Measurements of Enhanced Oxygen Diffusion in Silicon During Thermal Donor Formation: New Evidence for Possible Mechanism
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MEASUREMENTS OF ENHANCED OXYGEN DIFFUSION IN SILICON DURING THERMAL DONOR FORMATION: NEW EVIDENCE FOR POSSIBLE MECHANISMS A.R. BROWN, R. MURRAY, R.C. NEWMAN AND J.H. TUCKER Interdisciplinary Research Centre for Semiconductor Materials, The Blacket Laboratory, Imperial College of Science, Prince Consort Road, London, SW7 2BZ, U.K. ABSTRACT Czochralski silicon has been heated in a H-plasma at temperatures in the range 300-4500C, and compared with furnace annealed material. Plasma treatments produce enhanced rates of oxygen diffusion jumps, loss of oxygen from solution and tormation of thermal donor centres. The available evidence indicates that atomic hydrogen catalyses the enhancements via the oxygen diffusion rate. Donor concentrations greater than l017 cm- 3 have been observed in samples heated in a plasma at 3500C. Doubts have been raised about dimer formation being the primary mechanism for oxygen loss in furnace anneals at 3500C, but invoking enhanced diffusion leads to a conflict with stress dichroism data. INTRODUCTION Heat treatment of Czochralski silicon at 4500C leads to the formation of a sequence of at least nine helium-like double donors, designated thermal donors TDN which have progressively smaller ionisation energies as is increased [1]. EPR [21 and ENDOR [31 studies imply that the core of a TD centre incorporates at least four oxygen atoms. A commonly accepted model is that a TD(N+1) centre is produced when a TDN centre captures a mobile atom, which is usually presumed to be another oxygen impurity. Symmetry arguments which suggested that two atoms had to be captured at each stage can b discounted in the light of new evidence [41 but the possibility that the captured atom is a self-interstitial cannot be ruled out. Even if TDcentres incorporate only four oxygen atoms, it is necessary that the oxygen diffusion coefficient, D, be enhanced by a factor of about ten compared with the normal value, to account for the rate of TD-formation [5,6]. If TD-centres are larger oxygen agglomerates, the required enhancement factor is close to 104 [7]. Studies of the conversion of TD1 to TD2 by electrical measurements have suggested that there is indeed a fast diffusing species with an activation energy of 1.8+0.2eV [8]. It was proposed that the species was interstitial oxygen, Oi, itself which always transferred to an 'excited metastable state' at low temperatures. However measurements of the rate of relaxation of stress-induced dichroism carried out with sample temperatures in the range 330-4000C show that normal oxygen diffusion can occur with an activation energy of 2.5eV [9-11]. Nevertheless, this latter technique has shown that enhanced diffusion jumps can also occur with an activation energy of 1.8eV in samples given a pre-heat treatment for 2h at 9000C [10]. These results have been confirmed [111, but the mechanism of the enhancement has still not been determined. It has been speculated that it might be due to the interaction of Oi atoms with either self-interstitials (I-atoms) [12], or vacancies [131. Other
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