Nanosprings harvest light more efficiently

Nanotechnology presents versatile architectural designs for the purpose of utilization as a building block of 1D optoelectronic nanodevices because current nanowire-based schemes require more effective solutions for low absorption capacity of nanoscale volumes. We report on the potential of nanospring absorbers as an alternative light-harvesting platform with significant advantages over conventional nanowires. Absorption capacity of nanospring geometry is found to be superior to cylindrical nanowire shape. Unlike nanowires, they are able to trap a larger amount of light thanks to characteristic periodic behavior that boosts light collection for the points matched with Mie resonances. Moreover, nanospring shape supplies compactness to a resulting device with area preservation as high as twofold. By considering that a nanospring array with optimal periods yields higher absorption than individual arrangements and core-shell designs, which further promote light collection due to unique antireflection features of shell layer, these nanostructures will pave the way for the development of highly efficient self-powered nanosystems. © 2015 Optical Society of America

Fabrication and Characterisation of Nitride DBRs and Nitride Membranes by Electrochemical Etching Techniques

A Distributed Bragg Reflector (DBR) is an important component for semiconductor microcavities and optoelectronic devices, such as vertical cavity surface emitting lasers (VCSELs), resonant cavity light-emitting diodes (RCLEDs). In the past thirty years, epitaxially grown GaAs-based DBRs have made great achievements of the application of III-V VCSELs in communications and mobile applications. At the same time, III-nitrides have demonstrated excellent performance in solid-state lighting and advanced optoelectronic devices due to the wide bandgap and unique properties. In recent years, GaN-based semiconductors have made great progress in the application of blue VCSELs. However, the absence of high-performance DBRs is a challenge for developing higher-power GaN-based VCSELs. Currently, the typical epitaxial GaN-based DBRs are limited by a long growth period, low optical performance, and poor quality of growth. Therefore, this project proposes a method to fabricate nanoporous (NP)/GaN-based DBR by electrochemical etching (EC), which are grown using metalorganic vapour-phase epitaxy (MOVPE). The heavily silicon doped GaN layer is transformed into an NP structure by selective etching, resulting in a higher refractive index contrast in each periodic layer. Moreover, a lateral etching method is proposed to further improve the EC etching of DBRs. This method can confine the etching in each sacrificial layer and make the etching aperture directions highly uniform. The corresponding characterizations have been carried out to explore the mechanisms of different etching methods, by optical microscopy, scanning electron microscopy (SEM) and reflectance measurements. It further confirms that the laterally etched NP GaN-based DBRs exhibit a higher reflectivity and wider stopband. The GaN sacrificial layers required for the EC etching are typically heavily silicon doped (>1019cm-3), resulting in a rough surface and saturated conductivity. On the other hand, the heavily silicon doped AlGaN with a low Al content (≤5%) exhibits an atomically flat surface and an enhanced electrical conductivity. Therefore, in this work, we introduced multiple pairs of heavily doped n++-Al0.01Ga0.99N/GaN to replace the widely used multiple pairs of heavily doped n++-GaN/GaN to fabricate lattice-matched NP DBRs by EC etching. Consequently, the epitaxially grown n++-Al0.01Ga0.99N/GaN-based DBR demonstrates a smoother surface than the n++-GaN/GaN-based DBR. Moreover, the NP-Al0.01Ga0.99N/GaN-based DBR exhibits higher reflectivity and wider stopband after lateral EC etching compared to the NP-GaN/GaN-based DBR. This method has been successfully applied to fabrication of high-performance DBR structures with the wavelength range from blue to deep yellow by modifying the epitaxial growth conditions. Furthermore, it is found that a very thin Al-Si diffusion layer is formed at the interface between an AlN buffer layer and a silicon substrate when growing the low-temperature AlN buffer layer on the n-doped silicon substrate by MOVPE. The diffusion layer exhibits high conductivity and can be EC-etched and polished as a sacrificial layer. Therefore, this method is proposed for stripping large-area GaN membranes by EC etching. A sample with AlN/AlGaN/GaN layers is first epitaxially grown by MOVPE on an n-doped (111) silicon substrate, and then bonded upside-down to a new glass host substrate and EC etched. Finally, lift-off of a large size GaN-based membrane has been realized with an area of 2.625cm2 and a crack-free and nanoscale smooth surface. Compared to other lift-off methods such as laser lift-off (LLO), chemical lift-off (CLO), and mechanical release techniques, this method does not involve bulky and expensive equipment, which can be used to fabricate high-performance III-nitride devices on the membrane at low cost in the future

Growth and characterization of ZnO and SiC nanowires

The synthesis of semiconductor nanowires has been studied intensively worldwide for a wide spectrum of materials. Such low-dimensional nanostructures are not only interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications. Deeper understanding and sufficient control of the growth of nanowires are central to the current research interest. The objective of the thesis work is synthesizing semiconductor nanowires using various growth processes, with a focus on the spontaneous growth process, which offers an opportunity for the control of spatial positioning of nanowires. Zinc oxide (ZnO) based and Silicon carbide (SiC) based nanowires have been concentrated to synthesize using vapor-solid (VS) and vapor–liquid–solid (VLS) techniques respectively. ZnO is one of very interesting semiconductor material because of its physical and chemical properties. Also, it is well known that high n-type conductivity can be achieved by alloying zinc oxide with group III elements (such as Al, In or Ga) in ternary or even quaternary oxide compounds, in order to obtain transparent conducting oxides (TCOs). In this part of work, there were two major materials have been synthesized such as vertically aligned ZnO nanorods and ternary Zn(In,Ga,Sn)O nanorods using vapor phase technique. First, solution-free and catalyst-free vertically aligned ZnO nanorods have been synthesized by thermal CVD reactor at relatively low temperature (< 500 °C) to produce high-surface 3D photoanode on glass substrate. Different TCOs films such as Al doped ZnO films deposited by PED, RF-sputtering techniques and ITO were considered for the growth as starting seeding layer for the nanorods. The aim of this work is mainly focused to control the thickness and length of these nanostructures by varying not only the growth parameters, such as amount of Zn evaporation, but also substrate characteristics, such as grain size of Al doped ZnO and ITO seeding films. Second, Indium Zinc oxide nanorods (IZO-NRs) have been obtained at temperatures lower than 500°C using same CVD system, with a resulting indium concentration larger than 1%. The growth of these ternary oxide nanostructures has been obtained at relatively low temperature, starting from the corresponding metals, thanks to the direct deposition on the growth substrate of an In layer, which in its molten state and upon mixture with Zn acts as growth seed. The obtained indium concentration corresponds to the value required to get metallic behavior and make this ternary oxide a TCO (transparent conducting oxide), while the used temperature range makes it compatible also with commercial glass substrates. Same technique have been used to obtain GaZnO and SnZnO nanostructures. Among many kind of semiconductor, SiC is an important wide band gap IV-IV semiconducting material and it exhibit excellent, unique physical and mechanical properties at nano-scale, which lead to their potential applications for being used as the building blocks in nanoelectronics and nanooptoelectronics. Also, it has biocompatibility and inertness can be exploited for biomedical applications. In this part of work, there were two types of SiC nanowires have been synthesized using VLS growth technique. First, Cubic SiC nanowires were successfully grown using home-made induction heated Vapor Phase Epitaxy (VPE) reactor on Si (100) and Si (111) substrate using nickel (Ni) and Iron (Fe) as a catalysts. The main aim of this work is to optimize the condition to grow SiC nanowires with Ni and Fe catalyst. The size and shape of the nanowires has been controlled using temperature and gas flow rate. Second, self-assembled SiC core with SiO2 shell coaxial nanowires using Ni and Fe catalyst have been synthesized by thermal CVD reactor. The growth conditions were optimized for both catalyst using temperature, gas flow rate. This SiC /SiO2 coaxial core/shell nanowires (NWs) are intriguing as novel nanostructured to be functionalized because of the 3C-SiC biocompatibility and of the presence of a SiO2 native shell that favours surface functionalization. Those findings are encouraging in the prospective to employ this functionalized system for different nano-medical applications such as targeted therapy against deep tumor cells

Ultra flexible SiGe/Si/Cr nanosprings

The electrical and mechanical properties of Si/SiGe rolled-up nanosprings have been investigated. Micromanipulation has been employed to investigate the mechanical properties. For nanosprings under investigation, a linear dependence between applied force and extention is found until the spring is extended to 91% of its original length, moreover, the springs could be reproducibly extended to more than 180% of their original length. An extremely small spring constant of 0.003 N/m has been determined, which is an order of magnitude smaller than that of the most flexible available atomic force microscope (AFM) cantilever (similar to 10(-2) N/m). Thus, it is expected that these springs can be used as ultra-sensitive force sensors. A simple estimation assuming an imaging resolution of approximately 1 nm is adopted for displacement measurement and reveals that using a nanospring fabricated from a 300 nm wide mesa as a visual-based force sensor, a resolution of 3 pN/nm can be provided. The conductivity of nanospirals was analysed and current densities up to 530 kA/cm(2) were measured. Structures with metallic wires on top of the mesa structures were successfully employed to activate mechanical movements of the structure. (c) 2007 Elsevier Ltd. All rights reserved

2009 Annual Progress Report: DOE Hydrogen Program

This report summarizes the hydrogen and fuel cell R&D activities and accomplishments of the DOE Hydrogen Program for FY2009. It covers the program areas of hydrogen production and delivery; fuel cells; manufacturing; technology validation; safety, codes and standards; education; and systems analysis