The detection and amplification of molecular absorption lines from a chemical weapons simulant is demonstrated using plasmonic antennas fabricated from n-Ge epitaxially grown on Si. A free-standing Si0.25Ge0.75 microbolometer detector with n-Ge plasmonic antenna is demonstrated as an integrated mid-infrared plasmonic sensor

Baldassarre, Leonetta

Ballabio, Andrea

Biagioni, Paolo

Calandrini, Eugenio

Frigerio, Jacopo

Gallacher, Kevin

Giliberti, Valeria

Isella, Giovanni

Millar, Ross W.

Ortolani, Michele

Paul, Douglas J.

Pellegrini, Giovanni

Sakat, Emilie

English

Enlighten: Publications

  
 
 
 
 
Gallacher, K. et al. (2017) Mid-infrared n-Ge on Si Plasmonic Based Microbolometer 
Sensors. In: 2017 IEEE 14th International Conference on Group IV Photonics (GFP), 
Berlin, Germany, 23-25 Aug 2017, pp. 3-4. ISBN 9781509065684. 
 
   
There may be differences between this version and the published version. You are 
advised to consult the publisher’s version if you wish to cite from it. 
 
 
 
http://eprints.gla.ac.uk/152542/  
     
 
 
 
 
 
 
Deposited on: 27 November 2017 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Enlighten – Research publications by members of the University of Glasgow 
http://eprints.gla.ac.uk 
Mid-Infrared n-Ge on Si Plasmonic Based
Microbolometer Sensors
Kevin Gallacher∗, Ross W. Millar∗, Valeria Giliberti† Eugenio Calandrini† Leonetta Baldassarre† Jacopo Frigerio‡
Andrea Ballabio‡ Emilie Sakat§ Giovanni Pellegrini§ Giovanni Isella‡ Michele Ortolani†
Paolo Biagioni§ and Douglas J. Paul∗,¶
∗School of Engineering, University of Glasgow, Rankine Building, Oakfield Avenue, Glasgow G12 8LT, United Kingdom
†Dipartimento di Fisica, Sapienza Universita´ di Roma, Piazzale Aldo Moro 5, I-00185 Rome, Italy
‡L-NESS, Dipartimento di Fisica, Politecnico di Milano, Polo di Como, Via Anzani 42, I-22100 Como, Italy
§Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy
¶Douglas.Paul@glasgow.ac.uk
Abstract—The detection and amplification of molecular ab-
sorption lines from a chemical weapons simulant is demonstrated
using plasmonic antennas fabricated from n-Ge epitaxially grown
on Si. A free-standing Si0.25Ge0.75 microbolometer detector with
n-Ge plasmonic antenna is demonstrated as an integrated mid-
infrared plasmonic sensor.
I. INTRODUCTION
Plasmons are the free electron oscillations in a metal cou-
pled to photons [1] and allow the near field amplification of
radiation below the plasmon edge defined in wavelength as
λp = 2πc
√
ǫ0ǫ∞m∗
q2N
(1)
where N is the doping density, m∗ is the effective mass, ǫ0
is the permittivity of free space, ǫ∞ is the high frequency
dielectric constant, q is the electron charge, c is the speed of
light and λ is the wavelength [2]. In the visible, metals such as
Au and Ag can be used but these are incompatible with silicon
foundries due to fast diffusion and deep level trap states. Here
we demonstrate heavily doped semiconductors, doped well
above the Mott metal-insulator transition criteria, as plasmonic
materials for the mid-infrared wavelengths. Many molecules
can be identified at mid-infrared wavelengths due to the strong
molecular absorption lines related to individual bonds. We
demonstrate n-Ge epitaxially grown on Si substrates and use it
as a mid-infrared plasmonic material. Plasmonic antennas are
demonstrated to provide amplification of specific molecular
absorption lines of a chemical weapon simulant as an example.
Applications of mid-infrared plasmonic sensors include health-
care, environmental and chemical, biological, radiological,
nuclear and explosives (CBRNE) security sensing.
n-Ge epilayers on high resistivity Si (001) substrates were
grown using low-energy plasma enhanced chemical vapour
deposition [3]. The decrease in the wavelength (increase in
the wavenumber) of the plasmon edge as the doping density
increased for n-Ge epilayers of thicknesses between 600 nm
and 1000 nm [2][4][5] is demonstrated in Fig. 1. An activated
doping density of 1.3×1020 cm−3 at 300 K enables the lowest
wavelength of ∼ 3.9 µm to be achieved. This is sufficient to
010002000300040005000
20.06.74.02.92.2 10.05.03.32.52.0
0.0
0.2
0.4
0.6
0.8
1.0
Re
fle
ct
an
ce
Wavenumber (cm-3)
3.0x1019cm-3
3.4x1019cm-3
5.6x1019cm-3
1.3x1020cm-3
Wavelength (µm)
Fig. 1. The reflectance of 4 different n-Ge doped layers all with thickness
between 600 nm and 1000 nm at 300 K.
enable the whole of the important 6.7 to 20 µm molecular
fingerprint region which has only really been successfully
used with Fourier transform infrared (FTIR) systems and a
significant part of the 3 to 5 µm gas detection window.
Electron beam lithography and reactive ion etching [6] on
the 1000 nm thick n-Ge material (comparable to the skin depth
in the 10 to 20 µm wavelength region) [4] was used to fabricate
a range of single and gap antennas into arrays over a 5 × 5
mm2 array. Fig. 2 provides a scanning electron microscope
(SEM) image of a set of gap antennas. The substrate side
plasmonic resonance can be tuned by varying the antenna
length (1 to 4 µm) allowing the targeting of specific molecular
absorption lines for identification [4].
Fig. 3 demonstrates a chemical weapons application using
the n-Ge antennas in Fig. 2. The FTIR spectrum of the mustard
gas simulant chloroethyl methyl sulphide (CEMS - see Fig.
3 insert for chemical structure) has a C-Cl bond absorption
doublet at approximately 14 µm wavelength. Most explosives,
978-1-5090-6567-7/17/$31.00 ©2017 IEEE
10 µm 30 µm
Fig. 2. SEM images of an array of gap antennas with 3 µm long n-Ge sections
and 300 nm gaps fabricated on top of a Si substrate.
CEMS FTIR spectrum
CEMS on antennas
clean antennas
difference spectrum
n
oi
s
si
m
s
n
ar
T
n
oi t
c
elf
e
R
0.0
0.2
0.4
0.8
1.0
(x 4)
600 800 1000 1200 1400
Wavenumber (cm )
-1
b
16.7 12.5 10.0 8.3 7.1
Wavelength ( m)
Cl
CH
3
S
Fig. 3. Top: the transmission spectrum of CEMS measured by FTIR. Middle:
the reflection spectrum of a n-Ge plasmonic antenna array without CEMS
(dashed black line) and the array with CEMS (red line). Bottom: the difference
between the clean and CEMS coated plasmonic antenna spectrums.
chemical and biological weapons have unique molecular bond-
ing absorption lines in the spectroscopic fingerprint region of
6.7 to 15 µm. A few drops of CEMS liquid was deposited on a
set of antennas (red line in Fig. 3) and the difference was taken
from a clean set of antennas (black dashed line) to leave the
difference spectrum of CEMS (solid black line at bottom of
Fig. 3). Only the absorption lines for wavelengths longer than
the plasmon edge (10 µm for this material doped at 2.3×1019
cm−3) are amplified by the plasmonic antennas. n-Ge antennas
used to measure absorptions in polydimethylsiloxane (PDMS)
have demonstrated amplification up to two orders of magnitude
at 12.5 µm wavelength [4]. Whilst Au and Ag do provide
lower losses than n-Ge due to the higher electrical conduc-
tivity, this is counterbalanced by a significantly higher degree
of field confinement in n-Ge since the doped semiconductor
50 µm
5 µm
Fig. 4. A SEM image of a microbolometer sensor with single plasmonic
antenna array to provide amplification for detection of molecular absorption
at specific wavelengths. The insert provides a detailed SEM image of the
plasmonic antenna array.
demonstrates true plasmonic behaviour [5].
Fig. 4 is a SEM image of single n-Ge antenna fabricated on
a free standing microbolometer detector. Simulations indicate
that a 1 µW absorption from a plasmonic resonance illumi-
nated from a mid-infrared source raises the microbolometer
temperature by 5.5 K inside 150 ms whilst the microbolome-
ters have sensitivity to mK temperature changes.
II. CONCLUSION
Physical doping has been used to demonstrate plasmonic
behaviour in Ge antenna on Si substrates down to ∼3.9 µm
wavelengths. Examples of chemical weapons identification
and integrated mid-infrared sensors on a Si platform using
microbolometer detectors have been demonstrated.
ACKNOWLEDGMENT
The research leading to these results has received funding
from the European Unions Seventh Framework Programme
under grant agreement no. 613055.
REFERENCES
[1] A. Boltasseva and H. A. Atwater, “Low-loss plasmonic metamaterials,”
Science, vol. 331, no. 6015, pp. 290–291, 2011. [Online]. Available:
http://science.sciencemag.org/content/331/6015/290
[2] P. Biagioni et al., “Group-IV midinfrared plasmonics,” J.
Nanophotonics, vol. 9, no. 1, p. 093789, 2015. [Online]. Available:
http://dx.doi.org/10.1117/1.JNP.9.093789
[3] G. Isella et al., “Low-energy plasma-enhanced chemical vapor deposition
for strained Si and Ge heterostructures and devices,” Solid-State
Electron., vol. 48, no. 8, pp. 1317–1323, Aug. 2004. [Online]. Available:
http://linkinghub.elsevier.com/retrieve/pii/S0038110104000565
[4] L. Baldassarre et al., “Mid-infrared plasmon-enhanced spectroscopy
with germanium antennas on silicon substrates,” Nano Lett.,
vol. 15, no. 11, pp. 7225–7231, 2015. [Online]. Available:
http://dx.doi.org/10.1021/acs.nanolett.5b03247
[5] J. Frigerio et al., “Tunability of the dielectric function of heavily
doped germanium thin films for mid-infrared plasmonics,” Phys.
Rev. B, vol. 94, p. 085202, Aug 2016. [Online]. Available:
http://link.aps.org/doi/10.1103/PhysRevB.94.085202
[6] M. M. Mirza et al., “Determining the electronic performance limitations
in top-down-fabricated si nanowires with mean widths down to 4 nm,”
Nano Lett., vol. 14, no. 11, pp. 6056–6060, 2014. [Online]. Available:
http://dx.doi.org/10.1021/nl5015298


Mid-infrared n-Ge on Si Plasmonic Based Microbolometer Sensors

Kevin Gallacher

Ross W. Millar

Valeria Giliberti

Eugenio Calandrini

Leonetta Baldassarre

Jacopo Frigerio

Andrea Ballabio

Emilie Sakat

Giovanni Pellegrini

Giovanni Isella

Michele Ortolani

Paolo Biagioni

Douglas J. Paul

Crossref

Mid-infrared n-Ge on Si plasmonic based microbolometer sensors

Enlighten

      Gallacher, K. et al. (2017) Mid-infrared n-Ge on Si Plasmonic Based Microbolometer Sensors. In: 2017 IEEE 14th International Conference on Group IV Photonics (GFP), Berlin, Germany, 23-25 Aug 2017, pp. 3-4. ISBN 9781509065684.     There may be differences between this version and the published version. You are advised to consult the publisher’s version if you wish to cite from it.    http://eprints.gla.ac.uk/152542/             Deposited on: 27 November 2017                       Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk Mid-Infrared n-Ge on Si Plasmonic BasedMicrobolometer SensorsKevin Gallacher∗, Ross W. Millar∗, Valeria Giliberti† Eugenio Calandrini† Leonetta Baldassarre† Jacopo Frigerio‡Andrea Ballabio‡ Emilie Sakat§ Giovanni Pellegrini§ Giovanni Isella‡ Michele Ortolani†Paolo Biagioni§ and Douglas J. Paul∗,¶∗School of Engineering, University of Glasgow, Rankine Building, Oakfield Avenue, Glasgow G12 8LT, United Kingdom†Dipartimento di Fisica, Sapienza Universita´ di Roma, Piazzale Aldo Moro 5, I-00185 Rome, Italy‡L-NESS, Dipartimento di Fisica, Politecnico di Milano, Polo di Como, Via Anzani 42, I-22100 Como, Italy§Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, I-20133 Milano, Italy¶Douglas.Paul@glasgow.ac.ukAbstract—The detection and amplification of molecular ab-sorption lines from a chemical weapons simulant is demonstratedusing plasmonic antennas fabricated from n-Ge epitaxially grownon Si. A free-standing Si0.25Ge0.75 microbolometer detector withn-Ge plasmonic antenna is demonstrated as an integrated mid-infrared plasmonic sensor.I. INTRODUCTIONPlasmons are the free electron oscillations in a metal cou-pled to photons [1] and allow the near field amplification ofradiation below the plasmon edge defined in wavelength asλp = 2πc√ǫ0ǫ∞m∗q2N(1)where N is the doping density, m∗ is the effective mass, ǫ0is the permittivity of free space, ǫ∞ is the high frequencydielectric constant, q is the electron charge, c is the speed oflight and λ is the wavelength [2]. In the visible, metals such asAu and Ag can be used but these are incompatible with siliconfoundries due to fast diffusion and deep level trap states. Herewe demonstrate heavily doped semiconductors, doped wellabove the Mott metal-insulator transition criteria, as plasmonicmaterials for the mid-infrared wavelengths. Many moleculescan be identified at mid-infrared wavelengths due to the strongmolecular absorption lines related to individual bonds. Wedemonstrate n-Ge epitaxially grown on Si substrates and use itas a mid-infrared plasmonic material. Plasmonic antennas aredemonstrated to provide amplification of specific molecularabsorption lines of a chemical weapon simulant as an example.Applications of mid-infrared plasmonic sensors include health-care, environmental and chemical, biological, radiological,nuclear and explosives (CBRNE) security sensing.n-Ge epilayers on high resistivity Si (001) substrates weregrown using low-energy plasma enhanced chemical vapourdeposition [3]. The decrease in the wavelength (increase inthe wavenumber) of the plasmon edge as the doping densityincreased for n-Ge epilayers of thicknesses between 600 nmand 1000 nm [2][4][5] is demonstrated in Fig. 1. An activateddoping density of 1.3×1020 cm−3 at 300 K enables the lowestwavelength of ∼ 3.9 µm to be achieved. This is sufficient to01000200030004000500020.06.74.02.92.2 10.05.03.32.52.00.00.20.40.60.81.0ReflectanceWavenumber (cm-3)3.0x1019cm-33.4x1019cm-35.6x1019cm-31.3x1020cm-3Wavelength (µm)Fig. 1. The reflectance of 4 different n-Ge doped layers all with thicknessbetween 600 nm and 1000 nm at 300 K.enable the whole of the important 6.7 to 20 µm molecularfingerprint region which has only really been successfullyused with Fourier transform infrared (FTIR) systems and asignificant part of the 3 to 5 µm gas detection window.Electron beam lithography and reactive ion etching [6] onthe 1000 nm thick n-Ge material (comparable to the skin depthin the 10 to 20 µm wavelength region) [4] was used to fabricatea range of single and gap antennas into arrays over a 5 × 5mm2 array. Fig. 2 provides a scanning electron microscope(SEM) image of a set of gap antennas. The substrate sideplasmonic resonance can be tuned by varying the antennalength (1 to 4 µm) allowing the targeting of specific molecularabsorption lines for identification [4].Fig. 3 demonstrates a chemical weapons application usingthe n-Ge antennas in Fig. 2. The FTIR spectrum of the mustardgas simulant chloroethyl methyl sulphide (CEMS - see Fig.3 insert for chemical structure) has a C-Cl bond absorptiondoublet at approximately 14 µm wavelength. Most explosives,978-1-5090-6567-7/17/$31.00 ©2017 IEEE10 µm 30 µmFig. 2. SEM images of an array of gap antennas with 3 µm long n-Ge sectionsand 300 nm gaps fabricated on top of a Si substrate.CEMS FTIR spectrumCEMS on antennasclean antennasdifference spectrumnoissimsnarTnoi tcelfeR0.00.20.40.81.0(x 4)600 800 1000 1200 1400Wavenumber (cm )-1b16.7 12.5 10.0 8.3 7.1Wavelength ( m)ClCH3SFig. 3. Top: the transmission spectrum of CEMS measured by FTIR. Middle:the reflection spectrum of a n-Ge plasmonic antenna array without CEMS(dashed black line) and the array with CEMS (red line). Bottom: the differencebetween the clean and CEMS coated plasmonic antenna spectrums.chemical and biological weapons have unique molecular bond-ing absorption lines in the spectroscopic fingerprint region of6.7 to 15 µm. A few drops of CEMS liquid was deposited on aset of antennas (red line in Fig. 3) and the difference was takenfrom a clean set of antennas (black dashed line) to leave thedifference spectrum of CEMS (solid black line at bottom ofFig. 3). Only the absorption lines for wavelengths longer thanthe plasmon edge (10 µm for this material doped at 2.3×1019cm−3) are amplified by the plasmonic antennas. n-Ge antennasused to measure absorptions in polydimethylsiloxane (PDMS)have demonstrated amplification up to two orders of magnitudeat 12.5 µm wavelength [4]. Whilst Au and Ag do providelower losses than n-Ge due to the higher electrical conduc-tivity, this is counterbalanced by a significantly higher degreeof field confinement in n-Ge since the doped semiconductor50 µm5 µmFig. 4. A SEM image of a microbolometer sensor with single plasmonicantenna array to provide amplification for detection of molecular absorptionat specific wavelengths. The insert provides a detailed SEM image of theplasmonic antenna array.demonstrates true plasmonic behaviour [5].Fig. 4 is a SEM image of single n-Ge antenna fabricated ona free standing microbolometer detector. Simulations indicatethat a 1 µW absorption from a plasmonic resonance illumi-nated from a mid-infrared source raises the microbolometertemperature by 5.5 K inside 150 ms whilst the microbolome-ters have sensitivity to mK temperature changes.II. CONCLUSIONPhysical doping has been used to demonstrate plasmonicbehaviour in Ge antenna on Si substrates down to ∼3.9 µmwavelengths. Examples of chemical weapons identificationand integrated mid-infrared sensors on a Si platform usingmicrobolometer detectors have been demonstrated.ACKNOWLEDGMENTThe research leading to these results has received fundingfrom the European Unions Seventh Framework Programmeunder grant agreement no. 613055.REFERENCES[1] A. Boltasseva and H. A. Atwater, “Low-loss plasmonic metamaterials,”Science, vol. 331, no. 6015, pp. 290–291, 2011. [Online]. Available:http://science.sciencemag.org/content/331/6015/290[2] P. Biagioni et al., “Group-IV midinfrared plasmonics,” J.Nanophotonics, vol. 9, no. 1, p. 093789, 2015. [Online]. Available:http://dx.doi.org/10.1117/1.JNP.9.093789[3] G. Isella et al., “Low-energy plasma-enhanced chemical vapor depositionfor strained Si and Ge heterostructures and devices,” Solid-StateElectron., vol. 48, no. 8, pp. 1317–1323, Aug. 2004. [Online]. Available:http://linkinghub.elsevier.com/retrieve/pii/S0038110104000565[4] L. Baldassarre et al., “Mid-infrared plasmon-enhanced spectroscopywith germanium antennas on silicon substrates,” Nano Lett.,vol. 15, no. 11, pp. 7225–7231, 2015. [Online]. Available:http://dx.doi.org/10.1021/acs.nanolett.5b03247[5] J. Frigerio et al., “Tunability of the dielectric function of heavilydoped germanium thin films for mid-infrared plasmonics,” Phys.Rev. B, vol. 94, p. 085202, Aug 2016. [Online]. Available:http://link.aps.org/doi/10.1103/PhysRevB.94.085202[6] M. M. Mirza et al., “Determining the electronic performance limitationsin top-down-fabricated si nanowires with mean widths down to 4 nm,”Nano Lett., vol. 14, no. 11, pp. 6056–6060, 2014. [Online]. Available:http://dx.doi.org/10.1021/nl5015298

The detection and amplification of molecular absorption lines from a chemical weapons simulant is demonstrated using plasmonic antennas fabricated from n-Ge epitaxially grown on Si. A free-standing Si0.25Ge0.75microbolometer detector with n-Ge plasmonic antenna is demonstrated as an integrated mid-infrared plasmonic sensor

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Mid-infrared n-Ge on Si Plasmonic Based Microbolometer Sensors

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