SiGe Island Shape Transitions Induced by Elastic Repulsion

Abstract

The detailed morphological evolution during the transition between hut clusters and dome clusters is examined for Si 0.8 Ge 0.2 ͞Si͑001͒ heteroepitaxy. Simultaneous real-time light scattering and stress measurements directly demonstrate the correlation between island impingement and the shape transformation. We show that elastic interactions between islands can significantly reduce the equilibrium transition volume and may also modify the activation barrier for the transition. [S0031-9007(98)06223-1] PACS numbers: 68.35.Bs, 68.55. -a, 81.15. -zCoherent island formation provides a pathway to strain relaxation during lattice-mismatched heteroepitaxy that can bypass, or at least precede, dislocation introduction [1-6]. Relaxation results from the evolution of the film geometry rather than through shear, and is kinetically mediated by surface diffusion. Strain not only drives the initial 2D-to-3D transition during growth, but can also drive transitions from one island shape to another [6][7][8][9][10][11][12]. Island shape transitions are of interest because they probe the energetics and kinetic pathways associated with deterministic morphological evolution arising from the competition between strain energy and surface energy. These issues are critical to producing dense arrays of islands of controlled size for quantum dot applications.In a previous publication, we showed that the sequence of island transitions during Si 0.8 Ge 0.2 ͞Si͑001͒ heteroepitaxy (0.8% lattice mismatch strain) mirrors that observed for Ge͞Si(001) heteroepitaxy (4% strain), as long as the growth temperature is high enough to provide sufficient adatom mobility [11]. Longer surface diffusion lengths are required to overcome kinetic limitations imposed by the low strain. In particular, there is a natural length scale associated with strain-driven islanding that is proportional to DG͞M´2 coh , where DG is the change in surface energy, M is an elastic modulus, and´c oh is the lattice mismatch strain. Length scaling in the low strain regime can be exploited in the study of islanding phenomena, since transitions are a more gradual function of film thickness, allowing easier observation of intermediate transition stages, and since the increased length scales allow use of real-time optical probes.In this Letter, we examine the detailed transition from the hut cluster morphology (pyramidal islands bound by ͓501͔ facets [1]) to the dome cluster morphology (more isotropic island predominantly bound by ͓311͔ facets [8]) during Si 0.8 Ge 0.2 ͞Si͑001͒ heteroepitaxy. We show that island-island elastic interactions can strongly modify the transition energetics.Si 0.8 Ge 0.2 films were grown by molecular beam epitaxy (MBE) on Si(001) at 755 6 10 ± C and 0.

GeSi strained nanostructure self-assembly for nano- and opto-electronics.

“…In particular, if the substrate is only [10][11][12][13][14][15][16][17][18][19][20] o too warm, the film can destabilize to form extended 3D roughness that supercedes QDM formation, somewhat similar to what is seen in Fig. 3.…”

“…1 [9][10][11][12], and (6) misfit dislocation formation, possibly accompanied by superdome formation [5,11]. A similar transition sequence is observed with increasing Ge fraction in the alloy up to pure Ge, but the relevant length scales are reduced, and lower deposition temperatures can be used in order to observe the sequence [5].…”

Symmetry breaking in shape transitions of epitaxial quantum dots

“…In addition, during growth small asymmetries in the environment can develop, such as strain gradients or local misorientations, due to the presence of neighboring islands. 17 This could increase the lifetime of metastable configurations, making them easier to observe experimentally. Of course, in an asymmetrical environment even the ground state becomes asymmetrical, as for "miscut" crystal surfaces.…”

Dislocation-induced spatial ordering of InAs quantum dots: Effects on optical properties

“…8,9 Such step bunching-induced alignment has different effects in Ge/Si dots and results in increased lateral ordering 10 without the formation of strings of island on the step edges, due to the elastic repulsion between islands found in this system. 11 In contrast to the Ge/Si and InAs/GaAs systems where strain induced ordering is limited by their small elastic anisotropy, 12 very good long-range order has been achieved in lead salt crystals, in which strong elastic anisotropy leads to the formation of three-dimensional dot lattices, and the dot positioning shows anticorrelation with the underlying dot layers. 13 Good vertical alignment has been shown in InAs and InP QDs, in this case a positive correlation aligns the dots directly over existing islands when the capping layer is sufficiently thin to maintain a large enough residual strain field on the growth surface.…”