sensors Article Thermoelectric Photosensor Based on Ultrathin Single-Crystalline Si Films † Gustavo Gonçalves Dalkiranis 1, *, Pablo Ferrando-Villalba 1,‡ , Aitor Lopeandia-Fernández 1 , Llibertat Abad-Muñoz 2 and Javier Rodríguez-Viejo 1 1 Grup de Nanomaterials i Microsistemes, Departament de Física, Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain; pablo.ferrandovillalba@imec.be (P.F.-V.); aitor.lopeandia@uab.cat (A.L.-F.); javier.rodriguez@uab.cat (J.R.-V.) 2 Instituto de Microelectrónica de Barcelona—Centre Nacional de Microelectrònica, Campus UAB, Bellaterra, 08193 Barcelona, Spain; llibertat.abad@imb-cnm.csic.es * Correspondence: dalkiranis@gmail.com; Tel.: +34-93-581-1481 † This paper is an extension version of the conference paper: “Thermoelectric Microsensor Based on Ultrathin Si films”; Gustavo Gonçalves Dalkiranis, Pablo Ferrando-Villalba, Aitor Lopeandía-Fernández, Llibertat Abad-Muñoz, Javier Rodríguez-Viejo in Proceedings of the Eurosensors 2018 Conference, Graz, Austria, 9–12 September 2018. ‡ Current address: IMEC, Kapeldreef 75, 3001 Leuven, Belgium.  Received: 13 February 2019; Accepted: 19 March 2019; Published: 22 March 2019  Abstract: Ultrathin Si films have a reduced thermal conductivity in comparison to Si bulk due to phonon scattering at the surfaces. Furthermore, the small thickness guarantees a reduced thermal mass (in the µJ/K range), which opens up the possibility of developing thermal sensors with a high sensitivity. Based on these premises, a thermoelectric (TE) microsensor based on ultrathin suspended Si films was developed and used as a thermal photosensor. The photoresponse of the device was evaluated with an argon laser (λ = 457 nm) with a variable power ranging from 0 to 10 mW in air at atmospheric pressure, with laser diodes at 406 nm, 520 nm and 638 nm wavelengths, and fixed powers in high vacuum conditions. The responsivity per unit area, response time (τ) and detectivity (D*) of the device were determined in air at ambient pressure, being 2.6 × 107 V/Wm2 , ~4.3 ms and 2.86 × 107 cmHz(1/2) W −1 , respectively. Temperature differences up to 30 K between the central hot region and the Si frame were achieved during open-circuit voltage measurements, with and without laser diodes. During illumination, the photogeneration of carriers caused a slight reduction of the Seebeck coefficient, which did not significantly change the sensitivity of the device. Moreover, the measurements performed with light beam chopped at different frequencies evidenced the quick response of the device. The temperature gradients applied to the thermoelectric Si legs were corrected using finite element modeling (FEM) due to the non-flat temperature profile generated during the experiments. Keywords: photosensor; thermoelectric effect; microsensor 1. Introduction Photosensors are sensors that can detect light (electromagnetic waves). Depending on the desired spectral sensitivity, different physical principles should be considered. Among the sensors using semiconductors as active materials, this study highlights those working directly from [1–4]: (i) Photoelectric or photovoltaic effects, where photons have energies over the semiconductor band gap; Sensors 2019, 19, 1427; doi:10.3390/s19061427 www.mdpi.com/journal/sensors Sensors 2019, 19, 1427 2 of 8 (ii) Thermal-based absorption sensors, where the photon energies require the generation of mid-gap states to provide energy to the phonon bath. Thermal detectors, either bolometers or thermoelectric-based (thermopiles), have a broad spectral range of detection and rely on the temperature increase produced by the absorbed radiation. Among the wide family of thermal absorption photosensors, those based on the thermoelectric (TE) principle do not require an external current or voltage source, and therefore may offer extremely low energy detection limits depending on the design. In general, thermal detection is preferred over photon detection for applications involving small excitation energies. Strategies to enhance the efficiency of thermoelectric detectors entail the improvement of the thermoelectric performance of the materials by maximizing its power factor, S2 σ, which is the product of the square of the Seebeck coefficient and the electrical conductivity. In addition, low heat capacities per unit area and low thermal conductance are required to attain high temperature differences upon radiation absorption. A higher ∆T implies higher sensitivities. One method of achieving this goal is to use ultrathin highly doped Si layers as active elements of the n,p thermoelectric materials. It has been previously shown that reducing the dimensionality of Si can boost its thermoelectric performance reaching for small diameter Si nanowires, with ZT = S2 σTk-1 values close to 1 at 300 K due to their reduced thermal conductivity close or even below the amorphous limit [5,6]. Similar results can be obtained using ultrathin Si membranes where enhanced scattering at the membrane surface dramatically affects the lattice thermal conductivity [7,8]. Additionally, appropriate designs and materials can be used to boost absorption at the sensing/absorbing regions. The simplicity of these types of devices enable their miniaturization. Previous work has shown that this strategy can be applied satisfactorily to fabricate Si-based compatible thermoelectric microsensors complementary metal-oxide-semiconductor (CMOS)-compatible microgenerators as energy harvesters for low-power applications [9–12]. This study presents a thermoelectric microsensor that relies on an array of ultrathin single-crystalline n- and p-type thermoelectric legs to generate the output voltage. The device was microfabricated from a silicon-on-insulator (SOI) wafer where a central silicon free-standing membrane (0.25 mm2 area) was connected and supported to the bulk silicon frame with 40, 100 nm thick thermoelectric silicon legs (20 doped with B and 20 doped with P) [11]. 2. Materials and Methods As shown in Figure 1, the device consisted of an array of 40 n,p thermocouples connected thermally in parallel and electrically in series between a central region (hot side) and a cold region (outside the Si frame). For a given temperature difference, the output voltage of the device was the sum of the voltage induced in every thermocouple. The device was equipped with electrical actuators to permit a full thermoelectric characterization (Figure 1). In the center of the device, a platinum grid was used as a heater and/or thermometer, which permits byJoule effect to increase the temperature and to establish temperature differences between hot (center of the device) and cold (frame of the device) sides. In parallel, the electrical properties of the thermoelectric legs can be evaluated. To use the platinum grid as a thermometer, the temperature coefficient of resistance (TCR) was obtained through a low-current (thus negligible self-heating) measurement of its electrical resistance at different temperatures close to 300 K. The thermoelectric legs had a doping level of 6.5 × 1018 cm-3 for p-type Si and 2 × 1019 cm-3 for n-type Si, and were connected electrically in series, leading to an internal resistance of ~62 KΩ. The doping levels of the n- and p-type legs were achieved through a sequential ion implantation (P and B) and rapid thermal annealing to recrystallize and activate the dopant atoms. The Si thermoelectric legs were 100 nm thick and showed a reduction in the thermal conductivity from 150 W/mK for bulk silicon at room temperature down to 60 W/mK, as reported elsewhere. The reduction in thermal conductivity was due to phonon scattering at the surfaces/interfaces [7,8,13–15]. On the contrary, the Seebeck coefficient remained close to expected values for bulk Si with similar doping levels (~5 × 1019 at/cm3 ) [5,6]. The thermocouples supported the SiNX membrane that hosted the platinum (Pt) grid in the center of the device, which mechanically connected the membrane to the silicon frame. Sensors 2019, 19, 1427 3 of 8 The as amembrane-based thermal configuration photosensor. of the device, In the resulting device theguaranteed a reduced thermal sensing partthermal mass of 0.5 mm forthermally 2 was sensing Sensors 2019, 19 FOR PEER REVIEW 3 areas in the square millimeter range, making it ideal as a thermal photosensor. isolated from the supporting Si frame (6 mm2) with a thermal conductance of 52 µ W/K under In the resulting device, high the thermalconditions. sensing partFinite of 0.5 mm 2 was thermally isolated from the supporting Si frame (6 mm2 ) with vacuum element modeling (FEM) was required to consider the non-flat a temperature thermal conductance profiles ofofthe 52membrane µW/K under highthe during vacuum conditions.experiments. characterization Finite elementThemodeling (FEM) microfabrication was required to consider the non-flat temperature processes have been fully described by Perez-Marin et al. [11].profiles of the membrane during the characterization experiments. The microfabrication The measurements processeson were performed have thebeen fully described responsivity to lightbyabsorption Perez-Marin et al. [11]. the illuminating The measurements were performed on the responsivity to light absorption sensing zone with an Ar+ laser (λ = 457 nm) at different powers up to 10 mW in ambient conditions. illuminating the sensing zone with an + Ar was laserilluminated, (λ = 457 nm) attemperature different powers up to 10 While the sensing zone the difference andmW in ambient Seebeck voltage conditions. generated While the sensing zone was illuminated, the temperature difference were measured. The absorbance of the central multilayer stack (non-optimized) membrane and Seebeck voltage generatedwas were measured. around 52% at The 457 absorbance nm, as shown of the central in the multilayer stack transmittance (non-optimized) and reflectance spectramembrane was around in the Supplementary 52% at 457 nm, Materials as shown (Figure S1). Ininaddition, the transmittance using theand reflectance platinum gridspectra in the as a heat Supplementary source, open voltage Materials circuit (Figure S1). In addition, using the platinum grid as a heat source, open voltage measurements were carried out in a vacuum with and without illumination by using 406 nm, 520 nm circuit measurements were and carried 638 nm out indiodes laser a vacuum with and operating without at 2.94 mW,illumination 3.8 mW and by 4.2using mW, 406 nm, 520 nm respectively. and 638 To heat onlynmthe laser diodes central zone,operating the laser at beam2.94wasmW, 3.8 mW and collimated by an4.2objective mW, respectively. To heat only lens, as schematically the central shown zone, in Figure 1a. the laser beam was collimated by an objective lens, as schematically shown in Figure 1a. TE Legs Pt Grid Pt Pads (a) (b) Figure 1. (a) Schematic of the device showing the illuminated area. (b) Optical image of the device Pt grid presenting thermoelectric (TE) legs, Pt grid and and Pt Pt pads. pads. 3. Results 3. Results Due to Due to the thenon-flat non-flattemperature temperature profiles profilesgenerated generated during the experiments, during the experiments, FEMFEM corrections were corrections required. The correction consisted of establishing a relationship were required. The correction consisted of establishing a relationship between the averagebetween the average temperature measured by measured temperature the platinum bygrid the (heat source platinum and/or grid (heatthermometer) source and/or andthermometer) the average temperature at the and the average hot junction (Pt pads) of the thermoelectric Si legs (see Figure 1b). Based on temperature at the hot junction (Pt pads) of the thermoelectric Si legs (see Figure 1b). Based on this this analysis a correction factor was analysis calculatedfactor a correction and usedwasto obtain theand calculated realused temperature to obtain gradient applied on the the real temperature thermoelectric gradient applied Si legs. For the illuminated device, the heating caused by the light must on the thermoelectric Si legs. For the illuminated device, the heating caused by the light must be be also considered inalso the simulations due to local changes in the temperature distribution. Therefore, considered in the simulations due to local changes in the temperature distribution. Therefore, the the power of the light was power considered of the light aswas an additional consideredheat as ansource and the additional heatabsorption source andcoefficient of each the absorption elementofofeach coefficient the device was used to estimate the amount of heat absorbed from the laser element of the device was used to estimate the amount of heat absorbed from the laser beam. Figure beam. Figure 2 shows the temperature profile in each case, in dark conditions and when the device 2 shows the temperature profile in each case, in dark conditions and when the device was illuminated was illuminated by the different by laser diodes the different at 406 at laser diodes nm,406520 nm,nm520andnm638andnm 638operating nm operatingat 2.94atmW, 3.8 mW 2.94 mW, 3.8 and mW 4.2andmW,4.2 respectively. It is clear that the temperature increased and the temperature mW, respectively. It is clear that the temperature increased and the temperature profiles changed profiles changed due todue the incident to radiation. the incident It is also radiation. It important to note to is also important that the that note shapetheofshape the temperature distribution of the temperature changed distribution with the different wavelengths used, mainly due to the variation of changed with the different wavelengths used, mainly due to the variation of the absorption the absorption coefficients of the different wavelengths. coefficients of the different wavelengths. Sensors 2019, 19 FOR PEER REVIEW 4 Sensors 2019, 19, 1427 4 of 8 OFF 340 406 nm 520 nm 330 638 nm T(K) 320 310 300 -500 -250 0 250 500 Position (m) Figure 2. Temperature Figure as aas 2. Temperature function of the a function position of the from position thethe from center of the center device of the simulated device by by simulated finite finite element modeling (FEM). element modeling (FEM). The correction of the temperature drop in the thermoelectric legs was carried out as follows: The correction of the temperature drop in the  thermoelectric legs was carried out as follows: First, the mean temperature of the platinum grid TG ̅̅̅ was calculated using the FEM results. Then, First, the mean temperature of the platinum grid (𝑇𝐺 ) was calculated using the FEM results. Then, the mean temperature in the platinum pads TP ̅̅̅ at the hot junction was evaluated using the FEM the mean temperature in the platinum pads (𝑇𝑃 ) at the hot junction was evaluated using the FEM results, thus allowing the calculation of the correction factor as follows: results, thus allowing the calculation of the correction factor as follows: 𝑇̅̅̅ − 𝑇𝑆 f = TG − TS 𝐺 (1) 𝑓= TP − TS (1) ̅̅̅ 𝑇 − 𝑇 with TS being the substrate temperature. The values of𝑃the correction 𝑆 factor for the illuminated device with 𝑇 changed as𝑆 abeing functionthe of substrate temperature. the temperature The valuesbyofthe rise generated theplatinum correction factor grid, fromfor 1.45thetoilluminated 1.61 for the device different changed as a function temperature gradients.of the temperature In dark conditions, risethis generated by the platinum factor remained grid, from almost constant 1.45 to at 1.71. 1.61 Thus, theforreal thetemperature different temperature difference gradients. applied to In thedark conditions,legs thermoelectric this was factor remained obtained by almost dividing constant the at 1.71. Thus, the real temperature difference temperature difference measured experimentally by the correction factor. applied to the thermoelectric legs was obtained by dividing Figure 3the temperature shows the voltage difference generated measured in the TE experimentally by the legs as a function ofcorrection factor. The linear the laser power. increase Figure 3 showsvoltage, of the Seebeck the voltage generated VS , was due toin thethe TE legs as adifference temperature function induced of the laser power. by the laserThebeamlinear increase between theof the Seebeck central region voltage, VS, wasframe and the silicon due to the temperature (black circles). Thedifference responsivity, induced RS , ofbythethe TElaser beam sensor, between defined as the theratio central regionthe between and the silicon output voltageframe and(black circles).power the absorbed The responsivity, [16–18], wasRSdetermined , of the TE sensor, by the defined slope in as the ratio Figure 3. RSbetween amounted thetooutput 13 V/W voltage and thean and yielded absorbed power outstanding [16–18], value of 2.6was× 10 7 V/Wm2 by determined theconsidering in air, slope in Figure 3. RS amounted the small area of thetodevice. 13 V/W and yielded an outstanding value of 2.6 × 107 V/Wm2 in air, considering Figure 4 showsthe thesmall areaof variation ofthe theSeebeck device. open-circuit voltage as a function of the temperature Figure 4 shows the variation difference between the thermoelectric legs, both of the Seebeckunder open-circuit voltage the illumination ofas406a function nm, 520 nm of the and temperature 638 nm difference laser diodes and between in dark theconditions. thermoelectric The legs, both under temperature the illumination difference was imposed of 406bynm, 520 nmJoule producing and 638 nm laser heating in the diodes and ingrid platinum darkinconditions. the rangeThe temperature of 0.24 mW to 2.37 difference mW. The was incident imposed lightby producing at 406 nm Joule heating induced an in the platinum additional grid in the temperature riserange of ~2.7of K0.24 mWthe above to 2.37 small mW. The incident temperature light at 406 differences nm induced produced by anheating, Joule additional temperature which generatedrise of ~2.7 of a voltage K above ~17 mV theassmall seen temperature by the offset differences of the blue line produced in Figure by Joule 4. Theheating, which generated other incident wavelengths, a voltage 520 nm of ~17 andmV 638asnm, seenproduced by the offset of the blue temperature lineofin~2.6 rises FigureK and4. The ~3.4other incident wavelengths, K, respectively. This generated 520 nm and 638in an increase nm,theproduced temperature output voltage roughly rises of ~2.6 similar K and to the light~3.4 at K, 406respectively. nm, as indicated This by generated the offsetanofincrease the greenin the andoutput red lines voltage roughly in Figure 4. Thissimilar to the light contribution at 406 nm, decreased as indicated as the temperature by the offset of gradient the green increased in and red lines agreement in Figure with 4. This contribution a mechanism decreased as the in which photogenerated temperature carriers contribute gradient increased to increase in agreement the electrical with a mechanism conductivity in which at the expense photogenerated of reducing the Seebeck carriers contribute coefficient to increase [19–23]. The smallthe change electrical inconductivity at the expense the Seebeck coefficient fromof 208reducing µV/K tothe 200Seebeck µV/K did coefficient not [19–23]. The significantly smallthe modify change in the Seebeck sensitivity coefficient of the device, enablingfromits208use µ V/K as atothermal 200 µ V/K did notinsignificantly detector a broad Sensors 2019, 19 FOR PEER REVIEW 5 modify Sensors 2019, 19the FORsensitivity PEER REVIEWof the device, enabling its use as a thermal detector in a broad range 5 of wavelength Sensors 2019, 19, 1427radiation. The difference between the light power and the absorption coefficient of 5 ofsilicon 8 attributed modify to each wavelength the sensitivity generated of the device, enablinga its similar number use as of electrical a thermal detector carriers, which in a broad rangeledofto a comparable wavelength reduction radiation. Thein the Seebeck difference coefficient. between the light power and the absorption coefficient of silicon range of wavelength radiation. The difference between the light power and the absorption coefficient attributed to each wavelength generated a similar number of electrical carriers, which led to a of silicon attributed to each wavelength generated a similar number of electrical carriers, which led to comparable reduction in the Seebeck coefficient. a comparable reduction in the Seebeck coefficient. 21 T exc = 457 nm 0.12 V exc = 457 nm 21 T S 0.12 14 VS 0.08 345 K VS (V) T (K) 34014 0.08 345 K VS (V) T (K) 330 7 0.04 340 320 330 7 0.04 310 320 0.00 300 K 0 310 0.00 0 2 4 6 8 0 10 300 K P (mW) 0 2 4 6 8 10 Figure 3. Plot of the open-circuit voltage at and temperature difference as a function of laser power. P (mW) The inset plot shows the temperature profiles simulated by FEM. Figure3.3.Plot Figure Plotof ofthe theopen-circuit open-circuitvoltage voltageat atand andtemperature temperaturedifference difference as asaafunction function of of laser laser power. power. Theinset The insetplot plotshows showsthe thetemperature temperatureprofiles profilessimulated simulatedby byFEM. FEM. 0.25 OFF 406 nm 0.25 OFF 520 nm 0.20 406 nmnm 638 0.20 520 nm 0.15 VS (V) 638 nm SOFF = 208 V/K 0.15 V/K VS (V) 0.10 S406 = 200 SOFF = 208 V/K 0.10 0.05 S S520 = 200 = 200 V/K V/K 406 S638 S520 = 201 = 200 V/K V/K 0.05 0.00 0 5 10 15 S20 638 25 V/K = 201 30 35 0.00 T (K) 0 5 10 15 20 25 30 35 Figure 4. Plot Figure is illuminated 4.ofPlot the of Seebeck at 406 nm, 520 device is illuminated open-circuit the Seebeck nm nm, at 406 open-circuit and 520nm 638 nmand T (K) voltage as a function voltage and638nm without as a of the temperature function light. and difference. The of the temperature without light. device The difference. Figure 4. Plot of the Seebeck open-circuit voltage as a function of the temperature difference. The To verify To is the response verify of the the response ofdevice the to oscillating device light,light, an experimental setup withwith a homemade device illuminated at 406 nm, 520nm and to oscillating 638nm and without an experimental light. setup a homemade optical optical chopper system was used to measure the Seebeck voltage and the grid resistance, RGthe chopper system was used to measure the Seebeck voltage and the grid resistance, R G , while , while lightthe beam was chopped from 34from Hz 34to 239 Hz.239The measurements were were performed in airinatair at Tolight verifybeam was chopped the response of the device toHz to oscillating Hz. light,The an measurements experimental setup performed with + laser was used as a light source at 457 nm, operating at 5 mW. Figure 5 a homemade atmospheric atmosphericpressure. An Ar pressure. AnusedAr+ laser was used optical chopper system was to measure the as a light voltage Seebeck source at and457thenm, operating grid at 5RmW. resistance, Figure G, while shows the Seebeck voltage and the grid resistance measurements carried out at 34 Hz and 239 Hz. the 5light shows the was beam Seebeck voltage chopped fromand34theHz grid to resistance 239 Hz. The measurements measurements carried wereout at 34 Hz and performed in air239atHz. The Seebeck The voltage Seebeck and and voltage the grid the resistance grid oscillated resistance as a function oscillated as a of the function ofchopping the frequency chopping due due frequency to to atmospheric pressure. An Ar laser was used as a light source at 457 nm, operating at 5 mW. Figure + the temperature differences caused by the laser light. Both VS and RG presented the same behavior 5 shows the Seebeck voltage and the grid resistance measurements carried out at 34 Hz and 239 Hz. at the several frequencies used in this experiment. Furthermore, it was verified that the sensor The Seebeck voltage and the grid resistance oscillated as a function of the chopping frequency due to Sensors 2019, 19 FOR PEER REVIEW 6 the temperature differences caused by the laser light. Both VS and RG presented the same behavior at Sensors 2019, 19, 1427 6 of 8 the several frequencies used in this experiment. Furthermore, it was verified that the sensor can measure oscillations of light until at least 239 Hz. This value was limited by our experimental setup, therefore can measure it is oscillations expected that the device of light until atwill leastbe239 ableHz. to This efficiently measure value was variations limited in a higher by our experimental frequency range. itBy setup, therefore is fitting expectedthethat voltage curvewill the device in Figure be able 5a, it was also to efficiently possible measure to determine variations the in a higher response time of the device, which was ~4.3 ms. The detectivity was evaluated frequency range. By fitting the voltage curve in Figure 5a, it was also possible to determine the using the equation ∗ 𝐷response = 𝑅𝑆 ∙ √𝐴/4𝑘𝑇𝑅 time of the [16–18], device,where whichRwas S is ~4.3 the responsivity, A the was ms. The detectivity area evaluated of the sensing using zone, k the the equation √ 7 D ∗ = RS ·constant, Boltzmann A/4kTR T [16–18], the temperature where RandS is R the the internal resistance responsivity, A the of areatheofdevice. the D* = sensing 2.86 zone,× k10the (1⁄2) 𝑐𝑚𝐻𝑧 Boltzmann 𝑊 −1 compared constant, T the well with other temperature and R the thermoelectric-based internal resistance of CMOS-compatible the device. D* = 2.86 × 107 sensors cmHz ( considering 1/2 ) W − 1 the reduced compared thermal constant well with other of the device [16]. thermoelectric-based CMOS-compatible sensors considering the reduced thermal constant of the device [16]. 0.08 (a) exc = 457 nm f = 34 Hz i = 250 A VS RG 3375 RG () VS (V) 0.04 3300 0.00 0.900 0.925 0.950 0.975 1.000 (b) f = 239 Hz 0.060 3375 VS (V) RG () 0.045 0.030 3300 0.900 0.905 0.910 0.915 t (s) Figure 5. Seebeck open-circuit voltage and the grid resistance measured while the light was chopped at 34 Hz (a) and 239 Hz (b). The red and black lines are only an eye guide. Figure 5. Seebeck open-circuit voltage and the grid resistance measured while the light was chopped 4. Conclusions at 34 Hz (a) and 239 Hz (b). The red and black lines are only an eye guide. This study described the use as photosensor of a thermoelectric microsensor based on 4. Conclusions single-crystalline Si ultrathin films. By decreasing the thickness, the thermal conductance and heat This study capacity described of the Si thermopiles the were use as photosensor significantly of a thermoelectric reduced, microsensor therefore substantially based on improving thesingle- thermal crystalline insulationSiofultrathin films. ByThe the membrane. decreasing the an result was thickness, the thermal enhancement of theconductance detectivity of and theheat TE capacity device to ofvery the small Si thermopiles temperature were significantly differences. reduced, atherefore Furthermore, substantially small thermal improving time constant the thermal was obtained. FEM insulation of the membrane. The result was an enhancement of the detectivity simulations were used to obtain the real temperature gradients applied to the thermoelectric legs, of the TE device to very small temperature correcting differences. Furthermore, the non-flat temperature profile generateda small thermal during time constant the experiments. Thewas obtained. linear FEM dependence simulations of the output were used to voltage withobtain the real the input temperature power facilitatesgradients the use ofapplied to the thermoelectric the microsensor legs, as a photosensor. correcting The RS was thedetermined non-flat temperature being 13 V/Wprofile in generated during the air at atmospheric experiments. pressure. The linear dependence The measurements performed ofatthe output fixed power voltage with thetemperature and variable input power facilitates demonstrated differences the use of the amicrosensor as a photosensor. slight reduction of the Seebeck The RS was determined coefficient as a result of being 13 V/W in air at atmospheric the photogeneration pressure.byThe of electrical carriers measurements light absorption inperformed the visible atrange. fixed power and variable temperature The measurements carried out differences with a choppeddemonstrated light beama slight reduction evidenced the of the Seebeck quick response coefficient time of the as device a resultdue of the photogeneration to its very small thermal of electrical carriers mass, ~4.3 ms. by light absorption Finally, the D* of the in the visible device was range. The 7 2.86 × 10 cmHz ( 1/2 measurements ) − W . 1carried out with a chopped light beam evidenced the quick response time of the device due to its very small thermal mass, ~4.3 ms. Finally, the D* of the device was 2. 86 × 107 𝑐𝑚𝐻𝑧 (1⁄2) 𝑊 −1 . Materials: The following are available online at http://www.mdpi.com/1424-8220/19/6/1427/ Supplementary s1; Figure S1. Transmittance and reflectance spectra of the multilayer stack membrane. Author Contributions: Supplementary Materials:Conceptualization, The following G.G.D., A.L.-F. and are available J.R.-V.; online formal analysis, P.F.-V.; investigation, at www.mdpi.com/xxx/s1; Figure S1. G.G.D.; methodology, Transmittance P.F.-V.;spectra and reflectance projectof administration, the multilayer J.R.-V.; resources, Ll.A.-M.; software, P.F.-V.; supervision, stack membrane. A.L.-F. and J.R.-V.; writing—original draft, G.G.D., P.F.-V., A.L.-F., Ll.A.-M. and J.R.-V.; writing—review and editing, G.G.D., P.F.-V., A.L.-F. and J.R.-V. Acknowledgments: This work was supported by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund through projects FIS2013-50304-EXP, MAT2016-79579-R, Sensors 2019, 19, 1427 7 of 8 TEC2015-67462-C2-1-R and RyC-2013-12640. This work made use of the Spanish ICTS Network MICRONANOFABS, partially supported by the Spanish Ministry of Economy and Competitiveness. P.F.-V. was in receipt of an FPU grant from the Spanish Ministry of Education, Culture and Sport. G.G.D. acknowledges the Brazilian National Council for Scientific and Technological Development (CNPq) for his doctoral fellowship of SwB. The authors would like to thank Agustín Mihi and Pau Molet for assisting with the transmittance and reflectance measurements. Conflicts of Interest: The authors declare no conflict of interest. References 1. Dennis, P.J.; Welch, E.F.; Alarie, J.P.; Ramsey, J.M.; Jorgenson, J.W. Development of a Photothermal Absorbance Detector for Use with Microfluidic Devices. Anal. Chem. 2010, 8, 4063–4071. [CrossRef] [PubMed] 2. Patil, V.; Capone, A.; Strauf, S.; Yang, E.-H. Improved Photoresponse with Enhanced Photoelectric Contribution in Fully Suspended Graphene Photodetectors. Sci. Rep. 2013, 3, 2791. [CrossRef] [PubMed] 3. García de Arquer, F.P.; Mihi, A.; Konstantatos, G. Large-Area Plasmonic-Crystal–Hot-Electron-Based Photodetectors. ACS Photonics 2015, 2, 950–957. [CrossRef] 4. Chen, G.Y.; Wu, X.; Liu, X.; Lancaster, D.G.; Monro, T.M.; Xu, H. Photodetector Based on Vernier-Enhanced Fabry-Perot Interferometers with a Photo-Thermal Coating. Sci. Rep. 2017, 7, 41895. [CrossRef] [PubMed] 5. Boukai, A.I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J.-K.; Goddard, W.A., III; Heath, J.R. Silicon Nanowires as Efficient Thermoelectric Materials. Nature 2008, 451, 168. [CrossRef] 6. Hochbaum, A.I.; Chen, R.; Delgado, R.D.; Liang, W.; Garnett, E.C.; Najarian, M.; Majumdar, A.; Yang, P. Enhanced Thermoelectric Performance of Rough Silicon Nanowires. Nature 2008, 451, 163. [CrossRef] 7. Neogi, S.; Reparaz, J.S.; Pereira, L.F.C.; Graczykowski, B.; Wagner, M.R.; Sledzinska, M.; Shchepetov, A.; Prunnila, M.; Ahopelto, J.; Sotomayor-Torres, C.M.; et al. Tuning Thermal Transport in Ultrathin Silicon Membranes by Surface Nanoscale Engineering. ACS Nano 2015, 9, 3820–3828. [CrossRef] 8. Ferrando-Villalba, P.; Lopeandia, A.F.; Abad, L.; Llobet, J.; Molina-Ruiz, M.; Garcia, G.; Gerbolès, M.; Alvarez, F.X.; Goñi, A.R.; Muñoz-Pascual, F.J.; et al. In-Plane Thermal Conductivity of Sub-20 Nm Thick Suspended Mono-Crystalline Si Layers. Nanotechnology 2014, 25, 185402. [CrossRef] 9. Li, Y.; Buddharaju, K.; Singh, N.; Lo, G.Q.; Lee, S.J. Chip-Level Thermoelectric Power Generators Based on High-Density Silicon Nanowire Array Prepared with Top-down CMOS Technology. IEEE Electron Device Lett. 2011, 32, 674–676. [CrossRef] 10. Curtin, B.M.; Fang, E.W.; Bowers, J.E. Highly Ordered Vertical Silicon Nanowire Array Composite Thin Films for Thermoelectric Devices. J. Electron. Mater. 2012, 41, 887–894. [CrossRef] 11. Perez-Marín, A.P.; Lopeandía, A.F.; Abad, L.; Ferrando-Villaba, P.; Garcia, G.; Lopez, A.M.; Muñoz-Pascual, F.X.; Rodríguez-Viejo, J. Micropower Thermoelectric Generator from Thin Si Membranes. Nano Energy 2014, 4, 73–80. [CrossRef] 12. Haras, M.; Lacatena, V.; Morini, F.; Robillard, J.-F.; Monfray, S.; Skotnicki, T.; Dubois, E. Thermoelectric Energy Conversion: How Good Can Silicon Be? Mater. Lett. 2015, 157, 193–196. [CrossRef] 13. Li, D.; Wu, Y.; Kim, P.; Shi, L.; Yang, P.; Majumdar, A. Thermal Conductivity of Individual Silicon Nanowires. Appl. Phys. Lett. 2003, 83, 2934–2936. [CrossRef] 14. Cahill, D.G.; Braun, P.V.; Chen, G.; Clarke, D.R.; Fan, S.; Goodson, K.E.; Keblinski, P.; King, W.P.; Mahan, G.D.; Majumdar, A.; et al. Nanoscale Thermal Transport. II. 2003–2012. Appl. Phys. Rev. 2014, 1, 11305. [CrossRef] 15. Malhotra, A.; Maldovan, M. Impact of Phonon Surface Scattering on Thermal Energy Distribution of Si and SiGe Nanowires. Sci. Rep. 2016, 6, 25818. [CrossRef] 16. Dillner, U.; Kessler, E.; Meyer, H.-G. Figures of Merit of Thermoelectric and Bolometric Thermal Radiation Sensors. J. Sens. Sens. Syst. 2013, 2, 85–94. [CrossRef] 17. Wang, K.; Xue, C.; Liang, T.; Jiao, B.; Zhang, W.; Chen, D.; Xiong, J. Thermopile Infrared Detector with Detectivity Greater Than 108 CmHz(1/2)/W. J. Infrared Millim. Terahertz Waves 2010, 31, 810–820. [CrossRef] 18. Zhou, H.; Kropelnicki, P.; Tsai, J.M.; Lee, C. Development of a Thermopile Infrared Sensor Using Stacked Double Polycrystalline Silicon Layers Based on the {CMOS} Process. J. Micromech. Microeng. 2013, 23, 65026. [CrossRef] 19. Harper, J.G.; Matthews, H.E.; Bube, R.H. Photothermoelectric Effects in Semiconductors: N- and P-Type Silicon. J. Appl. Phys. 1970, 41, 765–770. [CrossRef] Sensors 2019, 19, 1427 8 of 8 20. Kwok, H.; Bube, R.H. Another Look at the Anomalous Photothermoelectric Effect in P-Type Silicon. J. Appl. Phys. 1970, 41, 5026–5027. [CrossRef] 21. Okazaki, R.; Horikawa, A.; Yasui, Y.; Terasaki, I. Photo-Seebeck Effect in ZnO. J. Phys. Soc. Jpn. 2012, 81, 114722. [CrossRef] 22. Mondal, P.S.; Okazaki, R.; Taniguchi, H.; Terasaki, I. Photo-Seebeck Effect in Tetragonal PbO Single Crystals. J. Appl. Phys. 2013, 114, 173710. [CrossRef] 23. Lv, Y.; Chen, J.; Zheng, R.-K.; Song, J.; Zhang, T.; Li, X.; Shi, X.; Chen, L. Photo-Induced Enhancement of the Power Factor of Cu2S Thermoelectric Films. Sci. Rep. 2015, 5, 16291. [CrossRef] [PubMed] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).