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Localized thinning for strain concentration in suspended germanium membranes and optical method for precise thickness measurement
Vaccaro, Pablo; Alonso, Maria Isabel; Garriga, M.; Gutiérrez, Joffre; Peró, D.; Wagner, Markus R.; Reparaz, J. S.; Sotomayor Torres, C. M.; Vidal, Xavier; Carter, E. A.; Lay, P. A.; Yoshimoto, M.; Goñi, Alejandro R.
We deposited Ge layers on (001) Si substrates by molecular beam epitaxy and used them to fabricate suspended membranes with high uniaxial tensile strain. We demonstrate a CMOS-compatible fabrication strategy to increase strain concentration and to eliminate the Ge buffer layer near the Ge/Si hetero-interface deposited at low temperature. This is achieved by a two-steps patterning and selective etching process. First, a bridge and neck shape is patterned in the Ge membrane, then the neck is thinned from both top and bottom sides. Uniaxial tensile strain values higher than 3% were measured by Raman scattering in a Ge membrane of 76 nm thickness. For the challenging thickness measurement on micrometer-size membranes suspended far away from the substrate a characterization method based on pump-and-probe reflectivity measurements was applied, using an asynchronous optical sampling technique.
