Effect of temperature and NO2 surface adsorption on electrical properties of screen printed ITO thin film

Applied Surface Science 355 (2015) 242–249

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Effect of temperature and NO2 surface adsorption on electrical properties of screen printed ITO thin film I. Madhi ∗ , W. Meddeb, B. Bouzid, M. Saadoun, B. Bessaïs Photovoltaic Laboratory, Research and Technology Centre of Energy, Borj-Cedria Technopark, BP 95, 2050 Hammam-Lif, Tunisia

a r t i c l e

i n f o

Article history: Received 3 March 2015 Received in revised form 16 July 2015 Accepted 18 July 2015 Available online 22 July 2015 Keywords: ITO NO2 XRD Dielectric properties Impedance spectroscopy Electrical conductivity

a b s t r a c t Indium tin oxide films with thicknesses of about 1 ␮m were prepared using the screen printing technique. Preliminary X-ray diffraction studies show that the formed ITO crystallizes in the cubic crystal system. The crystallite size (D) and the microstrain (εstr ) were investigated using Scherrer formula and Williamson–Hall analysis. Scanning electron microscopy and transmission electron microscopy show that the ITO films are granular, essentially composed of uniformly distributed sub-spherical – like grains. The variation of the DC conductivity with temperature confirms the presence of three activation energies, indicating the presence of different scattering mechanisms essentially dominated by oxygen adsorption and thermal excitation of electrons in the conduction band. Detailed studies of the dielectric parameters (i.e., ε* and tan ı) of the compound as a function of temperature and NO2 adsorption (at various range of frequencies) reveal that their values are strongly dependent on temperature and NO2 adsorption. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, metal oxides have been widely investigated because of their low cost and their high compatibility with semiconductor processing [1]. Their popularity is due to their natural physicalchemical properties, essentially due to their non-stoichiometric features [2], originating from oxygen vacancies which in turn generate free electrons [3,4]. Many techniques have been employed to prepare metallic oxides films; such as spray pyrolysis [5], RF-magnetron sputtering [6] and screen printing [7]. The screen printing technique is a promising simple and low cost method having the advantage of preparing n-type ITO films at ambient atmosphere. In addition, the screen printing method does not require the use of vacuum, as other sophisticated techniques. The use of ITO as a sensitive layer for gas sensor applications was demonstrated in many works [6–8]. Mbarek et al. [7] investigated the gas sensing properties of screen printed ITO, and reported its potential use as a sensitive layer in gas sensor applications. Screen printed ITO was also used in TiO2 –ITO nanocomposite that showed an improved solar photo-catalytic activity [9]. In general, the gases that we seek to detect are pollutants, which harm the human health and the environment balance like carbon

∗ Corresponding author. E-mail address: [email protected] (I. Madhi). http://dx.doi.org/10.1016/j.apsusc.2015.07.135 0169-4332/© 2015 Elsevier B.V. All rights reserved.

monoxide, nitrogen oxides or ozone [10–13]. The mechanism of gas detection can be explained as follows: when the ITO sensitive film is exposed to air, oxygen molecules adsorb on the surface of the material to form chemisorbed oxygen anions, which in turn capture electrons from the conductance band, resulting in the formation of a surface depletion layer and in an increase of the electrical resistance of the material. The reactions can be described as follows [14]: ½O2 + 2e− → O2− (ads)

(1)

O2 + 2e− → 2O2 − (ads)

(2)

O2 + 2e− → 2O− (ads)

(3)

However, when the ITO film is exposed to oxidizing/reducing gas, the concentration of electrons on the surface decreases/increase and, correspondingly, the electric resistance increases/decrease. Especially, for NO2 detection the process of the reaction can be described as follows [15]: NO2(gas) + e− → NO(gas) + O− (ads) −

NO2(gas) + e → NO2

−

(ads)

(4) (5)

These series of reactions result in the decrease of the electron concentration on the ITO surface, which lead to an increase of the resistivity of the material. This change in the resistivity can be used for NO2 detection [16]. Several works and approaches have been studied for understanding the gas/solid interaction mechanism during NO2

I. Madhi et al. / Applied Surface Science 355 (2015) 242–249

243

detection. The greater part of these studies was made in DC measurements. While direct current DC measurements give information on the global sensor response, AC impedance study is a powerful tool to understand the nature of the conduction processes and the mechanism of gas/solid interactions, as the processes of different time constants can be distinguished by varying the frequency [8,16]. Consequently, the objective of this work is to study the effect of temperature and NO2 adsorption on the dielectric properties of ITO. So, this work includes three parts: structural and morphological study, DC measurements and AC measurements. 2. Experimental details

3. Results and discussion

Fig. 1. EDX spectrum of the ITO.

2000

1000

In2O3 (440)

In2O3 (622)

800

1 .5 2 8 2 0 [A °]

1200

1 .7 8 9 7 4 [A °]

1400

In2O3 (400) 2 .5 2 6 6 2 [A °]

1600

In2O3 (222)

2 .9 2 2 1 9 [A °]

1800

in te n s ity (a .u .)

The ITO film is prepared by screen printing a viscous organometallic paste (ESL # 3050) of a dissolved combination of metallic indium and tin onto glass substrate [7]. The sample is firstly dried in air in an oven at a temperature of about 250 ◦ C during 15 min and then annealed in an infrared furnace at a temperature of 570 ◦ C during 45 min. Two Ag electrodes, 5 mm distant from one another, were deposited by screen printing method on the edges of the film. The structural analysis was carried out using a Panalytical X-Pert Pro X-ray diffractometer using the monochromatic Cu K␣ radia˚ the scanning angle (2) was varied from 20◦ tion ( = 1.54060 A); to 70◦ with scanning steps of 0.0181◦ . The average crystallite size dimensions were estimated using the Debye–Scherrer formula, Williamson–Hall analysis and verified by Transmission Electron Microscopy (XL 30-Philips) images. The atomic composition was determined by means of the energy dispersive spectroscopy (EDX) analysis during TEM observations. Alternating current (AC) measurements were acquired using HP4192A standard analyzer impedance in the 0.1 Hz to 13 MHz frequency range. We will note Z as the real part and Z as the imaginary part of the complex impedance. It is frequently mentioned that ITO is very sensitive to humidity, so variation of humidity ratio in test rig-humidity can alter the measurements. In this work all measurements were performed under a constant relative humidity (RH) of about 75%.

600 400

3.1. Atomic composition, structure and morphology

200

The objective of this work is to study the effect of temperature and NO2 adsorption on the dielectric properties of ITO. Many reports reveal that structure and morphology affect the dielectric properties of the material; Jin et al. [17] studied the microstructural and dielectric properties of porous TiO2 films synthesized on titanium alloys by micro-arc discharge oxidization, suggesting that the dielectric constants of the films varies as a function of chemical composition structure and morphology. Consequently, in this part we will study the atomic composition, structure and morphology of the screen printed ITO film. The obtained ITO film has a thickness of about 1 ␮m. The chemical composition was carried out by means of the energy dispersive spectroscopy (EDX) analysis (Fig. 1). EDX spectrum exhibits a strong intensity peak at 3.28 keV, which is characteristic of indium

30

40

50

60

2Theta (degree) Fig. 2. XRD patterns of the ITO film.

element. One mild peak at 3.44 keV also appears in the EDX spectrum, which is associated to tin. The characteristics peaks of Cu at 8 keV and 8.9 keV are present, which are attributed to the EDX grid. An analysis of different regions proclaims uniform chemical composition. The measured atomic percent composition is shown in Table 1, revealing that ITO film contain about 92.6 at.% of indium and 7.4 at.% of tin. Fig. 2 shows the XRD patterns of the as-prepared ITO film. XRD indicates that the ITO film is crystallized. The identification of the

Table 1 Atomic composition and lattice parameters of the ITO film. Atomic composition (at.%)

˚ Interplanar spacing dhkl (A)

In k

Sn K

d2 2 2

d4 0 0

d4 4 0

d6 2 2

a(2 2 2)

92.6

7.4

2.92219

2.52662

1.78974

1.52820

10.122

Lattice ˚ constant (A)

V (nm3 )

Scherrer’s method D (nm)

Williamson–Hall method D (nm)

1.037

12.00

11.04

I. Madhi et al. / Applied Surface Science 355 (2015) 242–249

diffraction lines was performed with reference to the In2 O3 powder (JCPDS data card no. 89-4595). The peaks appearing in the XRD patterns agree with that of the In2 O3 cubic structure. It is concluded that no SnO and/or SnO2 phases appear. The absence of tin oxide peaks in XRD pattern shows that tin remains in amorphous phase or doped In2 O3 structure; because the EDX analysis recognized the presence of 7.4 at.% of tin in the film. To check the possible introduction of tin into the In2 O3 structure; we calculate the lattice parameters. As ITO crystallizes in the cubic structure, the dhkl interplanar spacing (Miller indices h, k and l) values were calculated using Bragg equation: 2dhkl sin  = n

(6)

0,132 0,129

β cos(θ)

244

0,126 0,123 0,120 0,117 0,114

where n is the order of diffraction (usually n = 1) and  is the Xray wavelength. In the ITO cubic structure, the interplanar spacing plane is related to the lattice constants ‘a’ and the Miller indices as: dhkl =

a



(8)

where D is the average crystallite size, k = 0.9 is the Scherrer’s constant,  is the X-ray wavelength, ˇ is the full width at half maximum (FWHM) of the XRD line and  is the Bragg angle. D has been taken as average to all the peaks. Further, the particle size was also calculated, using the Williamson–Hall equation. By plotting the value of ˇ cos  against 4 sin , the lattice strain εstr can be obtained from the slope, and the crystallite size from the intercept on the vertical axis, as per equation: ˇ cos  =

k + 4εstr sin  D

1,4

1,6

1,8

2,0

4 sin(θ) Fig. 3. Williamson–Hall plot of the ITO film.

h2 + k2 + l2

k ˇ cos 

1,2

(7)

The lattice constant ‘a’ is calculated via the (2 2 2) line, which corresponds to the main diffraction peak. The value of the obtained ˚ is bigger than the value of the lattice constants (a1 = 10.122 A) ˚ standard In2 O3 lattice constant (a2 = 10.11 A). The fact that a1 > a2 may be interpreted by substitutional dissolution of Sn2+ ions into the In2 O3 lattice; because the In3+ ionic ˚ is smaller than Sn2+ ionic radius (1.22 A), ˚ which will radius (0.80 A) lead to the increase of the planar spacing of In2 O3 crystal lattice. As tin acts as a doping material and not an independent phase, XRD lines can be used to calculate the average crystallite size (D) and the microstrain (εstr ). In fact, X-ray profile analysis is a simple and powerful tool to estimate the crystallite size and lattice strain [18]. Among the available methods to estimate the crystallite size and lattice strain are the pseudo-Voigt function, Rietveld refinement, and Warren–Averbach method [19,20]. Williamson–Hall (W–H) analysis is a simplified integral breadth method where both size-induced and strain-induced broadening are deconvoluted by considering the peak width as a function of 2 [20,21]. In the present study, the average crystallite size has been calculated using Scherrer formula and Williamson–Hall analysis. Scherrer formula is defined as [18]: D=

1,0

(9)

Fig. 3 shows the Williamson–Hall plot of the ITO film. Average crystallite sizes calculated by Scherrer formula was 12.00 nm and by Williamson–Hall plot was 11.04 nm. The crystallite size obtained by Williamson–Hall method was less than those obtained by Scherrer formula. It was because the strain correction factor has been taken into account in case of Williamson–Hall method whereas it has not been taken into account in Scherrer’s method. The negative slope of the plot indicates that strain broadening must be very small [18]. The structural parameters of the ITO film are summarized the in Table 1.

Fig. 4 shows TEM and HR-TEM images of the ITO film. ITO is composed of spherical-like nanoparticles having various crystallographical orientations. The size of the nanoparticles varies in the range of 10–14 nm. This result is in agreement with the average crystallite size estimated from XRD measurements. HR-TEM shows clear lattice fringes indicating a highly crystalline structure. The interpalanar spacing of 0.29 nm, 0.17 nm and 0.25 nm correspond, respectively, to the (2 2 2), (4 4 0) and (4 0 0) plans of In2 O3 . This result is in agreement with that obtained from XRD analysis (Table 1). SEM surface view (Fig. 5) shows pseudospherical like grains uniformly distributed on the surface of the sample; the latter present a typical granular growth. 3.2. DC conductivity Fig. 6A depicts the variation of the resistance of the ITO film versus temperature in the range of 50–250 ◦ C. It is seems to be clear (Fig. 6A) that the electrical conductivity increases with increasing temperature. In first region, from 50 ◦ C to about 100 ◦ C, we observe a slight decrease of the resistance, which could be due to the thermal excitation of electrons into the conduction band. In second region, from about 100 ◦ C to about 180 ◦ C, the resistivity decreases quickly, probably due to the vigorous oxygen adsorption on the film surface. In third region, from about 180 ◦ C to about 250 ◦ C, resistance stabilization was obtained; in fact, resistance is not affected by the temperature increase [15]. The resistance behavior is a thermally activated process that can be described by the Arrhenius equation: R = R0 exp

E  a

(10)

kb T

where R is the resistance at temperature T, R0 is the pre-exponential factor, kb is the Boltzmann’s constant, T is the absolute temperature and Ea is the activation energy. By applying the Arrhenius equation (Fig. 6B), we found three slopes, and therefore three activation energies, indicating the presence of different scattering mechanisms essentially dominated by oxygen adsorption and thermal excitation of electrons in the conduction band. The phenomenon could be interpreted as a competition between adsorption and desorption rates which can be described by the Lennard–Jones model for adsorption [16,22,23]:



d Ea = Kads exp − Kb T dt



 E + H  a chem

− Kdes  exp −

Kb T

(11)

where d/dt is the rate of chemisorption, Kads and Kdes are, respectively, the rate constants for adsorption and desorption; Ea and

I. Madhi et al. / Applied Surface Science 355 (2015) 242–249

245

Fig. 4. TEM and HRTEM images of the ITO film.

Hchem are the activation barrier for chemisorption and the heat of chemisorption, respectively. In the Lennard–Jones model the rate of chemisorption is given by an activation barrier between a physiosorbed state and a chemisorbed state and an activation barrier of desorption. The rate of chemisorption d/dt is expressed as the difference in adsorption and desorption rates. Under steady state conditions d/dt = 0, i.e. the rate of adsorption is equal to the rate of desorption, the coverage  is dependent on the heat of chemisorptions and is given by: =

Kads exp Kdes

 H

chem

 (12)

Kb T

For a given temperature, it is known that the chemisorptions of molecules occur at high temperature, this process needs same activate energy Ea [16]. At low temperature, there is only a physisorption which decreases with increasing the temperature. The chemisorption activation leads to the maximum of adsorption; consequently, when kb T ≥ Ea , a chemisorption will be activated. In this situation, the rate of desorption became negligible. We notice that activation energy increases significantly in the second region of temperature: from about 100 ◦ C to about 180 ◦ C (Fig. 6B), so the chemisorption process dominates in this temperature range. According to Arrhenius and Lennard–Jones model, we

Fig. 5. SEM image of the ITO film.

3

9x10

9.0 3

8x10

Ea = 0.03 eV

3

7x10

8.5

3

A

3

5x10

Ln R (Ω)

R (Ω)

6x10

3

4x10

3

3x10

B

Ea= 0.41 eV

8.0

7.5

Ea= 0.01 eV

3

2x10

7.0

3

1x10

40

80

120

160

T (°C)

200

240

22

24

26

28

30

32 -1

1/kbT (ev )

Fig. 6. (A) Resistance versus operating temperature in air, (B) activation energies of the ITO film.

34

36

246

I. Madhi et al. / Applied Surface Science 355 (2015) 242–249

500

T=40°C T=80°C T=120°C T=160°C T=200°C

450

A

400

45 ppm NO2

2000

B Z'(Ω)

Z'(Ω)

350 300 250

T=80°C T=160°C T=200°C

2500

1500

1000

200 150

500

100 50

0

0

500

1000

1500

2000

2500

0

500

1000

1500

2000

Frequency (Hz)

Frequency (Hz)

Fig. 7. (A) Variation of the real part of the impedance (Z ) of the ITO film versus frequency at different temperatures in air, and (B) in presence of 45 ppm of NO2 .

ε Y Z =  =  ε Y Z

presume that the dielectric properties of ITO undergo a remarkable variation in the temperature range between 100 ◦ C and 180 ◦ C.

tan ı =

3.3. Dielectric properties

where ω = 2f is the angular frequency and f the frequency of the applied field, C0 = ε0 S/d is the empty cell capacitance with ε0 representing the permittivity of free space (ε0 = 8.854 × 10−12 F/m), d is the sample thickness (d = 10−6 m), S is the area of the sample (S = 25 × 10−6 m) and j2 = −1. These relations offer wide scope for a graphical analysis of the various parameters under different conditions of temperature or frequency [25]. Fig. 7A and B illustrate the variation of the resistive part of the impedance (Z ) with frequency in air and in presence of NO2 gas, respectively. It is clearly viewed from the pattern itself that Z decreases with the increase in frequency for all temperatures with and without NO2 . The decrement in the real part of the impedance (Z ) with the rise in frequency suggests that the AC conductivity increases with increasing frequency and attains a constant value at higher frequencies. Since Z strongly varies for low frequency values, this could be attributed to the reduction of the resistive grain boundaries in this frequency region [24]. The stabilization of the value of the real part of the impedance at higher frequencies suggests a possible release of the space charge and a consequent lowering of the barrier properties in the material [26]. At a fixed temperature and as compared to Z measured in air (Fig. 7A), Z increases in presence of NO2 ; this result can be ascribed to the decreased number of charge carriers subsequent to NO2 adsorption. Fig. 8 depicts the variation of the imaginary part Z of the impedance versus frequency at different temperatures in air (Fig. 8A) and in presence of 45 ppm of NO2 (Fig. 8B), respectively. It is found that Z present only one maximum for all films.

AC and DC measurements are the two ways to evaluate polarization effect. The resistance determined from AC measurements could be different from that of DC measurements [24]. The impedance spectroscopy method is widely used to characterize the electrical properties of materials and their interfaces with electronically conducting electrodes. This complex spectroscopic impedance technique enables us to separate the resistive (real part) and reactive (imaginary part) components of the electrical parameters and hence provides a clear picture of the material properties. The AC impedance spectroscopy is based on analyzing the AC response of a system submitted to a sinusoidal signal perturbation and subsequent calculation of the impedance as a function of the perturbation frequency. The frequency dependent properties of a material can be described with the complex permittivity (ε*), complex impedance (Z*), complex admittance (Y*) and dielectric loss or dissipation factor (tan ı). The real (ε , Z , Y ) and imaginary (ε , Z , Y ) parts of the complex parameters are in turn related to one another as follows: Z∗ = Z  + jZ  = ε∗ = ε + jε =

1 jC0 ε ∗ ω Z  ωC0

(Z  2

+ Z  2 )

(13) +j

Z ωC0

(Z  2

(14)

+ Z  2 )

Y ∗ = Y  + jY  = jωC0 ε∗

(15)

200

T=40°C T=80°C T=120°C T=160°C T=200°C

160

A

1000

T = 80°C T = 160°C T = 200°C

45 ppm NO2

800

600

I Z''(Ω) I

I Z''(Ω) I

120

(16)

80

400

40

200

0

0

0

500

1000

1500

Frequency (Hz)

2000

2500

B

0

500

1000

1500

2000

Frequency (Hz)

Fig. 8. (A) Variation of the imaginary part of the impedance (Z ) of the ITO film versus frequency at different temperatures in air, and (B) in presence of 45 ppm of NO2 .

1.5x10

4

1.2x10

4

9.0x10

3

6.0x10

3

3.0x10

3

1,2x10

T = 40°C T = 80°C T =120°C T = 160°C T = 200°C

A

5

9,0x10

4

6,0x10

4

in air

3,0x10

T=40°C T=80°C T=120°C T=160°C T=200°C

B

in air

4

0,0

0.0

0

0

500

1000

1500

2000

2500

50

T = 40°C T = 80°C T = 160°C T = 200°C

40

C

σac(Ω.m)

-1

30

in air

20 10

500

1000

1500

2000

2500

Frequency (Hz)

Frequency (Hz)

tang (δ)

247

ε''

ε'

I. Madhi et al. / Applied Surface Science 355 (2015) 242–249

6.0x10

-4

4.0x10

-4

2.0x10

-4

T = 40°C T = 80°C T = 160°C T = 200°C

D

0

in air 0

500

1000

1500

2000

Frequency (Hz)

0.0 0

400

800

1200

1600

Frequency (Hz)

Fig. 9. (A) real and (B) imaginary part of the dielectric constant of the ITO film in air, (C) dielectric loss and (D) AC conductivity versus frequency at different temperatures.

It is worth noting that Z increases with frequency until reach ing a maximum (Zmax ) at a particular frequency known as electrical relaxation frequency (ωH ), beyond this frequency Z decreases;  Zmax was found to shift toward higher frequency while varying temperature. Similar results have also been reported elsewhere [27,28].  In addition, the Zmax behavior is typical of a dipolar system involving a relaxation process [27]. The relaxation process may be due to the presence of electrons/immobile species at low temperatures and defects/vacancies at higher temperatures [29]. One can notice a systematic increase of Z in presence of NO2 at the same temperature, resulting from the presence of a polarization effect due to space charges since their electrical behavior depend in charge carriers after NO2 adsorption. Fig. 9 shows the variation of the real (A) and imaginary (B) parts of the dielectric constant of the ITO film in air, together with the with the variation of the dielectric loss (C) and the AC conductivity versus frequency at various temperatures. Fig. 10 shows the variation of the real (A) and imaginary (B) parts of the dielectric constant of the ITO film in presence of 45 ppm of NO2 , together with the variation of (C) dielectric loss and the (D) AC conductivity versus frequency at various temperatures. As observed from Figs. 9(A and B) and 10(A and B), the dielectric constant (ε and ε ) increases with increasing temperature and decreases with increasing frequency, in air or in presence of NO2 . On the other hand, both ε and ε decreases with NO2 adsorption. This behavior indicates a Debye-type dielectric dispersion [30,31]. The decrease of ε with the frequency (Figs. 9A and 10A) may be mainly due to the decrease of the number of dipoles and bond energies that contribute to polarization. In addition, at higher frequency, ε approaches a constant value, which probably results from a rapid polarization change. The value of ε decreases faster than ε both

in air and in presence of NO2 and becomes closer in the high frequency range. At low frequencies the dielectric constant decreases rapidly and reaches a frequency independent behavior as frequency increases (Figs. 9(A and B) and 10(A and B)). The large value of the dielectric constant at low frequencies can be attributed to the predominance effect of the grain boundary defects, oxygen vacancies, etc.; while the decrease in the dielectric constant with frequency is due to the fact that polarizability of any species existing in the material is found to show lagging behind higher frequencies [24]. Figs. 9(C and D) and 10(C and D)) show the variation of the dielectric loss factor with frequency with and without NO2 adsorption. Loss factor (tan ı) represents the energy dissipation in the dielectric system. It is considered to be caused by domain wall resonance [24]. It is observed that tan ı increases with the frequency increase. It is also noticed that tan ı decreases with increasing temperature and decreases with NO2 adsorption. Figs. 9D and 10D show the variation of the AC conductivity versus frequency with and without NO2 adsorption. The AC conductivity of a dielectric sample can be calculated using the relation [32]: ac = ε ε0 ω tan ı

(17)

where ω is the angular frequency. One may notice that the AC conductivity of the ITO film is initially high (as compared to sample placed in air) in presence of NO2 and was found to increase as the temperature increases (Figs. 9D and 10D). This may be due to the fact that NO2 adsorption initiates the defect ions and oxygen vacancies in the ITO system. As evidenced, all results exhibit the same behavior: the AC conductivity increases as the frequency increases. We can distinguish three characteristic frequency regions. At low and high frequency regions, where the conductivity is constant,

I. Madhi et al. / Applied Surface Science 355 (2015) 242–249

ε'

248

3

3x10

3

2x10

3

1x10

3

A

45 ppm NO2

5x10

4

4x10

4

3x10

4

2x10

4

1x10

4

ε''

4x10

T=80°C T=160°C T=200°C

B

45 ppm NO2

0

0 0

500

1000

1500

0

2000

4x10

2

3x10

2

1x10

C -4

2000

D

-1

45 ppm NO2

2

1500

1.2x10

σac(Ω.m)

2x10

1000

T = 80°C T = 160°C T = 200°C

-4

1.6x10

T = 80°C T = 160°C T = 200°C

500

Frequency (Hz)

Frequency (Hz)

tang (δ)

T = 80°C T = 160°C T = 200°C

2

45 ppm NO2

-5

8.0x10

-5

4.0x10 0

0.0 0

500

1000

1500

0

2000

500

1000

1500

2000

Frequency (Hz)

Frequency (Hz)

Fig. 10. (A) real and (B) imaginary part of the dielectric constant of the ITO film in presence of NO2 , (C) dielectric loss and (D) AC conductivity versus frequency at different temperatures.

charge carrier transport takes place on infinite paths. The electrical conduction mechanism can be explained in terms of the electron hopping model [25]. Indeed, for the frequency region where the conductivity increases, the transport phenomenon is dominated by charge carrier hopping in finite clusters. The frequency at which the AC conductivity begins increasing is known as hopping frequency. The impedance spectra can also been employed to evaluate the relaxation time ( ) of the electrical phenomena in the material. The relaxation time has been evaluated according to: 2fH = 1

(18)

130

in presence of 45ppm NO2

125

125

120

120

-5

115

110

without NO2

105

-5

115

110

τ x 10 (s)

τ x 10 (s)

130

105

100 100

20

40

60

80

100

120

140

160

180

200

Temperature (°C) Fig. 11. Variation of the relaxation time versus temperature in air and in presence of 45 ppm of NO2 .

 where fH is the frequency at Zmax . It is evident that the value of changes as a function of temperature and NO2 adsorption (Fig. 11). The steady increase/decrease of indicates the presence of electrical relaxation in the material [27].

4. Conclusions In this work, ITO films were prepared onto glass substrate using the screen printing technique. Preliminary XRD studies show the formation of well crystallized ITO phase in the cubic crystal system. The crystallite size (D) and the microstrain (εstr ) were evaluated using Scherrer formula and Williamson–Hall analysis. The morphological studies revealed a granular, dense and homogeneous surface. The nature of the variation of DC conductivity with temperature is due to the presence of three activation energies, which indicate the presence of different scattering mechanisms essentially dominated by oxygen adsorption and thermal excitation of electrons in the conduction band. Detailed studies of the dielectric parameters (i.e., ε* and tan ı) as a function of temperature and NO2 adsorption at a range of frequencies reveal that the values of these parameters are strongly dependent on temperature and NO2 adsorption. The nature of the variation of the AC conductivity reveals that NO2 adsorption initiates defect ions and oxygen vacancies in the ITO system. The electrical conduction mechanism was explained in terms of the electron hopping model. The calculated value of the relaxation time ( ) of the electrical phenomena of the material changes as a function of temperature and NO2 adsorption.

I. Madhi et al. / Applied Surface Science 355 (2015) 242–249

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