Self-aligned Ge and SiGe three-dimensional epitaxy on dense Si pillar arrays

In this report we present a novel strategy in selective epitaxial growth on top of Si pillars, which results in a tessellated Ge film, composed by self-aligned micron-sized crystals in a maskless process. Modelling by rate equations the morphology evolution of fully facetted crystal profiles is extensively outlined, showing an excellent prediction of the peculiar role played by flux shielding among microcrystals, in the case of dense array configuration. Crack formation and substrate bending, caused by the mismatch in thermal expansion coefficients, are eliminated by the mechanical decoupling among individual microcrystals, which are also shown to be dislocation- and strain-free. The method has been also tested for Si1-xGex alloys, with compositions ranging from pure silicon to pure germanium. There are ample reasons to believe that this approach could be extended to other material combinations and substrate orientations, actually providing a technology platform for several device applications.