Si Alloy Formation Research Articles

Layer-controlled single-crystalline graphene film with stacking order via Cu-Si alloy formation.

Multilayer graphene and its stacking order provide both fundamentally intriguing properties and technological engineering applications. Several approaches to control the stacking order have been demonstrated, but a method of precisely controlling the number of layers with desired stacking sequences is still lacking. Here, we propose an approach for controlling the layer thickness and crystallographic stacking sequence of multilayer graphene films at the wafer scale via Cu-Si alloy formation using direct chemical vapour deposition. C atoms are introduced by tuning the ultra-low-limit CH4 concentration to form a SiC layer, reaching one to four graphene layers at the wafer scale after Si sublimation. The crystallographic structure of single-crystalline or uniformly oriented bilayer (AB), trilayer (ABA) and tetralayer (ABCA) graphene are determined via nano-angle-resolved photoemission spectroscopy, which agrees with theoretical calculations, Raman spectroscopy and transport measurements. The present study takes a step towards the layer-controlled growth of graphite and other two-dimensional materials.

Effects of HF on the Lithiation Behavior of the Silicon Anode in LiPF6 Organic Electrolyte Solution.

As one of the major impurities in the organic electrolyte, HF can react with the alkali components in the solid electrolyte interphase (SEI), such as lithium alkoxide and lithium carbonate, to form more LiF-rich SEI. Here, the effects of HF on the lithiation behavior of the single crystal Si(111) anode were studied using scanning electron microscopy, soft X-ray emission spectroscopy, and windowless energy-dispersive X-ray spectroscopy. When the Li–Si alloy is formed in 1.0 M LiPF6 in the propylene carbonate solvent, it has a layered structure that contained the first layer of crystalline Li15Si4 (c-Li15Si4) alloy pyramids, the second layer of amorphous Li13Si4 (a-Li13Si4) alloy, and a third layer of Li-diffused LixSi alloy. When the more concentrated HF is in the electrolyte solution, less amount of the c-Li15Si4 alloy is formed in the first layer. It suggests that the Si lithiation can form only amorphous LixSi alloy relative to the components in the electrolytes. The study also explains why only amorphous LixSi alloy formation was observed in some previous studies.

Initial stages of Zr–Fe–Si alloy formation on Zr(0001) surface

We studied initial stages of Fe and Si atoms interaction with the Zr(0001) surface. Fe and Si were deposited in-situ on the clean Zr surface up to coverage of one monolayer. Chemical state of Zr as a function of annealing temperature and the surface composition was investigated. Characteristic (1×1) LEED (Low Energy Electron Diffraction) pattern of the clean Zr surface becomes significantly modified when Fe and Si are deposited and annealed at 400°C. The arrangement of new spots of the pattern corresponds to the (2√3×2√3)R30° structure, stable up to annealing at 750°C. Changes in the electronic structure were identified from the images of equi-energy cuts of the k-space covering the entire Brillouin zone. The results imply that hydrogen, typically present in the clean surface, was eliminated either by annealing or upon the deposition of Fe and Si.

Study of faceted Au nanoparticle capped ZnO nanowires: antireflection, surface enhanced Raman spectroscopy and photoluminescence aspects

We report a single step growth process of faceted Au nanoparticles (NPs) on highly c-axis oriented ZnO nanowires (NWs) and report that a system with a lower antireflection coefficient also showed higher surface enhanced Raman spectroscopy (SERS) enhanced factors. Well-dispersed Au NPs are grown on silicon substrate using a thin film-in-air-annealing method (using 1 nm and 5 nm thick Au films on silicon and subsequent annealing in air at 800 °C) wherein enhanced oxide growth at the Au–Si interface was used to inhibit inter-diffusion to avoid Au–Si alloy formation (Au/SiOx/Si). These substrates are used to grow aligned ZnO NWs using a high temperature (≈900 °C) chemical vapour deposition method. Depending on the size and areal density of initial catalytic Au NPs, the resultant photoluminescence, reflectance characteristics, and effectiveness as SERS substrates of the faceted Au NP capped ZnO NWs coatings are systematically studied. The highly oriented and faceted Au NPs on ZnO NWs have been used as free standing SERS substrates to detect sub-micro molar crystal violet molecules with an analytical enhancement factor (AEF) of ≥104 and with high repeatability. The substrate with high-density Au–ZnO heterostructures (5 nm Au case) found to have larger AEF, very low reflectance (≈0.75%) and more green emission.

Al–Si alloy formation in narrow p-type Si contact areas for rear passivated solar cells

For high efficiency silicon solar cells, the rear surface passivation by a dielectric layer has significant advantages compared to the standard fully covered Al back-contact structure. In this work the rear contact formation of the passivated emitter and rear cell device structure is analyzed. Contrary to expected views, we found that the contact resistivity of fine screen printed Al fingers alloyed on narrow p-type Si areas depends on the geometry of the Al–Si alloy formation below the contacts, and decreases by reducing the contact area, while the contact resistance remains constant. At the solar cell level, the reduction in the contact resistivity leads to a minimization of the fill factor losses. At the same time, narrow Al–Si alloy formations increased the passivated area below the contacts, improving the optical properties of the rear side, reducing the short-circuit current and open-circuit voltage losses. Scanning electron microscopy analysis of the Al–Si alloy geometry is performed, in order to understand its influence on the contact resistivity. The analysis presented in this article has application in Al–Si alloying processes and advanced solar cells concepts, like back-contact and rear passivated solar cells.

Is impurity gettering or passivation by hydrogen the improvement key of mc-Si solar cells during processing steps?

During conventional processing steps needed to make solar cells from mc-Si wafers the lifetime or diffusion length of minority carriers, are drastically improved. The first step, P diffusion, induces a marked gettering of fast diffusers. The second step is the deposition of an antireflection hydrogen rich layer on the front face and a thick aluminium layer on the back face, followed by a rapid thermal annealing which forms an Al–Si alloy. It was proposed that during this last step, the alloy formation induces an additional gettering and injects Si vacancies which favour the penetration of hydrogen in the bulk. We arrive to the conclusion that: the main improvement of the bulk properties results of gettering effects. P diffusion must be applied first, then Al–Si alloy formation improves drastically the material. Hydrogenation is also efficient provided the material was previously purified.

Near-eutectic Si–Cr alloy formation with electron-beam and laser pulses

A study is presented of amorphous Cr–Si alloy formation with intense electron-beam and laser pulses. Two different kinds of samples were used: the first consisted of a 100-nm Cr film deposited on a 200-nm Si layer; the second was of a 60-nm Cr film on a 700-nm Si layer. The substrates were SiO2. Formation of Cr–Si alloys, with a composition near the Cr45Si55 and Cr13Si87 eutectics in the first and second sample, respectively, was observed in a narrow range of energy fluences. The results are discussed with the aid of transient temperature profiles of the samples, obtained by computer simulations.

Evidence of a strong metal-support interaction and of NiSi alloy formation in silica-supported nickel catalysts

Evidence of a strong metal-support interaction and of NiSi alloy formation in silica-supported nickel catalysts