Fabrication of Scanning Probe Tips made of Epitaxial Germanium with Plasma Frequency in the Mid-Infrared

Valeria Giliberti Department of Physics, Sapienza University of Rome, Rome, Italy Emilie Sakat Department of Physics, Politecnico di Milano, Italy, Milano, Italy Leonetta Baldassarre Center for Life and Nano Sciences, Italian Institute of Technology, Rome, Italy Jacopo Frigerio LNESS, Laboratory for Nanostructure Epitaxy and Spintronics on Silicon, Como, Italy Giovanni Isella LNESS, Laboratory for Nanostructure Epitaxy and Spintronics on Silicon, Como, Italy Mauro Melli Lawrence Berkeley National Labs, Molecular Foundry, Berkeley, USA Alexander Weber-Bargioni Lawrence Berkeley National Labs, Molecular Foundry, Berkeley, USA Stefano Cabrini Lawrence Berkeley National Labs, Molecular Foundry, Berkeley, USA Marco Finazzi Department of Physics, Politecnico di Milano, Italy, Milano, Italy Michele Celebrano Department of Physics, Politecnico di Milano, Italy, Milano, Italy Paolo Biagioni Department of Physics, Politecnico di Milano, Italy, Milano, Italy Michele Ortolani Department of Physics, Sapienza University of Rome, Rome, Italy Monica Bollani LNESS, Laboratory for Nanostructure Epitaxy and Spintronics on Silicon, Como, Italy Institute for Photonics and Nanotechnologies, National Research Council of Italy, Milano, Italy

We target the nanofabrication of scanning probe tips made of high-crystal-quality epitaxial semiconductor material layers with high heterostructure complexity for optical applications at the nanoscale. Here the fabrication method for epitaxial germanium grown on a silicon substrate is demonstrated but the same approach can be extended to any heterostructure material.

The first step of the process is the fabrication of nanostructures out of planar epitaxial wafers in the form of pillars with arbitrary section and high aspect ratio by electron-beam lithography and deep reactive-ion etching. Electron-beam induced deposition is used to weld the cut tip of an Atomic Force Microscopy (AFM) cantilever to the top of one pillar, whose base is then cut by focused ion-beam (FIB) milling (figure 1a). Finally, the epitaxial germanium nanostructure is shaped into a pyramid tip by FIB milling (figure 1b and 1c).

Micro-photoluminescence and transmission electron microscopy analysis performed on the germanium scanning probe tips confirm that the high crystal quality typical of epitaxial layers grown on a large-area substrate is preserved throughout the different fabrication steps. First prototypes of the tips obtained from heavily-electron-doped germanium with plasma frequency in the mid-infrared have been successfully applied to perform scattering scanning near-field infrared microscopy on nanostructured samples.

Figure 1 (a) Scanning Electron Microscope (SEM) image during the lift-out of one pillar after the milling by FIB. (b), (c) SEM images of a commercial AFM cantilever with an epitaxial germanium tip welded on the flat top.

Work at the Molecular Foundry was performed under User Proposal # 1773. The research leading to these results has received funding from the European Union’s Seventh Framework Programme under grant agreement n°613055.

giliberti.valeria@gmail.com