Oxygen-related thermal donors in silicon: A new structural and kinetic model

Abstract

A structural model for the oxygen-related thermal donors produced at moderate temperatures (<500 °C) is presented, where electrical activity commences with clusters containing five or more oxygen atoms and arises from a silicon atom at the center of the cluster. The donor activity of a cluster is terminated upon the ejection of this central silicon atom in order to bring about stress relaxation. A large number of electrically active donor species are predicted, differing only in the number of oxygen atoms each species contains. This structural model satisfies the stringent symmetry requirements of electron paramagnetic resonance (EPR), explains the lack of hyperfine interaction with Si29 or O17 nuclei and the large multiplicity in the number of donor species observed by infrared spectroscopy and EPR. The proposed clusters are shown to be embryonic forms of the much larger rodlike defects examined in detail by electron microscopy. Chemical reaction theory is used to deduce the kinetic consequences of the structural model. Both approximate analytic and numerical solutions of the resulting coupled, nonlinear equations are presented. The numerical results provide a good description of the detailed infrared kinetic data on the growth of the individual donor species and, in addition, reproduce the well-known average kinetic data from resistivity measurements. In order to explain the dependence of the oxgyen diffusivity on the sample thermal history and the well-known diffusivity enhancement, a highly mobile oxygen-silicon interstitial complex is proposed, which provides a satisfactory description of the results of dichroic recovery experiments. It is shown, however, that the values of diffusivity deduced from donor formation studies are, in addition, strongly influenced by inhomogeneities in the oxygen distribution as well as by the presence of small, electrically inactive oxygen clusters prior to the anneal. It seems necessary to consider all these possibilities to explain the experimental data.