Enhanced Relaxation of Strained Ge x Si 1-x Layers Induced by Co/Ge x Si 1-x Thermal Reaction
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nucleation- and diffusion-controlled mechanisms of comparable magnitude [5]. On crystalline substrates, temperatures in excess In of -500 0 C are necessary for nucleation of the CoSi 2 phase. contrast, the formation of CoSi 2 on amorphous Si is dominated by In such samples, the diffusion-controlled mechanisms [6]. activation energy for nucleation of the CoSi phase is reduced when the free energy of crystallization o? amorphous Si is considered in the total free energy change of the system. Consequently, the minimum temperature for nucleation of the CoSi 2 phase on amorphous Si is less than that observed on crystalline With diffusion-controlled kinetics, superior planarity of Si. both the CoSi 2 /Si interface and the CoSi 2 surface has been the thermally-induced Co/amorphous-Si observed following reaction. For the present report, the nucleation and growth of CoSi 2 on amorphous and crystalline Ge. 1 7 Si 8 3 have been studied and Given the observatlons cited in the previous compared with Si. paragraph, CoSi 2 formation on amorphous Ge. 1 7 Si. 8 3 is a possible means of inhibiting enhanced relaxation of the underlying strained layer - a lower nucleation temperature could reduce processing temperatures and furthermore, improved planarity of the reacted-layer/GexSiI _ interface could reduce the number of dislocation nucleation sites. EXPERIMENTAL DETAILS Strained Ge. 1 7 Si. 8 3 layers of thickness 110 nm were grown on (100) Si substrates by molecular beam epitaxy as described elsewhere [7]. Amorphous surface layers of thickness 70 nm were formed by Si ion implantation at an energy and dose of 25 keV and 2 15 a /cm , respectively, with the samples maintained at 1X10 0 temperature
of
-- 196 C.
For
comparison,
amorphous
layers
were
utilizing the same formed in (100) Si substrates also Following amorphization, Co layers of implantation parameters. thickness 11 nm were deposited by electron-beam evaporation at a pressure of IX10- 6 torr. Samples were subsequently annealed for 0-240 min in forming gas (N 2 /H 2 95%/5%) at temperatures of 5000 600 C. The implantation and annealing conditions were deduced from a consideration of the rate of solid-phase epitaxial growth (SPEG) of the amorphous layer [8,9], as shown in Figure l(a), as compared with the rate of silicide formation [10], as shown in Figure 1(b). Given an initial amorphous layer of thickness -70 is nm and a final CoSi layer of thickness -22 nm [11], it apparent from Figs. 1(a) and (b) that CoSi layer formation should be complete prior to the recrystallization of 50 nm of amorphous Si (where 20 nm is the amount of Si consumed in the formation of 22 nm of CoSi from 11 nm of Co [11]). (Thus, a study of CoSi 2 nucleation on amorphous and crystalline Si was feasible over the Such samples could then be temperature range of interest. compared with Co/Ge. 1 7 Si. 8 3 /Si structures processed under the same conditions.) However, note that experimentally observed rates can deviate from those shown in Fig. 1 given the potential influences of strain [12] and impurities
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