Silicon nanomembranes (SiNMs) are defect-free, single-crystal Si sheets with thickness ranging from 2 to 500 nm. This thinness makes them flexible, transferable, bondable and, most importantly, mechanically ultracompliant. This compliance makes nanomembranes fundamentally different from bulk materials [1,2]. Growth of three-dimensional Ge islands (Ge “huts”, or quantum dots) on freestanding Si membranes exploits this unique mechanical behavior to induce self-organization of the dots, resulting in a periodic strain in Si nanomembranes or nanoribbons [1,2].

To understand better the effects of strain and substrate compliance on lattice-mismatched heteroepitaxy, we grow Ge islands on stretched (i.e., tensilely strained), freestanding, (001)-oriented SiNMs. We drape a SiNM over a substrate patterned with high ridges. The bending of the SiNM over the edges of the ridges creates regions of local strain that enhance and direct Ge island nucleation. The ridge height and separation are varied to manipulate the draped-membrane strain. Ge 3D islands are grown on the draped SiNM via chemical vapor deposition or molecular beam epitaxy. We demonstrate two-dimensional self-ordering of a single layer of Ge dots, with uniform dot size and spacing. The islands, however, differ from the classic [105]-facetted structures. We find that, while our new Ge dots still have square or rectangular bases, they have steeper facets than those of conventional Ge “huts”. We perform finite-element analysis to map the local strain in the draped membrane and to investigate the influence of membrane thickness and substrate ridge height on the dot ordering.

This work is supported by DOE, NSF, and AFOSR.

 

References:

 

[1] M. Huang et. al., ACS Nano 3, 721 (2009)

[2] H-J. Kim-Lee et. al., Phys. Rev. Lett., 102, 226103 (2009)