Mechanical properties of carbon nanotubes grown by pyrolysis of ferrocene

Carbon nanotubes (CNTs) have drawn a lot of attention during the last decades due to its promising mechanical and electrical properties. Extensive research regarding the mechanical properties of CNTs has been carried out during the last decades. A lot of effort has been put into developing methods to properly characterize features such as Young’s modulus and the deformation processes of carbon nanotubes. A detailed knowledge of these properties is important for many of the suggested applications of carbon nanotubes.Here we have examined multiwalled carbon nanotubes (MWCNTs) grown by pyrolysis of ferrocene. In some cases the carbon nanotube contained an iron core or traces of iron in the core. The carbon nanotubes ranged from 20 nm to 65 nm in radius and 1000 nm to 4000 nm in length.An atomic force microscope (AFM) was used inside a scanning electron microscope (SEM) for in situ force measurements. The AFM cantilever was used to displace individual carbon nanotubes from their equilibrium positions. The forces used to displace the carbon nanotubes have been plotted against the displacements of the tubes to obtain the characteristic force-displacement curves. From the slope of these curves the spring constants of the carbon nanotubes have been found. Young’s modulus for each tube was derived from the spring constant and the tube dimensions.We found that Young’s modulus ranged from 7 GPa to 340 GPa with no observed dependence on the radius or the length. Previous works suggest that deformation processes such as rippling and buckling will drastically change the spring constant of the tubes when displaced. The maximum values of the applied forces in our measurements were smaller than those needed to push the tubes into the deformation stages. The relatively low values of Young’s modulus indicate that these tubes are rich in defects which dominate their mechanical behaviour

Semiconductor Nanowires: Characterization and surface modification

The topic of III-V nanowires is still, after more than two decades, a growing and lively research area. The areas of application are wide and contain such important topics as energy harvesting, cheap and efficient lighting, high efficiency detectors and new types of electronics. III-V materials offer properties superior to the widely used Si. They can have considerably higher carrier mobility which paves the way for high-speed electronics and a great flexibility in band gap which is fundamental for optoelectronics. Producing these materials in the form of nanowires presents additional advantages as it gives the opportunity to tailor the material by altering the crystal structure and material composition in ways not possible in larger structures as well as integrating them with existing standard materials platforms.This combination of great materials in a small size has demonstrated potential for improved solar cells, high-speed low power transistors as well as energy efficient and flexible LEDs. But the promises come with challenges. The quality of the III-V surfaces is a significant factor for determining device performance, potentially both improving and limiting functionality. Further, the relevance of surfaces properties increases with decreasing sizes as the surface to bulk ratio goes up. This thesis focuses on characterization of semiconducting III-V surfaces with a special focus on nanowires, exploring recently developed methods, types of nanowires and nanowire synthesis.Native oxides forming on the surfaces of III-V materials in air are adverse and impede the development of top-tier devices. The surfaces have to be cleaned of the detrimental oxides and protected as part of the manufacturing of components. In the present work X-ray photoelectron spectroscopy (XPS) has been used to improve the understanding of different cleaning methods including the self-cleaning effect of atomic layer deposition (ALD) of high-k oxides.Aerotaxy growth of nanostructures, where nanowires grow in a carrier gas, has arisen as a more cost effective and scalable production method than epitaxial growth on a solid substrate. Yet little is known about both the mechanism involved in the growth as well as about the quality of the resulting nanostructure surfaces. Here XPS and scanning probe microscopy (SPM) have been used to unravel the effect of doping on the surfaces of aerotaxy nanowires. Further, also a technique using small-angle X-ray scattering (SAXS) for in-situ characterization of the aerotaxy seed particles has been developed.SPM has been a revolutionary tool for surface science since its invention. Central for the function of an SPM is the tip, which is usually made out of metal and formed by a simple electrochemical etching procedure. In this thesis the development of a high resolution GaN nanowire probe for SPM is described. It combines the optical and electronical advantages of GaN with the well-controlled tip formation at the end of a nanowire

In Vivo Detection and Absolute Quantification of a Secreted Bacterial Factor from Skin Using Molecularly Imprinted Polymers in a Surface Plasmon Resonance Biosensor for Improved Diagnostic Abilities

In this study, a surface plasmon resonance (SPR) biosensor was developed for the detection and quantification of a secreted bacterial factor (RoxP) from skin. A molecular imprinting method was used for the preparation of sensor chips and five different monomer-cross-linker compositions were evaluated for sensitivity, selectivity, affinity, and kinetic measurements. The most promising molecularly imprinted polymer (MIP) was characterized by using scanning electron microscopy, atomic force microscopy, and cyclic voltammetry. Limit of detection (LOD) value was calculated as 0.23 nM with an affinity constant of 3.3 × 10-9 M for the promising MIP. Besides being highly sensitive, the developed system was also very selective for the template protein RoxP, proven by the calculated selectivity coefficients. Finally, absolute concentrations of RoxP in several skin swabs were analyzed by using the developed MIP-SPR biosensor and compared to a competitive ELISA. Consequently, the developed system offers a very efficient tool for the detection and quantification of RoxP as an early indicator for some oxidative skin diseases especially when they are present in low-abundance levels (e.g., skin samples)

Low Trap Density in InAs/High-k Nanowire Gate Stacks with Optimized Growth and Doping Conditions

In this paper, we correlate the growth of InAs nanowires with the detailed interface trap density (Dit) profile of the vertical wrap-gated InAs/high-k nanowire semiconductor-dielectric gate stack. We also perform the first detailed characterization and optimization of the influence of the in situ doping supplied during the nanowire epitaxial growth on the sequential transistor gate stack quality. Results show that the intrinsic nanowire channels have a significant reduction in Dit as compared to planar references. It is also found that introducing tetraethyltin (TESn) doping during nanowire growth severely degrades the Dit profile. By adopting a high temperature, low V/III ratio tailored growth scheme, the influence of doping is minimized. Finally, characterization using a unique frequency behavior of the nanowire capacitance-voltage (C-V) characteristics reveals a change of the dopant incorporation mechanism as the growth condition is changed

InAs-oxide interface composition and stability upon thermal oxidation and high-k atomic layer deposition

Defects at the interface between InAs and a native or high permittivity oxide layer are one of the main challenges for realizing III-V semiconductor based metal oxide semiconductor structures with superior device performance. Here we passivate the InAs(100) substrate by removing the native oxide via annealing in ultra-high vacuum (UHV) under a flux of atomic hydrogen and growing a stoichiometry controlled oxide (thermal oxide) in UHV, prior to atomic layer deposition (ALD) of an Al2O3 high-k layer. The semiconductor-oxide interfacial stoichiometry and surface morphology are investigated by synchrotron based X-ray photoemission spectroscopy, scanning tunneling microscopy, and low energy electron diffraction. After thermal oxide growth, we find a thin non-crystalline layer with a flat surface structure. Importantly, the InAs-oxide interface shows a significantly decreased amount of In3+, As5+, and As0 components, which can be correlated to electrically detrimental defects. Capacitance-voltage measurements confirm a decrease of the interface trap density in gate stacks including the thermal oxide as compared to reference samples. This makes the concept of a thermal oxide layer prior to ALD promising for improving device performance if this thermal oxide layer can be stabilized upon exposure to ambient air

Hydrogen plasma enhanced oxide removal on GaSb planar and nanowire surfaces

Due to its high hole-mobility, GaSb is a highly promising candidate for high-speed p-channels in electronic devices. However, GaSb exhibits a comparably thick native oxide causing detrimental interface defects, which has been proven difficult to remove. Here we present full oxide removal from GaSb surfaces using effective hydrogen plasma cleaning, studied in-situ by synchrotron-based X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV). GaSb nanowires turn out to be cleaned faster and more efficiently than planar substrates. Since the UHV conditions are not scalable for industrial sample processing, H-plasma cleaning is furthermore used as pre-treatment prior to atomic layer deposition (ALD) of a protective high-k layer to demonstrate the use of the cleaning step in a more realistic fabrication situation. We observe a cleaning effect of the H-plasma even in the ALD environment, but we also find residual Ga- and Sb-oxides at the GaSb-high-k interface, which we attribute to re-oxidation of the cleaned surface. Our results indicate that an improved control of the ALD reactor vacuum environment could realize oxide- and defect-free interfaces in GaSb-based electronics

Surface smoothing and native oxide suppression on Zn doped aerotaxy GaAs nanowires

Aerotaxy, a recently invented aerosol-based growth method for nanostructures, has been shown to hold great promise in making III-V nanowires more accessible for cheap mass-production. Aerotaxy nanowire surface structure and chemistry, however, remains unexplored, which is unfortunate since this can influence (opto)electronic properties. We investigate the surfaces of aerotaxy grown GaAs nanowires using synchrotron based high resolution X-ray photoelectron spectroscopy and high resolution atomic force microscopy. We observe that increasing the concentration of the p-type dopant diethylzinc to very high levels during nanowire growth significantly changes the surface morphology and leads to a strong suppression of native surface oxide formation. Our findings indicate that up to 1.8 monolayers of Zn are present on the nanowire surface after growth. Finally, we find that this also influences the Fermi level pinning of the surface. We suggest that Zn present on the surface after growth could play a role in the strongly hindered oxidation of the III-V compound when exposed to air. The aerotaxy nanowires generally exhibit a round cross section, while a significant smoothening of the surface morphology along the nanowire appears for very high nominal doping levels likely as a result of slight reshaping during growth in the presence of Zn. Given that surface oxide and a rough morphology can be detrimental to nanowire electrical and optical performance, the ability to reduce them as a side effect of dopant introduction will benefit future applications. Finally, the observed hindering of oxidation during air transport can allow for reliable post-growth processing in separate systems

Atomic Layer Deposition of Hafnium Oxide on InAs : Insight from Time-Resolved in Situ Studies

III-V semiconductors, such as InAs, with an ultrathin high-κ oxide layer have attracted a lot of interests in recent years as potential next-generation metal-oxide-semiconductor field-effect transistors, with increased speed and reduced power consumption. The deposition of the high-κ oxides is nowadays based on atomic layer deposition (ALD), which guarantees atomic precision and control over the dimensions. However, the chemistry and the reaction mechanism involved are still partially unknown. This study reports a detailed time-resolved analysis of the ALD of high-κ hafnium oxide (HfOx) on InAs(100). We use ambient pressure X-ray photoemission spectroscopy and monitor the surface chemistry during the first ALD half-cycle, i.e., during the deposition of the metalorganic precursor. The removal of In and As native oxides, the adsorption of the Hf-containing precursor molecule, and the formation of HfOx are investigated simultaneously and quantitatively. In particular, we find that the generally used ligand exchange model has to be extended to a two-step model to properly describe the first half-cycle in ALD, which is crucial for the whole process. The observed reactions lead to a complete removal of the native oxide and the formation of a full monolayer of HfOx already during the first ALD half-cycle, with an interface consisting of In-O bonds. We demonstrate that a sufficiently long duration of the first half-cycle is essential for obtaining a high-quality InAs/HfO2 interface

Self-cleaning and surface chemical reactions during hafnium dioxide atomic layer deposition on indium arsenide

Atomic layer deposition (ALD) enables the ultrathin high-quality oxide layers that are central to all modern metal-oxide-semiconductor circuits. Crucial to achieving superior device performance are the chemical reactions during the first deposition cycle, which could ultimately result in atomic-scale perfection of the semiconductor-oxide interface. Here, we directly observe the chemical reactions at the surface during the first cycle of hafnium dioxide deposition on indium arsenide under realistic synthesis conditions using photoelectron spectroscopy. We find that the widely used ligand exchange model of the ALD process for the removal of native oxide on the semiconductor and the simultaneous formation of the first hafnium dioxide layer must be significantly revised. Our study provides substantial evidence that the efficiency of the self-cleaning process and the quality of the resulting semiconductor-oxide interface can be controlled by the molecular adsorption process of the ALD precursors, rather than the subsequent oxide formation