Mid-Infrared Sensing Using Heavily Doped Germanium Plasmonics on Silicon Substrates
Authors: Baldassarre Leonetta , Sakat Emilie , Frigerio Jacopo , Frigerio J. B. J. , Samarelli Antonio , Giliberti Valeria , Pellegrini Giovanni , Gallacher Kevin , Fischer Marco , Brida Daniele , Isella Giovanni , Biagioni Paolo , Paul Douglas J , Ortolani Michele 
Published: February 2020

The quest for novel plasmonic materials has been a lively area of research over the last few years. In the mid-infrared (mid-IR) spectral region, in particular, localized plasmon resonances in nanoparticles and nano-antennas hold promise for enhanced IR spectroscopies, with key applications in biology, medicine, and security. In this frame, the development of a CMOS-compatible plasmonic platform in the mid-IR could have disruptive effects for future technologies, allowing for cost-effective sensing devices integrated with electronics [1-2].
We report on the growth, fabrication and optical characterization of heavily-doped Ge antennas integrated on a Si substrate and we exploit them for the sensing of solid-phase and liquid-phase analytes [3-5]. Epitaxial Ge is grown on Si by plasma-enhanced chemical vapor deposition, exploiting phosphorous as the dopant and achieving plasma frequencies up to 1950 cm-1 (5.1 µm wavelength) corresponding to activated doping of 8.1 x 1019 cm–3. Through modelling and simulation of a series of key figures of merit we demonstrate that n-Ge has performance as a mid-infrared plasmonic material that is within an order of magnitude of noble metals such as gold and with properly designed antennas has the potential to provide enhancements of up to 3 orders of magnitude.
We demonstrate two-wire gap antennas fabricated by electron-beam lithography and reactive ion etching techniques, displaying localized plasmon resonances in the important 8 to 13 µm molecular fingerprint region. We target the sensing of a thin polydimethylsiloxane (PDMS) layer (thickness of about 40 nm) and demonstrate experimentally an enhancement of two orders of magnitude in the collected signal for resonances around 800 cm–1 (12.5 µm wavelength), as derived from a comparison with the results of detailed numerical simulations. Further measurements of the ~700 cm–1 (14.3 µm wavelength) C–Cl bond duplet of 2-chloroethyl methyl sulfide, a mustard chemical warfare (CW) agent simulant, demonstrates potential security applications. This part of the mid-infrared spectrum has a host of molecular absorption lines important for the sensing of both CW agents and explosives, both of which are of interest for airport and counter terrorism security screening.
Our results represent a first experimental benchmark for group-IV mid-IR plasmonics and confirm that future CMOS sensing platforms could benefit significantly from plasmonic enhancements provided by integrated heavily-doped Ge-based devices. The research leading to these results has received funding from the European Union’s Seventh Framework Programme under grant agreement n°613055.
[1] R. Soref, Nature Phot. 4, 495-497 (2010).
[2] R. Soref, J. Hendrickson, and J.W. Cleary, Opt. Exp. 20, 3814-3824 (2012).
[3] L. Baldassarre, E. Sakat, J. Frigerio, A. Samarelli, K. Gallacher, E. Calandrini, G. Isella, D.J. Paul, M. Ortolani, and P. Biagioni, Nano Lett. 15, 7225-7231 (2015).
[4] P. Biagioni, J. Frigerio, A. Samarelli, K. Gallacher, L. Baldassarre, E. Sakat, E. Calandrini, R.W. Millar, V. Giliberti, G. Isella, D.J. Paul, and M. Ortolani, J. Nanophot. 9, 093789 (2015).
[5] A. Samarelli, J. Frigerio, E. Sakat, L. Baldassarre, K. Gallacher, M. Finazzi, G. Isella, M. Ortolani, P. Biagioni, and D.J. Paul, Thin Solid Films 602, 52 – 55 (2016).

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