Semiconductor plasmonic resonances in the mid-IR range adjusted by thermal annealing

Introduction Mid-IR Biosensing, using surface enhanced infrared absorption, relies on resonant light absorption of molecules at certain characteristics wavelengths enhanced with plasmonic resonators. The tunability of... [ view full abstract ]

Introduction

Mid-IR Biosensing, using surface enhanced infrared absorption, relies on resonant light absorption of molecules at certain characteristics wavelengths enhanced with plasmonic resonators. The tunability of plasmonic antenna resonances is achieved mainly by their geometry, size or interaction. [1] With highly doped semiconductors, the doping level is an additional parameter to tune the plasma frequency [2], a noble-metal material constant. In this work, we investigate rapid thermal annealing of the semiconductor as a mean to fine tune the resonance wavelength to the molecular absorption features.

Methods

Highly Si-doped InAsSb films were epitaxially grown on GaSb substrates. Plasmonic resonators were fabricated by large-area surface patterning with UV lithography and selective chemical etching. Subsequent rapid thermal annealing were performed at temperatures from 500 to 650ºC with different protective cover masks of InAs or GaSb at the same for as-grown and nanostructured samples. The plasma wavelength was measured by FTIR spectroscopy in the Brewster configuration. [3] The localized surface plasmon resonances (LSPR) were measured by FTIR spectroscopy in reflection configuration.

Results and discussion

Both as-grown  and nanostructured samples show a red-shift of the plasma wavelength and the LSPR respectively, as the temperature increases, indicating a reorganization of dopants in the epitaxial structure. The GaSb cover during the annealing renders a stable surface up to 650ºC in comparison to only 590ºC with the InAs cover. The maximum shifts obtained are 109 and 158 cm^-1 for the plasma wavelength and the LSPR respectively. Moreover, no geometrical degradations were seen in scanning electron microscope images, correlated to an almost constant resonance full width half maximum of the LSPR. However, with the InAs cover mask, material droplets appear between the resonators and their surface density increases with the annealing temperature. Energy dispersive spectroscopy indicates a droplet composition of In and As for an annealing temperature above 545ºC, in correlation to a LSPR intensity decrease. At 590ºC, the final distortion of the plasmonic resonances corresponds to the resonators disintegration.

References

[1] F. Barho et al. Nanophotonics 7, 507 (2018)

[2] M.J. Milla et al. Nanotechnology 7, 425201 (2016)

[3] T. Taliercio et al., Opt. Exp. 22, 24294, (2014)