Steven T. Boles

Fabrication of silicon nanopillar-based nanocapacitor arrays

We report the fabrication of silicon nanopillar-based nanocapacitor arrays using metal-assisted etching in conjunction with electrodeposition. The high aspect ratio made possible by the catalyzed etching provides for an increased effective electrode area and hence a significant improvement in the capacitance density. Electroplated Ni electrode forms a conformal layer over the silicon nanopillars. Capacitance measurements show the expected trend as a function of pillar height and array period. The fabrication approach is simple, compatible with integration into standard silicon technology, and easily scalable.

In situ cycling and mechanical testing of silicon nanowire anodes for lithium-ion battery applications

In this work, we investigate the mechanical properties of silicon nanowires, which have been subjected to in situ electrochemical alloying and de-alloying with lithium inside a scanning electron microscope (SEM). Following de-alloying, in situ tensile testing of wires was performed inside a SEM using a microelectromechanical force sensor and a piezo-driven actuator. Compared to pristine silicon nanowires, the de-alloyed wires show a significant decrease in both their elastic modulus as well as in their ultimate tensile strength with indications that the newly formed amorphous silicon layer changes the mechanical properties of the wire.

Comparison of compressive and tensile relaxed composition-graded GaAsP and (Al)InGaP substrates

The authors present a comparison of metal organic chemical vapor deposition grown compositionally graded metamorphic buffers, which enable virtual substrates with very high quality crystal lattices with lattice constants from 5.45 to 5.65 A (threading dislocation density, ρt, around 104 cm−2). The structures, grown on GaP or GaAs, consist of graded In-fraction InGaP and AlInGaP or graded P-fraction GaAsP. They show that surface roughness and locally strained regions of phase separation (branch defects) limit misfit dislocation glide velocity and escalate threading dislocation density. High surface roughness and branch defects in (Al)InGaP lead to the lowest quality virtual substrates we observed, with ρt of around 3×106 cm−2. In contrast, graded mixed-anion films of GaAsP avoid branch defects and minimize surface roughness, giving superior defect densities, as low as 104 cm−2 at useful lattice constants halfway between that of Si and Ge. Tensile graded GaAs1−zPz layers yield the smoothest films (0.78 nm r...

In situ tensile and creep testing of lithiated silicon nanowires

We present experimental results for uniaxial tensile and creep testing of fully lithiated silicon nanowires. A reduction in the elastic modulus is observed when silicon nanowires are alloyed with lithium and plastic deformation becomes possible when the wires are saturated with lithium. Creep testing was performed at fixed force levels above and below the tensile strength of the material. A linear dependence of the strain-rate on the applied stress was evident below the yield stress of the alloy, indicating viscous deformation behavior. The observed inverse exponential relationship between wire radius and strain rate below the yield stress indicates that material transport was controlled by diffusion. At stress levels approaching the yield strength of fully lithiated silicon, power-law creep appears to govern the strain-rate dependence on stress. These results have direct implications on the cycling conditions, rate-capabilities, and charge capacity of silicon and should prove useful for the design and construction of future silicon-based electrodes.

Fast and slow stress evolution mechanisms during interruptions of Volmer-Weber growth

The evolution of mechanical stress during Volmer-Weber growth of thin films is complex, often including a reversible stress evolution during interruptions of film deposition. The underlying mechanism for stress evolution during growth interruptions has been extensively debated, but remains unclear. In this work, in situ measurements of stress evolution during growth interruptions of various time scales, film thicknesses, and substrate temperatures were made during deposition of gold and nickel films. It was found that at least two mechanisms lead to the observed stress evolution, one fast (time constant ∼102 s) and one slow (time constant ∼104 s). The fast process is reversible and weakly dependent on the film thickness, while the slow process is irreversible and strongly dependent on the film thickness. It is shown that grain growth during growth interruptions can account for a significant portion of the stress change associated with the slow process. The fast reversible process is likely to be associate...

Catalyst proximity effects on the growth rate of Si nanowires

Si nanowires grown by the vapor-liquid-solid (VLS) mechanism were fabricated using Au-catalyst nanoparticles and silane (SiH4) gas on Si substrates. Au was deposited on the substrate surface both by electron-beam evaporation and Au-colloid deposition. Both kinking defects and vertical nanowire epitaxy on Si ⟨111⟩ substrates were found to be directly related to SiH4 flow rate. A correlation between Au-colloid dilution and the nanowire growth rate was also observed, with the growth rate increasing with increasing concentrations of Au-catalyst particles on the wafer surface. Systematic experiments relating the nanowire growth rate to the proximity of nearest-neighbor Au particles and Au reservoirs were carried out, and the results were found to be in good agreement with a SiH4 reaction model, which associates decomposition to form SiH2 with higher nanowire growth rates. Implications toward the realization of VLS-grown single nanowire transistors are discussed.

Formation of Single Tiers of Bridging Silicon Nanowires for Transistor Applications Using Vapor–Liquid–Solid Growth from Short Silicon-on-Insulator Sidewalls

Single tiers of silicon nanowires that bridge the gap between the short sidewalls of silicon-on-insulator (SOI) source/drain pads are formed. The formation of a single tier of bridging nanowires is enabled by the attachment of a single tier of Au catalyst nanoparticles to short SOI sidewalls and the subsequent growth of epitaxial nanowires via the vapor-liquid-solid (VLS) process. The growth of unobstructed nanowire material occurs due to the attachment of catalyst nanoparticles on silicon surfaces and the removal of catalyst nanoparticles from the SOI-buried oxide (BOX). Three-terminal current-voltage measurements of the structure using the substrate as a planar backgate after VLS nanowire growth reveal transistor behaviour characteristics.

Ultra-fast polymer optical fibre Bragg grating inscription for medical devices

We report the extraordinary result of rapid fibre Bragg grating inscription in doped polymer optical fibres based on polymethyl methacrylate in only 7 ms, which is two orders of magnitude faster than the inscription times previously reported. This was achieved using a new dopant material, diphenyl disulphide, which was found to enable a fast, positive refractive index change using a low ultraviolet dose. These changes were investigated and found to arise from photodissociation of the diphenyl disulphide molecule and subsequent molecular reorganization. We demonstrate that gratings inscribed in these fibres can exhibit at least a 15 times higher sensitivity than silica glass fibre, despite their quick inscription times. As a demonstration of the sensitivity, we selected a highly stringent situation, namely, the monitoring of a human heartbeat and respiratory functions. These findings could permit the inscription of fibre Bragg gratings during the fibre drawing process for mass production, allowing cost-effective, single-use, in vivo sensors among other potential uses.

Mechanical measurements on lithium phosphorous oxynitride coated silicon thin film electrodes for lithium-ion batteries during lithiation and delithiation

The development of large stresses during lithiation and delithiation drives mechanical and chemical degradation processes (cracking and electrolyte decomposition) in thin film silicon anodes that complicate the study of normal electrochemical and mechanical processes. To reduce these effects, lithium phosphorous oxynitride (LiPON) coatings were applied to silicon thin film electrodes. Applying a LiPON coating has two purposes. First, the coating acts as a stable artificial solid electrolyte interphase. Second, it limits mechanical degradation by retaining the electrodes planar morphology during cycling. The development of stress in LiPON-coated electrodes was monitored using substrate curvature measurements. LiPON-coated electrodes displayed highly reproducible cycle-to-cycle behavior, unlike uncoated electrodes which had poorer coulombic efficiency and exhibited a continual loss in stress magnitude with continued cycling due to film fracture. The improved mechanical stability of the coated silicon electrodes allowed for a better investigation of rate effects and variations of mechanical properties during electrochemical cycling.

Leveraging Titanium to Enable Silicon Anodes in Lithium-Ion Batteries

Silicon is a promising anode material for lithium-ion batteries because of its high gravimetric/volumetric capacities and low lithiation/delithiation voltages. However, it suffers from poor cycling stability due to drastic volume expansion (>300%) when it alloys with lithium, leading to structural disintegration upon lithium removal. Here, it is demonstrated that titanium atoms inside the silicon matrix can act as an atomic binding agent to hold the silicon atoms together during lithiation and mend the structure after delithiation. Direct evidence from in situ dilatometry of cosputtered silicon-titanium thin films reveals significantly smaller electrode thickness change during lithiation, compared to a pure silicon thin film. In addition, the thickness change is fully reversible with lithium extraction, and ex situ post-mortem microscopy shows that film cracking is suppressed. Furthermore, Raman spectroscopy measurements indicate that the Si-Ti interaction remains intact after cycling. Optimized Si-Ti thin films can deliver a stable capacity of 1000 mAh g-1 at a current of 2000 mA g-1 for more than 300 cycles, demonstrating the effectiveness of titanium in stabilizing the material structure. A full cell with a Si-Ti anode and LiFePO4 cathode is demonstrated, which further validates the readiness of the technology.