NH3 sensor based SILAR coated undoped and aluminium doped

NH3 sensor based SILAR coated undoped and aluminium

doped ZnO thin films

K. Radhi Devi1, G. Selvan2, M. Karunakaran3*, K. Kasirajan3 1 PG and Research Department of Physic, Sethupathy Govt. Arts College, Ramanathapuram, India

2Department of Physic, Thanthai Hans Roever College, Perambalur, India

3PG and Research Department of Physics, Alagappa Govt Arts College, Karaikudi - 630003, India

[email protected]

Abstract: In this study undoped and Al doped ZnO thin films were coated on

SILAR method. The coated films were characterised their structural, surface

morphological, optical, PL and gas sensor using XRD, FE-SEM, UV-Vis

spectroscopy, Photoluminescence and homemade gas sensor setup. The XRD

results revealed that Wurtzite crystal structure matched with the JCPDS card No.

36-1451 and crystallite size was found to be 56 and 42 nm. Field emission

scanning electron microscopy conforms, spherical morphology of the prepared

films. Al-ZnO thin films-based sensors displayed improved response to NH3.

Keywords: TCO, SILAR, Aluminium, NH3, gas sensor.

1. INTRODUCTION

Significant research efforts have been dedicated to transparent conductive

oxide (TCO) thin films of metal oxide material with greater attention in the field

of emerging electronic devices. In recent years, ZnO Thin Films have become

relevant in new technologies and show a wide range of scientific and technical

applications, with their specific physical and chemical features that make them

more fascinating [1-4]. ZnO thin film is used in producing solar cells, gas sensors,

and catalysts. ZnO is a very promising replacement in flat display screens, in the

form of thin-film [5]. Semiconductor gas sensors are extensively used for the

detection of toxic or inflammable gases based on metal oxides. Metal oxides such

as Sb2O3, SnO2, ZnO and Fe2O3 etc., are potential candidates for the development

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Compliance Engineering Journal

Volume 11, Issue 1, 2020

of portable and inexpensive gas sensing devices with the advantages of simplicity,

high sensitivity, and rapid response. In recent years there has been a growing

interest in developing gas sensors for various polluting and harmful gases, such as

CO, NH3, CO2, NOx and H2S for environmental protection and the increased

demand for hazardous gasses monitoring in factories and homes [6-10]. Zinc

oxide (ZnO) has gained a great deal of attention both in the fundamental research

and various system applications, as a low-cost simple II-VI functional

semiconductor material. Zinc oxide has a wide band gap of 3.3 eV, low resistance

and high transparency in the visible range and high light trapping characteristics

[11, 12]. The incorporation of impurities to alter ZnO properties is another

important issue currently facing possible applications in numerous modern

technological gadgets. To satisfy the demands of several application fields, ZnO

can be doped with a wide variety of ions. The development of ZnO films has been

based on traditional dopant elements (F, B, Mn, Cu, Al, Ga, Ag, In, Sn,

etc.).Selective elemental doping in ZnO has been shown to be an effective method

of adaptation to their electrical, optical, and magnetic properties, which is crucial

for their practical applications. Al-doped ZnO thin films (AZO) have recently

attracted a great deal of attention due to their low cost material, stability, and non-

toxicity [13-16].

The various methods such as sputtering [17], RF magnetron sputtering

[18], chemical vapour deposition [19], sol-gel [20], spray pyrolysis technique [21],

and SILAR [22] are employed to deposit the ZnO film. Most of these deposition

processes like physical or chemical vapour deposition require high-temperature

films that do not always match the manufacturing technology for lift-offs. Process

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complexity needs to be overcome and production costs are low. The method, such

as the SILAR process, is not only inexpensive but also a simple and smooth

process for synthesizing large-area thin film of any material that is produced at

room temperature. It allows excellent compositional control, a high degree of

homogeneity at the molecular level and a low temperature of crystallization [23].

In previous studies, it has been proved that the sample-based on ‘Al’ source

showed a higher response, selectivity and short response/recovery time than the

remaining samples. Al- ZnO showed fast response times recovery times for NH3

gas concentrations of (100 ppm -200 ppm), respectively Fatma Ozutok [24]

reported. However, in the current research work, Al-doped ZnO thin films were

developed at a lower annealing temperature of 400 0C and have achieved

extensive results. The effect of the ‘Al’ doping level on film morphology,

structure, and gas sensing properties were investigated by X-ray diffraction

(XRD), scanning electron microscopy (SEM), UV-visible spectroscopy (UV),

Photoluminescence spectroscopy (PL), and gas sensing measurements.

2. EXPERIMENTAL DETAILS

2.1 Materials and Solvents

Analytical grade host materials of Zinc sulphate [ZnSO4], Sodium

Hydroxide [NaOH] and dopant precursor aluminium chloride (AlCl3·6H2O) were

purchased from Sigma-Aldrich and Alfa Aeser respectively. Distilled water was

used throughout the process.

2.2 Precursor and film preparation

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undoped zinc oxide films were deposited on glass substrates (microscope

slides) by alternatively dipping into 0.1 M sodium zincate [(Na2) ZnO2] bath kept

at room temperature and hot water maintained near the boiling point. The glass

substrate was cleaned, before deposition, by chromic acid followed by distilled

water rinse and ultrasonic cleaning with acetone and alcohol. The cleaned

substrate was tightly held in a holder so that only a requisite area for film

deposition was exposed. Thus, the area for film deposition could be easily varied

by adjusting the holder arrangement. Briefly, a precleaned substrate was

alternatively dipped in zinc complex solution (sodium zincate bath) kept at room

temperature and hot water bath maintained at ∼95–98C. One set of dipping

involves dipping in zincate bath for 20s and dipping in hot water bath for 20 s.

Fifty dipping’s were performed for this experiment. All the deposited films were

white and homogeneous. The ZnO films on the substrate are formed according to

the relation,

ZnSO4 + 2 NaOH → Na2ZnO2 + H2SO4

Na2ZnO2 + H2O → ZnO + 2 NaOH

Al-doped ZnO thin films were prepared by adding aluminium chloride

(Al’5 wt. %) in the sodium zincate bath. All the prepared films were dried in air

and then annealed at 300° C for about 2 hrs.

3. RESULT AND DISCUSSION

3.1 Structural analysis

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The XRD spectra of undoped and Al doped ZnO films are presented in

figure 1. The diffraction angle 2θ was scanned in the range 200 –800. The observed

XRD diffraction peaks were connected to the lattice planes (1 0 0), (0 0 2), (1 0 1),

(1 0 2), (1 0 3), (110), (103), (1 1 2) and (2 0 1), according to JCPDS card No. 36-

1451.All the samples are polycrystalline with a ZnO hexagonal Wurtzite structure.

There is no secondary of aluminium was observed. The intensity of (002) peak is

relatively higher than that of the other peaks i.e. (100) and (101) as the

concentration of ‘Al’ doping. In addition, no significant differences were observed

for undoped and Al-doped ZnO thin films with the exception of the peak position

of the (002) and (101) which are slightly shifts due to substitution of zinc ions by

aluminum ions into the hexagonal lattice. The sharp peaks and high intensity

reflect that the synthesized Al-doped ZnO thin films are well crystalline. This

shows the change in the films preferential growth along the c-axis. We assumed

that these growth behaviors came from the deformation of the lattice, which was

caused mainly by the replacement of the ‘Al’ atoms for the Zn sites in the ZnO

structure. Furthermore, when Al doping is higher, most ‘Al’ atoms do not replace

Zn atoms positions, they may occupy interstitial positions, causing severe

deformation of the lattice and degrading the crystallinity of the films [25].The

crystallite size of all the films deposited is measured using the well-known

Scherrer equation [22].

cos9.0

D --------------- (1)

where k is the shape factor, λ is the wavelength of incident X-ray, β is the FWHM

measured in radians and θ is the Bragg angle of diffraction peak.

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Figure 1. XRD spectra

The calculated crystallite size, dislocation density, strain, and lattice distortion of

crystal parameters are presented in Table 1. The crystallite size of the undoped

ZnO and Al-ZnO thin films is found 56 nm and 42 nm. The decrease in crystallite

size is due to the distortion in a host ZnO lattice by the addition of foreign

impurities and the presence of Al+ into the host lattice sites increases the

nucleation and subsequent growth rate of ZnO films.

The dislocation density of the prepared ZnO thin films was calculated from

the equation

21

D --------------- (2)

The substitution of Al+ ions in the interstitial position of host ZnO lattice

sites affects the concentration of the interstitial Zn, oxygen vacancies. This

observation of the small changes in 2 values and peak broadening of diffraction

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of diffraction peaks are due to decrease of micro-strain. The micro-strain is

calculated using the formula

4

cos --------------- (3)

When compared to pure ZnO sample, the micro-strain values decreases for

higher concentration of ‘Al’ doped thin films due to the relaxation of strain in the

respective unit cells which changes the size and shape of the particles. The Lattice

distortion (L.D) is calculated from using the formula

---------------(4)

Table 1: Structural parameters

Film Thickness (t)

(nm) Crystallite size

(nm)

Dislocation density ×1015

lines/m2

Strain ×10-3

Lattice Distortion ×103

ZnO

520

56.23

0.3162

0.6435

0.1218

Al-ZnO

480

42.16

0.5624

0.8582

0.162

3.2 SEM analysis

The surface morphology of undoped ZnO and Al-ZnO thin films is

characterized by SEM and presented in Figure 2. Spherical morphology with

bigger grains was observed for undoped ZnO (figure 2 (a)). However Al doped

ZnO, the spherical morphology (figure 2 (b)) with smaller grains are agglomerated

and some voids are appeared due to an incorporation of Al ions in Zn lattice sites

could influence the changes. The trend of the reduction of grain size of thin films

matches with the observations of XRD spectrum. This suggested that the

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incorporation of ‘Al’ take place by substitution and shrinkage in the lattice

structure takes place which in turn reduces the particle size.

Figure 2. SEM images (a) ZnO and (b) Al-ZnO

3.3 Photoluminescence spectrum

Figure 3 displays the PL emission spectrum of undoped and Al-ZnO thin

films using an excitation wavelength of 380 nm. The photoluminescence (PL)

emission is observed for all the samples covering from a short wavelength of 350

nm to a long wavelength of 600 nm. The emission spectra consist of UV emission

peaks 361, 393, 410, 437, 460,492, 520 and 538 nm. We measured the excitation

spectrum and emission spectrum of the undoped ZnO and Al-ZnO thin films; it

was observed that a strong broad peak was centered at 393 nm in the emission

spectrum, as shown in figure 3. The strong violet emission peak 393 occurs due to

the Zn interstitials. The characteristic of the peak is due to the strong violet-shift.

The peaks at 410, 460, and 492 nm corresponding to the blue emissions, while the

peak that arose at 520 nm represents green emissions. The luminescence

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properties are mainly decided by size of the particles. In that case, the strong

photoluminescence is attributed to the direct excitonic transition as the hexagonal

ZnO turns to a direct band gap semiconductor. Most of the reports about emission

properties of ZnO showed a higher intensity of UV or green emission compared to

the blue emissions. However the figure 3 shows that the violet emission intensity

is higher than UV and green emission. The violet emission at 393 nm is probably

due to the radiative defects of Zni and VZn related to the interface traps existing at

the grains boundaries of ZnO.The variation of emission properties can be clearly

seen from the figure for increased concentration of Al in ZnO. While increasing in

the doping concentration, the violet emissions are blue-shifted and intensity of the

peak also decreases when compared with pure ZnO. These shifts are occurring due

to the dopant of Al+ ions substituted in Zn2+ ions.

Figure3. Photoluminescence spectra

3.4 UV-visible spectroscopy

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Figure 4 (a) shows the room temperature UV–visible transmittance spectra

of undoped ZnO and Al-ZnO thin films respectively. The spectrum exhibits an

intense optical transmission in the region between 350 to 800 nm. Results shows

that undoped film has ~ 50 % transparency was obtained, Al-ZnO thin film

showed higher transmittance (~ 60 %) in the visible region. This rise in

transmittance can be attributed to the well-crystallization of films. High

transparency is related to strong systemic homogeneity and crystallinity.

Figure 4 (a): UV–visible transmittance spectra

From the transmission curve as a function of the wavelength we can

represent the variation of (αhν)2 with (hν) for calculated the band gap Eg from the

following equation

)E- A(h= ngh

(4)

Where hν is the energy of incident photons, Eg the optical gap and A is a constant.

Figure 4(b) shows the energy gaps for undoped ZnO and aluminium doped films.

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An obvious increase is observed for the values of the energy gap with the increase

in the concentration of aluminum. The obtained optical band gap a value is 3.20

eV and 3.22 eV for undoped and aluminium doped ZnO films respectively. This

increase is explained by the preposition that the Al-ZnO films are semiconductors

in which the Fermi level lies in the conducive band which means that the levels at

the bottom of the conductivity band are occupied by electrons and the shielding of

electronic traveling to these levels is termed the Burstein–Moss effect.

Figure 4: (b) Tauc plot

3.5 Gas Sensor analysis

The gas sensing analysis was performed using PC and Keithley

electrometer interconnected homemade gas sensing setup. The gas responses of

the prepared samples were tested at different NH3 concentrations (100 ppm – 200

ppm step of 25 ppm) and at a room temperature, are reported in Figure 5. Al

doped ZnO with film allows a remarkable enhancement of the sensing

characteristics toward NH3. Furthermore, the high response of these sensors can

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also be observed upon exposure to NH3 concentrations as low as 100 ppm. This

ammonia vapour sensing mechanism of thin film is as follows.

O2(atmosphere) + e− (ZnO surface)→O2

− (ZnO surface)(8)

4NH3 + 3O2−

(ad)→2N2 + 6H2O + 6e−(9)

Figure 5: NH3 gas response against various concentrations

Table 2: Structural parameters

Film Response time (sec)

Recovery time (sec)

Sensitivity

ZnO

60 18 680

Al-ZnO

38 12 8050

When the NH3 gas expose on the gas sensor (thin films) the change in

resistance was observed. From the variations in resistivity, we can estimate the

responses and recovery time of the undoped and Al doped ZnO.

Sensitivity �= Ra / Rg, here Ra and Rg stand for strengths of air and gas.

The response, recovery time and sensitivity are listed in table 2. From the sensor

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studies Al-ZnO had faster response and recovery times. The decrease of recovery

time attributes the distribution of more spherical particles with smaller grains.

4. CONCLUSION

Undoped and aluminium doped ZnO thin film were successfully

synthesised by SILAR method. The structural parameters were calculated form

XRD analysis. SEM imges conforms smaller grain size were observed at Al

doping. The high transmittance and 3.22 eV band gap was achieved for Al doping.

The gas sensing studies were carried out, and Al-ZnO thin films-based sensors

displayed improved response to NH3 in addition to rapid response-recovery times

under both dry and humid conditions compared to undoped ZnO NPs-based sensor

at an optimal room temperature.

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NH3 sensor based SILAR coated undoped and aluminium doped