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Nov 5, 2009 - used in various optoelectronic applications and solar energy conversion ... Figure 3 shows the SEM image of the film deposited at 500°C. It was ...

Indian J. Phys., Vol. 85, No. 4, pp 551-558 (2011)

Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method

P Saikia1, A Borthakur2 and P K Saikia1* 1

Thin Film Laboratory, Dibrugarh University, Dibrugarh- 786 004, Assam, India 2

Department of Physics, Tezpur University, Tezpur - 784 028, Assam, India E-mail : [email protected] Received 05 November 2009, accepted 16 June 2010

Abstract : Tin oxide (SnO2) thin films have been grown on glass substrates using atmospheric pressure chemical vapour deposition (APCVD) method. During the deposition, the substrate temperature was kept at 400°C–500°C. The structural properties, surface morphology and chemical composition of the deposited film were studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and Rutherford back scattering (RBS) spectrum. XRD pattern showed that the preferred orientation was (110) having tetragonal structure. The optical properties of the films were studied by measuring the transmittance, absorbance and reflectance spectra between O = 254 nm to 1400 nm and the optical constants were calculated. Typical SnO2 film transmits ~ 94% of visible light. The electrical properties of the films were studied using four-probe method and Hall-voltage measurement experiment. The films showed room temperature conductivity in the range 1.08 × 102 to 1.69 × 102 :-1cm-1. Keywords : Tin oxde, APCVD, XRD, RBS, Optical property, Electrical property PACS Nos. : 73.61.-r, 78.66.-w

1. Introduction Tin oxide (SnO2) thin film is an important n-type semiconducting material, which has shown low electrical resistance and high optical transparency due to which it can be well used in various optoelectronic applications and solar energy conversion [1,2]. The transparency and conductivity of these oxides are assumed to be caused by deviations from stoichiometry, doping and by the microstructure of the films [3]. A number of techniques are employed in the formation of high quality SnO2 thin films such as CVD [4-6], electron beam evaporation [7], spray pyrolysis [7-9], spin-coating method [10], filtered *Corresponding Author :

© 2011 IACS

552

P Saikia, A Borthakur and P K Saikia

arc vacuum deposition [11, 12], etc. The properties of the deposited films are strongly dependent on the deposition techniques employed. In this present work APCVD method is used for deposition of the SnO2 thin films and the structural, optical and electrical properties of the films deposited at different substrate temperatures are reported. 2. Experimental details APCVD method is simple and inexpensive method for preparation of good quality thin SnO2 films. SnO2 films were deposited on glass substrates by this method in an open tube system. The schematic diagram of the experimental set up is shown in Figure 1.

Figure 1. Experimental set-up for the deposition of SnO2 films by reaction with stannous chloride vapour and oxygen by APCVD method.

In this method films were grown by evaporating SnCl2 (supplied by Loba Chemie Pvt. Ltd) and allowed the vapour to react with oxygen inside the oven (Microprocessor Controlled Furnace, Mfg. Optochem International, New Delhi) at high temperature. SnO2 thin films were deposited on the cleaned glass substrates varying the oven temperature between 400°C to 500°C. The chemical reaction taken place during the process is as follows:

SnCl2  O2

o SnO2  Cl2 (n)

It was found that the properties of the films varied with the variation of temperature inside the oven and flow rate of the oxygen. For good quality films the flow rate of the oxygen must be within the range 1½-2 liter per minute. The thickness of the films was measured using the gravimetric method. The accuracy of this method is ± 20 Å. The thicknesses of the films were in the range between 157.2 to 284.8 nm. The structural, surface morphology and chemical compositions of the films were studied by XRD, SEM and RBS spectrum. The optical transmittance, absorbance and reflectance spectra were recorded using a U-4100 Spectrophotometer in the wavelength range 350 nm to 950 nm. The energy band gap, conductivity, mobility and density of charge carriers were studied using four-probe method and Hall-voltage measurement experiments.

Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method

553

3. Result and discussion

3.1 Structural properties : The XRD pattern of the film deposited at substrate temperature 500°C was recorded with the help of Rigaku Mini (Flex-2005G303) X-ray diffractometer using CuKD radiation. Figure 2 shows the XRD pattern for deposited film at substrate temperature 500°C. The XRD pattern of the film was taken within the range of Bragg angle 2T¸ from 10° to 70°. The XRD pattern of SnO2 thin film was compared with the standard diffraction data card. It was found that the SnO2 film had tetragonal structure and showed strong orientation in (110) plane together with other planes (101), (200), (211), (310) and (301). The lattice constants of the films were a = 4.7873 Å and c = 3.2063 Å (calculated from XRD pattern). 250

(110)

200 (101)

(200)

(211)

Intensity

150

(301) (310)

100 50 0 10

20

30

40

50

60

70

Bragg angle (2T)

Figure 2. XRD pattern of the SnO2 thin film.

Figure 3 shows the SEM image of the film deposited at 500°C. It was observed that the films deposited at higher temperature (500°C) were uniform and free from macroscopic defects like cracks etc.

Figure 3. SEM image of SnO2 thin film.

554

P Saikia, A Borthakur and P K Saikia

The Rutherford back scattering (RBS) spectrum of the film is shown in the Figure 4. As we have seen in the spectra, some chlorine residue is present which is due to use of chloride salt for the deposition of SnO2 thin films. The composition of the sample was calculated as O2- 65.54 %, Sn- 29.38 % and Cl- 5.08 %. The variation of the atomic percentage of Sn and O2 and presence of Cl contaminate as residue gave the film a nonstoichiometric nature due to which the film showed a higher conductivity without introducing any doping [4, 13].

Figure 4. RBS Spectrum of the SnO2 films.

3.2 Optical properties : The transmittance, absorbance and reflectance spectra of the SnO2 thin films deposited at different substrate temperatures are shown in Figure 5, 6 and 7 (a- 500°C, b- 450°C and c- 400°C) respectively. The transmittance of the films were varied from 82% to 94%. The reflectance of the films can be obtained from the relationship

R+T+A=1

(1)

where R, T and A are the reflectance, transmittance and absorbance of the film respectively. 1.7

96

1.6 1.5 Absorbance % o

Transmission % o

94 92 90 88 86

1.4 1.3 1.2 1.1 1.0

84

0.9

82

0.8 0.7

80 200

400

600 800 1000 1200 Wavelength (nm) o

1400

Figure 5. Transmittance versus incident wavelength.

200

400

600 800 1000 1200 Wavelength (nm) o

1400

Figure 6. Absorbance versus incident wavelength.

Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method

555

The real part of the refractive index, ‘n’, which is related with the reflectance spectrum of the film which relates with band structure and multilayer structure and antireflection coatings of the semiconducting film can be calculated using the relation [14,15].

R

n  1 2 n  1 2

(2)

18 16

Reflectance % o

14 12 10 8 6 4 2 200

400

600

800

1000

1200

1400

Wavelength (nm) o

Figurer 7. Reflectance versus incident wavelength.

Figure 8 shows the variation of refractive index with incident wavelength. The refractive indexes of the films varied from 1.6 to 3.6 which had good agreement with the values as indicated by K L Chopra and et al. [3] and slightly increase of the value was due to the increase in the reflectance with the film thickness. The imaginary part of the refractive index, which is also called extinction coefficient (k), is directly related with the absorption coefficient, ‘ D ’ by the relation [14]

D

4S k

O

(3)

where O is the incident wavelength. Figure 9 shows the variation of k with the incident wavelength. The absorption coefficient, D was calculated from expression [14, 16],

T

1  R 2 e D t

(4)

where, t is the film thickness. From absorption coefficient the optical bandgap of the semiconductor thin films were calculated. For a direct gap semiconductor the energy bandgap and D is related as [16, 17],

556

P Saikia, A Borthakur and P K Saikia

D hQ



B hQ  E g



1 2

(5)

where, ' hQ ' is the incident photon energy, ‘Eg’ represents the energy bandgap and ‘B’ is

characteristics parameter. Figure 10 shows the graph between D hQ against hQ in the visible region. The value of optical bandgap Eg was calculated by extrapolating the 2

linear portion of the respective curve to D hQ = 0. The values of optical bandgap observed from the graph ranged between 2.25 to 2.29 eV. 2

3.8 0.035

3.6 Extinction coefficient, k o

Refractive index, n o

3.4 3.2 3.0 2.8 2.6 2.4 2.2 2.0 1.8

0.030 0.025 0.020 0.015 0.010 0.005

1.6 0.000 200

400

600 800 1000 1200 Wavelength (nm) o

Figure 8. Refractive index versus incident wavelength.

1400

200

400

600 800 1000 1200 1400 Wavelength (nm) o

Figure 9. Extinction coefficient versus incident wavelength.

The optical conductivity, V of the films was calculated by using the relation [16]

V

D nc 4S

(6)

where, ‘c’ is the velocity of the light. Figure 11 shows the variation of optical conductivity with incident wavelength. The variation of increase and decrease of the optical conductivity was due to increase and decrease of the absorption respectively in that region.

3.3 Electrical properties : The variation of resistivity with temperature was studied from room temperature to 125°C temperature using four-probe method. Slight variations of resistivity with temperature for all the films were observed which indicted a semiconducting behavior of the deposited films. Figure 12 shows the variation of resistivity with inverse of temperature of SnO2 films at different substrate temperatures. From these results we have calculated the activation energy of the deposited films. The calculated activation energy lies between 0.0055 eV to 0.024 eV.

Structural, optical and electrical properties of tin oxide thin film deposited by APCVD method 4.5 × 1013

1×109 1×109

4.0 × 1013 3.5 × 1013

8×108

Optical conductivity, V o

(Dhv)2 cm2 eV2 o

9×108

7×108 6×10

557

8

5×108 4×108 3×108 2×108

3.0 × 1013 2.5 × 1013 2.0 × 1013 1.5 × 1013 1.0 × 1013

1×108 0

5.0 × 1012 200

1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

400

hQ (eV) o

Figure 10. D ˜ hQ

2

versus photon energy hQ .

600 800 1000 1200 1400 Wavelength (nm) o

Figure 11. Pptical conductivity versus incident wavelength.

The room temperature conductivities of the deposited films at different substrate temperatures were studied with the help of Hall-voltage measurement experiment. Table 1 shows the various electrical properties of the SnO2 films with substrate temperature. From the Table it can be seen that the variation in conductivity increases as we increase the substrate temperature of the films, which is mainly due to the non-stoichiometry of Sn and O2 in the films for same film thickness. –2.00 –2.05

log10 Uo

–2.10 –2.15 –2.20 –2.25 –2.30 –2.35 2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3

1/T × 10–3 K o

Figure 12. The electrical resitivity, U vs inverse temperature of SnO2 films at different substrates temperatures. Table 1. Electrical properties of SnO2 thin films. Carrier mobility, P

Substrate

Room temperature

temperature(°C)

conductivity, V ( : 1 cm-1)

Density of charge carrier, K (cm-3)

(cm2 V-1s-1)

400

108.45

1.37 × 1019

49.56

450

140.73

1.92 × 1019

45.68

169.98

19

44.27

500

2.40 × 10

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P Saikia, A Borthakur and P K Saikia

4. Conclusions Transparent conducting SnO2 thin films were deposited by APCVD method with substrate temperatures ranging between 400°C to 500°C. For good quality films we have to control the flow rate of O2, substrate temperature and deposition time. Films showed ~ 94% transparency in the visible region. The electrical studies indicated that the film deposited at higher temperature (500°C) showed higher room temperature conductivity. These studies also indicated that density of charge carrier increased with increasing temperature where as carrier mobility decreased with increasing temperatures. The films also showed slightly semiconducting behavior (nearly metallic) with activation energy ~ 10–2 eV. Thus, due to these properties SnO2 thin film prepared using this method can be used as a front contact in photovoltaic cells and other opto-electronic devices. Acknowledgment We would like to thank Indian Institute of Technology, Guwahati, for careful assistance in taking Rutherford back scattering (RBS) spectrum and Mr. Ratan Barua, Tezpur University, Assam for the SEM pictures. References: [1]

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[2]

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[3]

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[4]

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