Suppression of Intermixing in Strain-Relaxed Epitaxial Layers

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).

Figure 1

Ball and stick model of a typical $\mathrm{Ge}/\mathrm{Si}(001)$ structure with a Lomer dislocation (shuffle type) at the interface. The Burgers vector $\overrightarrow{b}$ lies in the (001) plane.

Figure 2

Equilibrium composition and stress in our system at 300 K. (a) Composition variations along the [001] growth axis of Ge and InAs dislocation-relaxed films on thin Si and GaAs substrates, respectively, before and after intermixing, as indicated in the legend. Points denote local atomic compositions averaged over slices having thicknesses of $\sim 0.63\mathrm{nm}$ ($\mathrm{Ge}/\mathrm{Si}$) and 1.03 nm ($\mathrm{InAs}/\mathrm{GaAs}$). Dashed horizontal line shows the limit of perfect mixing in the nondislocated case. Dotted vertical line denotes the interface position. (b) Biaxial stress at 300 K in the above structures, before and after intermixing. Points denote local atomic stresses averaged as in (a).

Figure 3

Dependence of Ge and InAs fraction in heterolayer on temperature. Filled symbols and dotted lines denote MC simulation and model results, respectively. Horizontal dashed line shows expected composition in coherent films.

Figure 4

Evolution of composition in InAs film on GaAs. Diamonds and circles are MC results for coherent and dislocation-relaxed layers, respectively. Solid curves are model results for the semi-infinite substrate. Inset emphasizes early-time behavior for comparing the model and MC results. Time units are arbitrary, with the mobility $m$ scaled to best match the MC results. Temperature is 300 K. Model results for 900 K are shown as dotted lines for comparison (horizontal alignment is arbitrary due to different time units at different T).