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.     

Ag-catalyzed growth of Ge nanostructures via the thermal evaporation of Ge powder

The Ag-catalyzed growth of straight Ge nanowires (GeNWs), serrated Ge nanobelts (GeNBs), and hexagonal Ge nanotowers (GeNTs) by thermal evaporation of Ge powder at 950 degrees C in Ar was studied. The growth of GeNWs and GeNBs at 550-600 degrees C followed the top-growth mode via the vapor-solid-solid process, while that of GeNTs at 700-750 degrees C followed the bottom-growth mode via the vapor-liquid-solid process. This result shows that the growth mode of Ge nanostructures catalyzed by Ag nanoparticles is temperature-dependent. The larger size of AgGe droplets assembled at high temperatures is beneficial to the growth of GeNTs with the bottom-growth mode. In addition, the growth mechanisms of Ge nanostructures are discussed.     

Direct growth of core-shell SiC-SiO(2) nanowires and field emission characteristics

A simple, direct synthesis method was used to grow core-shell SiC-SiO(2) nanowires by heating NiO-catalysed silicon substrates. A carbothermal reduction of WO(3) provided a reductive environment and carbon source to synthesize crystalline SiC nanowires covered with SiO(2) sheaths at the growth temperature of 1000-1100?°C. Transmission electron microscopy showed that the SiC core was 15-25?nm in diameter and the SiO(2) shell layer was an average of 20?nm in thickness. The thickness of the SiO(2) shell layer could be controlled using hydrofluoric acid (HF) etching. Field emission results of core-shell SiC-SiO(2) and bare SiC nanowires showed that the SiC nanowires coated with an optimum SiO(2) thickness (10?nm) have a higher field emission current than the bare SiC nanowires.     

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.     

Growth and characterization of germanium nanowires by electron beam evaporation

Germanium nanowires were grown on Au coated Si substrates at 380 °C in a high vacuum (5 × 10^− 5 Torr) by e-beam evaporation of Germanium (Ge). The morphology observation by a field emission scanning electron microscope (FESEM) shows that the grown nanowires are randomly oriented with an average length and diameter of 600 nm and 120 nm respectively for a deposition time of 60 min. The nanowire growth rate was measured to be ∼ 10 nm/min. Transmission electron microscope (TEM) studies revealed that the Ge nanowires were single crystalline in nature and further energy dispersive X-ray analysis (EDAX) has shown that the tip of the grown nanowires was capped with Au nanoparticles, this shows that the growth of the Ge nanowires occurs by the vapour liquid solid (VLS) mechanism. HRTEM studies on the grown Ge nanowire show that they are single crystalline in nature and the growth direction was identified to be along [110].     

Synthesis and characterization of germanium oxide nanowires

Germanium oxide (GeO2) nanowires have been prepared by heating a mixture of Ge powder and carbon in an Ar + O2 mixture at 900 degrees C. The nanowires have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and spectroscopic measurements. The nanowires are single crystalline with diameters in the 15-250-nm range. They exhibit characteristic blue luminescence.     

核壳结构SiC/SiO2纳米线的低温合成与表征

以酚醛树脂(PF)作为碳源, 纳米SiO₂为硅源, 在1300℃氩气气氛下通过碳热还原反应, 制备出具有核壳结构的SiC/SiO₂纳米线。采用X射线分析衍射仪(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)、高分辨率透射电子显微镜(HRTEM)、拉曼光谱(Raman)对产物的组成、形貌、微观结构等进行了表征。结果表明; SiC/SiO₂纳米线长可达数毫米, 单根SiC/SiO₂纳米线由直径30 nm的β-SiC晶体为内核和厚度约12 nm的无定形SiO₂壳层组成; 室温下SiC/SiO₂纳米线的PL发光峰与β-SiC单晶的发光特征峰相比有蓝移。最后, 讨论了核壳结构SiC/SiO₂纳米线的生成机制。     

Vertically standing Ge nanowires on GaAs(110) substrates

The growth of epitaxial Ge nanowires is investigated on (100), (111) B and (110) GaAs substrates in the growth temperature range from 300 to 380?°C. Unlike epitaxial Ge nanowires on Ge or Si substrates, Ge nanowires on GaAs substrates grow predominantly along the [Formula: see text] direction. Using this unique property, vertical [Formula: see text] Ge nanowires epitaxially grown on GaAs(110) surface are realized. In addition, these Ge nanowires exhibit minimal tapering and uniform diameters, regardless of growth temperatures, which is an advantageous property for device applications. Ge nanowires growing along the [Formula: see text] directions are particularly attractive candidates for forming nanobridge devices on conventional (100) surfaces.     

Synthesis and characterization of single-crystal strontium hexaboride nanowires

Catalyst-assisted growth of single-crystal strontium hexaboride (SrB6) nanowires was achieved by pyrolysis of diborane (B2H6) over SrO powders at 760-800 degrees C and 400 mTorr in a quartz tube furnace. Raman spectra demonstrate that the nanowires are SrB6, and transmission electron microscopy along with selected area diffraction indicate that the nanowires consist of single crystals with a preferred [001] growth direction. Electron energy loss data combined with the TEM images indicate that the nanowires consist of crystalline SrB 6 cores with a thin (1 to 2 nm) amorphous oxide shell. The nanowires have diameters of 10-50 nm and lengths of 1-10 microm.     

Stranski-Krastanow growth of germanium on silicon nanowires

There have been extensive studies of germanium (Ge) grown on planar silicon (Si) substrates by the Stranski-Krastanow (S-K) mechanism. In this study, we present S-K growth of Ge on Si nanowires. The Si nanowires were grown at 500 degrees C by a vapor-liquid-solid (VLS) method, using silane (SiH4) as the gaseous precursor. By switching the gas source from SiH4 to germane (GeH4) during the growth and maintaining the growth conditions, epitaxial Ge islands deposited on the outer surface of the initially formed Si nanowires. Transmission electron microscopy (TEM), scanning TEM, and energy-dispersive X-ray spectroscopy techniques were utilized to identify the thin wetting layer and the three-dimensional Ge islands formed around the Si core nanowires. Cross-sectional TEM verified the surface faceting of the Si core nanowires as well as the Ge islands.     

Parallel core-shell metal-dielectric-semiconductor germanium nanowires for high-current surround-gate field-effect transistors

Core-shell germanium nanowires (GeNW) are formed with a single-crystalline Ge core and concentric shells of nitride and silicon passivation layer by chemical vapor deposition (CVD), an Al2O3 gate dielectric layer by atomic layer deposition (ALD), and an Al metal surround-gate (SG) shell by isotropic magnetron sputter deposition. Surround-gate nanowire field-effect transistors (FETs) are then constructed using a novel self-aligned fabrication approach. Individual SG GeNW FETs show improved switching over GeNW FETs with planar gate stacks owing to improved electrostatics. FET devices comprised of multiple quasi-aligned SG GeNWs in parallel are also constructed. Collectively, tens of SG GeNWs afford on-currents exceeding 0.1 mA at low source-drain bias voltages. The self-aligned surround-gate scheme can be generalized to various semiconductor nanowire materials.     

Transport modulation in Ge/Si core/shell nanowires through controlled synthesis of doped Si shells

Appropriately controlling the properties of the Si shell in Ge/Si core/shell nanowires permits not only passivation of the Ge surface states, but also introduces new interface phenomena, thereby enabling novel nanoelectronics concepts. Here, we report a rational synthesis of Ge/Si core/shell nanowires with doped Si shells. We demonstrate that the morphology and thickness of Si shells can be controlled for different dopant types by tuning the growth parameters during synthesis. We also present distinctly different electrical characteristics that arise from nanowire field-effect transistors fabricated using the synthesized Ge/Si core/shell nanowires with different shell morphologies. Furthermore, a clear transition in the modification of device characteristics is observed for crystalline shell nanowires following removal of the shell using a unique trimming process of successive native oxide formation/etching. Our results demonstrate that the preferred transport path through the nanowire structure can be modulated by appropriately tuning the growth conditions.     

Growth of InAs/InP core-shell nanowires with various pure crystal structures

We have studied the epitaxial growth of an InP shell on various pure InAs core nanowire crystal structures by metal-organic vapor phase epitaxy. The InP shell is grown on wurtzite (WZ), zinc-blende (ZB), and {111}- and {110}-type faceted ZB twin-plane superlattice (TSL) structures by tuning the InP shell growth parameters and controlling the shell thickness. The growth results, particularly on the WZ nanowires, show that homogeneous InP shell growth is promoted at relatively high temperatures (～500?°C), but that the InAs nanowires decompose under the applied conditions. In order to protect the InAs core nanowires from decomposition, a short protective InP segment is first grown axially at lower temperatures (420-460?°C), before commencing the radial growth at a higher temperature. Further studies revealed that the InP radial growth rate is significantly higher on the ZB and TSL nanowires compared to WZ counterparts, and shows a strong anisotropy in polar directions. As a result, thin shells were obtained during low temperature InP growth on ZB structures, while a higher temperature was used to obtain uniform thick shells. In addition, a schematic growth model is suggested to explain the basic processes occurring during the shell growth on the TSL crystal structures.     

Catalyst-free synthesis of well-aligned ZnO nanowires on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates

Well-aligned ZnO nanowires have been synthesized vertically on In0.2Ga0.8N, GaN, and Al0.25Ga0.75N substrates, using a catalyst-free carbon thermal-reduction vapor phase deposition method for the first time. The as-synthesized nanowires are single crystalline wurtzite structure, and have a growth direction of [0001]. Each nanowire has a smooth surface, and uniform diameter along the growth direction. The average diameter and length of these nanowires are 120-150 nm, and 3-10 )m, respectively. We suggest that the growth mechanism follow a self-catalyzing growth model. Excitonic emission peaked around 385 nm dominates the room-temperature photoluminescence spectra of these nanowires. The room-temperature photoluminescence and Raman scattering spectra show that these nanowires have good optical quality with very less structural defects.     

Synthesis of chainlike In2Ge2O7/amorphous GeO2 core/shell nanocables and their luminescence

Novel chainlike In2Ge2O7/amorphous GeO2 core/shell nanocables were successfully synthesized by the simple thermal evaporation method without the presence of catalyst. The growth process of the nanocables is based on vapor-solid (VS) growth mechanism. Its morphology and microstructures were characterized by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, and photoluminescence spectroscopy. Studies indicate that typical chainlike nanocables consist of single crystalline In2Ge2O7 nanowires (core) with diameter of about 30 nm and amorphous GeO2 chainlike nanostructures (shell). Four emission peaks, namely 401 nm, 448.5 nm, 466.5 nm, and 491 nm, were observed in the room-temperature photoluminescence measurements.     

Low temperature growth of single crystalline germanium nanowires

Ge nanowires have been prepared at a low temperature by a simple hydrothermal deposition process using Ge and GeO₂ powders as the starting materials. These as-prepared Ge nanowires are single crystalline with the diameter ranging from 150 nm to 600 nm and length of several dozens of micrometers. The photoluminescence spectrum under excitation at 330 nm shows a strong blue light emission at 441 nm. The results of the pressure and GeO₂ content dependences on the formation and growth of Ge nanowires show that the hydrothermal pressure and GeO₂ content play an essential role on the formation and growth of Ge nanowires under hydrothermal deposition conditions. The growth of Ge nanowires is proposed as a solid state growth mechanism.     

A facile method for morphological control of MgZnO nanostructures on GaAs substrates and their optical properties

This research reports on morphological changes depending on the growth temperature in MgZnO nanostructures grown on GaAs substrates by metalorganic chemical vapor deposition as well as the investigation of their optical properties. As the growth temperature increased, the morphology of the MgZnO nanostructure changed from one-dimensional nanowires (480 degrees C) to pseudo-two-dimensional nanowalls (500 degrees C) to pyramid-shaped structures (520 degrees C). Among these structures, the nanowalls exhibited the best optical properties due to the large active surface area and high crystalline quality.     

Electrodeposited bismuth telluride nanowire arrays with uniform growth fronts

Bismuth telluride (Bi2Te3) nanowires were deposited into porous alumina templates with 35 nm diameter pores by a pulsed-potential electrodeposition method. For growth at temperatures between 1 and 4 degrees C, the nanowires filled 93% of the pores of the template, and the growth fronts were uniform with nanowire lengths of approximately 62-68 microm. There are over ten billion nanowires per square centimeter with aspect ratios approaching 2000:1. Samples were characterized by scanning and transmission electron microscopy, X-ray diffraction, and electron microprobe analysis. The crystalline nanowire arrays are highly oriented in the [110] direction, which is optimal for thermoelectric applications.     

Diffusion in Si(x)Ge(1-x)/Si nanowire heterostructures

Si0.48Ge0.52/Si tip/nanowire heterostructures were grown by pulsed laser vaporization (PLV) at a growth temperature of 1100 degrees C. Ge diffusion in [111]-growth Si nanowires was studied for different post-synthesis annealing temperatures from 200 degrees C to 800 degrees C. Ge composition profiles were quantified by energy-dispersive X-ray spectroscopy in a transmission electron microscope. The compositional profiles were modeled by a limited-source diffusion model to extract temperature-dependent diffusion coefficients. The Ge diffusion coefficients followed an Arrhenius relationship with an activation energy of 0.622 +/- 0.050 eV. This rather low activation energy barrier is similar to the previously reported activation energy barrier of 0.67 eV for Ge surface diffusion on Si, suggesting that surface diffusion may dominate in nanowires at this length scale.