Ge Nanowires Research Articles

Investigation of inversion, accumulation and junctionless mode bulk Germanium FinFETs

The characteristic performance of n-type and p-type inversion (IM) mode, accumulation (AC) mode and junctionless (JL) mode, bulk Germanium FinFET device with 3-nm gate length (LG) are demonstrated by using 3-D quantum transport device simulation. The simulated bulk Ge FinFET device exhibits favorable short channel characteristics, including drain-induced barrier lowering (DIBL<10 mV/V), sub threshold slope (SS∼64mV/dec.). Electron density distributions in ON-state and OFF-state also show that the simulated devices have large ION/IOFF ratios. Homogenous source/drain doping is maintained and only the channel doping is varied among different operating modes. Also, a constant threshold voltage |VTH| ∼ 0.31 V is maintained. Moreover, the calculated quantum capacitance (CQ) values of the Ge nanowire emphasizes the importance of quantum confinement effects (QCE) on the performance of the ultra-scaled devices.

Effects of temperature on the ion-induced bending of germanium and silicon nanowires

Nanowires can be manipulated using an ion beam via a phenomenon known as ion-induced bending (IIB). While the mechanisms behind IIB are still the subject of debate, accumulation of point defects or amorphisation are often cited as possible driving mechanisms. Previous results in the literature on IIB of Ge and Si nanowires have shown that after irradiation the aligned nanowires are fully amorphous. Experiments were recently reported in which crystalline seeds were preserved in otherwise-amorphous ion-beam-bent Si nanowires which then facilitated solid-phase epitaxial growth (SPEG) during subsequent annealing. However, the ion-induced alignment of the nanowires was lost during the SPEG. In this work, in situ ion irradiations in a transmission electron microscope at 400 °C and 500 °C were performed on Ge and Si nanowires, respectively, to supress amorphisation and the build-up of point defects. Both the Ge and Si nanowires were found to bend during irradiation thus drawing into question the role of mechanisms based on damage accumulation under such conditions. These experiments demonstrate for the first time a simple way of realigning single-crystal Ge and Si nanowires via IIB whilst preserving their crystal structure.

Hetero-Epitaxy and Self-Adaptive Stressor Based on Freestanding Fin for the 10 nm Node and Beyond**Supported by the National Key Research and Development Program of China (2016YFA0301701), and the Youth Innovation Promotion Association of CAS under Grant No 2016112.

A promising technology named epitaxy on nano-scale freestanding fin (ENFF) is firstly proposed for hetero-epitaxy. This technology can effectively release total strain energy and then can reduce the probability of generating mismatch dislocations. Based on the calculation, dislocation defects can be eliminated completely when the thickness of the Si freestanding fin is less than 10 nm for the epitaxial Ge layer. In addition, this proposed ENFF process can provide sufficient uniaxial stress for the epitaxy layer, which can be the major stressor for the SiGe or Ge channel fin field-effect transistor or nanowire at the 10 nm node and beyond. According to the results of technology computer-aided design simulation, nanowires integrated with ENFF show excellent electrical performance for uniaxial stress and band offset. The ENFF process is compatible with the state of the art mainstream technology, which has a good potential for future applications.

Solution synthesis of lead seeded germanium nanowires and branched nanowire networks and their application as Li-ion battery anodes

Herein, we report the high density growth of lead seeded germanium nanowires (NWs) and their development into branched nanowire networks suitable for application as lithium ion battery anodes. The synthesis of the NWs from lead seeds occurs simultaneously in both the liquid zone (solution–liquid–solid (SLS) growth) and solvent rich vapor zone (vapor–liquid–solid (VLS) growth) of a high boiling point solvent growth system. The reaction is sufficiently versatile to allow for the growth of NWs directly from either an evaporated catalyst layer or from pre-defined nanoparticle seeds and can be extended to allowing extensive branched nanowire formation in a secondary reaction where these seeds are coated onto existing wires. The NWs are characterized using TEM, SEM, XRD and DF-STEM. Electrochemical analysis was carried out on both the single crystal Pb–Ge NWs and the branched Pb–Ge NWs to assess their suitability for use as anodes in a Li-ion battery. Differential capacity plots show both the germanium wires and the lead seeds cycle lithium and contribute to the specific capacity that is approximately 900 mAh g−1 for the single crystal wires, rising to approximately 1100 mAh g−1 for the branched nanowire networks.

Behavior of Germanium and Silicon Nanowire Anodes with Ionic Liquid Electrolytes.

The electrochemical behavior of binder-free, germanium and silicon nanowires as high-capacity anode materials for lithium-ion battery systems is investigated in an ionic liquid electrolyte. Cyclic voltammetry, cycling tests, and impedance spectroscopy reveal a highly reversible lithium alloying/dealloying process, as well as promising compatibility between the Ge and Si materials and the electrolyte components. Reversible capacities of 1400 and 2200 mA h g-1 are delivered by the Ge and Si anodes, respectively, matching the values exhibited in conventional organic solutions. Furthermore, impressive extended cycling performance is obtained in comparison to previous research on Li alloying anodes in ionic liquids, with capacity retention overcoming 50% for Si after 500 cycles and 67% for Ge after 1000 cycles, at a current rate of 0.5C. This stable long-term cycling arises due to the ability of the electrolyte formulation to promote the transformation of the nanowires into durable porous network structures of Ge or Si nanoligaments, which can withstand the extreme volume changes associated with lithiation/delithiation. Remarkable capacity is exhibited also by composite Ge and Si nanowire electrodes. Preliminary tests with lithium cobalt oxide cathodes clearly demonstrate the feasibility of Ge and Si nanowires in full batteries.

Rapid, Low-Temperature Synthesis of Germanium Nanowires from Oligosilylgermane Precursors

New oligosilylgermane compounds with weak Ge–H bonds have been used as precursors for the rapid synthesis of germanium (Ge) nanowires in high yields (>80%), via a solution–liquid–solid (SLS) mechanism, using indium (In) nanoparticles as a seeding agent over a temperature range between 180 and 380 °C. Even at low growth temperatures, milligram quantities of Ge nanowires could be synthesized over a reaction period of between 5 and 10 min. The speed of release of Ge(0) into the reaction environment can be tuned by altering the precursor type, synthesis temperature, and the presence or lack of an oxidizing agent, such as tri-n-octylphosphine oxide (TOPO). Energy-dispersive X-ray analysis showed that silicon atoms from the precursors were not incorporated into the structure of the Ge nanowires. As both In and Ge facilitate reversible alloying with Li, Li-ion battery anodes fabricated with these nanowires cycled efficiently with specific capacities, i.e., >1000 mAh g–1

Anomalous Surface Doping Effect in Semiconductor Nanowires

Surface doping is being used as an effective approach to improve the mechanical, optical, electronic, and magnetic properties of various materials. For example, experimental studies have proven that rare-earth element doping can enhance the optical properties of silicon nanostructures. However, the majority of previous investigations focused on either bulk materials or nanosized spherical crystals. Here we present a comparative study on semiconducting germanium (Ge) nanowires with and without surface doping by using multiple integrated characterization probes, including high resolution scanning/transmission electron microscopy (SEM/TEM), in situ high pressure synchrotron X-ray diffraction (XRD), and Raman spectroscopy. Our results reveal that under pressure the stability of the Ge-I phase (diamond structure) in erbium (Er)-doped Ge nanowires is enhanced compared to undoped Ge nanowires. We also found an increased stability of the Ge-II phase (body centered tetragonal structure) in Er-doped Ge nanowires du...

Ge nanowire photodetector with high photoconductive gain epitaxially integrated on Si substrate

Efficient nanoscale photodetectors are desirable for future applications such as on-chip optical interconnect systems. High density integration, ideally at the transistor level, requires nanoscale photodetectors integrated on Si using a CMOS compatible process. In this study, we demonstrate 20 nm diameter Ge photodetectors based on individual Ge nanowires epitaxially grown on a Si substrate, with a photoconductive gain of 2000 and responsivity of 22.6 A/W operating at 1.55 μm wavelength. The abrupt heterojunction between the Ge nanowire and the Si substrate minimizes the leakage current including generation/recombination and tunneling. The photocurrent amplification can be attributed to the long lifetime of surface state trapped electrons in the Ge nanowires and Ge/Si core/shell nanowires. By increasing the number of active nanowires connected in parallel in a single detector, the photocurrent can be further increased.

A Hierarchical Nanostructure‐Based Surface‐Enhanced Raman Scattering Sensor for Preconcentration and Detection of Antibiotic Pollutants

A surface‐enhanced Raman scattering (SERS) sensor that integrates a plasmonic nanostructure with an electrostatic preconcentration unit has been developed. The silver nanoparticles (Ag‐NPs) grafted Ge nanowires (Ge‐NWs) are grown onto the surface of a commercial copper grid (Cu‐grid), which is usually used for transmission electron microscopy sample support. This leads to the formation of a hierarchical Ag‐NPs@Ge‐NWs@Cu‐grid substrate, which plays dual roles, i.e., preconcentration of analytes and amplification of SERS signal. This SERS detection platform based on the Ag‐NPs@Ge‐NWs@Cu‐grid substrate is able to selectively detect polar analytes such as antibiotic pollutants. This SERS sensor has achieved a limit of detection of 2.4 × 10−9m for 6‐aminopenicillanic acid and 0.9 × 10−9m for penicillin G, respectively. This work has demonstrated a promising SERS detection platform for monitoring polar SERS‐active pollutants in the aquatic environment.

Long Cycle Life Rutile TiO2 Inverse Opal Anodes for Li-Ion Batteries

With the ever advancing improvements in electronics and display technologies, it is crucial that Li-ion batteries will be able to rise to the challenge of powering future consumer electronics. Consumers are all too aware of the challenges facing next generation Li-ion batteries with issues such as poor cycle life and the requirement to charge devices everyday becoming commonplace. Consequently, in recent years there has been a tremendous amount of research into developing materials which are capable of delivering long cycle-life with suitable stability in terms of capacity values, capacity retention and voltage. (1-3) In order to significantly advance Li-ion battery technology, it is crucial to identify electrode materials that are capable of delivering stable capacities over a large number of cycles, instead of materials which offer high initial capacities and then significantly fade over the course of a relatively low number of cycles. In this work we detail the preparation of rutile TiO2 inverse opal (IO) structured samples and their application as an anode material for Li-ion batteries. We present X-ray diffraction, electron diffraction and Raman analysis to confirm that the IOs are pure rutile TiO2. The electrochemical performance of the TiO2 IOs is evaluated via cyclic voltammetry, rate capability testing and long cycle life galvanostatic tests. We demonstrate stable cycling of TiO2 IOs over 1000 and 5000 cycles, at specific currents of 75 and 450 mA g-1, respectively. The capacity values obtained when cycled at a specific current of 75 mA g-1 are greater than previously reported values for other TiO2 nanostructures, achieving an impressive capacity of ~ 140 mAh g-1 after 1000 cycles. (4-6) We demonstrate that our TiO2 IO anodes are capable of delivering high specific capacities with stable capacity rentention, achieving a capacity retention of ~ 82.4% between the 100th and 1000th cycles. We present SEM images of TiO2 IO samples after cycling showing that the IO morphology is retained after thousands of galvanostatic cycles. Previous reports for anode materials such as Si and Ge nanowires have shown that volume expansion during cycling can lead to substantial capacity fading due to significant changes in nanostructure morphology.(7, 8) However, the porosity of the IO structure is able to accommodate volume changes to the TiO2 nanoparticles that make up the walls of the IO material. The 3D porous structure and morphology is retained via expansion of the IO walls into the pores. The level of structural integrity retained after 5000 cycles is not only a first for TiO2 nanostructures but also for nanostructured oxide anode materials in general. Acknowledgements This work was also supported by Science Foundation Ireland (SFI) through an SFI Technology Innovation and Development Award under contract no. 13/TIDA/E2761. This publication has also emanated from research supported in part by a research grant from SFI under Grant Number 14/IA/2581.

High speed MSM photodetector based on Ge nanowires network

This paper presents the photoresponse characteristics of a high speed Ge nanowires (NWs) network metal-semiconductor-metal photodetector. Ge NWs with different diameters (30 nm–100 nm) were grown by a vapour-liquid-solid method on SiO2/Si (100) wafers. Responsivity up to 1.75 A W−1 has been observed for a 30 nm NWs device compared to 0.5 A W−1 for a 100 nm NWs detector. A large population of surface states results in higher responsivity in a smaller diameter NWs device. The high gain in photocurrent has been explained using back-to-back Schottky junctions in a NWs network. The 30 nm NWs detector shows a fast photoresponse with a rise time of 95 μs and a fall time of 100 μs. The observed diameter-dependent time response in network NWs devices has been explained using barrier-dominant photo-conductance.

Unified Compact Model for Nanowire Transistors Including Quantum Effects and Quasi-Ballistic Transport

We present a surface potential-based compact model for nanowire FETs, which considers 1-D electrostatics along with the effect of multiple energy subbands. The model is valid for any semiconductor material, cross-sectional geometry, and any channel length with transport regimes varying from drift-diffusive to quasi-ballistic. The model captures the phenomenon of quantum capacitance and the effect of temperature. We have validated it with numerical simulations and experimental data for Si, Ge, and InAs nanowires of different geometries. Circuit simulation has also been performed with the model. The physics-based model is accurate and can be used as a tool for analysis and prediction of the effects of geometry scaling, material dependence, and temperature variation on device and circuit characteristics. To the best of our knowledge, this is the first time a compact model for nanowire FETs is being presented, which includes multiple subbands along with geometry scaling while being valid for different degenerate and nondegenerate semiconductor materials.

Inducing imperfections in germanium nanowires

Nanowires with inhomogeneous heterostructures such as polytypes and periodic twin boundaries are interesting due to their potential use as components for optical, electrical, and thermophysical applications. Additionally, the incorporation of metal impurities in semiconductor nanowires could substantially alter their electronic and optical properties. In this highlight article, we review our recent progress and understanding in the deliberate induction of imperfections, in terms of both twin boundaries and additional impurities in germanium nanowires for new/enhanced functionalities. The role of catalysts and catalyst–nanowire interfaces for the growth of engineered nanowires via a three-phase paradigm is explored. Three-phase bottom-up growth is a feasible way to incorporate and engineer imperfections such as crystal defects and impurities in semiconductor nanowires via catalyst and/or interfacial manipulation. “Epitaxial defect transfer” process and catalyst–nanowire interfacial engineering are employed to induce twin defects parallel and perpendicular to the nanowire growth axis. By inducing and manipulating twin boundaries in the metal catalysts, twin formation and density are controlled in Ge nanowires. The formation of Ge polytypes is also observed in nanowires for the growth of highly dense lateral twin boundaries. Additionally, metal impurity in the form of Sn is injected and engineered via third-party metal catalysts resulting in above-equilibrium incorporation of Sn adatoms in Ge nanowires. Sn impurities are precipitated into Ge bi-layers during Ge nanowire growth, where the impurity Sn atoms become trapped with the deposition of successive layers, thus giving an extraordinary Sn content (>6 at.%) in Ge nanowires. A larger amount of Sn impingement (>9 at.%) is further encouraged by utilizing the eutectic solubility of Sn in Ge along with impurity trapping.

Controlled growth of hexagonal gold nanostructures during thermally induced self-assembling on Ge(001) surface

Nano-sized gold has become an important material in various fields of science and technology, where control over the size and crystallography is desired to tailor the functionality. Gold crystallizes in the face-centered cubic (fcc) phase, and its hexagonal closed packed (hcp) structure is a very unusual and rare phase. Stable Au hcp phase has been reported to form in nanoparticles at the tips of some Ge nanowires. It has also recently been synthesized in the form of thin graphene-supported sheets which are unstable under electron beam irradiation. Here, we show that stable hcp Au 3D nanostructures with well-defined crystallographic orientation and size can be systematically created in a process of thermally induced self-assembly of thin Au layer on Ge(001) monocrystal. The Au hcp crystallite is present in each Au nanostructure and has been characterized by different electron microscopy techniques. We report that a careful heat treatment above the eutectic melting temperature and a controlled cooling is required to form the hcp phase of Au on a Ge single crystal. This new method gives scientific prospects to obtain stable Au hcp phase for future applications in a rather simple manner as well as redefine the phase diagram of Gold with Germanium.

Enhanced sputtering of Ge nanowires under synergetic effect of Mn ion and electron beams

To monitor the damage evolution in Ge nanowires during Mn implantation, in situ transmission electron microscopy observations were carried-out as a function of the Mn fluence. Special interest lies in the sputtering of nanowires. We evidence an enhanced sputtering under the synergetic effects of Mn implantation and electron beam which may alert experimenters to some possible artefacts related to in situ observations in the case of nanostructures.

Ionic liquid electrodeposition of strain-released Germanium nanowires as stable anodes for lithium ion batteries.

With the growing demand for portable and wearable electronic devices, it is imperative to develop high performance Li-ion batteries with long life times. Germanium-based materials have recently demonstrated excellent lithium-ion storage ability and are being considered as the most promising candidates for the anodes of lithium-ion batteries. Nevertheless, the practical implementation of Ge-based materials to Li-ion batteries is greatly hampered by the poor cycling ability that resulted from the huge volume variation during lithiation/delithiation processes. Herein, we develop a simple and efficient method for the preparation of Ge nanowires without catalyst nanoparticles and templates, using ionic liquid electrodeposition with subsequent annealing treatment. The Ge nanowire anode shows improved electrochemical performance compared with the Ge dense film anode. A capacity of ∼1200 mA h g-1 after 200 cycles at 0.1 C is obtained, with an initial coulombic efficiency of 81.3%. In addition, the Ge nanowire anode demonstrates superior rate capability with excellent capacity retention and stability (producing highly stable discharge capacities of about 620 mA h g-1 at 5 C). The improved electrochemical performance is the result of the enhanced electron migration and electron transport paths of the nanowires, and sufficient elasticity to buffer the volume expansion. This approach encompasses a low energy processing method where all the material is electrochemically active and binder free. The improved cycling stability and rate performance characteristics make these anodes highly attractive for the most demanding lithium-ion applications.

UV-assisted, template-free electrodeposition of germanium nanowire cluster arrays from an ionic liquid for anodes in lithium-ion batteries

The structure of Ge nanowire cluster arrays is beneficial for capacity retention of the electrode.

Size Dependent Pore Formation in Germanium Nanowires Undergoing Reversible Delithiation Observed by In Situ TEM

Germanium (Ge) nanowires coated with an amorphous silicon (Si) shell undergoing lithiation and delithiation were studied using in situ transmission electron microscopy (TEM). Delithiation creates pores in nanowires with diameters larger than ∼25 nm, but not in smaller diameter nanowires. The formation of pores in Ge nanowires undergoing delithiation has been observed before in in situ TEM experiments, but there has been no indication that a critical diameter exists below which pores do not form. Pore formation occurs as a result of fast lithium diffusion compared to vacancy migration. We propose that a short diffusion path for vacancies to the nanowire surface plays a role in limiting pore formation even when lithium diffusion is fast.

Low temperature humidity sensor based on Ge nanowires selectively grown on suspended microhotplates

The gas sensing properties of germanium (Ge) monocrystalline nanowires (NWs) at temperatures up to 100°C have been demonstrated for the first time. The devices have been fabricated based on an energy efficient and site-specific vapor-liquid-solid growth of NW meshes on top of microhotplates, which contain a buried heater and top electrodes. The devices have been investigated for the detection of oxygen, nitrogen dioxide and carbon monoxide gases, showing the important effect played by pre-adsorbed surface oxygen in the response to the different gases. The Ge NW-based devices exhibit p-type conductivity and show high selectivity in their response towards water vapor. Water vapor interaction is not dependent on the presence of oxygen and the adsorption leads to electron donation in the Ge nanowires. TEM analysis of the NWs proves that they are covered by a thin, outer germanium oxide layer, which is stable and does not grow upon exposure to these gases and operation temperatures up to 100°C. The presence of this oxide layer plays a key role in the sensing mechanisms.

Design of Dipole-Allowed Direct Band Gaps in Ge/Sn Core–Shell Nanowires

Owing to the indirect band gap nature, Ge exhibits poor optical properties, limiting its usage for optical devices. However, since the direct band gap of Ge is only higher by 0.14 eV than the indirect band gap, band gap engineering has drawn much attention to realize the direct band gap. Here, we report a strategy to design the direct band gap in Ge/Sn core–shell nanowires (NWs), based on first-principles calculations. For [111]-oriented NWs, we show that the direct band gaps can be tuned by controlling the diameter and the core-to-shell ratio. We find that the intrinsic strain induced by the lattice mismatch between Ge and Sn drives an indirect-to-direct band gap transition. Even for Ge/Sn core–shell NWs with intrinsically indirect band gaps, the direct band gaps can be achieved by applying an external tensile strain lower than the critical values for pure Ge NWs and bulk Ge. The optical transitions of the direct band gaps are all dipole-allowed, suggesting that [111]-oriented Ge/Sn core–shell NWs are pr...