Ge Crystals Research Articles

Phonon dispersion of bulk Ge-rich SiGe: inelastic X-ray scattering studies

Group IV semiconductors, such as SiGe with a higher Ge fraction, are attracting attention as next-generation materials for high-mobility channel or thermal conductivity devices. Thus, we investigated the phonon dispersions of a Si1–xGex (x = 0.72) alloy crystal by the inelastic X-ray scattering (IXS) method. In this method, measurement with an X-ray beam with a small spot size of 75 μm and resolution of millielectronvolts is possible due to the synchrotron technique. The longitudinal and transverse phonon spectra of optical and acoustic modes were measured from the Γ to the X point in the phonon energy band (0–70 meV) with millielectronvolt energy resolution. SiGe showed dispersion characteristics similar to those of Si and Ge crystals. It was confirmed that the IXS method is useful for obtaining properties of phonon dispersion.

Au induced lateral crystallization of amorphous Ge with stress stimulation at 130 ○C

The effect of stress stimulation on gold (Au) induced lateral crystallization (GILC) of amorphous Ge on insulating substrates is investigated. As a result, the GILC is significantly enhanced by using compressive residual stresses of up to 200 MPa in TEOS-SiO2. In addition, it is found that the annealing temperature necessary to cause the GILC for a short annealing time (60 min) can be decreased to 130 °C. We have demonstrated that the GILC enhancement was caused by the stress stimulation contributed to bond rearrangement and Au easily diffused into Ge. This study proposes a unique low temperature crystallization technique that introduces a residual film stress on metal induced lateral crystallization, paving the way for the low-cost fabrication of flexible electronic devices on low-softening temperature plastic substrates.

New insights into thermomechanical behavior of GeTe thin films during crystallization

In this work, we reexamine Ge rejection in Ge-rich GeTe thin films with a slight deviation from stoichiometry using a unique combination of in situ measurements: curvature and x-ray diffraction as well as electrical resistance and x-ray diffraction and reflectivity during annealing. This unique combination of several experiments performed simultaneously on a synchrotron beamline allows to monitor in situ, during the crystallization and phase transformation, the microstructure, the strain and the stress changes, as well as electrical properties of GeTe films. Structural, electrical and thermomechanical evolutions of the GeTe thin films upon annealing are shown to follow three different steps. Stage I, before crystallization, is characterized by a tensile stress variation and a small decrease of the mass density. Stage II corresponds to the rhombohedral αGeTe phase crystallization leading to an abrupt tensile stress jump (+72 MPa), a mass density increase, and followed by a slight compressive stress evolution. During stage III, Ge crystallization is observed leading to a compressive stress jump (−54 MPa), an abrupt increase in αGeTe lattice spacing and diffracted intensity, whereas αGeTe diffraction peak widths decrease. During cooling a thermoelastic behavior is observed. A detailed analysis of stage III (Ge precipitation and crystallization) is performed and discussed regarding structural, stress, microstrain, electrical and thermomechanical properties. In particular, this study reveals that crystalline Ge precipitation results in important changes (volume of the unit cell, homogeneity of lattice spacing, average stress …) in the surrounding GeTe matrix. Different scenarios are proposed to understand these results.

Effects of surface oxidation on the crystallization characteristics of Ge-rich Ge-Sb-Te alloys thin films

We have studied the effect of surface oxidation on the crystallization of Ge-rich Ge-Sb-Te materials, promising for Phase Change Memories working at high temperatures (>350 °C). For this, we have compared the structural and chemical characteristics of films left exposed to air with those shown by TiN-encapsulated films. The effect of air exposure is to lower the temperature at which the onset of crystallization starts by 50–60 °C. Instead of homogeneous nucleation observed in encapsulated films, crystallization proceeds from the surface towards the bulk of the film and results in a massive redistribution of the chemical elements, forming Ge grains which grow until Ge concentration is low enough to allow the Ge2Sb2Te5 rocksalt phase to nucleate. In the air-exposed films, Ge crystallization preferentially occurs at the film surface while the Ge2Sb2Te5 grains develop later, at higher temperature, and deeper in the film. Our results strongly suggest that “seeds” are formed in or below the oxide during the early stage of annealing, promoting the heterogeneous nucleation of the Ge cubic phase at a lower temperature than observed in encapsulated films. These seeds necessarily involve oxygen and we speculate that crystalline Sb2O3 nuclei formed in the surface layer during annealing play this role.

The History of the German Association for Crystal Growth

The History of the German Association for Crystal Growth

Quantification of trace amounts of transmuted nuclides in transmutation detectors using SIMS

Neutron detectors based on neutron induced transmutation of nuclides of high-purity materials into other stable nuclides has been recently proposed to improve accuracy of long-term measurement of neutron fluence in fission nuclear reactors. The inductively coupled plasma mass spectrometry (ICP-MS) and the prompt gamma neutron activation analysis (PGNAA) have been used for the measurement of concentration of transmuted nuclides, which was then converted into neutron fluence using known neutron-reaction cross sections.In this work, the concentrations of transmuted elements were measured with a magnetic-sector secondary ion mass spectrometer (SIMS) instead of ICP-MS or PGNAA. Single crystals of Si, Ge and a polycrystalline metal foil of Ni were used as detector materials. The concentrations were measured down to sub ppb range in Si and Ge crystals. 9 prospective nuclides for transmutation detectors — 25Mg, 26Mg, 27Al, 31P, 67Zn, 69Ga, 71Ga, 59Ni, 63Ni were measured and compared to a reference neutron activation detector (NAD) method with neutron reaction cross section values taken from two different databases. With exception of 26Mg, the values determined using SIMS were within 20 % compared to those obtained using NADs with a cross section value from one or both databases. In case of 26Mg the difference between the results of SIMS and NAD methods was higher than 45 % for data from both databases. This discrepancy for 26Mg measurement was assigned tentatively to uncertainties of the neutron reaction cross section values. Nevertheless, the experiments proved distinct advantages of SIMS over ICP-MS in sample preparation and throughput, and in consuming micro-volumes of material and identified at least one well performing transmutation detector for thermal neutrons (Ge - single crystal) with agreement within 3 % between SIMS-TMD and NADs.

The environmental stability characterization of exfoliated few-layer CrXTe3 (X = Si, Ge) nanosheets

Two dimensional van der Waals layered CrXTe3 (X = Si, Ge) crystals have attracted great attention recently due to the observation of fascinating long range magnetic order. However, the detailed environmental stability of surface chemical reactivity of CrXTe3 has not been explored yet. Herein, we have studied the ambient air oxidation of exfoliated few-layer CrXTe3 (X = Si, Ge) nanosheets using optical microscopy, atomic force microscopy and Raman spectra. The optical contrast decreases, and surface roughness and thickness increase with exposure time for few-layer CrXTe3. The stability of CrGeTe3 flakes is superior to that of CrSiTe3. CrSiTe3 nanosheets degrade almost immediately after exposure to air, while CrGeTe3 above 11 nm can be air stable for 24 h. Under high power laser excitation or exposure to the air for sufficient time, it is found that amorphous Te and TeOx are formed initially and continually converted to TeO2, resulting in the degradation of CrXTe3. Our results shed light on environmental effects on the surface evolution of CrXTe3 and highlight the need for developing suitable protecting strategies in future CrXTe3-based devices.

Controlling the thermoelectric power of silicon–germanium alloys in different crystalline phases by applying high pressure

Si–Ge crystals are promising materials for use in various stress-controlled electronic junctions for next-generation nanoelectronic devices.

Quasi-Transient Calculation of Czochralski Growth of Ge Crystals Using the Software Elmer

A numerical scheme was developed to compute the thermal and stress fields of the Czochralski process in a quasi-time dependent mode. The growth velocity was computed from the geometrical changes in melt and crystal due to pulling for every stage, for which the thermal and stress fields were computed by using the open source software Elmer. The method was applied to the Czochralski growth of Ge crystals by inductive heating. From a series of growth experiments, we chose one as a reference to check the validity of the scheme with respect to this Czochralski process. A good agreement both for the shapes of the melt/crystal interface at various time steps and the change in power consumption with process time was observed.

Semiconductor Plasma Antennas Formed by Laser Radiation

Efficiency of the transmission of high-frequency signals by semiconductor plasma antennas based on Ge and Si single crystals with surface nonequilibrium electron-hole plasma generated by laser diode radiation has been experimentally studied. Dependences of the amplitude of a radiated 6- to 7.5-GHz microwave signal on the laser power and size of the laser-irradiated region on the semiconductor transmitting dipole antenna are determined. It is shown that a more than tenfold increase can be achieved in the efficiency of useful signal transmission by the plasma antenna formed in Ge crystals.

Micro ATR-FTIR spectroscopic imaging of colon biopsies with a large area Ge crystal.

A new large-area germanium ATR crystal is utilised with an FTIR microscope to improve the acquired images of de-paraffinized colon biopsy sections, without recourse to a synchrotron source. The large crystal (⌀ = 28 mm) offers significant improvements compared to slide-on small germanium crystal (⌀ = 3.5 mm); for example, it facilitates more uniform distribution of higher signal intensity within the field of view and more rapid acquisition time. Mapping of a larger sample area up to ca. 350 × 350 μm2 with this new set-up, coupled with imaging using an FPA detector, is demonstrated for the first time on biological specimens. The performance of k-means clustering algorithm applied to classify the different anatomical structures of the colon biopsies is greatly improved with mapping. Comparison of H&E stained adjacent tissue sections with false-colour k-means images strongly support differentiation of five distinct regions within tissues. The efficiency of the methodology to categorise colon tissues at various stages of malignancy is analysed via multivariate chemometrics. The second derivative spectra extracted from the crypt region of the colon were subjected to Partial Least Squares classification. Good separation between data in clusters occurs when projecting spectra on a PLS score plot on a plane made by the first 3 principal components. Important spectral biomarkers for colon malignancy classification were identified to exist mostly in the fingerprint region of the FTIR spectrum based on the chemometrics analysis.

Atomic Transport in Au-Ge Droplets: Brownian and Electromigration Dynamics.

The deposition of Au on Ge(111)-sqrt[3]×sqrt[3]-Au above the eutectic temperature results in the formation of AuGe liquid droplets that reach the liquidus composition by digging a hole in the Ge substrate. The combination of low-energy electron microscopy and atomic force microscopy measurements shows that AuGe droplets randomly migrate or electromigrate under an applied electric current dragging their underneath hole. The droplet motion is due to a mass transport phenomenon based on Ge dissolution at the droplet front and Ge crystallization at its rear. At high temperature the mass transport is limited by attachment or detachment at the solid-liquid interface and the activation energy is 1.05±0.3 eV. At low temperature the effective activation energy increases as a function of the droplet radius. This behavior is attributed to the nucleation of 2D layers at the faceted liquid-solid interface.

Band Structure of Strained $\mathrm{Ge}_{1-x}~\mathrm{Sn}_{x}$ Alloy: A Full-Zone 30-Band ${k}\cdot{p}$ Model

We extend the previous 30-band k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">.</sup> p model effectively employed for relaxed Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy to the case of strained Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy. The strain-relevant parameters for the 30-band k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">.</sup> p model are obtained by using linear interpolation between the values of single crystal of Ge and Sn that are from literatures and optimizations. We specially investigate the dependence of band-gap at L-valley and Γ-valley with different Sn composition under uniaxial and biaxial strain along [100], [110] and [111] directions. The good agreement between our theoretical predictions and experimental data validates the effectiveness of our model. Our 30-band k <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">.</sup> p model and relevant input parameters successfully applied to relaxed and strained Ge <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> alloy offers a powerful tool for the optimization of sophisticated devices made from such alloy.

Sn Nanodots-Induced Composition Enhancement (NICE) to Achieve 26 at. % Sn in GeSn for Mid-Infrared Integrated Photonics

We present a nanodot-induced compositionenhancement (NICE) approach for achieving high Sn composition GeSn. The approach involves the deposition of Sn dots (20-60nm in diameter) using physical vapor deposition (PVD) with a subsequent deposition of a Ge capping layer and crystallization annealing. Photoluminescence at λ=3-4 μm is demonstrated towards mid infrared photonic applications.

Orientation Relationships of Pure Tin on Single Crystal Germanium Substrates

The limited number of independent β-Sn grain orientations resulting from the difficulty in nucleating β-Sn during solidification of Sn-based solders has a large effect on the resulting β-Sn grain size and, hence, on overall solder joint performance and reliability. This study analyzes the efficacy of Ge as a heterogeneous nucleation agent for β-Sn by observing the morphologies and orientation relationships of as-deposited, solid-state annealed, and liquid-state annealed pure Sn films on single crystal Ge (100), (110), and (111) substrates. Results from scanning electron microscopy and electron backscatter diffraction showed that the as-deposited Sn films all deposited with a Sn (001)|| z-axis texture, regardless of the underlying Ge substrate orientation. Solid-state annealing at 150 °C for 5 min did not result in significant dewetting of the Sn films, and the films maintained their as-deposited texture of Sn (001)|| z-axis, regardless of the underlying Ge substrate orientation. Liquid-state annealing at 235 °C for 1 min resulted is large-scale dewetting of the Sn films and re-orientation of the Sn films on the various Ge substrates. After solidification, the Ge (100) and (110) single crystal substrates produced patches of dewetted grains of the same orientation but there were no consistent Sn grain textures after liquid-state annealing, suggesting no single orientation relationship. In contrast, solidification on Ge (111) single crystal substrates resulted in isolated grains with a single Sn film texture and an orientation relationship of \( \left( {100} \right)_{\rm{Sn}} \parallel \left( {111} \right)_{\rm{Ge}} {\hbox{and}} \left[ {100} \right]_{\rm{Sn}} \parallel \left[ {110} \right]_{\rm{Ge}} \). Density Functional Theory simulations of the experimentally observed Ge (111) sample orientation relationship and the Ge/Sn cube-on-cube orientation relationship suggest favorable relative interfacial binding energies for both interface orientations.

Removal mechanism and surface quality of crystal semiconductor materials in scratching tests with Berkovich indenter

Crystal semiconductor materials have been applied in a wide range of technology applications for their unique physical and mechanical properties. The groove depths of crystal semiconductor materials increased with the increasing scratching load while the effects of scratching speeds on the groove depths were not significant. Unlike the indentation process, the surfaces were sheared by the scratching load, leading to plastic flow and “ridge” at the edge of the scratches in the ranging of 10–40 mN. Cracks were observed at the edge of surface and extended to the free surface when the scratching load reached 60 mN, even resulting in the removal of materials due to brittle fracture. Meanwhile, the critical load of elastic-plastic deformation of single crystal Si and Ge was 2.24 mN and 2.26 mN, respectively. The original Si–I was converted to β-Si, and then to Si-XII (R8) and body-centered cubic (BC8), eventually returned to the Si-XII structure in the scratching process. Micro-chips and fracture debris indicated that brittle fracture and shear fracture occurred in the scratching experiments. In order to obtain relatively good surface quality of crystal semiconductor materials, the scratching load should be controlled below 40 mN and the scratching speed should be about 4 μm/s.

Investigating the formation of isotopically pure layers for quantum computers using ion implantation and layer exchange

Quantum computers have been proposed that exploit entangled quantum states between atoms that are isolated from environmental perturbations in a “semiconductor vacuum” which can be formed by cryogenically cooling an isotopically pure, defect free crystalline layer consisting of Si, or Ge. In a preliminary investigation of an implant and deposition layer exchange technique to produce such “vacuums”, a layer of aluminium was implanted with 28Si using a conventional implanter. After annealing and cross sectioning, layer exchange was observed to have produced multiple isolated crystals in a cross sectional TEM image. Further deposited Al layers were implanted with Ge using a SIMPLE (Single Ion Multispecies Positioning at Low Energy) implanter over a range of fluences. After anneals at 250 °C and Al removal, crystals of Ge (which also contained Si) were seen at areal densities that increased with implant fluence.

In situ dynamic transmission electron microscopy characterization of liquid-mediated crystallization of amorphous Ge

Crystal growth rates during laser-induced, liquid-mediated crystallization of amorphous Ge were measured with movie-mode dynamic transmission electron microscopy (MM-DTEM), a photoemission microscopy technique with nanosecond-scale time resolution. Films of 50-nm thick amorphous Ge were crystallized using a 12-ns laser pulse with a Gaussian spatial profile, which established high local temperature gradients in the specimen. Crystallization proceeded by the formation of a central zone with a high nucleation rate (zone I), followed by liquid-mediated outward growth of columnar grains (zone II), followed by spiraling growth (zone III) until the crystallization halted in cooler parts of the specimen. Zone II growth was imaged for several laser pulse energies with 20-ns electron pulses with 95 ns between frames. A thin liquid layer between the solid amorphous phase and the advancing crystallization front during zone II growth was imaged. The zone II growth rate for each experiment remained nearly constant although the crystallization front passes through a large temperature gradient. Measured growth rates ranged from 5.7 to 13.6 m/s, consistent with transient liquid layer mediated growth rather than solid-state growth. In contrast with a previous report, the growth rate did not increase systematically with laser energy or absorbed energy. The new results, together with previously reported data, suggest that both sets of experiments were conducted under conditions where the growth rate saturates near its maximum value. A phenomenological model based on the concept of upper and lower threshold temperatures for the zone II growth was fitted to the data from these experiments and previous MM-DTEM crystallization experiments.

Impurity concentration dependent electrical conduction in germanium crystals at low temperatures

A germanium single crystal of 7 N+ purity with a diameter of 45 mm and a length of 100 mm has been grown by the Czochralski method. The structural quality of the crystal has been characterized by Laue diffraction. Electrical conduction and Hall measurements are carried out on samples retrieved from different parts of the crystal along the growth axis. The top part of the crystal exhibits the lowest impurity concentration ($${\sim } 10^{12}\,\hbox {cm}^{-3})$$ that gradually increases towards the bottom ($$10^{13}\,\hbox {cm}^{-3})$$. The crystal is n-type at room temperature and the resistivity shows a non-monotonic temperature dependence. There is a transition from n-type to p-type conductivity below room temperature at which bulk resistivity shows a maximum and dip in carrier mobility. The intrinsic to extrinsic transition region shifts towards room temperature as the impurity concentration increases and it reflects the purity level of the crystal. A similar trend is observed in the boron-implanted high purity germanium (HPGe) crystal at different doping levels. The phenomena can be understood as a result of interplay between a temperature dependent conduction mechanism driven by an impurity band and an intrinsic carrier in Ge crystals having fairly low acceptor concentrations ($${<}10^{12}\,\hbox {cm}^{-3})$$.

Ge nanodot-mediated densification and crystallization of low-pressure chemical vapor deposited Si3N4 for advanced complementary metal-oxide-semiconductor photonics and electronics applications

A new phenomenon of highly localized, nanoscale densification and crystallization of silicon-nitride (Si3N4) layers has been observed. A drastic reduction in the thermal budget (temperature and processing time) for local densification and even nanocrystallization of low-pressure chemical vapor deposited amorphous Si3N4 layers is mediated by the presence of Ge, Si, and O interstitials in close proximity to the Si3N4. The enhancement of localized densification and nanocrystallization observed in Si3N4 layers appears to be catalyzed by proximal Ge quantum dots (QDs) ‘migrating’ through the Si3N4/Si layers and are influenced by the oxidation time and Ge QD size. Implications of the highly localized, nanoscale densification and crystallization of silicon-nitride (Si3N4) layers for photonic and electronic device applications are discussed.