Growth and Characterisation of Gold-seeded Indium Gallium Arsenide Nanowires for Optoelectronic Applications

III-V semiconductor nanowires have been shown as promising candidates to serve as building blocks in electronic and optoelectronic devices such as transistors, lasers, light emitting diodes, photodiodes and solar cells. Among the III-V semiconductors, ternary III-V alloy semiconductors such as InxGa1-xAs have the advantage of tunable bandgap by varying their alloy composition covering the important wavelengths used in optical telecommunication systems and sensing in near infra-red region. Therefore, it is essential to gain an understanding and control of ternary nanowires prior to incorporating them in device applications. This thesis presents a progressive advancement of Au-seeded InxGa1-xAs nanowire growth by metal-organic vapour phase epitaxy (MOVPE), towards achieving highly uniform composition, morphology and pure crystal phase. Several techniques have been employed to investigate the nanowire properties. Scanning and transmission electron microscopy, atomic force microscopy, X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) have been used for structural and compositional analysis, while photoluminescence (PL) has been used to provide insight into their optical properties. Pure zinc-blende (ZB) phase InxGa1-xAs nanowires are obtained via two-temperature growth method, which involves growing an initial stub at a higher temperature followed by a lower growth temperature. Low-temperature growth is found to favour high In incorporation rate either via the vapour-liquid-solid (VLS) mode or the vapour-solid (VS). InxGa1-xAs nanowires with highly homogenous composition and pure ZB phase are achieved when the In incorporation rates in both modes are equivalent. Homogenous composition InxGa1-xAs nanowires can also be achieved at relatively high temperatures with tunable crystal phase. Detailed TEM analysis in combination with the EDX show that the crystal phase is dependent on the V/III ratio, and correlates with the Ga incorporation rate in the nanowire. Pure wurtzite (WZ) phase, uniform and taper-free nanowires are obtained with a combination of relatively high growth temperature, low V/III ratio and small diameter Au seed particle. The optimized pure WZ phase nanowires capped with InP show luminescence properties around 1.54 um, a wavelength region of importance to the optical fibre telecommunications. Understanding the growth evolution of InxGa1-xAs nanowires is improved by developing a model based on a nucleation kinetics approach. The modelling correlates well with the experimental results revealing the key factors governing the composition and growth rate of InxGa1-xAs nanowires. Finally, tunable emission wavelengths of InxGa1-xAs /InGaP core-shell structures within the range of 1100 - 1420 nm are achieved by tuning the shell thickness. The growth of the complex ternary/ternary system is studied using TEM and EDX analyses, revealing some challenges in the growth of the shell. Despite the challenges, a strain related blue-shifting of the InxGa1-xAs bandgap is demonstrated. Overall the thesis makes a significant progress in understanding the growth of Au-seeded InxGa1-xAs nanowires. From the systematic study, the growth of highly uniform InxGa1-xAs nanowires grown via Au-seeded VLS method is demonstrated. A growth model is developed to further understand the growth mechanism. The optimized nanowires in combination with an InP or InGaP shell show luminescence properties tunable within the near infra-red region, promising as future optoelectronic building blocks

Synthesis and characterization of metal sulfates loaded Palm Empty Fruit Bunch (PEFB) for biodiesel production

Biodiesel has been globally accepted as a green substitute for diesel fuel. However, the insecurity of food raised with the application of edible sources in biodiesel production has caused much debate. The feasible alternative technique is the use of inedible and low-grade sources such as palm fatty acid distillate (PFAD). In this work, the production of biodiesel (FAME) from PFAD using solid acid catalysts (SACs) derived from palm empty fruit bunch (PEFB) is investigated. The SACs were synthesized through impregnation of different metal sulfate precursors, i.e. ferrous sulfate heptahydrate (FeSO4 .7H2 O), copper sulfate pentahydrate (CuSO4 .5H2 O), and magnesium sulfate heptahydrate (MgSO4 .7H2 O) over PEFB. SEM-EDX observations found that impregnation and then calcination resulted in attachment of sulfur (S) and improved surface porosity. FT-IR analysis showed that there were distinct interactions between metal sulfates and PEFB. XRD characterization showed that the prepared catalysts have a crystalline structure. Besides, the catalytic activity of the SACs was closely associated with their acid densities measured by the titration method. Fe-PEFB catalyst showed the highest acid density (2.44 mmol/g) among the catalysts studied. To study the effect of process parameters on FFA conversion (%), optimization of methanol: PFAD molar ratio, catalyst dosage, reaction temperature, and reaction time was conducted. Maximum FFA conversion of 89.1% was obtained over Fe-PEFB while Cu-PEFB and Mg-PEFB achieved an FFA conversion of 63 and 56.5%, respectively, under the optimum reaction conditions. Thus, the present study offers a sustainable and environmentally benign method for biodiesel production

Synthesis and characterization of metal sulfates loaded palm empty fruit bunch (PEFB) for biodiesel production

Biodiesel has been globally accepted as a green substitute for diesel fuel. However, the insecurity of food raised with the application of edible sources in biodiesel production has caused much debate. The feasible alternative technique is the use of inedible and low-grade sources such as palm fatty acid distillate (PFAD). In this work, the production of biodiesel (FAME) from PFAD using solid acid catalysts (SACs) derived from palm empty fruit bunch (PEFB) is investigated. The SACs were synthesized through impregnation of different metal sulfate precursors, i.e. ferrous sulfate heptahydrate (FeSO4.7H2O), copper sulfate pentahydrate (CuSO4.5H2O), and magnesium sulfate heptahydrate (MgSO4.7H2O) over PEFB. SEM-EDX observations found that impregnation and then calcination resulted in attachment of sulfur (S) and improved surface porosity. FT-IR analysis showed that there were distinct interactions between metal sulfates and PEFB. XRD characterization showed that the prepared catalysts have a crystalline structure. Besides, the catalytic activity of the SACs was closely associated with their acid densities measured by the titration method. Fe-PEFB catalyst showed the highest acid density (2.44 mmol/g) among the catalysts studied. To study the effect of process parameters on FFA conversion (%), optimization of methanol: PFAD molar ratio, catalyst dosage, reaction temperature, and reaction time was conducted. Maximum FFA conversion of 89.1% was obtained over Fe-PEFB while Cu-PEFB and Mg-PEFB achieved an FFA conversion of 63 and 56.5%, respectively, under the optimum reaction conditions. Thus, the present study offers a sustainable and environmentally benign method for biodiesel production

Investigation of the pH effect in hydrothermal growth of zinc oxide nanostructures

This study focuses on the influence of the pH value of hydrothermal solution on the morphology and the transmission spectrum of zinc oxide (ZnO) nanostructures using field-emission scanning electron microscope (FE-SEM) and ultraviolet-visible (UV-Vis) spectroscopy, respectively. The ZnO nanostructures were grown on glass substrates at 90oC for 5 hours. The pH and the concentration of the starting solutions were varied from 2.03 to 12.02 and 6 mM to 100 mM respectively. Various ZnO structures of neuron-like, flowerlike, and urchin-like morphologies were obtained at alkaline pH (~8.00 to ~9.00) while for pH solution lower than ~8.00, rod-like nanostructures were obtained. Solution of pH value 11.50 shows that growth of nanostructures was suppressed due to the high susceptibility to erosion in both acidic and alkaline solutions. By changing the concentrations of the solution, the density and size were also varied. The increase in concentration lead to the increase of nanostructure density and the diameter of ZnO nanorods. The trend shows that the concentration solution from lower (C 100mM) resulted ZnO nanostructures such as nanorods to thick film due to overgrowth and coalescence of the nanostructures with more available precursors

An Optimization of Nanostructure Aluminum on Porous Silicon at Different Aluminum Thickness

The growth of aluminum nanostructure was conducted on porous silicon substrate by depositing a layer of aluminum via thermal evaporation method. The deposition process of the aluminum nanostructure was under the annealing temperature at 350°C for 1 hour. The weight of aluminum was varied for each sample in order to obtain different thickness of aluminum deposited on the sample. The weight of aluminum used in this experiment were 12mg ,18mg ,50mg and 74mg with the corresponding aluminum thickness deposited of 112nm, 163nm, 205nm and 332nm. Characterization on the morphology of the sample are conducted by using Atomic force microscopy (AFM), Raman spectroscopy and IV measurements. Based on the result obtained, the optimum weight of aluminum was 50mg of aluminum since it is provide the higher conductivity value on the sample

Surface morphology of In0.5Ga0.5 quantum dots grown using Stranski-Krastanov growth mode

In this research an atomic force microscopy (AFM) study on self-assembled In0.5Ga0.5As/GaAs quantum dots (QDs) was performed. Surface morphology of self-assembled In0.5Ga0.5As QDs changes with different growth time. Increasing growth time increased the dots size and decreased the dots density. In addiditon, self-assembled In0.5Ga0.5As QDs was grown on In0.1Ga0.9As underlying layer with different after-growth AsH3 flow time during cooling-down. The underlying layer caused lattice strain relaxation in the QDs on the surface. Increasing the period of AsH3 flow during cooling-down reduced the diameter of the dots and increased the density. The migration of groups III species in the growth of In0.5Ga0.5As/GaAs system was influenced by AsH3 flow during cooling-down period. This was due to the increase in surface population of active arsenic species. Underlying layer and the period of AsH3 flow during cooling-down are the two key factors in the fabrication of small and dense In0.5Ga0.5As QDs

Formation of self-assembled Ge islands on Si (100) using magnetron sputtering and subsequent rapid thermal annealing

Formation of Ge island after an ex-situ thermal annealing of Ge-rich film on Si (100) were investigated. Ge film was deposited using radio-frequency magnetron sputtering with the substrate being at room temperature. The film was then thermal annealed at an elevated temperature using a rapid thermal processor (RTP) in nitrogen ambient. The structural changes of the annealed film were studied using atomic force microscopy (AFM). Following annealing at temperature above 500oC, island assembling from Ge film were observed. The size of these islands were about 80-180nm wide 4-30nm in height. Density of the islands increases as annealing temperature increases up to 700oC. The surface morphology of the samples after annealing varies with various thickness of the Ge layer deposited on Si (100). The formation of the island was explained to form via the same mechanism as Stranski-Krastanov mode though it is not epitaxially grown. The high temperature annealing causes the adatoms to migrate and forms to islands as to minimize the surface energy

Using computer technology in the ESL classroom

Using computer and the Internet allow students and teachers to interact with one another with no boundaries and this affects the students’ attitude and their motivation in learning. This study focuses on the attitude, motivation and performances of ESL learners in learning writing using Google Docs. The quantitative instruments consisted of questionnaire, pre-test, and post-test while the qualitative instruments consisted of interview recordings and the treatment sessions. The results from the questionnaire were used to identify the attitude and motivation of the students while the results from the pre-test and post-test were used to determine the students’ performances after the use of Google Docs. The result of the study showed that the students had positive attitude toward the use of Google Docs in learning and were also motivated to learn. Google Docs manage to encourage them to write better, and there were improvements in learners’ performances. They were able to generate more ideas and organized their writing accordingly. As a conclusion, Google Docs should be implemented by teachers to teach process writing as the tool can motivate the students to learn better

Morphological evolution of ternary inxga1-xas nanowires (NWs) grown with Au particle-assisted using vertical chamber MOCVD

The morphology and chemical composition of InxGa1-xAs NWs grown on undoped GaAs (111)B substrate have been investigated using scanning electron microscopy (SEM) and energy dispersive x-ray (EDX), respectively. SEM images show that InxGa1-xAs NWs underwent morphological evolution as temperature changes. By changing the growth temperature, the growth mechanism of NWs was assumed to have changed. Both characterizations results suggested the growth mechanism has strong influence to the evolution of the NWs morphologies and also to the distribution of the chemical composition of NWs

Understanding the growth and composition evolution of gold-seeded ternary InGaAs nanowires

InGaAs nanowires offer great promise in fundamental studies of ternary compound semiconductors with variable composition and opens up a wide range of applications due to their bandgap tunability and high carrier mobility. Here, we report a study on the growth of Au-seeded InGaAs nanowires by metal-organic vapour phase epitaxy and present a model to explain the mechanisms that govern the growth and composition evolution in ternary III-V nanowires. The model allows us to further understand the limitations on the growth rate and incorporation of the two group III species imposed by the deposition conditions and some intrinsic properties of the material transport and nucleation. Within the model, the evolution of InGaAs nanowire growth rate and composition with particle size, temperature and V/III ratio is described and correlates very well with experimental findings. The understanding gained in this study should be useful for the controlled fabrication of tunable ternary nanowires for various applications. This journal i