Precipitation, Phase Transformation, and Enhanced Diffusion in Ion-Implanted Silicon
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PRECIPITATION,
PHASE TRANSFORMATION, AND ENHANCED DIFFUSION IN ION-IMPLANTED SILICON
S. J. PENNYCOOK and R. J. CULBERTSON Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6024 S. D. BERGER Cavendish Laboratory, Madingley Road, Cambridge, U.K. ABSTRACT Z-contrast scanning transmission electron microscopy has been used to study the connection between dopant precipitation and phase transformation in high dose In+ and Sb+ implanted Si. In the case of In, the observations Images of the precursor precipiconfirm a heterogeneous nucleation model. tates give the first measurement of the diffusion coefficient in amorphous Si, with an enhancement of 107 over tracer crystalline values. With Sb+ implants enhanced homogeneous nucleation is observed. The connection between these results and the transient enhanced diffusion observed in crystallized Si is discussed. INTRODUCTION The amorphous Si (a-Si) to polycrystalline Si (p-Si) transformation in high dose In-implanted Si has been extensively studied by Nygren et al.1 They showed that the transformation occurred independently of solid phase epitaxial (SPE) growth and proposed a mechanism involving precipitation and migration of In-rich droplets. We have studied this system using Z-contrast scanning transmission electron microscopy (STEM), 2 which provides a direct image of the In droplets in complete confirmation of the Nygren model. We have also imaged the precursor precipitates in a largely untransformed sample and obtained the first estimate for the diffusion coefficient of In in a-Si. These precursor precipitates also explain the origin3 of the pileup phenomenon observed for certain SPE growth conditions. We have extended the studies to high-dose Sb+-implanted Si, expecting to find similar behavior for annealing temperatures above the eutectic temperature (assumed to be close to the 630 0C melting point of pure Sb). In fact, we have found no redistribution of Sb above or below this temperature and believe that enhanced homogeneous nucleation of p-Si is occurring. We believe that all these effects, and the transient-enhanced diffusion we have observed previously in SPE recrystallized Si have a common origin. Z-CONTRAST STEM This is a technique which can directly image low concentrations of heavy dopants in Si, independent of the phase of the dopants or the matrix. It can, therefore, follow their behavior right through a phase transformation process and is ideally suited to the present problem. The technique utilizes electrons scattered through large angles (typically 50), where the electron scattering approaches the Rutherford limit and is therefore strongly Z sensitive. Furthermore, the scattering is essentially incoherent, and independent of atomic arrangement as long as channeling of the incident electron beam is avoided. It forms a complementary image to conventional diffraction contrast or high-resolution imaging techniques which are primarily sensitive to atomic arrangement, i.e., structure, but relatively insensitive to atomic composition. Rutherford scattere
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