sno2 thin film sensor with enhanced response for no2 gas at lower temperatures
TRANSCRIPT
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Sensors and Actuators B 156 (2011) 743– 752
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical
journa l h o mepage: www.elsev ier .com/ locate /snb
nO2 thin film sensor with enhanced response for NO2 gas at lower temperatures
njali Sharmaa, Monika Tomarb, Vinay Guptaa,∗
Department of Physics and Astrophysics, University of Delhi, Delhi 110007, IndiaDepartment of Physics, Miranda House, University of Delhi, Delhi 110007, India
r t i c l e i n f o
rticle history:eceived 3 December 2010eceived in revised form 15 February 2011ccepted 17 February 2011vailable online 24 February 2011
ACS:ilm deposition by sputtering1.15.Cdas sensors7.07.Df
a b s t r a c t
Semiconducting SnO2 thin films having higher value of electrical conductivity have been deposited usingRF sputtering technique in the reactive gas environment (30% O2 + 70% Ar) using a metallic tin (Sn) targetfor detection of oxidizing NO2 gas. The effect of growth pressure (12–18 mTorr) on the surface morphol-ogy and structural property of SnO2 film was studied using Atomic force microscopy (AFM), Scanningelectron microscopy (SEM) and X-ray Diffraction (XRD) respectively. Film deposited at 16 mTorr sput-tering pressure was porous with rough microstructure and exhibits high sensor response (∼2.9 × 104)towards 50 ppm NO2 gas at a comparatively low operating temperature (∼100 ◦C). The sensor responsewas found to increase linearly from 1.31 × 102 to 2.9 × 104 while the response time decrease from 12.4to 1.6 min with increase in the concentration of NO2 gas from 1 to 50 ppm. The reaction kinetics of targetNO2 gas on the surface of SnO2 thin film at the Sn sites play important role in enhancing the response
anocrystalline materials1.07.Bc
eywords:as sensoremiconducting thin film
characteristics at lower operating temperature (∼100 ◦C). The results obtained in the present study areencouraging for realization of SnO2 thin film based sensor for efficient detection of NO2 gas with lowpower consumption.
© 2011 Elsevier B.V. All rights reserved.
putteringO2 gas
. Introduction
Environmental protection policy of most of the countries isriented towards the regulation, precise measurement and controlf the noxious gases such as H2S, O3, CO, H2 and NO2 in thetmosphere. Now a days, air pollution by nitrogen oxides (NOx),ainly NO and NO2, is becoming an important environmental
ssue. As a matter of fact, NO2 associated with other pollutantsike volatile organic compounds (VOC) is responsible for theormation of ozone [1] in lower atmosphere, smog in urban areasnd also chemical reaction of NO2 gas with water vapour causescid rain [2]. Therefore, the development of a NO2 gas sensor fornvironmental monitoring has become a necessary task.
Semiconducting tin oxide (SnO2) based gas sensors have
eceived much attention for more than four decades due to its suit-ble physical–chemical properties, and possibility to detect wideariety of gases (reducing as well as oxidizing) with high response3–6]. Thin films are more advantageous over their bulk counter-∗ Corresponding author at: Department of Physics and Astrophysics, Universityf Delhi, Delhi 110007, India.
E-mail addresses: [email protected], drvin [email protected]. Gupta).
925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.snb.2011.02.033
part for gas sensing applications due to higher surface to volumeratio and controlled surface morphology [3,4]. SnO2 is naturallynon-stoichiometric having a rutile phase that eases the adsorptionof oxygen on its surface and thus is highly sensitive towards manytoxic and harmful gases [7]. However, requirement of higheroperating temperature (>200 ◦C) is a major issue. It is reported thatthe quality of SnO2 film with desired surface morphology plays animportant role in reducing the operating temperature and enhanc-ing the sensing response characteristics towards a target gas whichin turn depends on the growth kinetics [3,6]. Few reports [Table 1]are available on the detection of NO2 gas using pure semiconduct-ing SnO2 or SnO2 doped thin films with suitable catalysts (WO3,In, In2O3 etc.) deposited using various techniques showing rea-sonably good sensing response but at relatively higher operatingtemperatures (>150 ◦C). Currently, worldwide efforts are towardsthe development of inexpensive, compact and maintenance freeNO2 gas sensors exhibiting higher response at low operatingtemperature.
Since, NO2 is an oxidizing gas, the higher sensing response
is expected for SnO2 sensor having much lower value of theinitial sensor resistance (in air). In the present work, SnO2 thinfilms with lower electrical resistivity are deposited using RFsputtering technique, and the deposition conditions are optimizedto obtain enhanced sensing response characteristics for lower
744 A. Sharma et al. / Sensors and Actuators B 156 (2011) 743– 752
Table 1Brief summary of results reported for NO2 gas sensors based on semiconducting SnO2 thin films.
Material used Deposition technique Catalyst used Temperature (◦C) Sensor response (gas concentration) Reference no.
SnO2 hollowspheres Chemical route – 160 2031 (20 ppm) [8]SnO2 nanowires Thermal evaporation 200 180 (5 ppm) [9]SnO2 thin film Sol gel WO3 (D) 150 33,359 (500 ppm) [5]SnO2 thin film Sol gel Indium (D) 150 72 (500 ppm) [10]SnO2 thin film RF Sputtering – 200 18 (100 ppb) [11]SnO2 thin film RF Induction Plasma – 180 10 (100 ppb) [12]SnO2 thin film Sol gel – 180–260 100 (100 ppb) [13]SnO2thick film Screen Printing – 2SnO2 thin film Sol gel In2O3In2O3–SnO2 2SnO2 thin film Chemical spray – 3SnO2 thin film Vapour Phase Deposition – 3
Table 2Deposition parameters for SnO2 thin film deposition using RF Sputtering technique.
Target Metallic 4 in. dia. Tin (99.999% pure)
Target to substrate distance 7.5 cmSputtering pressure 12–18 mTorr
cisd
2
u(Tifiitpfi
RF power 150 WGas composition (Ar:O2) 7:3Substrate temperature No heating
oncentrations (1–50 ppm) of NO2 gas at relatively lower operat-ng temperature. The origin of possible mechanism for enhancedensing response of deposited SnO2 thin film for NO2 gas is alsoiscussed in detail.
. Experimental
Tin oxide (SnO2) thin films of 430 nm thickness were depositedsing RF diode sputtering technique using a metallic tin target99.999% pure) in a reactive ambient of Ar and O2 gas mixture.ypical deposition conditions for the SnO2 thin film are listedn Table 2. The sensing response characteristics of SnO2 thinlms were studied using inter digital electrodes (IDEs) of plat-
num (Pt) as shown in Fig. 1. The Pt IDEs were patterned overhe corning glass substrates using conventional photolithogra-hy, prior to deposition of SnO2 thin films. The platinum thinlm of 90 nm thickness was deposited by RF sputtering using
Fig. 1. Schematic of SnO2 thin film gas sensor.
00 28.5 (5 ppm) [14]50 20 (5 ppm) [15]50 60 (500 ppm) [16]00 9 (200 ppb) [17]
platinum metal target in 100% Ar. In order to improve the adhe-sion of Pt on corning glass substrate an ultra thin (10 nm) bufferlayer of Titanium was sputtered prior to Pt deposition. Thick-ness and surface roughness of deposited thin films were measuredusing a Veeco Dektak 150 surface profiler. Crystalline structureand surface morphology of SnO2 thin films were studied usingBragg–Brentano (�–2�) scan of a X-ray Diffractometer (Bruker D8Discover) using the CuK�1 source (� = 0.154 nm), Atomic forcemicroscopy (Veeco DICP2) and Scanning electron microscopy (ZeissUltra Plus) respectively. A Double Beam UV–visible Spectropho-tometer (Perkin Elmer, Lambda 35) was used to study the opticalproperties of SnO2 thin films.
NO2 gas sensing characteristics of sensors were studied in aspecially designed “gas sensor test rig (GSTR)” having a glass testchamber. Different concentrations of NO2 gas (50, 10 and 1 ppm)were introduced into the glass test chamber using calibrated leaksthrough needle valves. Volume of the test chamber was taken to be11.0 L and target NO2 gas was injected in the test chamber througha syringe of 0.5 ml at the time of taking response for 50 ppm of NO2gas. A pirani gauge with a rotary pump was used to control the flowof target gas in the test chamber. Vacuum of the order of ∼10−3 Torrwas first created in the test chamber and subsequently a mixtureof the known concentration of target gas and clean (dry synthetic)air was introduced till the test chamber acquired the atmosphericpressure to ensure that the target gas was free from any other dis-turbing gas. The creation of vacuum ensures the removal of anyforeign gas molecules from the test chamber. The measurementswere carried out in static mode. At the time of recovery of the senor,target gas was flushed out of the test chamber (by creating vacuumagain) and the clean dry air was introduced. The sensor was placedon a temperature controlled heating block inside the glass testchamber and spring loaded platinised contacts were used to mea-sure the sensor response as a function of temperature (75–225 ◦C).At each temperature the sensor was first stabilized in air to obtaina stable resistance value. Target gas (NO2) of specific concentrationwas introduced into the test chamber and changes in the sensorresistance were recorded after every second using a data acqui-sition system consisting of a digital multi-meter (model: Keithley2700) interfaced with a computer.
The sensor response for an oxidizing gas such as NO2 is definedas:
S = Rg − Ra
Ra(1)
where, Ra and Rg are the resistances of the sensor in the presence ofatmospheric air and target gas respectively. The response time wasmeasured as the time taken by the sensor to acquire the 90% of its
maximum resistance value in the presence of target oxidizing gas.Once the maximum resistance value is attained, the target gas wasflushed out of the test chamber and sensor was allowed to regain itsinitial resistance value in atmospheric air while keeping the sensorat the same temperature. Time taken by the sensor to reacquire
A. Sharma et al. / Sensors and Actuators B 156 (2011) 743– 752 745
Table 3Estimated values of lattice parameters (‘a’ and ‘c’), crystalline size and stress modulus for SnO2 films deposited at different sputtering pressures from X-ray diffractogram.
Sputtering pressure XRD peaks at 2� (◦) a = b (Å) c (Å) Crystallite size (nm) Stress modulus (�) %
(1 1 0) (1 0 1) (2 1 1)
12 mTorr 26.48 34.01 51.57 4.761 3.178 13 0.25161 3.178 12 0.25141 3.162 10 0.75340 3.101 8 2.66737 3.186 – –
aa
3
bmscf(gtfi1udpisfi‘a
20 30 40 50 60 70 80
18 mTor r
16 mTor r
14 mTor r
12 mTor r
(110) (101)
(211)
2θ (o)
Inte
nsity
(a.u
)
F
14 mTorr 26.51 34.0 51.56 4.716 mTorr 26.54 34.1 51.54 4.718 mTorr 26.60 34.4 51.47 4.7Bulk SnO2 26.61 33.89 51.78 4.7
bout 10% higher value of its initial resistance in the presence oftmospheric air is considered as the recovery time.
. Results and discussion
The as-grown SnO2 thin films were found to be amorphous, andecome nanocrsytalline after a post deposition annealing treat-ent at 300 ◦C for 3 h in air. The annealed SnO2 thin films were
mooth, transparent and strongly adherent to the substrate. Theontent of oxygen in the reactive gas mixture (Ar + O2) was variedrom 20% to 100%. SnO2 thin film having lower value of resistancein air) of about 4 k� was obtained with 30% oxygen in the reactiveas mixture. Since the surface morphology and porous microstruc-ure of film is influenced by the processing pressure, the SnO2 thinlms were deposited under varying sputtering pressure from 12 to8 mTorr. Fig. 2 shows the XRD pattern of SnO2 thin films depositednder varying sputtering pressures (12–18 mTorr). Broad and wellefined reflections corresponding to (1 1 0), (1 0 1) and (2 1 1)lanes of SnO2 were observed for all the deposited films and are
n good agreement to the corresponding values reported for rutile
tructure [18]. The values of lattice constants (‘a’ and ‘c’) estimatedrom XRD data for SnO2 thin films deposited at different sputter-ng pressures are summarized in Table 3. The lattice parametersa’ and ‘c’ for all SnO2 thin films were found to be slightly highernd lower respectively in comparison to the corresponding valuesig. 3. AFM images of the SnO2 thin films deposited at (a) 12 mTorr, (b) 14 mTorr, (c) 16 m
Fig. 2. X-ray diffractogram of the SnO2 thin films deposited at varying sputteringpressures and annealed in air at 300 ◦C for 3 h.
Torr and (d) 18 mTorr sputtering pressures in 30% O2 and 70% Ar gas environment.
7 d Actu
rim%rfiswtwabttt
dFffpao(ap
taps
46 A. Sharma et al. / Sensors an
eported for bulk SnO2 [Table 3]. The deviation in lattice parameters associated with the presence of stress in the thin films. The esti-
ated values of stress modulus in percentage {= [(cb − cf) × 100/cb], where cb and cf are lattice constants for SnO2 bulk and thin filmespectively} are also presented in Table 3. It is important to noterom Table 3 that the stress modulus % is increasing with increasen sputtering pressure from 12 to 18 mTorr which depicts that thetress is causing contraction in lattice. The values of crystallite sizeere estimated by fitting the width of (1 1 0) reflection peak using
he Scherrer’s formula and is found to decrease from 13 to 8 nmith increase in deposition pressure from 12 to 18 mTorr. This is
ttributed to the fact that at higher sputtering pressure, plasmaecomes denser and ion bombardment starts taking place insidehe plasma. Therefore, less number of sputtered species bombardshe substrate surface resulting in reduction in the grain size of SnO2hin films with increase in the sputtering pressure.
Fig. 3 shows the AFM images of annealed SnO2 thin filmseposited at different sputtering pressures. It is clearly seen fromig. 3 that the annealed SnO2 films are nanocrystalline having uni-ormly distributed crystallites. Grain size of the SnO2 film wasound to decrease from 80 nm to 16 nm with increase in sputteringressure from 12 to 18 mTorr. The formation of subgrains within
bigger grain are observed (Fig. 3) having an approximate sizef about 10 nm in the SnO2 thin films grown at lower pressure12–14 mTorr). Uniformly distributed nanopores are observed inll the films and are found to be increasing with increase in growthressure indicating an increase in the porosity.
Fig. 4 shows the SEM images of the surface morphology of SnO
2hin films grown at three different sputtering pressures (14, 16nd 18 mTorr). A rough morphology has been observed for the allrepared SnO2 thin films. The SnO2 thin films grown at low pres-ure (14 mTorr) shows the formation of densely packed large sizedFig. 4. SEM images of the SnO2 thin films deposited at (a) 14 mTorr, (b) 16 mTorr a
ators B 156 (2011) 743– 752
grains which are separated by micro-cracks. A decrease in the grainsize and an increase in the porosity have been seen with increasingthe sputtering pressure from 14 to 18 mTorr and is attributed tothe intensive in situ bombardment of the sputtered species in theplasma at higher pressure. The lesser number of sputtered speciesreaching the substrate surface at higher pressure results in thegrowth of small sized grains having large grain boundaries and highsurface to volume ratio resulting in a porous microstructure. Theobtained results are in agreement to the AFM studies discussedearlier. Similar findings have also been reported for WO3 thin filmsdeposited by magnetron sputtering, where an increase in the effec-tive surface area of WO3 thin film from 2.6 to 19.1 m2/gm with anincrease in sputtering pressure from 4 to 90 mTorr has been identi-fied using Brunauer–Emmet–Teller (BET) plots of Kr gas adsorptionisotherms [19].
Fig. 5 shows the transmittance spectra of the SnO2 filmsdeposited at different sputtering pressures as a function of wave-length. All the films exhibit a high transmission (>80%) in the visibleregion and show a sharp fundamental absorption edge at around340 nm. The presence of well defined interference fringes pat-tern in the optical transmittance spectra indicates the growth ofgood quality SnO2 thin films free from any type of inhomogenity.The value of optical bandgap was estimated from the Taue plot(linear portion of the plot between (˛h�)2 with photon energy(h�) where, ̨ is absorption coefficient and � is the optical fre-quency). The estimated value of band gap was found to increasefrom 3.7 to 4.1 eV with increase in sputtering pressure from 12to 18 mTorr which is close to the reported value raging from
3.4 eV to 4.0 eV for SnO2 thin film by other workers [20]. Theobserved increase in band gap is attributed to a decrease in thegrain size of SnO2 thin film with increasing sputtering pressure[Table 3].nd (c) 18 mTorr sputtering pressures in 30% O2 and 70% Ar gas environment.
A. Sharma et al. / Sensors and Actuators B 156 (2011) 743– 752 747
100 20 0 30 0 40 0 50 0 60 0 70 0 80 0 90 0 100 0 11 00-10
0
10
20
30
40
50
60
70
80
90
100
%T
Wavelen gth (nm)
12 mTorr 14 mTorr 16 mTorr 18 mTorr
F(p
l11tcfi
t
n
wtpdfibtoswitfimsSss
TRSn
60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 0
0
5000
10000
15000
20000
25000
30000
35000
Res
pons
e ((
Rg-R
a) / R
a)
Tempe rature (°C)
12 mTorr 14 mTorr 16 mTorr 18 mTorr
at different sputtering pressures (12–18 mTorr) in the presence of50 ppm NO2 gas as a function of temperature. Sensor response forall the SnO2 films deposited at different sputtering pressure (exceptat 12 mTorr) is found to increase with increase in temperature
8
10
12
)
12 mTorr 14 mTorr 16 mTorr 18 mTorr
ig. 5. Transmittance spectra of SnO2 films deposited at varying sputtering pressure12–18 mTorr). Insert: variation of bandgap of the SnO2 thin films with sputteringressure.
Refractive index n(�) of all thin film samples was found to beower than the corresponding bulk value and is decreasing from.97 to 1.85 (� = 470 nm) with increase in deposition pressure from2 to 18 mTorr [Table 4]. The lower value of refractive index of SnO2hin film in comparison to that of bulk is attributed to the latticeontraction along c-axis and the presence of porosity in SnO2 thinlms [21].
The relation of refractive index of a thin film with lattice con-raction and the packing density (or porosity) is given as [21].
f =√(
(1 − p)n4v + (1 + p)n2
vn2b
(1 + p)n2v + (1 − p)n2
b
)+ 5
2
(n2
b − 1
nbcb
)(cb − cf) (2)
here, nf is the refractive index of the deposited thin film, nb ishe refractive index of SnO2 bulk, n� is the index of the voidsresent in the film (equal to one for air) and p is the packingensity (p = 1 for bulk). The packing density (p) of all SnO2 thinlms deposited at different sputtering pressures were estimatedy fitting the experimental values of the refractive indices with theheoretical values obtained from Eq. (2) using the estimated valuef lattice parameter [Table 4]. The estimated values of packing den-ity are listed in Table 4 and are found to reduce from 0.98 to 0.85ith increase in sputtering pressure from 12 to 18 mTorr, indicat-
ng an increase in the porosity. The results are in agreement withhe AFM analysis discussed earlier. Surface roughness of SnO2 thinlms deposited at different sputtering pressures (12–18 mTorr) is
easured using the surface profiler and the values are also pre-ented in Table 4. It may be seen that the surface roughness ofnO2 film is increasing from 6.6 nm to 29.8 nm with increase inputtering pressure from 12 to 18 mTorr. The presence of higherurface roughness and lower packing density in SnO2 thin film lead
able 4efractive index and packing density values estimated from UV visible spectra ofnO2 thin films deposited at different sputtering pressures. Variation in film rough-ess determined using the surface profiler is also included.
Sputtering pressure Refractiveindex (at470 nm)
Packing density(�)
Surfaceroughness(nm)
12 mTorr 1.97 0.98 6.614 mTorr 1.91 0.95 12.316 mTorr 1.87 0.91 13.118 mTorr 1.85 0.85 29.8Bulk SnO2 2.00 – –
Fig. 6. Sensor Response to 50 ppm NO2 gas as a function of temperature for theSnO2 films deposited at sputtering pressures varied from 12 to 18 mTorr. Insert:variation of sensor response at an operating temperature of 100 ◦C as a function ofthe deposition pressure of SnO2 films.
to a higher value of surface to volume ratio. Since, gas sensing isa surface phenomenon, SnO2 thin films with porous microstruc-ture exhibiting large surface to volume ratio are expected to resultin higher sensing response. Therefore, SnO2 thin films having lowvalues of packing density (0.91–0.85) and higher value of surfaceroughness (13.1 and 29.8 nm) seem to be advantageous for gassensing applications.
3.1. Gas sensing characteristics
Figs. 6–8 show the variation of sensor response, response timeand recovery time respectively for the SnO2 thin films deposited
60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 0
0
2
4
6
Res
pons
e tim
e (m
in
Tempe rature (°C)
Fig. 7. Variation of response time as a function of temperature for the SnO2 filmsdeposited at different sputtering pressures (12–18 mTorr) when exposed to 50 ppmNO2 gas.
748 A. Sharma et al. / Sensors and Actuators B 156 (2011) 743– 752
60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 0
0
5
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45
50
55
60
65
70
Rec
over
y tim
e (m
in)
Tempe rature (°C)
12 mTorr 14 mTorr 16 mTorr 18 mTorr
Fdw
ufias(ttstSTesrt
tt(ssewttfia((ti
TSs
60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 0 26 0
0
1
2
3
4
5
6
7
8
9
Ra (
kΩ)
o
12 mTorr 14 mTorr 16 mTorr 18 mTorr
ig. 8. Variation of recovery time as a function of temperature for the SnO2 filmseposited at different sputtering pressures (12–18 mTorr) when 50 ppm NO2 gasas flushed out.
pto 100 ◦C and decreases continuously thereafter. The SnO2 thinlm sensors show very high response (∼104) for 50 ppm NO2 gast a relatively low operating temperature of 100 ◦C (Fig. 5). Theensing response of SnO2 thin film at the operating temperature100 ◦C) increases from 4.3 × 103 to 2.9 × 104 with increase inhe sputtering pressure from 12 to 16 mTorr (insert of Fig. 6) andhereafter decreases to 1.0 × 104 for the SnO2 film grown at higherputtering pressure (18 mTorr). The observed higher response inhe present study is attributed to the formation of nanocrystallinenO2 thin films having nanoporous microstructure (Figs. 3 and 4).he nanocrystalline SnO2 films provide a large surface area fornhanced adsorption of target (NO2) gas resulting in higherensor response. To the best of our knowledge, such higher sensoresponse for SnO2 thin film based sensor has not been reported inhe literature for the detection of oxidizing NO2 gas.
The sensor response characteristics (sensor response, responseime and recovery time) obtained at 100 ◦C for the sensor struc-ures having SnO2 films deposited at different sputtering pressures12–18 mTorr) are listed in Table 5. It may be noted from Table 5 thatensor response is very high for SnO2 films deposited at 16 mTorrputtering pressure with comparatively fast response time. How-ver, the recovery time of the SnO2 thin film sensor is increasingith increase in sputtering pressure. It is observed from Figs. 7 and 8
hat with increase in the temperature, response time and recoveryime for all the sensor structures for 50 ppm NO2 gas decrease. SnO2lms deposited at 12, 14 and 16 mTorr sputtering pressures showlmost similar response time (∼1–2 min) at a fixed temperature
Fig. 7). However, SnO2 film deposited at higher sputtering pressure18 mTorr) exhibits much higher response time (>6 min) at loweremperature (<100 ◦C), and response time decreases rapidly withncrease in temperature (Fig. 7). Both response and recovery timesable 5ensor response characteristics of the SnO2 sensing element deposited at varyingputtering pressures at an operating temperature of 100 ◦C.
Sputtering pressure Sensorresponse
Response time(min)
Recovery time(min)
12 mTorr 4.3 × 103 2.1 19.214 mTorr 1.6 × 104 2.2 21.316 mTorr 2.9 × 104 1.6 42.718 mTorr 1.0 × 104 6.8 35.9
Tempe rature ( C)
Fig. 9. Variation of sensor resistance in air (Ra) for the SnO2 films deposited atdifferent sputtering pressures (12–18 mTorr).
(Figs. 7 and 8) for all the sensor structures are seen to be improv-ing with increase in temperature. This may be related to the fasterrates of adsorption and desorption of NO2 gas molecules on the sur-face of SnO2 sensing film at higher temperatures. It is important topoint out that at low temperatures (<100 ◦C) the sensor resistance(after removal of NO2 gas) is recovering back to its initial resistancevalues (Ra) after a long recovery time (>30 min) especially for theSnO2 film grown at higher sputtering pressure (>14 mTorr). Shenet al. [19] have reported that the sensing response of WO3 thin filmincreases with increase in effective surface area. They obtained aresponse of about 450 at a higher operating temperature of 200 ◦Cfor 1 ppm NO2 gas. However, the sensor was not able to recovercompletely even after few hours. It is important to point out thatthe sensor prepared in the present study is exhibiting enhancedresponse at much lower operating temperature (∼100 ◦C) and ableto recover completely in about 40 min.
Fig. 9 shows the variation of sensor resistance measured inatmospheric air (Ra) for the SnO2 thin films deposited at differentsputtering pressure (12–18 mTorr) as a function of temperature.The room temperature values of Ra for all prepared films are rel-atively lower (<10 k�) and found to increase with increase insputtering pressure. The observed increase in Ra is attributed tothe presence of nanoporous morphology of SnO2 film grown athigher sputtering pressure which is responsible for the adsorptionof large amount of oxygen on its surface. The adsorbed oxygen cap-tures the free electrons from the conduction band of SnO2 thin filmthereby resulting in an increase in sensor resistance. It may be seenfrom Fig. 8 that for all sensor structures, resistance in air is foundto decrease continuously with increase in temperature indicatinga typical semiconducting behaviour of SnO2 thin film. However,a hump in the value of Ra in the temperature range 140–200 ◦Cwas also observed. The overall behaviour of Ra could be dividedinto three distinct temperature regions (i) low temperature region(<140 ◦C) where oxygen is physisorbed on the surface of SnO2 thinfilm and capture the electrons from interior of SnO2, (ii) moder-ate temperature region (140–200 ◦C), where physisorbed oxygenspecies (O−
2) are converted into chemisorbed oxygen (2O−) result-
ing in an increase in Ra giving a hump, and (iii) higher temperatureregion (>200 ◦C), where Ra shows a decrease with increase in tem-perature due to semiconducting nature of SnO2. Similar behaviourof resistance of SnO2 thin films with temperature has been reportedearlier for sputtered SnO2 film having much higher value of Ra [22]
A. Sharma et al. / Sensors and Actuators B 156 (2011) 743– 752 749
60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 0
0
20
40
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120R
g (MΩ
)
o
12 mTorr 14 mTorr 16 mTorr 18 mTorr
Fa
ao
tfiTititgmtraiaet1oos
3
aettshgdthg
3
v
Tempe rature ( C)
ig. 10. Variation of sensor resistance in presence of 50 ppm NO2 gas with temper-ture for the SnO2 films deposited at different sputtering pressures (12–18 mTorr).
nd is related to the conversion of molecular oxygen to atomicxygen species.
Fig. 10 shows the variation of sensor resistance (Rg) measured inhe presence of 50 ppm NO2 gas with temperature for the SnO2 thinlms deposited at different sputtering pressures (12–18 mTorr).he value of Rg for all sensor structures was found to increase onnteraction with sensing gas molecules at all measured tempera-ures (Figs. 8 and 9) and is due to oxidizing nature of NO2 gas. It isnteresting to note from Fig. 9 that the Rg becomes maximum at aemperature of 100 ◦C for all prepared samples (except SnO2 filmrown at 12 mTorr), therefore the sensing response was found to beaximum at 100 ◦C. As the sputtering pressure is increased from 12
o 16 mTorr, the value of Rg increases at a fixed temperature, and aelatively lower value of Rg is obtained for the SnO2 film depositedt 18 mTorr. As a result, SnO2 film deposited at 16 mTorr sputter-ng pressure exhibits the enhanced sensing response (2.9 × 104) at
low operating temperature of 100 ◦C (insert Fig. 6). The origin ofnhanced response obtained at relatively low operating tempera-ure (100 ◦C) in the present study for SnO2 thin films deposited at6 mTorr sputtering pressure may be correlated with the presencef porosity and the availability of free Sn sites for the interactionf oxidizing NO2 gas molecules, and are discussed in the followingections.
.1.1. SelectivityThe experiment on the selectivity of SnO2 thin film deposited
t 16 mTorr was carried out by monitoring the change inlectrical resistance towards different interfering gases (concentra-ion = 200 ppm) including NH3, CH4, LPG, H2 and H2S at an operatingemperature of 100 ◦C. The prepared sensor is found to be highlyelective towards NO2 gas (insert of Fig. 11) and exhibits muchigher increase in the sensor resistance after interaction with NO2as molecules at a lower temperature of 100 ◦C. However, a smallecrease in the sensor resistance was observed when exposedo other interfering gases (Fig. 11) indicating the development ofighly selective SnO2 thin film based sensor for detection of NO2as.
.2. Sensing mechanism
The enhanced sensing response will be obtained if either thealue of Ra is lower or Rg is higher (Eq. (1)) and if both the
Fig. 11. Sensor Response of SnO2 films deposited at 16 mTorr sputtering pressuretowards various target gases at different concentrations.
changes occur simultaneously it is more advantageous. The oxy-gen to argon ratio (O2:Ar) in the reactive sputtering ambienthas been optimized (3:7) such that the value of Ra is minimum(∼4 k�) at room temperature. It may be noted that the value ofRa is smaller (Fig. 9) especially in the lower temperature region(<140 ◦C) for SnO2 thin films deposited at low sputtering pres-sures (<18 mTorr). An increase in Ra was observed for the SnO2thin film grown under higher sputtering pressure (Fig. 9), andbecomes much higher (>6 k�) for the film deposited at 18 mTorr.The observed increase in Ra is due to increase in the porosity inthe SnO2 thin films grown with increasing sputtering pressure. Theporous morphology provides a large surface region for the adsorp-tion of molecular oxygen (O2) from atmosphere and the resistanceof the film increases because of capturing of free electrons fromthe conduction band of SnO2. The possibility of some small con-tribution towards increase in the value of Ra is due to decrease ingrain size and increase in grain boundaries in the SnO2 thin filmgrown at higher sputtering pressure (>18 mTorr) may not be ruledout.
The presence of hump observed in the value of Ra in the tempera-ture region 140–200 ◦C (Fig. 9) is due to the conversion of molecularoxygen (O−
2) to atomic oxygen (O−) as:
O2 + e− ⇒ O−2 + e− ⇒ 2O− (3)
Therefore, more number of electrons from the conduction bandhave been captured during this conversion resulting in the presenceof hump in Ra at moderate temperature. The activity of adsorbedoxygen on the surface of SnO2 thin film is reported to be enhancedat moderate temperature (∼140 ◦C) [22]. However, no such activityhas been observed in the present study and may be due to lowerresistivity of the SnO2 thin films grown under lower oxygen partialpressure (30% O2).
3.3. Adsorption and desorption of NO2 gas at the SnO2 filmsurface
NO2 is an oxidizing gas which traps the free electrons fromthe SnO2 surface after adsorption and therefore increases the sen-sor resistance. At lower temperatures (<140 ◦C), the predominantoxygen species on the SnO2 surface are still O−
2 but NO2 gasmolecules interact directly with tin ionic sites instead of reacting
7 d Actuators B 156 (2011) 743– 752
ww
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etiStgsaiwNossocgi
2
aHfa
3
ffigtstinafw(
fscusvepi
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60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 01
10
100
1000
10000
100000
Res
pons
e ((
Rg-R
a) / R
a)
Tempe rature (oC)
50 pp m 10 pp m 1 pp m
50 A. Sharma et al. / Sensors an
ith O−2 species [23]. SnO2 responds to NO2 gas in the following
ay [10].
O2 + Sn2+adsorption−→ (Sn3+ − NO−2 )
desorption−→ (Sn3 − O−) + NO (4)
NO2 gas molecules when adsorbed on the surface of SnO2 sensorlement attack the available Sn sites and take away electrons fromhe conduction band of SnO2 forming NO2
− species. The reductionn the concentration of charge carriers in the conduction band ofnO2 results in an increase in sensor resistance (Rg) as observed inhe present study. However, this is a slow phenomenon and under-oes a reversible oxidizing interaction at Sn sites available on theurface of SnO2 film [23]. Therefore value of Rg depends upon thevailability of Sn sites on the surface of SnO2 thin films, whichn turn are influenced by the physisorbed oxygen for interaction
ith target NO2 gas molecules. During the recovery, these adsorbedO2
− species desorb as NO gas molecules leaving chemisorbedxygen species behind (Eq. (4)) on SnO2 surface. These oxygenpecies have trapped electrons due to which the SnO2 surface istill deficient of free electron carriers and therefore original valuef the resistance (Ra) is not regained. Subsequently the left behindhemisorbed oxygen species on the SnO2 surface are released as O2as according to Eq. (5) [12] and the SnO2 thin film sensor regainsts initial resistance (Ra) of 3.5 k�.
(Sn3+ − O−)desorption−→ 2Sn2+ + O2 (5)
The entire process takes a lot of time and hence, slow recoveryt lower operating temperature is observed (∼20–40 min) (Fig. 8).owever, the process of recovery for SnO2 thin film sensor becomes
aster with increase in temperature and is attributed to thermallyctivated electron transfer reactions (Eqs. (4) and (5)) [23].
.4. Available free Sn sites
As explained earlier, at lower temperatures, oxygen speciesrom the atmosphere are physisorbed on the surface of SnO2 thinlm and enough Sn sites are available for interaction with NO2as molecules. As NO2 molecules are physisorbed at Sn sites, elec-rons are trapped from bulk of SnO2 leading to an increase in theensor resistance from Ra to Rg (Figs. 9 and 10). At higher tempera-ures (>140 ◦C) the physisorbed oxygen species (O2
−) are convertednto chemisorbed oxygen (2O−) resulting into availability of lessumber of free Sn sites on the surface of SnO2 thin film for inter-ction with NO2 gas molecules. Decrease in the concentration ofree Sn sites result in a reduction in the sensor response alongith fast recovery for all prepared samples at higher temperatures
Figs. 6 and 8).The value of Rg was found to increase continuously (Fig. 10)
or the SnO2 thin films deposited with increasing sputtering pres-ure (12–16 mTorr). The observed increase in Rg is due to increasingoncentration of Sn sites available on the surface of SnO2 thin filmsnder extensive bombardment of sputtered species at higher pres-ure. The film grown at 18 mTorr is expected to have much higheralue of Rg in comparison of that obtained for other samples. How-ver, much lower value of Rg for the SnO2 films deposited at highressure (18 mTorr) is obtained in the present study (Fig. 10) and
s attributed to the presence of high porosity [Table 4].SnO2 thin film deposited at 18 mTorr shows lower sensing
esponse as compared to that obtained with film deposited at6 mTorr (Fig. 6) despite the fact that SnO2 film grown at highressure (18 mTorr) was more porous and rough. Since an increase
n the amount of physisorbed oxygen on the surface of sensingnO2 layer increases with increasing porosity, the film deposited at8 mTorr exhibits a much higher value of Ra (Fig. 9) due to capturingf large concentration of free electrons from the conduction band.owever, a small increase in the porosity and surface roughness of
Fig. 12. Variation of sensor response with temperature to different concentrationsof NO2 gas (50, 10 and 1 ppm) for the SnO2 film deposited at 16 mTorr sputteringpressure.
films were observed with increasing sputtering pressure from 12to 16 mTorr (Table 4), therefore a small increase in Ra is observed inthese samples (Fig. 9). The presence of large amount of physisorbedoxygen on the surface of SnO2 thin film grown at higher pressure(18 mTorr) leads to the availability of a relatively much lowerconcentration of free Sn sites where the sensing NO2 gas moleculescan interact. Therefore the interaction of target gas on the surfaceof SnO2 film grown at 18 mTorr does not show an expected increasein the value of sensor resistance (Rg), and was even lower than thecorresponding value obtained for film grown at 16 mTorr (Fig. 10).The significant increase in Ra and lowering of Rg for SnO2 thin filmgrown at 18 mTorr results in relatively small sensing response(Eq. (1)) as compared to that obtained for the film deposited at16 mTorr. The contribution of porosity towards the reduction in thevalue of Rg for SnO2 thin films grown at other sputtering pressuresmay not be ruled out. The relatively higher packing density (>0.90)and availability of large concentration of free Sn sites results inthe much higher value of Rg along with lower value of Ra forSnO2 thin film grown at 16 mTorr thereby exhibiting enhancedresponse.
The sensor prepared in the present case is having a porousmicrostructure and is operating at a comparatively lower temper-ature of 100 ◦C. The high sensor response observed in the presentstudy is due to the porous surface morphology and the availabil-ity of more number of free Sn sites on the surface of sensing SnO2thin film. The NO2 gas molecules are adsorbed at the free Sn sitesavailable not only on the surface but also in the interior of poresor grain boundaries of the SnO2 thin film resulting in enhancedresponse with a relatively poor response time. Similarly, sufficienttime is required for desorption of NO2 gas molecules from theinterior of the pores and the grain boundaries of SnO2 thin filmespecially at lower operating temperature, giving higher recoverytime.
3.5. Effect of NO2 gas concentration
Since, the SnO2 thin film grown at 16 mTorr exhibits enhancedresponse for 50 ppm NO2 gas, the gas sensing response character-istics of the same sensor were examined at different concentrationof NO2 gas. Fig. 12 shows the variation of sensor response obtainedat three different concentrations (50, 10 and 1 ppm) of NO2 gas as
A. Sharma et al. / Sensors and Actu
60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 0
0
2
4
6
8
10
12
14
16
50 pp m 10 pp m 1 pp m
10 pp m
1 pp m
50 pp m
Res
pons
e tim
e (m
in)
o
Ft
asgtwTtrfrtt
crofti
Ft
Tempe rature ( C)
ig. 13. Response time of the SnO2 thin film sensor (sputtering pressure = 16 mTorr)o 50 ppm, 10 ppm and 1 ppm concentrations of NO2 gas.
function of temperature. Sensor response was found to exhibitimilar behaviour with temperature at all concentration of NO2as and shows a maximum response at around 100 ◦C. However,he response was found to decrease from 2.9 × 104 to 1.3 × 102
ith a decrease in NO2 gas concentration from 50 ppm to 1 ppm.he observed reduction in response could be attributed to the facthat the number of NO2 gas molecules interacting with the surfaceeduces with decrease in concentration of target gas and there-ore give rise to lower sensor response (Fig. 12). An increase in theesponse time and a decrease in recovery time with reduction inhe concentration of NO2 gas are also observed at all measuredemperatures (Figs. 13 and 14).
The slow response time and recovery times are the major con-ern with NO2 gas sensing at lower operating temperatures. Theise in temperature increases the adsorption and desorption rates
f gaseous molecules on the surface of sensing layer which causesaster response and recovery times. Therefore, if the operatingemperature of the SnO2 thin film grown at 16 mTorr is slightlyncreased to 150 ◦C (Fig. 6), the sensor will respond at much faster60 80 10 0 12 0 14 0 16 0 18 0 20 0 22 0 24 0
0
5
10
15
20
25
30
35
40
45
50
Rec
over
y tim
e (m
in)
Tempe rature (oC)
50 pp m 10 pp m 1 pp m
ig. 14. Recovery time of the SnO2 thin film sensor (sputtering pressure = 16 mTorr)o 50 ppm, 10 ppm and 1 ppm concentrations of NO2 gas.
[
[
[
[
ators B 156 (2011) 743– 752 751
speed (Figs. 7 and 8) with response time and recovery time of about0.8 min and 14.0 min respectively.
4. Conclusions
SnO2 thin films of desired morphology and higher electricalconductivity have been deposited by RF sputtering under vary-ing processing pressure for efficient detection of NO2 gas. SnO2film deposited at 16 mTorr sputtering pressure exhibits enhancedresponse (2.9 × 104) with moderate response time (1.6 min) andrecovery time (42.7 min) at a relatively much lower operating tem-perature (100 ◦C). The number of free Sn sites available on thesurface of SnO2 thin film is identified to be crucial for enhancedresponse characteristics at low operating temperature which inturn depends on the growth kinetics and adsorption of oxygen fromatmosphere at the porous micro-surface of SnO2 film. The sensorresponse, response time and recovery time of the SnO2 thin filmsensor is governed by the adsorption and desorption of NO2 gasmolecules at the free Sn sites. The conversion of physisorbed molec-ular oxygen (O2
−) to chemisorbed atomic oxygen (2O−) resultsin the reduction of sensor response at higher temperatures. Thesensing response of SnO2 thin film at the operating temperature isinfluenced significantly by the processing pressure. The availabil-ity of large amount of free Sn sites and formation of desired surfacemorphology having moderate porosity results in an enhanced sens-ing response to NO2 gas (1–50 ppm) at low operating temperature(100 ◦C) for SnO2 film grown at 16 mTorr pressure.
Acknowledgement
The authors are thankful to Department of Science and Tech-nology (DST), Department of Information Technology (DIT) andUniversity of Delhi for the financial support for carrying out thisresearch work. One of the author (AS) is grateful to the Coun-cil of Scientific and Industrial Research (CSIR) for the researchscholarship.
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Biographies
Anjali Sharma was born in Delhi, India, in June 1985. She received her B.Sc. and M.Sc.degrees in Physics in the year 2006 and 2008 respectively, from University of Delhi.Presently she is a Junior Research Fellow pursuing her Ph.D. program from Universityof Delhi, Delhi. Her research interests are in gas sensor systems that include sensorcharacterization and development of metal oxide films for sensor coatings. She isalso working towards the fabrication of MEMS based Electronic-Nose for gas sensingapplications.
Monika Tomar was born in Delhi, India, in April 1976. She received her B.Sc., M.Sc.and Ph.D. degrees in Physics in 1996, 1998 and 2005 respectively, from the Universityof Delhi. Presently she is Assistant Professor at Miranda House, University of Delhi,India. Her research interests include piezoelectric thin films for Surface acousticswave devices and sensors, oxide thin films and nanostructures for gas sensing andbiosensing applications, photonic devices etc.
Vinay Gupta was born in Mujjaffar Nagar (U.P.), India, in March, 1967. He receivedhis B.Sc., M.Sc., and Ph.D. degrees in physics in 1987, 1989 and 1995, respectively
from the University of Delhi, India. Subsequently he joined DDU College, Univer-sity of Delhi as Assistant professor. Presently he is Professor in the Department ofPhysics and Astrophysics, University of Delhi, India. He is a senior member of IEEE.His current research interests are in piezoelectric, ferroelectric and semiconduct-ing thin films, gas/bio sensors, Electro-optic applications, oxide nanostructures formulti functional applications etc.