Shallow Thermal Donor Defects in Silicon

An ab initio local density functional cluster program, AIMPRO, is used to examine nitrogen related shallow thermal donor defects in silicon. We find the bonding of oxygen with interstitial nitrogen in ${\mathrm{N}}_i$- ${\mathrm{O}}_{2i}$ to be almost “normal,” but the O atoms move slightly out of their bond centered sites causing the deep donor level of ${\mathrm{N}}_i$ to become shallow. The defect has properties consistent with those experimentally observed for shallow thermal donors. We also find that a ${\mathrm{C}}_i\mathrm{H}$- ${\mathrm{O}}_{2i}$ defect has very similar electronic properties, and suggest that shallow thermal donors do not have a unique composition.