Sensors and Actuators B 103 (2004) 344–349

Porous a/nc-Si:H films produced by HW-CVD as ethanolvapour detector and primary fuel cell

Isabel Ferreira∗, Rui Igreja, Elvira Fortunato, Rodrigo MartinsCENIMAT, Department of Materials Science, Faculty of Science and Technology of New University of Lisbon,

CEMOP/UNINOVA, Campus da Caparica, 2829-516 Caparica, Portugal

Available online 28 May 2004

Abstract

This work reports the use of undoped porous amorphous/nanocrystalline hydrogenated silicon (a/nc-Si:H) thin films produced by hotwire chemical vapour deposition (HW-CVD) as ethanol detector above 50 ppm and as a primary fuel cell where a power of 4W/cm2

was obtained in structures of the type glass/ITO/i-a-nc-Si:H/Al. The porous silicon looks like a sponge constituted by grains and cluster ofgrains that determines the type of surface morphology and the behaviour of the structure under the presence of vapour moisture. Apart fromthat, the detector/device performances will also depend on the type of interlayer and interfaces with the metal contacts. The sponge likestructure adsorbs the OH groups in uncompensated bonds, which behave as donor-like carriers, leading to an increase in the current flowingthrough the material, directly dependent on the ethanol vapour pressure. The corresponding role of the components of the microstructureon this detector was investigated by spectroscopic impedance. The response time of the current of the sensor and its recovery time are inthe range of 10–50 s at room temperature.© 2004 Elsevier B.V. All rights reserved.

Keywords: HW-CVD technique; Porous silicon; Ethanol detector

1. Introduction

Porous silicon (PS) obtained by electrochemical etch-ing of crystalline silicon, besides being employed in lightemitting diodes has also been applied in the detection ofvapours from organic compounds such as ethanol at roomor higher temperatures[1–3]. The detection of ethanolusing ZnO [3], SnO2 [3] and Bi2(MoO4)3 [4] materialshas also been demonstrated. Nevertheless, their sensi-tivity is temperature-dependent being significant around400–600C.

In previous reports we demonstrate that intrinsic or dopedsilicon films produced by HW-CVD, under certain deposi-tion conditions, have a porous sponge-like structure[5]. Duethis structure a chemisorption of the environmental gases isexpected, changing the surface state and charge distributionof the a/nanocrystalline silicon hydrogenated agglomerates(a/nano c-Si:H). Reduction/oxidation reactions of the OH−group from ethanol and the SiOx bonds are also involvedand responsible for the creation of extra free electrons for

∗ Corresponding author. Tel.:+351-21-2948564;fax: +351-21-2957810.E-mail address: imf@fct.unl.pt (I. Ferreira).

conduction. Therefore, an extra current flow is observedwhen the material is in the presence of the ethanol.

In this paper we present data concerning the correla-tion of the transport properties of these structures with thecorresponding microstructure, before and after exposing toethanol vapour, through the analysis of the spectroscopicimpedance data (dependence of the capacitance and ofthe ac conductivity on the frequency)[6]. Besides thatwe also observed the existence of a potential developedon glass/ITO/i-a-nc-Si:H/Al structure imbedded in ethanolvapour, when in the open circuit mode. This phenomenonis analysed and discussed in this paper.

2. Sample preparation and experimental procedures

The intrinsic porous silicon (amorphous or nanos-tructured) thin films were obtained by HW-CVD tech-nique. Details concerning the deposition system usedare described elsewhere[5]. The sensors studied have aglass/ITO/i-a-nc-Si:H/Al structure (seeFig. 1). The thick-ness of the layers is≈200, 800 and 250 nm for ITO, a-Si:Hand Al, respectively. The intrinsic porous silicon film wasproduced using: silane gas diluted in hydrogen in the

0925-4005/$ – see front matter © 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.snb.2004.04.064

I. Ferreira et al. / Sensors and Actuators B 103 (2004) 344–349 345

Fig. 1. Sketch of the sample structure.

proportion of 5 sccm (SiH4):500 sccm (H2); a total gas pres-sure of 66.5 Pa; filament temperature (Tf ) of 2123 K andsubstrate temperature (Ts) of 473 K. For testing the sensorsthe ethanol vapour was introduced into the test chamberthrough a mass flow meter[7]. Voltages between−1 and1 V were applied and the current was measured by an elec-trometer connected to a computer in order to measure thetime dependence of the sensor response. All the tests wereperformed at room temperature and the pressure inside thechamber (Pethanol) was varied from 10 Pa to atmosphericpressure by introducing ethanol vapour. The sensitivity ofthe structure was determined through the ratio between thecurrent of the sensor in presence of ethanol vapour and invacuum—1 Pa (Ieth/Ivac).

The morphology of the films analysed was performed us-ing a scan electron microscope (Hitachi) while the film’scompactness was determined through spectroscopic ellip-sometry (SE) measurements using a Jobin Yvon UVISELDH10 Ellipsometry, where the analysis of the spectra wasperformed with DELTAPSI 2 WindowsTM based software.Apart from that, the impedance spectroscopy measurementswere done using a precision impedance analyser (Agilent4294 A) in the frequency range from 40 Hz to 110 MHz.

Fig. 2. SEM micrograph of porous silicon used as ethanol detector. The size of the grains is below 100 nm.

Fig. 3. Spectrum of the real and imaginary components of the dielectricfunction obtained by spectroscopic ellipsometry of the film shown inFig. 2.

3. Results and discussion

3.1. Microstructure and morphology

Fig. 2shows scanning electron microscopy (SEM) imageof the porous silicon used in the sensor. The micrographreveals an agglomerate-like structure with variable size(<100 nm), high surface roughness and porosity. The filmsporosity was not quantified but as the films are formedby agglomerates, the existence of empty space betweenagglomerates are well-identified in the SEM micrograph.This is corroborated by the spectroscopic ellipsometry data(Fig. 3) where we observe the dependence of the real andimaginary components (εr and εi ) of the films dielectricfunction. There, we notice that the peak value ofεi isof about 6, which it is five times smaller than the valuerecorded in compact crystalline silicon and about 4.5 timessmaller than the values recorded in high compact a-Si:H[5].

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Fig. 4. Plot of Z′′ vs. Z′ of a structure behaviour based on porous Siunder atmosphere (a) and ethanol pressure conditions (b). All plots weretaken at the same environmental pressure.

These data clear show that the films produced are porousand so, the behaviours of the structures built upon them area function of the films porosity, size of grains and how theyare aggregated.

3.2. Spectroscopic impedance

Spectroscopic impedance measurements were performedto analyse the role of the microstructure on the device perfor-mances. For the interpretation of the results we assume thatdifferent regions of the film can be considered as constitutedby a resistance and a capacitance associated in parallel. Theindividual components may then be quantified. In this par-ticular case, we assume the existence of at least of the fol-lowing contributors for the overall impedance recorded: bulkand boundary grain contributions, interface or inter-layerscontribution. Such contributors’ lead to different impedanceswith an imaginary (Z′′, capacitive) and a real components(Z′, resistive) where each RC gives rise to a semicircle (ide-ally) in a Z′′–Z′ system of axes (also called Cole–Cole plot[8]) that can be interpreted in terms of the contribution of asingle parallel element. Apart from that, low frequency spikerepresents charge build up at the blocking metal electrode.

In Fig. 4(a and b)we show theZ′′–Z′ plot of the filmsanalysed under normal environment conditions and underethanol vapour pressure conditions (STP). The data show a

clear change on the films’ impedance behaviour with andwithout ethanol vapour, due to vapour chemisorptions. Apartfrom that, we also notice a change on the curve shape.Two poorly resolved semicircles and a straight line matchthe data. The poor resolution of the semicircles can be at-tributed to the role of electron transfer from the interfacelayer between the metal and the bulk grain boundaries ofthe semiconductor. The first large semicircle corresponds tothe bulk impedance while the second to the grain bound-ary. The spike line at about 45 is associated with blockingcontact and surface effects. If we consider that the grains inthe amorphous tissue have in average al1 dimension sep-arated from each other by a boundary of thicknessl2, theration between the bulk and grain boundary capacitances isdirectly proportional to the ratio ofl2/l1 (Cb/Cgb = l2/l1)and so correlated to the data depicted inFig. 4. From thisdata we estimate a relation over 103, for this relation. Wealso notice a high change in the impedance recorded withand without ethanol vapour present, translated by a changeon Z′ andZ′′ over about four orders of magnitude. This be-haviour is dependent on the type of porous structure and soon the role of the surface grains, inter-layers and type ofvoids/spaces existing between aggregates of grains. To cer-tify that the change observed is due to porous structure of thesilicon film, we perform the spectroscopy impedance mea-surements on a structure base on a compact silicon film. Thedependence ofZ′ andZ′′ on the frequency for such film isdepicted inFig. 5. No difference onZ′ andZ′′ behaviour wasobserved with and without the presence of ethanol vapour,proving so that the changes observed are only related to theset of reduction/oxidation reactions that take place on theporous structure.

3.3. The model

The behaviour observed in the porous Si based struc-ture is in favour of a chemisorptions reaction mechanism

101 102 103 104 105 106 107 108 109101

102

103

104

105

106

107

108

109

1010

1011

1012

Z''

Z' (

Ω),

Z''

(Ω)

f (Hz)

Z'

Fig. 5. Plot ofZ′ andZ′′ as a function of frequency for a structure usinga compact Si film. There were no changes observed before and afterexposure of the structure to ethanol vapour.

I. Ferreira et al. / Sensors and Actuators B 103 (2004) 344–349 347

----

---

D

D+/0

0/-

--

-

-

-

-

-

D

D+/0

0/-

EC

EV

EF

O

O

Inner boundary Outer boundary

InnerOuter boundary boundary

InnerOuter boundary boundary

Fig. 6. Model proposed for the electronic behaviour of the porous structure.

in the surface of the agglomerates due to the existence ofdangling bonds where OH− is adsorbed, originating an ex-cess of electrons in the conduction band of the semicon-ductor. To this we should add the surface modification re-action mechanisms due to gas interaction with the surfacesstates associated to adsorbed oxygen in a/nc-SiO:H films,similarly as the one described for ZnO films [9]. Accord-ing to the mechanism suggested for gas detection in ZnO,one of the possible reaction between a/nc-SiO:H and ethanolis the adsorption mechanism where OH− interacts with thesurface of the porous material acting as a reducing agentand injecting one or two electrons into the semiconductor[8]. Assuming that the films have a columnar growth ofstacked grains, we may consider two types of grain bound-aries, the inner boundaries (between grains) and the outerboundaries (between columns), as depicted in Fig. 6. Bothexhibit oxygen adsorption originating a SiO:H film, as itwas observed by FTIR measurements [5]. When oxygenis adsorbed the donor charges are blocked in the grainsboundaries. Thus, a space charge region is formed wherethe Fermi level is in the middle of optical gap and the dan-gling bonds are occupied. If the oxygen is desorbed a neg-ative charge is emitted to conduction band and the Fermilevel is shifted upwards and the dangling bonds diminish[10].

From the above model we can consider that free carriersare generated due to the reaction of OH− with the oxygenof the silicon. Such reactions could be for instance: SiO +H2O SiH2 + O2 (resulting from 4e + SiO + 3H2O SiH2 + 4OH− and 4OH− O2 + 2H2O); 2SiO + H2O

2SiH + 3/2O2 (resulting from 3e + SiO + 2H2O SiH +3OH− and 6OH− 3/2O2 + 3H2O + 6e).

3.4. Static and dynamic I–V characteristics

The static |I|–V curves of the glass/ITO/i-a-nc-Si:H/Alstructure at different Pethanol show to be bias dependent. ForPethanol = 1 atm. pressure, Ieth/Ivac is ∼102 at V = 0 V andis ∼104 at V = −0.9 V. Apart from that, we notice that theplot of Ieth/Ivac as a function of Pethanol, for V = −0.9 Vhas an almost square power dependence on Pethanol (Ieth/Ivac= 6 × 10−3P2.1

ethanol), indicating that the number of inducedcarriers into porous a-Si:H films is proportional to Pethanol(see Fig. 7).

Fig. 8 shows the time dependence of Ieth/Ivac before andafter sustaining the surface to oxidation. The data show thatthe ethanol is only detected after oxidation of the sample.This demonstrates that the mechanism leading to the currentenhancement, when the sample is in the presence of ethanol,is related to the oxidation of the a/nano crystalline poroussilicon films, which corroborates the model proposed. Wealso observe constant time dependence, indicating that nofurther carriers-like donors recombination or formation areundergoing. On the other hand, the sensor shows a responsetime of the order of 15 s and the recovery time is similar [7].

3.5. Behaviour of the structure as a primary fuel cell

From the I–V characteristics we notice that under forwardbias, an inversion of the current signal is observed, Pethanol

348 I. Ferreira et al. / Sensors and Actuators B 103 (2004) 344–349

Fig. 7. Ieth/Ivac vs. Pethanol for V = −0.9 V and SEM micrograph of filmused in the sensor structure.

dependent. That is, the value of the voltage to which the sig-nal of the current is inverted increases as Pethanol increases.This reveals the appearance of a built-in potential on theglass/ITO/i-a-nc-Si:H/Al structure.

The appearance of a potential can be explained takinginto account the model proposed (see Fig. 6). That is, theappearance of a potential related to the electric field effectcreated between the bulk and the boundaries of the grains(or agglomerates), which originates an ethanol-induced cur-rent, when the glass/ITO/i-a-nc-Si:H/Al structure is shortcircuited, Pethanol dependent, as depicted in Fig. 9. The ori-gin of this phenomenon could be also related to possiblethe ionic conduction established between Al (anode) andITO–In2O3 (cathode) due to ethanol filling the nanoporosi-ties and consequently allowing the formation of a primarybattery.

In order to eliminate other possible mechanisms con-tributing to the observed phenomenon, such as photoin-duced phenomenon, all the previous measurements wereperformed under dark conditions. In order to confirm thatthe behaviour observed is not related to the contacts (barrier

Fig. 8. Time dependence of the ethanol-induced current of theglass/ITO/i-a-nc-Si:H/Al sensor before and after oxidation of the a/nc-Si:Hporous film.

1 10 100 1000

0.2

0.4

0.6

0.8

ethanol pressure (mbar)

Vol

tage

(I=

0) (

V)

10-9

10-8

10-7

Cur

rent

(V=

0) (

A)

a)

Fig. 9. Potential obtained in the open circuit mode and ethanol-inducedcurrent (V = 0 V) as a function of Pethanol.

-0.8 -0.4 0.0 0.4 0.810-13

10-12

10-11

10-10

10-9

Cur

rent

(A

)

Voltage (V)

Fig. 10. I–V characteristic of a glass/ITO/a-nc-Si:H/Al using compacta-Si:H film produced by PECVD technique, taken under vacuum (squares)and exposed to 103 mbar ethanol vapour pressure (circles).

formation between Al and ethanol), a glass/ITO/a-Si:H/Alsensor where a compact a-Si:H obtained by plasma en-hanced chemical vapour deposition technique was tested.Fig. 10 shows that the obtained I–V characteristics of thesensor in vacuum (∼1 Pa) and under ethanol vapour aresimilar. No ethanol-induced current and/or built-in potentialwas detected for the sensor employing a-Si:H compact films.These data supports the suggestion that the current–voltagegeneration observed becomes from the carriers inducedby the ethanol in the porous a/nc-SiO:H film and it is notdue to the formation of an Al/C2H5OH surface barrier.However, if the generated current in porous films come upfrom the ethanol donors then the open circuit voltage arisesprobably from a potential barrier formed between agglom-erates, due to the adsorbed oxygen and or hydroxyl groups(Si:H/Si:OH), as suggested before.

4. Conclusions

We demonstrated the possibility to use a/nc-Si:Hporous undoped films produced by HW-CVD techniquein glass/ITO/a-nc-Si:H/Al structure as ethanol vapourdetectors, whose sensitivity is highly dependent on themicrostructure. A model based on the role of grains and ag-gregates of grains was successfully built up to interpret the

I. Ferreira et al. / Sensors and Actuators B 103 (2004) 344–349 349

results achieved, namely the high sensitivity of the struc-ture to ethanol vapour (four orders of magnitude). The dataalso show that the response time of the sensor is lower than30 s, being the recovery time similar. We also verify theappearance of an ethanol-induced current for V = 0 V andof a built-in potential in the glass/ITO/a-nc-Si:H/Al sensorstructure for the short circuit mode, leading to power gen-eration of 4 W/cm2. Hence, we suggest the use of thesestructures not only as ethanol detectors but also as fuel cellusing ethanol as catalytic precursor.

Acknowledgements

The authors thank A. Lopes (U. Aveiro) and R.Igreja (CENIMAT) for the help given, respectively, forSEM and impedance spectroscopy analysis. This workwas supported by Fundação da Ciencia e Tecnologiathrough ‘Financiamento Plurianuais’ of CENIMAT, andprojects PRAXIS/P/CTM/12094/1998 and H-Alpha Solar,ERK&CT-1999-00004.

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