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57 %\title{\bf Characterisation of nanostructured thin films}
58 %\author{Sam Moore\\ School of Physics, University of Western Australia}
63 B.Sc. (Hons) Physics Project \par
64 {\bf \Large Thesis} \par
66 School of Physics, University of Western Australia \\
69 \section*{Characterisation of Nanostructured Thin Films}
70 {\bf \emph{Keywords:}} surface plasmons, nanostructures, spectroscopy, metallic-blacks \\
71 {\bf \emph{Supervisers:}} W/Prof. James Williams (UWA), Prof. Sergey Samarin (UWA) \\
76 \section*{Acknowledgements}
82 \item Workshop (for producing electron gun mount?)
83 \item Peter Hammond (?)
84 \item JA Woolam (for providing replacement alignment detector pins)
85 \item CMCA at UWA (for SEM images) (Is there a specific person I should thank?)
88 \section{Introduction}
90 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.
93 \item {\bf Aim of Project}
95 \item The aim is to employ a range of techniques for characterisation of metallic thin films
96 \item In particular, metallic-black films vs ``shiny'' (``bright'' might sound better)
97 \item TODO: Find a result (plasmonic behaviour?) so that I can say I was aiming to find it
99 \item {\bf Motivation for Project}
101 \item Talk about the surface playing an increasingly important role as semiconductor devices become smaller
102 \item Although metallic-black films are well known, they are extremely complicated and difficult to characterise
104 \item Fractal like structure, remarkable
105 \item Mechanism for formation is not well understood
106 (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)
108 \item Metallic-black films have had numerous applications as good absorbers of optical wavelengths
110 \item High absorbsion coatings for radiometer vanes (Pfund)
111 \item Infra-red detectors; due to being almost transparent in the far infra-red
112 \item (Very recently; 2011), as scattering centres to increase the efficiency of thin film solar cells (Deep R Panjwani)
113 \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}
118 \item {\bf Past Research}
120 \item Pfund - First mentions, preparation, optical transmission, resistivity
121 \item Harris - Later work, extends Pfund's experiments, introduces theoretical discussion of structure
122 \item Some other authors repeat or extend Harris' work. Metallic-black in different atmospheres, etc.
123 \item Modern research - Tends towards ``artificially'' blackened films, which suppress light reflection through plasmonic effects.
125 \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
127 \item Can't find much research on plasmons in the ``naturally'' blackened films, except Panjwani
128 \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}
134 \section{Overview of Theory}
136 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.
139 \item {\bf What a nanostructured film is, how it differs from the bulk material}
141 \item The surface of a solid is the interface for physical/chemical interactions with it's surrounding environment
142 \item The physical and chemical properties of a material are largely determined by the electron spectra at the surface
144 \item Electron spectra is determined by the lattice potential
145 \item Characterised by
147 \item Band structure of energy states - due to periodic lattice potential
148 \item Density of States
150 \item Surface differs from bulk due to
152 \item Termination of periodic lattice
153 \item Adsorbed particles on surface (thin films)
154 \item Relocation of lattice sites near the surface
156 \item Band structure for Metal's vs Semi-conductors
159 \item Semiconductors:
165 \item {\bf Surface Plasmons}
167 \item A collective oscillation of the electron gas in a metal
168 \item Surface plasmons are confined to the surface region; 2 dimensional, differs from bulk plasmons
170 \item In nanostructured materials, plasmons can be localised
172 \item Bohms and Ritchie
173 \item Past studies at CAMSP and UWA
174 \item May be caused due to excitations from
176 \item Electrons - refer to next section
179 \item Only light polarised in the plane of the surface can excite plasmons
180 \item Need to provide an extra wavevector to ``match'' the momenta of the photon and plasmon
181 \item Possibility for rough structure of metallic films to provide this wavevector
183 \item Refer to papers on ``artificially'' blackened films
184 \item Similar topic to Nikita's thesis; look at some of his references
189 \item {\bf Interactions between Electrons and Metallic Thin Films}
191 \item Electron-Surface Interaction
193 \item How an incoming electron interacts with the surface as a whole
194 \item Elastic reflection from potential barrier
195 \item Phonon vibrations of lattice (quasi-elastic - low energy losses)
197 \item Electron-Electron Interaction
199 \item Inelastic scattering processes determined by interaction of primary electron with the electron gas
200 \item Low energy interactions (focus of low energy TCS)
202 \item Outer electron transitions between valence and conduction band (result of interaction between primary electron and an individual bound electron)
203 \item Plasmon excitation (result of interaction between incoming electron and the electron gas as a whole)
205 \item Higher energy interactions (focus of other forms of 2nd Electron Spectroscopy)
207 \item Auger processes due to excitation of inner band electrons
208 \item ``True'' secondary electrons; bound electrons given sufficient energy to leave the surface
211 \item General structure of secondary electron energy distribution (not investigated by TCS)
212 \item Mention that secondary electrons have an angular distribution (not investigated by TCS)
217 \section{Experimental Methods}
219 Here I will give overviews of each method used in the study, including:
222 \item {\bf Scanning Electron Microscopy}
224 \item Not used directly by me, so I will be very brief
225 \item Very useful for understanding the structural differences between metallic-black and metallic-bright films
228 \item {\bf Total Current Spectroscopy}
230 \item Experimental setup
232 \item Refer to appendices for detailed description of control circuit and electron optics
234 \item Formation of the signal $S(E) = \der{I(E)}{E}$, and relation to theory of {\bf Interactions between Electrons and Metallic Thin Films}
237 \item {\bf Ellipsometry}
239 \item Optical technique commonly used for characterising thin films
241 \item Measures change in polarisation of light
248 \item Sensitive to optical properties of materials; can be used to determine optical constants of a sample
250 \item Can relate the Ellipsometric parameters $\psi$ and $\Delta$ to the optical constants ($n$ or $\epsilon$) of the
252 \item {\bf Variable Angle Spectroscopic Ellipsometry}
254 \item Aquires a large amount of data automatically over different $\lambda$ and $\theta$
255 \item Using fitting algorithms, can construct multi-layered model of the surface, and determine characteristics of each layer based on known information
256 \item Can be complemented by reflection and transmission spectroscopy, performed with the same instrument
260 \item {\bf Optical Spectroscopy}
262 \item Brief section, may not include if the other sections are sufficient
263 \item May combine with Ellipsometry section, since the Ellipsometer is used for these measurements
265 \item The important concepts will already have been discussed in the Ellipsometry section
266 \item Also used OceanOptics spectrometer early in the year, but repeated the same measurements using the Ellipsometer
267 \item Will need to review results before deciding whether to include or not
272 \section{Results and Discussion}
274 In this section I will discuss the results from each of the experimental methods described above, in order.
279 \item Images prepared by CMCA
281 \item The secondary electron current is imaged. The secondary electron current due to an Si substrate alone is substracted.
282 \item This gives an idea of the spatial distribution of the density of Au deposited on the Si substrate
283 \item TODO: Learn exactly how they are related... can I assume intensity $\propto$ density?
285 \item Discuss the clear difference in structure
287 \item Well defined regions (metallic nanoparticles) vs ``smoke'' like connected strands
289 \item Perform image fourier transforms
291 \item Au-Bright shows a elliptical distribution of low frequencies; indicates a preferred orientation for the Au
292 \item Au-Black shows a wider, circular distribution of low frequencies, and significantly larger high frequencies
293 \item Phase plots appear random except for some sharp lines near the centre of the FFT image (not sure how to interpret yet)
294 \item Validity of the transforms?
296 \item Error is introduced due to discontinuities in the periodic extension of the image
297 \item However, since the image is a region taken out of a periodically structured surface, these should be small
298 \item No window has been applied
300 \item Relation of image fourier transforms to theory?
302 \item I would love to be able to do this, but may not have time to understand how to do it
303 \item Can at least give a numerical approximation of fourier transform of density distribution of electron gas
304 \item Surely this can be used somehow? Approximate the structure factor of system?
305 \item Predict plasmonic behaviour?
312 \item {\bf TCS of Stainless Steel}
314 \item Establishes the location of the primary electron peak for Stainless steel
315 \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
316 \item Appears to change over time; will discuss this behaviour below in relation to Si
319 \item {\bf TCS of Si substrate}
321 \item Changing of TCS vs Time
322 \item I am pretty sure it is not due to a ``mistake'' in the electron gun circuit
323 \item Possibly due to adsorbsion of oxide layer on the surface
324 \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.
326 \item I can also show that a sudden jump in $E$ causes $I$ to tend towards an assymptote
328 \item One reason why I changed to use $E$ steps of $0.4$V per second, instead of $2$ V per
329 \item I probably won't discuss this in the thesis; maybe in an appendix
333 \item {\bf TCS of Au}
340 \item {\bf Ellipsometry}
342 \item {\bf General Application to thin films}
344 \item {\bf Jeremy's Sample - Permalloy}
346 \item Already have a good idea of the thicknesses
347 \item Can construct a model which gives good agreement with these thicknesses
348 \item Lorentz Oscillator model for the Permalloy
349 \item NOTE: I need to repeat this modelling procedure with more care
352 \item {\bf Mikhail's Sample}
354 \item White Ni compared to Normal Ni
358 \item {\bf Metallic-Black Films}
360 \item Difficult to measure Au-black directly
361 \item Measured Au on Au-black on Si
366 \item Compare with Au-bright on Si
370 \item {\bf Optical Spectroscopy}
372 \item Need to review data before deciding whether to include this section
373 \item Mostly conducted transmission spectroscopy experiments
375 \item Transmission spectra of Au-Black agrees qualitatively with published spectra by Pfund and Harris
377 \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)
379 \item Can show a difference between Au-Bright on Glass and Au-Black on Glass transmission spectra
380 \item Also have some transmission spectra of Ag, agree with expected Ag transmission spectra
387 In this section, I will hopefully find something intelligent to say about my results
389 \section*{Appendices}
391 Mostly to do with the practical side of setting up the TCS experiment, and therefore (sadly) of little interest to the markers.
394 \item The TCS experiment in more detail
396 \item Electron Gun control circuit
397 \item Electron Optics - focusing the gun
399 \item This is extremely important for optimising the resolution of TCS
400 \item Include results of 2D simulation, but for qualitative purposes only (not actually used to focus the real gun)
402 \item ADC/DAC Card for control of $E$ and measurement of $I(E)$ in TCS
405 \item Monitoring of the Vacuum Chamber pressure
407 \item This one will be short (I pointed a webcam at the pressure gauge, and wrote some software)
408 \item Include graphs of pressure over time
413 \item I may use this to put more detailed theory if the {\bf Overview of Theory} section is too long
414 \item Then again, I may not have enough detailed theory to need this.
419 \item I will probably just make all software available on my website and link to it
420 \item The software is not really written with ``someone else may want to use this'' in mind
421 \item The software includes:
423 \item Atmel AVR Butterfly
430 %\bibliographystyle{unsrt}
431 %\bibliography{thesis}