SnO2 Nanostructured Thin Films Using Spectroscopic Ellipsometry Technique: Effects of Annealing Remperature

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SnO pectroscopic 2

S sing U

ilms F

hin T anostructured N

Ellipsometry Technique: Effects of Annealing Remperature

Sahar Moradi1, Hassan Sedghi1

1Department of Physics, Faculty of Science, Urmia University, Urmia, Iran

Moradisahar884@gmail.com

Abstract- Nickel-doped tin dioxide (Ni:SnO2) thin films coated on glass substrates using sol-gel spin coating method. All samples were preheated on hot plate for 30 min at 100ºϹ and then divided to three groups annealed for 2 h at temperatures including 450, 500 and 550ºϹ.

Effects of annealing temperature on optical characteristics of samples were investigated by spectroscopic ellipsometry (SE) technique. SE measurements were conducted in the wavelength range of 300-800 nm at fixed incidence angle of 70º. SE measured (Ψ,Δ) parameters. Then by construction a proper optical model for synthesized thin films and choosing the Leng- Oscillator model as an appropriate dielectric function, the new (Ψ,Δ) parameters were produced. Optical properties of thin films including the refractive index (n), extinction coefficient (k) and complex dielectric constant (ε(ε1, ε2)), were determined by fitting the measured (Ψ,Δ) by SE technique and the one resulted from the analysis based on the optical model. Results Showed by increasing the annealing temperature from 450ºϹ to 550ºϹ the refractive index and extinction coefficient values decreased and also the behavior of real and imaginary parts of the dielectric constant (ε1,ε2) were almost the same as the refractive index and extinction coefficient, respectively. By the dielectric constant and refractive index results obtained using SE technique, real and imaginary parts of the conductivity of Ni:SnO2 thin films (σ(σ1,σ2)) were calculated.

Keywords: Spectroscopic ellipsometry, Sol-gel spin-coating, Nickel-doped tin dioxide, complex dielectric constant, conductivity.

The 1^th Conference on Optoelectronics, Applied Optics and Microelectronics (OAM). Namin, Ardabil, Iran.

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1. Introduction

SnO2 thin films as TCFs have many applications in solar cells, sensors and Li-Ion batteries [1-3]. Doping SnO2 thin film with transition metals such as Co, Fe and Ni is a common way to improve its optical properties and band gap energy modification for desired applications [4-6]. Ni-doped SnO2 thin films have been produced by several coating methods [7-9]. The method used to synthesize a thin film has undeniable impacts on its optical, structural and electrical properties. Among applied coating methods, sol-gel has remarkable advantages. It is easy and low-cost process and has high productivity and low temperature processing. Production of porous and multilayer structures is also possible by sol-gel method [10]. Spectroscopic ellipsometry (SE) technique is widely used to optical analysis of thin films. SE determines optical properties such as the refractive index, extinction coefficient, transmittance and reflectance of thin films. SE is capable to evaluate the thickness and complex dielectric constant of a multilayer, simultaneously. Furthermore, by using SE analysis, valuable information about the band structure and band gap energy of a film can be extracted [11]. In this work, Ni-doped SnO2 thin films were synthesized at different annealing temperatures by sol-gel spin coating method and their optical properties were investigated by using SE technique.

2. Materials and coating method

Ni-doped SnO2 thin films deposited on glass substrates by sol-gel spin coating method.

1.082 g tin (II) chloride dehydrate [SnCl2.2H2O, Mw=225.63, Merck] was dissolved in 10 ml pure ethanol. Then 0.0451 g nickel (II) chloride hexahydrate [NiCl2.6H2O, Mw=237.7, Merck] as a dopant source was added to the solution. The sol was stirred at room temperature for 1 h and then at 60ºϹ for 2 h. Before spin coating, glass substrates were washed and cleaned by deionized water and acetone and dried by a hot gun. Spin coating conducted at 3300 rpm. Coated thin films were preheated on a hot plate at 100ºϹ for 30 min. Finally, all samples divided to 3 groups and were annealed at 450, 500 and 550ºϹ for 2 h in an air furnace.

3. Results and discussion

The SE 800 DUV (SENTECH) instrument in the wavelength range of 300-800 nm was used for thin films analysing. Ellipsometry method uses detection of polarization state of light to characterize thin films. It measures (Ψ,Δ) parameters which represent the amplitude ratio and phase difference between p and s polarized light waves, respectively.

Ellipsometric parameters (Ψ,Δ) are defined in terms of the amplitude reflection coefficient for p and s polarizations (rp,rs) [12]:

ρ ≡ tan(Ψ)exp(iΔ) ≡ rp / rs (1)

Since ellipsometry is an indirect method to characterize optical properties of a thin film, the analysis based on an optical model is required.therefore, in ‘Spectra Ray’, the instrument’s analytical software, an optical model for synthesized Ni:SnO2 thin films was defined as air/ surface roughness layer/Ni:SnO2 thin layer/glass substrate. The refractive index and extinction coefficient for prepared thin films were determined by choosing Leng-Oscillator dielectric function model and through fitting error (MSE) minimization using linear regression analysis. In order to surface roughness layer fitting, the Bruggemann's effective medium approximation (EMA) was applied. Figure 1(a,b) shows the experimental and fitted data of (Ψ,Δ) parameters for Ni-doped SnO2 thin films annealed at 450, 500 and 550°Ϲ for 2 h.

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Fig 1. The experimental (dotted line) and fitted (solid line) data of a) Ψ and b) Δ parameters for Ni:SnO2 thin films annealed at 450, 500 and 550°Ϲ.

The refractive index (n) and extinction coefficient (k) for synthesized Ni:SnO2 thin films were shown in figure 2(a,b). It can be seen by increasing the annealing temperature from 450°Ϲ to 550°Ϲ, n and k values were decreased. These seem probably because of the decrease of light scattering and absorption due to the formation of thin films with more crystalline structure.

Fig 2. a) the refractive index and b) extinction coefficient for Ni:SnO2 thin films annealed at 450, 500 and 550°Ϲ.

In figure 2a), it can be observed by increasing the wavelength, the n values of thin films were reduced to a constant value which means a normal dispersion in the visible wavelength region. According to figure 2b), increasing the wavelength leads to decrease of k values for all samples which can be deduced the layers get more transparent. The complex dielectric constant is defined as ε = ε1 + iε2, where ε1 and ε2 are the real and imaginary parts, respectively. The behaviour of ε1 is similar to n while the ε2 mainly depends on values of k. ε1 and ε2 defined as follows:

ε1 = n² – k² (2)

The 1^th Conference on Optoelectronics, Applied Optics and Microelectronics (OAM). Namin, Ardabil, Iran.

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ε2 = 2nk (3)

figure 3(a,b) shows the real (ε1) and imaginary (ε2) parts of dielectric constant for Ni:SnO2 thin films with different annealing temperatures of 450, 500 and 550°Ϲ. It can be deduced, by increasing annealing temperature from 450°Ϲ to 550°Ϲ both ε1 and ε2 were reduced. Also values of the real part of dielectric constant is higher than the imaginary part in visible light range which shows low energy loss of light through Ni:snO2 thin films in visible light region. In figure 3a), it can be seen ε1 nearly keeps constant in the visible light range and sharply increases near the optical absorption edge. According to figure 3b) ε2 values were decreased by increasing the wavelength in visible light region.

Fig 3. a) the ε1 and b) ε2 for Ni:SnO2 thin films annealed at 450, 500 and 550°Ϲ.

Figure 4(a,b) displays the real (σ1) and imaginary (σ2) parts of conductivity σ(σ1 , σ2) for Ni:SnO2 thin

Films annealed at 450, 500 and 550°Ϲ which are defined through the equations [13]:

σ1 = ε2 ω / 4π (4) σ2 = (1-ε1 ) ω / 4π (5) Where ω is the angular frequency.

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Fig 4. a) the σ1 and b) σ2 for Ni:SnO2 thin films annealed at 450, 500 and 550°Ϲ.

It can be observed real and imaginary parts of the conductivity were decreased in visible light region by increasing the annealing temperature from 450°Ϲ to 550°Ϲ. According to figure 4b) for Ni:SnO2 thin films annealed at 450 and 500°Ϲ there is a peak at about 324 nm wavelengths while it is shifted to wavelength of 334 nm for the film annealed at 550°Ϲ.

4. Conclusion

Ni-doped SnO2 thin films have been deposited on glass substrates by sol-gel spin coating method. The films were annealed at 450, 500 and 550°Ϲ for 2 h. The effects of annealing temperature on optical properties of synthesized thin films were investigated using spectroscopic ellipsometry technique. Results showed the refractive index and extinction coefficient were decreased by increasing the annealing temperature from 450°Ϲ to 550°Ϲ. Ni-doped SnO2 thin films annealed at 450°Ϲ and 550°Ϲ showed the highest and lowest ε1 and ε2 values, respectively. The peaks of ε1 near ultra- violet wavelength region represent the optical absorption edge. Using complex dielectric constant and refractive index results obtained by SE technique, the conductivity of Ni:SnO2 thin films were calculated.

Results showed conductivity decrement by increasing annealing temperature from 450°Ϲ to 550°Ϲ. It seems by regulating the annealing temperature in Ni:SnO2 thin film synthesis process, optical properties can be modified for desired applications.

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