Oxygen '96

Early Stages of Oxygen Precipitation in Silicon

---

INTERACTION OF POSITRONS WITH VACANCY-OXYGEN COMPLEXES AND OXYGEN CLUSTERS IN Si

M. Fujinami

Advanced Technology Research Laboratories, Nippon Steel Corporation 1618 Ida, Nakahara-ku, Kawasaki 211, JAPAN

Keywords: positron, silicon, oxygen clusters, vacancy.

Positron annihilation spectroscopy is one of the most useful techniques to detect oxygen-related defects as well as vacancy-type defects in Si. In this study variable-energy positron annihilation spectroscopy has been applied to study the behaviour of oxygen-related defects in Si caused by implantation of oxygen ions. It is expected that excess vacancies and interstitial oxygen atoms can interact during the annealing stages and that oxygen-related and extended defects are formed ultimately. The interaction of such defects with positrons may give vital clues as to their composition.

The samples were prepared by implanting 2x10¹⁵ O⁺ ions/cm² at 180 keV into p-type CZ-Si(100) wafers at room temperature. Annealing of the samples from 573 to 10 K for 30 min was carried out in a nitrogen atmosphere. The Nippon Steel Corporation slow positron beam was utilised to measure the Doppler broadening of the annihilation gamma-ray, characterised in terms of the S parameter, with respect to the incident positron energy (S-E data). The S parameter is defined as the ratio of the counts in a central region of the annihilation photopeak to those in the whole of the photopeak. The value of the S parameter increases when the positrons are trapped at vacancy-type defects, since the overlap of the positron density with (low momentum) valence electrons increases. When the positrons are trapped at vacancy-type defects coupled with oxygen atoms, the value of S is strongly dependent on the contribution of electrons in the oxygen atoms.

The S values for the as-implanted sample are much higher than those of the unirradiated one. The multivacancy-oxygen complexes such as V₃O are the primary defect found to be formed by room temperature oxygen implantation and are stable up to 673 K. The first annealing stage (873 K) is strongly dependent on the oxygen implantation profile and hence, the depth in the sample. In the implanted region, the S parameter becomes lower than that for the Si bulk, suggesting the formation of multivacancy-multioxygen complexes. While multivacancy-based defects are still evolved in the proximity of the surface, where the oxygen concentration is low, as the S parameter is large. Upon annealing at 1073 K, high S values are no longer observable, indicating that vacancy-type defects are eliminated near the surface. But the S-E data do not return to that of the unirradiated Si, and new positron trapping sites with very low S are created. The level of S parameter depends on the dose of oxygen in the range from 2x10¹⁴ /cm² to 2x10¹⁵ /cm². These results suggest that these defects are due to the oxygen clusters which the multivacancy-multioxygen complexes are transformed into. From the ratio of the density of oxygen clusters to the average atomic oxygen concentration, the mean number of oxygen atoms contained in the oxygen clusters is estimated to be several tens of atoms. It is clearly proven that positrons are a sensitive probe to detect multivacancy-multioxygen complexes and oxygen clusters, whose size is not observable by transmission electron microscopy.

Last modified: Mon Feb 19 12:11:15 GMT 1996 JG