Fabrication parameters dependent morphology variation of silicon thin film

Achieving two dimensional quantum structure of silicon with welldefined tuneable morphology is an outstanding issue. We present the preliminary results on fabrication parameters dependent silicon thin film production using VHF-PECVD method. Five samples are prepared on Si(100) substrate with gold (Au) catalyst by adjusting different parameters such as deposition time, temperature and the flow of precursor gas. The samples morphology are analysed using FESEM. The results reveal that the silicon thin film appear to be smooth and crystal-like after an enormous amount of hydrogen is inserted together with the precursor gas (silane) during the deposition process. More interestingly, the films exhibit silicon nanowires as the deposition time is increased up to 1 hour. This morphological transformation is attributed to the vapour-liquid-solid (VLS) mechanism related to the deposition process. Our results may contribute towards the development of nanosilicon based optoelectronics

Surface-enhanced Raman scattering of graphene caused by self-induced nanogating by GaN nanowire array

A constant height of gallium nitride (GaN) nanowires with graphene deposited on them is shown to have a strong enhancement of Raman scattering, whilst variable height nanowires fail to give such an enhancement. Scanning electron microscopy reveals a smooth graphene surface which is present when the GaN nanowires are uniform, whereas graphene on nanowires with substantial height differences is observed to be pierced and stretched by the uppermost nanowires. The energy shifts of the characteristic Raman bands confirms that these differences in the nanowire height has a significant impact on the local graphene strain and the carrier concentration. The images obtained by Kelvin probe force microscopy show clearly that the carrier concentration in graphene is modulated by the nanowire substrate and dependent on the nanowire density. Therefore, the observed surface enhanced Raman scattering for graphene deposited on GaN nanowires of comparable height is triggered by self-induced nano-gating to the graphene. However, no clear correlation of the enhancement with the strain or the carrier concentration of graphene was discovered

Hybrid III–V/Silicon Nanowires

International audienceSemiconducting nanowires are emerging as a route to combine heavily mismatched materials. The nanowire dimensions facilitate the defect-free integration of the two most powerful semiconductor classes, group IVs and group III-Vs. These combinations may enhance the performance of existing device concepts, and also create new applications. In this chapter we review the recent progress in heteroepitaxial growth of III-V andIVmaterials. We highlight the advantage of using the small nanowire dimensions to facilitate accommodation of the lattice strain at the surface of the structures. Another advantage of the nanowire system is that anti phase boundaries are not formed, as there is only one nucleation site per wire. In this chapter, we will discuss three different heteroepitaxial III-V/Si morphologies, III-V nanowires on group IV substrates, and axial and radial heterojunctions. Advanced analysis techniques are used tocharacterise the quality of the heterointerfaces. Finally, we address potential applications of III-V/Si nanowires

Single-step growth of graphene and graphene-based nanostructures by plasma-enhanced chemical vapour deposition

The realization of many promising technological applications of graphene and graphene-based nanostructures depends on the availability of reliable, scalable, high-yield and low-cost synthesis methods. Plasma enhanced chemical vapor deposition (PECVD) has been a versatile technique for synthesizing many carbon-based materials, because PECVD provides a rich chemical environment, including a mixture of radicals, molecules and ions from hydrocarbon precursors, which enables graphene growth on a variety of material surfaces at lower temperatures and faster growth than typical thermal chemical vapor deposition. Here we review recent advances in the PECVD techniques for synthesis of various graphene and graphene-based nanostructures, including horizontal growth of monolayer and multilayer graphene sheets, vertical growth of graphene nanostructures such as graphene nanostripes with large aspect ratios, direct and selective deposition of monolayer and multi-layer graphene on nanostructured substrates, and growth of multi-wall carbon nanotubes. By properly controlling the gas environment of the plasma, it is found that no active heating is necessary for the PECVD growth processes, and that high-yield growth can take place in a single step on a variety of surfaces, including metallic, semiconducting and insulating materials. Phenomenological understanding of the growth mechanisms are described. Finally, challenges and promising outlook for further development in the PECVD techniques for graphene-based applications are discussed

太陽電池応用に向けた半導体ナノ構造体配列の形成と評価に関する研究

関西大

Plasma catalysis using low melting point metals.

Plasma catalysis is emerging as one of the most promising alternatives to carry out several reactions of great environmental importance, from the synthesis of nanomaterials to chemicals of great interest. However, the combined effect of a catalyst and plasma is not clear. For the particular case of 1-D nanomaterials growth, the low temperatures synthesis is still a challenge to overcome for its scalable manufacturing on flexible substrates and thin metal foils. Herein, the use of low-melting-point metal clusters under plasma excitation was investigated to determine the effectiveness in their ability to catalyze the growth of 1-D nanomaterials. Specifically, plasma catalysis using Gallium (Ga) was studied for the growth of silicon nanowires. The synthesis experiments using silane in hydrogen flow over Ga droplets in the presence of plasma excitation yielded tip-led growth of silicon nanowires. In the absence of plasma, Ga droplets did not lead to silicon nanowire growth, indicating the plasma-catalyst synergistic effect when using Ga as catalyst. The resulting nanowires had a 1:1 droplet diameter to nanowire diameter relationship when the droplet diameters were less than 100 nm. From 100 nm to a micron, the ratio increased from 1:1 to 2:1 due to differences with wetting behavior as a function of droplet size. The growth experiments using Ga droplets derived from the reduction of Gallium oxide nanoparticles resulted in silicon nanowires with size distribution similar to that of Gallium oxide nanoparticles. Systematic experiments over 100 ºC – 500 ºC range suggest that the lowest temperature for the synthesis of silicon nanowires using the plasma-gallium system is 200 ºC. A set of experiments using Ga alloys with aluminum and gold was also conducted. The results show that both Ga rich alloys (Ga-Al and Ga-Au) allowed the growth of silicon nanowires at a temperature as low as 200 ºC. This temperature is the lowest reported when using either pure Al or Au. The estimated activation energy barrier for silicon nanowire growth kinetics using Al-Ga alloy (~48.6 kJ/mol) was higher compared to that using either pure Ga or Ga-Au alloy (~34 kJ/mol). The interaction between Ga and hydrogen was measured experimentally by monitoring pressure changes in a Ga packed batch reactor at constant temperature. The decrease of the pressure inside the reactor when the Ga was exposed to plasma indicated the absorption of hydrogen in Ga. The opposite effect is observed when the plasma is turned off suggesting that hydrogen desorbed from Ga. This experimental observation suggests that Ga acts as hydrogen sink in the presence of plasma. The formation of Ga-H species in the Ga surface and in the bulk as intermediate is suggested to be responsible for the dehydrogenation of silyl radicals from the gas phase and subsequently for selective dissolution of silicon into molten Ga. The proposed reaction mechanism is also consistent with the experimentally determined activation barrier for growth kinetics (~34 kJ/mol). In addition, theoretical simulations using VASP (Vienna Ab-initio Simulation Package) were used to study atomic hydrogen – molten Ga interactions. The simulation results suggest significant interaction of atomic hydrogen with molten Ga through formation of Ga-H species on the surface and fast diffusion through bulk Ga while supporting the proposed model to explain the Plasma-Ga synergistic effect. Finally, plasma synthesis of silicon nanotubes using sacrificial zinc oxide nanowire thin film as a template was investigated for lithium ion battery anode applications. The silicon nanotube anode showed high initial discharge capacity during the first cycle of 4600 mAh g−1 and good capacity retention (3600 mAh g−1 after 20 cyles). The silicon nanotubes preserved their morphology after cycling and the observed performance was attributed to the change in phase from nanocrystalline silicon hydrogenated (nc-Si:H) to amorphous silicon hydrogenated (a-Si:H) during lithiation. This dissertation demonstrated the plasma synergism with molten metals during vapor-liquid-solid growth of silicon nanowires. A model based on atomic hydrogen interactions with molten metals under plasma excitation has been proposed and validated through systematic experimental studies involving Ga and its alloys with gold and aluminum and theoretical studies involving first principles computations. Finally, the plasma-Ga system has been used to grow successfully silicon nanowires on various technologically useful substrates at temperatures as low as 200 ºC

Novel synthesis techniques for nanostructures.

Crystalline beta-Ga2O3 nanowires with two distinct morphologies have been synthesized through simple physical evaporation of Te doped GaAs powder in argon atmosphere. The structure of the nanowires was characterized by SEM, TEM, XRD, EDX, and raman spectroscopy. Nanowires as long as hundreds of micrometers with diameters in the range of 20-200 nm have been produced with a high yield. Absence of Tellurium in the nanowires indicates that the growth mechanism is not VLS based. The role of Tellurium in the growth process is not clear. Substitution of sulfur in place of tellurium resulted in similar nanostructures. One of the morphologies of the nanowires exhibits herringbone structure and the TEM images show hexagonal crystallites ordered in regular spacing along the nanowire axis. The crystal plane of the nanowire is parallel to one of the facets of the crystallite. The other morphology is essentially platelets and free of hexagonal crystallites. Laser assisted catalyst growth process has been employed to synthesize various semiconducting nanowires, heterostructural nanowires and single walled carbon nanotubes. Structural characterization and physical properties of individual nanostructures have been explored