Gold-catalysed porous silicon for NOx sensing

Sensors and Actuators B 68 Ž2000. 74–80 www.elsevier.nlrlocatersensorb

Gold-catalysed porous silicon for NO x sensing C. Baratto a,) , G. Sberveglieri a , E. Comini a , G. Faglia a , G. Benussi a , V. La Ferrara b, L. Quercia b, G. Di Francia b, V. Guidi c , D. Vincenzi c , D. Boscarino d , V. Rigato d a

Department of Chemistry and Physics and INFM, UniÕersity of Brescia, Via Valotti 9, I-25133 Brescia, Italy b National Research Center, ENEA, Via Vecchio Macello, I-80055 Portici (NA), Italy c Department of Physics, INFM and INFN, Via Paradiso 12, I-44100 Ferrara, Italy d Legnaro National Laboratory INFN, Via Romea 4, I-35020 Legnaro (PD), Italy

Abstract Porous silicon ŽPS., obtained by electrochemical anodization of an n-type silicon wafer, was catalysed by sputtering gold onto the surface Ž4, 8, 15 and 40-nm nominal thickness.. Investigation by Rutherford backscattering spectroscopy ŽRBS. and by electron microscopy showed that gold did not form a continuous layer, but rather formed clusters penetrating into the pores of PS by about 1 mm. A variation of the sample conductivity in the presence of a few parts per million of NO 2 and NO was recorded at room temperature. We demonstrated that, as a result of Au catalysation, PS is suitable for sensing nitrogen oxides with negligible influence by interfering gases such as CO, CH 4 or methanol. Indeed, we found that humidity appreciably affected the response. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Porous silicon; Gas sensor; Gold catalysis; Nitrogen oxides; Rutherford backscattering spectroscopy

1. Introduction NO and NO 2 — harmful pollutants in urban air — are produced as a result of combustion at relatively high temperature; the major source of NO, the precursor of NO 2 , is fuel combustion. The need for air-quality monitoring demands development of a sensor specifically selective to NO x . Unfortunately, a stable and sensitive device working at RT has not been developed yet. A very interesting new material in the field of gas sensing is porous silicon ŽPS., because of its huge surfaceto-volume ratio and its high reactivity w1x. Harper and Sailor reported quenching in photoluminescence of n-type PS in the presence of very low concentrations of NO and NO 2 in inert gas Žnitrogen. w2x, and Schetchter et al. w3x reported variations of both photoluminescence and electrical properties of PS in the presence of organic vapour mixtures in dry nitrogen. For air-quality monitoring, however, humidity and oxygen must be taken into account, since they are always present in environmental air. For this reason, we performed tests using humid synthetic air as gas carrier.

)

Corresponding author. Tel.: q39-30-3715771; fax: q39-30-2091271. E-mail address: [email protected] ŽC. Baratto..

PS obtained from n-type wafers is less reactive than the one obtained from p and pq wafers, because of its lower specific surface, but its macroporous structure can easily be impregnated or catalysed by wet or dry technique. Dry impregnation of gold by DC sputtering has been carried out to activate the response of the material to nitrogen oxides. As a consequence of the morphology of the porous layer, gold did not form a uniform film onto the surface, but penetrated into the pores as clustered particles. An extensive characterisation has been carried out by scanning electron microscopy ŽSEM. and Rutherford backscattering spectroscopy ŽRBS. to study Au distribution in the pores. The electrical behaviour of Au-catalysed PS as a gas sensor will be addressed, too. The variation of the DC conductance of the device as a consequence of gas introduction into the test chamber has been tested using a volt-amperometric technique at constant relative humidity ŽRH. and at RT.

2. Experimental PS was obtained by electrochemical etching of 1 V cm n-type silicon ²100: wafer 500 mm thick, with a current

0925-4005r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 Ž 0 0 . 0 0 4 6 4 - 0

C. Baratto et al.r Sensors and Actuators B 68 (2000) 74–80

Fig. 1. Sketch of the PS sensor. Gold was deposited on the top surface of PS in different amounts Žfour different samples were tested..

density of 40 mArcm2 under exposure to a 250-W Hg lamp w4x. Etching solution was prepared by adding 30% volume of isopropyl alcohol Žas surfactant agent. to 70% volume of an HF aqueous solution Ž50% weight.. Current density and etching time were adjusted in order to obtain a porous layer characterised by an average porosity of 45% and a thickness of 40 mm. The average porosity, i.e. the void fraction in the porous layer, can be obtained by gravimetry via the equation: P s Ž m1 y m 2 . r Ž m1 y m 3 . , where m1 is the mass of the sample before the etching, m 2 is the mass after the formation of the porous layer, and m 3 is the mass after the removal of the porous layer in a 1-M solution of NaOH. The PS layer thickness was determined with a step-profiler after the removal of the PS layer. After formation, PS samples were rinsed in pentane Žlow vapour pressure. in order to avoid superficial cracks and then cut to obtain 1-cm2 specimens. Gold was DC-sputtered onto PS surface at RT. Gold deposition was pursued up to a nominal thickness of 4, 8, 15 and 40 nm. Nominal thickness means the size of the layer that would be deposited on a plane surface under the same conditions. Hereinafter, the samples will be labeled as AU4, AU8, AU15 and AU40 for gold thickness of 4, 8, 15 and 40 nm, respectively. Two golden pads for electrical contacts, 50 nm thick, were deposited on the top of catalysed PS as depicted in Fig. 1. Two gold wires were stuck to the golden pads by means of a silver paste.

3. Morphological characterisation A study on the penetration of gold into the porous layer was carried out by SEM. Comparison between secondary and backscattered electron images highlighted the presence of gold inside the porous layer, thus determining the penetration depth. In fact, the range of secondary electrons is limited to a few nanometers and the image they form is

75

essentially in topographic contrast, while backscattered electrons are reflected deeper into the material. For the latter method, the yield varied as a function of the atomic number w5x so that the image is both compositional and topological. Since the atomic number of gold is largely greater than that of silicon, the contrast of backscattered images is to be regarded as mainly compositional. A PS layer has a quite complex morphology, mainly depending on etching condition and substrate type w1,6,7x. Two different types of morphology can be found: spongelike and filament-like. The former refers to a highly interconnected and homogeneous pore network where no preferential direction can easily be observed. In the latter, clear channels can be observed, all exhibiting ²100: direction as the preferential growth axis. Our samples showed mixed morphology where larger pores in the ²100: direction were characterised by a typical diameter of 1–2 mm, as can be seen in Fig. 2a. This picture was achieved after analysis of the secondary electrons emitted by the sample. For comparison, we reported the photo taken over the same area ŽFig. 2b. through backscattered electrons. The huge difference in atomic number between silicon and gold allowed one to firmly determine the position where gold was located Žbright areas.. Sample AU8 revealed a less-significant compositional contrast, owing to a less amount of gold on the porous layer. In order to obtain more detailed information on the distribution of gold as a function of depth, we prepared several cross-sections of the samples AU40 and AU8 by an automatic dicing saw. After scribing, the samples were neither grinded nor polished in order to prevent any damage on the porous layer. A cross-sectional view of AU40 sample is illustrated in Fig. 3 as taken by secondary electrons Ža. and backscattered electrons Žb.. A 1-mm-thick region with spread gold clusters permeating the whole porous layer is clearly visible. The figure also shows a bright area surrounding the clusters, which is ascribed to be the presence of gold inside the bulk of the porous film behind the plane of the cross-section. After extensive analysis over the sample, one deduced a penetration depth of 1–2 mm for the AU40 sample, i.e. with the same size as the mean pore’s diameter. As expected, sample AU8 showed a thinner region, less than 1 mm, characterised by the presence of clusters and, in particular, a less-evident contrast between film permeated by gold and the bulk. The contrast for AU8 sample Žnot shown. was not so high as that for AU40 sample, consistently with a lower amount of gold sputtered onto the surface. A qualitative description of gold-concentration profile was carried out by EDXS technique; in-depth analysis was executed, confirming the expected thickness of gold inclusions. However, the Afluorescence dropB induced by impinging electrons was in the order of 1 mm in size, and, therefore, the resolution of the method was comparable to

76

C. Baratto et al.r Sensors and Actuators B 68 (2000) 74–80

Fig. 2. On the left side Ža., secondary electrons image of the AU40 sample; on the right side Žb., backscattered electrons image of the same area. Bright areas owed to the presence of gold.

the penetration depth of gold we wanted to investigate. In order to better understand this point, we also executed measurements by RBS. 4. Surface analysis RBS measurements were performed using the HVEC AN2000 Van de Graaff accelerator at Laboratori Nazionali

di Legnaro, with a 2-MeV 4 Heq beam, 08 and 308 tilt angles Žangles between the beam and surface normal, the surface normal lying on the plane formed by the incident and scattering directions., and 1608 scattering angle. The RBS spectra at 08 tilt angle were reported in Fig. 4. The spectra at 308 tilt angle Žnot reported. were qualitatively similar and exhibit the same features and trends. By assuming that Au and PS were uniformly mixed on a scale

Fig. 3. On the left side Ža., secondary electrons image of a cross-section of the sample AU40; on the right side Žb., backscattered electrons image of the same area. Bright areas owed to the presence of gold.

C. Baratto et al.r Sensors and Actuators B 68 (2000) 74–80

Fig. 4. RBS spectra of the PS samples covered by various nominal thickness of Au, using a 2-MeV 4 Heq beam at normal incidence and 1608 scattering angle. The spectra are multiplied by a scale factor of 5 in the region below channel 360. The high-energy edges of Au, Si and O are marked by arrows.

of a few nanometers, the spectra could be simulated as if Au were atomically diffused into PS. The results of such simulations of the spectra at 08 tilt angle, performed using the RUMP code w8x, were reported as Au-depth profiles in Fig. 5. Similar results were obtained from the spectra at 308 tilt angle. The depth scale was calculated by taking into account that, due to the presence of oxygen, the Si atomic fraction was about 0.4, as determined by RBS Žthis low value may be due to hydrocarbon layer build-up on the pore surfaces during the irradiation.. Furthermore, the density of the Si atoms was assumed to be equal to the value in the crystalline material Žabout 5.0 = 10 22 atomsrcm3 .. Finally, the depth was multiplied by the factor 1rŽ1 y P ., with P s 0.45, to take into account the

Fig. 5. Au depth profiles as calculated from the spectra in Fig. 4, under the assumption of PS–Au mixing.

77

porosity. Penetration depths were lower than those indicated by the SEM cross-sectional images. However, the assumptions leading to the results reported in Fig. 5 were questionable for these samples. As suggested by the SEM images, these samples had pores running in random directions Žsponge-like PS. with micrometric dimensions. The deposited Au atoms may not be mixed with PS, and most of them may be traversed by the beam before traversing PS. Furthermore, different amounts of Au may be traversed by ions striking the sample in different points. In this case, the tails of Au peaks in the RBS spectra should be attributed to a distribution of Au thickness coating the pore walls. According to this point of view, an alternative interpretation of the RBS spectra can be given by introducing the concept of the fractional coverage F, which can be defined as follows: F Ž t . s Y Ž E .rYB Ž E ., where Y Ž E . is the Au yield at the detected energy E, Y B Ž E . is the yield at the same energy for a bulk Au sample, and t is the depth corresponding to E in the bulk of the Au sample Ži.e. the particle scattered at depth t was detected at energy E .. F is the fraction of the surface covered by a thickness of gold larger than t. The calculation of F Ž t . from the experimental spectra at 08 tilt angle was reported in Fig. 6. The results for the spectra at 308 tilt angle are similar, but with slightly higher tails. The peaks in Fig. 6 did not correspond to the peaks of the fractional coverage, since they were due to the finite resolution of the measurements. Support to this interpretation of the RBS spectra was also given by the deficient leading edges of the Si signal ŽFig. 4., which were qualitatively explained by a shift of the high-energy edge of Si covered by a distribution of Au thickness. Calculations of F from this spectral feature gave higher values than those obtained from the Au signal, indicating that other effects were present, such as those mentioned in the introduction of Ref. w9x.

Fig. 6. Fractional coverage calculated from the spectra in Fig. 5.

C. Baratto et al.r Sensors and Actuators B 68 (2000) 74–80

78

The disagreement between the SEM cross-sectional images and the depth profile obtained from RBS under the assumption of PS–Au mixing indicated that the amount of Au penetrated into the pores is small. This is consistent with a spongy Žnot columnar. PS micrometric structure of the top surface. With such a structure, the Au atoms could reach only the outer part of the pore convolutions, within a depth from the surface approximately equal to the average pore size.

5. Electrical characterisation We studied the response to gas of the PS layers by monitoring the change in conductivity when NO or NO 2 traces were fed into the test chamber. Since the device was biased at constant voltage Ž5 V., the change in conductivity was revealed by a variation of the current flowing through the device w10x. All measurements were performed at RT by keeping the test chamber at 208C at constant RH. Since our interest was to monitor NO x in environmental condition, we used synthetic air as a gas carrier. NO is an unstable species in the presence of oxygen, even at RT, according to the reaction 2NO q O 2

l 2NO , 2

Ž 1.

hence, cannot be stored in air bottles. However, direct reaction is of the third order in NO concentration, and therefore is very slow at low NO concentration. Thus, the desired concentration was obtained by employing an air bottle with a certified content of NO 2 and a molybdenum converter kept at 3208C, outside the test chamber, which dissociated NO 2 according to the reverse of reaction Ž1.. While the gas passed through the tube

Fig. 7. Dynamic electrical response to 5 ppm of NO 2 at 20% RH and RT. The response is greater for the AU15 sample than for the AU4 sample. No current variation was observed for bare PS.

Fig. 8. Dynamic electrical response of sample AU4 to 1, 2, 4 and 5 ppm of NO at 20% RH and RT, compared to that of noncatalysed PS.

connecting the converter to the stainless-steel chamber, it cooled down to 208C. By employing a constant flow equal to 300 cm3rmin, we were ensured that back conversion of NO into NO 2 did not take place before the gas had reached the test chamber. The conductance of the gold-catalysed samples was similar to that of bare PS, except for the AU40 sample. RBS analysis proved that exaggerate coverage took place over the surface for the AU40 sample, thereby creating a current path through the gold clusters. Therefore, AU40 sample was much more conductive than the others Žabout five orders of magnitude higher. and is not suitable for gas sensing. Fig. 7 reports the current variations of the film when 5 ppm of NO 2 were fed into the test chamber: it resulted in an increase of the sensor resistance. We found that as-prepared PS was insensitive to NO 2 , while gold-catalysed PS showed an increasing normalised response while augmenting the amount of gold dispersed in the porous layer, i.e. passing from 4 to 15 nm of nominal thickness, the relative response raised from 0.6 to 1.6. The response time of PS sensors Žtime taken to reach 90% of equilibrium value. was of the order of some minutes, while the recovery time Žtime taken to reach 63% of current in air. was about 10 min. For each sample, complete recovery of initial current was observed at RT. Fig. 8 reports the kinetic response to an increasing square concentration of NO for a bare PS layer and AU4 sample. The response of gold-activated PS devices to NO resulted comparable as that with NO 2 . When a few parts per million of NO were introduced into the chamber, no variation was detected for bare PS, while the resistance of AU4 sample increased. The normalised response D RrR was slightly higher than that shown towards the same concentration of NO 2 . The relationship between the two

C. Baratto et al.r Sensors and Actuators B 68 (2000) 74–80

79

influence of humidity on the sensor response. Operation at RT envisaged interesting development of a low-cost gas sensor based on PS. Best results were obtained for AU15 sample Ž D RrR s 1.6 at 5 ppm of NO 2 .. This case was a trade-off that was set between the maximum amount of catalyst deposited onto the surface and the formation of a current path through the gold clusters. Influence by humidity was strong in the low-humidity range while remarkable selectivity towards interfering gases such as CO, CH 4 and methanol was proven.

Acknowledgements Fig. 9. Normalised response of as-prepared PS and gold-catalysed samples toward 5 ppm of NO 2 at RT vs. RH.

mechanisms of interaction of NO and NO 2 with PS will be further investigated in a future work. All PS samples were found to be insensitive towards interfering gases such as CO Ž1000 ppm., CH 4 Ž5000 ppm. and methanol Ž1000 ppm. at 20% constant RH and RT. PS has been known to possess considerable reactivity to humidity w11x so that an investigation on a possible influence by NO x detection of RH needs to be examined closely. Fig. 9 reports the relationship between D RrR vs. RH for AU4, AU8 and AU15 samples at 5 ppm of NO 2 and RT. The response of each catalysed sample to NO 2 decreased as RH was increased from 0% to 60%. A possible explanation is that, at high RH, the dangling bonds of PS are saturated with water molecules, so fewer sites are available for reactions with NO 2 . However, further work is needed to validate this explanation. Indeed, a comparison of all samples at constant RH showed that D RrR increased while the amount of gold over PS was increased, recovering the results shown in Fig. 8 for 20% RH and adding the results for other values of RH.

6. Conclusions Preliminary results showed that the gold deposited by sputtering on n-type PS was dispersed on the porous surface of the material instead of forming a continuous layer, as inspected by SEM and RBS. These analyses confirmed that penetration of gold into the pores was 1 mm deep until occlusion was reached when a gold layer nominally as thick as 40 nm was deposited. Clusterization of gold favoured the catalytic behaviour of gold deposited on n-type PS for sensitive and selective sensing of NO x . Experimental results showed a strong

This work has been partially supported by INFM-SUD project AStudy, Characterisation and Fabrication of Prototypes based on PS for the Detection of Toxic GasesB.

References w1x L. Canham, Properties of Porous Silicon, INSPEC, London, 1997, pp. 44–86. w2x J. Harper, M.J. Sailor, Detection of nitric oxide and nitrogen dioxide with photoluminescent porous silicon, Anal. Chem. 68 Ž1996. 3713– 3717. w3x I. Schetchter, M. Ben-Chorin, A. Kux, Gas sensing properties of porous silicon, Anal. Chem. 67 Ž1995. 3727. w4x P. Gupta, V.L. Corvin, S.M. George, Hydrogen desorption kinetics from monohydride and dihydride species on silicon surfaces, Phys. Rev. B 37 Ž1998. 8234–8243. w5x C.W. Oatley, The Scanning Electron Microscope, Cambridge Univ. Press, Cambridge, 1972. w6x V. Lehman, U. Gosele, in: Z.C. Feng, R. Tsu ŽEds.., Porous Silicon, World Scientific, Singapore, 1994, p. 17. w7x R.L. Smith, S.D. Collins, Porous silicon formation mechanism, J. Appl. Phys. 71 Ž1992. R1–R21. w8x L.R. Doolittle, Algorithms for the rapid simulation of Rutherford backscattering spectra, Nucl. Instrum. Methods, B 9 Ž1985. 334. w9x Z. Hajnal, E. Szilagyi, F. Paszti, G. Battistig, Channeling-like effects ´ ´ due to the macroscopic structure of porous silicon, Nucl. Instrum. Methods, B 118 Ž1996. 617. w10x G. Sberveglieri, L.E. Depero, S. Groppelli, P. Nelli, WO 3 -sputtered thin films for NO x monitoring, Sens. Actuators, B 26–27 Ž1995. 89–92. w11x J.J. Mares, J. Kristofik, E. Hulicius, Influence of humidity on transport in porous silicon, Thin Solid Films 255 Ž1995. 272–275.

Biographies Camilla Baratto was born in Brescia in 1972 and received her degree in Applied Physics at the University of Parma in November 1997 with a thesis on Raman and EXAFS characterisation of iron oxide thin films. In 1998, she started her PhD at the Gas Sensor Laboratory. She is working on the development of porous silicon-based gas sensors. Her research topics include structural and electrical characterisations of porous silicon devices by SEM and DC–AC technique.

80

C. Baratto et al.r Sensors and Actuators B 68 (2000) 74–80

Elisabetta Comini was born in 1972. She received her graduate degree in Physics at the University of Pisa in 1996. She is presently working on chemical sensors, with particular reference to deposition of thin films by PVD technique and electrical characterisation of MOS thin films. She is now finishing her PhD in Material Science at the University of Brescia. Giorgio SberÕeglieri was born on July 17, 1947 and received his degree in Physics from the University Parma, where he started his research activities on the preparation of semiconducting thin film solar cells in 1971. He was appointed as Associate Professor at the University of Brescia in 1987. In the following year, he established the Thin Film Laboratory, afterwards called Gas Sensor Laboratory, which is mainly devoted to the preparation and characterisation of thin film chemical sensors. He has been the director of the GSL since 1988. In 1994, he was appointed as Full Professor in Physics, formerly at the Faculty of Engineering of the University of Ferrara, and, then, in 1996, at the Faculty of Engineering of the University of Brescia. He is referee of AThin Solid FilmsB, ASensors and ActuatorsB, ASensors and MaterialsB and other journals. He is also a member of the Scientific Committee of Conferences on Sensor and Materials Science. During his 25 years of scientific activities, Giorgio Sberveglieri has published more than 140 papers on international reviews, has presented more than 50 oral communications to international congresses as well as numerous ones to national congresses. Guido Faglia received an MS degree from the Polytechnic of Milan in 1991 with a thesis on gas sensors. Since then, he has been studying to obtain his PhD in Electronics at the University of Brescia. In 1992, he was appointed as a researcher by the Thin Film Laboratory at the University of Brescia. He is involved in the study of the interactions between gases and the tin oxide surface, and in sensor–electrical characterisation. In 1996, he received his PhD degree after discussing a thesis on semiconductor gas sensors.

Vincenzo Guidi received his graduate degree in Physics in 1990 from the Ferrara University and his PhD degree, also in Physics, from the same university in 1994. Vincenzo Guidi was awarded by the Italian Physics Society for scientific merits in 1992. He has been a researcher at the Faculty of Engineering, Ferrara University since 1994. Vincenzo Guidi worked for some years in the field of photoemission from semiconductors. He is presently working on chemical sensor and laser cooling. He has 70 papers published in several international journals, 60 contributions to international conferences and one patent. He is also referee to major journals on applied physics and has been a guest editor of an international conference. Donato Vincenzi was born in Mantova, Italy in 1975. He received his graduate degree in Physics in 1998 from the Ferrara University with a thesis on porous silicon photoluminescence. He is presently pursuing his PhD at the Semiconductors and Sensors Laboratory in the same university, where he is involved in the design of a hybrid gas analysis microsystem. Luigi Quercia, Doctor in Physics, was born in Naples, Italy in 1961. He has been working at the ENEA Research Center in Portici ŽNA. since 1992. He is presently involved in the research activity on porous silicon gas-sensor devices and has previously worked on CuInSe 2 and amorphous silicon solar cells, high-resolution spectroscopy, molecular beams and cluster formation. Girolamo Di Francia, Doctor in Physics, was born in Naples, Italy in 1958. He works at the ENEA Research Center in Portici ŽNA.. At present, he is in-charge of the research activity on porous silicon gassensor devices and has been previously in-charge of the ENEA activity on GaAs and silicon solar cells.