Oxygen and Carbon impurities in Silicon

 

 

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.