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?
1. Why do we study Oxygen in Silicon?
The problem of oxygen impurities in silicon is one that has plagued
the semiconductor industry for decades. Oxygen is the most common
impurity in silicon, and can be introduced in many different ways. At
high temperatures it forms aggregates or clusters within the silicon,
as well as interstitial dislocation loops. These all disrupt the
lattice and lead to defect states in the silicon. At lower
temperatures oxygen can still be a problem, for example through the
formation of thermal donors which occur when silicon is
annealed at temperatures of around 450 degrees celsius. These defects
are electrically active (double donors), and act as electron / hole
recombination centres, lowering the mobility, etc. For this reason, it
is essential to understand exactly how oxygen behaves in silicon. A
full understanding of the bonding and structure, as well as the
diffusion and kinetics of oxygen, will allow silicon manufacturers to
optimise their manufacturing processes in order to minimise the oxygen
problem. This in turn should lead to faster, cheaper, and more
efficient microchip technology.
Despite the large amount of research that has been conducted in this
area, remarkably little is known about the microscopic behaviour of
these defects. Thermal donors were first discovered over 40 years ago,
yet there is still a lot of debate about their structure, with no firm
conclusions yet. Indeed, there is even controversy about the number of
atoms that they contain.