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%\title{\bf Characterisation of nanostructured thin films}
%\author{Sam Moore\\ School of Physics, University of Western Australia}
%\date{April 2012}
%\maketitle

\begin{center}
	B.Sc. (Hons) Physics Project \par
	{\bf \Large Thesis} \par
	Samuel Moore \\
	School of Physics, University of Western Australia \\
	April 2012
\end{center}
\section*{Characterisation of Nanostructured Thin Films}
{\bf \emph{Keywords:}} surface plasmons, nanostructures, spectroscopy, metallic-blacks \\
{\bf \emph{Supervisers:}} W/Prof. James Williams (UWA), Prof. Sergey Samarin (UWA) \\


%\tableofcontents

\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 (?)
	\item JA Woolam (for providing replacement alignment detector pins)
	\item CMCA at UWA (for SEM images) (Is there a specific person I should thank?)
\end{itemize}

\section{Introduction}

In this section I will give the overview of research done on metallic-black thin films, and explain the motivation for this project given the conclusions of past research.

\begin{itemize}
	\item {\bf Aim of Project}
	\begin{itemize}
		\item The aim is to employ a range of techniques for characterisation of metallic thin films
		\item In particular, metallic-black films vs ``shiny'' (``bright'' might sound better)
		\item TODO: Find a result (plasmonic behaviour?) so that I can say I was aiming to find it
	\end{itemize}
	\item {\bf Motivation for Project}
	\begin{itemize}
		\item Talk about the surface playing an increasingly important role as semiconductor devices become smaller
		\item Although metallic-black films are well known, they are extremely complicated and difficult to characterise
		\begin{itemize}
			\item Fractal like structure, remarkable 
			\item Mechanism for formation is not well understood
			(Probably due to metallic nanoparticles having insufficient energy to form a regular lattice, due to energy losses through collisions with the high pressure atmosphere... I need to find a reference for this)
		\end{itemize}
		\item Metallic-black films have had numerous applications as good absorbers of optical wavelengths
		\begin{itemize}
			\item High absorbsion coatings for radiometer vanes (Pfund)
			\item Infra-red detectors; due to being almost transparent in the far infra-red
			\item (Very recently; 2011), as scattering centres to increase the efficiency of thin film solar cells (Deep R Panjwani)
			\emph{NOTE: This work was done in an honours thesis, although I believe the supervisor has also published a (much shorter) paper that I may reference if the honours thesis is not a good reference}
		\end{itemize}

		\item 
	\end{itemize}
	\item {\bf Past Research}
	\begin{itemize}
		\item Pfund - First mentions, preparation, optical transmission, resistivity
		\item Harris - Later work, extends Pfund's experiments, introduces theoretical discussion of structure
		\item Some other authors repeat or extend Harris' work. Metallic-black in different atmospheres, etc.
		\item Modern research - Tends towards ``artificially'' blackened films, which suppress light reflection through plasmonic effects. 
		\begin{itemize}
			\item The goal is to develop films that exhibit similar effects to metallic-black films, but are simpler to describe theoretically. These films can then be used in applications requiring high absorbsion, as the original films
		\end{itemize}
		\item Can't find much research on plasmons in the ``naturally'' blackened films, except Panjwani
		\emph{NOTE: Panjwani seems to have modelled the black films as semi-spherical nanoparticles, which in the light of other research (and the SEM images) may be inaccurate}
	\end{itemize}
	
\end{itemize}


\section{Overview of Theory}

I will use this section to introduce general concepts of solid state physics. The Experimental Methods section will concentrate on the theory of each method, and how this relates to the overall theory.

\begin{itemize}
	\item {\bf What a nanostructured film is, how it differs from the bulk material}
	\begin{itemize}
		\item The surface of a solid is the interface for physical/chemical interactions with it's surrounding environment
		\item The physical and chemical properties of a material are largely determined by the electron spectra at the surface
		\begin{itemize}
			\item Electron spectra is determined by the lattice potential
			\item Characterised by
			\begin{enumerate}
				\item Band structure of energy states - due to periodic lattice potential
				\item Density of States
			\end{enumerate}
			\item Surface differs from bulk due to
			\begin{enumerate}
				\item Termination of periodic lattice
				\item Adsorbed particles on surface (thin films)
				\item Relocation of lattice sites near the surface
			\end{enumerate}
			\item Band structure for Metal's vs Semi-conductors
			\begin{enumerate}
				\item Metals: 
				\item Semiconductors:
			\end{enumerate}
		\end{itemize}
		
		
	\end{itemize}
	\item {\bf Surface Plasmons}
	\begin{itemize}
		\item A collective oscillation of the electron gas in a metal
		\item Surface plasmons are confined to the surface region; 2 dimensional, differs from bulk plasmons
		\begin{itemize}
			\item In nanostructured materials, plasmons can be localised
		\end{itemize}
		\item Bohms and Ritchie
		\item Past studies at CAMSP and UWA
		\item May be caused due to excitations from
		\begin{enumerate}
			\item Electrons - refer to next section
			\item Photons
			\begin{itemize}
				\item Only light polarised in the plane of the surface can excite plasmons
				\item Need to provide an extra wavevector to ``match'' the momenta of the photon and plasmon
				\item Possibility for rough structure of metallic films to provide this wavevector
				\begin{itemize}
					\item Refer to papers on ``artificially'' blackened films
					\item Similar topic to Nikita's thesis; look at some of his references
				\end{itemize}
			\end{itemize}
		\end{enumerate}
	\end{itemize}
	\item {\bf Interactions between Electrons and Metallic Thin Films}
	\begin{enumerate}
		\item Electron-Surface Interaction
		\begin{itemize}
			\item How an incoming electron interacts with the surface as a whole
			\item Elastic reflection from potential barrier
			\item Phonon vibrations of lattice (quasi-elastic - low energy losses)
		\end{itemize}
		\item Electron-Electron Interaction
		\begin{itemize}
			\item Inelastic scattering processes determined by interaction of primary electron with the electron gas
			\item Low energy interactions (focus of low energy TCS)
			\begin{itemize}
				\item Outer electron transitions between valence and conduction band (result of interaction between primary electron and an individual bound electron)
				\item Plasmon excitation (result of interaction between incoming electron and the electron gas as a whole)
			\end{itemize}
			\item Higher energy interactions (focus of other forms of 2nd Electron Spectroscopy)
			\begin{itemize}
				\item Auger processes due to excitation of inner band electrons
				\item ``True'' secondary electrons; bound electrons given sufficient energy to leave the surface
			\end{itemize}
		\end{itemize}
		\item General structure of secondary electron energy distribution (not investigated by TCS)
		\item Mention that secondary electrons have an angular distribution (not investigated by TCS)
		
	\end{enumerate}
\end{itemize}

\section{Experimental Methods}

Here I will give overviews of each method used in the study, including:

\begin{enumerate}
	\item {\bf Scanning Electron Microscopy}
	\begin{itemize}
		\item Not used directly by me, so I will be very brief
		\item Very useful for understanding the structural differences between metallic-black and metallic-bright films
	\end{itemize}
	
	\item {\bf Total Current Spectroscopy}
	\begin{itemize}
		\item Experimental setup
		\begin{itemize}
			\item Refer to appendices for detailed description of control circuit and electron optics
		\end{itemize}
		\item Formation of the signal $S(E) = \der{I(E)}{E}$, and relation to theory of {\bf Interactions between Electrons and Metallic Thin Films}

	\end{itemize}
	\item {\bf Ellipsometry}
	\begin{itemize}
		\item Optical technique commonly used for characterising thin films
		\begin{itemize}
			\item Measures change in polarisation of light
			\begin{enumerate}
				\item 
			\end{enumerate}
			
		\end{itemize}

		\item Sensitive to optical properties of materials; can be used to determine optical constants of a sample
		\begin{itemize}
			\item Can relate the Ellipsometric parameters $\psi$ and $\Delta$ to the optical constants ($n$ or $\epsilon$) of the
		\end{itemize}
		\item {\bf Variable Angle Spectroscopic Ellipsometry}
		\begin{enumerate}
			\item Aquires a large amount of data automatically over different $\lambda$ and $\theta$
			\item Using fitting algorithms, can construct multi-layered model of the surface, and determine characteristics of each layer based on known information
			\item Can be complemented by reflection and transmission spectroscopy, performed with the same instrument
		\end{enumerate}
	\end{itemize}

	\item {\bf Optical Spectroscopy}
	\begin{itemize}
		\item Brief section, may not include if the other sections are sufficient
		\item May combine with Ellipsometry section, since the Ellipsometer is used for these measurements
		\begin{itemize}
			\item The important concepts will already have been discussed in the Ellipsometry section
			\item Also used OceanOptics spectrometer early in the year, but repeated the same measurements using the Ellipsometer
			\item Will need to review results before deciding whether to include or not
		\end{itemize}
	\end{itemize}
\end{enumerate}

\section{Results and Discussion}

In this section I will discuss the results from each of the experimental methods described above, in order.

\begin{enumerate}
	\item {\bf SEM}
	\begin{itemize}
		\item Images prepared by CMCA
		\begin{itemize}
			\item The secondary electron current is imaged. The secondary electron current due to an Si substrate alone is substracted.
			\item This gives an idea of the spatial distribution of the density of Au deposited on the Si substrate
			\item TODO: Learn exactly how they are related... can I assume intensity $\propto$ density?
		\end{itemize}
		\item Discuss the clear difference in structure
		\begin{itemize}
			\item Well defined regions (metallic nanoparticles) vs ``smoke'' like connected strands
		\end{itemize}
		\item Perform image fourier transforms
		\begin{itemize}
			\item Au-Bright shows a elliptical distribution of low frequencies; indicates a preferred orientation for the Au
			\item Au-Black shows a wider, circular distribution of low frequencies, and significantly larger high frequencies
			\item Phase plots appear random except for some sharp lines near the centre of the FFT image (not sure how to interpret yet)
			\item Validity of the transforms?
			\begin{itemize}
				\item Error is introduced due to discontinuities in the periodic extension of the image
				\item However, since the image is a region taken out of a periodically structured surface, these should be small
				\item No window has been applied
			\end{itemize}
			\item Relation of image fourier transforms to theory?
			\begin{itemize}
				\item I would love to be able to do this, but may not have time to understand how to do it
				\item Can at least give a numerical approximation of fourier transform of density distribution of electron gas
				\item Surely this can be used somehow? Approximate the structure factor of system?
				\item Predict plasmonic behaviour?
			\end{itemize}
		\end{itemize}
	\end{itemize}

	\item {\bf TCS}
	\begin{itemize}
		\item {\bf TCS of Stainless Steel}
		\begin{itemize}
			\item Establishes the location of the primary electron peak for Stainless steel
			\item Useful because it allowed me to tell when the electron gun was focused so that the beam struck both the sample holder and the sample of interest
			\item Appears to change over time; will discuss this behaviour below in relation to Si
		\end{itemize}

		\item {\bf TCS of Si substrate}
		\begin{itemize}
			\item Changing of TCS vs Time
			\item I am pretty sure it is not due to a ``mistake'' in the electron gun circuit
			\item Possibly due to adsorbsion of oxide layer on the surface
			\item I can show that depositing a thin layer of Au has the effect of ``resetting'' the TCS, which then begins to evolve over time.

			\item I can also show that a sudden jump in $E$ causes $I$ to tend towards an assymptote
			\begin{itemize}
				\item One reason why I changed to use $E$ steps of $0.4$V per second, instead of $2$ V per 
				\item I probably won't discuss this in the thesis; maybe in an appendix
			\end{itemize}
		\end{itemize}

		\item {\bf TCS of Au}
		\begin{itemize}
			
		\end{itemize}

	\end{itemize}

	\item {\bf Ellipsometry}
	\begin{itemize}
		\item {\bf General Application to thin films}
		\begin{itemize}
			\item {\bf Jeremy's Sample - Permalloy}
			\begin{itemize}
				\item Already have a good idea of the thicknesses
				\item Can construct a model which gives good agreement with these thicknesses
				\item Lorentz Oscillator model for the Permalloy
				\item NOTE: I need to repeat this modelling procedure with more care
			\end{itemize}

			\item {\bf Mikhail's Sample}
			\begin{itemize}
				\item White Ni compared to Normal Ni
			\end{itemize}
		\end{itemize}

		\item {\bf Metallic-Black Films}
		\begin{itemize}
			\item Difficult to measure Au-black directly
			\item Measured Au on Au-black on Si
			\begin{itemize}

			\end{itemize}

			\item Compare with Au-bright on Si
		\end{itemize}
	\end{itemize}

	\item {\bf Optical Spectroscopy}
	\begin{itemize}
		\item Need to review data before deciding whether to include this section
		\item Mostly conducted transmission spectroscopy experiments
		\begin{itemize}
			\item Transmission spectra of Au-Black agrees qualitatively with published spectra by Pfund and Harris
			\begin{itemize}
				\item However, the range of my experiments is extremely small (visible wavelengths to short infra-red) compared to Harris (visible to extremely far infra-red)
			\end{itemize}
			\item Can show a difference between Au-Bright on Glass and Au-Black on Glass transmission spectra
			\item Also have some transmission spectra of Ag, agree with expected Ag transmission spectra
		\end{itemize}
	\end{itemize}
\end{enumerate}

\section{Conclusion}

In this section, I will hopefully find something intelligent to say about my results

\section*{Appendices}

Mostly to do with the practical side of setting up the TCS experiment, and therefore (sadly) of little interest to the markers.

\begin{enumerate}
	\item The TCS experiment in more detail
	\begin{enumerate}
		\item Electron Gun control circuit
		\item Electron Optics - focusing the gun
		\begin{itemize}
			\item This is extremely important for optimising the resolution of TCS
			\item Include results of 2D simulation, but for qualitative purposes only (not actually used to focus the real gun)
		\end{itemize}
		\item ADC/DAC Card for control of $E$ and measurement of $I(E)$ in TCS
	\end{enumerate}

	\item Monitoring of the Vacuum Chamber pressure
	\begin{itemize}
		\item This one will be short (I pointed a webcam at the pressure gauge, and wrote some software)
		\item Include graphs of pressure over time
	\end{itemize}

	\item Theory
	\begin{itemize}
		\item I may use this to put more detailed theory if the {\bf Overview of Theory} section is too long
		\item Then again, I may not have enough detailed theory to need this.
	\end{itemize}

	\item Software
	\begin{itemize}
		\item I will probably just make all software available on my website and link to it
		\item The software is not really written with ``someone else may want to use this'' in mind
		\item The software includes:
		\begin{enumerate}
			\it
		\end{enumerate}
	\end{itemize}
	
\end{enumerate}

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