Fabrication and characterisation of semiconductor nanowires and metal interconnects

One-dimensional semiconductor nanowires are considered to be promising materials for future nanoelectronic applications. However, before these nanowires can be integrated into such applications, a thorough understanding of their growth behaviour is necessary. In particular, methods that allow the control over nanowire growth are deemed especially important as it is these methods that will enable the control of nanowire dimensions such as length and diameter (high aspect ratios). The production of nanowires with high-aspect ratios is vital in order to take advantage of the unique properties experienced at the nanoscale, thus allowing us to maximise their use in devices. Additionally, the development of low-resistivity interconnects is desirable in order to connect such nanowires in multi-nanowire components. Consequently, this thesis aims to discuss the synthesis and characterisation of germanium (Ge) nanowires and platinum (Pt) interconnects. Particular emphasis is placed on manipulating the nanowire growth kinetics to produce high aspect ratio structures. The discussion of Pt interconnects focuses on the development of low-resistivity devices and the electrical and structural analysis of those devices. Chapter 1 reviews the most critical aspects of Ge nanowire growth which must be understood before they can be integrated into future nanodevices. These features include the synthetic methods employed to grow Ge nanowires, the kinetic and thermodynamic aspects of their growth and nanowire morphology control. Chapter 2 outlines the experimental methods used to synthesise and characterise Ge nanowires as well as the methods used to fabricate and analyse Pt interconnects. Chapter 3 discusses the control of Ge nanowire growth kinetics via the manipulation of the supersaturation of Ge in the Au/Ge binary alloy system. This is accomplished through the use of bi-layer films, which pre-form Au/Ge alloy catalysts before the introduction of the Ge precursor. The growth from these catalysts is then compared with Ge nanowire growth from standard elemental Au seeds. Nanowires grown from pre-formed Au/Ge alloy seeds demonstrate longer lengths and higher growth rates than those grown from standard Au seeds. In-situ TEM heating on the Au/Ge bi-layer films is used to support the growth characteristics observed. Chapter 4 extends the work of chapter 3 by utilising Au/Ag/Ge tri-layer films to enhance the growth rates and lengths of Ge nanowires. These nanowires are grown from Au/Ag/Ge ternary alloy catalysts. Once again, the supersaturation is influenced, only this time it is through the simultaneous manipulation of both the solute concentration and equilibrium concentration of Ge in the Au/Ag/Ge ternary alloy system. The introduction of Ag to the Au/Ge binary alloy lowers the equilibrium concentration, thus increasing the nanowire growth rate and length. Nanowires with uniform diameters were obtained via synthesis from AuxAg1-x alloy nanoparticles. Manifestation of the Gibbs-Thomson effect, resulting from the dependence of the mean nanowire length as a function of diameter, was observed for all of the nanowires grown from the AuxAg1-x nanoparticles. Finally, in-situ TEM heating was used to support the nanowire growth characteristics. Chapter 5 details the fabrication and characterisation of Pt interconnects deposited by electron beam induced deposition of two different precursors. The fabrication is conducted inside a dual beam FIB. The electrical and structural characteristics of interconnects deposited from a standard organometallic precursor and a novel carbon-free precursor are compared. The electrical performance of the carbon-free interconnects is shown to be superior to that of the organometallic devices and this is correlated to the structural composition of both interconnects via in-situ TEM heating and HAADF-STEM analysis. Annealing of the interconnects is carried out under two different atmospheres in order to reduce the electrical resistivity even further. Finally, chapter 6 presents some important conclusions and summarises each of the previous chapters.

Beyond simple imaging with energetic beams – nanopatterning, correlative microscopy and statistical analysis of inclusions

Electron microscopy (EM) has advanced in an exponential way since the first transmission electron microscope (TEM) was built in the 1930’s. The urge to ‘see’ things is an essential part of human nature (talk of ‘seeing is believing’) and apart from scanning tunnel microscopes which give information about the surface, EM is the only imaging technology capable of really visualising atomic structures in depth down to single atoms. With the development of nanotechnology the demand to image and analyse small things has become even greater and electron microscopes have found their way from highly delicate and sophisticated research grade instruments to key-turn and even bench-top instruments for everyday use in every materials research lab on the planet. The semiconductor industry is as dependent on the use of EM as life sciences and pharmaceutical industry. With this generalisation of use for imaging, the need to deploy advanced uses of EM has become more and more apparent. The combination of several coinciding beams (electron, ion and even light) to create DualBeam or TripleBeam instruments for instance enhances the usefulness from pure imaging to manipulating on the nanoscale. And when it comes to the analytic power of EM with the many ways the highly energetic electrons and ions interact with the matter in the specimen there is a plethora of niches which evolved during the last two decades, specialising in every kind of analysis that can be thought of and combined with EM. In the course of this study the emphasis was placed on the application of these advanced analytical EM techniques in the context of multiscale and multimodal microscopy – multiscale meaning across length scales from micrometres or larger to nanometres, multimodal meaning numerous techniques applied to the same sample volume in a correlative manner. In order to demonstrate the breadth and potential of the multiscale and multimodal concept an integration of it was attempted in two areas: I) Biocompatible materials using polycrystalline stainless steel and II) Semiconductors using thin multiferroic films. I) The motivation to use stainless steel (316L medical grade) comes from the potential modulation of endothelial cell growth which can have a big impact on the improvement of cardio-vascular stents – which are mainly made of 316L – through nano-texturing of the stent surface by focused ion beam (FIB) lithography. Patterning with FIB has never been reported before in connection with stents and cell growth and in order to gain a better understanding of the beam-substrate interaction during patterning a correlative microscopy approach was used to illuminate the patterning process from many possible angles. Electron backscattering diffraction (EBSD) was used to analyse the crystallographic structure, FIB was used for the patterning and simultaneously visualising the crystal structure as part of the monitoring process, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed to analyse the topography and the final step being 3D visualisation through serial FIB/SEM sectioning. II) The motivation for the use of thin multiferroic films stems from the ever-growing demand for increased data storage at lesser and lesser energy consumption. The Aurivillius phase material used in this study has a high potential in this area. Yet it is necessary to show clearly that the film is really multiferroic and no second phase inclusions are present even at very low concentrations – ~0.1vol% could already be problematic. Thus, in this study a technique was developed to analyse ultra-low density inclusions in thin multiferroic films down to concentrations of 0.01%. The goal achieved was a complete structural and compositional analysis of the films which required identification of second phase inclusions (through elemental analysis EDX(Energy Dispersive X-ray)), localise them (employing 72 hour EDX mapping in the SEM), isolate them for the TEM (using FIB) and give an upper confidence limit of 99.5% to the influence of the inclusions on the magnetic behaviour of the main phase (statistical analysis).

Growth and characterization of anodic Films on InP in KOH and (NH4)2S

The current-voltage characteristics of InP were investigated in (NH4)2S and KOH electrolytes. In both solutions, the observation of current peaks in the cyclic voltammetric curves was attributed to the growth of passivating films. The relationship between the peak currents and the scan rates suggests that the film formation process is diffusion controlled in both cases. The film thickness required to inhibit current flow was found to be much lower on samples anodized in the sulphide solution. Focused ion beam (FIB) secondary electron images of the surface films show that film cracking of the type reported previously for films grown in (NH4)2S is also observed for films grown in KOH. X-ray and electron diffraction measurements indicate the presence of In2O3 and InPO4 in films grown in KOH and In2S3 in films grown in (NH4)2S.

Nanofabrication towards biophotonics

This thesis explores methods for fabrication of nanohole arrays, and their integration into a benchtop system for use as sensors or anti-counterfeit labels. Chapter 1 gives an introduction to plasmonics and more specifically nanohole arrays and how they have potential as label free sensors compared to the current biosensors on the market. Various fabrication methods are explored, including Focused Ion Beam, Electron Beam Lithography, Nanoimprint lithography, Template stripping and Phase Shift Lithography. Focused Ion Beam was chosen to fabricate the nanohole arrays due to its suitability for rapid prototyping and it’s relatively low cost. In chapter 2 the fabrication of nanohole arrays using FIB is described, and the samples characterised. The fabricated nanohole arrays are tested as bulk refractive index sensors, before a bioassay using whole molecule human IgG antibodies and antigen is developed and performed on the senor. In chapter 3 the fabricated sensors are integrated into a custom built system, capable of real time, multiplexed detection of biomolecules. Here, scFv antibodies of two biomolecules relevant to the detection of pancreatic cancer (C1q and C3) are attached to the nanohole arrays, and detection of their complementary proteins is demonstrated both in buffer (10 nM detection of C1q Ag) and human serum. Chapter 4 explores arrays of anisotropic (elliptical) nanoholes and shows how the shape anisotropy induces polarisation sensitive transmission spectra, in both simulations and fabricated arrays. The potential use of such samples as visible and NIR tag for anti-counterfeiting applications is demonstrated. Finally, chapter 5 gives a summary of the work completed and discusses potential future work in this area.

Modifying germanium nanowires for future devices: an in situ TEM study

Germanium was of great interest in the 1950’s when it was used for the first transistor device. However, due to the water soluble and unstable oxide it was surpassed by silicon. Today, as device dimensions are shrinking the silicon oxide is no longer suitable due to gate leakage and other low-κ dielectrics such as Al2O3 and HfO2 are being used. Germanium (Ge) is a promising material to replace or integrate with silicon (Si) to continue the trend of Moore’s law. Germanium has better intrinsic mobilities than silicon and is also silicon fab compatible so it would be an ideal material choice to integrate into silicon-based technologies. The progression towards nanoelectronics requires a lot of in depth studies. Dynamic TEM studies allow observations of reactions to allow a better understanding of mechanisms and how an external stimulus may affect a material/structure. This thesis details in situ TEM experiments to investigate some essential processes for germanium nanowire (NW) integration into nanoelectronic devices; i.e. doping and Ohmic contact formation. Chapter 1 reviews recent advances in dynamic TEM studies on semiconductor (namely silicon and germanium) nanostructures. The areas included are nanowire/crystal growth, germanide/silicide formation, irradiation, electrical biasing, batteries and strain. Chapter 2 details the study of ion irradiation and the damage incurred in germanium nanowires. An experimental set-up is described to allow for concurrent observation in the TEM of a nanowire following sequential ion implantation steps. Grown nanowires were deposited on a FIB labelled SiN membrane grid which facilitated HRTEM imaging and facile navigation to a specific nanowire. Cross sections of irradiated nanowires were also performed to evaluate the damage across the nanowire diameter. Experiments were conducted at 30 kV and 5 kV ion energies to study the effect of beam energy on nanowires of varied diameters. The results on nanowires were also compared to the damage profile in bulk germanium with both 30 kV and 5 kV ion beam energies. Chapter 3 extends the work from chapter 2 whereby nanowires are annealed post ion irradiation. In situ thermal annealing experiments were conducted to observe the recrystallization of the nanowires. A method to promote solid phase epitaxial growth is investigated by irradiating only small areas of a nanowire to maintain a seed from which the epitaxial growth can initiate. It was also found that strain in the nanowire greatly effects defect formation and random nucleation and growth. To obtain full recovery of the crystal structure of a nanowire, a stable support which reduces strain in the nanowire is essential as well as containing a seed from which solid phase epitaxial growth can initiate. Chapter 4 details the study of nickel germanide formation in germanium nanostructures. Rows of EBL (electron beam lithography) defined Ni-capped germanium nanopillars were extracted in FIB cross sections and annealed in situ to observe the germanide formation. Chapter 5 summarizes the key conclusions of each chapter and discusses an outlook on the future of germanium nanowire studies to facilitate their future incorporation into nanodevices.