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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}
83 \item Workshop (for producing electron gun mount?)
84 \item Peter Hammond (?)
87 \section{Introduction}
89 \item Waffle about motivation for the project
91 \item Metal-Black films may have application for ... something.
93 \item Radiometer vanes, IR detectors
94 \item Number of applications where high absorbance into IR is required
95 \item These have all been studied before though.
97 \item The electron spectra of metal-blacks have not yet been examined.
98 \item Remarkable difference between Metal-Black films (bad vacuum) and normal metal films (UHV)
100 \item No (detailed/satisfactory) explanation (that I can find...) for difference
102 \item Talk about plasmonic based computing? Moore's law? Applications to thin film solar cells?
105 \item Specific aims of project
107 \item Surface density of states / band structure of Black-Au films using TCS (The main aim)
108 \item Identification of plasmonic effects in Black-Au films (?) (If they even exist!)
110 \item Identify plasmonic effects in Au and Ag films with Ellipsometry (this is fairly simple to do)
112 \item Combination of Ellipsometry and TCS to characterise thin films (not just Black-Au)
114 \item Ie: How can one technique be used to support the other?
117 \item Structure of thesis
120 \section{Overview of Theory}
121 Summarise the literature, refer to past research etc
123 \subsection{Electron Spectra of Solids and Surface}
132 \item Description of the near surface region
134 \item All real solids occupy finite volumes in space.
135 \item The surface of a solid is important because interactions between the solid and its surroundings occur in the near surface region.
136 \item Characterised physically by:
138 \item Termination of periodic crystal lattice
139 \item Violation of geometric order
140 \item Distortion of interatomic distances and hence interaction forces
141 \item There is a transition ``near surface'' region between bulk and surface properties, roughly 5 atomic distances.
143 \item Potential seen by an electron at a surface can differ greatly from the bulk
144 \item $\implies$ the electron spectra of the near surface region differs from the bulk spectra
145 \item Simplest case: Step potential at surface.
149 \item The Electron Spectra
151 \item Electron Spectra describes the energy eigenstates for an electron in a Bulk or Surface potential
152 \item Characterised by
154 \item Energy dispersion $E(\vect{k})$
156 \item Dependence of Energy on electron wave vector
157 \item Obtained theoretically by solving Scrhrodinger's Equation
158 \item For a free electron gas, $E = \frac{\hbar^2 k^2}{2m}$
159 \item Periodic potential in bulk solid leads to band gap structure of $E(\vect{k})$
160 \item Periodic potential $\implies$ E is periodic. Only needs to be defined in first Brillouin zone.
162 \item Density of States $N(E)$
164 \item $N(E) = \frac{\Delta N}{\Delta E} = \frac{1}{4\pi^3}\int_S\left(\der{E}{k}\right)^{-1} dS$
165 \item Integral is in momentum space over the isoenergetic surface of energy $E$
166 \item For a free electron gas, $N(E) = $
173 \item Simplest model: Step potential
174 \item Two major models
176 \item Tamm States: Periodic potential in solid, free space outside, jump at surface
178 \item Energy eigenvalues lie in the forbidden band of the bulk spectra
179 \item Attenuation of eigenvalues from surface to vacuum, oscillation of state within surface
180 \item Max electron density occurs on the crystal surface
182 \item Shockley states: Potential of surface and bulk cells equal
184 \item Corresond to free valences (dangling bonds) at the surface
187 \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.
188 \item These states arise from termination of the lattice; but the surface cells are assumed undistorted
189 \item In reality surface cells are distorted by relaxation and reconstruction of the surface
192 \item Main reference: Komolov "Total Current Spectroscopy"
193 \item "Solid State Physics" textbooks and "Electron Spectroscopy" textbooks
196 \subsection{Plasmonics}
197 I really think I should actually find plasmonic effects before writing too much about them...
199 \item Charge density oscillations
200 \item Surface and bulk plasmons
202 \item Review article from T.W.H Oates et al about using Ellipsometry to characterise plasmonic effects
205 \subsection{Metallic-Black Thin Films}
207 \item How they are made (bad vacuum, in air or a noble gas)
209 \item If made in air, there are usually tungsten oxides present (from filament). Refer to paper by Pfund.
211 \item Structural difference between Black-Au and ``Shiny'' (need a better term) Au
213 \item Can include electron microscopy images?
214 \item An actual photograph of a Black-Au film? Not necessary?
216 \item Pfund (earliest publisher, preparation and general properties)
217 \item Louis Harris (most research in 50s and 60s)
219 \item L. Harris mostly did transmission spectroscopy in the far infra red (well beyond the ellipsometer and Ocean Optics spectrometer ranges)
220 \item The really crappy measurements I did with the Ocean Optics spectrometer seem to agree with these measurements
222 \item L. Harris' $\lambda$ has a range of 1nm to $100\mu$m; my measurements are only to $1\mu$m
223 \item Agreement in first $1\mu$m anyway
224 \item I should probably re-do those measurements with a less crappy setup, if I actually want to use them
226 \item Harris related the optical properties to the structure of the film (condensor strands) via the electronic properties
228 \item Plasmonic effects - Deep R. Panjwani (honours thesis)
230 \item Not sure if I can use an honours thesis as a reference.
231 \item Concluded that surface plasmon resonance in Black-Au film on solar cells lead to increase in solar cell efficiency
232 \item Used simulation that modelled Black-Au film as spherical balls to show E field increased by plasmon resonance
234 \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.
236 \item Need to read this reference more thoroughly
240 \section{Experimental Techniques}
242 \subsection{Secondary Electron Spectroscopy}
244 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.
246 \subsubsection{Electron-Surface Interactions}
249 \subsubsection{Methods of Secondary Electron Spectroscopy}
251 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.
253 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
255 Both Energy-resolved and Total Current methods can be performed
257 \subsection{Total Current Secondary Electron Spectroscopy}
259 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.
261 \subsubsection{Electron Optics}
263 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.
267 The full circuit diagram for the electron gun control circuit is shown in Appendix A.
269 \subsubsection{Automatic Data Acquisition}
271 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.
273 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.
275 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.
282 \item Black-Au - 1e-2 mbar vacuum
283 \item ``Shiny'' - 1e-6 / 1e-7
284 \item Current of ~3.5A through W wire filament spot welded onto Ta strips in turn spot welded to Mo posts
285 \item Voltage through filament is ~1 V; quote the power?
286 \item Filament isotropically coats sample with desired material.
287 \item Possibly get a curve of Au thickness estimated with Ellipsometry vs exposure time?
289 \item Probably too much work and too unreliable
290 \item Maybe do it, but only use 2/3 data points
295 \subsection{Electron Spectroscopy}
297 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}.
299 Total Current Spectroscopy is a group of electron secondary
303 \item Total Current Spectroscopy methods measure the total current of secondary electrons as a function of primary electron energy.
304 \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.
307 \item Low energy beam of electrons incident on sample
308 \item Measure slope of resulting I-V curve
309 \item Relate to density of states and electron band structure (Komolov chapter 3.2)
311 \item Description of apparatus
313 \item Electron gun and filament
314 \item Electron gun control box
315 \item ADC/DAC control box and data processing
317 \item Photographs vs Diagrams
319 \item Prefer diagrams to photographs
320 \item Especially for the ADC/DAC control box circuit. Because it looks like a horrible mess.
324 \subsection{Ellipsometry and Transmission Spectroscopy}
326 \item Overview of techniques
327 \item Description of apparatus (use VASE manual)
328 \item Ocean Optics spectrometer? Usable?
329 \item Application of Ellipsometry to finding plasmonic effects
331 \item Surface plasmons = E oscillation parallel to surface $\implies$ only $p$ component of light excites plasmons
335 \section{Experimental Results and Discussion}
336 \subsection{TCS Measurements}
339 \item TCS for Si + Au
340 \item TCS for Si + Black-Au
341 \item Affect of preparation pressure on TCS for Si + Black-Au
342 \item Repeat for Si + Ag and Si + Black-Ag (?)
345 \subsection{Ellipsometric Measurements}
347 \item Ellipsometry to estimate thickness of SiO2 layer on Si
348 \item Estimate thickness of Au/Ag on Si+SiO2
349 \item Ellipsometric measurements of Si+Black-Au/Ag
351 \item Modelling procedures to characterise Black-Au/Ag
353 \item Ellipsometric measurements of Glass+Black-Au/Ag (?)
354 \item Transmission spectra of Glass+Black-Au/Ag from earlier in year (?)
357 \section{Achievements}
359 \item Deposition of thin films of Au and Black-Au in vacuum chamber
360 \item Ellipsometric and spectroscopic measurements on these films
361 \item Repurpose vacuum chamber for sample preparation and TCS experiments
362 \item Designed and built electronics for TCS experiments
364 \item Electron gun control box
367 \item Wrote software for data aquisition and data processing
370 \section{General notes}
373 \item Optimise setup of gun
375 \item Emission current. How much does it vary, why does it vary.
376 \item Why does Is/Ie curve shift with successive sweeps? Does sweep modify sample's surface?
377 \item Is sample holder acceptable? Are ceramic washers accumulating charge?
378 \item How do I tell when the setup is optimised...
379 ``The setup was optimised by looking for an S curve''. Very scientific.
380 \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.
382 \item Obtain TCS spectra for Si that compares well with literature
384 \item How to relate TCS spectrum to $n(E)$ and $E(\vect{k})$
386 \item Prepare Au films, obtain TCS spectra that compares with literature
387 \item Obtain TCS spectra of Black-Au films
388 \item Use results to compare properties of films with results from other methods in the literature
391 \item Oscilloscope measurements of inputs to ADC channels under controlled conditions
393 \item Expected values are +/-3mV due to ADC channel, +/-300mV due to 610B, +/-1mV due to 602
394 \item 610B and 602 will probably be worse because they are ancient
395 \item There is about 200mV of noise between the GND of the ADC box and the electron control box.
396 \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.
398 \item Stupid 50Hz AC noise... how to reduce with filters and/or averaging
400 \item Create circuit diagrams for Electron gun circuit
401 \item Create circuit diagrams for ADC/DAC box
403 \item Simulate behaviour of circuit
404 \item Use of instrumentation amplifier on ADC5 to make off-ground measurements
405 \item Use of low pass filter on ADC5
407 \item Include references to all datasheets, etc
410 \item Base pressure with rotary pump? Was 1e-3 after 30 minutes at start of year, but probably introduced leaks since then
411 \item Lowest pressure achieved with turbo pump is 1.1e-7 mbar as of 25/07.
412 \item Viton gaskets on some seals. Copper on other.
415 \item View window (large, view of sample \& sputtering filaments)
416 \item Rotation manipulator \& sample mount
418 \item Filament flanges 1 (used earlier in year, not anymore) and 2
419 \item Inlet with leak valve (for introducing gases into chamber)
420 \item Vent valve on turbo pump
421 \item Electron gun flange
422 \item View window (small, view of back of electron gun)
427 \section*{Appendix A - Electron Gun Control and Current Measurement Circuit}
429 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}.
431 BLARGH IT IS AN ELECTRON GUN BLARGH
434 \section*{Appendix B - DAC/ADC Box - Hardware}
436 \subsection*{Overview}
438 In order to automate TCS experiments, both Digital to Analogue and Analogue to Digital Convertors were required (DAC and ADC). To satisfy these requirements, a custom DAC/ADC Box was designed and constructed. The box can be controlled by any conventional computer with available RS-232 serial communication (COM) ports. Most modern computers no longer feature COM ports; a commercially available convertor can be used to interface between the box's RS-232 output and a standard Universal Serial Bus (USB) port.
441 The key components of the DAC/ADC box hardware include:
444 \item Microprocessor (AVR Butterfly ATMega169)
445 \item Four Analogue to Digital Converter (ADC) inputs
446 \item Single Digital to Analogue Converter (DAC) output (Microchip MCP4922)
447 \item Analogue electronics for amplification at ADC inputs and DAC outputs
448 \item Seperate power supply circuitry for Digital and Analogue electronics
449 \item RS-232 communications for control by a conventional PC or laptop
452 \subsection*{Microprocessor}
453 The DAC/ADC box has been based upon Atmel's AVR Butterfly; an inexpensive and simple demonstration board for the ATMega169 16 Bit microprocessor. The features of the AVR Butterfly include easily accessible ports for Analogue to Digital Convertor (ADC) inputs and digital input/output, an onboard Universal Asynchronous Reciever/Transmitter (USART) for RS-232 serial communications, and a 6 character Liquid Crystal Display (LCD). The AVR Butterfly can be programmed using a conventional computer over the USART using a RS-232 COM port. For modern computers (which do not usually posess COM ports), a RS-232 to USB converter may be used.
457 Unless otherwise stated, all voltage differences are specified relative to the power supply ground of the AVR Butterfly.
459 \subsection*{ADC Inputs}
461 The AVR Butterfly offers easy access to four of the ATMega169's ADCs through PORTF. Each ADC is capable of measuring voltages of $0 < V_{\text{adc}} < V_{cc}$ with 10 Bit resolution. For measuring voltages outside this range, some circuitry is required between the input voltage and the ADC input. In addition, it is desirable to provide the ADC with some form of input protection against accidental overloading. Figure \ref{fig_ADC_normal} shows the typical input circuit which was used for three of the four available ADCs.
464 For making voltage measurements above $V_{cc}$, a voltage divider allows reduction of the voltage at the ADC. By constructing the voltage divider using a variable resistor, the range of measurable inputs could be manually adjusted.
466 The diodes shown in Figure Figure \ref{fig_ADC_normal} ensure that the ADC is protected from accidental exposure to voltages outside the acceptable range. In normal operation both diodes are off. If $V_{\text{adc}}$ were to become greater than the reference point $V_{cc}$, current would flow between the ADC input and the reference point, acting to reduce $V_{\text{adc}}$ until it reached $V_{cc}$. Similarly, if $V_{\text{adc}}$ fell below ground, current would flow from ground to the ADC input, acting to increase $V_{\text{adc}}$ until it reached ground.
468 The voltage at the ADC input can be related to the input of the voltage divider using Kirchoff's Voltage Law and Ohm's Law:
470 V_{\text{adc}} &= \frac{R_1}{R_1 + R_2} V_{\text{in}}
472 Where $V_{\text{in}}$ is the voltage at the input of the circuit, $R_1$ is a fixed resistor, and $R_2$ is variable resistor.
474 $V_{\text{in}}$ can be therefore be determined from the registered ADC counts by:
476 V_{\text{in}} &= \frac{\text{ADC counts}}{2^{10}} \times \frac{R_1 + R_2}{R_1} V_{cc}
479 \subsubsection*{Differential ADC Input}
481 During the testing of the TCS experimental apparatus, it became desirable to measure the emission current of the electron gun. The electrometer used for this current measurement was capable of producing an analogue output in the range of $0-1V$. However, the negative terminal of this output was not at ground potential, but rather at the same terminal as the negative input terminal. Directly connecting the electrometer output to one of the ADC inputs discussed above would create a short circuit between the initial energy power supply, and ground (refer to Figures \ref{} and \ref{}). Therefore, it was decided to add a differential stage before the input of one of the ADCs.
483 Figure \ref{} shows the modification made to the input for ADC5 on the AVR Butterfly. The original voltage divider and input protection discussed above are still present. The modifications include the addition of an instrumentation amplifier, and low pass filters.
485 The instrumentation amplifier consists of two stages of operational amplifiers (op-amps); input buffers, and a difference amplifier.
486 The difference amplifier can be shown using the ideal op-amp model to produce an output voltage proportional to the difference between its inputs:
489 V_{out} &= \frac{R_2}{R_1} \left(V_{2} - V_{1}\right)
492 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.
494 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.
496 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.
498 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.
500 \subsection*{Power Supplies}
501 Due to the presence of both analogue and digital electronics in the DAC/ADC box, three seperate supply voltages were required:
503 \item Digital logic in the range $3 \to 4.5$V
504 \item Positive op-amp supply in the range $10 \to 15$V
505 \item Negative op-amp supply in the range $-10 \to -15$V
508 Circuitry was designed which allowed two seperate single pole power supplies to be used for Digital logic and the op-amps. A dual 0-30V DC power supply has been used for both digital and analogue circuitry.
510 \subsubsection*{Logic Power Supply}
511 The AVR Butterfly runs off $3V < V_{cc} < 4.5V$ DC. Since $V_{cc}$ was also used as the reference voltage for the ADCs and DAC output, it was desirable that $V_{cc}$ be kept constant, despite the absolute level of the power supply. A $3.3V$ voltage regulator has been used for this purpose. The capacitor further smooths the output by shorting high frequency fluctuations to ground.
513 When the DAC/ADC box was first constructed $V_{cc}$ was supplied by three $1.5V$ batteries. However, due to higher than expected power usage, and the unreliability of the voltage regulator as the input voltage fell below $4V$, inputs for an external power supply were later added.
515 \subsubsection*{Op-amp Power Supply}
516 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.
518 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.
520 To simplify circuit diagrams, op-amps will be drawn with the power supply connections ommitted from this point onwards.
522 \subsection*{DAC Output}
523 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.
525 The ET-Mini DAC can only be powered off $3V$ to $5V$. Using $V_{cc} = 3.3V$ means that the DAC output cannot exceed $V_{cc} = 3.3V$. For TCS, energies of up to $15eV$ are required, so amplification of the DAC output was clearly necessary. A simple non-inverting amplifier with a manually adjustable gain was used to amplify the $3.3V$ DAC output to $10V$. This output was then used to control a laboratory power supply to produce the full range of initial energies.
527 \subsection*{RS-232 Communications}
529 The AVR Butterfly features an onboard USART, which can be used both for programming and communication with the ATMega169 processor. The RS-232 communications requires only three wires; Recieve (RX), Transmit (TX) and a common ground.
531 The requirement that the AVR Butterfly share a common ground with the controlling computer lead to increased noise through ground loops. This is discussed in more detail in Appendix D.
533 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.
536 \bibliographystyle{unsrt}
537 \bibliography{thesis}