Analysis of Oxygen Gettering and Dislocation Locking in Silicon
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ANALYSIS OF OXYGEN GETTERING AND DISLOCATION LOCKING IN SILICON DIMITRIOS MAROUDAS AND ROBERT A. BROWN Massachusetts Institute of Technology, Department of Chemical Engineering, Cambridge, MA 02139 ABSTRACT The motion under an applied stress of dislocations in silicon crystals is slowed and stopped by the presence of oxygen in the material. A model is presented that quantitatively describes the inhibition of dislocation motion by accounting for the oxygen gettering to dislocations caused by diffusion and stress-enhanced migration. Drag on the dislocation motion is modelled using the elastic interactions between the interstitial oxygen and the dislocation and the energy needed to break bonds formed between silicon and aggregated oxygen atoms within the dislocation core. The predictions of the model agree quantitatively with the experimental data of Imai and Sumino. 1. INTRODUCTION Impurity gettering to crystalline defects plays a crucial role in establishing the electrical and mechanical properties of semiconductor materials. In particular, the gettering of oxygen, the ubiquitous impurity in silicon crystals, has been demonstrated to retard dislocation motion [1] and thereby leads to solid solution hardening of the crystal. A systematic model that describes the effects of the impurity on dislocation motion must account for both the migration of the impurity to the dislocation, caused by diffusion and drift due to the elastic force between the dislocation and each impurity atom, and the resulting drag on the dislocation caused by the elastic interactions with the impurity distribution and the presence of the impurity in the dislocation core. We present such an analysis for oxygen, an interstitial and electrically inactive impurity, acting on 600 dislocations in silicon crystals at high temperatures. Without the effect of an impurity, the velocity v of these dislocations under an applied stress is well described by the Alexander and Haasen model for dislocation dynamics in a diamond-cubic lattice
[2]:
v = B0 exp(--Q-)hp,
(1)
where Q is a thermal activation energy, B 0 is a mobility coefficient, k is the Boltzmann constant and Thp is the applied stress. The action of the oxygen impurity on the dislocation velocity is modelled by a combination of elastic and interatomic interactions between the dislocation and oxygen around and inside the dislocation core, which retard dislocation motion. These forces axe computed using a self-consistent model of oxygen gettering to the dislocation, as described by Maroudas and Brown [3]. The drag force due to interstitial oxygen outside the dislocation core is modelled as an elastic interaction, following the pioneering work of Cottrell et al. [4]. The effect of oxygen in supersaturation in the dislocation core is accounted for using the ab initio calculations of Needels et al. [5] to model the force necessary to break ionic bonds formed between silicon and aggregated oxygen atoms.
Mat. Res. Soc. Symp. Proc. Vol. 209. Q1991 Materials Research Society
598
The predictions of the
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