Growth Of Germanium Nanowires Research Articles

Analysis of catalyst surface wetting: the early stage of epitaxial germanium nanowire growth.

The dewetting process is crucial for several applications in nanotechnology. Even though not all dewetting phenomena are fully understood yet, especially regarding metallic fluids, it is clear that the formation of nanometre-sized particles, droplets, and clusters as well as their movement are strongly linked to their wetting behaviour. For this reason, the thermodynamic stability of thin metal layers (0.1–100 nm) with respect to their free energy is examined here. The decisive factor for the theoretical considerations is the interfacial energy. In order to achieve a better understanding of the interfacial interactions, three different models for estimating the interfacial energy are presented here: (i) fully theoretical, (ii) empirical, and (iii) semi-empirical models. The formation of nanometre-sized gold particles on silicon and silicon oxide substrates is investigated in detail. In addition, the strengths and weaknesses of the three models are elucidated, the different substrates used are compared, and the possibility to further process the obtained particles as nanocatalysts is verified. The importance of a persistent thin communication wetting layer between the particles and its effects on particle size and number is also clarified here. In particular, the intrinsic reduction of the Laplace pressure of the system due to material re-evaporation and Ostwald ripening describes the theoretically predicted and experimentally obtained results. Thus, dewetting phenomena of thin metal layers can be used to manufacture nanostructured devices. From this point of view, the application of gold droplets as catalysts to grow germanium nanowires on different substrates is described.

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.

Direct growth of germanium nanowires on glass

We report a detailed characterization of Ge NWs directly grown on glass by a MOVPE system, showing how different growth parameters can affect the final outcome and comparing NWs grown on a monocrystalline Ge(111) substrate with NWs grown on amorphous glass. Our experimental results indicate that the choice of the substrate does not affect any of the relevant morphological, crystallographic or electrical properties of Ge NWs. Lengths are in the 20–30 micrometer range with minimal tapering, while growth rates are very similar to to NWs grown on Ge(111); TEM and Raman characterization show a very good crystallinity of measured nanostructures. We have also analyzed the growth process on glass and we were able to reach a conclusion on the specific growth mechanism for Ge NWs on amorphous substrates. Our findings demonstrate that glass is a valid option as cheap substrate for the mass production of these nanostructures.

The model for in-plane and out-of-plane growth regimes of semiconductor nanowires

In this work we present the model capable of prediction whether the vapor-liquid-solid growth would preferably result in the formation of in-plane (horizontal) or out-of-plane (vertical or inclined) nanowires. Within the model, we analyze the particular case of gold-catalyzed germanium nanowire growth on Ge(111), Ge(110) and Si(100) substrates. We focus on two aspects of the growth process: detachment of the catalyst from the substrate and stabilization of horizontal growth by nucleation at the nanowire-substrate-liquid line.

Growth of germanium nanowires with isobuthyl germane

We demonstrate the feasibility of the use of isobutyl germane, a novel germanium source, for the vapor–liquid–solid growth of germanium nanowires (NWs) on Si (111) substrates, using a thin gold layer as catalyst. The density and the diameter of the NWs were controlled by varying the Au layer thickness and the isobutyl germane flow. The NWs grow along (111) directions and show perfect crystallinity and lengths from several hundreds of nm to 3–4 μm. The use of isobutyl germane gives a considerable technological advantage in the growth of germanium NWs since it is a safer and more manageable germanium source and it allows to grow Ge NWs in a standard vapor phase epitaxy system at 400 °C.

Reaction Kinetics of Germanium Nanowire Growth on Inductively Heated Copper Surfaces

This article describes the chemical kinetics of germanium nanowire growth on inductively heated copper surfaces using diphenylgermane as a precursor. Inductive heating of metal surfaces presents a simple, rapid, and contact-free method to activate the direct growth of nanowires on metal surfaces. We show the main effects of synthesis temperature, duration, precursor concentration on the morphology, and loading of the nanowire film. We describe the complex interplay of precursor degradation, nucleation, and growth in context of a multistep reaction mechanism. We studied the temporal evolution of nanowire loading and morphology to develop a kinetic model, which predicts critical thresholds that define the onset of sequential axial and radial nanowire growth modes. These results may be used to commercially scale a nanowire growth process.

Synthesis of Group IV Nanowires on Graphene: The Case of Ge Nanocrawlers.

In recent years, there has been a growing interest in using graphene as a synthesis platform for polymers, zero-dimensional (0D) materials, one-dimensional materials (1D), and two-dimensional (2D) materials. Here, we report the investigation of the growth of germanium nanowires (GeNWs) and germanium nanocrawlers (GeNCs) on single-layer graphene surfaces. GeNWs and GeNCs are synthesized on graphene films by gold nanoparticles catalyzed vapor-liquid-solid growth mechanism. The addition of hydrogen chloride gas (HCl) at the nucleation step increased the propensity toward GeNCs growth on the surface. As the time lag before HCl introduction during the nucleation step increased, a significant change in the number of out-of-plane GeNWs versus in-plane GeNCs was observed. The nucleation temperature and time played a key role in the formation of GeNCs as well. The fraction of GeNCs (χNCs) decreased from 0.95 ± 0.01 to 0.66 ± 0.07 when the temperature was kept at 305 °C for 15 s versus maintained at 305 °C throughout the process, respectively. GeNCs exhibit ⟨112⟩ as the preferred growth direction whereas GeNWs exhibit both ⟨112⟩ and ⟨111⟩ as the preferred growth directions. Finally, our growth model suggests a possible mechanism for the preference of an in-plane GeNC growth on graphene versus GeNW on SiO2. These findings open up unique opportunities for fundamental studies of crystal growth on graphene, as well as enable exploration of new electronic interfaces between group IV materials and graphene, potentially toward designing new geometries for hybrid materials sensors.

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.

A catalyst-free synthesis of germanium nanowires obtained by combined X-ray chemical vapour deposition of GeH4 and low-temperature thermal treatment techniques

A catalyst-free innovative synthesis, by combined X-ray chemical vapour deposition and low-temperature thermal treatments, which has not been applied since so far to the growth of germanium nanowires (Ge-NWs), produced high yields of the nanoproducts with the GeH4 reactant gas. Nanowires were grown on both surfaces of a conventional deposition quartz substrate. They were featured with high purity and very large aspect ratios (ranging from 100 to 500). Products were characterized by scanning electron microscopy with energy-dispersive atomic X-ray fluorescence and transmission electron microscopies, X-ray powder diffraction diffractometry, thermogravimetric analysis with differential scanning calorimetry, vibrational infrared and Raman and ultraviolet–visible–near infrared spectroscopies. A quantitative nanowire bundles formation was observed in the lower surface of the quartz substrate positioned over a heating support, whilst spots of nanoflowers constituted by Ge-NWs emerged from a bulk amorphous germanium film matter, deposited on the upper surface of the substrate. The nanoproducts were characterized by crystalline core morphology, providing semiconductive features and optical band gap of about 0.67 eV. The possible interpretative base-growth mechanisms of the nanowires, stimulated by the concomitant application of radiant and thermal conditions with no specific added metal catalyst, are hereafter investigated and presented.

Nanosize effect in Germanium Nanowire Growth with Binary Metal Alloys

ABSTRACTThis article describes feasible and improved ways towards enhanced nanowire growth kinetics by reducing the equilibrium solute concentration in the liquid collector phase in a vapor-liquid-solid (VLS) like growth model. Use of bi-metallic alloy seeds (AuxAg1-x) influences the germanium supersaturation for a faster nucleation and growth kinetics. Nanowire growth with ternary eutectic alloys shows Gibbs-Thompson effect with diameter dependent growth rate. In-situ transmission electron microscopy (TEM) annealing experiments directly confirms the role of equilibrium concentration in nanowire growth kinetics and was used to correlate the equilibrium content of metastable alloys with the growth kinetics of Ge nanowires. The shape and geometry of the heterogeneous interfaces between the liquid eutectic and solid Ge nanowires were found to vary as a function of nanowire diameter and eutectic alloy composition.

In-situ observations of nanoscale effects in germanium nanowire growth with ternary eutectic alloys.

Vapour-liquid-solid (VLS) techniques are popular routes for the scalable synthesis of semiconductor nanowires. In this article, in-situ electron microscopy is used to correlate the equilibrium content of ternary (Au0.75 Ag0.25 -Ge and Au0.65 Ag0.35 -Ge) metastable alloys with the kinetics, thermodynamics and diameter of Ge nanowires grown via a VLS mechanism. The shape and geometry of the heterogeneous interfaces between the liquid eutectic and solid Ge nanowires varies as a function of nanowire diameter and eutectic alloy composition. The behaviour of the faceted heterogeneous liquid-solid interface correlates with the growth kinetics of the nanowires, where the main growth facet at the solid nanowire-liquid catalyst drop contact line lengthens for faster nanowire growth kinetics. Pronounced diameter dependent growth kinetics, as inferred from liquid-solid interfacial behaviour, is apparent for the synthesised nanowires. Direct in-situ microscopy observations facilitates the comparison between the nanowire growth behaviour from ternary (Au-Ag-Ge) and binary (Au-Ge) eutectic systems.

MBE growth of germanium nanowires along 〈1 1 0〉

Germanium nanowires using gold as catalyst were grown by MBE on differently oriented germanium substrates in order to study growth direction and faceting. All experiments led exclusively to 〈110〉 grown nanowires independent of the chosen substrate orientation. The growth angles as determined by SEM correspond to those directions which are expected for the cubic crystal structure. Furthermore, the faceting of sidewalls and tip of the nanowires was elucidated by TEM studies. The results are discussed regarding surface energies especially in comparison with silicon nanowires where gold wetting has a strong influence on the growth directions of nanowires.

Real-Time Observation of Collector Droplet Oscillations during Growth of Straight Nanowires

A liquid droplet sitting on top of a pillar is crucially important for semiconductor nanowire growth via a vapor-liquid-solid (VLS) mechanism. For the growth of long and straight nanowires, it has been assumed so far that the droplet is pinned to the nanowire top and any instability in the droplet position leads to nanowire kinking. Here, using real-time in situ scanning electron microscopy during germanium nanowire growth, we show that the increase or decrease in the droplet wetting angle and subsequent droplet unpinning from the growth interface may also result in the growth of straight nanowires. Because our argumentation is based on terms and parameters common for VLS-grown nanowires, such as the geometry of the droplet and the growth interface, these conclusions are likely to be relevant to other nanowire systems.

Recent advances in the growth of germanium nanowires: synthesis, growth dynamics and morphology control

One-dimensional semiconductor nanostructures have been studied in great depth over the past number of decades as potential building blocks in electronic, thermoelectric, optoelectronic, photovoltaic and battery devices. Silicon has been the material of choice in several industries, in particular the semiconductor industry, for the last few decades due to its stable oxide and well documented properties. Recently however, Ge has been proposed as a candidate to replace Si in microelectronic devices due to its high charge carrier mobilities. A number of various ‘bottom-up’ synthetic methodologies have been employed to grow Ge nanowires, including chemical vapour deposition, thermal evaporation, template methods, supercritical fluid synthesis, molecular beam epitaxy and solution phase synthesis. These bottom-up methods afford the opportunity to produce commercial scale quantities of nanowires with controllable lengths, diameters and crystal structure. An understanding of the vapour–liquid–solid (VLS) and vapour–solid–solid (VSS) mechanism by which most Ge nanowires are produced, is key to controlling their growth rate, aspect ratio and morphology. This article highlights the various bottom-up growth methods that have been used to synthesise Ge nanowires over the past 5–6 years, with particular emphasis on the Au/Ge eutectic system and the VLS mechanism. Thermodynamic and kinetic models used to describe Ge nanowire growth and morphology control will also be discussed in detail.

Size-controlled growth of germanium nanowires from ternary eutectic alloy catalysts

We report the simultaneous manipulation of both solute and equilibrium concentration of Ge to synthesise high aspect-ratio Ge nanowires.

Diffusion induced effects on geometry of Ge nanowires.

We report diffusion induced germanium nanowire growth and its dependence on the Ge evaporation flux. The wires show a growth rate (dL/dt) in agreement with the previously reported models, but detection of anomalies in the grown wires may indicate the prevalence of the direct Ge impinging effect on large diameter wires. Additionally, we demonstrate that change in deposition flux could directly affect the diffusion length of the Ge adatoms on the wire sidewalls. This in turn modifies the geometry of the grown wires by introducing a lateral growth starting from the base of the wire. A detailed understanding of the deposition flux effect on the growth and geometry of wires will result in improved knowledge of physical properties of wires.

Kinetics of germanium nanowire growth by the vapor-solid-solid mechanism with a Ni-based catalyst

The kinetics of vapor-solid-solid (VSS) Ge nanowire growth using a Ni-based catalyst were investigated to probe the rate-limiting step for this complex nanoscale crystal growth process. The effects of key parameters such as temperature and precursor partial pressure on the nanowire growth rate were studied in order to gain detailed insights into the growth kinetics. Two different regimes were observed for VSS growth of Ge nanowires as function of temperature. At higher temperatures (345 °C–375 °C), kinetics data suggest that mass transport of germane precursor to the catalyst surface is rate limiting. At lower temperatures (<345 °C), either surface reaction of the GeH4 precursor on the catalyst or incorporation of Ge into the nanowire across the wire/catalyst interface is rate limiting.

Engineering the Growth of Germanium Nanowires by Tuning the Supersaturation of Au/Ge Binary Alloy Catalysts

The synthesis of Ge nanowires with very high-aspect ratios (greater than 1000) and uniform crystal growth directions is highly desirable, not only for investigating the fundamental properties of nanoscale materials but also for fabricating integrated functional nanodevices. In this article, we present a unique approach for manipulating the supersaturation, and thus the growth kinetics, of Ge nanowires using Au/Ge bilayer films. Ge nanowires were synthesized on substrates consisting of two parts: a Au film on one-half of a Si substrate and a Au/Ge bilayer film on the other half of the substrate. Upon annealing the substrate, Au and Au/Ge binary alloy catalysts were formed on both the Au and Au/Ge-sides of the substrates, respectively, under identical conditions. The mean lengths of Ge nanowires produced were found to be significantly longer on the Au/Ge bilayer side of the substrate compared to the Au-coated side, as a result of a reduced incubation time for nucleation on the bilayer side. The mean length ...

Gold catalytic Growth of Germanium Nanowires by chemical vapour deposition method

Germanium nanowires (GeNWs) were synthesized using chemical vapor deposition (CVD) based on vapor–liquid–solid (VLS) mechanism with Au nanoparticles as catalyst and germanium tetrachloride (GeCl4) as a precursor of germanium. Au catalysts were deposited on silicon wafer as a thin film, firstly by sputtering technique and secondly by submerging the silicon substrates in Au colloidal solution, which resulted in Au nanoparticles with different sizes. GeNWs were synthesized at 400 °C, which is a low temperature for electrical device fabrication. Effect of different parameters such as Au nanoparticles size, carrier gas (Ar) flow and mixture of H2 with the carrier gas on GeNWs diameter and shape was studied by SEM images. The chemical composition of the nanostructure was also examined by energy dispersive X-ray spectroscopy (EDS).

Self-Seeded Growth of Germanium Nanowires: Coalescence and Ostwald Ripening

We report the controlled self-seeded growth of highly crystalline Ge nanowires, in the absence of conventional metal seed catalysts, using a variety of oligosilylgermane precursors and mixtures of germane and silane compounds (Ge:Si ratios between 1:4 and 1:1). The nanowires produced were encased in an amorphous shell of material derived from the precursors, which acted to isolate the Ge seed particles from which the nanowires were nucleated. The mode diameter and size distribution of the nanowires were found to increase as the growth temperature and Ge content in the precursors increased. Specifically, a model was developed to describe the main stages of self-seeded Ge nanowire growth (nucleation, coalescence, and Ostwald ripening) from the oligosilylgermane precursors and, in conjunction with TEM analysis, a mechanism of growth was proposed.