Nitrogen in germanium: Identification of the pair defect

Several nitrogen-related centers have been introduced by ion implantation of nitrogen into germanium and studied by infrared-absorption spectroscopy. Two local vibrational modes at 825.3 and 658.6 cm^-1 were especially prominent. Measurements on annealed samples implanted with either pure ¹⁴N or ¹⁵N, or implanted with both isotopes showed these modes to arise from a nitrogen pair defect analogous to one previously suggested to occur in silicon. Based on this and ab initio pseudopotential cluster theory, a model of the pair is proposed which is consistent with the observed infrared absorption. This model is similar to that of the nitrogen pair in silicon and offers an explanation of the low donor efficiency of nitrogen in germanium. Several other nitrogen-related local vibrational modes are also observed in the implanted material.