Electric Field Driven Growth of Tin Oxide Thin Films

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Energy Procedia

Energy Procedia (2011) Energy Procedia 15 00 (2012) 318000–000 – 324 www.elsevier.com/locate/procedia

International Conference on Materials for Advanced Technologies 2011, Symposium O

Electric Field Driven Growth of Tin Oxide Thin Films Shikha Bansal, Sudesh Sharma, Dinesh K. Pandya* and Subhash C. Kashyap Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi - 110016, India

Abstract This paper presents our results on the significant effect of application of electric field during deposition on various properties of thin SnO2 films, especially in the ultrathin regime. SnO2 films of 25-75 nm deposited on glass substrates without and with the presence of electric field (32 V/cm) during deposition were compared. It is found that the presence of field has improved the crystallinity and stoichiometry of the film along with its transmittance. Significantly high mobility of 43 cm2/V.s at low thickness of 25 nm resulted in a reasonably low resistivity of 1.5 ohm-cm as a consequence of the field assisted growth. ©©2011 Published by Elsevier Ltd. Selection and/or and/or peer-review under responsibility of the organizing committee 2011 Published by Elsevier Ltd. Selection peer-review under responsibility of Solar Energyof International Conference on Materials(SERIS) for Advanced Technologies. Research Institute of Singapore – National University of Singapore (NUS). Keywords: Tin oxide; electric field assisted deposition; structural properties; optical properties; electrical properties; spray pyrolysis

1. Introduction The extensive use of multifunctional transparent conducting oxide (TCO) thin films has prompted their widespread investigations [1, 2] especially in low emissivity windows, gas sensors, thin film transistors, and solar cells etc. [3-6]. Recent device applications such as touch-sensitive screens, flat panels, and OLEDs [1-4] demand ultra thin high quality transparent conducting oxide films. These films show unique properties on the nanometre scale owing to quantum size effects and high surface to volume ratio of atoms. The commonly used TCO materials encompass n-type semiconductors namely, F- or Sb-doped tin

* Corresponding author. Tel.: +91 011 26591347; fax: +91 11 26581114 E-mail address: [email protected].

1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of International Conference on Materials for Advanced Technologies. doi:10.1016/j.egypro.2012.02.038

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oxide (SnO2), Sn- or Zn-doped indium oxide (In 2O3) and Al-, In- or Ga-doped zinc oxide (ZnO). Among these, tin oxide is an important metal oxide semiconductor due to its excellent chemical stability, high band gap (3.6 eV), high work function, good electrical conductivity and high transparency in the visible region [7, 8]. But for new applications in energy sector it is important to modify the morphology, stoichiometry and surface quality to achieve the desired multifunctional properties. This can be done in two ways either by proper choice of deposition technique or by applying some driving force during the growth of the films. The latter approach can be more effective in modifying morphology and microstructure because it can control the properties right from the nucleation stage. Yet there are very few reports dealing with this concept in growing thin films of oxides in general, and ultrathin films in particular. Though there are several techniques that can be employed for the fabrication of oxide thin films, spray pyrolysis is a simple and low cost deposition technique for depositing highly transparent and conducting tin oxide ultrathin films of uniform thickness [9, 10] and high conductivity and transparency. We have developed a modified spray pyrolysis process by using electric field to induce and modify nucleation by presence of electric field during growth. In the present work, a systematic study of effect of electric field applied during deposition on the physical properties of ultrathin (25-75 nm) SnO2 films grown by spray pyrolysis technique have been done. We have seen a significant enhancement of the mobility and control of carrier concentration due the presence of electric field during film growth. 2. Experimental details Undoped tin oxide films were deposited by spray pyrolysis technique without and with the application of electric field (E-field) on the glass substrates during growth stage. A substrate temperature of 450 C was maintained during deposition. These films were prepared by using 3-7 ml of 0.1M solution of SnCl4.5H2O in absolute ethanol. The flow rate (5 ml/min) of solution was controlled by the nitrogen gas (99 % purity) used as a carrier gas at a pressure of 0.5 kg/cm2. The nozzle was kept at a distance of 24 cm from the substrate during deposition. A maximum potential difference of 32 V was applied across the contacts with 1 cm gap pre-deposited on the substrate for growing tin oxide thin film in the presence of Efield. To maintain the thickness of both the series (with and without E-field) same every time during depositions two substrates were placed parallel to each other and potential was applied to the one substrate. The films were characterised for their structural, electrical and optical properties. The film thickness was measured by using spectroscopic ellipsometry (SE) in the wavelength range of 200-1000 nm using an M-2000F ellipsometer from J.A. Woollam Co., Inc., Nebraska. Crystallographic structure of the films was determined from the X-ray diffractograms recorded in the 2θ range of 20 to 80. The electron transport was studied by resistivity and Hall Effect using Vander Pauw configuration. Optical transmission in the wavelength range of 300-1000 nm relative to air of all films was determined using a UV-VIS-NIR double beam spectrophotometer (Model: Perkin-Elmer). 3. Results and discussion 3.1. Structural analysis Thickness of tin oxide films was found to be in the range 25-75 nm. X-ray diffractograms (Fig. 1) for the films of different thickness deposited without and with E-field show that the SnO2 films are polycrystalline. All the patterns contain the characteristic SnO2 peaks that are correlated with the tetragonal structure of tin oxide. The calculated average lattice parameter values for all the films are about a = 4.7 Å and c = 3.2 Å, that are in good agreement with the standard data. The films show a preferred orientation along the (110) plane. The intensity ratio of (110) to (211) peak in all films is about 2.5, in

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comparison to its value of 1.6 in powder sample. The ratio shows a slight increase in its value for field deposited films of all thicknesses.

800

25 nm

(211) (220)

(101)

(110)

1600

(200)

2400

Intensity (a.u.)

2400 1600 800

37 nm

2400 1600

50 nm

800 2400

64 nm

1600 800 2400

75 nm

1600 800 20

30

40

50

2 degree

60

70

Fig. 1. XRD patterns of SnO2 films of different thicknesses deposited (a) without E-field; (b) with E-field.

Fig. 2. Intensity variation of (110) and (220) peaks with thicknesses of SnO2 films deposited (a) without E-field; (b) with E-field.

Figure 2 shows the variation of intensities of (110) and (220) peaks with thickness of SnO2 films deposited with and without E-field. The intensities of both (110) and (220) peaks are higher for the films deposited in the presence of E-field. It is clear from Fig. 2 that due to the field applied during deposition the SnO2 films possess better crystallinity than the films deposited without E-field. The grain size in the SnO2 thin films was calculated by using Scherrer’s formula [11], and it was found that the films grown in

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the presence of E-field have higher grain size than the grain size of the films grown without E-field. Also the grain size increases in both cases as the thickness of the films increases from 25 to 75 nm. Variation of grain size of SnO2 films deposited without and with E-field and the corresponding FWHM values of the peaks are given in Table 1. Table 1. Variation of FWHM of (110) and (211) peaks and grain size of SnO2 films with thickness when deposited without E-field and with E-field. S. No. 1 2 3 4 5

Thickness (nm) 25 37 50 64 75

FWHM (110) 0.8894 0.8274 0.7536 0.5872 0.5545

Without E-field FWHM (211) -0.8602 0.7896 0.5884 0.5792

Grain size (nm) 9.31 10.13 11.35 14.12 14.75

FWHM (110) 0.9488 0.7421 0.6684 0.5868 0.4753

With E-field FWHM Grain size (211) (nm) 1.1524 8.10 0.8002 11.05 0.7242 12.26 0.6118 14.12 0.5186 17.45

3.2. Optical properties Figure 3 shows the variation of the transmittance with thickness for films deposited without E-field and with E-field. It is found that the transmittance of the films deposited in the presence of field is increased. The optical throughput, a measure of cumulative transmission, of all the SnO2 ultra thin films is also calculated by using the following equation:

O.T . 

 n( )T ( )  n( )

(1)

where n(λ) is the number of photons of wavelength λ and T(λ) is the transmittance at that wavelength. For the calculation of optical throughput over the wavelength range 300-2000 nm the standard AM1.5G solar spectrum [12] data is used. It is clear from the Fig. 4 that the optical throughput is higher for the films deposited with E-field.

Fig. 3. Variation of transmittance of SnO2 films with thickness of the film deposited (A) without E-field and (B) with E-field.

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Fig. 4. Variation of optical throughput of SnO2 films with thickness of the film deposited (A) without E-field and (B) with E-field.

Since the films are very thin and reflectance and transmittance based normal method of band gap calculation is not applicable, the energy corresponding to 8 % transmittance has been taken as the band gap of the film. The optical band gap energy E g of SnO2 films are given in Table 2. The accuracy of determination of band gap is about 0.01 eV. It is observed that the films deposited in the presence of Efield posses lower band gap than the films deposited without E-field. Lowering of the band gap is due to the Burstein-Moss shift [13]. This is because the films deposited in the presence of field are more stoichiometric and hence these possess low carrier concentration. Though there is not much change in the transmission behaviour of films of different thicknesses, overall the band gap decreases and optical throughput increases for films deposited in presence of field. Table 2. Variation of optical bandgap of SnO2 films deposited without E-field and with E-field with the film thickness. S. No. 1 2 3 4 5

Thickness (nm) 25 37 50 64 75

Bandgap (eV) without E-field 3.97 3.93 3.94 3.97 3.95

Bandgap (eV) with E-field 3.64 3.56 3.56 3.69 3.56

3.3. Electrical properties The room temperature values of Hall mobility µ, carrier concentration n, and electrical resistivity ρ for SnO2 films grown without and with E-field are obtained from the combined measurements of resistivity and Hall coefficient. Figure 5 shows plot of the three parameters vs. thickness for the comparative study of the electrical properties of SnO2 films deposited without and with E-field. Resistivity of the films is decreasing with increase in thickness due to increase in carrier concentration of the films. The value for n is always lower for E-field deposited films. It is clear from the Fig. 5 that the mobility increases significantly with the decrease in thickness for the films deposited with E-field, especially so for 25 nm film. The higher mobility of the thinner films is due to better crystallinity of the E-field deposited films as

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well as the possible dominance of specular scattering over the diffuse scattering of electrons at such a low film thickness where electron mean free path is comparable to film thickness. At higher thicknesses the contribution of specular scattering decreases with the increase in grain boundary scattering leading to the observed decrease in mobility with increasing thickness.

Carrier Conc. Resistivity 17 ( x 10 per cc) (ohm.cm)

2.0 1.5 1.0 0.5

0.0 100 80 60 40 20 0

Mobility 2 (cm / V s)

40 30 20 10 20

30

40

50

60

Thickness (nm)

70

80

Fig. 5. Variation of (a) resistivity; (b) carrier concentration and (c) mobility of SnO2 films with thickness for films deposited without E-field () and with E-field ().

The films deposited in the presence of E-field show higher resistivity than those deposited without Efield. This increase is much higher at lower thickness that is the resistivity increases from 0.5 to 1.55 cm due to field assisted growth, but becomes much smaller at higher thickness. This shows that the presence of E-field is more effective at lower thicknesses that at higher thickness. This is consistent with our XRD results that show an improvement in crystallinity and preferred grain growth in the field assisted growth mode, in particular at lower thicknesses. The increase in resistivity is due to the lowering of the carrier concentration of the film deposited in the presence of E-field. The observed lowering of the carrier concentration of the E-field deposited films can be explained on the basis of increase in the stoichiometry of the films, most probably due to higher reactivity of oxygen in the presence of E-field leading to lowering of oxygen vacancies. Lesser scattering of charge carriers is implied from the lower vacancies and also from the increased grain size. These in turn lead to increase in the mobility of the films deposited in the presence of E-field. So E-field helps in the deposition of films with high mobility at lower thicknesses and thus resistivity is kept relatively low even when the films are ultrathin with thickness as low as 25 nm.

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4. Conclusions Undoped ultrathin SnO2 films of 25-75 nm thickness possessing high electrical conductivity were deposited by spray pyrolysis technique with the application of electric field (E-field) during film growth. It is found that the films deposited in the presence of E-field possess higher stoichiometry, better crystallinity, larger grain size and higher transparency than the films deposited without E-field. The electrical resistivity was found to be relatively higher for the films deposited in the presence of field due to improved stoichiometry as evidenced from the decrease in carrier concentration of the films. But the mobility of these films has become significantly high reaching a value of 43 cm2/V.s due to field assisted growth that incorporates lesser oxygen vacancies. So we can say that the E-field helps in the deposition of high mobility ultra thin films. It is concluded that the presence of electric field during deposition has a significant effect on the physical properties of ultra thin SnO2 thin films. Acknowledgements SB is thankful to CSIR for providing a research scholarship. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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