Optical characterizations of ZnO, SnO2, and TiO2

thin films for butane detection

Thomas Mazingue, Ludovic Escoubas, Lorenzo Spalluto, François Flory, Patrick Jacquouton,Alessio Perrone, Eliana Kaminska, Anna Piotrowska, Ion Mihailescu, and Peter Atanasov

The optogeometric properties of various sensitive thin films involved in gas sensing applications areinvestigated by using the m-line technique and atomic force microscopy. Variations of these opticalproperties are studied under butane and ozone exposure. © 2006 Optical Society of America

OCIS codes: 310.2790, 310.6860.

1. Introduction

Several types of gas sensor are currently availableand intensively studied: solid electrochemical sen-sors, infrared spectroscopic sensors, metal oxidesensors,1 quartz microbalance (QMB) sensors2 andsurface acoustic wave (SAW) sensors.3 For the lastthree categories, an interaction between a sensitivematerial and the chemical agent occurs. The physicalproperties of the material are modified under gasexposure.

Metal oxide sensors are based on the static conduc-tivity variation of a thin semiconductor layer (typi-cally SnO2, ZnO, WO3, TiO2, etc.1) subjected to thetarget gas exposure. The presence of ambient oxygennear the material leads to the adsorption of oxygen bytrapping (usually) free electrons and creating ionssuch as O2

�, O� or O2�, which remain adsorbed. Thiseffect leads to the formation of a depletion zone near

the grain boundary or the nanoparticle surface. Themacroscopic effect is the reduction of conductivity dueto the reduction of available carriers. This phenome-non is usually thermally activated. Sensitivity de-pends on the gas type and the working temperature.Selectivity between gases of the same chemical fam-ily remains low. However, it can be improved by dop-ing the films with catalysts such as Pd and Pt.4

In the case of semiconductor oxides, it is possible toshow that a strong variation in surface conductivityof a thin film under gas exposure (several decades)leads to a variation in the refractive index. Indeed,the electromagnetic field in a harmonic regime in ahomogeneous nonmagnetic material is described bythe electric field E��� and the magnetic field H���:Maxwell’s equations are given by the following ex-pressions:

rot E��� � ��0

�H����t , (1)

rot H��� ��D���

�t � J���, (2)

where D��� is the displacement vector and J��� is thecurrent density vector. If we consider that the currentis due only to a free-charge displacement under theelectric-field effect, we can use the expression of Ohm:

J��� � �E���, (3)

where � is the surface dynamic conductivity and is ascalar if the material is isotropic. Equations (1) and(2) can therefore be written in the following way:

T. Mazingue (thomas.mazingue@fresnel.fr) and P. Jacquoutonare with Cybernétix, Technopole de Château–Gombert, 306 rueAlbert Einstein, BP94, 13382 Marseille Cedex 13, France. L.Escoubas, L. Spalluto, and F. Flory are with Institut Fresnel, UnitéMixte de Recherche–Centre National de la Recherche Scientifique6133, Domaine Universitaire de Saint Jérôme, 13397 MarseilleCedex 20, France. A. Perrone is with the Department of Physicsand National Institute of Nuclear Physics (INFN), 73100 Lecce,Italy. E. Kaminska and A. Piotrowska are with the Institute ofElectron Technology, Al. Lotnikow 32�46, 02-668 Warsaw, Po-land. I. Mihailescu is with the National Institute for Laser,Plasma, and Radiation Physics, Bucharest–Magurele RO-77125,P.O. Box MG-36, Romania. P. Atanasov is with the Institute ofElectronics, Bulgarian Academy of Sciences, 72 TsarigradskoShose Boulevard, Sofia 1784, Bulgaria.

Received 1 March 2005; accepted 31 August 2005.0003-6935/06/071425-11$15.00/0© 2006 Optical Society of America

1 March 2006 � Vol. 45, No. 7 � APPLIED OPTICS 1425

rot E��� � j��0H���, (4)

rot H��� � �j�D��� � �E���. (5)

By introducing the following expression to Eq. (5),

D��� � �0�r���E���, (6)

where �0 is the dielectric permittivity of the vacuumand �r is the relative dielectric permittivity of thematerial, we obtain

rot H��� � �j��0��r � j�

��0�E��� (7)

or

rot H��� � �j��0n2E���, (8)

where n2 is the complex refractive index of the mate-rial and can be written as

n2 � �r� � �r � j�

��0. (9)

Since n is complex, it can be decomposed into theform n � n� � jn�. That induces n2 � �n�2 � n�2�� j�2 n� n��, where n� corresponds to the absorp-tion of the material and may be null in special cases.By identification with Eq. (9), we obtain the followingsystem of equations:

n�2 � n�2 � �r,

2 n� n� ��

��0. (10)

Solutions having a physical meaning are

n� ���r

2�1 � �1 � � �

2�0�r��21�2

,

n � ���2�0�

��r

2�1 � �1 � � �

2�0�r��21�2. (11)

Equations (11) show that the refractive index of amaterial is affected by a variation in its dynamicconductivity, which is the basis of our approach. Weselected materials that are used in metal oxide sen-sors, which are sensitive to gas by means of a con-ductivity variation when the gas is adsorbed at thesurface. Note that some other interactions that leadto a variation in the refractive index may occur (e.g.,variation in the thickness or in the mass of the thinfilm). Thus we describe how it is possible to build anew type of gas sensor based on the measurement of

the variation in the refractive index by use ofwaveguiding methods.

One way to measure the refractive variation is touse a sensitive waveguide. The coupling of an inci-dent laser beam by a prism into a planar waveguideis governed by the incident angle s of the beam on theprism base (Fig. 1). The resonant coupling of the laserbeam into the waveguide is observed for a specificincident angle, through the appearance of a dark line(called a mode line or an m line) in the reflected beam.Consequently, by measuring the angles correspond-ing to the m lines, one can calculate the propagationconstants of the guided modes determined from theseangles and can obtain separately the refractive indexn and the thickness t of the waveguide layer. Anaccuracy of 10�2 deg on the measurement of the an-gle, which is easy to achieve, corresponds to a goodaccuracy �1 or 2 10�3� on the absolute value of therefractive index n and on the thickness t of singlelayers ��2 nm�.5

A guided mode corresponds to a resonance that willbe strongly affected by a perturbation due to an in-teraction between the material and its surroundingatmosphere. The effective index N of the guided modeis linked to the incident angle s of the light by Eq.(12) (Ref. 5):

N � np sin�arcsin�sin s

np�� Ap, (12)

where np is the refractive index of the prism and Ap isits characteristic angle. From Eq. (12) we deduce theexpression of �s:

�N �

np cos�s�cos�arcsin�sin s

np�� Ap

�np2 � sin2 s�1�2 �s.

(13)

The waveguide configuration allows the propaga-tion of light into the film and therefore an interactionwith the material for a distance greater than tens ofmicrometers. This method is appropriate for accuratemeasurement of the variation in the refractive in-dex.6

The aim of the research presented in the followingparagraphs is to characterize the optical and morpho-logical properties of ZnO, TiO2, and SnO2. Indeed,these materials are suitable n-type semiconductorswhen used as metal oxide sensors. The variations ofthe optical properties of the films are then investi-

Fig. 1. Totally reflecting prism coupler.

1426 APPLIED OPTICS � Vol. 45, No. 7 � 1 March 2006

gated under gas exposure in waveguide configurationto confirm the concept of a new optical gas sensor.

2. Experiment

A. Film Deposition

The deposition parameters of thin films realized bypulsed laser deposition (PLD) are given in Table 1. Inthe PLD process excimer-laser pulses with high en-ergy density are focused on a target of an originalsubstance under an oxidized atmosphere. A plasmaplume is formed above the exposed target area, ex-tending to a distance of several tens of millimeters.The ablation products are deposited on a closely lo-calized substrate, forming a thin layer of depositedsubstance.7

The sample named ZNIET was deposited by rfmagnetron sputtering from a ZnO target at the In-stitute of Electron Technology, Poland. The vacuum-chamber base pressure was below 5 10�4 Pa beforedeposition. Films were grown on unheated quartzsubstrates. Before deposition, the substrates were de-greased in hot organic solvents and were in situ Ar�

ion cleaned. The target was cleaned by Ar� ion pre-sputtering for 10 min. Optimum sputter depositionconditions yielding stoichiometric ZnO were as fol-lows: total pressure during deposition, 1 Pa; cathodecurrent, 160 mA. The resultant deposition rate was60 nm�min.

B. Atomic Force Microscope Measurements

Samples were analyzed with a Topometrix S�N:EX359607 atomic force microscope (AFM) to deter-mine their surface morphology. For gas sensing appli-cations, roughness has to be as high as possible. Indeeda maximal film–gas interaction surface is needed toyield a maximum number of adsorption sites so that avariation of conductivity can be achieved. A compro-mise between high roughness for good sensitivity andsmoothness for good optical properties has to be found.

C. Determination of Refractive Index and Thickness bythe m-Line Technique

The total reflection prism coupler (m-line technique)was used to determine the refractive index and thethickness of all films. This method is described inSection 1 and depicted in Fig. 1. The refractive indi-ces of the thin films were examined at 633 nm for thetwo polarization states of light.

D. Gas Sensitivity Measurements

The principle of the gas sensitivity measurement is tofollow the shift, under gas exposure, of the dark linein the Fresnel field due to a mode coupled in a sen-sitive layer. The aim is to link the variation in theangular position of the dark line to the concentrationof the injected gas. Figure 2 describes the experimen-tal setup.

In Fig. 2 the core of the testing facility is theprism coupler on its rotation plate and the sample,which is the thin film deposited on a glass sub-strate, pressed on the prism’s base. The physicalinteractions between the gas and the sensitive ma-terial that lead to the variation in the refractiveindex take place at the interface of the air-sensitivematerial. Calculations show that the electromag-netic field is enhanced at this interface for TM po-larization states of light and for a thin-filmthickness that is close to the cutoff thickness (thethickness below which the mode can no longer prop-agate in the thin film) of the guided mode.8 Thisconfiguration allows the highest sensitivity torefractive-index variation under gas exposure. Themeasurement arm is placed in front of the reflectedbeam at the angle corresponding to the m-line po-sition. A small Hamamatsu Si photodiode, polarizedwith reverse voltage and collecting the light, is fixedon a motorized translation stage with high resolu-tion ��1 �m corresponding to an angle of 8 10�5

deg). The profile of the m line is recorded by acomputer under a LabVIEW application. In casethere is a variation in the refractive index of the

Table 1. Deposition Parameters of PLD-Deposited Films

Sample TargetO2 Pressure

(Pa)Substrate

Temperature (°C)

Target–Substrate

Distance (cm)Fluence(J cm�2) Pulses (nb)

TO29 TiO2:Pt3% 26 550 6 3.6 50,000ZO25 ZnO 13 350 5 3.6 50,000ZO27 ZnO 13 150 5 4 50,000ZO28 ZnO 13 25 5 2.5 50,000

SnPd02 SnO2:2.6% 10 25 6 10 15,000SnPd25 SnO2 10 350 6 10 20,000SnPd27 SnO2:2.6% 10 500 6 10 20,000SnPd29 SnO2:2.6% 10 350 6 10 20,000

Zn23114 ZnO 5 300 4 2 10,800Ti150903 TiO2 10 500 4 1.5 14,400

1 March 2006 � Vol. 45, No. 7 � APPLIED OPTICS 1427

material (for example, under gas exposure), the lineis translated.6,9 The photodiode is therefore placed

at the position at which the slope of the intensitycurve of the m line is the highest to obtain the

Fig. 2. Testing facility for waveguide-coupling interrogation schemes.

Fig. 3. ZN28 surface morphology.

1428 APPLIED OPTICS � Vol. 45, No. 7 � 1 March 2006

largest signal variation with optical sensitivity. Thesignal is amplified by a lock-in amplifier (StanfordResearch System RS830) synchronized with a chop-per. The aim is to detect concentrations of butanebelow 800 parts per million �ppm� (or 1900 mg�m3,maximum daily authorized exposition by the Amer-

ican Code of Federal Regulations10). Since it wasvery difficult to obtain a mixture with such preciseconcentrations, we chose to take a ready-to-use mix-ture with 1000 ppm of butane diluted in nitrogen�N2�. By mixing it with another bottle of N2, one canobtain concentrations C ranging from 100 to

Fig. 4. SNPDQ29 surface morphology.

Fig. 5. Zn23114 surface morphology.

1 March 2006 � Vol. 45, No. 7 � APPLIED OPTICS 1429

1000 ppm, according to the following formula:

C �Dm

Dm � DN2

Cm, (14)

where Dm is the flow of the mixture, DN2the flow of

N2, and Cm the concentration of the mixture. TheDm and DN2

are controlled by two numeric mass-flowAera FC7700CDs. The concentration we aim at issent within the glass measurement cell, which iscylindrical and transparent, after establishmentof a primary vacuum to remove impurities. The

pressure is controlled by a Pirani gauge InficonPSG400.

The valves prevent the gas mixture from returningback to the setup during the rupture of the vacuum orduring any manipulation mistake. The experimentalprotocol is described as follows:

• Creation of a primary vacuum to get rid of impu-rities contained in the measurement cell (duration 1min 30 s).

• Introduction of N2 (carrier gas) to fill in the mea-surement cell and be at the atmospheric pressure(duration 2 min 30 s).

Fig. 6. Ti150903 surface morphology.

Fig. 7. ZNIET surface morphology.

1430 APPLIED OPTICS � Vol. 45, No. 7 � 1 March 2006

• Introduction of the mixture N2 � butane (dura-tion 3 min).

• Stop the mixture inlet and introduction of N2 tostudy the return to the baseline (duration 3 min).

Time sequences were chosen according to the vol-ume of the measurement cell ��3 l� and the chosenflow �1 l�mn�.

3. Results and Discussion

A. Atomic Force Microscope Characterizations

ZnO, SnO2, and TiO2 samples were characterized byAFM. Figures 3–7 compare typical surface morphol-ogies among the three materials. All the roughnessmeasurements are presented in Table 2.

The average roughness is about several nanome-ters for all films, regardless of the deposition process.This roughness is adapted for waveguiding condi-tions, but it should be higher (average roughness inthe range of a few tens of nanometers) to enhance theinteraction surface between the sensitive materialand the gas without decreasing the optical smooth-ness of the film. The typical size of the grains of all thematerials is in the micrometer range.

B. m-Line Characterizations

The m-line characterizations are given in Table 3. ForZnO, n is in good accordance with the values found inthe literature �n � 1.97 at � 633 nm; see Ref. 11) inwhich the measurements are usually performed withnonpolarized light and induce no consideration of an-isotropy. It was not possible to calculate the refractiveindex for all the samples and the thickness for thefollowing reasons:

• There is only one mode by polarization state(SnPdQ02 and SnPd25).

• The m lines are too large and curved because ofinternal stresses when strong coupling is needed(SnPdQ29, TO29, ZO25, and Zn11034).

A slight anisotropy is observed for all films forwhom nTE and nTM were calculated. The followingfilms with well contrasted and thin lines in the TMpolarization state were selected for exposure to bu-tane for the sensitivity tests: ZO27, SnPdQ27,Zn23114, Ti150903, and ZNIET.

C. Gas Sensitivity Tests

The ZO27, SnPdQ27, Zn23114, Ti150903, and ZNIETsamples were exposed to different concentrations ofbutane diluted in nitrogen and in air at room tem-perature. The protocol described above is repeatedtwice to observe the response reproducibility. Opticalresponses are given in Figs. 8–12.

The optical response of ZnO is repeatable, as can beseen in Fig. 8. The vacuum stabilizes the signal at520 mV, and the mixture of butane stabilizes thesignal at 700 mV. There is a slight background in-crease in the signal owing to a laser power drift. Thevacuum changes the concentration of the surround-ing atmosphere, and it thus modifies the equilibriumof adsorbed species linked to adsorbed oxygen ions atthe surface of ZnO and therefore the refractive indexof the film. This leads to a signal variation. Thisequilibrium is not changed when N2 is introduced,since the vacuum is removed by a neutral gas (asobserved in Fig. 8). Butane molecules interact withactive sites (adsorbed oxygen ions at the surface),leading to a variation in equilibrium and thus in therefractive index. When all the active sites are occu-pied, the surface is saturated and there is no furtherrefractive-index variation. When the butane is intro-duced just after the vacuum pumping, a signal vari-ation is observed. This is the proof that the signalvariations are due to the presence of butane and can-not be attributed to pressure effects. Moreover, thesignal always reaches the same level independentlyof the precedent phase of the protocol, a sign of greatreproducibility and reversibility. The reversibility of

Table 2. Roughness of Thin Films Measured by AFM

SampleAverage Roughness

(nm)Peak–Valley Distance

(mm)

ZO28 3.5 34SnPd29 2.5 19.5Zn23114 1.3 23Ti150903 4.6 65ZNIET 2.5 21.5

Table 3. Optical Characterizations of Thin Films by the m-Line Technique

Sample Modes nTE nTM Thickness (nm)

TO29 TE0, TE1, TM0, TM1 — — —Z025 TE0 to TE3, TM0 to TM3 — 2.000 770Z027 TE0, TE1, TE2, TM0, TM1, TM2 1.968 1.978 626Z028 TE0, TE1, TM0, TM1 1.955 1.952 435SnPdQ02 TM0 — — —SnPdQ25 TE0, TM0, TM1 — — —SnPdQ27 TE0, TE1, TM0, TM1, TM2 1.979 1.964 402SnPdQ29 TE0, TE1, TM0, TM1 — — —Zn11034 TE0, TE1, TE2 TM0, TM1, TM2 1.988 — —Ti150903 TE0, TE1, TM0, TM1 2.334 2.344 296ZNIET TE0, TE1, TE2 TM0, TM1, TM235 1.884 1.899 644

1 March 2006 � Vol. 45, No. 7 � APPLIED OPTICS 1431

the response is obtained through the vacuum pump-ing. The most common way to get a reversible sensoris to heat the film to desorb the gas molecules at thesurface of the material. The ZO27 sample shows asimilar response when the concentration of butanedecreases. The shape of the signal variation remainsthe same when the concentration of butane decrea-ses, but the kinetics is slower at lower concentrations.Information about the butane concentration couldtherefore be deduced with a derivation of the signalwith time. A possible explanation is that, with fewermolecules of butane in the mixture, it takes more totime to saturate the surface of the material.

Figures 9, 10, and 12 show that similar remarks

can be made for the SnPdQ27 �SnO2�, Zn23114, andZNIET (ZnO) films. For SnPdQ27 and ZNIET, thecurve is reversed compared with that of ZO27. Thismay be attributed to a variation in the refractiveindex opposed to that of ZO27 under gas exposure.Concentrations as low as 100 ppm can be detected byusing SnO2 or ZnO.

Concerning TiO2, Fig. 11 shows a slight optical re-sponse of the Ti150903 sample, but there is no repro-ducibility from one concentration to another. There isa drift in laser power that prevents us from observingstrong variations (for example, for 1000 ppm). It istherefore impossible to evaluate the time response forthe different concentrations.

Fig. 8. Sensitivity of ZO27 to different concentrations of butane diluted in nitrogen.

Fig. 9. Sensitivity of Pd-doped SnPdQ27 to different concentrations of butane diluted in nitrogen.

1432 APPLIED OPTICS � Vol. 45, No. 7 � 1 March 2006

The ZO27 sample was exposed to butane dilutedin air. A similar response is observed, as shown onFig. 13. When air is introduced, oxygen is not ad-sorbed at the surface since only other adsorbed spe-cies were removed during vacuum pumping. Thesignal variation occurs only when butane is intro-duced in the cell. The amplitudes of the signal vari-ation and the time response are the same whennitrogen is used as a carrier gas.

From Eq. (12) we calculate the variation in theeffective index of a guided mode with the shift �s

of the line. Here �s � �d�R, where �d is the shiftof the line deduced from the profile with a signal

variation (see Fig. 14) and R is the length of themeasurement arm �75 cm�. The refractive indexof the prism is np � 2.5821, and its angle is Ap

� 45°. The variation in the effective index of severalfilms under butane exposure is presented in Table4.

Table 4 shows that effective-index variations aslow as 0.005 can be detected. Concerning ZO27, thesignal variation is twice as important as that for theZNIET sample (see Figs. 8 and 12), but theeffective-index variation �N is lower (0.005 versus0.037, respectively). This is due to the line profileshape as shown in Fig. 14. This underscores the

Fig. 10. Sensitivity of Zn23114 to different concentrations of butane diluted in nitrogen.

Fig. 11. Sensitivity of TiO2 to different concentrations of butane diluted in nitrogen.

1 March 2006 � Vol. 45, No. 7 � APPLIED OPTICS 1433

importance of having a layer with high optical prop-erties (homogeneity, low absorption, etc.) and a linewith high contrast to observe low variations in therefractive index.

All samples were exposed to a concentration of3 ppm of ozone diluted in dry air at room tempera-ture. No optical response was observed. This prob-ably results from the thermal activated aspect ofozone adsorption. The study of the influence of tem-perature on the sensitivity of all materials to bu-tane and the reversibility of the response withoutvacuum pumping are topics for future research.

4. ConclusionsSensitive materials used in different kinds of gassensor and deposited in thin-film form by differenttechniques were characterized by AFM and the

Fig. 12. Sensitivity of ZNIET to different concentrations of butane diluted in nitrogen.

Fig. 13. Sensitivity of ZO27 to different concentrations of butane diluted in air.

Table 4. Variation in the Effective Index of ZnO Films

Sample Mode �d (mm) �s (°) ��s (°) �N

ZO27 TM0 0.1 8.832 0.0076 0.005Zn23114 TM1 0.45 11.116 0.0340 0.022ZNIET TM2 0.65 �28.265 0.050 0.037

1434 APPLIED OPTICS � Vol. 45, No. 7 � 1 March 2006

m-line technique. AFM measurements showed a lowroughness for all films, all of which presented a lowoptical anisotropy. The ZnO and Pd-doped SnO2 sam-ples presented a significant optical response to lowconcentrations of butane diluted in nitrogen and inair, down to 100 ppm at room temperature. Thesematerials are therefore good candidates for activelayers in a new type of integrated optical gas sensor.TiO2 showed no obvious response to the presence ofbutane.

Part of this research is funded by the EuropeanCommission in the Nanophos Project [Information So-ciety Technologies (IST)–5th Programme-Cadre de Re-cherche et de Développement, contracts IST-2001-39112 and 2003–2005) and Region Provence–AlpesCôte d’Azur (Microsystème pour la détection optiqued’espèces chimiques, contracts 2004-05869 andDEB04-64). The authors thank Jean-Pierre Spinelli,Julie Desrousseaux, and Geraldine Loesel for theircontribution to this study.

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Fig. 14. Shown are the m-line profiles of ZnO films. For ZO27, the signal variation is between Vmin � 650 mV and Vmax � 850 mV (seeFig. 8). The corresponding linear shift is 0.1 mm; ��s � 1.33 � 10�3 rad � 0.0076°.

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