Oxygen defects in Silicon
- Why do we study Oxygen in Silicon?
- What are oxygen defects made of?
- How do we study them?
- 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:
In combination these can also form a range of other defects. Of
particular recent interest is the interstitial oxygen pair,
where two oxygen atoms sit on neighbouring bond centres. A pair such
as this is believed to be able to diffuse much more rapidly than
single interstitial oxygens, and could be responsible for rapid
thermal donor formation at seemingly quite low temperatures.
- 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
- 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
- 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
- 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.