Effect of Strain on the Epitaxy of B-Doped Si0.5Ge0.5 Source/Drain Layers

Abstract

Pseudomorphic B-doped Si1-xGex is commonly used as a source/drain (S/D) material for p-type MOSFETs. The lattice mismatch between the Si1-xGex and the Si substrate induces a compressive strain within the epilayer, which is beneficial to device performance. Indeed, it increases the hole mobility in the channel [1] and increases the B solubility limit in the S/D thanks to a strain compensation effect that enables higher substitutional boron concentrations ([B]sub) compared to pure Si or Ge [2], [3]. However, with device downscaling and a reduction in the active area where epitaxial deposition needs to occur, maintaining the high amounts of strain required is challenging [4]. Most of the compressive strain is released through the elastic relaxation allowed by the presence of free surfaces. This sets the need of assessing the impact of strain on the epitaxy of in-situ doped Si1-xGex and how it affects the material properties.In this study, we investigate these effects through three series of B-doped Si1-xGex films grown on Si(001) substrates whereby three Ge concentrations ranging from 25 to 65% are considered. For each Ge concentration, epilayers of different thicknesses are grown to achieve both fully strained and partially relaxed layers. Subsequently fabricated Ti / Si1-xGex contacts are then evaluated using the multiring circular transmission line method [5]. The resistivity (ρ) and contact resistivity (ρc) are both found to show an enhancement with increasing degree of strain relaxation (Fig. 1a). The thinnest layer in each Ge concentration also yields degraded contact and electrical properties due to a surface scattering effect. Correspondingly, SIMS analyses reveal a decrease in B incorporation during the epitaxial growth upon relaxation, which also coincides with a slight increase in Ge content (Fig. 1b).A similar behavior is observed when B-doped Si0.5Ge0.5 epilayers are grown onto Si1-yGey strain relaxed buffers (SRB) with different Ge contents (Fig. 1c). A change in virtual substrate modifies the magnitude and type of strain in the epilayer. B-incorporation during epitaxial growth is thereby severely impacted. This confirms that the change in adatom incorporation is not linked to the mechanisms of plastic strain-relaxation, but it is determined by the strain status of the growing epilayer.Mechanisms responsible for this behavior are being assessed by ab-initio calculations using Density Functional Theory (DFT). A 128 atoms surface slab of Si0.5Ge0.5 is modelled using Perdew–Burke-Ernzerhof (PBE) exchange-correlation functional. The surface presents a 2x1 reconstruction with all the dangling bonds passivated by H atoms (Fig. 1d). Four strain conditions are applied to the slab, ranging from a fully compressively strained to a fully relaxed Si0.5Ge0.5:B epilayer on unstrained Si. The adsorption and incorporation steps corresponding to B surface doping from a B2H6 molecule are subsequently simulated. The precursor interaction with the surface, its chemisorption and decomposition are found not to be affected by the magnitude of strain. However, the incorporation probability of a B in the first atomic layer of the Si0.5Ge0.5 surface increases with the applied stress, corroborating the observed experimental results.In summary, we report a modified incorporation of adatoms during Si1-xGex epitaxy when the biaxial compressive strain is relaxed. In particular, the B incorporation is reduced, while the Ge concentration is progressively increased. From DFT simulations we get indications that, indeed, a decrease in strain may cause a decrease in the B incorporation probability during the epitaxial deposition.[1] D. Yu et al., Phys. Rev. B, 78 (24), p. 245204, 2008.[2] B. Tillack et al., Appl. Phys. Lett., 67 (8), p. 1143, 1995.[3] R. Loo et al., ECS J. Solid State Sci. Technol. 6 (1), p. 14, 2017.[4] M. Bargallo Gonzalez et al., Mat. Sci. Semicon. Proc. 11 (5), p. 285, 2008.[5] H. Yu, et al., IEEE Electr. Dev . Let. , 36 (6), p. 600, 2015. Figure 1