GaSb oxidation for plasmonic enhanced mid-IR molecular spectroscopy

Introduction Plasmonic mid-IR resonators enable surface-enhanced infrared (SEIRA) spectroscopy to improve the detection of minute quantities of molecules. While the field of fabricating plasmonic SEIRA resonators is dominated... [ view full abstract ]

Introduction

Plasmonic mid-IR resonators enable surface-enhanced infrared (SEIRA) spectroscopy to improve the detection of minute quantities of molecules. While the field of fabricating plasmonic SEIRA resonators is dominated by metals¹, there is an interest to work with highly-doped semiconductors (HDSC) in the mid-IR range for their tunable optical properties² and their potential to combine plasmonic resonator and detection on one chip. We exploit the oxidation of GaSb (supporting layer) to fabricate pedestals and to blue-shift the mid-IR resonances of our InAs_0.9Sb_0.1 resonators. Additionally, the GaSb oxide layer allows to apply phosphonate-oxide bonding for surface functionalization.

Methods

The resonators used in this study were grown by solid-source molecular beam epitaxy (MBE) and were surface patterned by UV-lithography and acid etching. Self-assembled monolayers (SAM) of phosphonates were deposited by a wet chemical process. The samples were characterized by standard optical techniques including Raman spectroscopy, ellipsometry and FTIR-spectroscopy.

Results and Discussion

We show that 50 nm of GaSb on an InAs_0.9Sb_0.1 supporting layer undergoes a material transition upon exposure to water (Fig. 1). After being exposed for 10h to water, the refractive index of the 50 nm toplayer is reduced by more than 50% and Raman measurements indicate that the material transition involves an evolution of different oxidation states. We demonstrate that this oxidation of GaSb allows to fabricate a pedestal for InAs_0.9Sb_0.1 gratings (Fig. 2). The change of refractive index upon oxidation is a post-fabrication technique to tune plasmonic resonances in the mid-IR range. Finally, we present that phosphonate monolayers are formed by a self-assembling wet-chemical process on native GaSb oxide layers. By depositing a phosphonate monolayer on a InAs_0.9Sb_0.1/GaSb grating and by comparing the reflection spectra with those of a monolayer on a non-patterned GaSb layer, we can demonstrate enhanced molecular sensing (Fig. 3).

We conclude that the InAsSb:Si / GaSb material system and in particular the oxidation of GaSb can be exploited for surface enhanced molecular spectroscopy.

References

¹ F. Neubrech, C. Huck, K. Weber, A. Pucci, and H. Giessen, Chem. Rev. (2017).

² Y. Zhong, S.D. Malagari, T. Hamilton, and D. Wasserman, J. Nanophotonics 9, 093791 (2015).