A brief introduction to defect physics

 
“Perfection has one grave defect : it is apt to be dull”
William Somerset Maugham

Dislocations in metalThe behaviour of materials is often dominated, much like people, by their defects.  Early theoretical predictions suggested metals should be many times stronger than they actually are, and arguments raged for some time as to whether the calculations were flawed or not.  The paradox was only resolved in 1935 when it was realised that real metals are not perfect crystals but instead are riddled with dislocations, defective ripples in the crystal lattice that allow the material to bend and flex in the way that metals do.

Czochralski Silicon GrowthHowever defects don’t only weaken materials.  Commercial silicon used in microchips is grown from large vats of liquid silicon, but the silicon attacks and gradually dissolves the silica vats.  Hence the silicon produced contains lots of dissolved oxygen impurities.  These impurities can scatter electrons and generally detract from the electrical properties of ‘perfect’ silicon, but they also contribute something, namely strength – you can crumble ‘perfect’ silicon between your fingers.

And sometimes, defects might be just what you need.  Components on chips such as diodes and transistors are formed by doping the silicon to introduce a local excess or reduction in electrons – doping is just the deliberate introduction of impurities with the electrical properties that you require.

“In all science, error precedes truth, and it is better it should go first than last”
Hugh Walpole

Defect physics is one area of science where Hugh gets it back to front!  Early modelling of new materials generally concentrates on a ‘perfect’ model of the material, simply because that’s the easiest place to start.  It’s only later in the day that the defects and imperfections get a look in, yet they are often the things that control the material properties.

Early modelling of the exciting new nanostructures emerging in carbon science largely focussed on the properties of ideal carbon tubes and cages.  It’s only more recently that we’ve begun to examine the effect of defects in the lattice such as missing atoms, unusual bonding, and even deliberate doping with impurity atoms.  These calculations are revealing strange and unexpected results.  It is known experimentally that most of these nanostructures are riddled with defects, as we now have sufficiently powerful microscopes that we can actually see them.  Now is the time for theory to catch up and study these defects, so that we can not only learn to live with them, but also learn to use them to their best advantage.


 
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Last modified November 7, 2006