  Point defect complexes in crystalline silicon composed of hydrogen, nitrogen,
and oxygen atoms are studied within density-functional theory (DFT). Ab initio
Random Structure Searching (AIRSS) is used to find low-energy defect
structures. We find new lowest-energy structures for several defects: the
triple-oxygen defect, {3O}, triple oxygen with a nitrogen atom, {N, 3O}, triple
nitrogen with an oxygen atom, {3N,O}, double hydrogen and an oxygen atom,
{2H,O}, double hydrogen and oxygen atoms, {2H,2O} and four
hydrogen/nitrogen/oxygen complexes, {H,N,O}, {2H,N,O}, {H,2N,O} and {H,N,2O}.
We find that some defects form analogous structures when an oxygen atom is
replaced by a NH group, for example, {H,N,2O} and {3O}, and {H,N} and {O}. We
compare defect formation energies obtained using different oxygen chemical
potentials and investigate the relative abundances of the defects.
