Growth Of Ge Nanowires Research Articles

Synthesis of Sn-catalyzed Ge nanowires and Ge/Si heterostructures via a gradient method

In this study, we investigate the growth of Ge nanowires (NWs) using a gas supply gradient method during plasma-enhanced chemical vapor deposition (PECVD), focusing on the effects of GeH4 partial pressure and total chamber pressure on NWs morphology. By adjusting either the GeH4 flow rate or the total pressure, we explored a gradient method to manipulate the growth process. Scanning electron microscopy (SEM) images revealed that, even with constant GeH4 partial pressure, variations in total pressure significantly influenced NWs diameter and structure. Furthermore, elemental mapping and compositional analysis demonstrated the presence of a Ge/Si heterostructure with a Sn catalyst at the apex and an amorphous Si layer encapsulating the NW surface.

Tapering-free monocrystalline Ge nanowires synthesized via plasma-assisted VLS using In and Sn catalysts

In and Sn are the type of catalysts which do not introduce deep level electrical defects within the bandgap of germanium (Ge). However, Ge nanowires produced using these catalysts usually have a large diameter, a tapered morphology, and mixed crystalline and amorphous phases. In this study, we show that plasma-assisted vapor–liquid–solid (PA-VLS) method can be used to synthesize Ge nanowires. Moreover, at certain parameter domains, the sidewall deposition issues of this synthesis method can be avoided and long, thin tapering-free monocrystalline Ge nanowires can be obtained with In and Sn catalysts. We find two quite different parameter domains where Ge nanowire growth can occur via PA-VLS using In and Sn catalysts: (i) a low temperature-low pressure domain, below ∼235 °C at a GeH4 partial pressure of ∼6 mTorr, where supersaturation in the catalyst occurs thanks to the low solubility of Ge in the catalysts, and (ii) a high temperature-high pressure domain, at ∼400 °C and a GeH4 partial pressure above ∼20 mTorr, where supersaturation occurs thanks to the high GeH4 concentration. While growth at 235 °C results in tapered short wires, operating at 400 °C enables cylindrical nanowire growth. With the increase of growth temperature, the crystalline structure of the nanowires changes from multi-crystalline to mono-crystalline and their growth rate increases from ∼0.3 nm s−1 to 5 nm s−1. The cylindrical Ge nanowires grown at 400°C usually have a length of few microns and a radius of around 10 nm, which is well below the Bohr exciton radius in bulk Ge (24.3 nm). To explain the growth mechanism, a detailed growth model based on the key chemical reactions is provided.

A Review of Self-Seeded Germanium Nanowires: Synthesis, Growth Mechanisms and Potential Applications.

Ge nanowires are playing a big role in the development of new functional microelectronic modules, such as gate-all-around field-effect transistor devices, on-chip lasers and photodetectors. The widely used three-phase bottom-up growth method utilising a foreign catalyst metal or metalloid is by far the most popular for Ge nanowire growth. However, to fully utilise the potential of Ge nanowires, it is important to explore and understand alternative and functional growth paradigms such as self-seeded nanowire growth, where nanowire growth is usually directed by the in situ-formed catalysts of the growth material, i.e., Ge in this case. Additionally, it is important to understand how the self-seeded nanowires can benefit the device application of nanomaterials as the additional metal seeding can influence electron and phonon transport, and the electronic band structure in the nanomaterials. Here, we review recent advances in the growth and application of self-seeded Ge and Ge-based binary alloy (GeSn) nanowires. Different fabrication methods for growing self-seeded Ge nanowires are delineated and correlated with metal seeded growth. This review also highlights the requirement and advantage of self-seeded growth approach for Ge nanomaterials in the potential applications in energy storage and nanoelectronic devices.

Growth of Ge Nanowires by Chemical Vapor Deposition at Atmospheric Pressure Using Readily Available Precursors GeO2 and C2H5OH

The aim of the present study is to investigate chemical vapor deposition of Ge nanowires from readily available precursors solid GeO2 and liquid ethanol (C2H5OH) at atmospheric pressure. Gaseous GeO was generated in situ by the reactions between the reactants in the source temperature range from 1000 K to 1200 K. Ge wires were grown from the gaseous species transported from the source to the Au-coated Si substrate heated to 723 K for 5 min and 15 min. The diameter of the Ge wires slightly increased with increasing source temperature for the growth time of 5 min. The mean diameter (~ 220 nm) of the wires grown from the species generated at 1200 K for 15 min was greater than those of the other samples (range ~ 120 nm to 145 nm) owing to excessive Ge deposition on previously formed Ge wires. The growth of the Ge nanowires is discussed in terms of the vapor–liquid–solid mechanism and the reduction reactions between the species derived from the precursors.

Chemical vapor deposition of Ge nanowires from readily available GeO2 and CH4 precursors

The present study aimed to investigate growth of Ge nanowires on a Si substrate by chemical vapor deposition technique at atmospheric pressure using readily available precursors GeO2 and CH4. The process consists of two consecutive steps: generation of gaseous GeO and growth of Ge wires. The species was in-situ produced at 1100 K by the reactions between GeO2 and CH4 pyrolysis products. It was transported by the main gas flow to the Au-coated Si substrate heated to 1073 K and to 723 K. At 1073 K, large droplets and thick Ge wires were observed on the substrate. Ge nanowires with high aspect ratios were obtained at 723 K. Growth of Ge wires from the GeO2-and CH4- derived species was discussed in terms of vapor–liquid-solid mechanism and the reduction reactions.

In-plane growth of germanium nanowires on nanostructured Si(001)/SiO2 substrates

Germanium (Ge) nanowires (NWs) were grown in-plane on nano-structured Si(001)/SiO2 substrates by molecular beam epitaxy using gold (Au) as the solvent. The site-selective NW growth was enabled by a rectangular array of gold droplets on silicon (Si) tips with an Au nuclei density below 0.25 µm−2 on the surrounding silicon oxide (SiO2). The initial growth of Ge NWs starting from Si–Au droplets with SixGe1−x nucleation from ternary alloy is discussed from a thermodynamic point of view. The in-plane NW elongation occurred within 〈110〉 directions on the substrate and NWs were mainly bounded by two 55° inclined 111 facets and a less pronounced planar (001) top facet. Fully relaxed crystal lattices of Ge NWs were observed from two-dimensional reciprocal space maps of x-ray diffraction measurements.

In Situ Transmission Electron Microscopy Measurements of Ge Nanowire Synthesis with Liquid Metal Nanodroplets in Water.

The growth of Ge nanowires in water inside a liquid transmission electron microscope (TEM) holder has been demonstrated at room temperature. Each nanowire growth event was stimulated by the incident electron beam on otherwise unsupported liquid Ga or liquid In nanodroplets. A variety of conditions were explored, including liquid metal nanodroplet surface condition, liquid metal nanodroplet size and density, formal concentration of dissolved GeO2, and electron beam intensity. The cumulative observations from a series of videos recorded during growth events suggested the following points. First, the conditions necessary for initiating nanowire growth at uncontacted liquid metal nanodroplets in a liquid TEM cell indicate the process was governed by solvated electrons generated from secondary electrons scattered by the liquid metal nanodroplets. The attained current densities were comparable to those achieved in conventional electrochemical liquid-liquid-solid (ec-LLS) growths outside of a TEM. Second, the surface condition of the liquid metal nanodroplets was quite influential on whether nanowire growth occurred and surface diffusion of Ge adatoms contributed to the rate of crystallization. Third, the Ge nanowire growth rates were limited by the feed rate of Ge to the crystal growth front rather than the rate of crystallization at the liquid metal/solid Ge interface. Estimates of an electrochemical current for the reduction of dissolved GeO2 were nominally in line with currents used for Ge nanowire growth by ec-LLS outside of the TEM. Fourth, the Ge nanowire growths in the liquid TEM cell occurred far from thermodynamic equilibrium, with supersaturation values of 104 prior to nucleation. These collective points provide insight on how to further control and improve Ge nanowire morphology and crystallographic quality by the ec-LLS method.

Local lateral integration of 16-nm thick Ge nanowires on silicon on insulator substrates

In this contribution, we report on the growth of horizontal Ge nanowires inside extremely thin tunnels surrounded by oxide. This is achieved through selective lateral growth of Ge on silicon-on-insulator (001) substrates. The 16 nm high tunnels are formed by HCl vapor etching of Si followed by Ge growth in the same epitaxy chamber. First, the benefit of growing the Ge nanowires at high temperature was highlighted to homogenize the length of the nanowires and achieve a high growth rate. Afterwards, we showed that increasing the tunnel depth led to a significant reduction in the growth rate. Finally, transmission electron microscopy showed that no defects were present in the Ge nanowires. These results are encouraging for the planar co-integration of heterogeneous materials on Si.

Catalyst–substrate interaction and growth delay in vapor–liquid–solid nanowire growth

Understanding of the initial stage of nanowire growth on a bulk substrate is crucial for the rational design of nanowire building blocks in future electronic and optoelectronic devices. Here, we provide in situ scanning electron microscopy and Auger microscopy analysis of the initial stage of Au-catalyzed Ge nanowire growth on different substrates. Real-time microscopy imaging and elementally resolved spectroscopy clearly show that the catalyst dissolves the underlying substrate if held above a certain temperature. If the substrate dissolution is blocked (or in the case of heteroepitaxy) the catalyst needs to be filled with nanowire material from the external supply, which significantly increases the initial growth delay. The experiments presented here reveal the important role of the substrate in metal-catalyzed nanowire growth and pave the way for different growth delay mitigation strategies.

Direct Synthesis of Hyperdoped Germanium Nanowires.

A low-temperature chemical vapor growth of Ge nanowires using Ga as seed material is demonstrated. The structural and chemical analysis reveals the homogeneous incorporation of ∼3.5 at. % Ga in the Ge nanowires. The Ga-containing Ge nanowires behave like metallic conductors with a resistivity of about ∼300 μΩcm due to Ga hyperdoping with electronic contributions of one-third of the incorporated Ga atoms. This is the highest conduction value observed by in situ doping of group IV nanowires reported to date. This work demonstrates that Ga is both an efficient seed material at low temperatures for Ge nanowire growth and an effective dopant changing the semiconductor into a metal-like conductor.

Vapor-solid-solid grown Ge nanowires at integrated circuit compatible temperature by molecular beam epitaxy

We demonstrate Au-assisted vapor-solid-solid (VSS) growth of Ge nanowires (NWs) by molecular beam epitaxy at the substrate temperature of ∼180 °C, which is compatible with the temperature window for Si-based integrated circuit. Low temperature grown Ge NWs hold a smaller size, similar uniformity, and better fit with Au tips in diameter, in contrast to Ge NWs grown at around or above the eutectic temperature of Au-Ge alloy in the vapor-liquid-solid (VLS) growth. Six ⟨110⟩ growth orientations were observed on Ge (110) by the VSS growth at ∼180 °C, differing from only one vertical growth direction of Ge NWs by the VLS growth at a high temperature. The evolution of NWs dimension and morphology from the VLS growth to the VSS growth is qualitatively explained by analyzing the mechanism of the two growth modes.

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.

The Synergic Effect of Atomic Hydrogen Adsorption and Catalyst Spreading on Ge Nanowire Growth Orientation and Kinking

Hydride precursors are commonly used for semiconductor nanowire growth from the vapor phase and hydrogen is quite often used as a carrier gas. Here, we used in situ scanning electron microscopy and spatially resolved Auger spectroscopy to reveal the essential role of atomic hydrogen in determining the growth direction of Ge nanowires with an Au catalyst. With hydrogen passivating nanowire sidewalls the formation of inclined facets is suppressed, which stabilizes the growth in the ⟨111⟩ direction. By contrast, without hydrogen gold diffuses out of the catalyst and decorates the nanowire sidewalls, which strongly affects the surface free energy of the system and results in the ⟨110⟩ oriented growth. The experiments with intentional nanowire kinking reveal the existence of an energetic barrier, which originates from the kinetic force needed to drive the droplet out of its optimum configuration on top of a nanowire. Our results stress the role of the catalyst material and surface chemistry in determining the nanowire growth direction and provide additional insights into a kinking mechanism, thus allowing to inhibit or to intentionally initiate spontaneous kinking.

The synergic effect of atomic hydrogen and catalyst spreading on Ge nanowire growth orientation and kinking

The synergic effect of atomic hydrogen and catalyst spreading on Ge nanowire growth orientation and kinking

Lead-supported germanium nanowire growth

The Pb-assisted growth of Ge nanowires (NWs) has been investigated under high and low pressure conditions via thermal decomposition of diphenylgermane. Highly crystalline Ge NWs were obtained and Pb was established as a viable growth promoter with the Pb particle being in the solid and liquid state.

P-doped germanium nanowires with Fano-broadening in Raman spectrum

The optimized growth conditions for high density germanium (Ge) nanowires and P-doped Ge nanowires on Si (111) substrate were investigated, the phosphorus (P)-doping in Ge nanowires was also characterized. Vapor liquid solid-low pressure chemical vapor deposition (VLS-LPCVD) of Ge nanowires was conducted with different thicknesses of Au film as catalyst, different flow rates of GeH4 as precursor and PH3/Ar as co-flow. The morphologies of the Ge nanowires were characterized by scanning electron microscopy (SEM), the P-doping was verified by micro Raman spectroscopy via measuring the P local vibrational peak (342-345 cm-1) and asymmetric broadening of Ge-Ge vibrational peak (about 300 cm-1), respectively. The characterization results show that 1 nm thickness of Au catalyst is the most suitable condition among thicknesses of 0.1, 1, 5, and 10 nm for the growth of high density Ge nanowires at 300 and 350 °C, and 0.5 sccm is the best flow rate of PH3/Ar to grow high density and large scale P-doped Ge nanowires among flow rates of 0.5, 1 and 2 sccm. The P impurity can be doped into Ge nanowires effectively during LPCVD process at 350 °C.

Supersaturation state effect in diffusion induced Ge nanowires growth at high temperatures

We report on supersaturation state effect in diffusion-induced vapor-liquid-solid growth of Ge nanowires at high temperature. Our experimental investigation establishes that at T≥550°C the growth is hindered while the growth limitation is not resulted from a high value of the desorption rate. We demonstrate that the suppressed growth is a result of the droplets large chemical potential that inhibit the supersaturation state. This results either in a strong growth limitation due to a significant droplets enlargement or to a growth cessation.

The Role of Surface Passivation in Controlling Ge Nanowire Faceting.

In situ transmission electron microscopy observations of nanowire morphologies indicate that during Au-catalyzed Ge nanowire growth, Ge facets can rapidly form along the nanowire sidewalls when the source gas (here, digermane) flux is decreased or the temperature is increased. This sidewall faceting is accompanied by continuous catalyst loss as Au diffuses from the droplet to the wire surface. We suggest that high digermane flux and low temperatures promote effective surface passivation of Ge nanowires with H or other digermane fragments inhibiting diffusion and attachment of Au and Ge on the sidewalls. These results illustrate the essential roles of the precursor gas and substrate temperature in maintaining nanowire sidewall passivation, necessary to ensure the growth of straight, untapered, ⟨111⟩-oriented nanowires.

Direct Observation of Transient Surface Species during Ge Nanowire Growth and Their Influence on Growth Stability.

Surface adsorbates are well-established choreographers of material synthesis, but the presence and impact of these short-lived species on semiconductor nanowire growth are largely unknown. Here, we use infrared spectroscopy to directly observe surface adsorbates, hydrogen atoms and methyl groups, chemisorbed to the nanowire sidewall and show they are essential for the stable growth of Ge nanowires via the vapor-liquid-solid mechanism. We quantitatively determine the surface coverage of hydrogen atoms during nanowire growth by comparing ν(Ge-H) absorption bands from operando measurements (i.e., during growth) to those after saturating the nanowire sidewall with hydrogen atoms. This method provides sub-monolayer chemical information at relevant reaction conditions while accounting for the heterogeneity of sidewall surface sites and their evolution during elongation. Our findings demonstrate that changes to surface bonding are critical to understand Ge nanowire synthesis and provide new guidelines for rationally selecting catalysts, forming heterostructures, and controlling dopant profiles.

Structure and morphology of Ge nanowires on Si (001): Importance of the Ge islands on the growth direction and twin formation

Understanding and controlling the structural properties of Ge nanowires are important for their current and future use in technological applications. In this study, the initial stages of the heteroepitaxial growth of Ge nanowires on Si(001) via the Au catalyzed vapor-liquid-solid (VLS) method are investigated. We observe a Ge island located at the base of each nanowire. We propose that these islands are formed by the VLS mechanism and initiate the nanowire growth. Analysis of the islands morphology helps to explain the 〈011〉 growth direction commonly observed for Ge nanowires. Moreover, our observations provide an insight into the formation of twins that propagate along the growth direction.