5 ML and C2 = 0.1 ML with mean diameter and dot density of 28.3 nm and 9.6 × 1010 cm-2, respectively. However, the dot size and density of C2/Ge/C1/Si structure was 1.3 times lager and 0.6 times lower than those of Ge/C1/Si structure in the case of C1 = 0.5 ML. As a result, it was found that Ge dots became large due to interference with the effect of reducing dot size in Ge/C1/Si structure.

Self-Assemble Formation of Ge Dots on Si(100) via C/Ge/C/Si Structure

Self-assemble formation of Ge quantum dots on Si is one of the promising candidates to implement optical devices, such as laser diodes [1], photo detectors [2] and solar cells [3], on a Si ULSI platform. It has been reported that the usage of sub-monolayer (ML) carbon (C) gave a beneficial way to form Ge dots as a result of c(4×4) reconstruction on Si(100) [4]. In our previous studies, self-assemble formations of Ge dots by C mediation were investigated as follow. In the case of sequential deposition of C and Ge on Si (Ge/C1/Si structure) at low temperature and subsequent annealing, it was confirmed that Si-C bonds promoted to form Ge dots during solid phase epitaxy [5]. In the case of C2/Ge/Si structure, an incorporation of C atoms into Ge layer during annealing induced to decrease the bulk free energy due to the formation of Ge-C bonds in the nucleation process, and led to scale down dot size and increase dot density [6]. On the basis of these backgrounds, by combining both structures, Ge dot formation via C2/Ge/C1/Si structure was investigated in this study. This is because the formation of Si-C bonds at the Ge/Si interface and Ge-C bonds in Ge dots is expected to help to obtain small and high-density Ge dots. The samples were prepared by solid-source molecular beam epitaxy system. C2/Ge(7.5 ML)/C1/Si(100) structures by changing C1 and C2 coverage were formed at substrate temperature of 200°C and were annealed at 650°C in the MBE chamber. As a result, small (20 nm) and dense (1.7 × 1010 cm-2) Ge dots were obtained at C1 = 0.25 ML and C2 = 0.1 ML. This indicates that the C2/Ge/C1/Si structure, which enables to control the binding states of both Si-C and Ge-C bonds, is an effective way to form Ge quantum dots self-assembly. In the meeting, the influence of the relationship between C1 and C2, the dot formation mechanism related to C binding states, and the optimization of the C2/Ge/C1/Si structure will be presented in detail. Acknowledgments This study was partially supported by JSPS KAKENHI Grant Number 24246003 and 15H03554 and, JSPS Core-to-Core Program, A. Advanced Research Networks. Reference [1] F. Ross et al., Phys. Rev. Lett. 80 (1998) 984. [2] G. Abstreiter et al., Semicond. Sci. Technol. 11 (1996) 1521. [3] A. Alguno et al., Appl. Phys. Lett. 83 (2003) 1258. [4] O. Leifeld et al., Mat. Sci. Eng. B 74 (2000) 222. [5] S. Hatakeyama et al., Microelec. Eng. 125 (2014) 28. [6] Y. Itoh et al., Materials Today Asia 2014 O2.2 (2014).

Vapor-liquid-solid epitaxial growth of Si1−xGex alloy nanowires: Composition dependence on precursor reactivity and morphology control for vertical forests

Growth of high-density group IV alloy nanowire forests is critical for exploiting their unique functionalities in many applications. Here, the compositional dependence on precursor reactivity and optimized conditions for vertical growth are studied for Si1−xGex alloy nanowires grown by the vapor-liquid-solid method. The nanowire composition versus gas partial-pressure ratio for germane-silane and germane-disilane precursor combinations is obtained at 350 °C over a wide composition range (0.05 ≤ x ≤ 0.98) and a generalized model to predict composition for alloy nanowires is developed based on the relative precursor partial pressures and reactivity ratio. In combination with germane, silane provides more precise compositional control at high Ge concentrations (x > 0.7), whereas disilane greatly increases the Si concentration for a given gas ratio and enables more precise alloy compositional control at small Ge concentrations (x < 0.3). Vertically oriented, non-kinking nanowire forest growth on Si (111) substrates is then discussed for silane/germane over a wide range of compositions, with temperature and precursor partial pressure optimized by monitoring the nanowire growth front using in-situ optical reflectance. For high Ge compositions (x ≈ 0.9), a “two-step” growth approach with nucleation at higher temperatures results in nanowires with high-density and uniform vertical orientation. With increasing Si content (x ≈ 0.8), the optimal growth window is shifted to higher temperatures, which minimizes nanowire kinking morphologies. For Si-rich Si1−xGex alloys (x ≈ 0.25), vertical nanowire growth is enhanced by single-step, higher-temperature growth at reduced pressures.

Formation of Ge dots on Si(100) using reaction of Ge with sub-monolayer carbon on top

To form small and dense Ge dots, an availability of simple bottom-up method by using reaction of Ge and carbon (C) formed on Si(100) at low temperature through post-annealing has been investigated. Ge dots were formed at annealing temperature (TA) above 450°C. Small, dense and relatively uniform dots were formed for Ge=7.5MLs and C=0.05–0.1ML at TA=650°C. From the dependence of dot size and density on Ge thickness and C coverage, the effect of C is considered to decrease in bulk free energy of Ge in nucleation process, that is, C led to reduce nucleation barrier height and to decrease critical radius.

High performing ITO/Ge heterojunction photodetector for broad wavelength detection

High-performing photodetector was fabricated by the ITO/Ge heterojunction device. Due to a small bandgap Ge semiconductor, ITO/Ge photodetector showed a significantly high photoresponse for long wavelengths. For ITO/Ge photodetector fabrication, an optically transparent and electrically conductive ITO layer was deposited on a Ge substrate and spontaneously formed a rectifying junction. The HRTEM image revealed the presence of GeOx at the interface of the ITO and Ge. The XPS and UPS studies were carried out to realize the band-offset structure of ITO/Ge photodiode. When the p-type Ge and ITO are brought into contact, the Fermi levels of both the materials are lined up. Besides, the energy bands of Ge are pinned to the Fermi level of the ITO layer at the junction. The band bending effect is depicted from the band offset of the ITO/Ge photodetector. We discuss an effective method of the ITO/Ge heterojunction device fabrication and suggest a strong route for high-performing broad range wavelength (600–900 nm) photodetectors.

Structure and effects of annealing in colloidal matrix-free Ge quantum dots.

The structure of small (2-5 nm) Ge quantum dots prepared by the colloidal synthesis route is examined. Samples were synthesized using either GeO2 or GeCl4 as precursor. As-prepared samples were further annealed under Ar or H2/Ar atmosphere at different temperatures in order to understand the effect of annealing on their structure. It was found that as-prepared samples possess distinctly different structures depending on their synthesis route as indicated by their long-range ordering. An appreciable amount of oxygen was found to be bound to Ge in samples prepared with GeO2 as a precursor; however, not for GeCl4. Based on combined transmission electron microscope, Raman, X-ray diffraction and X-ray absorption measurements, it is suggested that as-prepared samples are best described by the core-shell model with a small nano-crystalline core and an amorphous outer layer terminated either with oxygen or hydrogen depending on the synthesis route. Annealing in an H2Ar atmosphere leads to sample crystallization and further nanoparticle growth, while at the same time reducing the Ge-O bonding. X-ray diffraction measurements for as-prepared and annealed samples indicate that diamond-type and metastable phases are present.

Formation of GeSn alloy on Si(100) by low-temperature molecular beam epitaxy

GeSn alloys grown on Si(100) by the low-temperature (100 °C) molecular beam epitaxy are studied using scanning tunneling microscopy and Raman spectroscopy. It is found that the effect of Sn as a surfactant modifies substantially the low-temperature growth mechanism of Ge on Si. Instead of the formation of small Ge islands surrounded by amorphous Ge, in the presence of Sn, the growth of pure Ge islands appears via the Stranski-Krastanov growth mode, and a partially relaxed Ge1−xSnx alloy layer with the high Sn-fraction up to 40 at. % is formed in the area between them. It is shown that the observed growth mode induced by high surface mobility of Sn and the large strain of the pseudomorphic state of Ge to Si ensures the minimum elastic-strain energy of the structure.

Structural origin of light emission in germanium quantum dots.

We used a combination of optically-detected x-ray absorption spectroscopy with molecular dynamics simulations to explore the origins of light emission in small (5 nm to 9 nm) Ge nanoparticles. Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces. We show that optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to reveal the different origins of light emission in these two sets of samples. We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission. In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence. Using a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy we show that these disordered regions correspond to the disordered layer a few Å thick at the surface of the simulated nanoparticle.

Size dependent light absorption modulation and enhanced carrier transport in germanium quantum dots devices

Quantum confinement in closely packed arrays of Ge quantum dots (QDs) was studied for energy applications. In this work, we report an efficient tuning mechanism of the light harvesting and detection of Ge QDs. Thin films of SiGeO alloys, produced by rf-magnetron sputtering, were annealed at 600°C in N2 to induce precipitation of small amorphous Ge QDs into the oxide matrix. Varying the Ge content, the QD size was tailored between 2 and 4nm, as measured by high resolution transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) measurements indicate the formation of pure SiO2, as well as the presence of a sub-stoichiometric Ge oxide shell at the QD interface. Light absorption spectroscopy shows a clear size-dependent shift of the QD optical bandgap (Eg), between 1.4 and 2.1eV, and was modeled using the standard effective mass approximation (EMA) or a spatially dependent effective mass approximation (SPDEM) model. The reported quantum confinement effect was exploited to enhance light harvesting capability in Ge QDs-based devices. Metal–insulator–semiconductor devices with Ge QDs in the insulating layer exhibit a significant photo-response under reverse bias. Whereby, we demonstrate a large photoconductive gain (up to 1500%) that is tunable with QD size and is based on a preferential trapping of photo-generated holes by QDs, which enhances the charge carrier collection. Our results provide a new route for the application of Ge QDs in light harvesting devices and solar cells.

Electrochemical Liquid-Liquid-Solid Deposition of Crystalline Ge Nanowires as a Function of Ga Nanodroplet Size

Direct preparation of crystalline Ge nanowires from dissolved aqueous solutions of GeO2 via electrodeposition through an electrochemical liquid-liquid-solid (ec-LLS) process using a variety of Ga nano/microdroplets has been demonstrated. Templated silica films featuring regularly spaced openings with defined sizes were prepared on degenerately doped Si substrates by a modified natural lithography patterning process. These platforms were used to electrodeposit Ga droplet arrays with defined sizes and pitch. The Ga nano/microdroplets subsequently facilitated Ge nanowire ec-LLS in water as a function of droplet size. The amperometric responses recorded during ec-LLS depended on the Ga nanodroplet size. However, none of the recorded current-time profiles fit either Cottrell-type or standard nucleation models. Similarly, electron micrographs of the as-produced Ge nanowires showed a strong correlation between the base size of the Ge nanowires and the Ga droplet diameter. All nanowires featured a consistent taper, with a cone angle of approximately 10°. For the largest Ga droplets, no nanowires were observed under the employed ec-LLS conditions. Instead, a large isotropic Ge deposit covered with small Ge nanowires was observed. These cumulative results provide the first probe of the effects that liquid metal size(volume) have on electrodeposition by ec-LLS.

(Invited) Undoped Ge Core-Si(Ge) Shell Nanowires: Synthesis, Local Composition and Strain Characterization

Germanium-silicon core-shell nanowire (NW) heterostructures are promising building blocks to enable improved performance of nanophotonic and nanoelectronic devices. However, surface passivation is an important requirement for realization of NW-based devices: Small diameter Ge nanowires have large surface area-to-volume ratio, and carrier scattering and recombination at the surface defects may significantly compromise the electronic and optoelectronic properties of nanowire devices. On promising approach to passivating Ge NWs is to deposit an epitaxial shell of Si or SiGe around them. This paper describes the details of chemical vapor deposition growth of single crystal Ge core-Si(Ge) shell nanowires that are not intentionally doped and have coherent core-shell interfaces. Methods for quantitative local composition and strain measurements of these coaxial heterostructures are highlighted.

Charge deep-level transient spectroscopy of SiO2 and Al2O3 layers with embedded Ge nanocrystals

The present study of deep level transient spectroscopy (DLTS) is focused on a comparison of the trap states in two types of Ge nanocrystallites (NCs)-insulator composites. The investigated systems were the dielectric matrices Al2O3 and SiO2 in which the Ge NCs were embedded. We have found couples of traps with related values of activation energies in both the Ge:Al2O3 and the Ge:SiO2 films. In the films with a relatively low Ge content (where only small NCs sized 3–5 nm could have been detected by means of Raman spectroscopy), we observed traps with an energy level ∼50 meV in the Ge:Al2O3 films and 120 and 50 meV in the Ge:SiO2 films. In both systems, we found that the electron traps have a small carrier capture cross-section (10−21–10−23 cm2). We have identified the levels of the traps to be the quantum confinement levels in the small Ge NCs. For samples of higher Ge contents, where the NC size reaches about 20 nm and where an appreciable portion of the dielectric matrix consists of amorphous Ge (α-Ge), we found traps with an energy of 0.22–0.24 eV in the Ge:Al2O3, and 0.26–0.27 eV in the Ge:SiO2 samples. We suggest that this peak in the charge DLTS (Q-DLTS) spectra is associated with a trap at the Ge-NC/α-Ge interface. We have also identified the energy position of a defect level in the Ge:Al2O3 layers, which lies 0.46–0.49 eV below the conduction-band edge of the Si substrate.

Morphology and ferromagnetism of Ge nanostructure

A series of Ge films with varying thickness is prepared by plasma enhanced chemical vapor deposition technology. With the thickness of the sample becoming thinner, the sample shows ferromagnetism. When the 12-nm-thick sample is annealed at 300℃, the partile size becomes smaller, and thus the number of interface defects between the particles increases, so the sample gives a largest magnetic signal at room temperature (50 emu/cm3). FC-ZFC measurement shows that Curie temperature is 350 K. For a higher temperature (600 ℃, the coalescence of small Ge particles makes surface area decline, so magnetic signal becomes weak. With the thickness being 6 nm, the paramagnetism and the ferromagnetism coexist in the 6-nm-thick Ge film. When the 6-nm-thick sample is annealed under nitrogen atmosphere at 300 ℃, the sample only shows ferromagnetism. However, annealed at 600 ℃, the sample only presents paramagnetism. With the annealing temperature changing, the 12-nm-thick film and the 6-nm-thick film show different magnetic phenomena. Particle size and particle distribution cause different magnetic phenomena. It is supposed that the Ge nannostructure unpaired electrons are provided mainly by the interface defect between Si matyix and Ge film and the surface defect of Ge particles. The ferromagnetism coupling of the unpaired electrons is related to the distribution of sample particles and the junction among particles. The fusion between particles will reduce the ferromagnetism of the sample.

Germanium chemistry and MC-ICPMS isotopic measurements of Fe–Ni, Zn alloys and silicate matrices: Insights into deep Earth processes

Chemical separation and isotopic measurement of germanium using hexapole-collision cell-MC-ICPMS were developed in various Fe–Ni, ZnS and silicates matrices in order to investigate the potentiality of Ge as an isotopic tracer of planetary differentiation and rock-forming processes. Analytical procedures are described for the critical step of silicate dissolution in HF+HNO3 medium, as well as for Ge chemical purification using a single cationic-exchange resin step for Fe–Ni and ZnS matrices, and two anionic and cationic resin steps for silicate matrices. Germanium isotopic measurements using MC-ICPMS were performed with appropriate Ar+H fluxes in the collision cell to eliminate argide interferences on Ge masses. Three methods of mass bias correction, including sample standard bracketing, external Ga mass bias correction using the exponential law, and the empirical “regression method”, give similar results and demonstrate the use of Ga as an appropriate element for mass bias correction of Ge. Results are presented as delta values with respect to JMC Ge standard, and NIST3120a Ge standard for comparison. We show a long-term 2SD reproducibility of less than 0.24‰ on the δ74Ge.These analytical methods have been applied to Fe-meteorites, sphalerite (ZnS) deposits, and geostandard silicates ranging from ultramafic to basaltic to granitic compositions, and to an iron formation composition. Fe-meteorites and terrestrial silicate samples display small variations of δ74GeJMC=+1.77±0.22‰ and +0.89±0.16‰ (2SD reproducibility), respectively. This contrasts with the large variations seen in low-temperature rocks, such as the ZnS ores (δ74GeJMC=−0.37 to −1.69‰), and banded iron formations (IF-G Isua, δ74GeJMC=+1.38‰). A slight δ74GeJMC–NBO/T negative tendency in silicate samples indicates that polymerisation of silicate melt would control the small Ge isotope fractionation among mantle silicates.A comparison of δ74Ge values of iron meteorites and Earth silicate mantle opens new perspectives in deep Earth processes. On the basis of theoretical metal-silicate isotopic equilibrium processes, the low δ74Ge of silicate Earth cannot reconcile one-stage process of core–mantle segregation. It is proposed that the δ74Ge(JMC) value of silicate earth samples of +0.89±0.16‰ (or δ74Ge(NIST3120a)=+0.53‰) represents the composition of the accessible Earth modern mantle. The δ74Ge of the silicate mantle in equilibrium with the core at time of core formation would be distinct to that of the present mantle in result of distinct thermodynamic parameters, e.g. fO2, pressure, inducing changes in coordination and valence state of Ge in the silicate crystallographic structure. In addition, the light isotopic composition of the Earth's mantle could result from reverse diffusive processes induced by an increase in oxidation state at the end of core formation. This would have some implications on core formation modelling and the use of Ge isotopes for tracing the origin of deep mantle plumes.