Germanium Nanostructures Research Articles

Engineering thermal transport in SiGe-based nanostructures for thermoelectric applications

Abstract

Operando X-ray scattering and spectroscopic analysis of germanium nanowire anodes in lithium ion batteries.

X-ray diffraction (XRD) and Fourier transform extended X-ray absorption fine structure (EXAFS) analysis of X-ray absorption spectroscopy (XAS) measurements have been employed to determine structural and bonding changes, as a function of the lithium content/state of charge, of germanium nanowires used as the active anode material within lithium ion batteries (LIBs). Our data, collected throughout the course of battery cycling (operando), indicate that lithium incorporation within the nanostructured germanium occurs heterogeneously, preferentially into amorphous regions over crystalline domains. Maintenance of the molecular structural integrity within the germanium nanowire is dependent on the depth of discharge. Discharging to a shallower cutoff voltage preserves partial crystallinity for several cycles.

EXAFS and XPS studies of germanium and gallium arsenide nanostructures in porous aluminum oxide matrices

The local atomic structure and chemical bonds of germanium and gallium arsenide, specimens of an isoelectronic series, are studied. Both materials are obtained via thermal evaporation of the materials’ powders onto porous matrices of anodic aluminum oxide and polycorr substrates. The mechanism responsible for the formation of local atomic structure is shown to be almost identical for Ge and GaAs. When compared to the films on polycorr surfaces, changes in local atomic structure are observed in the material obtained in porous matrices due to the difference between the mechanisms of condensation on rough and smooth surfaces.

Preparation of a porous nanostructured germanium from GeO2via a “reduction–alloying–dealloying” approach

Electrochemical reduction-cum-alloying in molten salt and subsequent water-etching dealloying for the controllable, affordable and environmentally friendly preparation of nanoporous germanium.

One-dimensional silicon and germanium nanostructures with no carbon analogues.

In this work we report new silicon and germanium tubular nanostructures with no corresponding stable carbon analogues. The electronic and mechanical properties of these new tubes were investigated through ab initio methods. Our results show that these structures have lower energy than their corresponding nanoribbon structures and are stable up to high temperatures (500 and 1000 K, for silicon and germanium tubes, respectively). Both tubes are semiconducting with small indirect band gaps, which can be significantly altered by both compressive and tensile strains. Large bandgap variations of almost 50% were observed for strain rates as small as 3%, suggesting their possible applications in sensor devices. They also present high Young's modulus values (0.25 and 0.15 TPa, respectively). TEM images were simulated to help in the identification of these new structures.

Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium Ion Batteries: Comparative Phase Conversion of Germanium and Tin

Germanium and tin sulfide nanostructures are considered the most promising candidates for useful alternative materials in commercial Li–graphite anodes of lithium ion batteries. Selenides have received less attention, but the electrochemical reaction mechanism is still being debated. We report the novel synthesis of GeSex and SnSex (x = 1 and 2) nanocrystals by a gas-phase laser photolysis reaction and their excellent reversible capacity for lithium ion batteries. The capacity was 400–800 (mA h)/g after 70 cycles, which is close to the theoretical capacity (Li4.4Ge or Li4.4Sn). Remarkably, SnSex exhibited higher rate capabilities than GeSex. Ex situ X-ray diffraction and Raman spectroscopy revealed the cubic–tetragonal phase conversion of Ge and Sn upon lithiation/delithiation to support their distinctive lithium ion battery capacities. First-principles calculations of the Li intercalation volume change indicate that the smallest volume expansion in the cubic Sn phase can guarantee the enhanced cycling ca...

Impact of annealing on surface morphology and photoluminescence of self-assembled Ge and Si quantum dots.

Controlled growth and characterization of germanium (Ge) and silicon (Si) nanostructure are the key issues for optoelectronic device fabrication. The role of post-annealing on the structural and optical properties of radio frequency (rf) magnetron sputtering grown of Ge and Si quantum dots (QDs) deposited on Si(100) substrate is studied. Atomic force microscopy confirmed the formation of Si and Ge QDs with estimated sizes lower than -17 nm and -14 nm respectively. The X-ray diffraction analysis revealed the formation of Si and Ge QDs accompanied by SiO2 with estimated sizes of -5 and -7 nm for post-annealed Si and pre-annealed Ge QDs respectively. The room temperature photoluminescence spectra for Ge and Si demonstrated an emission peak at 3.20 and 2.72 eV respectively, which are attributed to the electron and hole recombination within QDs. A shift in the PL peak is observed through annealing which is ascribable to the changes in size of QDs and quantum confinement effect. The thermal annealing at 600 degrees C is found to play an important role in controlling the shape, number density, root mean square (rms) roughness and the energy shift of the luminescence band for both Si and Ge QDs. The influence of annealing on growth morphology for Ge QDs is appeared to be stronger than Si.

DFT characterization of nanostructured germanium surfaces induced by cobalt atoms

By deposition in ultra-high vacuum of cobalt on a Ge(111)–c(2 × 8) surface, Mocking et al. obtained a surface reconstruction. In the present paper, we analyse the related atomic structure, proposed by these authors, by means of density functional theory calculations. The surface presents ordered clusters that consist of six Ge atoms arranged in a triangle, lying above three Co atoms. The latter are located at substitutional positions within the top plane of the Ge(111) first bilayer. These clusters are similar to what is obtained on part of the Co-induced Si(111) surface. For this surface, the clusters are terminated either by six Si atoms or by one, two or three adatoms above the six Si atoms. As the Co–Ge clusters systematically display six protrusions in the scanning tunnelling microscopy measurements by Mocking et al., we investigated why Ge adatoms are not present. Comparison of the Gibbs energy, interatomic distances, as well as charge density indicates that Ge adatoms on top of Co-Ge clusters are less stable than Si adatoms in the Co-Si system. Copyright © 2013 John Wiley & Sons, Ltd.

(Invited) Germanium Nanostructures in High-K Materials

In this paper our recent research on Ge nanoparticles embedded in ZrO2 will be reviewed. Ge nanoparticles have been deposited by rf-cosputtering of Ge1.6ZrO2/ZrO2 superlattices and subse-quent annealing. TEM measurements confirmed the phase separation of the two compounds and the forming of ex-tended nanocrystalline Ge layers at 650°. These layers show a luminescence signal at 2.5 eV, which is contributed to defect luminescence.

Long Wavelength {greater than or equal to}1.9 μm Germanium for Optoelectronics Using Process Induced Strain

The photoluminescence of tensile strained germanium nanostructures is reported. Sub-micron gratings and pillars were fabricated before bein embedded in a strained silicon nitride films. Using different deposition conditions and different sizes of structures the stress in the nanostructures can be controlled. The measured optical properties of the samples show that the direct band-gap is shifted drastically towards higher wavelength over 1.9 um wavelength. This process of local control of the stress in germanium nanostructures is compatible with integrated photonic devices in waveguides geometry. That opens the route for both emitters and photo-detectors above 1.6 um wavelength which are not easily available and also potentially towards a germanium laser.

(Invited) Germanium Nanostructures in High-K Materials

not Available.

Mapping the local elastic properties of nanostructured germanium surfaces: from nanoporous sponges to self-organized nanodots

Due to their reduced dimensions, the mechanical properties of nanostructures may differ substantially from those of bulk materials. Quantifying and understanding the nanomechanical properties of individual nanostructures is thus of tremendous importance both from a fundamental and a technological point of view. Here we employ a recently introduced atomic force microscopy mode, i.e., peak-force quantitative nanomechanical imaging, to map the local elastic properties of nanostructured germanium surfaces. This imaging mode allows the quantitative determination of the Young’s modulus with nanometer resolution. Heavy-ion irradiation was used to fabricate different self-organized nanostructures on germanium surfaces. Depending on the sample temperature during irradiation, nanoporous sponge-like structures and hexagonally ordered nanodots are obtained. The sponge-like germanium surface is found to exhibit a surprisingly low Young’s modulus well below 10 GPa, which furthermore depends on the ion energy. For the nanodot patterns, local variations in the Young’s modulus are observed: at moderate sample temperatures, the dot crests have a lower modulus than the dot valley whereas this situation is reversed at high temperatures. These observations are explained by vacancy dynamics in the amorphous germanium matrix during irradiation. Our results furthermore offer the possibility to tune the local elastic properties of nanostructured germanium surfaces by adjusting the ion energy and sample temperature.

Self-assembled Ge islands and nanocrystals by RF magnetron sputtering and rapid thermal processing: The role of annealing temperature

Germanium (Ge) nanostructures were fabricated through the rapid thermal processing of radio frequency sputtered Ge on silicon (Si). The substrates were unheated during the growth, resulting in a post-growth Ge deposited layer that was 250nm thick with a surface that had no evidence of nanostructure formation. The samples subsequently underwent rapid annealing from 400°C to 800°C for 15s. Dramatic nanostructuring was observed at the surface following annealing above 600°C. Scanning electron microscopy showed that as annealing temperatures increased, the Ge layer evolved into circular islands. The sizes of the Ge islands increased and the islands became more uniform and dense at 700°C. Viable Ge nanocrystals were obtained at 600°C and 700°C given the increased annealing temperatures that improved crystallinity. The calculated Raman line shape based on the phonon confinement model agreed well with the experimental spectrum which constituted crystalline Ge nanostructures with an estimated size of 2.65–3.5nm. In addition, no Ge–Si intermixing was observed at the interface. High resolution X-ray diffraction revealed the tetragonal Ge phases and the samples were of polycrystalline structures. The annealing temperature also enhanced photo currents of the fabricated metal semiconductor metal photodetector. These results suggest that 700°C for 15s is an optimum annealing temperature for the production of viable crystalline Ge nanostructures.

High-field chemistry of organometallic precursors for direct-write of germanium and silicon nanostructures

The fabrication of integrated multimaterial structures with nanoscale accuracy is a challenging pursuit essential for novel advances in electronics and photonics. The atomic force microscope precisely controls the synthesis of these nanostructures by triggering chemical reactions at determined substrate locations. Using a biased atomic force microscope tip, we have recently demonstrated the localized synthesis of germanium and silicon from organometallic liquid precursors. This localized synthesis yields the direct-write of germanium and silicon nanostructures onto the substrate. Here, we elucidate the reactions occurring during this synthesis in the tip–sample proximity. Reactions for both precursors are triggered by field-emitted electrons that fragment the precursor molecules into the desired products. In this reaction pathway, the synthesis of germanium is more favourable than the synthesis of silicon from precursors containing the same ligands. We explain these differences by comparing experimental data of reaction onset and kinetics to continuum/VASP simulations. This data is supplemented with electron impact mass spectrometry of the precursors and compared with the simulation results.

In situ one-step synthesis of a 3D nanostructured germanium–graphene composite and its application in lithium-ion batteries

A germanium–graphene nanocomposite material with three-dimensional nanostructures has been synthesized by an efficient one-step, in situ, and aqueous-based method. The electrochemical properties of the germanium–graphene nanocomposite have been evaluated by galvanostatic discharge–charge cycling, cyclic voltammetry, and electrochemical impedance spectroscopy. Results show that the germanium–graphene nanocomposite has a much more stable cycling performance than that of the pure germanium, with a capacity of about 832 mA h g−1 after 50 cycles. The rate capability is also improved significantly. The superior performance is attributed to the graphene content, which increases the material's conductivity, enlarges the specific surface area, delivers enough sites to allow dispersion of the Ge nanoparticles without excessive agglomeration, and provides void space to buffer the volume change during discharge–charge cycles.

Erratum: “Giant enhancement of n-type carrier mobility in highly strained germanium nanostructures” [J. Appl. Phys. 109, 113703 (2011)

Erratum: “Giant enhancement of <i>n</i>-type carrier mobility in highly strained germanium nanostructures” [J. Appl. Phys. 109, 113703 (2011)

Low-temperature (210 °C) deposition of crystalline germanium via in situ disproportionation of GeI2

A new approach is reported for depositing crystalline germanium films. The deposition occurs at low temperatures (210–260°C) via in situ disproportionation of GeI2 and is thereby useful for depositing Ge onto a wide range of surfaces. Deposition onto glass and polymer substrates is demonstrated. The rate of deposition onto glass is found to be 25ngmin−1. New synthetic routes to GeI2, GeI4, and Cu3Ge are also reported. These are valuable precursors for the synthesis of germanium nanostructures and organo-germanium compounds.

Long Wavelength ≥1.9 μm Germanium for Optoelectronics Using Process Induced Strain

not Available.

Morphology Control of Self-Catalyzed Germanium Nanostructures with Graphitic Carbon Shell

The crystalline germanium nanowires (GeNWs) with a uniform graphitic carbon shell were prepared via a conventional low-pressure chemical vapor deposition method without any external catalyst. The GeNWs grown at low temperature (Tg < 500 degrees C) have a uniform diameter with a large expect ratio of more than 10(3). With increasing the growth temperature (Tg > 500 degrees C), however, the nanowire morphology is dramatically changed into a hybrid structure where highly dense Ge nanoparticles (GeNPs) with a diameter of 5-10 nm are attached onto the Ge nanowires. The nanostructures consist of crystalline Ge-core and very thin graphitic carbon shell. The possible mechanism of anisotropic growth and the control of morphological transition from uniform nanowires to NW/NP hybrid structures are discussed and demonstrated.

Low-Temperature Fabrication of Germanium Nanostructures by Ion Irradiation: Effect of Supplied Particle Species

Low-Temperature Fabrication of Germanium Nanostructures by Ion Irradiation: Effect of Supplied Particle Species