Nanoscale View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
PAPER
Cite this: Nanoscale, 2014, 6, 1390
View Journal | View Issue
Integrated architecture for the electrical detection of plasmonic resonances based on high electron mobility photo-transistors† Davide Sammito,‡*ab Davide De Salvador,‡*c Pierfrancesco Zilio,ac Giorgio Biasiol,b Tommaso Ongarello,ac Michele Massari,a Gianluca Ruffato,ac Margherita Morpurgo,d Davide Silvestri,d Gianluigi Maggioni,ce Gianluca Bovo,c Michele Gaioc and Filippo Romanatoabc We report the design of an integrated platform for on-chip electrical transduction of the surface plasmon resonance supported by a nanostructured metal grating. The latter is fabricated on the active area of a GaAs/AlGaAs photo-HEMT and simultaneously works as the electronic gate of the device. The gold
Received 2nd September 2013 Accepted 28th October 2013
plasmonic crystal has a V-groove profile and has been designed by numerical optical simulations. By showing that the numerical models accurately reproduce the phototransistors experimental response,
DOI: 10.1039/c3nr04666d
we demonstrate that the proposed architecture is suitable for the development of a new class of
www.rsc.org/nanoscale
compact and scalable SPR sensors.
Introduction Surface Plasmon Polaritons (SPPs) are electromagnetic waves that can be excited and guided on metallic surfaces and nanostructures. Due to the possibility they offer to be conned in subwavelength sized spaces, SPPs have been proposed to merge the technological gap between photonics and electronics, which poses a real obstacle to the realization of high-bandwidth fullyintegrated optoelectronic chips.1,2 Additionally, the opportunity for electrical signals to be carried on the same metallic structures supporting SPPs3–5 is a key factor fostering the integration of plasmonic and electronic functionalities in the same devices. Another notable application area of SPPs is in biosensing, which takes advantage of the high near eld enhancement close to metallic surfaces and nanostructures to sense the local variations in refractive index due to the binding of molecular layers.
Laboratory for Nanofabrication of Nanodevices, LaNN – Veneto Nanotech, Corso Stati Uniti 4, 35127 Padova, Italy. E-mail: [email protected]
a
Istituto Officina dei Materiali, IOM – CNR, SS. 14 km 163.5 in Area Science Park, 34149 Basovizza, Trieste, Italy
b
c
University of Padova, Department of Physics and Astronomy, Via Marzolo 8, 35131 Padova, Italy. E-mail: [email protected] d
University of Padova, Department of Pharmaceutical and Pharmacological Sciences, Via Marzolo 5, 35131 Padova, Italy
e
INFN Legnaro National Laboratories, Viale dell'Universit` a 2, 35020 Legnaro, Pd, Italy
† Electronic supplementary information (ESI) available: HEMTs optical response, device fabrication ow chart, estimate of device resolution based on numerical simulations, synthesis of thiolated biotin, biotin functionalization and avidin binding procedure. See DOI: 10.1039/c3nr04666d ‡ These authors contributed equally to the experimental work reported in the paper.
1390 | Nanoscale, 2014, 6, 1390–1397
The analyte’s binding kinetics is conventionally monitored by characterizing the properties of scattered waves in the far-eld by using external photodetectors coupled to spectroscopic and/ or goniometric systems. The current trend is to obtain compact on-chip electronic processing of the sensing signal6 to enable multichannel detection.7 Thus, an essential feature to be implemented in both optoelectronic and biosensing chips is the detection of plasmonic signals or, in other words, the local and direct transduction of plasmonic excitations into electrical signals. This objective can be achieved in semiconductor detectors by plasmonic waves-mediated photogeneration in the near-eld regime.5,8–10 This kind of approach to integrated biosensing architectures imposes antithetical optimization constraints: on the one hand the photodetector performances are intrinsically improved by increasing the absorption and the photogeneration in the active area;11–14 on the other hand the absorption in the semiconductor should be minimized to increase the energy density at the metal–analyte interface, crucial for biosensing. In this work we describe an innovative SPPs detection platform that overcomes the above trade-off. It is based on a High Electron Mobility photo-Transistor (photo-HEMT) suitable for photonic circuitry applications. Impinging light is coupled to SPPs by a V-shaped grating at the air–metal interface of a relatively thick Au lm (180 nm). Only a small fraction of the incident irradiance (between 103 and 104) is transmitted by the plasmonic crystal and absorbed by the underlying highly sensitive photo-HEMT, allowing it to perform an accurate optical-to-electrical transduction of the intensity modulation signal of visible light. Since it is based on a planar phototransistor, the present architecture has the advantage of being
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
scalable and a rst indication of this characteristic is provided by showing that the output signal is not affected by the different surface areas of two prototypes. Furthermore, we demonstrate that the nanopatterned metallic surface simultaneously works as a plasmonic structure and an electronic gate to the transistor, providing gain in SPP signal detection through the application of a small bias voltage. We prove the potentialities of this plasmonic resonances detection architecture by showing that the photo-HEMT’s response is affected by the presence of a few nanometres-thick organic lm coating the gold surface. Thus, we propose this innovative detection scheme for the development of SPR biosensors based on intensity modulation and grating coupling. The choice of this SPP coupling scheme over prism coupling, the xed design wavelength and incidence angle and the direct electrical read-out of the plasmonic related signal can make the sensing setup particularly compact and suited to multiplexing.
Device structure design and realization The phototransistor is made of a GaAs/Al0.42Ga0.58As modulation doped heterostructure. The schematics of the device are reported in Fig. 1a and b. For the chosen Al mole fraction, the AlGaAs layer is transparent to the incident radiation at 633 nmwavelength. The lattice matched multilayer, grown by Molecular Beam Epitaxy on semi-insulating (100) GaAs wafers consists of a 300 nm-thick i-GaAs buffer, a 500 nm-thick GaAs (2 nm)/ Al0.42Ga0.58As (8 nm) superlattice, followed by a 500 nm-thick GaAs layer, a 100 nm-thick i-AlGaAs deep spacer, a 30 nm-thick Si-doped (4 1018 cm3) AlGaAs layer, a 100 nm thick i-AlGaAs top spacer and a 300 nm-thick GaAs capping layer. This multilayer scheme resembles that typical of high electron mobility heterostructures, which were demonstrated to feature a non linear photoresponse with high sensitivity at low optical power levels.15,16 We modied the standard semiconductor heterostructure to adapt it to our purposes. The thick capping layer has been added in order to integrate the V-shaped grating by
Nanoscale
anisotropic wet etching of GaAs. As a consequence, the top AlGaAs spacer is needed to keep the potential well at the top GaAs/AlGaAs heterointerface barely populated, at least in dark conditions, as shown in the band diagram and electron density prole simulated by Schr¨ odinger–Poisson modelling (Fig. 1c). A batch of test heterostructures has been grown and characterized by Van der Pauw Hall effect measurements to optimize the doping density, the epitaxial growth conditions and the layer thicknesses and to assess their inuence on the 2DEG resistance under illumination. The semiconductor substrates used to fabricate the devices have a 2DEG carrier density of the order of 5 1010 cm2 and a mobility of the order of 4000 cm2 V1 s1 in dark conditions and at room temperature. The devices have a Hall bar layout (Fig. 1b) for the measurement of the channel resistance under controlled illumination conditions. A gold lm coats the nanostructured GaAs surface forming a Schottky contact on the active area and is connected by a lead to a pad for the electrical biasing of the gate. A current is driven between source and drain (Ni–Ge–Au multilayer annealed to form ohmic contacts with the 2DEG) and the relative voltage drop is measured with a four-point probes architecture (see Fig. 1b). The transistor characteristics are acquired by two channel source meters (Keithley 2612A) that allow the measurement of the voltage drop at V–V+ terminals (Vds) as a function of the source–drain current (Ids), and to set the gate to source voltage while monitoring the gate leakage current. Therefore, the conductance of the 2DEG channel can be measured by computing Ids vs. Vds linear ts for different gate voltages. A polarized HeNe laser beam is focused within the squared active area of the device (500 mm-side) with a divergence of 0.1 . The sample socket is placed on a goniometric stage with the rotation axis on the gold surface and intersecting the incident beam axis for angle-resolved characterizations. The photoresponse curve of the device is reported in Fig. 2a. It is measured by applying different neutral density lters to tune the irradiance at a xed incidence angle. The light
Fig. 1 (a) Schematic cross section of the active area of the phototransistor integrating a V-groove-shaped gold plasmonic crystal. (b) Optical microscope image of the device showing the terminals for electronic driving and measurement. (c) Band diagram and electron density profile of the flat epitaxial multilayer in dark conditions simulated by Schro ¨ dinger–Poisson numerical modelling.
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 1390–1397 | 1391
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Nanoscale
Paper
reduction of the transistor threshold voltage and thus an increase of channel conductance, with a logarithmic dependence on the incident optical irradiance.16 Our photo-response data are well tted by the analytical models proposed,17 as shown in the ESI† section. Furthermore, unlike photoresistors and photodiodes, which measure the photogenerated current, in phototransistors the conductance signal does not scale with the active area of the device working with xed incidence irradiance values. Using the detector in the working regime above the threshold irradiance, the conductance has a linear dependence on the logarithm of the relative irradiance: s ¼ ms ln(aT ) + q
(2)
This implies that, at xed attenuation in the high intensity regime, variations of conductance depend on the relative transmittance variations. Indeed, by differentiating eqn (2), we get: Ds ¼ ms
(a) Interpolated conductance of the device as a function of incident irradiance for different voltage biases applied to the gate. The incident irradiance values are normalized to the non-attenuated laser beam, having an integrated optical power of about 1 mW and an 80 mm spot size at beam waist. (b) Slope of the responsivity curves shown in (a) in the high irradiance region (aT ¼ 100.2), calculated as derivative of the conductance with respect to the logarithm of relative irradiance as a function of the gate voltage bias. Fig. 2
irradiance reaching the 2DEG (Ipd) is the product of the incident irradiance (I0), the transmittance of the top gold structure (T) and the attenuation factor due to the lters (a): Ipd ¼ I0aT
(1)
In Fig. 2a the 2DEG conductance is plotted as a function of the relative irradiance Ipd/I0 ¼ aT for different voltage biases applied to the gate. Above a threshold irradiance value, the channel conductance shows a steep dependence on the incident power ux. Considering that the patterned gold lm has a transmittance of the order of 104 (see below) and that the non attenuated power of the incident laser beam is 1 mW, from this graph it appears that the heterostructures are sensitive to relatively low light levels, of the order of 10 nW. As demonstrated in the literature, the optical response of HEMTs at low frequency is dominated by the photovoltaic effect, whereby the accumulation of photogenerated holes in the channel induces a
1392 | Nanoscale, 2014, 6, 1390–1397
DT T
(3)
The proportionality factor ms can be considered as the sensitivity of the semiconductor heterostructure: the higher its value, the higher will be the sensitivity to modulations of the transmittance due to the plasmonic structure. Interestingly, this gure of merit can be enhanced and optimized by applying a small negative gate to source voltage bias, as can be seen in Fig. 2b. On the other hand, by increasing the absolute value of this negative bias, the threshold irradiance increases while the dark conductance decreases due to depletion of the 2DEG population in the channel.
Plasmonic grating simulation and fabrication At xed incident optical power, the irradiance incident upon the absorbing substrate can be modulated by the nanostructured front surface by means of the supported surface plasmon resonance. In the present work, we incorporate on the active area of the device, a plasmonic grating obtained by conformal evaporation of a gold layer on top of the periodic V-groove array etched in the topmost GaAs capping layer. The aperture angle of the bottom vertex of the V-grooves is 70.53 , which is the typical angle formed by {111} crystal planes. Grooves with a triangular cross section milled in metal lms have raised much interest in plasmonics for nanofocusing and waveguiding applications because they support Channel Plasmon Polariton (CPP) modes featuring subwavelength connement with relatively low propagation losses.18 Arrays of such grooves were investigated by Søndergaard and Bozhevolnyi,19 who highlighted the existence and interplay of three types of optical resonances under monochromatic illumination, namely: geometric resonances related to the shape of the individual grooves; standing-wave SPP resonances due to coherent scattering by the periodic grooves structure; Wood–Rayleigh anomalies corresponding to congurations for which a diffraction order lies in the plane of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
the grating. These resonances were detected in the reection spectra and it was shown that their minima correspond to peaks in the spectra of eld enhancement on groove valleys. To the best of our knowledge, in the literature the transmission properties of this kind of plasmonic gratings have not been analyzed yet. The optical response of the present structure has been simulated by means of full-eld 2D Finite Elements simulations, using the soware COMSOL Multiphysics.22 Considering the peculiar logarithmic photoresponse of our detector, the period, V-groove width and metal thickness of the plasmonic structure have been optimized by optical simulations in order to maximize its relative transmittance variation (|DT|/T) with respect to the variation of the refractive index of a 5 nm-thick dielectric coating from n ¼ 1 to n ¼ 1.45. In other words, the plasmonic structure is designed to maximize the surface sensitivity of the device in view of the application as an affinity biosensor, working with normally impinging TM-polarized 633 nm-wavelength plane waves. The obtained map of |DT|/T as a function of period and wedge width-to-period ratio (duty cycle) is reported in Fig. 3a. As can be seen, a sharp peak is observed around a grating pitch of 628 nm and a duty cycle of 0.58 (groove width 364 nm), where |DT|/T reaches values as high as 7 for an optimal gold thickness of 180 nm. Such a high metal thickness allows most of the coupled light energy to be conned to the upper metal–dielectric interface, maximizing the interaction with the coating to be detected. The optical properties of the grating are then expected to be almost identical to those of a groove array on a semi-innite metal slab. As pointed out in the literature,23 nanostructured SPR sensors feature a signicant dependence of the optimized surface prole on the refractive index of the target analyte to be detected and on the dielectric environment. For the simulations shown in Fig. 3a, we assumed 1.45 as the refractive index of a typical protein-based analyte lm in an air environment. We also performed the parametric optical design of the grating structure for the same thin layer in a water-based environment (refractive index 1.33) in place of air. The result (not shown here) is that the optimal period and duty cycle shi to 450 nm and 0.5 respectively. The new optimal conguration still corresponds to the excitation of the same SPP mode coupled at the gold surface in the case of an air environment, conrming that the plasmonic structure can be properly re-designed considering the specic experimental conditions for its application as an SPR substrate. The origin of the high-sensitivity peak in the optimization map (Fig. 3a) can be ascribed to the optical properties of the nanostructure. The latter supports a radiative plasmonic Bloch mode propagating normally to the grooves (in the x-direction), which can be excited by impinging TM-polarized light, according to the well-known grating coupling formula k0 sin(q) + nG ¼ kSPP, where k0 is the vacuum wave vector, q the incidence angle, G ¼ 2p/d the reciprocal lattice vector, d being the period, and kSPP the Bloch mode propagation constant. We calculated the value of kSPP by means of the nite elements based modal analysis developed in ref. 24. In Fig. 3b we report the reectance map of the optimal grating structure (in an air environment) as
This journal is © The Royal Society of Chemistry 2014
Nanoscale
a function of period and incidence angle, for l ¼ 633 nm TMpolarized light. We superimpose the Bloch mode geometrical dispersion (white dots), calculated by means of the modal analysis and using the aforementioned grating coupling relation, together with same curve for SPPs at a at metal–dielectric interface (yellow line) and the Wood's anomalies locations (black line). A clear dip of reectance is observed in correspondence of the excitation of the Bloch mode, which departs signicantly from the at-surface SPP mode, owing to the strong surface corrugation25 caused by the grooves. The eigenmode electric eld norm is reported in Fig. 3b (inset). Interestingly, it shows an increased eld at the grooves bottom. Indeed, the excited Bloch mode combines a vertical groove resonance, which determines an enhanced transmission toward the underlying layers, with the horizontal SPP grating resonance,19 which is sensitive to the dielectric environment above the grating surface.7,26 Therefore, the resulting Fano-like resonance of the structure shows an enhanced transmittance sensitivity to surface refractive index variations. This is clearly seen in Fig. 3c, where we report the transmittance spectra at normal incidence for the optimized gold grating, with and without the thin dielectric lm (refractive index 1.45) on top, in conjunction with the Figure Of Merit (FOM) |DT|/T. The eld distributions at the design wavelength in absence and presence of the dielectric layer (Fig. 3d) show a signicant difference in the eld concentration and in the power ow inside the grooves, which translates in a considerable transmittance variation. It is worth emphasizing that, for the optimal working parameters which maximize the FOM, the transmittance of the structure is close to a minimum (about 104 without the dielectric lm, blue line in Fig. 3c). Aer the mesa and metal contacts lithographic patterning steps (see the ESI† for details on the ow chart), the designed gratings have been integrated on the active area by Electron Beam Lithography (100 keV Jeol JBX-6300FS at LaNN Labora] crystory) of an array of slits in PMMA aligned with the [011 tallographic orientation. The resist is used as a mask for the anisotropic wet etching of (100) GaAs27 in a H2SO4 : H2O2 : H2O solution (1 : 8 : 40 by volume) and then stripped. Tuning the resist slit-width and etching time, the optimized grating geometric parameters can be closely matched. Furthermore, as can be seen in the optimization map (Fig. 3a), the design is tolerant to effective groove widths narrower than the target value. The groove side walls meet with sharp corners at the bottom of the structures, as shown in the SEM micrograph of the grating aer gold conformal coating by e-gun evaporation (Fig. 4a). Optical characterizations of the plasmonic crystals integrated on the gate of the devices have been performed by spectroscopic ellipsometry. The specular reectance at 15 incidence angle for TM polarization has been measured and compared with the spectrum simulated by FEM (Fig. 4b). The geometric parameters (grating pitch and groove width) measured by SEM images have been used as input for the simulation. Specically, the sample under consideration has a pitch of 628 nm, corresponding to the optimized conguration, and an effective groove width of 340 nm. The agreement
Nanoscale, 2014, 6, 1390–1397 | 1393
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Nanoscale
Paper
Fig. 3 (a) FEM simulation of the relative transmittance variation of the gold grating between the bare surface and the same surface coated by a 5 nm-thick dielectric film with a refractive index of 1.45. This figure of merit is mapped versus the grating pitch and groove width-to-period ratio (duty cycle) in order to optimize the nanostructure geometry for the device application as a biosensor working with normally incident TMpolarized light at 633 nm wavelength. (b) Simulated reflectance map as a function of grating pitch and incidence angle for a fixed groove width of 364 nm. Overplotted on the map are the Bloch mode dispersion relation (white dots), the analytical dispersion relation for a SPP at a flat gold–air interface (yellow line) and the Wood–Rayleigh grating anomaly (black line). The blue dashed lines refer to the two grating periods of the batches of devices fabricated: 628 nm and 610 nm. Inset: electric field norm of the Bloch SPP eigenmode supported by the grating (d ¼ 610 nm). (c) FEM simulated transmittance spectra of the plasmonic grating without (blue line) and with (green line) the thin dielectric film modelling the bioanalyte layer to be detected. The relative transmittance variation spectrum is also plotted (red line). (d) Distribution of electric field norm enhancement generated by a normally incident plane wave without (left) and with (right) the thin dielectric film coating on the gold surface. Arrows represent the Poynting vector. The single unitary cell of the grating has been simulated setting Bloch–Floquet boundary conditions for the fields at the sides and Perfectly Matched Layers above and below the grating to limit the computational domain. The adopted relative permittivity of gold has been measured by ellipsometry, while for GaAs and AlGaAs tabulated data have been used.20,21
between simulation and experiment is excellent, as shown in Fig. 4b, where the simulated reectance at normal incidence is reported as additional reference.
Plasmonic resonance detection by the photo-HEMT Channel resistance characterizations under TM laser illumination have been performed aligning the rotation axis of the motorized stage along the V-grooves and acquiring the signal within few degrees incidence angle. The angular scans are bellshaped and peaked at normal incidence as shown in Fig. 5. Coherently with considerations about the photoresponse curves
1394 | Nanoscale, 2014, 6, 1390–1397
reported in Fig. 2, the device signal is signicantly enhanced by applying negative voltages to the gate terminal. Thus the nanostructured metal lm effectively performs both as a plasmonic structure and as an electronic gate of the transistor, modulating the 2DEG population and the device sensitivity to transmitted light. In order to verify that the shape of the plasmonic resonance is accurately transduced into an electrical signal, the experimentally acquired angular scans have been quantitatively compared with the transmittance curves calculated by numerical simulations. With this aim in mind, the channel conductance variations were transformed into transmittance variations by means of eqn (2), where ms is deduced by tting of Fig. 2a, and all the variations were conventionally computed with
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
Fig. 4 (a) SEM image of the gold-coated V-groove grating etched in the GaAs capping layer. (b) Specular reflectance spectrum of a Vgroove grating at 15 incidence angle for TM polarization, measured by ellipsometry (black line) and simulated by FEM (red line) using the experimental geometric parameters obtained by SEM. The blue dashed line is the simulated reflectance at normal incidence while the vertical dashed line marks the design wavelength.
Fig. 5 Channel resistance variation as a function of laser beam incidence angle for TM polarization and different gate-source voltage biases. The value at q ¼ 5 is taken as reference.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
respect to the “out of resonance” value at q ¼ 5 . Finally a comparison between simulated (black dotted line) and measured (black solid line) transmittance curves can be achieved and is reported in Fig. 6a for the batch of devices having 628 nm-pitch and 340 nm-grooves width. The curves relative to the non-resonant TE polarization (in blue) are plotted together with those relative to the TM polarization, which interacts with the SPP mode of the grating. The agreement with the optical model turns out to be satisfactory. A transmittance modulation value in excess of 60% is measured between normal incidence and q ¼ 5 for TM polarization. The difference compared to the simulated maximum transmittance modulation could be attributed to surface roughness and minor discrepancies between the modelled and fabricated nanostructures’ geometry, which reduce the strength of SPP coupling with incident light. The transmittance variation with respect to the incidence angle is much weaker for TE polarization and is also coherent with the simulated trend. As a proof of principle of the photo-HEMT operation as an integrated SPR sensor and with the aim of characterizing its sensitivity to the binding of bioanalytes on the active area, we measured the optoelectronic response aer coating the surface with a thin tetraphenylporphyrin (TPP) lm evaporated under vacuum. For details about the deposition see ref. 28. The thickness (12.5 nm) and the complex refractive index (1.88 + 0.15i at 633 nm) of the organic lm have been measured by spectroscopic ellipsometry and these parameters have been used to model the optical response of the grating by FEM simulations. The measured and simulated device responses aer the thin lm coating are reported in Fig. 6a, together with the signal acquired on the same device before deposition. Again, the trend of the transmitted irradiance signal modulated by the plasmonic grating is well described by the model. A signicant shi between the device signal before and aer coating of the sensitive surface is measured at normal incidence and xed wavelength, the working parameters for which the grating layout has been designed. It is worth noting that a stronger variation in the signal can be obtained by a smaller coating thickness. Specically, the maximum |DT|/T value achievable for this experimental conguration amounts to +1.6 at normal incidence for a 5 nm-thick TPP layer, as derived from FEM simulations (not shown). We estimated the performance of the device in terms of surface sensitivity by calculating the derivative of the conductance signal with respect to the bulk (or effective) refractive index neff of the dielectric environment surrounding the plasmonic grating. The epitaxial heterostructure sensitivity ms has been evaluated from Fig. 2b (3.7 105 U1) while the plasmonic structure sensitivity T1vT/vneff has been calculated by optical simulations to be about 4.6 104 % RIU1 (Refractive Index Unit, see the ESI† section for details). Thus the estimated sensitivity of the plasmonic photo-HEMT is of the order of 1.7 102 U1 RIU1 which, considering the rms noise in conductance measurements of the order of 3 107 U1, gives a resolution of about 1.8 105 RIU, which is comparable to other high performing intensity-based SPR biosensors.29,30
Nanoscale, 2014, 6, 1390–1397 | 1395
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Nanoscale
Paper
Fig. 6 (a) Measured (dotted lines on the left) and simulated (solid lines on the right) relative transmittance modulation curves as a function of incidence angle for the batch of devices integrating triangular gold gratings with 628 nm-pitch and 340 nm-groove width. Blue lines refer to the bare surface illuminated with TE polarization. The sensing experiment is carried out with a 633 nm-wavelength TM polarized incident beam: the black lines refer to the bare surface while the red lines refer to the gold surface coated with a 12.5 nm-thick tetraphenylporphyrin film. (b) Sensing experiment and relative simulations carried out on a batch of devices integrating a plasmonic crystal having 610 nm-pitch and 364 nm-groove width. The black lines (for the sake of simplicity called “bare” in the legend) refer to the gold surface functionalized with a thiolated biotin layer while the red lines refer to the device response after the avidin binding.
As further evidence of the understanding of the physical mechanisms governing the nanostructures’ optical properties, we fabricated another batch of devices integrating a plasmonic crystal with a 610 nm-pitch. As shown in Fig. 3b, for this grating geometry the reectance angular spectrum features a marked plasmonic dip at slightly off normal incidence, which corresponds to an enhanced power ow toward the 2DEG. Even if this layout does not match a conguration of maximum sensitivity to surface binding events, the resulting angular trend provides clear proof of the plasmonic nature of the resonances involved. Though not constituting a demonstration of operation as a mature biosensor, we implemented a more realistic functionalization/binding procedure on these devices in view of the development of the proposed platform as an affinity biosensor. The device surface has been exposed rst to an ethanolic solution of thiolated biotin, for the functionalization of the gold active area, and then to a water solution of avidin molecules, whose binding has to be detected aer rinsing and drying (details on the synthesis of thiolated biotin and on the experimental procedure are provided in the ESI† section). The photo-HEMTs responses have been acquired both aer functionalization and aer the target analyte binding step and are reported in Fig. 6b, where they are compared to the relative optical simulations. The avidin coating has been modelled with a 5 nm-thick layer having a refractive index of 1.45.31,32 A good quantitative agreement between experimental data and theoretical predictions is obtained, with the correct reproduction of the plasmonic transmittance features and their relative shi with surface coating. The electro-optical response of this additional batch of devices highlights a key feature of the photoHEMTs. In spite of their smaller active area (60 240 mm2) compared to that (500 500 mm2) of the batch of devices with the optimized grating pitch, the amplitude of the extracted signal is not affected. This result constitutes a rst indication of
1396 | Nanoscale, 2014, 6, 1390–1397
the scalability properties of the developed architecture. In both batches of devices fabricated, the plasmonic resonance induced-modulation of power ow towards the 2DEG accurately reveals the binding of target molecules on the nanostructured surface, notwithstanding the signicant physical thickness of the gold layer (180 nm) to be passed through by the incident radiation. The non-linear responsivity of the semiconductor detector enables us to work with low values of transmittance of the plasmonic gate (104), which optimize the optical FOM. The latter condition maximizes the fraction of incident energy used to probe the dielectric environment close to the grating surface by the evanescent plasmonic eld, with a really limited outgoing power ow needed to perform the electronic transduction function.
Conclusions Summarizing, we have presented an original and integrated platform for on-chip detection and electrical transduction of the surface plasmon resonance supported by a nanostructured metal grating. The latter is fabricated on the active area of a GaAs/AlGaAs photo-HEMT and constitutes the electronic gate of the device, besides being the active plasmonic substrate. Both the 2DEG channel population and the sensitivity to transmitted light can be tuned by biasing the gate terminal. We have demonstrated that the proposed architecture is suitable for the development of a new class of compact SPR sensors based on intensity modulation and grating coupling, not requiring external photodetectors to monitor the optical properties of far elds related to the material properties probed by evanescent elds. The gold plasmonic crystal has a V-groove prole, realized by high resolution lithography and anisotropic wet etching, and has been designed by numerical optical simulations in order to maximize the device sensitivity to the surface binding
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
of a thin bioanalyte layer. The optical model describes faithfully the characteristics measured on two batches of plasmonic phototransistors having different active areas. Indeed scalability constitutes one of the main features of this architecture, which makes it particularly suited to the realization of microarrays for parallel detection. A necessary requirement to be met toward a mature biosensing chip is constituted by the capability to detect analytes present in a liquid environment, for which the plasmonic grating layout has already been optimized. This task involves the integration of a microuidic platform and the passivation of the chip surface, apart from the active area, by techniques developed for semiconductor device-based sensors.33 Finally, being made on high mobility III–V semiconductor epitaxial heterostructures, the device is naturally prone to monolithic integration with high-bandwidth signal processing circuits and could be used for the development of innovative plasmonic chips based on SPP–2DEG interaction.
Acknowledgements The authors gratefully acknowledge Dr G. Zacco and Dr V. Giorgis for the fruitful scientic discussions and technical advices, and Luca Bacci for the technical support about the electro-optical measurements. This work has been supported by a Grant from “Fondazione Cariparo” – Surface PLasmonics for Enhanced Nano Detectors and Innovative Devices (SPLENDID) – Progetto Eccellenza 2008 and partially by Nanobrain project funded by CNR.
Notes and references 1 E. Ozbay, Science, 2006, 311, 189–193. 2 R. Zia, J. Schuller, A. Chandran and M. Brongersma, Mater. Today, 2006, 9, 20–27. 3 M.-G. Kang, T. Xu, H. J. Park, X. Luo and L. J. Guo, Adv. Mater., 2010, 22, 4378–4383. 4 P. Neutens, L. Lagae, G. Borghs and P. Van Dorpe, Nano Lett., 2010, 10, 1429–1432. 5 P. Neutens, P. Van Dorpe, I. De Vlaminck, L. Lagae and G. Borghs, Nat. Photonics, 2009, 3, 283–286. 6 F. Mazzotta, G. Wang, C. H¨ agglund, F. H¨ o¨ ok and M. P. Jonsson, Biosens. Bioelectron., 2010, 26, 1131–1136. 7 J. Dost´ alek and J. Homola, Sens. Actuators, B, 2008, 129, 303– 310. 8 I. De Vlaminck, P. Van Dorpe, L. Lagae and G. Borghs, Nano Lett., 2007, 7, 703–706. 9 A. L. Falk, F. H. L. Koppens, C. L. Yu, K. Kang, N. de Leon Snapp, A. V. Akimov, M.-H. Jo, M. D. Lukin and H. Park, Nat. Phys., 2009, 5, 475–479.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
10 P. Zilio, D. Sammito, G. Zacco and F. Romanato, Opt. Express, 2010, 18, 19558. 11 J. a. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White and M. L. Brongersma, Nat. Mater., 2010, 9, 193–204. 12 L. Tang, S. E. Kocabas, S. Latif, A. K. Okyay, D.-S. Ly-Gagnon, K. C. Saraswat and D. a. B. Miller, Nat. Photonics, 2008, 2, 226–229. 13 J. A. Shackleford, R. Grote, M. Currie, J. E. Spanier and B. Nabet, Appl. Phys. Lett., 2009, 94, 083501. 14 J. S. White, G. Veronis, Z. Yu, E. S. Barnard, A. Chandran, S. Fan and M. L. Brongersma, Opt. Lett., 2009, 34, 686– 688. 15 B. Nabet, M. a. Romero, A. Cola, F. Quaranta, and M. Cesareo, Proc. 2001 SBMO/IEEE MTT-S Int. Microw. Optoelectron. Conf. (Cat. no. 01TH8568), 2001, vol. 1, pp. 57–60. 16 M. a. Romero, M. a. G. Martinez and P. R. Herczfeld, IEEE Trans. Microwave Theory Tech., 1996, 44, 2279–2287. 17 Y. Takanashi, K. Takahata and Y. Muramoto, IEEE Electron Device Lett., 1998, 19, 472–474. 18 S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.-Y. Laluet and T. W. Ebbesen, Nature, 2006, 440, 508–511. 19 T. Søndergaard and S. Bozhevolnyi, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80, 1–9. 20 E. D. Palik, Handbook of Optical Constants of Solids, Academic Press, 1997. 21 D. E. Aspnes, S. M. Kelso, R. a. Logan and R. Bhat, J. Appl. Phys., 1986, 60, 754. 22 Comsol Inc. 23 S. J. Yoon and D. Kim, J. Opt. Soc. Am. A, 2008, 25, 725–735. 24 G. Parisi, P. Zilio and F. Romanato, Opt. Express, 2012, 20, 16690–16703. 25 W. Barnes, T. Preist, S. Kitson and J. Sambles, Phys. Rev. B: Condens. Matter, 1996, 54, 6227–6244. 26 G. Ruffato and F. Romanato, Opt. Lett., 2012, 37, 2718–2720. 27 S. Adachi and K. Oe, J. Electrochem. Soc., 1983, 130, 2427. 28 G. Maggioni, S. Carturan, M. Tonezzer, M. Buffa, a. Quaranta, E. Negro and G. Della Mea, Eur. Polym. J., 2008, 44, 3628–3639. 29 J. Homola, Chem. Rev., 2008, 108, 462–493. 30 A.-P. Blanchard-Dionne, L. Guyot, S. Patskovsky, R. Gordon and M. Meunier, Opt. Express, 2011, 19, 15041–15046. 31 P. Zijlstra, P. M. R. Paulo and M. Orrit, Nat. Nanotechnol., 2012, 7, 379–382. 32 S. Busse, V. Scheumann, B. Menges and S. Mittler, Biosens. Bioelectron., 2002, 17, 704–710. 33 Semiconductor device-based sensors for gas, chemical, and biomedical applications, ed. F. Ren and S. Pearton, CRC Press, Taylor and Francis Group, Boca Raton (USA), 2011.
Nanoscale, 2014, 6, 1390–1397 | 1397
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
PAPER
Cite this: Nanoscale, 2014, 6, 1390
View Journal | View Issue
Integrated architecture for the electrical detection of plasmonic resonances based on high electron mobility photo-transistors† Davide Sammito,‡*ab Davide De Salvador,‡*c Pierfrancesco Zilio,ac Giorgio Biasiol,b Tommaso Ongarello,ac Michele Massari,a Gianluca Ruffato,ac Margherita Morpurgo,d Davide Silvestri,d Gianluigi Maggioni,ce Gianluca Bovo,c Michele Gaioc and Filippo Romanatoabc We report the design of an integrated platform for on-chip electrical transduction of the surface plasmon resonance supported by a nanostructured metal grating. The latter is fabricated on the active area of a GaAs/AlGaAs photo-HEMT and simultaneously works as the electronic gate of the device. The gold
Received 2nd September 2013 Accepted 28th October 2013
plasmonic crystal has a V-groove profile and has been designed by numerical optical simulations. By showing that the numerical models accurately reproduce the phototransistors experimental response,
DOI: 10.1039/c3nr04666d
we demonstrate that the proposed architecture is suitable for the development of a new class of
www.rsc.org/nanoscale
compact and scalable SPR sensors.
Introduction Surface Plasmon Polaritons (SPPs) are electromagnetic waves that can be excited and guided on metallic surfaces and nanostructures. Due to the possibility they offer to be conned in subwavelength sized spaces, SPPs have been proposed to merge the technological gap between photonics and electronics, which poses a real obstacle to the realization of high-bandwidth fullyintegrated optoelectronic chips.1,2 Additionally, the opportunity for electrical signals to be carried on the same metallic structures supporting SPPs3–5 is a key factor fostering the integration of plasmonic and electronic functionalities in the same devices. Another notable application area of SPPs is in biosensing, which takes advantage of the high near eld enhancement close to metallic surfaces and nanostructures to sense the local variations in refractive index due to the binding of molecular layers.
Laboratory for Nanofabrication of Nanodevices, LaNN – Veneto Nanotech, Corso Stati Uniti 4, 35127 Padova, Italy. E-mail: [email protected]
a
Istituto Officina dei Materiali, IOM – CNR, SS. 14 km 163.5 in Area Science Park, 34149 Basovizza, Trieste, Italy
b
c
University of Padova, Department of Physics and Astronomy, Via Marzolo 8, 35131 Padova, Italy. E-mail: [email protected] d
University of Padova, Department of Pharmaceutical and Pharmacological Sciences, Via Marzolo 5, 35131 Padova, Italy
e
INFN Legnaro National Laboratories, Viale dell'Universit` a 2, 35020 Legnaro, Pd, Italy
† Electronic supplementary information (ESI) available: HEMTs optical response, device fabrication ow chart, estimate of device resolution based on numerical simulations, synthesis of thiolated biotin, biotin functionalization and avidin binding procedure. See DOI: 10.1039/c3nr04666d ‡ These authors contributed equally to the experimental work reported in the paper.
1390 | Nanoscale, 2014, 6, 1390–1397
The analyte’s binding kinetics is conventionally monitored by characterizing the properties of scattered waves in the far-eld by using external photodetectors coupled to spectroscopic and/ or goniometric systems. The current trend is to obtain compact on-chip electronic processing of the sensing signal6 to enable multichannel detection.7 Thus, an essential feature to be implemented in both optoelectronic and biosensing chips is the detection of plasmonic signals or, in other words, the local and direct transduction of plasmonic excitations into electrical signals. This objective can be achieved in semiconductor detectors by plasmonic waves-mediated photogeneration in the near-eld regime.5,8–10 This kind of approach to integrated biosensing architectures imposes antithetical optimization constraints: on the one hand the photodetector performances are intrinsically improved by increasing the absorption and the photogeneration in the active area;11–14 on the other hand the absorption in the semiconductor should be minimized to increase the energy density at the metal–analyte interface, crucial for biosensing. In this work we describe an innovative SPPs detection platform that overcomes the above trade-off. It is based on a High Electron Mobility photo-Transistor (photo-HEMT) suitable for photonic circuitry applications. Impinging light is coupled to SPPs by a V-shaped grating at the air–metal interface of a relatively thick Au lm (180 nm). Only a small fraction of the incident irradiance (between 103 and 104) is transmitted by the plasmonic crystal and absorbed by the underlying highly sensitive photo-HEMT, allowing it to perform an accurate optical-to-electrical transduction of the intensity modulation signal of visible light. Since it is based on a planar phototransistor, the present architecture has the advantage of being
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
scalable and a rst indication of this characteristic is provided by showing that the output signal is not affected by the different surface areas of two prototypes. Furthermore, we demonstrate that the nanopatterned metallic surface simultaneously works as a plasmonic structure and an electronic gate to the transistor, providing gain in SPP signal detection through the application of a small bias voltage. We prove the potentialities of this plasmonic resonances detection architecture by showing that the photo-HEMT’s response is affected by the presence of a few nanometres-thick organic lm coating the gold surface. Thus, we propose this innovative detection scheme for the development of SPR biosensors based on intensity modulation and grating coupling. The choice of this SPP coupling scheme over prism coupling, the xed design wavelength and incidence angle and the direct electrical read-out of the plasmonic related signal can make the sensing setup particularly compact and suited to multiplexing.
Device structure design and realization The phototransistor is made of a GaAs/Al0.42Ga0.58As modulation doped heterostructure. The schematics of the device are reported in Fig. 1a and b. For the chosen Al mole fraction, the AlGaAs layer is transparent to the incident radiation at 633 nmwavelength. The lattice matched multilayer, grown by Molecular Beam Epitaxy on semi-insulating (100) GaAs wafers consists of a 300 nm-thick i-GaAs buffer, a 500 nm-thick GaAs (2 nm)/ Al0.42Ga0.58As (8 nm) superlattice, followed by a 500 nm-thick GaAs layer, a 100 nm-thick i-AlGaAs deep spacer, a 30 nm-thick Si-doped (4 1018 cm3) AlGaAs layer, a 100 nm thick i-AlGaAs top spacer and a 300 nm-thick GaAs capping layer. This multilayer scheme resembles that typical of high electron mobility heterostructures, which were demonstrated to feature a non linear photoresponse with high sensitivity at low optical power levels.15,16 We modied the standard semiconductor heterostructure to adapt it to our purposes. The thick capping layer has been added in order to integrate the V-shaped grating by
Nanoscale
anisotropic wet etching of GaAs. As a consequence, the top AlGaAs spacer is needed to keep the potential well at the top GaAs/AlGaAs heterointerface barely populated, at least in dark conditions, as shown in the band diagram and electron density prole simulated by Schr¨ odinger–Poisson modelling (Fig. 1c). A batch of test heterostructures has been grown and characterized by Van der Pauw Hall effect measurements to optimize the doping density, the epitaxial growth conditions and the layer thicknesses and to assess their inuence on the 2DEG resistance under illumination. The semiconductor substrates used to fabricate the devices have a 2DEG carrier density of the order of 5 1010 cm2 and a mobility of the order of 4000 cm2 V1 s1 in dark conditions and at room temperature. The devices have a Hall bar layout (Fig. 1b) for the measurement of the channel resistance under controlled illumination conditions. A gold lm coats the nanostructured GaAs surface forming a Schottky contact on the active area and is connected by a lead to a pad for the electrical biasing of the gate. A current is driven between source and drain (Ni–Ge–Au multilayer annealed to form ohmic contacts with the 2DEG) and the relative voltage drop is measured with a four-point probes architecture (see Fig. 1b). The transistor characteristics are acquired by two channel source meters (Keithley 2612A) that allow the measurement of the voltage drop at V–V+ terminals (Vds) as a function of the source–drain current (Ids), and to set the gate to source voltage while monitoring the gate leakage current. Therefore, the conductance of the 2DEG channel can be measured by computing Ids vs. Vds linear ts for different gate voltages. A polarized HeNe laser beam is focused within the squared active area of the device (500 mm-side) with a divergence of 0.1 . The sample socket is placed on a goniometric stage with the rotation axis on the gold surface and intersecting the incident beam axis for angle-resolved characterizations. The photoresponse curve of the device is reported in Fig. 2a. It is measured by applying different neutral density lters to tune the irradiance at a xed incidence angle. The light
Fig. 1 (a) Schematic cross section of the active area of the phototransistor integrating a V-groove-shaped gold plasmonic crystal. (b) Optical microscope image of the device showing the terminals for electronic driving and measurement. (c) Band diagram and electron density profile of the flat epitaxial multilayer in dark conditions simulated by Schro ¨ dinger–Poisson numerical modelling.
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 1390–1397 | 1391
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Nanoscale
Paper
reduction of the transistor threshold voltage and thus an increase of channel conductance, with a logarithmic dependence on the incident optical irradiance.16 Our photo-response data are well tted by the analytical models proposed,17 as shown in the ESI† section. Furthermore, unlike photoresistors and photodiodes, which measure the photogenerated current, in phototransistors the conductance signal does not scale with the active area of the device working with xed incidence irradiance values. Using the detector in the working regime above the threshold irradiance, the conductance has a linear dependence on the logarithm of the relative irradiance: s ¼ ms ln(aT ) + q
(2)
This implies that, at xed attenuation in the high intensity regime, variations of conductance depend on the relative transmittance variations. Indeed, by differentiating eqn (2), we get: Ds ¼ ms
(a) Interpolated conductance of the device as a function of incident irradiance for different voltage biases applied to the gate. The incident irradiance values are normalized to the non-attenuated laser beam, having an integrated optical power of about 1 mW and an 80 mm spot size at beam waist. (b) Slope of the responsivity curves shown in (a) in the high irradiance region (aT ¼ 100.2), calculated as derivative of the conductance with respect to the logarithm of relative irradiance as a function of the gate voltage bias. Fig. 2
irradiance reaching the 2DEG (Ipd) is the product of the incident irradiance (I0), the transmittance of the top gold structure (T) and the attenuation factor due to the lters (a): Ipd ¼ I0aT
(1)
In Fig. 2a the 2DEG conductance is plotted as a function of the relative irradiance Ipd/I0 ¼ aT for different voltage biases applied to the gate. Above a threshold irradiance value, the channel conductance shows a steep dependence on the incident power ux. Considering that the patterned gold lm has a transmittance of the order of 104 (see below) and that the non attenuated power of the incident laser beam is 1 mW, from this graph it appears that the heterostructures are sensitive to relatively low light levels, of the order of 10 nW. As demonstrated in the literature, the optical response of HEMTs at low frequency is dominated by the photovoltaic effect, whereby the accumulation of photogenerated holes in the channel induces a
1392 | Nanoscale, 2014, 6, 1390–1397
DT T
(3)
The proportionality factor ms can be considered as the sensitivity of the semiconductor heterostructure: the higher its value, the higher will be the sensitivity to modulations of the transmittance due to the plasmonic structure. Interestingly, this gure of merit can be enhanced and optimized by applying a small negative gate to source voltage bias, as can be seen in Fig. 2b. On the other hand, by increasing the absolute value of this negative bias, the threshold irradiance increases while the dark conductance decreases due to depletion of the 2DEG population in the channel.
Plasmonic grating simulation and fabrication At xed incident optical power, the irradiance incident upon the absorbing substrate can be modulated by the nanostructured front surface by means of the supported surface plasmon resonance. In the present work, we incorporate on the active area of the device, a plasmonic grating obtained by conformal evaporation of a gold layer on top of the periodic V-groove array etched in the topmost GaAs capping layer. The aperture angle of the bottom vertex of the V-grooves is 70.53 , which is the typical angle formed by {111} crystal planes. Grooves with a triangular cross section milled in metal lms have raised much interest in plasmonics for nanofocusing and waveguiding applications because they support Channel Plasmon Polariton (CPP) modes featuring subwavelength connement with relatively low propagation losses.18 Arrays of such grooves were investigated by Søndergaard and Bozhevolnyi,19 who highlighted the existence and interplay of three types of optical resonances under monochromatic illumination, namely: geometric resonances related to the shape of the individual grooves; standing-wave SPP resonances due to coherent scattering by the periodic grooves structure; Wood–Rayleigh anomalies corresponding to congurations for which a diffraction order lies in the plane of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
the grating. These resonances were detected in the reection spectra and it was shown that their minima correspond to peaks in the spectra of eld enhancement on groove valleys. To the best of our knowledge, in the literature the transmission properties of this kind of plasmonic gratings have not been analyzed yet. The optical response of the present structure has been simulated by means of full-eld 2D Finite Elements simulations, using the soware COMSOL Multiphysics.22 Considering the peculiar logarithmic photoresponse of our detector, the period, V-groove width and metal thickness of the plasmonic structure have been optimized by optical simulations in order to maximize its relative transmittance variation (|DT|/T) with respect to the variation of the refractive index of a 5 nm-thick dielectric coating from n ¼ 1 to n ¼ 1.45. In other words, the plasmonic structure is designed to maximize the surface sensitivity of the device in view of the application as an affinity biosensor, working with normally impinging TM-polarized 633 nm-wavelength plane waves. The obtained map of |DT|/T as a function of period and wedge width-to-period ratio (duty cycle) is reported in Fig. 3a. As can be seen, a sharp peak is observed around a grating pitch of 628 nm and a duty cycle of 0.58 (groove width 364 nm), where |DT|/T reaches values as high as 7 for an optimal gold thickness of 180 nm. Such a high metal thickness allows most of the coupled light energy to be conned to the upper metal–dielectric interface, maximizing the interaction with the coating to be detected. The optical properties of the grating are then expected to be almost identical to those of a groove array on a semi-innite metal slab. As pointed out in the literature,23 nanostructured SPR sensors feature a signicant dependence of the optimized surface prole on the refractive index of the target analyte to be detected and on the dielectric environment. For the simulations shown in Fig. 3a, we assumed 1.45 as the refractive index of a typical protein-based analyte lm in an air environment. We also performed the parametric optical design of the grating structure for the same thin layer in a water-based environment (refractive index 1.33) in place of air. The result (not shown here) is that the optimal period and duty cycle shi to 450 nm and 0.5 respectively. The new optimal conguration still corresponds to the excitation of the same SPP mode coupled at the gold surface in the case of an air environment, conrming that the plasmonic structure can be properly re-designed considering the specic experimental conditions for its application as an SPR substrate. The origin of the high-sensitivity peak in the optimization map (Fig. 3a) can be ascribed to the optical properties of the nanostructure. The latter supports a radiative plasmonic Bloch mode propagating normally to the grooves (in the x-direction), which can be excited by impinging TM-polarized light, according to the well-known grating coupling formula k0 sin(q) + nG ¼ kSPP, where k0 is the vacuum wave vector, q the incidence angle, G ¼ 2p/d the reciprocal lattice vector, d being the period, and kSPP the Bloch mode propagation constant. We calculated the value of kSPP by means of the nite elements based modal analysis developed in ref. 24. In Fig. 3b we report the reectance map of the optimal grating structure (in an air environment) as
This journal is © The Royal Society of Chemistry 2014
Nanoscale
a function of period and incidence angle, for l ¼ 633 nm TMpolarized light. We superimpose the Bloch mode geometrical dispersion (white dots), calculated by means of the modal analysis and using the aforementioned grating coupling relation, together with same curve for SPPs at a at metal–dielectric interface (yellow line) and the Wood's anomalies locations (black line). A clear dip of reectance is observed in correspondence of the excitation of the Bloch mode, which departs signicantly from the at-surface SPP mode, owing to the strong surface corrugation25 caused by the grooves. The eigenmode electric eld norm is reported in Fig. 3b (inset). Interestingly, it shows an increased eld at the grooves bottom. Indeed, the excited Bloch mode combines a vertical groove resonance, which determines an enhanced transmission toward the underlying layers, with the horizontal SPP grating resonance,19 which is sensitive to the dielectric environment above the grating surface.7,26 Therefore, the resulting Fano-like resonance of the structure shows an enhanced transmittance sensitivity to surface refractive index variations. This is clearly seen in Fig. 3c, where we report the transmittance spectra at normal incidence for the optimized gold grating, with and without the thin dielectric lm (refractive index 1.45) on top, in conjunction with the Figure Of Merit (FOM) |DT|/T. The eld distributions at the design wavelength in absence and presence of the dielectric layer (Fig. 3d) show a signicant difference in the eld concentration and in the power ow inside the grooves, which translates in a considerable transmittance variation. It is worth emphasizing that, for the optimal working parameters which maximize the FOM, the transmittance of the structure is close to a minimum (about 104 without the dielectric lm, blue line in Fig. 3c). Aer the mesa and metal contacts lithographic patterning steps (see the ESI† for details on the ow chart), the designed gratings have been integrated on the active area by Electron Beam Lithography (100 keV Jeol JBX-6300FS at LaNN Labora] crystory) of an array of slits in PMMA aligned with the [011 tallographic orientation. The resist is used as a mask for the anisotropic wet etching of (100) GaAs27 in a H2SO4 : H2O2 : H2O solution (1 : 8 : 40 by volume) and then stripped. Tuning the resist slit-width and etching time, the optimized grating geometric parameters can be closely matched. Furthermore, as can be seen in the optimization map (Fig. 3a), the design is tolerant to effective groove widths narrower than the target value. The groove side walls meet with sharp corners at the bottom of the structures, as shown in the SEM micrograph of the grating aer gold conformal coating by e-gun evaporation (Fig. 4a). Optical characterizations of the plasmonic crystals integrated on the gate of the devices have been performed by spectroscopic ellipsometry. The specular reectance at 15 incidence angle for TM polarization has been measured and compared with the spectrum simulated by FEM (Fig. 4b). The geometric parameters (grating pitch and groove width) measured by SEM images have been used as input for the simulation. Specically, the sample under consideration has a pitch of 628 nm, corresponding to the optimized conguration, and an effective groove width of 340 nm. The agreement
Nanoscale, 2014, 6, 1390–1397 | 1393
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Nanoscale
Paper
Fig. 3 (a) FEM simulation of the relative transmittance variation of the gold grating between the bare surface and the same surface coated by a 5 nm-thick dielectric film with a refractive index of 1.45. This figure of merit is mapped versus the grating pitch and groove width-to-period ratio (duty cycle) in order to optimize the nanostructure geometry for the device application as a biosensor working with normally incident TMpolarized light at 633 nm wavelength. (b) Simulated reflectance map as a function of grating pitch and incidence angle for a fixed groove width of 364 nm. Overplotted on the map are the Bloch mode dispersion relation (white dots), the analytical dispersion relation for a SPP at a flat gold–air interface (yellow line) and the Wood–Rayleigh grating anomaly (black line). The blue dashed lines refer to the two grating periods of the batches of devices fabricated: 628 nm and 610 nm. Inset: electric field norm of the Bloch SPP eigenmode supported by the grating (d ¼ 610 nm). (c) FEM simulated transmittance spectra of the plasmonic grating without (blue line) and with (green line) the thin dielectric film modelling the bioanalyte layer to be detected. The relative transmittance variation spectrum is also plotted (red line). (d) Distribution of electric field norm enhancement generated by a normally incident plane wave without (left) and with (right) the thin dielectric film coating on the gold surface. Arrows represent the Poynting vector. The single unitary cell of the grating has been simulated setting Bloch–Floquet boundary conditions for the fields at the sides and Perfectly Matched Layers above and below the grating to limit the computational domain. The adopted relative permittivity of gold has been measured by ellipsometry, while for GaAs and AlGaAs tabulated data have been used.20,21
between simulation and experiment is excellent, as shown in Fig. 4b, where the simulated reectance at normal incidence is reported as additional reference.
Plasmonic resonance detection by the photo-HEMT Channel resistance characterizations under TM laser illumination have been performed aligning the rotation axis of the motorized stage along the V-grooves and acquiring the signal within few degrees incidence angle. The angular scans are bellshaped and peaked at normal incidence as shown in Fig. 5. Coherently with considerations about the photoresponse curves
1394 | Nanoscale, 2014, 6, 1390–1397
reported in Fig. 2, the device signal is signicantly enhanced by applying negative voltages to the gate terminal. Thus the nanostructured metal lm effectively performs both as a plasmonic structure and as an electronic gate of the transistor, modulating the 2DEG population and the device sensitivity to transmitted light. In order to verify that the shape of the plasmonic resonance is accurately transduced into an electrical signal, the experimentally acquired angular scans have been quantitatively compared with the transmittance curves calculated by numerical simulations. With this aim in mind, the channel conductance variations were transformed into transmittance variations by means of eqn (2), where ms is deduced by tting of Fig. 2a, and all the variations were conventionally computed with
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
Fig. 4 (a) SEM image of the gold-coated V-groove grating etched in the GaAs capping layer. (b) Specular reflectance spectrum of a Vgroove grating at 15 incidence angle for TM polarization, measured by ellipsometry (black line) and simulated by FEM (red line) using the experimental geometric parameters obtained by SEM. The blue dashed line is the simulated reflectance at normal incidence while the vertical dashed line marks the design wavelength.
Fig. 5 Channel resistance variation as a function of laser beam incidence angle for TM polarization and different gate-source voltage biases. The value at q ¼ 5 is taken as reference.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
respect to the “out of resonance” value at q ¼ 5 . Finally a comparison between simulated (black dotted line) and measured (black solid line) transmittance curves can be achieved and is reported in Fig. 6a for the batch of devices having 628 nm-pitch and 340 nm-grooves width. The curves relative to the non-resonant TE polarization (in blue) are plotted together with those relative to the TM polarization, which interacts with the SPP mode of the grating. The agreement with the optical model turns out to be satisfactory. A transmittance modulation value in excess of 60% is measured between normal incidence and q ¼ 5 for TM polarization. The difference compared to the simulated maximum transmittance modulation could be attributed to surface roughness and minor discrepancies between the modelled and fabricated nanostructures’ geometry, which reduce the strength of SPP coupling with incident light. The transmittance variation with respect to the incidence angle is much weaker for TE polarization and is also coherent with the simulated trend. As a proof of principle of the photo-HEMT operation as an integrated SPR sensor and with the aim of characterizing its sensitivity to the binding of bioanalytes on the active area, we measured the optoelectronic response aer coating the surface with a thin tetraphenylporphyrin (TPP) lm evaporated under vacuum. For details about the deposition see ref. 28. The thickness (12.5 nm) and the complex refractive index (1.88 + 0.15i at 633 nm) of the organic lm have been measured by spectroscopic ellipsometry and these parameters have been used to model the optical response of the grating by FEM simulations. The measured and simulated device responses aer the thin lm coating are reported in Fig. 6a, together with the signal acquired on the same device before deposition. Again, the trend of the transmitted irradiance signal modulated by the plasmonic grating is well described by the model. A signicant shi between the device signal before and aer coating of the sensitive surface is measured at normal incidence and xed wavelength, the working parameters for which the grating layout has been designed. It is worth noting that a stronger variation in the signal can be obtained by a smaller coating thickness. Specically, the maximum |DT|/T value achievable for this experimental conguration amounts to +1.6 at normal incidence for a 5 nm-thick TPP layer, as derived from FEM simulations (not shown). We estimated the performance of the device in terms of surface sensitivity by calculating the derivative of the conductance signal with respect to the bulk (or effective) refractive index neff of the dielectric environment surrounding the plasmonic grating. The epitaxial heterostructure sensitivity ms has been evaluated from Fig. 2b (3.7 105 U1) while the plasmonic structure sensitivity T1vT/vneff has been calculated by optical simulations to be about 4.6 104 % RIU1 (Refractive Index Unit, see the ESI† section for details). Thus the estimated sensitivity of the plasmonic photo-HEMT is of the order of 1.7 102 U1 RIU1 which, considering the rms noise in conductance measurements of the order of 3 107 U1, gives a resolution of about 1.8 105 RIU, which is comparable to other high performing intensity-based SPR biosensors.29,30
Nanoscale, 2014, 6, 1390–1397 | 1395
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Nanoscale
Paper
Fig. 6 (a) Measured (dotted lines on the left) and simulated (solid lines on the right) relative transmittance modulation curves as a function of incidence angle for the batch of devices integrating triangular gold gratings with 628 nm-pitch and 340 nm-groove width. Blue lines refer to the bare surface illuminated with TE polarization. The sensing experiment is carried out with a 633 nm-wavelength TM polarized incident beam: the black lines refer to the bare surface while the red lines refer to the gold surface coated with a 12.5 nm-thick tetraphenylporphyrin film. (b) Sensing experiment and relative simulations carried out on a batch of devices integrating a plasmonic crystal having 610 nm-pitch and 364 nm-groove width. The black lines (for the sake of simplicity called “bare” in the legend) refer to the gold surface functionalized with a thiolated biotin layer while the red lines refer to the device response after the avidin binding.
As further evidence of the understanding of the physical mechanisms governing the nanostructures’ optical properties, we fabricated another batch of devices integrating a plasmonic crystal with a 610 nm-pitch. As shown in Fig. 3b, for this grating geometry the reectance angular spectrum features a marked plasmonic dip at slightly off normal incidence, which corresponds to an enhanced power ow toward the 2DEG. Even if this layout does not match a conguration of maximum sensitivity to surface binding events, the resulting angular trend provides clear proof of the plasmonic nature of the resonances involved. Though not constituting a demonstration of operation as a mature biosensor, we implemented a more realistic functionalization/binding procedure on these devices in view of the development of the proposed platform as an affinity biosensor. The device surface has been exposed rst to an ethanolic solution of thiolated biotin, for the functionalization of the gold active area, and then to a water solution of avidin molecules, whose binding has to be detected aer rinsing and drying (details on the synthesis of thiolated biotin and on the experimental procedure are provided in the ESI† section). The photo-HEMTs responses have been acquired both aer functionalization and aer the target analyte binding step and are reported in Fig. 6b, where they are compared to the relative optical simulations. The avidin coating has been modelled with a 5 nm-thick layer having a refractive index of 1.45.31,32 A good quantitative agreement between experimental data and theoretical predictions is obtained, with the correct reproduction of the plasmonic transmittance features and their relative shi with surface coating. The electro-optical response of this additional batch of devices highlights a key feature of the photoHEMTs. In spite of their smaller active area (60 240 mm2) compared to that (500 500 mm2) of the batch of devices with the optimized grating pitch, the amplitude of the extracted signal is not affected. This result constitutes a rst indication of
1396 | Nanoscale, 2014, 6, 1390–1397
the scalability properties of the developed architecture. In both batches of devices fabricated, the plasmonic resonance induced-modulation of power ow towards the 2DEG accurately reveals the binding of target molecules on the nanostructured surface, notwithstanding the signicant physical thickness of the gold layer (180 nm) to be passed through by the incident radiation. The non-linear responsivity of the semiconductor detector enables us to work with low values of transmittance of the plasmonic gate (104), which optimize the optical FOM. The latter condition maximizes the fraction of incident energy used to probe the dielectric environment close to the grating surface by the evanescent plasmonic eld, with a really limited outgoing power ow needed to perform the electronic transduction function.
Conclusions Summarizing, we have presented an original and integrated platform for on-chip detection and electrical transduction of the surface plasmon resonance supported by a nanostructured metal grating. The latter is fabricated on the active area of a GaAs/AlGaAs photo-HEMT and constitutes the electronic gate of the device, besides being the active plasmonic substrate. Both the 2DEG channel population and the sensitivity to transmitted light can be tuned by biasing the gate terminal. We have demonstrated that the proposed architecture is suitable for the development of a new class of compact SPR sensors based on intensity modulation and grating coupling, not requiring external photodetectors to monitor the optical properties of far elds related to the material properties probed by evanescent elds. The gold plasmonic crystal has a V-groove prole, realized by high resolution lithography and anisotropic wet etching, and has been designed by numerical optical simulations in order to maximize the device sensitivity to the surface binding
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 29 October 2013. Downloaded by University of Western Ontario on 31/10/2014 21:57:31.
Paper
of a thin bioanalyte layer. The optical model describes faithfully the characteristics measured on two batches of plasmonic phototransistors having different active areas. Indeed scalability constitutes one of the main features of this architecture, which makes it particularly suited to the realization of microarrays for parallel detection. A necessary requirement to be met toward a mature biosensing chip is constituted by the capability to detect analytes present in a liquid environment, for which the plasmonic grating layout has already been optimized. This task involves the integration of a microuidic platform and the passivation of the chip surface, apart from the active area, by techniques developed for semiconductor device-based sensors.33 Finally, being made on high mobility III–V semiconductor epitaxial heterostructures, the device is naturally prone to monolithic integration with high-bandwidth signal processing circuits and could be used for the development of innovative plasmonic chips based on SPP–2DEG interaction.
Acknowledgements The authors gratefully acknowledge Dr G. Zacco and Dr V. Giorgis for the fruitful scientic discussions and technical advices, and Luca Bacci for the technical support about the electro-optical measurements. This work has been supported by a Grant from “Fondazione Cariparo” – Surface PLasmonics for Enhanced Nano Detectors and Innovative Devices (SPLENDID) – Progetto Eccellenza 2008 and partially by Nanobrain project funded by CNR.
Notes and references 1 E. Ozbay, Science, 2006, 311, 189–193. 2 R. Zia, J. Schuller, A. Chandran and M. Brongersma, Mater. Today, 2006, 9, 20–27. 3 M.-G. Kang, T. Xu, H. J. Park, X. Luo and L. J. Guo, Adv. Mater., 2010, 22, 4378–4383. 4 P. Neutens, L. Lagae, G. Borghs and P. Van Dorpe, Nano Lett., 2010, 10, 1429–1432. 5 P. Neutens, P. Van Dorpe, I. De Vlaminck, L. Lagae and G. Borghs, Nat. Photonics, 2009, 3, 283–286. 6 F. Mazzotta, G. Wang, C. H¨ agglund, F. H¨ o¨ ok and M. P. Jonsson, Biosens. Bioelectron., 2010, 26, 1131–1136. 7 J. Dost´ alek and J. Homola, Sens. Actuators, B, 2008, 129, 303– 310. 8 I. De Vlaminck, P. Van Dorpe, L. Lagae and G. Borghs, Nano Lett., 2007, 7, 703–706. 9 A. L. Falk, F. H. L. Koppens, C. L. Yu, K. Kang, N. de Leon Snapp, A. V. Akimov, M.-H. Jo, M. D. Lukin and H. Park, Nat. Phys., 2009, 5, 475–479.
This journal is © The Royal Society of Chemistry 2014
Nanoscale
10 P. Zilio, D. Sammito, G. Zacco and F. Romanato, Opt. Express, 2010, 18, 19558. 11 J. a. Schuller, E. S. Barnard, W. Cai, Y. C. Jun, J. S. White and M. L. Brongersma, Nat. Mater., 2010, 9, 193–204. 12 L. Tang, S. E. Kocabas, S. Latif, A. K. Okyay, D.-S. Ly-Gagnon, K. C. Saraswat and D. a. B. Miller, Nat. Photonics, 2008, 2, 226–229. 13 J. A. Shackleford, R. Grote, M. Currie, J. E. Spanier and B. Nabet, Appl. Phys. Lett., 2009, 94, 083501. 14 J. S. White, G. Veronis, Z. Yu, E. S. Barnard, A. Chandran, S. Fan and M. L. Brongersma, Opt. Lett., 2009, 34, 686– 688. 15 B. Nabet, M. a. Romero, A. Cola, F. Quaranta, and M. Cesareo, Proc. 2001 SBMO/IEEE MTT-S Int. Microw. Optoelectron. Conf. (Cat. no. 01TH8568), 2001, vol. 1, pp. 57–60. 16 M. a. Romero, M. a. G. Martinez and P. R. Herczfeld, IEEE Trans. Microwave Theory Tech., 1996, 44, 2279–2287. 17 Y. Takanashi, K. Takahata and Y. Muramoto, IEEE Electron Device Lett., 1998, 19, 472–474. 18 S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, J.-Y. Laluet and T. W. Ebbesen, Nature, 2006, 440, 508–511. 19 T. Søndergaard and S. Bozhevolnyi, Phys. Rev. B: Condens. Matter Mater. Phys., 2009, 80, 1–9. 20 E. D. Palik, Handbook of Optical Constants of Solids, Academic Press, 1997. 21 D. E. Aspnes, S. M. Kelso, R. a. Logan and R. Bhat, J. Appl. Phys., 1986, 60, 754. 22 Comsol Inc. 23 S. J. Yoon and D. Kim, J. Opt. Soc. Am. A, 2008, 25, 725–735. 24 G. Parisi, P. Zilio and F. Romanato, Opt. Express, 2012, 20, 16690–16703. 25 W. Barnes, T. Preist, S. Kitson and J. Sambles, Phys. Rev. B: Condens. Matter, 1996, 54, 6227–6244. 26 G. Ruffato and F. Romanato, Opt. Lett., 2012, 37, 2718–2720. 27 S. Adachi and K. Oe, J. Electrochem. Soc., 1983, 130, 2427. 28 G. Maggioni, S. Carturan, M. Tonezzer, M. Buffa, a. Quaranta, E. Negro and G. Della Mea, Eur. Polym. J., 2008, 44, 3628–3639. 29 J. Homola, Chem. Rev., 2008, 108, 462–493. 30 A.-P. Blanchard-Dionne, L. Guyot, S. Patskovsky, R. Gordon and M. Meunier, Opt. Express, 2011, 19, 15041–15046. 31 P. Zijlstra, P. M. R. Paulo and M. Orrit, Nat. Nanotechnol., 2012, 7, 379–382. 32 S. Busse, V. Scheumann, B. Menges and S. Mittler, Biosens. Bioelectron., 2002, 17, 704–710. 33 Semiconductor device-based sensors for gas, chemical, and biomedical applications, ed. F. Ren and S. Pearton, CRC Press, Taylor and Francis Group, Boca Raton (USA), 2011.
Nanoscale, 2014, 6, 1390–1397 | 1397
