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Energy Conversion and Management 48 (2007) 2911–2917

Optical system for CO and NO gas detection in the exhaustmanifold of combustion engines

M. Mello a, M. De Vittorio a,b, A. Passaseo a, M. Lomascolo c, A. de Risi b,*

a National Nanotechnology Laboratory of INFM-CNR, Distretto Tecnologico ISUFI, Universita del Salento, via per Arnesano, 73100 Lecce, Italyb Dipartimento Ingegneria dell’innovazione, Universita del Salento, University Campus, Strada Prov. Lecce-Monteroni km 1,200, 73100 Lecce, Italy

c IMM-CNR, Institute for Microelectronics and Microsystems, Department of Lecce, University Campus, Strada Prov.

Lecce-Monteroni km 1,200, 73100 Lecce, Italy

Available online 19 September 2007

Abstract

The experimental characterization of an innovative optical system for detection of carbon monoxide (CO) and nitride oxide (NO) inthe exhaust manifold of otto and diesel engines is reported. A photodetector based on gallium nitride (GaN) and an UV light source areintegrated in a chamber of analysis and form the detection system. The UV light source, consisting of a spark produced by an arc dis-charge, induces electronic transitions in the gas molecules flowing between the light source and the GaN photodetector. The transitionsmodify the fraction of light in the UV spectral region which is detected by the GaN photodetector, as a function of the species concen-tration. By means of its structural properties, gallium nitride (GaN) allows to operate at high temperature and high speed and to workin situ in the exhaust manifold of combustion engines at temperatures as high as 600 �C, at which the deposited organic residuals on thedetector can be oxidized. This assures a clear surface necessary for a real time optical measurement of the species concentration to beused for a closed loop control of the fuel injection process.

The system was applied to the detection of CO and NO with concentration between 0% and 2% in a buffer of pure nitrogen gas, show-ing an increase in the measured photocurrent as a function of the above gases.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Gallium nitride photodetector; Exhaust manifold sensor; CO measurement; NO measurement; Combustion control

1. Introduction

Nowadays, the capability to monitor and detect chemi-cal species has become more and more important toimprove engineer control strategies. The aim is to optimizecombustor operation, monitor the process and alleviatetransitory condition and their severe consequences on emis-sion [1–3].

Moreover, the control on the exhaust gases and in par-ticular on the polluting gases emissions is required by envi-ronmental legislations, which become more and morerestrictive. In urban area, motor vehicles such as cars,buses, trucks, motorcycles are the main source of pollu-

0196-8904/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enconman.2007.07.034

* Corresponding author. Tel.: +39 0832 297757; fax: +39 0832 297777.E-mail address: arturo.derisi@unile.it (A. de Risi).

tants gas emissions which mainly consist of nitrogen oxides(NOx), carbon monoxide (CO) and carbon dioxide (CO2)[3,4]. The fast and in situ monitoring and detection of thesechemical species and the use of these informations by afeedback signal will allow the improvement of the combus-tion engine efficiency (closed loop control).

So far, the gas sensors available on the market are typ-ically chemical sensors [3,5,6] or optical sensors [7–10]. Thefirst group of sensors detects gases from a change in theelectrical resistance of a porous sensing material whichadsorbs the molecules of gas under observation; responsetime, in this case, is limited by the diffusivity of the gas mol-ecules into the sensitive element of the sensor, whichtypically is of the order of seconds. Whereas the secondgroup, thanks to a higher selectivity and high speed, allowsa measure of gas concentration in real time. These

Nomenclature

A optical areaEg energy gapEt,12 energy ht related to 1–2 transitiong photocurrent gainh Planck’s constanti photocurrenti0 light source intensityi12 intensity of 1–2 transitionIA,12 absorption share of 1–2 transitionIE,12 emission share of 1–2 transitionl distance between two electrodesL optical pathNi population of the i-level[Ni] concentration of the i speciepi,12 (t) probability of 1–2 transition with energy Et,12 in

the i specie

P light intensityq electron chargeR responsivity of the sensorV bias voltage

Greek letters

g quantum efficiencyle electron mobilitylp hole mobilitym light frequencyr absorption cross sections average life-time of the light-induced carriers[a],[b] transitions observed in the NO flow[c],[d],[�],[x] transitions observed in the CO flow

Fig. 1. Temporal response of the photocurrent signal in MSM GaNphotodetector to light pulses.

2912 M. Mello et al. / Energy Conversion and Management 48 (2007) 2911–2917

detectors typically operate in the infrared (IR) spectralrange and are mainly used for the analysis of the carbonoxides and unburned hydrocarbons.

To date, these sensors do not satisfy all the requirementsfor automotive applications. The major limitations are onone hand poor selectivity and reproducibly [9] and on theother large in size, weight and especially cost [3].

The only closed loop control of car emissions is thelambda probe [5], which is an electrochemical sensor thatdetects the excess of air by sensing the O2 directly in theexhaust manifold of petrol engine. Within diesel enginethere is not any sensor. Regarding to the lambda probe itis important to note the transitory condition present whenthe internal combustion engine is under high load; in thiscondition the oxygen sensor no longer operates, and theengine automatically enriches the mixture to both increasepower and protect the engine. Any modifications to theoxygen sensor will be ignored in this transient state, whilemodifications to the air flow meter will give the risk oflower performance due to the mixture being too rich ortoo lean, and the risk of damaging the engine due to deto-nation if the mixture is too lean. Moreover the responsetime of lambda probe is not fast enough to perform acycle-by-cycle control.

It is a common practice to analyze exhaust emissionsex situ through Non-Dispersive IR (NDIR) instruments(COx analysis) and chemiluminescence instruments (NOx

analysis) and thus setup an open loop control. Opticaldetectors have not been used for exhaust gas analysisdue to the hard environmental conditions and to thedeposition of organic compounds particles on the activearea of detectors. In fact, mechanical and physical proper-ties of the traditional semiconductor materials, such as Si,Ge, GaAs and their compounds, drastically decay at hightemperature.

Gallium nitride (GaN) and related alloys by virtue oftheir intrinsic robustness and resistance to high tempera-tures allow to realize sensors working in critical thermalconditions (T > 550 �C) [11,12] and harsh environments.The absorption edge of III-Vs nitride compounds can betuned from 365 nm (Eg GaN = 3.44 eV) down to 200 nm(Eg AlN = 6.2 eV) through the epitaxial synthesis of Alx-

Ga1 � xN alloys. This tunability of the detection edge isrealized by just varying the Al mole fraction [13]. To date,several researchers have reported encouraging results forGaN-based photodetectors in different architectures, suchas p–n junction diodes, p–i–n diodes, the p–p–n diode,Schottky barrier detectors and the metal–semiconductor–metal (MSM) photodetectors [14]. Among these structures,MSM photodetectors are an attractive choice for UVdetectors, because of their fast response, simplicity [15]and small size.

This paper report on a GaN based MSM photodetectorrealized and characterized for the measurements of com-bustion species.

Fig. 3. MSM photodetector with optical area A of L · L and two Cr/AuSchottky interdigitated contacts.

M. Mello et al. / Energy Conversion and Management 48 (2007) 2911–2917 2913

The choice of MSM photodetector based on GaN is dueeither, as reported, to mechanical and electronic character-istic of GaN and to the advantages of the MSM configura-tion in terms of fast response.

The photocurrent signal of the MSM GaN photodetec-tor biased at 1 V when excited by a light source operatingat 1 kHz is shown in Fig. 1. The results indicated that thedevices allows the cycle-by-cycle control of the combustionprocess in automotive engines.

In order to test the sensor, controlled NO and CO con-centrations are continuously forced in the analysis system,embedding a GaN MSM UV photodetector placed in frontof an arc discharge source (a spark). The change of theintensity of the detected UV light was a function of thegas mixture; this intensity variation caused a modified pho-tocurrent excited in the GaN photodetector, which wascorrelated with the gas concentration. The schematic repre-sentation of the potential application of this detector sys-tem is shown in Fig. 2, where the geometry of an engineis reproduced. The circle in Fig. 2 showed the detection sys-tem proposed in this work. The system has been success-fully applied to measure CO concentrations and NOconcentrations (0–2% respectively) in pure nitrogen buffergas. Photocurrent of GaN detector as function of the COand NO and the spectral response of the spark have beenanalyzed and correlated.

2. Experimental setup

The GaN samples investigated in this paper were grownin a horizontal low pressure metal organic chemical vapourdeposition (MOCVD) system (Aixtron 200 AlX rf),equipped with a rotating substrate holder, with TMGaand pure NH3 as source materials; palladium purified H2

was used as a carrier gas. The growth was performed ona sapphire substrate (crystalline Al2O3) cleaned with sol-vents and then annealed in situ at 1100 �C. In order to relaxthe lattice mismatch between GaN and sapphire, a lowtemperature GaN buffer layer with a thickness of a fewnanometers was deposited before the growth of the activeepitaxial film on the sapphire substrate. The thickness ofthe epitaxial layer grown at high temperature was about1.5 lm. After the optical lithography with a negative tone

Fig. 2. Potential application of the detector system.

resist, interdigitated contacts were realized by lift-off proce-dures. The device consists of two Cr/Au Schottky interdig-itated contacts over an optical area A of 0.5 mm · 0.5 mm,the fingers thickness and the interspacing between the inter-digitated contacts are 2 lm and 3 lm respectively (Fig. 3).

The MSM GaN photodetector was placed in front of aspark at a distance of about 30 mm. The spark generated byan arc discharge lamp as UV light source was chosen dueto its robustness and its resistance at high temperature.Moreover the voltaic arc generated between two closemetal electrodes excites some of the molecules of the gasunder study, thus allowing their monitoring and quantita-tive analysis. The gas flow was continuously forcedbetween the spark and the GaN MSM photodetector insidean analysis chamber.

The MSM GaN photodetector sensitivity is related tothe photocurrent If, which is the difference between thetotal current (I) when the spark is on and the noise current(Id) when the spark is off, which includes also the bias cur-rent. The photocurrent was measured by a Keithley 2400current/voltage source/meter and its value was analyzedat 5 V and 10 V bias voltage.

By using two digital gas flowmeters the total flow ratewas kept constant to 500 sccm and the system has beenapplied for the measure of CO and NO concentrations var-ied between 0% and 2%, diluted in pure nitrogen.

The light radiation is generated by the lamp and col-lected by the photodetector. The gas molecules eitherabsorb a fraction of this light or are excited by the spark

and afterwards relax to the ground state thus emittingaccording to molecular optical transitions; the GaN photo-detector can detect only in the UV spectral region andtherefore can only measure species which emit and/oradsorb in this spectral region.

3. Results and discussion

Fig. 4 shows the photocurrent values as function of COconcentration (Fig. 4a) and NO concentration (Fig. 4b),respectively. The photocurrent intensity, which dependson the bias voltage, is strongly modified when the COand NO concentrations increase from 0% to 2%. As itcan be seen in Fig. 4b, a saturation of the photocurrent sig-nal is observed at 10V for NO concentration higher than1.4%.

0.00% 0.20% 0.40% 0.80% 1.20% 1.60% 2.00%

0.6

0.7

0.8

0.9

1.0

1.1

1.2

I f [m

A]

% CO in pure nitrogen flow

5V 10V

0.00% 0.40% 1.00% 1.40% 1.80% 2.00%0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

1.3

1.4

I f [m

A]

%NO in pure nitrogen flow

5V 10V

a

b

Fig. 4. Photocurrent intensity measured as a function of the CO (a) andNO (b) in pure nitrogen.

200 400 600 800 1000

1000

10000

contemporary3 sec7 sec

Light spectra

Inte

nsity

(a.u

.)

Wavelength [nm]

Fig. 5. Spectra of the light emitted by the spark in air to tree ignition time.

2914 M. Mello et al. / Energy Conversion and Management 48 (2007) 2911–2917

The increasing trend of the photocurrent If as a functionof bias voltage is due to the dependence between current I

and bias voltage V. The noise current Id is negligible in allour measurements, so it is possible to consider the photo-current If like the total current I generated by the spark.Under this consideration the photocurrent If is defined bythe equation:

I f ffi I ¼ R � PA

ð1Þ

where R is the responsivity of the sensor and P the opticalpower incident on the optical area A.

The responsivity R, in the particular case of MSM pho-todetector, can be expressed by the following equation [16]:

R ¼ ggqhm¼ gqsleV

l21þ le=lp

� �ð2Þ

where g, q, g, h, m, s, le, lp, V and l are photocurrent gain,electron charge, quantum efficiency, Planck’s constant,light frequency, average life-time of the light-induced carri-ers, electron mobility, hole mobility, bias voltage and thedistance between two electrodes, respectively. All of theseparameters depend on the material properties.

The increasing trend of the photocurrent as a functionof the gas under analysis has been attributed to the excita-tion of the gas molecules from the arc discharge. This exci-tation is a function of the gases concentration and caused avariation of the light intensity on the photodetector andtherefore the photocurrent measured is different (Eq. (1)).

In order to understand the origin of the increase of thephotocurrent signal, the variation of the radiation emittedby the spark has been spectrally analyzed as a function ofthe CO and NO concentrations. An Optical Emission Spec-troscopy (OES) has been performed [17] by replacing theMSM GaN photodetector with a quartz window and thesignal has been acquired by a 460 HR spectrometerequipped with a 300 ll/mm grating. In this case, the dis-tance between the arc lamp and the quartz window was60 mm. The spark emitted light has been analyzed in thespectra region between 100 nm and 1000 nm. Fig. 5 showsthe result obtained in air by varying spark duration (instan-taneous, after 3 s and after 7 s of ignition time respectively).It can be noticed that the spark emitted only at a wave-length higher than 300 nm and that all the spectra, varyingthe spark duration, overloop up to 600 nm. Differences forwavelength higher than 600 nm are caused by the heatingof the electrodes. Considering the spectral characteristicsof the spark and of the GaN detector only species withoptical transition in the spectral region between 300 nmand 360 nm could be detected.

The spectral characterization of the arc discharge lamphas been realized by varying CO and NO concentrationsin the same range between 0% and 2% respectively, in abuffer of pure nitrogen.

In the case of pure nitrogen flow only transitions corre-sponding to the ionization of N2 in Nþ2 [18,19] wereobserved. In presence of small concentrations of eitherCO or NO, the spectral features are, as expected, relatedto the electronic transition in the CO or NO molecules.

As NO molecules were added to the flow, the spectralanalysis of the emitted light showed several transitions

330 340 350 360

2000

4000

6000 onl y N2

1.4% NO 2.0% NO

[ α ]

[ β ]

Wavelength λ[nm]

Inte

nsity

(a.u

.)

0.0 0.4 0.8 1.2 1.6 2.0 2.41000

2000

3000

4000

5000

6000

7000

8000

[ β ]

[ α ]

% NO in the flow

Inte

nsity

of t

he s

ingl

e tra

nsiti

on (a

.u.)

Region IIIRegion IIRegion I

Fig. 6. Intensity of the NO transition.

Fig. 7. Emission and absorption trend for NO end N2 molecules and theircombination.

M. Mello et al. / Energy Conversion and Management 48 (2007) 2911–2917 2915

resulting from N2, NO and Nþ2 and NO+ molecules. In thespectral region between 330 nm and 360 nm, the two tran-sitions [a] and [b] are clearly distinguished, which areattributed in the literature, both to N2 and to NO [18,19].Fig. 6 shows the intensity of the NO transition at about336 nm [a] and about 357 nm [b] as a function of NO gasconcentration. Three regions can definitely be distinguishedin Fig. 6. When the NO concentration is varied from 0% to0.4% the collected light is reduced due to an absorptionprocess in the volume between the spark and the quartzwindow. A further increase of the NO concentration, upto 1.4%, leads to an increase of the collected light. Whenthe NO concentration is in the range between 1.4% and2% the intensities of the [a] and [b] peaks are reduced again.

This behaviour has been attributed to the simultaneousabsorption and emission processes for both NO and N2

molecules. In this hypothesis, the superposition of emissionand absorption transitions of NO and N2 molecules is con-sidered. The NO and N2 molecules, interacting with thespark, produce a radiation which can be reabsorbed inthe volume between the spark and the quartz window.

For a general optical transition from energy state 1 toenergy state 2, the intensity of the emission IE,12 can berelated, for a single specie i, to the concentration [Ni] andto the probability (pi,12(t)) that the transition with Et,12

energy happens:

IE;12 / ½Ni�pi;12ðtÞEt;12 ð3Þ

On the other hand the absorption intensity (IA,12) isdescribed by the Lambert-Beer law:

IA;12 ¼ I0 exp½�rðN 1 � N 2ÞL� ð4Þwhere I0 is the source intensity and corresponds to theintensity of the emitted light IE,12, while N1 and N2 areproportional to the concentration [Ni] of the generic speciei. By fixing the geometry and the specie under analysis, theoptical path (L) and the absorption cross section (r) are

constant and the total intensity of the transition 1–2 (I12),resulting from the two absorption and emission processescan be obtained from (3) and (4) as follows:

I12 / ½N i� expð�½Ni�Þ ð5ÞThe concentration of NO is varied between 0% and 2%

and the I12jNO is strictly dependent of this value as pre-dicted by Eq. (5). The increase of the NO corresponds toa reduction of the N2 concentration in the range between100% and 98%. This slight change in the N2 concentrationcorrespond to a negligible change in its absorption inten-sity, which can be kept constant in the considered concen-tration range (IA;12jN2

constant), whereas the N2 emissionintensity linearly increase (IE;12jN2

) with [Ni].Fig. 7 shows, the different contributions leading to the

measured optical signal. The intensity of NO transition(dot line) has a bell-like shape as expected from Eq. (5).This curve combined with the emission and absorptiontrend (dash line) for N2 molecules leads to the total inten-sity of the transition (solid line) in good agreement with theexperimentally detected signal (Fig. 6). The measured sig-nal is therefore strongly dependent on which mechanism,among those considered before, is dominating.

The reader should notice that by using the GaN photo-detector the distance between the spark and the detectorwas only 30 mm instead of the 60 mm used in the OESanalysis. For this reason, the fraction of absorption radia-tion was reduced correspondingly, thus in the measuredphotocurrent of Fig. 4 only trends corresponding to regionII could be observed.

By using a GaN photodetector, the distance between thespark and the detector can be made smaller than 6 cm, thusdecreasing the absorption fraction of N2 and NOmolecules.

The difference between the NO transitions detected byour sensor and the NO transitions traditionally used todetect the NO composition in the exhaust gas, is associatedto the different energy. The NO transitions traditionallyused for the exhaust gas analysis are blazed to 230 nmand obviously have energy higher than the [a] and [b] tran-

2916 M. Mello et al. / Energy Conversion and Management 48 (2007) 2911–2917

sitions observed in the inset of Fig. 6. In fact the wave-length around 230 nm corresponds to the transition withenergy of Et, 3 � 9 = 5,396 eV from t1 = 3 to t2 = 9. [19]Instead the [a] and [b] transitions correspond to the transi-tion at 336,6 nm with energy of Et,11–25 = 3,687 eV fromt1 = 11 to t2 = 25 and to the transition with energy ofEt,0–10 = 3,468 eV from t1 = 0 to t2 = 10.

A similar analysis has been carried out using CO flowdiluted in pure nitrogen. As CO molecules were added tothe nitrogen flow, an increase of the intensity in the spectralregion between 380 nm and 390 nm was observed. Fig. 8shows the differences in this spectral range as a functionof CO concentration, the main transitions in the gas mix-ture are indicated as [c],[d], [e] and [x]. These transitions,in agreement with the data reported in the literature[18,20], have been attributed to a decomposition of theCO molecules in C ([c]), O ([d] and [x]) and C2 ([e]).

The intensity of these transitions, showed in the inset ofFig. 8, increases with CO concentration due to the incre-ment of the number of CO decomposed molecules (andso the increment of P in Eq. (1)).

Light reabsorption by gas volume in the optical pathtoward the quartz window has not been observed becausein presence of CO, the dissociation of the molecules leadsto an emitted radiation which cannot be readsorbed bythe nitrogen gas present in the volume, due to the observa-tion of the only CO transition in the spectral regionbetween 380 nm and 390 nm (Fig. 8).

It should be mentioned that at room temperature thespectral region around 385 nm lies in the low-energy tailof the band edge of the GaN material (about 3,44 eV),where the absorption of the photodetector is stronglyreduced. Anyway, photocurrent measurements on GaNMSM photodetector have shown (B.Potı et al. [21]) thepresence of a large tail absorption at energy lower than3,44 eV in GaN bulk.

Therefore this tail allows GaN photodetector to detecttransitions associated to CO decomposition which are

376 380 384 3880

10000

20000

30000

40000

50000

only N2

2% CO5% CO9% CO

[ ω ]

[ ε ][ δ ][ γ ]

Inte

nsity

(a.u

.)

Wavelength [nm]

0 2 4 6 8 10

0

10000

20000

30000

40000

50000 Intensity of single transition (a.u.)

% CO in the mixture

Fig. 8. Different spectra at increasing CO concentrations and in the insetthe intensity of the single transitions.

associated to wavelengths longer than 360 nm. Moreoversince this detector is conceived to work at high temperature(550–600 �C) the efficiency of the system will increase byvirtue of a shift of the GaN cut-off toward higher wave-lengths [22]. The use of (Al,Ga)N and (In,Ga)N alloys willmake also possible to tune precisely the cut-off wavelengthso as to avoid interference with other gases.

4. Conclusion

A MSM photodetector for the analysis of the exhaustgas in combustion engines has been realized and character-ized. The system consists of a MSM GaN photodetectorand an arc discharge light source and it discriminates vari-ations of concentration of CO and NO between 0% and 2%in a buffer of pure nitrogen. A spectral analysis of the lightwas been realized acquiring a light radiation with a mono-chromator. Thanks to this analysis it has been possible toobserve the CO transitions which are connected to thedecomposition of CO molecules. In the case of NO it is dif-ficult to discriminate the transition of NO from N2 becausethese transitions are at the same wavelength. In order toexplain the measured optical signal a dual behaviour forthe NO and N2 molecules has been proposed. In thishypothesis, the superposition of emission and of absorp-tion transitions of NO and N2 molecules is considered.

The GaN photodetector allows us to operate at hightemperature and high speed, which will enable the in situoperation of the sensor in the exhaust manifold of combus-tion engines. The high temperature present in the exhaustmanifold and a micro-heater integrated with the GaN pho-todetector allow the oxidation of particulate matter depos-ited on it, allowing the photodetector to remain alwaysclean. This application will make possible real time mea-surements and control of the pollutant species generatedby non-stationary combustion processes.

Acknowledgements

The authors gratefully acknowledge dott. Roberto Rella– CNR-IMM Department of Lecce for allowing the use ofthe flowmeter test bench used in the present investigation.

This research has been partially supported by the Minis-try of Instruction, University and Research (MIUR) withContract FIRB RBAU01CXNP.

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