Ge NW Research Articles

Ge nanowires on top of a Ge substrate for applications in anodes of Li and Na ion batteries: a first-principles study.

In this study, density functional theory (DFT) was used to research the adsorption and diffusion features of Li and Na on Ge nanowires on top of a Ge substrate. The adsorption energies at different positions are 0.71–1.28 eV for Na and −2.96–−2.13 eV for Li. The adsorption energies can be further reduced by surface modification with one or two P atoms. In particular, the sidewall of the Ge nanowire modified by two P atoms is most favorable to adsorb Li/Na. In addition, we used the nudged elastic band (NEB) method to study the diffusion pathways of Li/Na on the sidewall of Ge NW and the Ge substrate and computed their energy barriers. When Li or Na diffuses across the Ge NW, the energy barrier is 0.65 or 0.79 eV, indicating that the Ge NW can be applied to anodes in lithium and sodium ion batteries. Finally, the insertion of more lithium and sodium atoms into the Ge NW would cause volume expansion and the average length of Ge–Ge bonds to increase. This work will contribute to studying the adsorption and diffusion of Li and Na on nanowires with a substrate and the volume expansion caused by the insertion of Li/Na into the nanowires. Additionally, it provides guidance for designing Ge anodes for sodium ion batteries.

Self-induced ferroelectric 2-nm-thick Ge-doped HfO2 thin film applied to Ge nanowire ferroelectric gate-all-around field-effect transistor

This paper reports a self-induced ferroelectric 2-nm-thick Ge-doped HfO2 (Ge:HfO2) thin film. Ge thermal desorption, incorporation into HfO2, and further Ge:HfO2 crystallization were all performed through rapid thermal annealing simultaneously. The ferroelectric property of a 2-nm-thick Ge:HfO2/2-nm-thick Al2O3 dielectric stack was confirmed using the polarization-electric field measurement. X-ray photoelectron spectroscopy was used to confirm that Ge bonded to HfO2 as Hf-germanates. Piezoresponse force microscopy was used to demonstrate the piezoelectric property of Ge:HfO2/Al2O3. Furthermore, a dielectric stack of Ge:HfO2/Al2O3 was applied as a gate insulator in a Ge nanowire gate-all-around ferroelectric field-effect transistor (Ge NW Fe-GAAFET). The device exhibited a minimum steep-sub-threshold slope of 47 mV/dec, a high ION/IOFF ratio of >106, and low gate leakage current; moreover, it was free of a drain-induced barrier lowering effect. The proposed self-induced ferroelectric Ge:HfO2 Ge NW Fe-GAAFET is feasible for future ultra-low power integrated circuit applications.

Ferroelectric Enhanced Performance of a GeSn/Ge Dual-Nanowire Photodetector.

GeSn offers a reduced bandgap than Ge and has been utilized in Si-based infrared photodetectors with an extended cutoff wavelength. However, the traditional GeSn/Ge heterostructure usually consists of defects like misfit dislocations due to the lattice mismatch issue. The defects with the large feature size of a photodetector fabricated on bulk GeSn/Ge heterostructures induce a considerable dark current. Here, we demonstrate a flexible GeSn/Ge dual-nanowire (NW) structure, in which the strain relaxation is achieved by the elastic deformation without introducing defects, and the feature dimension is naturally at the nanoscale. A photodetector with a low dark current can be built on a GeSn/Ge dual-NW, which exhibits an extended detection wavelength beyond 2 μm and enhanced responsivity compared to the Ge NW. Moreover, the dark current can be further suppressed by the depletion effect from the ferroelectric polymer side gate. Our work suggests the flexible GeSn/Ge dual-NW may open an avenue for Si-compatible optoelectronic circuits operating in the short-wavelength infrared range.

In Situ Transmission Electron Microscopy Analysis of Copper-Germanium Nanowire Solid-State Reaction.

A promising approach of making high quality contacts on semiconductors is a silicidation (for silicon) or germanidation (for germanium) annealing process, where the metal enters the semiconductor and creates a low resistance intermetallic phase. In a nanowire, this process allows one to fabricate axial heterostructures with dimensions depending only on the control and understanding of the thermally induced solid-state reaction. In this work, we present the first observation of both germanium and copper diffusion in opposite directions during the solid-state reaction of Cu contacts on Ge nanowires using in situ Joule heating in a transmission electron microscope. The in situ observations allow us to follow the reaction in real time with nanometer spatial resolution. We follow the advancement of the reaction interface over time, which gives precious information on the kinetics of this reaction. We combine the kinetic study with ex situ characterization using model-based energy dispersive X-ray spectroscopy (EDX) indicating that both Ge and Cu diffuse at the surface of the created Cu3Ge segment and the reaction rate is limited by Ge surface diffusion at temperatures between 360 and 600 °C. During the reaction, germanide crystals typically protrude from the reacted NW part. However, their formation can be avoided using a shell around the initial Ge NW. Ha direct Joule heating experiments show slower reaction speeds indicating that the reaction can be initiated at lower temperatures. Moreover, they allow combining electrical measurements and heating in a single contacting scheme, rendering the Cu-Ge NW system promising for applications where very abrupt contacts and a perfectly controlled size of the semiconducting region is required. Clearly, in situ TEM is a powerful technique to better understand the reaction kinetics and mechanism of metal-semiconductor phase formation.

A Triode Device with a Gate Controllable Schottky Barrier: Germanium Nanowire Transistors and Their Applications.

Electrical contacts often dominate charge transport properties at the nanoscale because of considerable differences in nanoelectronic device interfaces arising from unique geometric and electrostatic features. Transistors with a tunable Schottky barrier between the metal and semiconductor interface might simplify circuit design. Here, germanium nanowire (Ge NW) transistors with Cu3 Ge as source/drain contacts formed by both buffered oxide etching treatments and rapid thermal annealing are reported. The transistors based on this Cu3 Ge/Ge/Cu3 Ge heterostructure show ambipolar transistor behavior with a large on/off current ratio of more than 105 and 103 for the hole and electron regimes at room temperature, respectively. Investigations of temperature-dependent transport properties and low-frequency current fluctuations reveal that the tunable effective Schottky barriers of the Ge NW transistors accounted for the ambipolar behaviors. It is further shown that this ambipolarity can be used to realize binary-signal and data-storage functions, which greatly simplify circuit design compared with conventional technologies.

Highlighting the Importance of Full-Cell Testing for High Performance Anode Materials Comprising Li Alloying Nanowires

Herein, the electrochemical performance of directly grown Ge nanowire anodes in full-cell Li-ion configurations (using lithium cobalt oxide cathodes) are examined. The impacts of voltage window, anode/cathode balancing and anode preconditioning are assessed. The cells had a useable upper cutoff of 3.9 V, with a higher voltage cutoff of 4.2 V shown by SEM analysis to lead to Li plating on the anode surface. The rate performance of Ge NW anodes was shown to be boosted within full-cells compared to half-cells, meaning that existing studies may underestimate the rate performance of alloying mode anode materials if they are only based on half-cell investigations. The capacity retention of the full-cells is lower compared to equivalent half-cells due to progressive consumption of cyclable Li. This phenomenon is demonstrated using a parallel anode and cathode delithiation approach that could be extended to other full-cell systems. The findings stress the importance of testing promising anode materials within full-cell configurations, to identify specific capacity fade mechanisms that are not relevant to half-cells and aid the development of higher energy density storage systems.

Nanowires enabling strained photovoltaics

Photovoltaic nano-devices have largely been relying on charge separation in conventional p-n junctions. Junction formation via doping, however, imposes major challenges in process control. Here, we report on a concept for photovoltaic energy conversion at the nano scale without the need for intentional doping. Our approach relies on charge carrier separation in inhomogeneously strained germanium nanowires (Ge NWs). This concept utilizes the strain-induced gradient in bandgap along tapered NWs. Experimental data confirms the feasibility of strain-induced charge separation in individual vapor-liquid-solid grown Ge NW devices with an internal quantum efficiency of ∼5%. The charge separation mechanism, though, is not inherently limited to a distinct material. Our work establishes a class of photovoltaic nano-devices with its opto-electronic properties engineered by size, shape, and applied strain.

High-Density Germanium Nanowire Arrays via Supercritical Fluid-Liquid-Solid Growth in Porous Alumina Templates

High-density germanium (Ge) nanowire arrays grown from porous alumina templates via supercritical fluid-liquid-solid (SFLS) growth were successfully synthesized. Before the growth of Ge nanowire arrays, the gold metal nanoparticles as growth catalyst layer were synthesized at the bottoms of anodic porous alumina (AAO) templates by pulsed electro-deposition. Partial exposure of Ge NW arrays from AAO or free standing of Ge NW arrays was demonstrated by using different etching solutions. The growth of single crystalline Ge NW arrays from AAO significantly provides a low-cost with high yield approach to control the size of Ge NWs by the AAO template. © 2013 The Electrochemical Society. [DOI: 10.1149/2.010306ssl] All rights reserved.

Solution phase synthesis of silicon and germanium nanowires

Si and Ge nanowires have attracted widespread interest due to their suitability for photovoltaic, nanoelectronic, sensing and energy storage applications. As a result, a variety of synthetic protocols have emerged which include chemical vapour deposition and solution phase approaches. Early solution based approaches were hindered by the high nucleation temperatures required for Ge and Si NW formation but these have been overcome by using either supercritical fluid (SCF) approaches or high boiling point organic solvent (HBS) routes. This feature article examines the various synthetic strategies which have been successful in forming Ge and Si NWs within solvent based systems with particular emphasis on the reaction media and catalytic method employed.

Control of Lateral Dimension in Metal-Catalyzed Germanium Nanowire Growth: Usage of Carbon Sheath

We report on the catalytic growth of thin carbon sheathed single crystal germanium nanowires (GeNWs), which can solve the obstacles that have disturbed a wide range of applications of GeNWs. Single crystal Ge NW core and amorphous carbon sheath are simultaneously grown via vapor-liquid-solid (VLS) process. The carbon sheath completely blocks unintentional vapor deposition on NW surface, thus ensuring highly uniform diameter, dopant distribution, and electrical conductivity along the entire NW length. Furthermore, the sheath not only inhibits metal diffusion but also improves the chemical stability of GeNWs at even high temperatures.

(Invited) Low-Temperature Growth of Ge Nanowires by Vapor-Liquid-Solid Chemical Vapor Deposition

This paper reports the growth of the germanium nanowires (Ge NWs) with a single-step temperature method via vapour-liquid-solid (VLS) mechanism in the low pressure chemical vapour deposition (CVD) reactor at 300, 280 and 260°C. The Ge NWs grown at 300°C tend to have a tapered structure and the sidewalls of the nanowires were observed to be decorated with gold. The tapering was caused by the uncatalysed deposition of Ge via CVD mechanism at the sidewalls of the nanowires and significantly minimised at 260°C which leads to the formation of straight and ultra-thin Ge NWs. The sidewalls of the Ge NWs grown at 260°C were also observed not to be decorated with gold. The Ge NWs grown from 0.1-nm-thick Au at 260°C had diameter as small as ~3 nm which offers an opportunity to fabricate high-performance p-type ballistic Ge NW transistor and to realise nanowire solar cell with higher efficiency.

Germanium nanowires-based carbon composite as anodes for lithium-ion batteries

Lithium-ion batteries have been actively researched in recent years due to it being one of the most promising energy storage systems. Herein, we report a novel approach where germanium nanowires (Ge NW) are grown in gold-seeded porous carbon via the solution–liquid–solid mechanism, and the corresponding improvement observed in terms of the specific capacity of this porous carbon–germanium nanowires (PC–Ge NW) composite anode. At a current density of 160mAg−1 and voltage window of 0.001–1.5V, a specific capacity of 789mAhg−1 during the 50th cycle for PC–Ge NW is achieved as compared to 624mAhg−1 during the 50th cycle for pure Ge NW. Even though the content of the Ge is only 53.5 weight percent in the PC–Ge NW composite, it yields a better stability and higher specific capacity, indicating a synergistic effect between porous carbon and Ge nanowires. There is also potential cost savings since the use of a lower amount of Ge can bring about good cycling properties.

Influence of Radial Stress on the Performance of Gate-All-Around Ge(110) NW FETs with HfO2 Dielectric

In this work, a simulation method from strained valence band structures to strained mobility calculation to consider a radial stress at the boundary of HfO2 gate dielectric surrounding Ge(110) nanowire is developed. The simulation implements the radial stress to strain distribution calculation via finite element method and then to valence band calculation. The radial stress at the boundary of gate dielectric pushes the valence subbands downwards in contrast with lattice mismatch strain effects between Ge NW and gate dielectric. The impact of the radial stress on the hole effective masses and density of states of HfO2 gate dielectric surrounding Ge(110) nanowire are also investigated. The potential distribution and holes density distribution are calculated by solving the 2D Poisson equation and Schrödinger equation self-consistently in NW cross section. Hole mobility is obtained by modified Kubo-Greenwood formula. Based on strained valence band structures, the hole density distribution in cross-sectional Ge(110) NW reduces with larger radial stress value. The phonon scattering-limited hole mobility in NW significantly increases as the radial stress increases.

Germanium Nanowires with 3-nm-Diameter Prepared by Low Temperature Vapour–Liquid–Solid Chemical Vapour Deposition

We report the growth of germanium nanowires (Ge NWs) with single-step temperature method via vapour-liquid-solid (VLS) mechanism in the low pressure chemical vapour deposition (CVD) reactor at 300 degrees C, 280 degrees C, and 260 degrees C. The catalyst used in our experiment was Au nanoparticles with equivalent thicknesses of 0.1 nm (average diameter approximately 3 nm), 0.3 nm (average diameter approximately 4 nm), 1 nm (average diameter approximately 6 nm), and 3 nm (average diameter approximately 14 nm). The Gibbs-Thomson effect was used to explain our experimental results. The Ge NWs grown at 300 degrees C tend to have tapered structure while the Ge NWs grown at 280 degrees C and 260 degrees C tend to have straight structure. Tapering was caused by the uncatalysed deposition of Ge atoms via CVD mechanism on the sidewalls of nanowire and significantly minimised at lower temperature. We observed that the growth at lower temperature yielded Ge NWs with smaller diameter and also observed that the diameter and length of Ge NWs increases with the size of Au nanoparticles for all growth temperatures. For the same size of Au nanoparticles, Ge NWs tend to be longer with a decrease in temperature. The Ge NWs grown at 260 degrees C from 0.1-nm-thick Au had diameter as small as approximately 3 nm, offering an opportunity to fabricate high-performance p-type ballistic Ge NW transistor, to realise nanowire solar cell with higher efficiency, and also to observe the quantum confinement effect.

Vertical Germanium Nanowire Arrays in Microfluidic Channels for Charged Molecule Detection

We demonstrate the fabrication of vertical germanium nanowire (Ge NW) molecular sensor arrays that are suitable for biological field-effect-transistor sensing, using conventional semiconductor fabrication processes. The proposed fabrication process enabled production of freestanding vertical NW arrays with a Schottky junction top contact. The arrays’ dc current–voltage characteristics could be modulated by altering the solution pH in contact with the gate insulator-coated nanowire surface. A sensitivity of with sensor area is demonstrated for undoped vertical Ge NW arrays.

Carrier Transport in One-dimensional Ge Nanowires/Si Substrate Heterojunctions

ABSTRACTSamples of Ge Nanowires (Ge NWs) grown by chemical vapor deposition (CVD) on single crystal, (100) and (111) oriented Si substrates were studied with respect to their electrical properties. Using different contact geometries, direct current (DC) and alternating current (AC) electrical and photoelectrical measurements were carried out at room temperature. A rectifying junction behavior was observed indicating a low defect density at NWs/substrate heterointerface. AC conductance exhibits significant frequency dependence with a power law behavior, suggesting that carrier transport in Ge NW volume is associated with hopping processes.