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{\bf \emph{Keywords:}} surface plasmons, nanostructures, spectroscopy, metallic-blacks \\
{\bf \emph{Supervisers:}} W/Prof. James Williams (UWA), Prof. Sergey Samarin (UWA) \\
+\pagebreak
+\tableofcontents
-%\tableofcontents
+\pagebreak
\section*{Acknowledgements}
-\begin{itemize}
- \item Sergey Samarin
- \item Jim Williams
- \item Paul Guagliardo
- \item Nikita Kostylev
- \item Workshop (for producing electron gun mount?)
- \item Peter Hammond (?)
-\end{itemize}
+I am extremely grateful for the support offered to me by many individuals during this project. There aren't many synonyms for ``Thanks'', so I'm afraid this section may be a little repetitive.
+
+Thanks to my supervisors Prof Sergey Samarin and W/Prof Jim Williams for envisioning the project, and their invaluable support throughout the year. I would also like to thank staff members at CAMSP for assisting with the supervision of this project. In particular I am extremely grateful for the help and advice given by Dr Paul Gualiardo during the construction and testing of the Total Current Spectroscopy experiment.
+
+Thanks to Alexie ??? from CMCA for producing the SEM images which proved a invaluable aid for discussing the structure of the metallic-black films. Thanks to Nikita Kostylev for helping me learn the art of operating the ellipsometer. Thanks to both Prof Mikhail Kostylev and Jeremy Hughes for lending me some samples for ellipsometric analysis. I would also like to endorse the team at J.A Woolam, who provided replacement pins for the ellipsometer alignment detector at no charge after one of the original pins became mysteriously damaged.
+
+Congratulations to Jeremy Hughes who successfully predicted that the emission current of the electron gun was varying periodically less than a quarter of the way through the first period. Condolences to Alexander Mazur, whose theory that the vacuum chamber contained a pulsar proved unfounded.
+
+Thanks to all my family and friends for their support and for continuing to put up with my slow descent into madness during the last 12 months.
+
+Finally, perhaps as a result of the aforementioned madness, I would also like to thank the various pieces of equipment and inanimate objects which have been crucial to the success of this project. This includes the ellipsometer, the ADC/DAC box, my laptop computer ``Cerberus'', and the two ammeters upon which I relied upon so heavily. Rest in peace Keithly 610B. Your death was not in vain.
+
+\pagebreak
\section{Introduction}
-\begin{itemize}
- \item Waffle about motivation for the project
- \begin{itemize}
- \item Metal-Black films may have application for ... something.
- \begin{itemize}
- \item Radiometer vanes, IR detectors
- \item Number of applications where high absorbance into IR is required
- \item These have all been studied before though.
- \end{itemize}
- \item The electron spectra of metal-blacks have not yet been examined.
- \item Remarkable difference between Metal-Black films (bad vacuum) and normal metal films (UHV)
- \begin{itemize}
- \item No (detailed/satisfactory) explanation (that I can find...) for difference
- \end{itemize}
- \item Talk about plasmonic based computing? Moore's law? Applications to thin film solar cells?
-
- \end{itemize}
- \item Specific aims of project
- \begin{enumerate}
- \item Surface density of states / band structure of Black-Au films using TCS (The main aim)
- \item Identification of plasmonic effects in Black-Au films (?) (If they even exist!)
- \begin{itemize}
- \item Identify plasmonic effects in Au and Ag films with Ellipsometry (this is fairly simple to do)
- \end{itemize}
- \item Combination of Ellipsometry and TCS to characterise thin films (not just Black-Au)
- \begin{itemize}
- \item Ie: How can one technique be used to support the other?
- \end{itemize}
- \end{enumerate}
- \item Structure of thesis
-\end{itemize}
-\section{Overview of Theory}
-Summarise the literature, refer to past research etc
+The report will be organised as follows; first we will discuss literature relating to surface science and nanostructured thin films in particular. We will then give an overview of the primary experimental techniques employed during this project, before presenting experimental results. Finally, we will discuss stuff.
-\subsection{Electron Spectra of Solids and Surface}
+\pagebreak
-\begin{itemize}
- \item Overview of section
-\end{itemize}
+\section{Overview}
-In this section, we will first introduce the basic concepts needed to describe the electron spectra of solids. A short description of methods for calculating the electron spectra will be given, and the results shown by these calculations. We will then discuss the electron spectra for the near surface region of solids, compared to the ``bulk'' spectra far from the surface.
+In this section we provide an overview of theoretical and experimental literature related to the properties of nanostructured thin films. We also include a summary of the past research which focuses on metallic-black films.
+\subsection{Secondary Electron Emission}
-\subsubsection{Description of Matter in the Solid State}
+% What is it?
+Secondary electron emission refers to the emission of electrons from a target surface caused by interactions with an incident electron, or more commonly a beam of electrons. Electrons from the incident beam are referred to as primary electons, whilst the term ``secondary electron'' applies to all electrons reflected or emitted from the surface under bombardment.
-\begin{itemize}
- \item Define and describe solid geometrically
- \begin{itemize}
- \item Basis + Lattice
- \item Basis groups are assumed to be fixed relative to the lattice
- \end{itemize}
- \item Describe general properties of the potentials seen by electrons in solids
-\end{itemize}
+% First mentions and early research
-In the simplest models, a solid is represented by an infinite crystalline lattice; a geometrically repeated arrangement of some basis group of atoms. The nuclei of atoms are assumed to remain in fixed positions.
-The potential seen by an electron in the lattice is periodic. For a single nuclei, the potential seen by an electron is
+% General description of secondary electron spectrum
-\subsubsection{Calculation of Electron Spectra}
+Figure \ref{se_dist.pdf} shows the general shape of an energy distribution of secondary electrons. for a primary electron energy of $E_p$. The narrow peak centred at $E = E_p$ is due to elastically scattered electrons; the width of this peak is determined by the distribution of primary electron energies, as well as the resolution of the detector. The broad peak in the low energy part of the spectrum is due to inelastically scattered electrons.
-\begin{itemize}
- \item Define electron spectra
- \begin{enumerate}
- \item
- \end{enumerate}
- \item Free electron gas
- \item Free electron entering a periodic potential
- \item Potentials of real solids
- \item Single vs Multiple electrons
- \item Numerical calculations
- \begin{itemize}
- \item TODO: Actually research this
- \item Find a comparison with a real experimental result
- \end{itemize}
- \item
-\end{itemize}
+\begin{center}
+ \includegraphics[scale=0.40]{figures/se_dist.pdf}
+ \captionof{figure}{{\bf Model of Secondary Electron Distribution}}
+ \label{se_dist.pdf}
+\end{center}
-\subsubsection{The Near-Surface Region}
+% How real secondary electron spectra depend upon the surface
-\begin{itemize}
- \item Real solids have surfaces
- \item Differences between surface region and bulk
- \begin{itemize}
- \item Resulting differences in potential
- \end{itemize}
- \item How the electron spectra may differ from bulk values
- \begin{itemize}
- \item Surface states
- \begin{itemize}
- \item Tamm \& Shockley States
- \end{itemize}
- \end{itemize}
-\end{itemize}
+Real secondary electron distributions also show fine structure imposed upon the inelastic part of simplified spectrum described above. This fine structure is characteristic of the electron spectra of the target surface. Near to the elastic peak, fine structure is caused by energy loss to interband transitions and plasma vibration excitation. The central part of the distribution contains fine structure due to Auger electron emission, and energy losses due to excitation of inner electrons. Fine structure at low energies is due to the structure of empty states of the solid.
-In the preceeding sections, solids were assumed to have infinite spatial extent. In practice, any real solid occupies a finite volume in space. Any interactions between a solid and its environment take place at the surface of the solid. As the volume of the solid is decreased, the role of the surface region in determining the behaviour of the solid in its environment is increased.
+% Research of Komolov
+\subsection{Plasmons}
+% What are they?
+A plasmon is a quasi-particle arising due to charge density oscillations in a solid.
+% Early research
-\begin{itemize}
- \item Description of the near surface region
- \begin{itemize}
- \item All real solids occupy finite volumes in space.
- \item The surface of a solid is important because interactions between the solid and its surroundings occur in the near surface region.
- \item Characterised physically by:
- \begin{itemize}
- \item Termination of periodic crystal lattice
- \item Violation of geometric order
- \item Distortion of interatomic distances and hence interaction forces
- \item There is a transition ``near surface'' region between bulk and surface properties, roughly 5 atomic distances.
- \end{itemize}
- \item Potential seen by an electron at a surface can differ greatly from the bulk
- \item $\implies$ the electron spectra of the near surface region differs from the bulk spectra
- \item Simplest case: Step potential at surface.
-
- \end{itemize}
-
- \item The Electron Spectra
- \begin{itemize}
- \item Electron Spectra describes the energy eigenstates for an electron in a Bulk or Surface potential
- \item Characterised by
- \begin{enumerate}
- \item Energy dispersion $E(\vect{k})$
- \begin{itemize}
- \item Dependence of Energy on electron wave vector
- \item Obtained theoretically by solving Scrhrodinger's Equation
- \item For a free electron gas, $E = \frac{\hbar^2 k^2}{2m}$
- \item Periodic potential in bulk solid leads to band gap structure of $E(\vect{k})$
- \item Periodic potential $\implies$ E is periodic. Only needs to be defined in first Brillouin zone.
- \end{itemize}
- \item Density of States $N(E)$
- \begin{itemize}
- \item $N(E) = \frac{\Delta N}{\Delta E} = \frac{1}{4\pi^3}\int_S\left(\der{E}{k}\right)^{-1} dS$
- \item Integral is in momentum space over the isoenergetic surface of energy $E$
- \item For a free electron gas, $N(E) = $
- \end{itemize}
- \end{enumerate}
- \end{itemize}
-
- \item Surface states
- \begin{itemize}
- \item Simplest model: Step potential
- \item Two major models
- \begin{enumerate}
- \item Tamm States: Periodic potential in solid, free space outside, jump at surface
- \begin{itemize}
- \item Energy eigenvalues lie in the forbidden band of the bulk spectra
- \item Attenuation of eigenvalues from surface to vacuum, oscillation of state within surface
- \item Max electron density occurs on the crystal surface
- \end{itemize}
- \item Shockley states: Potential of surface and bulk cells equal
- \begin{itemize}
- \item Corresond to free valences (dangling bonds) at the surface
- \end{itemize}
- \end{enumerate}
- \item Tamm and Shockley states arise from two extreme models (large change and small change respectively between bulk and surface). In reality, a combination of Tamm and Shockley states appear.
- \item These states arise from termination of the lattice; but the surface cells are assumed undistorted
- \item In reality surface cells are distorted by relaxation and reconstruction of the surface
- \end{itemize}
-
- \item Main reference: Komolov "Total Current Spectroscopy"
- \item "Solid State Physics" textbooks and "Electron Spectroscopy" textbooks
-\end{itemize}
+\subsection{Metallic-Black Films}
-\subsection{Plasmonics}
-I really think I should actually find plasmonic effects before writing too much about them...
-\begin{itemize}
- \item Charge density oscillations
- \item Surface and bulk plasmons
- \item Pines and Bohm
- \item Review article from T.W.H Oates et al about using Ellipsometry to characterise plasmonic effects
-\end{itemize}
+% What are they?
+So called metallic-black films are the result of deposition of metal elements at a relatively high pressure (of the order of $10^{-2}$ mbar). The films are named due to their high absorbance at visible wavelengths; they appear black to the naked eye. There is a remarkable contrast between such films and films deposited under low pressure (less than $10^{-6}$mbar), which are typically highly reflective and brightly coloured.
+
+% First mentions and early research; Pfund
+This phenomenom has been known since the early 20th century, with the first papers on the subject published by Pfund in the 1930s \cite{pfund1930}, \cite{pfund1933}. Pfund established the conditions for formation of metallic-blacks \cite{pfund1930}, and showed that the transmission spectrum of metallic black films is almost zero in visible wavelengths, but increases to a plateau in the far infrared \cite{pfund1933}. More extensive research on the structural and optical properties of these films by Louis Harris and others during the 1940s and 1950s \cite{harris1948}, \cite{harris1952}, \cite{harris1953}. It has been established that metallic blacks may be prepared in either air or inert gases
-\subsection{Metallic-Black Thin Films}
-\begin{itemize}
- \item How they are made (bad vacuum, in air or a noble gas)
- \begin{itemize}
- \item If made in air, there are usually tungsten oxides present (from filament). Refer to paper by Pfund.
- \end{itemize}
- \item Structural difference between Black-Au and ``Shiny'' (need a better term) Au
- \begin{itemize}
- \item Can include electron microscopy images?
- \item An actual photograph of a Black-Au film? Not necessary?
- \end{itemize}
- \item Pfund (earliest publisher, preparation and general properties)
- \item Louis Harris (most research in 50s and 60s)
- \begin{itemize}
- \item L. Harris mostly did transmission spectroscopy in the far infra red (well beyond the ellipsometer and Ocean Optics spectrometer ranges)
- \item The really crappy measurements I did with the Ocean Optics spectrometer seem to agree with these measurements
- \begin{itemize}
- \item L. Harris' $\lambda$ has a range of 1nm to $100\mu$m; my measurements are only to $1\mu$m
- \item Agreement in first $1\mu$m anyway
- \item I should probably re-do those measurements with a less crappy setup, if I actually want to use them
- \end{itemize}
- \item Harris related the optical properties to the structure of the film (condensor strands) via the electronic properties
- \end{itemize}
- \item Plasmonic effects - Deep R. Panjwani (honours thesis)
- \begin{itemize}
- \item Not sure if I can use an honours thesis as a reference.
- \item Concluded that surface plasmon resonance in Black-Au film on solar cells lead to increase in solar cell efficiency
- \item Used simulation that modelled Black-Au film as spherical balls to show E field increased by plasmon resonance
- \begin{itemize}
- \item Was this model appropriate? Black-Au is more ``smoke'' or ``strand'' like according to other references. Images also do not show ``blob'' like structure.
- \end{itemize}
- \item Need to read this reference more thoroughly
- \end{itemize}
-\end{itemize}
+
+% Pretty pictures for purpose of this discussion
+% Not really "results", since I didn't make the images
+Secondary Electron Microscope (SEM) images of Au deposited on Si at high and low pressures (in air) were produced at the Centre for Microscopy Characterisation and Analysis (CMCA), UWA. The film imaged on the left (high pressure) appears black at visible wavelengths, whilst the film on the right (low pressure) appears golden yellow.
+
+\begin{center}
+
+
+\begin{tabular}{cc}
+ \includegraphics[scale=0.2]{figures/Au_BLACK_200nm.png} & %\captionof{figure}{Au-Black SEM Image} \label{Au_BLACK_200nm.png} &
+ \includegraphics[scale=0.2]{figures/Au_semi-shiny_1_SEM.png} %\captionof{figure}{Au SEM Image} \label{Au_semi-shiny_1_SEM.png}
+
+ \label{SEM_images}
+\end{tabular}
+
+ \captionof{figure}{{\bf SEM images of Au deposited on Si at $2\times10^{-2}$mbar (left) and $1\times10^{-6}$mbar.} Note that the scales are very similar for both images.}
+
+\end{center}
+
+% Explanation of structure?
+The structural difference between the two films is striking, and yet the exact mechanism behind the formation of the metallic-black film is not well understood. The most widely accepted explanation is that the evaporated metal particles reaching the target surface have insufficient energy to form a regular crystal lattice due to cooling through collisions with the atmosphere \cite{}. As of yet, there is no detailed theoretical description of this behaviour.
+
+% Research by Harris concluding ``condensor'' like structure
+Harris et al. have produced experimental results of the transmission of metallic-black films from visible wavelengths to the far-infrared \cite{}. By modelling the film as a layer of metallic strands, acting as ``condensors'', Harris et al. arrived at an expression for the electron relaxation time of [element]-black \cite{}, leading to a a transmission spectrum in good agreement with experimental results.
+
+% Mckenzie
+Mckenzie has established that the presence of oxygen effects the optical and electrical properties of metallic-blacks \cite{mckenzie2006}.
+
+% Model of structure
+Nanostructured metal films prepared at low pressure are often approximated by an isotropic layer of spherical blobs upon the substrate, or even as a uniform layer with an ``effective'' thickness \cite{}. As the right image in Figure \ref{SEM_images} shows, this is a good representation of the structure of such a film. In contrast, the metallic-black film is highly non-uniform; as a result, detailed characterisation of the properties of such a film is difficult.
+
+
+
+
+% More recent research
+More recently, it was shown that Au-black coatings increased the efficiency of thin film solar cells \cite{}. In this study, a simulation approximating an Au-black film as a layer of semi-spherical structures showed plasmonic behaviour which lead to an increase in electric field behind the film.
+
+
+% Artificially ``blackened'' thin films
+Metallic-black films have proven useful in applications requiring efficient absorption of light, including the. Recently there has been interest in artificial ``blackening'' of metal surfaces in ways which simplify the characterisation of the surfaces for practical applications.
+
+Sondergaard et al. have produced metallic-black surfaces capable of suporting surface plasmon modes \cite{sondergaard2012}. These films exhibit similar optical properties to the previously considered ``evaporated'' metallic-black surfaces.
+
+% What I will be doing with metallic-black films
+This project will employ Total Current Specroscopy, Ellipsometry and Optical Spectroscopy methods to investigate the difference between metallic films deposited at low pressure, and high pressure (metallic-blacks). The production and study of artificially blackened films is beyond the scope of this research.
+
+
+\begin{center}
+ \includegraphics[scale=0.2]{figures/300V-01.jpg}
+ \captionof{figure}{{\bf Au-black film viewed at magnifications of x20000, x50000, x100000 and x200000} (top left, top right, bottom left, bottom right). The film appears non-isotropic, and possibly fractal like upon magnification. This structure has lead some reasearchers to refer to the deposited films as ``smokes'' \cite{}.}
+ \label{300V-01.jpg}
+\end{center}
+
+\pagebreak
\section{Experimental Techniques}
\subsection{Secondary Electron Spectroscopy}
-Secondary Electron Spectroscopy encompasses a large group of techniques used for studying the electron spectra of surfaces and solids. In these methods a beam of primary electrons is directed at the surface of a solid. The interactions between primary electrons and the surface give rise to a flux of secondary electrons scattered from the surface. Analysis of this secondary electron flux gives information about the interaction between primary electrons and the surface.
+Secondary Electron Spectroscopy encompasses a large group of techniques which exploit secondary electron emission for studying the electron spectra of surfaces and solids. In these methods a beam of primary electrons is directed at a surface. The interactions between primary electrons and the surface give rise to an energy distribution of electrons elastically and inelastically scattered from the surface. Analysis of the distribution of the scattered ``secondary'' electrons gives information about the electron energy spectrum of the target surface.
+
+Techniques of Secondary Electron Spectroscopy can be divided into two classes. Energy-resolved methods are based upon observation of the secondary electron distribution at a fixed primary electron energy. These methods aim to examine specific secondary emission processes which occur within a selected energy interval. The angular distribution of emitted electrons is often also recorded.
+
+In contrast to Energy-resolved methods, Total Current (or Yield) methods measure the total current of secondary electrons as a function of primary electron energy. The focus of this project has been low energy Total Current Spectroscopy (TCS). While Total Current methods provide less detailed information about secondary emission processes within a solid, they are generally simpler to realise experimentally as they do not require energy analysers, and current measurement may be performed external to the vacuum chamber, using a conventional ammeter.
+
-\subsubsection{Electron-Surface Interactions}
+\subsection{Total Current Spectroscopy}
+Figure \ref{} shows a simplified schematic for the Total Current Spectroscopy experiments conducted during this study. An electron gun is used to produce the beam of primary electrons. Electrons are emitted from a cathode held at negative potential relative to the target. These electrons are focused into a beam and accelerated onto the target through the electric field produced by a series of electrodes. A detector is used to measure the total current passing through the target.
-\subsubsection{Methods of Secondary Electron Spectroscopy}
-Energy-resolved methods of Secondary Electron Spectroscopy are based upon observation of the secondary electron energy distribution at a fixed primary electron energy. The primary electron energy determines which processes are possible, whilst the observed distribution can be related to the probability distribution for the possible processes.
-In contrast to Energy-resolved methods, Total Current (or Yield) methods are based on observation of the total current of secondary electrons as a function of primary electron energy. As the primary electron energy is increased, the threshold energies for particular processes are passed. This
+If the current incident upon the sample is $I_{\text{total}}$, and the current of secondary electrons scattered from the surface is $I_r$, then the transmitted current $I_t$ is given by:
-Both Energy-resolved and Total Current methods can be performed
+\begin{align*}
+ I_t &= I_{\text{total}} - I_r
+\end{align*}
-\subsection{Total Current Secondary Electron Spectroscopy}
+Generally $I_{\text{total}}$ is assumed to be independent of initial energy $E$. This assumption is valid if the initial energy is small compared to the accelerating potential of the gun, and the distance of the sample from the gun is sufficiently large.
-Figure \ref{} shows a simplified schematic for the Total Current Spectroscopy experiments conducted during this study. Electrons are emitted from a cathode held at negative potential relative to the target. The electron beam is focused and accelerated onto the target by the electric field of an electron gun. A detector is used to measure the total current passing through the target.
+In this case, differentiating with respect to $E$:
+\begin{align*}
+ \der{I_t}{E} &= - \der{I_r}{E}
+\end{align*}
+
+Figure \ref{block_diagram.pdf} is a block diagram of the experimental setup including measurement and control systems external to the vacuum chamber.
+Note that the emission current ammeter is optional, and was used for testing purposes.
+
+\begin{center}
+ \includegraphics[scale=0.60]{figures/block_diagram.pdf} \label{block_diagram.pdf}
+\end{center}
\subsubsection{Electron Optics}
The electron gun used for this experiment was repurposed from an old Cathode-Ray Oscilloscope (CRO). Figure \ref{} shows a simplified diagram of the electron gun, whilst Figure \ref{} shows a photograph of the gun.
+The full circuit diagram for the electron gun control circuit is shown in Appendix A. %\cite{}
+\subsubsection{Automatic Data Acquisition}
-The full circuit diagram for the electron gun control circuit is shown in Appendix A.
+In order to collect data on the large number of planned samples for the study, some form of automation was required. The automated system needed to be able to set the initial energy by adjusting the potential of the cathode relative to the sample, and simultaneously record the total current through the sample.
-\subsubsection{Automatic Data Acquisition}
+The available power supplies at CAMSP featured analogue inputs for external control. This meant that a Digital to Analogue Convertor (DAC) card was needed to interface between the control computer and the power supply. In addition, the available instruments for current measurement produced analogue outputs. As a result, Analogue to Digital Convertors (ADCs) were required to automate the recording of total current.
-In order to collect data on the large number of planned samples for the study, some form of automation was required. The automated system needed to be able to incrementally set the initial energy by controlling a power supply, and record the total current measured by an ammeter.
+Although an external DAC/ADC box was already available for these purposes, initial tests showed that the ADCs on the box did not function. The decision was made to design and construct a custom DAC/ADC box, rather than wait up to two months for a commercial box to arrive. The design of the custom DAC/ADC box is discussed in detail in Appendix B, and the software written for the on-board microprocessor and the controlling computer are presented in Appendix D.
-The available power supplies at CAMSP only featured analogue inputs for external control. This meant that a Digital to Analogue Convertor (DAC) card was needed to interface between the control computer and the power supply. In addition, the available instruments for current measurement at CAMSP produced analogue outputs. As a result, Analogue to Digital Convertors (ADCs) would be required to automate the recording of total current.
-Although an external DAC/ADC box was already available for these purposes, initial tests showed that the ADCs on the box did not function. The decision was made to design and construct a custom DAC/ADC box, rather than wait up to two months for a commercial box to arrive. The design of the custom DAC/ADC box is discussed in detail in Appendix B, and the software written for the on-board microprocessor and the controlling computer are presented in Appendix C.
+\subsection{Ellipsometry}
+Ellipsometry is an optical technique most commonly used to determine the thickness of multilayered thin films. Ellipsometry can also be used to determine the optical constants and properties of unknown materials.
+Essentially, ellipsometry measures the change in polarisation of light reflected from a surface. This change in polarisation can be related to properties of the surface if knowledge of the surface is correctly applied. For a bulk sample, the change in polarisation can be directly related to the optical constants of the material.
+A
-\begin{itemize}
- \item Black-Au - 1e-2 mbar vacuum
- \item ``Shiny'' - 1e-6 / 1e-7
- \item Current of ~3.5A through W wire filament spot welded onto Ta strips in turn spot welded to Mo posts
- \item Voltage through filament is ~1 V; quote the power?
- \item Filament isotropically coats sample with desired material.
- \item Possibly get a curve of Au thickness estimated with Ellipsometry vs exposure time?
- \begin{itemize}
- \item Probably too much work and too unreliable
- \item Maybe do it, but only use 2/3 data points
- \item Low priority
- \end{itemize}
-\end{itemize}
-\subsection{Electron Spectroscopy}
+\subsubsection{Variable Angle Specroscopic Ellipsometry}
+A single ellipsometric measurement involves recording $r_p$ and $r_s$ at one angle and wavelength. The earliest ellipsometers were
-Secondary electron spectroscopy methods are a broad class of methods which investigate surface electron spectra through observing processes in which the surface electrons participate directly \cite{komolov}.
-Total Current Spectroscopy is a group of electron secondary
+A Variable Angle Spectroscopic Ellipsometer at CAMSP has been used to perform a variety of measurements on metallic thin films.,
+
+The VASE
+
+It is also possible to conduct reflection and transmission spectroscopy experiments using the VASE.
+
+
+\subsection{Vacuum Techniques and Sample Preparation}
+
+Both the TCS experiments and the deposition of films must be performed in a vacuum. For convenience and simplicity, a single vacuum chamber at CAMSP has been repurposed to perform both of these tasks. The chamber can be pumped by a molecular turbo pump, backed by a rotaray pump, to a base pressure of $2\times10^{-8}$ mbar, or by the rotary pump alone to a base pressure of $1\times10^{-3}$ mbar. The pressure is monitored using either a pirani or ion gauge (for pressures greater than and less than $10^{-3}$ mbar respectively).
+
+%TODO: Insert graphs of pressure in chamber
+
+Figure \ref{} shows a diagram of the vacuum chamber used both for the creation of nanostructured thin films and their study using TCS. A rotatable sample holder is positioned in the centre of the chamber. One flange of the chamber houses the electron gun used for TCS measurements, whilst the opposite flange contains feedthroughs on which tungsten filament evaporators are mounted. This setup allows for almost immediate study of evaporated films by simple rotation of the sample holder to face the gun.
+
+
+The evaporators consist of a tungsten wire filament attached between two feedthroughs. A piece of a desired metal is folded over the apex of the tungsten wire. The metal can be heated by passing a current through the filament; near the metal's melting point it begins to evaporate. To clean the metal surface and ensure uniform evaporation, this procedure is first performed at low pressure (below $10^{-6}$ mbar) with no sample in the chamber, with the current increased until the metal piece begins to melt and forms a ball on the wire. Figure \ref{} shows an image of an evaporator that has been prepared for use.
+
+This study focused primarily on depositing Au films on an Si substrate, at both high and low pressures. The substrates and sample holders were cleaned in an acetone bath immediately prior to insertion in the vacuum chamber.
-\begin{itemize}
- \item
- \item Total Current Spectroscopy methods measure the total current of secondary electrons as a function of primary electron energy.
- \item These methods are distinguished from ``differential'' methods (such as Auger electron spectroscopy and energy loss spectroscopy) which measure the secondary electron spectrum at a fixed primary electron energy.
- \item
- \begin{itemize}
- \item Low energy beam of electrons incident on sample
- \item Measure slope of resulting I-V curve
- \item Relate to density of states and electron band structure (Komolov chapter 3.2)
- \end{itemize}
- \item Description of apparatus
- \begin{itemize}
- \item Electron gun and filament
- \item Electron gun control box
- \item ADC/DAC control box and data processing
- \end{itemize}
- \item Photographs vs Diagrams
- \begin{itemize}
- \item Prefer diagrams to photographs
- \item Especially for the ADC/DAC control box circuit. Because it looks like a horrible mess.
- \end{itemize}
-\end{itemize}
-\subsection{Ellipsometry and Transmission Spectroscopy}
-\begin{itemize}
- \item Overview of techniques
- \item Description of apparatus (use VASE manual)
- \item Ocean Optics spectrometer? Usable?
- \item Application of Ellipsometry to finding plasmonic effects
- \begin{itemize}
- \item Surface plasmons = E oscillation parallel to surface $\implies$ only $p$ component of light excites plasmons
- \end{itemize}
-\end{itemize}
\section{Experimental Results and Discussion}
\subsection{TCS Measurements}
-\begin{itemize}
- \item TCS for Si
- \item TCS for Si + Au
- \item TCS for Si + Black-Au
- \item Affect of preparation pressure on TCS for Si + Black-Au
- \item Repeat for Si + Ag and Si + Black-Ag (?)
-\end{itemize}
+
\subsection{Ellipsometric Measurements}
-\begin{itemize}
- \item Ellipsometry to estimate thickness of SiO2 layer on Si
- \item Estimate thickness of Au/Ag on Si+SiO2
- \item Ellipsometric measurements of Si+Black-Au/Ag
- \begin{itemize}
- \item Modelling procedures to characterise Black-Au/Ag
- \end{itemize}
- \item Ellipsometric measurements of Glass+Black-Au/Ag (?)
- \item Transmission spectra of Glass+Black-Au/Ag from earlier in year (?)
-\end{itemize}
+
+\subsection{Transmission Spectra of Metal Films}
+
+Using the VASE and a commercial spectrometer (OceanOptics) in independent experiments, we obtained transmission spectra for metallic-black and some other metallic films.
+
+
+
\section{Achievements}
-\begin{itemize}
- \item Deposition of thin films of Au and Black-Au in vacuum chamber
- \item Ellipsometric and spectroscopic measurements on these films
- \item Repurpose vacuum chamber for sample preparation and TCS experiments
- \item Designed and built electronics for TCS experiments
- \begin{itemize}
- \item Electron gun control box
- \item ADC/DAC box
- \end{itemize}
- \item Wrote software for data aquisition and data processing
-\end{itemize}
-\section{General notes}
-\subsection{TCS}
-\begin{itemize}
- \item Optimise setup of gun
- \begin{itemize}
- \item Emission current. How much does it vary, why does it vary.
- \item Why does Is/Ie curve shift with successive sweeps? Does sweep modify sample's surface?
- \item Is sample holder acceptable? Are ceramic washers accumulating charge?
- \item How do I tell when the setup is optimised...
- ``The setup was optimised by looking for an S curve''. Very scientific.
- \item The gun was focused on the phosphor screen... and then I turned it around, changing the distance from the gun to the sample. Brilliant.
- \end{itemize}
- \item Obtain TCS spectra for Si that compares well with literature
- \begin{itemize}
- \item How to relate TCS spectrum to $n(E)$ and $E(\vect{k})$
- \end{itemize}
- \item Prepare Au films, obtain TCS spectra that compares with literature
- \item Obtain TCS spectra of Black-Au films
- \item Use results to compare properties of films with results from other methods in the literature
- \item Uncertainties
- \begin{itemize}
- \item Oscilloscope measurements of inputs to ADC channels under controlled conditions
- \begin{itemize}
- \item Expected values are +/-3mV due to ADC channel, +/-300mV due to 610B, +/-1mV due to 602
- \item 610B and 602 will probably be worse because they are ancient
- \item There is about 200mV of noise between the GND of the ADC box and the electron control box.
- \item How to reduce ground loops? Not much I can do. Rack is now also grounded to water pipe, but this doesn't seem to make a difference.
- \end{itemize}
- \item Stupid 50Hz AC noise... how to reduce with filters and/or averaging
- \end{itemize}
- \item Create circuit diagrams for Electron gun circuit
- \item Create circuit diagrams for ADC/DAC box
- \begin{itemize}
- \item Simulate behaviour of circuit
- \item Use of instrumentation amplifier on ADC5 to make off-ground measurements
- \item Use of low pass filter on ADC5
- \end{itemize}
- \item Include references to all datasheets, etc
- \item Vacuum chamber
- \begin{itemize}
- \item Base pressure with rotary pump? Was 1e-3 after 30 minutes at start of year, but probably introduced leaks since then
- \item Lowest pressure achieved with turbo pump is 1.1e-7 mbar as of 25/07.
- \item Viton gaskets on some seals. Copper on other.
- \item Flanges:
- \begin{enumerate}
- \item View window (large, view of sample \& sputtering filaments)
- \item Rotation manipulator \& sample mount
- \item Pump inlet
- \item Filament flanges 1 (used earlier in year, not anymore) and 2
- \item Inlet with leak valve (for introducing gases into chamber)
- \item Vent valve on turbo pump
- \item Electron gun flange
- \item View window (small, view of back of electron gun)
- \end{enumerate}
- \end{itemize}
-\end{itemize}
+\pagebreak
\section*{Appendix A - Electron Gun Control and Current Measurement Circuit}
Figure \ref{} shows the complete electron gun control circuit. The circuit was designed and constructed as part of this project. The design is based upon examples found in \cite{komolov} and \cite{Moore}.
-s
+\subsection*{Electron Optics}
+The electron gun has been recycled from a Cathode Ray Oscilloscope. Figure \ref{} shows a diagram and photograph of the gun. The gun contains a total of 9 electrodes; several electrodes are held at the same potential, as shown in the figure. As shown in figure \ref{}, the electrode potentials are referenced to the cathode, not signal ground. Because of the relatively large distance between the gun and sample (held at ground), this ensures that changes in initial energy do not significantly effect the focusing properties of the gun.
+
+The optimum potentials of the gun electrodes were determined manually by focusing the gun on an Au film (deposited on Si). I-V curves obtained by measuring current through the sample as a function of initial energy were obtained. The electrode potentials were systematically altered to ensure the curves were as close as possible to the ideal model.
+
+\subsection*{Model of I-V curves}
+The current detected through the sample is due to electrons which have sufficient energy to overcome the potential barrier between the surface and vacuum, entering the conduction band
+
+
+\subsection*{}
+
+The filament is surrounded by a conducting cylindrical electrode commonly called the ``Venault''. The potential of the venault has little effect on the focusing properties of the gun, but is largely responsible for determining the current leaving the gun. Figure \ref{} shows I-V curves for several venault settings, including the optimum setting.
+
+\subsection*{Einzel Lens}
+
+%Electrodes passing through the aperture of the venault are accelerated towards the target through six electrodes. The first and last pair of electrodes are at the same potential; the central pair are held at a different potential. Such an arrangement can be considered as an ``Einzel'' or ``zoom'' lens. It can be shown \ref{Moore} that the
+
+\pagebreak
\section*{Appendix B - DAC/ADC Box - Hardware}
\label{adc5.pdf}
\end{center}
-
+asdfa
The instrumentation amplifier consists of two stages of operational amplifiers (op-amps); input buffers, and a difference amplifier.
The difference amplifier can be shown using the ideal op-amp model to produce an output voltage proportional to the difference between its inputs:
V_{out} &= \frac{R_2}{R_1} \left(V_{2} - V_{1}\right)
\end{align*}
-The two op-amps at the inputs to the differential amplifier act as unity gain buffers. Although the output of the unity gain buffer is equal to the input on its positive terminal, the buffer prevents current from flowing from the positive terminal to ground. With the buffer amplifiers absent, a current of: would flow between each of the input terminals and ground.
-
-Instrumentation amplifiers are usually constructed in the schematic shown in Figure \ref{}. In this version, the gain of the amplifier can be changed by altering a single resistor. However, more resistors are required. The version actually constructed was designed based upon the small number of resistors available, within a short time frame. Although the design could have later been changed, this would have been of no real benefit, since there was no requirement to adjust the gain of the amplifier.
+The two op-amps at the inputs to the differential amplifier are unity gain buffers. Although the outputs of the op amps are equal to their inputs, current is prevented from flowing from the circuit under measurement, and is instead drawn from the op amp power supply.
In principle, two ADC channels could be used to record the positive and negative outputs of the electrometer seperately, with differencing done in software. However this would require modification to the output cable of the electrometer, which may prove inconvenient for future uses.It was decided that the modification of the cable and added complexity of the software required would be more time consuming than differencing the two inputs using the hardware methods described above.
The low pass filters were added to the inputs of ADC5 after it was found that an unacceptable level of AC noise was being output by the electrometer. The level of noise was too high to be filtered in software, for reasons that will be discussed in Appendix D.
+\subsection*{Temperature Measurement}
+
+The AVR Butterfly features an onboard thermistor connected to ADC0. Reading ADC0 and applying the formula given in the AVR Butterfly User's Guide \cite{} results in a temperature measurement. This was useful in establishing a link between the changing chamber pressure and the temperature of the laboratory (see Appendix C).
+
\subsection*{Power Supplies}
Due to the presence of both analogue and digital electronics in the DAC/ADC box, three seperate supply voltages were required:
\begin{enumerate}
\end{center}
\subsubsection*{Op-amp Power Supply}
-The DAC/ADC box circuitry involves several operational amplifiers (LF356), which require dual $\pm 10-15V$ supplies. As there were no dual $\pm$ power supplies available, a single $30V$ power supply was used, with the circuit shown in figure \ref{fig_opamp_supply} used to produce $\pm 15V$ relative to ground.
+The DAC/ADC box circuitry involves several operational amplifiers (LF356), which require dual $\pm 10-15V$ supplies. As there were no dual $\pm$ power supplies available, a single $30V$ power supply was used, with the circuit shown in figure \ref{} used to produce $\pm 15V$ relative to ground.
The buffer amplifier ensures that negligable current can flow from the power supply into the logic and ADC circuits, whilst the capacitor removes high frequency fluctuations of the power supply relative to ground.
-To simplify circuit diagrams, op-amps will be drawn with the power supply connections ommitted from this point onwards.
-
\subsection*{DAC Output}
A commercial DAC board was used to produce the DAC output. The Microchip MCP4922 ET-Mini DAC is controlled by the AVR Butterfly using Motorola's Serial Peripheral Interface (SPI) Bus. The software used to implement SPI between the MCP4922 and the AVR Butterfly is discussed in Appendix D.
Although the RS-232 is relatively simple to implement, which makes it ideal for non-proprietry microprocessor applications, most modern computers no longer feature RS-232 COM ports. Although a computer with COM ports was available at CAMSP, due to the extreme unreliability of this computer, it was quickly replaced with a laptop that did not possess COM ports, and a commercial RS-232 to USB converter was used to interface with the laptop.
-\section*{Pressure Monitoring}
+\section*{Appendix C - Pressure Monitoring}
+
+The pressure in the chamber was monitored by a ion gauge at low pressure (below $10^{-3}$ mbar), and a pirani gauge at high pressure. The gauge included a flurescent Liquid Crystal Display (LCD). In order to automate monitoring of pressure, a USB webcam was placed in front of the gauge LCD. Software was written using the Python Imaging Library (PIL) to convert the image produced by the webcam into a pressure reading. In this way, the pressure could be recorded as a function of time, independent from other measurements performed using the ADC/DAC control box.
+
+Figures \ref{pressure_a.jpg} to \ref{pressure_c.jpg} show the process by which an image taken with the webcam was converted into a pressure reading. The software first identifies bounding rectangles for each individual digit. These are then further subdivided into 7 segments. If enough pixels in a given segment match the colour LCD segments, then the segment can be identified as activated. The software then creates a string corresponding to the activated segments, and looks up the digit in a dictionary.
+
+\begin{center}
+ \includegraphics[scale=0.50]{figures/pressure_a.jpg}
+ \captionof{figure}{An unprocessed image}
+ \label{pressure_a.jpg}
+\end{center}
+
+\begin{center}
+ \includegraphics[scale=0.50]{figures/pressure_c.jpg}
+ \captionof{figure}{Individual digits identified}
+ \label{pressure_b.jpg}
+\end{center}
+
+\begin{center}
+ \includegraphics[scale=0.50]{figures/pressure_d.jpg}
+ \captionof{figure}{Activated segments (green) for a single digit}
+ \label{pressure_c.jpg}
+\end{center}
+
+\section*{Appendix D - Sources of Error}
+
+GROUND LOOOOOOPS!
+
+\section*{Appendix E - Software}
+No really, you don't want to know
\pagebreak
\bibliographystyle{unsrt}
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