Bulk Ge Research Articles

Mineralogy, geochemistry and classification of the new Smolenice iron meteorite from Slovakia

Abstract: A single 13.95 kg mass of a slightly weathered iron meteorite was found in the forest near Smolenice (48°31.2’N, 17°23.9’E; Trnava County, Slovakia). The bulk chemical composition (in wt. %) is: Fe 88.78, Ni 8.16, Co 0.38, P 0.05, S˂0.006 and (in μg/g): Ge˂0.18, Ir 1.67, Ga 1.80, Cr 87.3, Cu 135.1, As 4.52, Mo 5.82, Sn 1.53, W 0.56, Re 0.18, Ru 3.56, Rh 0.90, Pd 4.12, Pt 5.35, Au 1.19, Zn˂5, B˂0.68, Pb˂0.06. Bulk geochemistry, and Ni, Ga, Ge and Ir contents in particular suggest that the meteorite is an octahedrite belonging to the IVA group. The average thickness of kamacite lamellae is 0.22 mm, ranking it as fine octahedrite (Of). The mineral composition is simple, the most abundant minerals being iron (kamacite) (5.16–7.36 wt. % Ni) followed by taenite (16.73–33.93 wt. % Ni). Troilite nodules and daubreelite inclusions and thin veinlets are rare. The Widmanstatten pattern is uniform across the meteorite and plessite structure is developed locally. Analyses of cosmogenic radionuclides (14C and 26Al) indicate that the radius of the Smolenice meteorite could be 30±10 cm and its terrestrial age 11±2 kyr.

Selective Epitaxial Growth of In Situ Doped SiGe on Bulk Ge for p+/n Junction Formation

Epitaxial in situ doped Si0.73Ge0.27 alloys were grown selectively on patterned bulk Ge and bulk Si wafers. Si0.73Ge0.27 layers with a surface roughness of less than 3 nm were demonstrated. Selectively grown p+Si0.73Ge0.27 layers exhibited a resistivity of 3.5 mΩcm at a dopant concentration of 2.5 × 1019 boron atoms/cm3. P+/n diodes were fabricated by selectively growing p+- Si0.73Ge0.27 on n-doped bulk Ge and n-doped Si wafers, respectively. The geometrical leakage current contribution shifts from the perimeter to the bulk as the diode sizes increase. Extracted near midgap activation energies are similar to p+/n Ge junctions formed by ion implantation. This indicates that the reverse leakage current in p+/n Ge diodes fabricated with various doping methods, could originate from the same trap-assisted mechanism. Working p+/n diodes on Ge bulk substrates displayed a reverse current density as low as 2.2·10−2 A/cm2 which was found to be comparable to other literature data. The layers developed in this work can be used as an alternative method to form p+/n junctions on Ge substrates, showing comparable junction leakage results to ion implantation approaches.

Temperature dependence of Raman scattering in GeSn films

AbstractTemperature dependence of Raman scattering from Ge0.95Sn0.05 and Ge0.92Sn0.08 films, grown on Ge (001) substrates by low temperature molecular beam epitaxy, was studied over the temperature range of 90 to 850 K. Nonlinear temperature dependencies in the Raman shift of Ge‐Ge and Ge‐Sn modes have been observed. Although they could be well described by an empirical formula, much different values were found in first‐order temperature coefficient of the Raman shift for the Ge‐Ge mode in bulk Ge, the Ge0.95Sn0.05, and Ge0.92Sn0.08 films, being 1.7, 2.4, and 2.7 × 10−2 cm−1/K, respectively. Two factors, phonon–phonon coupling and thermal expansion, contribute to the temperature dependence of Raman shift. Detailed analysis shows that the thermal expansion contribution increases slightly with the Sn content, whereas the phonon–phonon coupling contribution increases markedly with the Sn content. In other words, the anharmonic decay process is much enhanced by the alloy perturbation in GeSn alloys. In addition, an abrupt change was observed at temperature of 650 K in Raman shift and linewidth of Ge‐Ge mode for the Ge0.92Sn0.08 film, which is caused by Sn segregation, concomitant lattice relaxation, and crystallization as well.

Atypical reversed pressure-induced phase transformation in Ge nanowires

Phase transformations of Ge under compression/decompression cycle at room temperature were studied in a diamond anvil cell (DAC) using in situ synchrotron x-ray diffraction, Raman spectroscopy and near infrared absorption techniques. Upon compression similar behavior is observed in nanowires and in bulk although a higher stability is observed in nanowires. The cubic-diamond phase (Ge-3C), the most energetically favorable phase, transforms into the β-tin metallic phase at high pressure and the reverse Ge-β-tin to Ge-3C transformation is generally inhibited by kinetics when pressure is released. While the transformation in Ge bulk leads mostly to Ge-ST12 phase, the loading/unloading cycle of Ge nanowires in DAC leads back to Ge-3C, exhibiting unprecedented size effects. A comprehensive characterization of the final states is described.

Degradation Mechanism of Ge N+-P Shallow Junction With Thin GeSn Surface Layer

In this article, the degradation mechanism of the Ge N+-P junctions with a thin GeSn surface layer is investigated, aiming for source/drain (S/D) application in emerging field-effect transistors. GeSn is a promising channel material with high carrier mobility, which can offer a better performance of the field-effect transistors. By performing thermal annealing at different temperatures, the reverse-biased leakage current increases apparently when annealing temperature exceeds 550 °C. X-ray diffraction (XRD) analysis indicates most Sn atoms escape from lattice sites while secondary-ion mass spectroscopy (SIMS) analysis indicates enhanced Sn diffusion in Ge bulk after ion implantation. Therefore, a degradation model considering Sn-defects interaction is proposed to explain the degradation of junction leakage current and possible solution is proposed.

Electron and proton irradiation effect on the minority carrier lifetime in SiC passivated p-doped Ge wafers for space photovoltaics

We report on the effect of electron and proton irradiation on effective minority carrier lifetimes (τeff) in p-type Ge wafers. Minority carrier lifetimes are assessed using the microwave-detected photoconductance decay (μW-PCD) method. We examine the dependence of τeff on the p-type doping level and on electron and proton radiation fluences at 1 MeV. The measured τeff before and after irradiation are used to estimate the minority carriers’ diffusion lengths, which is an important parameter for solar cell operation. We observe τeff ranging from ≈ 50 to 230 μs for Ge doping levels between 1 × 1017 and 1 × 1016 at.cm−3, corresponding to diffusion lengths of ≈ 500–1400 μm. A separation of τeff in Ge bulk lifetime and surface recombination velocity is conducted by irradiating Ge lifetime samples of different thicknesses. The possible radiation-induced defects are discussed on the basis of literature.

Ex-situ doping of epitaxially grown Ge on Si by ion-implantation and pulsed laser melting

The integration in Si technology of highly doped of Ge layers with a controlled amount of strain is crucial for nanoelectronic and photonic applications. N-type doping of Ge layers epitaxially grown on Si by P ion-implantation and pulsed laser melting is reported. In particular, samples with or without a post-growth annealing cycle in order to reduce the amount of threading dislocations (TDs) have been studied, in comparison with bulk Ge. Samples have been characterized by Secondary Ion Mass Spectrometry, Van der Paw-Hall and High-Resolution X-ray Diffraction. A very low out-diffusion, 1 × 1020 cm−3 carrier concentration with 100% P activation, as well as an increase of the tensile strain together with an improvement of the crystalline quality is reported, regardless of the as-grown TD density.

Temperature dependent dielectric function and direct bandgap of Ge

The dielectric function of bulk Ge is determined between 0.5 and 6.3 eV in a temperature range of 10–738 K using spectroscopic ellipsometry. The authors provide the data in a tabulated format that can be interpolated as a function of photon energy and temperature using commercial software. Another focus of this paper lies on the analysis of critical points, in particular, on the investigation of the temperature dependence of the direct bandgap E0 and the critical point E0+Δ0, where Δ0 is the spin–orbit splitting. To explore the temperature dependence of critical points, the parameters that characterize their line shapes are calculated using three different techniques. First, the common method of numerically calculating and analyzing the second derivatives of the dielectric function works well for critical points at higher energies. Second, an analysis in reciprocal space by performing a discrete Fourier transform and analyzing the resulting Fourier coefficients yields values for the energies of E0 and E0+Δ0. Third, the energy determined from a parametric semiconductor model is shown as a function of temperature. The authors observe a temperature dependent redshift of the E0 and E0+Δ0 critical point energies as well as an increase in the broadening of E0 with temperature.

Enhancement of the electronic thermoelectric properties of bulk strained silicon-germanium alloys using the scattering relaxation times from first-principles calculations

We use first-principles electronic structure methods to calculate the electronic thermoelectric properties (i.e., due to electronic transport only) of single-crystalline bulk n-type silicon-germanium alloys vs Ge composition, temperature, doping concentration, and strain. We find excellent agreement to available experiments for the resistivity, mobility, and Seebeck coefficient. These results are combined with the experimental lattice thermal conductivity to calculate the thermoelectric figure of merit ZT, finding very good agreement with experiments. We predict that 3% tensile hydrostatic strain enhances the n-type ZT by 50% at carrier concentrations of n=1020cm−3 and a temperature of T=1200K. These enhancements occur at different alloy compositions due to different effects: at 50% Ge composition, the enhancements are achieved by a strain induced decrease in the Lorenz number, while the power factor remains unchanged. These characteristics are important for highly doped and high temperature materials, in which up to 50% of the heat is carried by electrons. At 70% Ge, the increase in ZT is due to a large increase in the electrical conductivity produced by populating the high mobility Γ conduction band valley, lowered in energy by strain.

Modelling and simulation of SEU in bulk Si and Ge SRAM

In this work, the random-walk drift-diffusion (RWDD) model has been coupled to a circuit simulator to investigate single event upsets (SEU) induced by alpha particles in SRAM cells with silicon or germanium as bulk material. The impact of semiconductor charge generation and transport properties on the SEU mechanisms is quantified and discussed in a first-order approach considering ideal materials and generic devices. Our results suggest that the radiation response of Ge-based SRAM should be similar to the one observed for Si-SRAM, the benefits of higher mobilities at circuit level for germanium offsetting the negative impact of a relatively low energy value for electron-pair creation on transient current pulse magnitudes.

Observation of Phonon Propagation in Germanium Nanowires Using Femtosecond Pump–Probe Microscopy

The excited-state dynamics in individual Ge nanowires (NWs) are imaged using ultrafast pump–probe microscopy with high spatial (∼600 nm) and temporal (∼500 fs) resolution. Photoexcitation of the NW by a focused femtosecond laser pulse promotes electrons from the valence band to the conduction band within a 400 nm segment of the 20–30 μm long NW. The localized excitation is then probed by a delayed femtosecond pulse, whose position with respect to the pump pulse is precisely controlled. The pump–probe signals contain contributions from free carriers, thermal excitation, and impulsively excited acoustic phonons, the latter of which are detected in the time domain as a coherent oscillation in the pump–probe signal. Examination of an ensemble of NWs with diameters (d) ranging from 50 to 300 nm shows that the coherence frequency is inversely proportional to d, consistent with excitation of a radial breathing mode. In addition, experiments performed with spatially separated pump and probe beams show that the vibrational motion is not confined to the excitation region but, rather, spreads as much as 2.5 μm along the NW axis during the first 3 ns after excitation. Spatially separated pump–probe microscopy also reveals the creation of a longitudinally propagating wave that travels along the NW at ∼6500 m/s or ∼20% faster than the speed of sound in bulk Ge.

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.

Versatile Electronic Devices Based on WSe2/SnSe2 Vertical van der Waals Heterostructures.

Van der Waals heterostructures formed by stacking of various two-dimensional materials are promising in electronic applications. However, the performances of most reported electronic devices based on van der Waals heterostructures are far away from those of existing (Si, Ge, and III-V bulk material based) technologies. Here, we report high-performance heterostructure devices based on vertically stacked tungsten diselenide and tin diselenide. Due to the unique band alignment and the atomic thickness of the material, both charge carrier transport and energy barrier can be effectively modulated by the applied electrical field. As a result, the heterostructure devices show superb characteristics, with a high current on/off ratio of ∼3 × 108, an average subthreshold slope of 126 mV/dec over 5 dec of current change due to band-to-band tunneling, an ultrahigh rectification ratio of ∼3 × 108, and a current density of more than 104 A/cm2. Furthermore, a small signal half-wave rectifier circuit based on a majority-carrier-transport-dominated diode is successfully demonstrated, showing great potential in future high-speed electronic applications.

Vacancy complexes in nonequilibrium germanium-tin semiconductors

Depth-profiled pulsed low-energy positron annihilation lifetime spectroscopy and Doppler broadening spectroscopy were combined to identify vacancy-related complexes and probe their evolution as a function of Sn content in GeSn epitaxial layers. Regardless of the Sn content in the 6.5–13.0 at. % range, all GeSn samples showed the same depth-dependent increase in the positron annihilation line broadening parameters, relative to that of epitaxial and bulk Ge references, thus confirming the formation of open volume defects during growth. The measured average positron lifetimes were found to be the highest (380–395 ps) in the region near the surface and monotonically decrease across the analyzed thickness but remain above 350 ps. All GeSn layers exhibit average lifetimes that are 20–160 ps higher than those recorded for the Ge reference. Surprisingly, these lifetimes were found to decrease as the Sn content increases in GeSn layers. These measurements indicate that divacancies are the dominant defect in the as-grown GeSn layers. However, their corresponding lifetime was found to be shorter than in epitaxial Ge, thus suggesting that the presence of Sn may alter the structure of divacancies. Additionally, GeSn layers were also found to contain a small fraction of vacancy clusters, which become less important as the Sn concentration increases. The interaction and possible pairing between Sn and vacancies have been proposed to explain the reduced formation of larger vacancy clusters in GeSn when the Sn content increases.

Germanium-on-Nothing for Epitaxial Liftoff of GaAs Solar Cells

Summary Solar cells from III-V materials offer outstanding light conversion efficiency and power densities and have a proven reliability record. Nevertheless, the utilization of III-V devices has been hindered by high production costs that partially stem from expensive substrates for the growth of III-V materials. Here, we present an ultrathin epitaxially ready single-crystal Ge membrane, formed by germanium-on-nothing (GON) technology, which employs morphological evolution of an arrayed porous Ge during hydrogen annealing. This new process, inspired by a silicon-on-nothing (SON) process, significantly improves the reformed Ge surface compared with previous porous Ge studies such that low-defect-density heteroepitaxy of GaAs is achievable. We demonstrated the growth of a 14.44% efficient GaAs solar cell on GON with nearly identical open circuit voltage to a control cell grown on a bulk Ge and successfully transferred it onto an inexpensive handle by employing the plate-like void under the Ge film as a release layer.

Effects of surface passivation by hydrogen on the structural and electronic properties of a germanium nanowire: A sp3 tight binding study

Abstract The surface passivation is an important technique that suppresses the effects of surface dangling bonds on the one-dimensional (1D) nanostructure and changes their electronic properties due to the elimination of surface states. Hydrogen (H) is one of the most renowned surface passivants that can effectively passivate the dangling bonds on the surfaces of group IV materials such as carbon (C), silicon (Si), and germanium (Ge). In this paper, the sp3 tight binding calculation has been performed to investigate the effects of surface passivation by H atoms on the structural and electronic properties of a germanium nanowire (GeNW) oriented along 〈100〉, 〈110〉 and 〈111〉 crystallographic directions over a wide diameter range. The results show that when the entire surface dangling bonds of a GeNW are passivated with H atoms both the nature and magnitude of the band gap are tailored for each diameter and growth direction. Without H passivation of GeNWs surfaces, (i) the 〈100〉-oriented GeNW show indirect band gap semiconductors nature for all diameter values with magnitude of band gap is smaller than the band gap value of a Bulk Ge, (ii) the first smaller diameter 〈110〉-oriented GeNW show zero band gap semiconductor (metallic in nature) while for all the others diameter values they exhibit indirect band gap semiconductors nature with magnitude of band gap is smaller than the Bulk Ge, and (iii) the first smaller diameter 〈111〉-oriented GeNW show indirect band gap semiconductor nature while the two show zero band gap semiconductors (metallic) as well as the others three exhibits narrow direct band gap semiconductors. Further analysis show that when the surface of a GeNW are passivated by H atoms, all the 〈100〉-oriented, 〈110〉-oriented, and 〈111〉-oriented GeNWs exhibits direct band gap semiconductors nature with a magnitude of band gap inversely proportional to their diameter. This shows that the magnitude of band gap for each oriented H passivated surface GeNW is larger at smaller diameter which further decreases with the increase in their diameter and approaches the band gap value of a Bulk Ge at a larger diameter value. These results confirm the suitability of an H-passivated GeNW for designing high quality nanoelectronic devices and for optoelectronic applications particularly for making optical devices such as light emitting diodes and semiconductor lasers.

Inelastic electron-exciton scattering in bulk germanium

We investigate the destructive inelastic as well as the elastic scattering of a hot electron-hole plasma with an incoherent exciton population in bulk Ge by means of optical pump-terahertz probe spectroscopy. An incoherent exciton population evolves from a first optical pulse while a delayed second optical pulse creates the electron-hole plasma. The interaction of the plasma with the exciton population is monitored via the intraexcitonic transitions by a probing terahertz pulse. Analyzing the density-dependent decay of the intraexcitonic transitions after the arrival of the second optical pulse yields an inelastic scattering rate of $2.0\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. An analysis of the corresponding linewidth of the $1s\ensuremath{-}2p$ transition yields a total scattering rate of $3.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. This allows us to experimentally distinguish between elastic and inelastic scattering and we obtain an elastic scattering rate of $1.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$.

80 °C synthesis of thermoelectric nanocrystalline Ge film on flexible plastic substrate by Zn-induced layer exchange

Zn-induced layer exchange allowed us to fabricate nanocrystalline Ge on insulators at 80 °C. The Ge layer had a high hole concentration (>1020 cm−3) and was degenerate, leading to a high electrical conductivity (>200 S cm−1). The power factor reached 240 and 160 μW mK−2 for Ge on glass and even flexible plastic substrates, respectively. For the sample with glass, grain boundary phonon scattering reduced the thermal conductivity to 3 W mK−1, which is an order of magnitude smaller than that of bulk Ge. These findings will accelerate the design of flexible thermoelectric devices based on reliable group IV semiconductors.

Impact of Punch-through Stop Implants on Channel Doping and Junction Leakage for Ge <inline-formula> <tex-math notation="LaTeX">${p}$ </tex-math> </inline-formula>-FinFET Applications

Fin field-effect transistor (FinFET) scaling beyond the 10-nm node requires formation of a junction isolation region between the source and the drain to suppress sub-fin leakage current. In this paper, heavy species such as Sb and As were implanted at room temperature to form a punch-through stop (PTS) layer in n-Ge substrates. The impact of PTS implants on channel doping and defects, as well as junction leakage, was investigated for bulk Ge p-FinFET applications. We report retrograde doping profiles with a low surface (channel) dopant concentration of 4-5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> for Sb PTS implants post 550 °C and 650 °C activation anneals. Besides achieving low effective channel doping, the 650 °C high-temperature activation anneal post PTS implant also helps in the reduction of channel defect density. Furthermore, low junction leakage currents are also demonstrated for Sb and As PTS implants with 550 °C and 650 °C activation anneals. To avoid damaging the FinFET channel layer due to PTS ion implantation, Ge homoepitaxy was also studied post Sb PTS implants. However, Sb PTS implants activated at 650 °C without Ge epitaxy exhibit the best results in terms of low channel dopant and defect concentrations, besides junction leakage.

Ge field-effect transistor with asymmetric metal source/drain fabricated on Ge-on-Insulator: Schottky tunneling source mode operation and conventional mode operation

An asymmetric Schottky tunneling source field-effect transistor (STS FET) is a prospective device structure to suppress the short-channel effect. Recently, we succeeded in the fabrication and operation of a Ge-STS n-channel FET with TiN and PtGe asymmetric metal source/drain (S/D) on a bulk Ge substrate. However, the Ge-STS p-channel FET has not been demonstrated yet. In this study, we fabricated an asymmetric metal S/D FET with the same S/D structure on a bulk Ge and a Ge-on-Insulator (GOI) substrate. The GOI was made by using the Smart-CutTM technique. The device fabricated on a bulk Ge did not operate. On the other hand, the fabricated FET on a GOI, which has a taper-shaped TiN/Ge source interface, showed STS p-FET behavior. These results suggest that the carrier injection can be improved by the optimization of the device structure. As an auxiliary effect, conventional metal-oxide-semiconductor (MOS) FET operation was also observed, thanks to GOI introduction. We demonstrated both STS mode and MOSFET mode operation in the same device on GOI.