Title Goes Here

Highly Transparent Amorphous Tin Oxide Thin Films By Sol-Gel Spin Coating Technique

Soumya.S.S

Department of Physics, University College, Thiruvananthapuram

soumyadevanss@gmail.com

Abstract. This paper reports the preparation and characterization of amorphous tin oxide thin films by sol-gel spin coating technique. The structural properties were analyzed by using XRD, FESEM and EDAX measurements. XRD analysis reveals that the as prepared tin oxide thin films were amorphous in nature. No crystalline peaks corresponding to the tetragonal rutile structure of tin oxide were present in it. FESEM analysis reveals that the particles were uniformly distributed in the tin oxide thin films. EDAX analysis shows that the as prepared tin oxide thin films contain tin and oxygen. Optical study was done by using UV-Vis Spectrophotometer and Photoluminescence spectroscopy. Transmittance spectra show that as prepared tin oxide thin film has highly transparent up to 85%. The optical band gap was calculated using Tauc plot and the value of optical band gap was obtained as 3.64eV. PL study shows an intense and broad blue asymmetric emission band centered at 389nm. Electrical study was done by using Keithley’s four probe techniques. The resistivity was obtained as 1.21x10-3Ωm. The Figure of Merit value was calculated using Haacke formula and the obtained value was 2.2139x105Ω-1. Thus good quality amorphous tin oxide thin films were successfully prepared by sol-gel spin coating technique.

INTRODUCTION

Transparent conducting oxides received considerable attention in last decades due to their vast applications in nanotechnology. A transparent conducting oxide is a wide band gap semiconductor with relatively high concentration of free electrons in its conduction band. Tin oxide is an n-type semiconductor and it is a transparent conducting oxide material with wide band gap of ~3.8eV. Its high optical transparency, electrical conductivity and chemical stability make it as a very attractive material for solar cells, heat mirrors, catalysis, dye sensitizing purposes and gas sensing applications1. Tin oxide thin films are the most attractive films that are used in automotive sectors as an anticorrosive surface treatment of a carbonaceous bipolar plate in proton exchange membrane fuel cells2. Tin oxide has so many applications in the field of flat panel displays, window layers, optoelectronic devices3,4, lithium ion batteries5, thin film transistors6, Spintronics, Photonic device applications7, humidity sensors8,energy storage applications9, etc.

Several techniques were available for the synthesis of tin oxide nano particles as thin films. Co-precipitation method, Ball milling method, Solvothermal method, Pulsed laser deposition method10, Reactive co-sputtering6,wet chemical process11,Thermal oxidation method2,Sol-gel Dip coating technique12,13,14,Sol-gel Spin coating technique15,16,17, Hydrothermal method, Chemical vapour deposition18,etc. Sol-gel method is a chemical method. This method was cheap and uniformly coated films were obtained. In this paper sol-gel spin coating method was used for the synthesis of amorphous tin oxide thin films by using Spin NXG-P1A spin coating unit.

EXPERIMENT TO SYNTHESIS TIN OXIDE THIN FILMS

The starting materials were SnCl2.2H2O (Purified by Merck) and absolute ethanol. 1.128gm of SnCl2.2H2O was dissolved in 10ml of absolute ethanol and the solution was stirred for 1hr at 323K using a magnetic stirrer to get a homogenous solution. The resulting solution was kept at 24hrs under room temperature for gelation. This solution was coated on the pre-cleaned glass plate using a spin coating machine (SpinNXG-P1A). The spinning was done at 1500rpm for 30s to get a uniform coating of the tin oxide film on the surface of the glass substrate. The coated glass plates were heated at 1000C for 15minutes using a hot plate to remove the volatile solvents. Thus the required SnO2 thin films were obtained.

Structural, optical and electrical characterizations of as prepared tin oxide thin films were performed. The structure of the tin oxide thin film was characterized by XRD measurements in 2θ angles from 10 - 800 using CuKα radiation (BRUKER D8 ADVANCE). Surface morphology of the tin oxide thin films was studied using Field Emission Scanning Electron Microscope-FESEM (NOVA NANOSEM 450) analysis. The elemental composition was done by using Energy Dispersive X-ray analysis (EDAX METEK Materials Analysis Division). The thickness of the tin oxide thin film was measured using Dektek Stylus profilometer. Optical studies of all the films were recorded using a UV – Vis Double beam spectrophotometer (JASCO V-730) in the spectral range of 300-900nm. Photoluminescence spectroscopy was done by using HORIBA FL3-211 spectroflurometer. Electrical study was done by using Keithley’s four probe apparatus (6220 Precision current source and 2182A Nanovoltmeter).

RESULTS AND DISCUSSIONSX-ray diffraction

Figure.1 shows the XRD spectrum of tin oxide thin films prepared by sol-gel spin coating method. The 2θ values were measured from 10- 800. XRD spectrum shows broad amorphous peak related to the glass substrate19. Figure.1. shows that the films were amorphous in nature because no crystalline peaks corresponding to the tetragonal rutile structure of the tin oxide nano particles were found here. Amorphous tin oxide thin films were used for thin film transistor applications19.

FIGURE 1.XRD spectra of as prepared tin oxide thin film

FESEM Analysis

Figure.2 shows the FESEM picture of tin oxide thin films at high magnification. It shows that the as prepared tin oxide thin films forms an agglomerated structure. The morphology of amorphous SnO2 is fine particles smaller than 1 µm in diameter20. The particles shown in Figure.2 were fine with particle size in the order of nanometers. So the films obtained were amorphous in nature. FESEM picture was in agreement with the result obtained from XRD analysis.

FIGURE 2.FESEM micrograph of as prepared tin oxide thin film

EDAX Analysis

The compositional and quantitative analysis was done by using Energy Dispersive X-ray Spectrophotometer. The spectrum was shown in Figure.3 and it reveals the presence of tin and oxygen in the sample. The spectrum confirms the formation of tin oxide thin films. The mapping of tin oxide, tin and oxygen in the as prepared tin oxide thin films were shown in Figure.4. Table.1 shows the atomic percentage of tin and oxygen present in the as prepared tin oxide thin films. It shows that the atomic percentage of tin and oxygen present in the as prepared tin oxide thin

FIGURE 3.EDAX spectra of as prepared tin oxide thin film

(a) (b) (c)

FIGURE 4.Mapping of (a) tin oxide, (b) oxygen and (c) tin in the as prepared tin oxide thin film

films were 29.58% and 70.42% respectively.

TABLE 1.Atomic percentage of tin and oxygen in the as prepared tin oxide thin film

Element

Atomic%

Tin

29.58

Oxygen

70.42

UV-Vis Spectroscopy

UV-Vis study gives the transmittance as well as absorbance spectra of as prepared tin oxide thin films. Figure.5 (a) and (b) shows the transmittance spectra and absorbance spectra of as prepared tin oxide thin films. The transmittance of as prepared tin oxide films were high and its average transparency was 85%. The highest value of transmittance was 87% at a wavelength of 700nm. The absorbance spectra shows that the as prepared tin oxide thin films have low absorbance in the visible region. The lowest value absorbance was at 700nm. The as prepared tin oxide thin films have high transmittance in the visible region.

The optical band gap of as prepared tin oxide thin films were calculated using Tauc plot by the realtion21

(αhν) = A(hν – Eg)n(1)

Where α is the absorption coefficient, h is the Planck’s constant, A is a constant, ν is the transition frequency, Eg is the band gap corresponding to a particular transition occurring in the film and n gives transition type. n=1/2 for direct allowed, n=2 for indirect allowed, n=3/2 for direct forbidden and n=3 for indirect forbidden. The absorption coefficient α can be calculated using the equation

α = 2.303A/t (2)

Where A is the absorbance and t is the thickness of the as prepared film. The thickness of the as prepared tin oxide thin film was measured using Dektek Stylus Profilometer and was obtained as 250nm.

FIGURE 5.(a)Transmittance and (b) absorbance spectra of as prepared tin oxide thin film

The optical band gap was calculated and the value of optical band gap of as prepared tin oxide thin film was obtained as 3.64eV and is shown in the Figure.6. This was comparable with the previously reported band gap values of tin oxides22,23,24.The optical conductivity of as prepared tin oxide thin films was calculated using the formula25

σ = αnc/4π (3)

Where n is the refractive index of tin oxide and c is the velocity of light. The calculated values of optical conductivity were plotted against photon energy and were shown in the Figure.7. The optical conductivity of as prepared tin oxide thin films increases sharply around 3.5eV due to high absorbance of SnO2 thin films reported by N.B.Ibrahim etat25. The optical conductivity of as prepared tin oxide thin films increases sharply at 3.6eV in this work.

FIGURE 6.The optical Band gap of as prepared tin oxide FIGURE 7.Optical conductivity versus photon energy

thin film using Tauc Plot of as prepared tin oxide thin film

Photoluminescence study

Photoluminescence (PL) spectroscopy has been proven to be a sensitive technique for providing valuable information of the crystal defects, surface defects and exciton fine structures in the nanostructures26. The room temperature photoluminescence (PL) spectroscopy technique was a selective and extremely sensitive probe of discrete electronic states27. Figure.8 Show the room temperature photoluminescence spectra of as prepared tin oxide thin film excited at 325nm. The spectra were recorded from 350nm- 600nm Wavelength range. The film show an intense and broad blue asymmetric emission band centered at 389nm and extend up to 500nm. This 389 nm near band edge (NBE) emission was due to the recombination process between trapped charges from the valence band and the electrons from the conduction band26. A weak emission at 535nm was observed in the photoluminescence spectrum of as prepared tin oxide thin films. Since the position of the emission maxima were lower than the respective band gap of tin oxide, this emission cannot be attributed to the direct recombination of a conduction electron in the Sn 4p band and a hole in the O 2p valence band. Therefore, this emission may be attributed to the different luminescent centers such as defect energy levels arising due to the tin interstitials and the oxygen vacancies as well as dangling bonds in the nano crystals28.

FIGURE 8.Photoluminescence spectra of as prepared tin oxide thin film

Electrical properties

Electrical study was done by using Keithley’s four probe technique. The outer two leads measures the current and the inner two leads measures the voltage. The shape of I-V curve was almost linear and shows Ohmic behavior and is shown in Figure.9. The sheet resistances was calculated using the equation29

Rs = (4)

The calculated value of sheet resistance was 4.85KΩ. The resistivity of the film was calculated using the relation29

ρ = Rs t (5)

Where’t’ was the thickness of the film. The calculated value of resistivity was 1.21x10-3Ωm. The resistivity was lower than the value obtained by O.Oluwaleye etal30. The conductivity of the as prepared tin oxide thin film was calculated and obtained as 0.826x103Ω-1m-1. This was higher than the value obtained by A.M.EI Sayed etal16.

The Figure of merit (FOM) values were calculated using Haacke’s relation31,32

φ = T10/Rs (6)

Where T is the transmittance at 550nm and Rs is the sheet resistance. This formula gives more weight to the transparency and thus was better adapted to solar cell technology. It was clear that the figure of merit was dependent on the sheet resistance32. The value of transmittance at 550nm was 80%. The calculated value of FOM was 2.2139x10-5Ω-1. The FOM values obtained in the present study was higher than the value of FOM obtained in the study of Eyüp Fahri Keskenler etal32. The as prepared tin oxide thin films prepared and studied in the present work was of good quality and is a promising candidate for solar cell applications.

FIGURE 9.I-V Characteristics of as prepared tin oxide thin film

CONCLUSIONS

Tin oxide thin films were prepared successfully by sol-gel spin coating technique. XRD and FESEM analysis were shown that the as prepared tin oxide thin films possess amorphous nature. EDAX analysis confirms the formation tin oxide in the prepared thin films. UV-Vis spectra study reveals that the films were highly transparent and the average transparency was 85%. Band gap value of the as prepared tin oxide thin film was 3.64eV. The optical conductivity of the as prepared tin oxide thin films increases sharply at 3.6eV. The Photoluminescence spectrum shows a broad emission peak at 389nm. Electrical study shows that the resistivity of the film was very low and the calculated value of resistivity was 1.21x10-3Ωm and the conductivity was 0.826x103Ω-1m-1. So as prepared tin oxide thin films are good for transparent conducting oxide applications. The FOM value of the as prepared tin oxide thin film was 2.2139x10-5Ω-1. So the as prepared tin oxide thin film has good quality and are used for the promising candidate for solar cell and optoelectronic applications.

ACKNOWLEDGEMENTS

Author acknowledges RUSA for funding to Spin coating unit and UV-Vis Spectrophotometer.

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400500600700800900

40

50

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90

Transmittance(%)

Wavelength(nm)

(a)

400500600700800900

0.05

0.10

0.15

0.20

0.25

0.30

Absorbance(a.u)

Wavelength(nm)

(b)

1.52.02.53.03.54.0

0.00E+000

2.00E+013

4.00E+013

6.00E+013

8.00E+013

(

h

)

2

(

cm

-1

eV

)

2

h(eV)

SnO

2

: E

g

= 3.64eV

1.52.02.53.03.54.0

2.00E+013

4.00E+013

6.00E+013

8.00E+013

1.00E+014

1.20E+014

Optical conductivity

(

s

-1

)

Photon energy(eV)

350400450500550600

60000

90000

120000

150000

180000

210000

240000

Intensity(a.u)

Wavelength(nm)

0.00.51.01.52.02.53.03.54.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Current(mA)

Voltage(V)

1020304050607080

100

200

300

400

Intensity(a.u)

2 (degree)