Abstract
The use of heavily doped semiconductors to achieve plasma frequencies in the mid-IR has been recently proposed as a promising way to obtain high-quality and tunable plasmonic materials. We introduce a plasmonic platform based on epitaxial n-type Ge grown on standard Si wafers by means of low-energy plasma-enhanced chemical vapor deposition. Due to the large carrier concentration achieved with P dopants and to the compatibility with the existing CMOS technology, SiGe plasmonics hold promises for mid-IR applications in optoelectronics, IR detection, sensing, and light harvesting. As a representative example, we show simulations of mid-IR plasmonic waveguides based on the experimentally retrieved dielectric constants of the grown materials.
Highlights
IntroductionOver the last two decades, the development of plasmonics for operation around the visible spectral window has heavily relied on gold as the preferred material, mainly because of its excellent properties in terms of stability, easy chemical synthesis, and biocompatibility.[1,2,3,4,5,6] Already in these high-frequency ranges, other metals have sometimes been preferred when it comes to specific spectral regions, in particular, Ag and Al for the blue region and, more recently, the near-UV region.[7,8] Longer midinfrared (mid-IR) wavelengths have been approached by exploiting gold,[9,10,11,12,13] which in the IR range behaves as a very good conductor, with low penetration depths for the electromagnetic fields
Biagioni et al.: Group-IV midinfrared plasmonics. Another relevant issue when it comes to the use of plasmonic interfaces in optoelectronics and integrated devices is the compatibility with the existing Si photonics and CMOS platforms, something that cannot be solved with Au-based nanostructures
Ge should be preferred both in standard mid-IR photonic devices, due to its inherently lower losses, and for plasmonic applications based on heavy doping, since the lower effective mass (m à ≈0.12 for Ge compared to m à ≈0.26 for Si) allows a higher plasma frequency to be reached for a given doping level
Summary
Over the last two decades, the development of plasmonics for operation around the visible spectral window has heavily relied on gold as the preferred material, mainly because of its excellent properties in terms of stability, easy chemical synthesis, and biocompatibility.[1,2,3,4,5,6] Already in these high-frequency ranges, other metals have sometimes been preferred when it comes to specific spectral regions, in particular, Ag and Al for the blue region and, more recently, the near-UV region.[7,8] Longer midinfrared (mid-IR) wavelengths have been approached by exploiting gold,[9,10,11,12,13] which in the IR range behaves as a very good conductor, with low penetration depths for the electromagnetic fields. A few seminal works have already outlined the very interesting possibilities that will be opened by the use of such materials for mid-IR plasmonics.[14,15,16] The idea of turning attention to semiconductors as “metals” in the mid-IR comes from the dependence of the plasma frequency (marpkinffiffiffigffiffiffiffitffihffiffiffie onset of conducting behavior) on the carrier density n, according to the relation ωp ∝ n∕mÃ, where mà represents the effective mass of the free carriers involved in the plasma oscillations Another relevant issue when it comes to the use of plasmonic interfaces in optoelectronics and integrated devices is the compatibility with the existing Si photonics and CMOS platforms, something that cannot be solved with Au-based nanostructures. A natural choice in terms of semiconductor materials, from the point of view of integration, points toward Si and Ge.[17,18,19] Between these two, Ge should be preferred both in standard mid-IR photonic devices, due to its inherently lower losses, and for plasmonic applications based on heavy doping, since the lower effective mass (m à ≈0.12 for Ge compared to m à ≈0.26 for Si) allows a higher plasma frequency to be reached for a given doping level
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