Theoretical Physics
Oxygen defects in Silicon

1. Why do we study Oxygen in Silicon?
2. What are oxygen defects made of?
3. How do we study them?
4. What are local vibrational modes?

2. What are oxygen defects made of?

There's a number of basic ingredients which can combine to form various different oxygen / silicon defects. These include:

• Interstitial Oxygen atoms. These are oxygen atoms that sit very close to the centre of a Si-Si bond (with a Si-O-Si bond angle of around 170 degrees). They can easily rotate around this bond. The bond is roughly 30% longer than an ideal Si-Si bond, leading to strain in the Si lattice. Most oxygen in Si occurs in this form.
• Interstitial Silicon atoms. Silicon atoms can also form self-interstitials, either naturally during formation of the lattice, or else by forming a vacancy-self-interstitial pair, by knocking a silicon atom out of its lattice site (this can occur when the lattice is bombarded with ions, for example when deliberately doping it with impurity atoms). They tend to form split interstitials, a structure where the interstitial atom and a lattice Si atom sit either side of a normal lattice site. Si split interstitials prefer to lie along the <110> direction, however <100> orientated interstitials are also very similar in energy.
• Vacancies. Vacancies occur naturally in silicon but can also be introduced through radiation damage, in the same way as silicon self-interstitials. A vacancy is a lattice site where a Si atom is missing, a 'hole' in the lattice. They tend to combine with many other defect types and can diffuse very rapidly through the lattice.
• Substitutional Oxygen. If a single oxygen atom sits in a vacancy it positions itself somewhere between the substitutional Si site and a bond centred site between two of the neighbouring silicons. When negatively charged, this is called the 'A' Centre. It is also possible to put more than one oxygen atom into a vacancy (see our recent paper).
• Other impurities. Several other impurity atoms are known to form complexes with oxygen in silicon. The most important of these are probably
  • Hydrogen. Hydrogen can help to passivate electrically active defects, and can diffuse extremely rapidly through the Si lattice. It is thought to aid oxygen diffusion.
  • Carbon and Nitrogen. These both form interesting complexes with interstitial oxygen, notably square-like structures with a carbon and an oxygen atom on neighbouring interstitial sites. We have recently investigated various nitrogen - oxygen defects, particularly the NNO defect which is a N2 square next to an interstitial oxygen. There is much current controversy as to whether nitrogen-oxygen defects may be the shallow donors which are sometimes observed in Si.