The Role of Vacancies in Enhancing Oxygen Diffusion in Silicon
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THE ROLE OF VACANCIES IN ENHANCING OXYGEN DIFFUSION IN SILICON A.S. OATES*+, R.C. NEWMAN*, J.M. TUCKER*, G. DAVIES** and E.C. LIGHTOWLERS** * J.J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading RG6 2AF, U.K. ** Physics Department, King's College, Strand, London WC2R 2LS, U.K. + Current address: A.T. & T. Bell Laboratories, Allentown, Penn.18103. ABSTRACT Measurements of optical bands in irradiated Si are combined with numerical modelling of the radiation induced reactions. No evidence is found for appreciable interaction of self-interstitials with0 0 atoms for irradiations carried out at temperatures between 25 and 500 C. The reduction during electron irradiation of stress-induced dichroism in the 9pm oxygen band is shown to occur by sequential capture of a vacancy and a selfinterstitial at the oxygen for irradiations carried out between 25 and 280 0 C. At higher temperatures repetitive capture and release of vacancies at oxygen atoms appears to dominate in the oxygen migration process. INTRODUCTION
Thermal diffusion of interstitial oxygen in silicon occurs with a single activation enthalpy H = 2.50 ± 0.05 eV over the temperature range 4000 to 1240'C [eg.1-3]. Diffusion can also occur at considerably enhanced rates in some "as-received" Si [4] or in Si with metallic (Cu or Fe) contamination [5]. Enhanced migration is important concerning thermal donor formation. Most current theories of thermal donors assume that they are aggregates of 0 atoms. If so, the diffusion of the 0 atoms must be enhanced over its thermal value to produce the assumed rate of clustering. Mechanisms proposed for this enhancement are: the formation and diffusion of di-oxygen molecules; oxygen-vacancy interactions; and interactions between the self-interstitials and 0 atoms. (For a review see Bourret[6]). Two of these processes involve the interaction of 0 with native defects (vacancies or self-interstitials). It is important to establish if these reactions are possible. Recently, Newman [7] has suggested that the thermal donors are formed from the clusteriig of self-interstitials, generated by pairing of 0 atoms during normal (non-enhanced) migration. Formation of thermal donors in float-zone Si [eg. 8] possibly supports this suggestion. Any interaction of the self-interstitials with 0 atoms will have an important effect on this mechanism. This paper examines the reactions of vacancies (V) and selfinterstitials (I) with oxygen atoms (0). A controlled way of introducing V's and I's into Si is by radiation damage. Many of the defects produced by room temperature irradiation are at least partially understood. In the following section this knowledge is exploited to present a quantitative model of the radiation-induced reactions. We show that interactions between I and 0 are of negligible importance at room temperature. Next we extend this result to temperatures above the thermal donor formation threshold 0 ('\450 C). Then we show that during room temperature irradiation oxygen enhanced migration can occur through succes
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