+\begin{center}
+ \includegraphics[scale=0.40]{figures/se_dist.pdf}
+ \captionof{figure}{{\bf Model of Secondary Electron Distribution}}
+ \label{se_dist.pdf}
+\end{center}
+
+Figure \ref{se_dist.pdf} shows the general shape of the secondary electron distribution 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. The thick curve shows
+
+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 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.
+
+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. 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 useful for characterisation of the electron structure. Total Current methods are also simpler to realise experimentally, as they do not require energy analysers, and current measurement may be performed external to the vacuum chamber.
+
+
+\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.
+
+
+A major advantage of Total Current Spectroscopy methods is the relative simplicity of the experimental setup. Because energy resolution of secondary electrons is not required, current measurement can be performed external to the vacuum chamber, using a conventional ammeter.
+
+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:
+
+\begin{align*}
+ I_t &= I_{\text{total}} - I_r
+\end{align*}
+
+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.
+
+In this case, differentiating with respect to $E$:
+\begin{align*}
+ \der{I_t}{E} &= - \der{I_r}{E}
+\end{align*}
+
+Figure \ref{} is a block diagram of the experimental setup including measurement and control systems external to the vacuum chamber.
+
+\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}
+
+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.
+
+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.
+
+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.
+
+
+\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.
+
+\subsubsection{Description of the Polarisation state of Light}
+
+
+
+
+\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
+
+
+A Variable Angle Spectroscopic Ellipsometer at CAMSP has been used to perform a variety of measurements on metallic thin films.,
+
+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.