Stress driven spontaneous mesoscopic alloy decomposition and ordering along step bunches during epitaxial Si 1 − yC y alloy growth on Si

Abstract

In this report attention is paid to gain understanding about alloy decompositions associated with spontaneous phase orderings observed in the course of C accommodation in epitaxial Si 1 − y C y alloy growths upon reconstructed Si(0 0 1)2 × 1 and Si(1 1 1)7 × 7 surfaces. These observations in the projection plane of cross-sectional transmission electron microscopy (XTEM) are only possible in a restricted window of growth temperature (around 600°C) and kinetic conditions (low deposition rates). They consist in a C accommodation form which is intermediate between random C substitution and SiC clustering with carbon — in this projection — agglomerated in nanometric and ordered C-rich aggregates (1–3 nm). Alternating sequences —in the [0 0 1] or [1 1 1] growth directions— of dark and bright XTEM contrasts leading to striations parallel to the surface reveal that these nanoaggregates are all self-organized in 2D layers like in a “natural” superlattice. This indicates a periodic C composition modulation along the growth direction corresponding to a spontaneous, growth induced, alloy decomposition with separation between pseudomorphic C-rich regions and a nearly unstrained, pure and defect free Si matrix. Secondary ion mass spectrometry measurements have allowed us to study and to delimit the conditions of emergence of this particular growth mode as a function of the mean C concentration y and to connect the self-organization pseudoperiod with y. Another striking breakthrough to the comprehension is provided by XTEM observation of a particularly clear connection between surface steps and C accumulation planes. Actually, each C-rich layer seen as a TEM striation emerges at the surface of a misoriented (1 1 1) surface at the bottom and the top of step bunches whose constant height corresponds to the striation period. This strongly suggests that C incorporation is stress driven below the growing surface and agglomerates at the less tensile place, i.e. near the upper edge of step bunches related to an island growth mode. A stress determined constant height or plateau-like shape of these islands determines the periodicity since a progression during growth of the step front in a step flow regime would ensure a laminar C incorporation with a periodicity corresponding to that of the step bunch height. These results on the SiC system provide a first confirmation of a general theoretical model proposed very recently by Tersoff (Phys. Rev. Lett. 77 (1996) 2017) on self-organization processes.