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Synthesis And Characterization Of Germanium Nanowires And Germanium Silicon Radially Heterostructured Nanowires
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Book Synopsis Synthesis and Characterization of Germanium Nanowires and Germanium/silicon Radially Heterostructured Nanowires by : Irene Anne Goldthorpe
Download or read book Synthesis and Characterization of Germanium Nanowires and Germanium/silicon Radially Heterostructured Nanowires written by Irene Anne Goldthorpe and published by . This book was released on 2009 with total page 240 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Growth and Characterization of Silicon and Germanium Nanowires and Heterostructures by : Prashanth Madras
Download or read book Growth and Characterization of Silicon and Germanium Nanowires and Heterostructures written by Prashanth Madras and published by . This book was released on 2010 with total page 346 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Synthesis of Silicon and Germanium Nanowires and Si/Ge Nanowire Heterostructures by : Teresa J. Clement
Download or read book Synthesis of Silicon and Germanium Nanowires and Si/Ge Nanowire Heterostructures written by Teresa J. Clement and published by . This book was released on 2007 with total page 272 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Synthesis and Characterization of Silicon and Germanium Nanowires, Silica Nanotubes, and Germanium Telluride/tellurium Nanostructures by : Hsing-Yu Tuan
Download or read book Synthesis and Characterization of Silicon and Germanium Nanowires, Silica Nanotubes, and Germanium Telluride/tellurium Nanostructures written by Hsing-Yu Tuan and published by . This book was released on 2007 with total page 179 pages. Available in PDF, EPUB and Kindle. Book excerpt: Heterostructured nanomaterials are interesting since they merge the properties of the individual materials and can be used in diverse applications. GeTe/Te heterostructures were synthesized by reacting diphenylgermane (DPG) and TOP-Te in the presence of organic surfactants. Aligned Te nanorods were grown on the surface facets of micrometer-size germanium telluride particles.
Book Synopsis Synthesis, Fabrication and Characterization of Ge/Si Axial Nanowire Heterostructure Tunnel FETs by :
Download or read book Synthesis, Fabrication and Characterization of Ge/Si Axial Nanowire Heterostructure Tunnel FETs written by and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Axial Ge/Si heterostructure nanowires allow energy band-edge engineering along the axis of the nanowire, which is the charge transport direction, and the realization of asymmetric devices for novel device architectures. This work reports on two advances in the area of heterostructure nanowires and tunnel FETs: (i) the realization of 100% compositionally modulated Si/Ge axial heterostructure nanowires with lengths suitable for device fabrication and (ii) the design and implementation of Schottky barrier tunnel FETs on these nanowires for high-on currents and suppressed ambipolar behavior. Initial prototype devices resulted in a current drive in excess of 100 [mu]A/[mu]m (I/[pi]D) and 105 I{sub on}/I{sub off} ratios. These results demonstrate the potential of such asymmetric heterostructures (both in the semiconductor channel and metal-semiconductor barrier heights) for low-power and high performance electronics.
Book Synopsis Germanium Nanowires by : Tobias Hanrath
Download or read book Germanium Nanowires written by Tobias Hanrath and published by . This book was released on 2004 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis The Synthesis and Characterization of Germanium Nanoparticles and Nanowires and the Study of Their Potential in Photovoltaics by : Stephen Corey Codoluto
Download or read book The Synthesis and Characterization of Germanium Nanoparticles and Nanowires and the Study of Their Potential in Photovoltaics written by Stephen Corey Codoluto and published by . This book was released on 2009 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The increasing energy demand of an overpopulated society has bolstered the interest in exploring renewable energy forms, one of which is solar energy. Current solar cell technology is neither an efficient nor cost-effective alternative to currently used fossil fuels. Nanostructured semiconductor building blocks are expected to play a central role in the development of next-generation cost-effective solar cell technology. Among the various materials that have been explored and studied, Ge holds particular promise due to it favorable band gap and good transport characteristic. A method to produce colloidal Ge nanocrystals, however, has not yet been established. Colloidal synthesis provides a scalable and cost-effective route to nanocrystalline semiconductor material as building blocks in low-cost PV energy conversion devices. This work describes the synthesis and characterization of Ge nanoparticles and Ge nanowires and their potential applications. Ge nanoparticles, 1.9 - 16.0 nm, are synthesized via colloidal synthesis by reducing germanium iodide using a strong reducing agent in various coordinating solvents. The effects of reaction and injection temperature, reaction time, and initial concentration are studied. A minimum temperature of 250 °C is required to crystallize Ge in a colloidal synthesis, below which only amorphous material is formed. An increase in reaction temperature from 250 to 300 °C has little effect on the final nanocrystal size and structure. A temperature of 200 °C was found to minimize crystal growth defects. Increasing or decreasing the injection temperature increased the crystal defects. The final crystalline products are analyzed using XRD, FTIR, TEM, HR-TEM, SEM, UV-vis spectroscopy, and PL to study oxidation, crystal structure, and optical properties. Spin coated germanium nanoparticles are combined with sputtered a-Si to create a polysilicon-Ge matrix which could direct charge transfer and decrease recombination of photogenerated charges. As a complementary nanocrystalline Ge building block nanowires were also synthesized by the thermal decomposition of DPG and TMG in supercritical hexane using a batch and a semicontinuous supercritical reactor. Up to 210 mg are synthesized and collected using this process with a diameter range of 20 nm to 60 nm and lengths up to 15 [MICRO SIGN]m. The continuously grown nanowire experimental yield is ~35%, compared to the batch experimental yield of 15%. The Ge nanowires were easily extracted from the collection vessel and characterized using TEM, SEM, and XRD to confirm the presence of Ge and to study the structure of the wires.
Book Synopsis Synthesis of Silicon/germanium Nanowires and Field Emission Studies of 1-D Nanostructures by : Joonho Bae
Download or read book Synthesis of Silicon/germanium Nanowires and Field Emission Studies of 1-D Nanostructures written by Joonho Bae and published by . This book was released on 2007 with total page 218 pages. Available in PDF, EPUB and Kindle. Book excerpt: Using the vapor-liquid-solid (VLS) growth method, silicon nanowires and germanium nanowires are grown. We find the high growth rate is responsible for the silicon nanowires with less growth defects when they are grown by use of silicon tetrachloride as a precursor and hydrogen as a carrier gas. Based on this funding, large area, high aspect ratio, h111i oriented silicon nanowires are successfully grown on Si (111) and Si (100). Novel growth mechanisms of VLS growth method were discovered in SiOx nanoflowers and silicon nanocones. In SiOx nanoflowers grown at the tip of silicon nanowires, it is found that they are produced via the enhanced oxidation of silicon at the gold-silicon interface. Furthermore, the analysis of the flower pattern reveals that it is the observation of the dense branching morphology on nanoscale and on spherical geometry. For the silicon nanocones, they are grown by the in situ etching of the catalysts of Ga/Al by HCl during the growth. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) reveal that the nanocones are composed of amorphous silicon oxides and crystalline Si. Based on the similar chemistry of hydrogen reduction of SiCl4 for the growth of silicon nanowires, single crystalline germanium nanowires are grown by use of GeCl4 as a precursor and H2 as a carrier gas. As one of important application of one dimensional nanostructures, the field emission properties of 1-D nanostructures are explored. The field emission properties of a single graphite nanocone are measured in SEM. The inter-electrode separation is controlled using scanning tunneling microscopy (STM) approach method, allowing the precise and ne determination of the separation. Its Fowler-Nordheim plot shows it emits currents in accordance with the Fowler-Nordheim field emission. Its onset voltage, field enhancement factor show that its basic field emission parameters are comparable to those of a single carbon nanotube. It is observed that single nanocone is damaged after emitting a current of about 100 nA, which seems to be due to its hollow interior structure.
Book Synopsis Synthesis and Characterisation of Highly Ordered Arrays of Semiconducting and Core-shell Nanowires by : Brian Daly
Download or read book Synthesis and Characterisation of Highly Ordered Arrays of Semiconducting and Core-shell Nanowires written by Brian Daly and published by . This book was released on 2006 with total page 184 pages. Available in PDF, EPUB and Kindle. Book excerpt: The work described in this thesis reports the synthesis and characterisation of novel arrays of both semiconductor and core shell heterostructured nanowires. In Chapter 3, an investigation into the contacting and electrical characterisation of highly ordered arrays of germanium nanowires is carried out. C-AFM was used to determine the electrical transport properties of individual nanowires within the arrays, whilst macro-contacts were used to measure the mean current-voltage characteristics of groups of nanowires. The results demonstrate that practically all the nanowires within the array were conducting and that good ohmic contact can be attained to these arrays with careful substrate preparation. In Chapter 4, the germanium nanowire arrays are utilised to create a novel photoresistor device. The photocurrent in the GeNW-AAO system was measured by illumination of the membrane through the light sensitive macrocontact. The photokinetics of the germanium nanowire system was also investigated. The compositional structure of high density arrays of coaxial nanocables, consisting of germanium nanowires surrounded by cobalt nanotube sheaths, within AAO were investigated by various x-ray analysis techniques in Chapter 5. An investigation of the interface between the cobalt shell and germanium core of these nanocables was particularly carried out with a view to understanding the unique magnetic properties demonstrated by these arrays. An investigation into the synthesis of magnetically tunable high density arrays of coaxial nanocables, consisting of magnetite nanowires surrounded by cobalt nanotube sheaths and cobalt nanowires surrounded by magnetite nanotube sheathes within anodic aluminium oxide membranes is presented in Chapter 6. These materials are a combination of separate hard (Co) and soft (Fe3O4) magnetic materials in a single nanocable structure. The combination of two or more magnetic materials in such a radial structure are seen as very powerful building blocks for the future fabrication of magnetoresistive, spin valve and ultrafast spin injection devices.
Book Synopsis Synthesis and Properties of Germanium Nanowires by : Dunwei Wang
Download or read book Synthesis and Properties of Germanium Nanowires written by Dunwei Wang and published by . This book was released on 2005 with total page 346 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Ge and Si Based Nanowires/nanotubes Synthesis and Their Applications in Wearable Device, Biochemical Sensor, and Thermoelectronics by : Soonshin Kwon
Download or read book Ge and Si Based Nanowires/nanotubes Synthesis and Their Applications in Wearable Device, Biochemical Sensor, and Thermoelectronics written by Soonshin Kwon and published by . This book was released on 2015 with total page 133 pages. Available in PDF, EPUB and Kindle. Book excerpt: Si and Ge nanowires and their heterostructure have been received widespread attention in various research fields because of the inherent advantages and the major historical roles played by these materials in contemporary microelectronics. From decades of research on two materials, integrated in-depth knowledge on the nature of material properties and manufacture process provide useful guidelines to design nanostructures and related devices with increased structural and functional complexity. In this dissertation, synthesis and applications of Ge and Si based nanowires and nanotubes in electronics, photonics, biochemical sensor, and thermoelectrics are discussed. In chapter 2, self-organizing characteristics of misfit-guided Ge quantum dots growth on Si core nanowires are systematically demonstrated. Unique Ge quantum dots growth mode caused by strain supperlattice along the Si nanowire backbone can be controlled by the choice of core diameter. Such strain-guided growth opens up a new avenue towards growth of self-organized nanoscale heterostructures. In chapter 3, fundamental study of crystalline Si nanotubes properties as a platform for electrically and biochemically functional devices is demonstrated. Four-probe current-voltage characterization of precisely probe the inherent electrical properties of crystalline Si nanotubes. Selective functionalization and loading of fluorescence dye and biomolecule inside the core of nanotubes are demonstrated lighting the potential as in-vivo drug carrier. In chapter 4, characterization of thermal transport behavior of crystalline and amorphous Si nanotubes are presented. Ultra-low thermal conductivity of crystalline nanotube below the amorphous counterpart is observed. Study on elastic properties of those nanotubes reveals new possible control mechanism of phonon transport behavior. In chapter 5, fabrication of optical polarizer by printing Ge or Ge/Si core/shell nanowires into highly compacted and ordered fashion is presented. Transmission measurement under various mechanically stressed circumstances reveals potential of nanowire polarizer as high flexible and stretchable optical filter.
Book Synopsis Study of Catalyzed Growth of Germanium Nanowires by : Shruti Vivek Thombare
Download or read book Study of Catalyzed Growth of Germanium Nanowires written by Shruti Vivek Thombare and published by . This book was released on 2013 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Semiconductor nanowires are of great interest for application in nanoelectronics, nanophotonics, sensors and energy technologies. Particular attention has focused on Si and Ge nanowires because of their compatibility with Si integrated circuit technology. The great majority of literature studies of Ge nanowire growth have used Au as a catalyst for "bottom-up" synthesis of deposited wires. In most cases, growth occurs by the vapor-liquid-solid (VLS) mechanism. Gold has been a popular choice as a catalyst in part because it forms a eutectic liquid with Ge at temperatures below 400 °C, permitting generally high-quality and spatially-controlled crystal growth at very low temperatures. However, concerns exist about possible Au contamination of VLS-grown nanowire devices and, in particular, semiconductor processing facilities used to fabricate them. Gold is a fast diffuser in diamond cubic crystals and produces trap levels deep in the band gap of both Ge and Si, making it a significant cross-contamination hazard in semiconductor fabrication. Moreover, VLS is not well suited to synthesis of nanowires with abrupt grown-in p-n junctions or semiconductor heterostructures, which are interesting for many device applications. This has provided additional motivation to investigate the growth of semiconductor nanowires via the vapor-solid-solid (VSS) mechanism using alternative catalysts to Au. VSS nanowires can be grown at reduced temperatures compared to VLS, as the catalyst is not molten. Literature reports indicate that the morphology of Ge nanowires grown using alternative catalysts via VSS is not as easily controlled as that of VLS grown Ge nanowires with Au as a catalyst; mixtures of straight, twisted, and defective nanowires are reported for different catalysts and growth conditions. A size-dependent wire morphology transition from straight to tortuous nanowires in VSS growth of Ge nanowires using a Ni-based catalyst is discussed here. The catalyst phase was identified as orthorhombic NiGe, which is reported to be a state-of-the-art contact material in Si-compatible semiconductor devices. Using detailed transmission electron microscopy analysis of the nanowire and catalyst morphology and composition, the role of sidewall and catalyst/nanowire interface energetics as well as crystal defects in dictating the observed wire diameter effect on VSS growth morphology was analyzed. Development of optimized processes for bottom-up synthesis of these nanowires requires a more quantitative understanding of the VSS wire growth mechanism. Therefore, the kinetics of VSS nanowire growth, probing the rate-limiting step for various growth conditions was investigated. The effect of growth parameters such as growth temperature and precursor partial pressure on the nanowire growth rate was studied in order to gain an insight into the growth kinetics. Two different growth regimes were observed for VSS grown Ge nanowires at different temperature ranges. At higher temperatures (345-375 oC), the diffusion or mass transport of germane precursor to the catalyst surface was found to be rate limiting. At lower temperatures (300-345 oC) either the surface reaction or incorporation of Ge at growth step could be rate limiting.
Book Synopsis Synthesis, Assembly, and Integration of Semiconductor Nanowires by : Elena P. Pandres
Download or read book Synthesis, Assembly, and Integration of Semiconductor Nanowires written by Elena P. Pandres and published by . This book was released on 2020 with total page 206 pages. Available in PDF, EPUB and Kindle. Book excerpt: Semiconductor nanowires are a class of highly anisotropic crystalline materials with nanoscale diameters and lengths that range from micrometers to millimeters. The electronic, optical, and mechanical properties of semiconductor nanowires can be considerably different than their bulk counterparts, making them attractive for a range of applications including sensors, energy storage, and quantum information systems. Solution-based synthesis is a promising strategy to produce semiconductor nanowires in a scalable, cost-effective matter. However, many solution-based methods are limited in their ability to produce nanowires with increasingly complex compositions-including doped, alloyed, and heterostructured architectures-as well as to rapidly screen synthetic parameters for combinatorial discovery and optimization. In addition, chemistries and growth dynamics can be difficult to track with nanowire syntheses that require high temperature and extreme pressure equipment. Moreover, the widespread integration of semiconductor nanowires into devices will also require new methods of assembly as well as careful consideration of surface chemistry. After an introduction to current methods of semiconductor nanowire synthesis, existing tactics for nanowire assembly, and strategies to improve the energy density of lithium ion batteries with group IV nanomaterials, this dissertation will cover three main topics related to (i) new synthetic methods for nanowire growth, (ii) a novel light-based nanowire assembly process, and (iii) the integration of nanowires into high-energy-density composite electrodes for lithium ion batteries. Herein, we demonstrate a new continuous-flow, laser-driven, nanowire growth process that exploits the light absorption of colloidal metal nanocrystals to drive semiconductor nanowire growth in an optically accessible reactor on the benchtop, potentially opening the door for both rapid screening of synthetic parameters as well as in situ studies of nanowire growth dynamics. Investigations of solution-based nanowire growth using this system establish that laser-driven syntheses can achieve rapid, on-demand growth of semiconductor nanowires. Importantly, the integration of nanowires into future device architectures will require a wide range of assembly strategies. While current solution-based nanowire assembly processes struggle to create deterministic heterojunctions, here, we demonstrate a novel example of nanowire assembly in a high-Prandtl-number organic solvent system, using an optical trap to orient, align, and "solder" metal-seeded semiconductor nanowires into periodic axial heterostructures. Finally, we investigate the role of surface functionalization on the integration of group-IV nanowires into high-capacity alloying electrodes for lithium ion batteries. We demonstrate that interfacial chemistry affects electrochemical access to different phases of lithiated germanium, and by carefully controlling the nanowire surface chemistry, we eliminate the need for the fluorinated electrolyte additives typically required for the stable cycling of group-IV-based, lithium-ion battery electrodes. In addition, we demonstrate that by balancing precursor decomposition kinetics, alloyed silicon-germanium (SiGe) nanowires can be synthesized through supercritical-fluid-based processes, potentially improving the rate capability of high-capacity silicon-based electrode materials produced via scalable processes. We anticipate that the information gained from these solution-based synthetic methods, assembly techniques, and surface chemistry studies will inform synthetic compositional control, elucidate relationships between solution-based reaction parameters and emergent properties, and advance the integration of solution-grown semiconductor nanowires into next-generation devices.
Book Synopsis The Synthesis of Silicon and Germanium Nanowires for Energy Storage and Single Nanowire Devices by : Dylan Storan
Download or read book The Synthesis of Silicon and Germanium Nanowires for Energy Storage and Single Nanowire Devices written by Dylan Storan and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis One-Dimensional Nanostructures by : Tianyou Zhai
Download or read book One-Dimensional Nanostructures written by Tianyou Zhai and published by John Wiley & Sons. This book was released on 2012-10-19 with total page 857 pages. Available in PDF, EPUB and Kindle. Book excerpt: Reviews the latest research breakthroughs and applications Since the discovery of carbon nanotubes in 1991, one-dimensional nanostructures have been at the forefront of nanotechnology research, promising to provide the building blocks for a new generation of nanoscale electronic and optoelectronic devices. With contributions from 68 leading international experts, this book reviews both the underlying principles as well as the latest discoveries and applications in the field, presenting the state of the technology. Readers will find expert coverage of all major classes of one-dimensional nanostructures, including carbon nanotubes, semiconductor nanowires, organic molecule nanostructures, polymer nanofibers, peptide nanostructures, and supramolecular nanostructures. Moreover, the book offers unique insights into the future of one-dimensional nanostructures, with expert forecasts of new research breakthroughs and applications. One-Dimensional Nanostructures collects and analyzes a wealth of key research findings and applications, with detailed coverage of: Synthesis Properties Energy applications Photonics and optoelectronics applications Sensing, plasmonics, electronics, and biosciences applications Practical case studies demonstrate how the latest applications work. Tables throughout the book summarize key information, and diagrams enable readers to grasp complex concepts and designs. References at the end of each chapter serve as a gateway to the literature in the field. With its clear explanations of the underlying principles of one-dimensional nanostructures, this book is ideal for students, researchers, and academics in chemistry, physics, materials science, and engineering. Moreover, One-Dimensional Nanostructures will help readers advance their own investigations in order to develop the next generation of applications.
Book Synopsis Silicon and Germanium Nanostructures by : Xiaotang Lu
Download or read book Silicon and Germanium Nanostructures written by Xiaotang Lu and published by . This book was released on 2015 with total page 404 pages. Available in PDF, EPUB and Kindle. Book excerpt: A variety of chemical routes exist for a wide range of nanomaterials with tunable size, shape, composition and surface chemistry. Of these materials, silicon (Si) and germanium (Ge) nanomaterials have been some of the most challenging to synthesize. Solution-liquid-solid (SLS) growth of Si was studied using tin (Sn) as the seeding metal. Si nanorods with narrow diameters can be grown by the decomposition of trisilane in hot squalane in the presence of Sn nanocrystals. Photoluminescence could be obtained from the Si nanorods by thermal hydrosilylation passivation. This colloidal synthesis could be further simplified to a single-step reaction procedure by the in situ formation of Sn seed particles. In addition to trisilane as a Si source, isotetrasilane, neopentasilane and cyclohexasilane were studied for Si nanorod growth: all three reactants enabled nanorod formation at lower growth temperatures. A monophenylsilane (MPS) enhanced growth was discovered for supercritical fluid-liquid-solid (SFLS) growth of Ge nanowires that enables the Ge conversion of ~100%. A variety of metalorganic compounds were studied for replacing pre-synthesized metal nanoparticles to induce Ge nanowire growth. Si and Ge nanowires are some of the most promising anode materials in lithium ion batteries (LIBs) because of their high lithium storage capacity. However, the significant chemical and physical changes that occur during cycling hamper their practical uses. In situ transmission electron microscopy (TEM) techniques were conducted to observe and understand structural and interfacial changes of the Si and Ge nanowires during electrochemical cycling; and, therefore, resolving the problems with current anodes by materials modification. The in situ TEM experiments showed that the incorporation of Sn into Si nanowires can enhance their rate capability. But the enhanced Li diffusion leads to the premature pore formation in Si nanowires. Ge nanowires has been discovered the potential as sodium ion battery anodes after an initial activation with a lithiation step to amorphize the nanowires.
Book Synopsis Core-shell Germanium/germanium-tin Nanowires by : Andrew Chengsi Meng
Download or read book Core-shell Germanium/germanium-tin Nanowires written by Andrew Chengsi Meng and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Germanium-tin is a promising material for novel devices for light sources and optical sensing in the mid-IR region. For sufficiently high Sn compositions, the material has a direct band-gap near 0.5 eV, and could have applications either as a detector or as an emitter. The main challenge to growth of high-quality single crystals is the large lattice mismatch of the system (~14% for diamond cubic Sn on Ge), the low equilibrium solubility of Sn in Ge (~1 at%), and compressive misfit strain counteracting the transition to a direct band-gap induced when growing thin films on Si (001) or Ge (001) substrates. We demonstrate that core-shell Ge/Ge1-xSnx nanowire structures take advantage of an elastically compliant, small-diameter nanowire substrate for high quality single crystal growth. Ge1-xSnx growth can be compatible with complementary metal-oxide semiconductor (CMOS) processing techniques. Therefore, there is potential for monolithic integration of Ge1-xSnx light sources onto silicon for photonic applications. In this thesis, we demonstrate growth, characterization, and optimization of core-shell Ge/Ge1-xSnx nanowire structures. First, growth of core-shell Ge/Ge0.96Sn0.04 using a low temperature chemical vapor deposition (CVD) process is demonstrated using typical industrial precursors, GeH4 and SnCl4. In contrast to Ge1-xSnx epitaxial thin films, free-standing nanowires deposited on misfitting Ge or Si substrates can avoid compressive elastic strains that inhibit formation of a direct gap. The Sn incorporation is several times greater than the equilibrium solubility limit of Sn in Ge. Structural characterization by lab source x-ray diffraction and transmission electron microscopy are performed along with optical characterization by photoluminescence. Second, core-shell Ge/Ge0.96Sn0.04 nanowires are cross-sectioned and characterized extensively to analyze the interplay between the composition distribution and the core-shell strain and how optical properties are affected. The nanowire cross section reveals six Sn-poor radial spokes approximately 60 degrees apart in the Ge1-xSnx shell. Phase field simulations provide an estimate of the expected strain, which matches well with experimental results and explain the stability of the Sn-poor spokes by considering their effect on the elastic strain energy. There is a two-fold synergistic effect on the optical properties produced by the core-shell nanowire geometry: the Ge core acts as an elastically compliant substrate for growth of an axially lattice-matched epitaxial Ge1-xSnxshell, which facilitates growth of high-quality single crystal Ge1-xSnx having intense photoluminescence; at the same time, the tensile misfit strain in the Ge core serves to decrease its direct gap transition energy with respect to the indirect gap transition energy, thus enhancing its optical emission. Finally, we examine the parameters affecting Ge1-xSnx shell growth and optimize the CVD parameter space for high Sn incorporation in the shell. Ge1-xSnx CVD chemistry with GeH4 and SnCl4 precursors undergoes a transition from growth at higher temperatures to etching at lower temperatures, in keeping with the entropies of the respective chemical reactions. Also, Sn composition increases when temperature is decreased because Sn at the growth front can be kinetically trapped into the Ge1-xSnx shell. We demonstrate that the degree of axial and radial growth of Ge/Ge1-xSnx core-shell nanowire heterostructures can be controlled by varying precursor to H2 partial pressure ratio during CVD growth, with the SnCl4:GeH4 partial pressure ratio fixed. By increasing the SnCl4 partial pressure, radial growth rate increases and axial growth rate decreases. This is consistent with SnCl4 disruption of H-passivation of Ge sidewall facets. Examining shell thickness variation with shell growth time showed slow initial growth that approached a constant volumetric growth rate, which is consistent with precursor mass transport-limited shell growth. Controlling nanowire density by the vapor-liquid-solid (VLS) catalyst loading per unit substrate area as a means to probe the effect of SnCl4 mass transport on Ge1-xSnx shell growth, we found that very sparse nanowires tend to be decorated by Sn precipitates while very dense nanowires exhibited bending induced by Sn composition variation across the wire circumference caused by local SnCl4 depletion. Thus, mass transport of SnCl4 plays an important role in the Ge1-xSnx shell growth. It is likely that a balance between Sn precursor flux and the available surface sites for Sn incorporation is required to prevent these undesirable effects. We are also able to achieve different Sn compositions up to 14 at% by varying SnCl4 partial pressure and growth temperature. With control over geometry, morphology, and composition of Ge1-xSnx heterostructures, a wide range of potential device architectures can be achieved.
