Point-Defect Migration in Crystalline Si: Impurity Content, Surface and Stress Effects
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ABSTRACT The results of several recent experiments aimed at assessing the room temperature migration properties of interstitials (D)and vacancies (V) in ion implanted crystalline Si are reviewed. We show that combining the results of ex-situ techniques (deep level transient spectroscopy and spreading resistance profilometry) and in-situ leakage current measurements new and interesting information can be achieved. It has been found that at room temperature I and V, generated by an ion beam, undergo fast long range migration (with diffusivities higher than 10-1 cm2 /sec) which is interrupted by trapping at impurities (C, 0) or dopant atoms and by recombination at surface. Analysis of two-dimensional migration of point defects injected through a photolithographically defined mask shows that a strong I recombination (characterized by a coefficient of 30 gm-1) occurs at the sample surface. Moreover, we have found that the strain field induced by an oxide or a nitride mask significantly affects defect migration and produces a strong anisotropy of the defect diffusivity tensor. Finally, using in-situ leakage current measuremens, performed both during and just after ion irradiation, the time scale of point defect evolution at room temperature has been determined and defect diffusivities evaluated. The implications of these results on our current understanding of defect and diffusion phenomena in Si are discussed. INTRODUCTION The properties of intrinsic defects, vacancies (V) and self-interstitials (I), in crystalline Si have been the target of extensive experimental and theoretical investigations"' motivated by both
scientific and technological reasons. In fact, point defects drive several phenomena such as transient enhanced dopant diffusion' 2 ' 3 , dopant clustering' 4 and secondary defect formation Full comprehension and proper modeling of these phenomena is far from being achieved as many parameters, such room temperature defect diffusivity and surface recombination rate, are poorly known. For example, the different experimental determinations of I and vacancies V diffusivities at room temperature (RT) in crystalline Si span over a surprisingly wide range. Measurements performed "in-situ" under electron irradiation 9 at cryogenic temperatures gave extremely high values for the I (- 3.2x10-4 cm 2/s) and V diffusivity (- 4.2x10 9 cm 2/s). On the other hand, extremely low values (-10-20_10.30 cm2/sec) are obtained by extrapolating to RT the I and V diffusivity determined from metal diffusion' and gettering4 experiments performed at high temperatures. Indeed several processes contribute to defect diffusivity and the interpretration of diffusion experiments is extremely difficult. First of all, due to the high formation energy (3-4 eV) of the point defects, their equilibrium concentration is very low even close to the melting point' and only indirect assessment of their migration properties is possible. Secondly, I and V in Si can exist into various charge states 7' 9 having different equilibrium atomic structures 7 and
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