Oxygen
in Silicon is an unintentional impurity arising from the dissolution of the
crucible during growth.
Less
dissolution of the crucible occurs as the melt level is lowered in the
crucible, and thus less oxygen impurity is available for incorporation. Other
factors that determine the level and distribution of oxygen in the crystal are
rotation speeds, ambient partial pressure, and free melt surface.
A
novel method to reduce crucible erosion is to suppress thermal convection
currents by applying a magnetic field to the melt.
High
levels of impurity doping also affect the level of oxygen in the melt. High
levels of boron doping tend to enhance the dissolution rate of the silica in
crucible and increase the oxygen level while antimony doping reduces the oxygen
level in the crystal.
Effects of oxygen impurity are as
follows:
1.
Donor FormationIn the crystal as grown, over 95% of the oxygen atoms occupy
interstitials lattice sites. The remainder of the oxygen polymerizes into
complexes, such as, SiO4. This configuration acts as a donor and
changes the resistivity of the crystal caused by intentional doping.
2.
Oxygen in
interstitial lattice sites also acts to increase the yield strength of silicon.
This beneficial effect increases with concentration until the oxygen begins to
precipitate.
3.
Defect generation by
oxygen precipitation when the oxygen concentration exceeds a threshold value of
about 6.4 X 1017 atoms/cm3.
4.
Point defects are
involved in the nucleation process of the oxygen precipitates. The precipitates
represent a SiO2 phase. A volume mismatch occurs as the precipitates
grown in size. A variety of defects including stacking faults are associated
with precipitate formation These defects attract fast-diffusing metallic
species which give rise to large junction leakage currents.[2]
Carbon Impurity
Carbon
is introduced from graphite parts in the furnace and occupies substitutional
lattice site in silicon. Its presence is undesirable because it aids the
formation of defects.