Semi-empirical PM3 cluster calculations are used to show that stable,
electrically active NO complexes may exist in silicon. Based on their
relative stability with respect to oxygen and nitrogen pairs, the reta
rdation of thermal double donor formation in the presence of nitrogen
is explained, but an equilibrium concentration much less than that of
NN pairs is predicted. It is also shown that interaction of NO with a
single nitrogen atom creates a bistable NNO defect, while encounter wi
th an oxygen or an NN pair preserves the electrical activity of the NO
centre. The possible role of the NO complex in shallow thermal donor
formation is discussed.