Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics

Frigerio, J. et al. (2016) Tunability of the dielectric function of heavily doped germanium thin films for mid-infrared plasmonics. Physical Review B, 94(8), 085202. (doi:10.1103/PhysRevB.94.085202)

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Heavily doped semiconductor thin films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in the 5 to 50 μm wavelength range at least. In this work, we investigate the electrodynamics of heavily n-type-doped germanium epilayers at infrared frequencies beyond the assumptions of the Drude model. The films are grown on silicon and germanium substrates, are in situ doped with phosphorous in the 10 17 to 10 19 cm−3 range, then screened plasma frequencies in the 100 to 1200 cm−1 range were observed. We employ infrared spectroscopy, pump-probe spectroscopy, and dc transport measurements to determine the tunability of the plasma frequency. Although no plasmonic structures have been realized in this work, we derive estimates of the decay time of mid-infrared plasmons and of their figures of merit for field confinement and for surface plasmon propagation. The average electron scattering rate increases almost linearly with excitation frequency, in agreement with quantum calculations based on a model of the ellipsoidal Fermi surface at the conduction band minimum of germanium accounting for electron scattering with optical phonons and charged impurities. Instead, we found weak dependence of plasmon losses on neutral impurity density. In films where a transient plasma was generated by optical pumping, we found significant dependence of the energy relaxation times in the few-picosecond range on the static doping level of the film, confirming the key but indirect role played by charged impurities in energy relaxation. Our results indicate that underdamped mid-infrared plasma oscillations are attained in n-type-doped germanium at room temperature.

Item Type:Articles
Additional Information:The research leading to these results has received funding from the European Unions Seventh Framework Programme under Grant Agreement No. 613055.
Glasgow Author(s) Enlighten ID:Gallacher, Dr Kevin and Paul, Professor Douglas
Authors: Frigerio, J., Ballabio, A., Isella, G., Sakat, E., Pellegrini, G., Biagioni, P., Bollani, M., Napolitani, E., Manganelli, C., Virgilio, M., Grupp, A., Fischer, M. P., Brida, D., Gallacher, K., Paul, D. J., Baldassarre, L., Calvani, P., Giliberti, V., Nucara, A., and Ortolani, M.
College/School:College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
Journal Name:Physical Review B
Publisher:American Physical Society
ISSN (Online):1550-235X
Published Online:15 August 2016
Copyright Holders:Copyright © 2016 American Physical Society
First Published:First published in Physical Review B 94(8):085202
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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Project CodeAward NoProject NamePrincipal InvestigatorFunder's NameFunder RefLead Dept
634451GEMINIDouglas PaulEuropean Commission (EC)613055ENG - ENGINEERING ELECTRONICS & NANO ENG