I've tried to make this simple for newcomers. Work with me and don't be terrified. ;-)
In my PhD research I studied the surfaces of III-V semiconductors. Most III-V semiconductors take the form of cubic crystals, made up of regularly alternating atoms (like table salt, although III-Vs make better chips than condiments). The goal of my research was to determine the structure of the top few layers of atoms on the surface of these crystals.
Deep inside a cubic crystal (in the bulk), each atom is completely surrounded by other atoms. Like someone squashed in the middle of a festival crowd, each atom experiences similar forces acting from all sides. Because every atom experiences similar forces, they all behave the same way, and that's why they tend to line up to form a nearly perfect regular crystal structure.
However, atoms at surfaces behave differently from atoms deep within a crystal. At the top surface, the atoms don't have forces acting from above. To stick with the festival analogy, it's like the people at the back of a crowd who are relatively free to wander off. Without the balancing forces from atoms above them, the surface atoms naturally tend to re-organize themselves or reconstruct. They tend to lie in regular patterns on top of the surface. To stretch the analogy, imagine synchronised swimmers at our festival, who decide to crowd-surf - staying in a pattern together above the crowd below. Understanding these reconstructions is useful if you want to design small-scale electronic and optical devices. It's the atomic structure that fixes the physical (and so the electronic) properties of the material.
There are many reconstructions which are relatively stable. The actual pattern depends on how they are created (for instance you often see different reconstructions which are stable at different temperatures). Most reconstructions have their structure repeating at simple multiples of the bulk crystal pattern (or unit cell). You see these described by shorthands such as 4x4 or 1x3 (where the pattern repeat is three times less frequent in one direction than the other). However, whilst the dimensions may be simple multiples of the bulk structure, the actual arrangement of atoms within many of these reconstructions is unknown or only partly understood. You may be able to see that there is a pattern, but not what makes it up.
My work used surface science analytical techniques which are sensitive only to the top few atomic layers of crystals, such as low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). These techniques require an ultra-high vacuum (UHV) of around one millionth of a millionth of an atmosphere (give or take) to keep the surfaces free from contamination, and to ensure that the electrons and ion beams used can move without being affected by the atoms of the air itself.
However, most of my work was performed using two separate UHV systems to perform ion-scattering spectroscopy. One was the CAICISS system (pronounced kye-siss), of the Surface Physics group in the Department of Physics at the University of Warwick. CAICISS is a low-energy ion-scattering spectroscopic technique, described in detail elsewhere. The other main system is at the MEIS facility at CCLRC Daresbury Labs, the UK's central facility for medium-energy ion-scattering spectroscopy. My work was funded by a studentship from the EPSRC.
I was admitted to the degree of Doctor of Philosophy in Physics, of the University of Warwick, on 12/Jan/2001. My doctoral thesis is available for download in PDF format (screen-resolution, 3.42M). If you use it, please let me know - I'd be interested to hear if it proves valuable to future generations!
Since then, I gained the professional qualification of Chartered Physicist,
alongside full membership of the Institute of Physics. So, I'm now:
Dr Nick Kaijaks, CSci CPhys MInstP
Woo-hoo!
VEGASmagic is a program for converting files produced by the VEGAS simulation code for medium energy ion scattering spectroscopy (MEIS). The program runs only on 32-bit Windows systems (95/98/ME/NT/2000). It was developed as part of my doctoral work.
It extracts atomic position information from VEGAS input, output and crystal files, and writes new crystal files. It can also read and write Tripos' Alchemy and Brookhaven Data Bank format (PDB) files. The latter ability is particularly useful as it enables VEGAS models to be viewed in macromolecular visualization software, such as RasMol.
The program is provided as is. Full (but simple) conditions
are described within the zipfile.
Version 1.21a, 26/Mar/2000, 226K
As much as anything, the following are just useful links I have discovered in my travels on the Web.