Epitaxial growth versus nucleation in amorphous Si doped with Cu and Ag
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D. J. Eaglesham, D. C. Jacobson, and J. M. Poate AT&T Bell Laboratories, Murray Hill, New Jersey 07974 (Received 13 July 1992; accepted 23 November 1992)
The competition between solid phase epitaxy and random nucleation in amorphous Si implanted with Cu and Ag has been studied. At low metal concentrations, solid phase epitaxy proceeds with slight deviations from the intrinsic rate, with the impurity segregated and evenly distributed in the amorphous layer. At an impurity concentration of 0.12 at. %, rapid nucleation occurs, transforming the remaining layer into polycrystalline Si. The nucleation rate is 3=108 the intrinsic homogeneous rate. The effects of the metals on epitaxy scale with the amount of metal-Si interaction. Nucleation appears to occur when the metal impurities exceed their absolute solubility limit and begin to phase separate.
I. INTRODUCTION Solid phase epitaxy (SPE) of amorphous Si (a-Si) to crystalline Si (c-Si) has numerous current and potential applications within the electronic industry. For example, since the early 1970s, ion implantation has been the standard technique for the introduction of electrical dopants. Thermal anneals inducing SPE, necessary to restore the crystallinity and to activate these dopants, are subsequently required. Recently, silicon-on-insulator structures have been produced by lateral SPE of a-Si over S1O2,1 and it has been proposed that SPE could even be used to produce delta-doped layers in molecular beam epitaxy.2 However, as well as crystallizing epitaxially, the metastable amorphous phase can also transform to c-Si through random nucleation and subsequent grain growth, producing polycrystalline Si (p-Si) instead of epitaxial Si. Since both the SPE and nucleation processes are known to be influenced by impurities, it is important to understand the fundamental mechanisms leading to these effects for a wide range of impurities. The epitaxial growth rate of pure a-Si on a cSi substrate follows an Arrhenius relation, v(T) = VQ exp(—E a /kT), with a single velocity prefactor, vo, and an activation energy, Ea = 2.68 eV, across the entire temperature range from 400 to 1200 °C.3'4 Impurities such as B, P, and As, i.e., electrically active dopants, are known to enhance the growth rate.5'6 These impurities are thought to affect SPE by shifting the Fermi level, resulting in, for example, changes in the
"'Present address: FOM Institute for Atomic and Molecular Physics, 1098 SJ Amsterdam, The Netherlands. 820 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 4, Apr 1993 Downloaded: 02 Apr 2015
concentration of charged and uncharged defect states.7'8 Low levels of electrically inactive impurities (N, C, F, O, Ne, Ar, and Kr) reduce the growth rate,6'9 possibly by blocking active growth sites. In general, the effect of impurities on the nucleation rate is the inverse of an impurity's effect on SPE. For example, B, P, and As decrease the nucleation rate while O and F increase the rate. 610 In these cases, both the enhancements and retardations appear to act on the hom
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