Suppression of Intermixing in Strain-Relaxed Epitaxial Layers T. Leontiou, 1 J. Tersoff, 2 and P. C. Kelires 1 1 Department of Mechanical & Materials Science Engineering, Cyprus University of Technology, P.O. Box 50329, 3036 Limassol, Cyprus 2 IBM T.J. Watson Research Center, Yorktown Heights, New York 10598, USA (Received 23 August 2010; published 3 December 2010) Misfit strain plays a crucial role in semiconductor heteroepitaxy, driving alloy intermixing or the introduction of dislocations. Here we predict a strong coupling between these two modes of strain relaxation, with unexpected consequences. Specifically, strain relaxation by dislocations can suppress intermixing between the heterolayer and the substrate. Monte Carlo simulations and continuum modeling show that the suppression, though not absolute, can be surprisingly large, even at high temperatures. The effect is strongest for a large misfit (e.g., InAs on GaAs) or for thin substrates (e.g., Ge on silicon on insulator). DOI: 10.1103/PhysRevLett.105.236104 PACS numbers: 68.35.Dv, 61.72.Lk, 64.75.Ef, 81.10.Aj Since the early days of semiconductor heteroepitaxy, the role of misfit strain has been a constant theme. Initially the focus was on avoiding the relaxation of misfit strain by dislocations. More recently, strain-relaxed layers have shown great promise as templates with variable lattice spacing, enabling structures with dramatically strain- enhanced electron mobilities for high-performance transis- tors [1]. Strain can also be relaxed by intermixing of the two different-sized components, a process which blurs the heterointerface and so is highly detrimental for hetero- structures. Intriguingly, these two modes of strain relaxa- tion may couple [2]. This has been widely discussed in the context of ‘‘strain-enhanced diffusion’’ [3]. Here we show that these two modes of strain relaxation can couple in an entirely different way, with important implications for the thermal stability of strain-relaxed tem- plate layers. Specifically, strain relaxation by dislocations can suppress intermixing between the heterolayer and the substrate. Intermixing is suppressed because, once the strain is fully relaxed by dislocations, intermixing would actually increase the strain. This assumes that any dislocations are immobile; and indeed, heterolayers that are almost fully relaxed by sessile dislocations have been reported for Ge on Si (001) [4] and InAs on GaAs (001) [5]. Thus systems already exist that would be sufficient for a proof of principle. (In these systems the relaxation was unintentional, and doubtless left high densities of threading dislocations, so further optimization would be needed to approach device-quality material.) In order to demonstrate this effect and study it, we carry out Monte Carlo (MC) atomistic simulations. We also develop a continuum model based on classic principles [2], and we verify that it reproduces the MC results. In this way we can predict the extent and rate of intermixing in thin heterolayer films as a function of temperature, degree of relaxation, and other parameters. Our MC equi- librium results for Ge on effectively thin substrates like silicon on insulator (SOI) [6] show a significant suppres- sion of alloying in the film. The effect is even stronger for InAs on GaAs, due to the larger misfit. Continuum model- ing allows us to extend these results to very thick (effec- tively semi-infinite) substrates, where we find a dramatic slowing-down of intermixing over long times. We consider first the equilibrium of an epitaxial layer on a substrate where intermixing is limited to a thin region, as for SOI substrates. We treat Ge on Si (001) and InAs on GaAs (001), in each case either coherent or relaxed by dislocations. The epilayers contain 60 Ge or InAs mono- layers and are terminated by reconstructed (dimerized) surfaces. We allow equilibration within these 60 layers plus the first 238 layers of substrate, with deeper layers ‘‘frozen’’ to represent, e.g., the oxide in an SOI substrate. This corresponds to a substrate thickness of 32 nm, read- ily achievable with SOI. The lattice constants parallel to the layer are fixed by the epitaxial constrain of zero strain deep in the substrate, leaving the film under biaxial stress. The computational cells are periodically repeated in two dimensions. For the relaxed case, we include one dislocation per cell in each direction, with sessile 90  (Lomer type) disloca- tions. The dislocation spacing is set by the cell size (L  L), which is chosen here to correspond to nearly complete relaxation of misfit strain. The core structure is shown in Fig. 1, and it conforms with experimental observations [7]. We note that details of core structure and surface recon- struction do not affect the results presented here. The system is allowed to equilibrate, both geometrically and compositionally, using a continuous-space MC method that has been extensively tested in similar environments [8–10]. Three types of random moves are involved in the MC algorithm: atomic displacements and volume changes, which lead to geometrical relaxation, and mutual identity PRL 105, 236104 (2010) PHYSICAL REVIEW LETTERS week ending 3 DECEMBER 2010 0031-9007= 10=105(23)=236104(4) 236104-1 The American Physical Society