Bending of Nanoscale Thin Si Film Induced by Growth of Ge Islands: Hut vs. Dome
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Q6.4.1
Bending of Nanoscale Thin Si Film Induced by Growth of Ge Islands: Hut vs. Dome
Minghuang Huang1, Martin Cuma1, M. G. Lagally2 and Feng Liu1 1
University of Utah, Salt Lake City, Utah 84112
2
University of Wisconsin, Madison, Wisconsin 53706
ABSTRACT We perform atomistic simulations to compute bending of freestanding nanoscale thin Si film induced by strained Ge islands. We show that a larger Ge dome island can induce smaller bending than a smaller hut island and explain the surprising experimental observation for growth of Ge islands on patterned silicon-on-insulator substrate (SOI) with Si template layer thinned down to nanometer scale. This counterintuitive bending behavior is caused by strain sharing between the film and the ultra thin substrate.
INTRODUCTION In growth of coherent Ge film on Si(001), strain relaxation not only drives the initial island formation (2D to 3D transition) [1], but also induces the island shape transition from small-size pyramids (huts) having shallow (105) facets [1] to large-size domes having steeper (113) and (102) facets [1-3] with increasing Ge coverage. The bending of Si substrate induced by strained Ge islands can be used as a unique probe to monitor the island shape transition [4]. In general, when Ge grows on a thick Si substrate, the larger dome induces always a larger bending of Si substrate than does the smaller hut, as predicted by the classical Stoney’s formula. However, when Ge is grown on SOI substrate with an ultra thin Si template layer, very recently, it has been observed that the larger dome induces a smaller bending than does a smaller hut, exactly the opposite of that on thick Si substrate [5].
Q6.4.2
In order to understand such a counterintuitive behavior, we have performed atomistic simulations to compute directly the bending of an ultra thin freestanding Si layer induced by both Ge hut and dome island. Our calculations confirm that the larger dome can indeed induce a smaller bending than does a smaller hut, when Si substrate thickness is reduced to be comparable to the height of Ge islands. We explain this bending behavior by large amount of strain sharing between the Ge island and Si film, i.e. the compliancy of the ultra thin Si film.
RESULTS AND DISCUSSION In the experiment, the Si template layer in SOI is thinned down to ~10 nm, and Ge islands grow to a height ranging from 10~20 nm [5]. It has been observed that both dome and hut induce a very large localized bending underneath the individual island. The bending curvature induced by dome ( Κ ~ 5 × 10 −4 nm −1 ) is about one order of magnitude smaller than that by hut ( Κ ~ 3 × 10 −3 nm −1 ). Continuum mechanics calculations [6] show that the magnitude of the large local bending approaches the maximum value of a freestanding Si film, which is assisted by the substantial local viscosity flow of SiO2 underneath the Si. Quantitatively, the local bending mode can only occur when the thickness of substrate is reduced to be comparable to the height of islands in nanometer regime and those island
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