Structural and optical properties of indium tin oxide (ITO) thin films with different compositions prepared by electron beam evaporation

doi:10.1016/j.vacuum.2009.11.017

Vacuum

Volume 84, Issue 6, 4 February 2010, Pages 864-869

https://doi.org/10.1016/j.vacuum.2009.11.017 Get rights and content

Abstract

Tin-doped Indium oxide thin films in different compositions (Sn = 0,5,10,15,20 at.wt%) were prepared on glass substrates at the substrate temperature of 250 °C in an oxygen atmosphere by electron beam evaporation. The structural and morphological studies were carried out by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The grain size of the ITO films decreased when increasing the dopant concentration of Sn in the In₂O₃ lattice. Optical properties of the films were studied in the UV-Visible-NIR region (300–1000 nm). The optical energy band gap (E_g), as determined by the dependence of the absorption coefficient on the photon energy at short wavelengths was found to increase from 3.61 to 3.89 eV revealing the ascending loading profile of dopant concentration. Optical Parameters, such as absorption depth, refractive index (n), extinction coefficient (k), packing density, porosity, dispersion energy and single effective oscillator energy were also studied to show the composition dependence of tin-doped indium oxide films.

Introduction

The transparent semiconducting oxide materials have extended their broad spectrum of applications in the material science world because of the fortunate combination of visible light transmission that may exceed 0.9 and specific resistivity below 10⁻³ Ω cm. Consequently oxides are utilized for wavelength-selective coatings as electrodes on opto-electronic semiconducting devices and gas-sensing coatings [1], [2]. In this area, Indium tin oxide (ITO) has proven to be an advanced semiconducting material opening a new window in many electronic and optical industries due to its optical band gap of ∼3.6 eV with a plasma frequency lying in the near-IR spectral region [3], [4], [5]. Due to its distinct character and salient features, ITO shows excellent transparency in the visible region but is fairly reflective in the infrared spectral regions. ITO is an insulator in its stoichiometric form and a conductor in its non-stoichiometric form with a wide direct band gap of ∼3.6 eV [6]. This high electrical property arises from the formation of a conducting carrier–oxygen vacancy with the addition of a dopant tetravalent cation Sn⁺⁴ to the matrix In₂O₃. The oxygen vacancies act as doubly-ionised donors and contribute a maximum two electrons to the electrical conductivity. Since In₂O₃ exists in two different crystallographic structures, i.e. rhombohedral and cubic forms, high electrical conduction is normally achieved by the stabilization of the cubic phase [7]. Over the past decades, many researchers have investigated the preparation of ITO thin films because of their wide utilization in fabricating photovoltaic cells [8], sensor modules [9] and transparent electrode materials for both electrochromic cells [10] and liquid-crystal display devices [6]. Hitherto, numerous techniques have been developed to prepare ITO thin films. Among those, the electron beam evaporation technique is found to be very promising for fabricating ITO films with lower resistivity and higher transmittance [11]. In this paper, we present and discuss an investigation of the structural and optical properties of ITO films deposited by electron beam evaporation.

Section snippets

Preparation of source materials

The indium tin oxide (ITO) powders were prepared by a co-precipitation process using anhydrous Indium chloride (InCl₃, 99.9%) and Tin chloride (SnCl₂.2H₂O, 99.9%) in different compositions (Sn/In = 0,5,10,15 and 20 at.%). The starting chemicals of an anhydrous InCl₃ and an appropriate amount of SnCl₂.2H₂O were dissolved in 30 ml of distilled water. The solution containing 7 ml of acetylacetone and 13 ml of methanol was slowly added to an aqueous solution of indium and tin chlorides by rapid

Results and discussion

X-ray diffraction spectra recorded for Sn-doped In₂O₃ films for different Sn concentrations deposited at a substrate temperature of 250 °C using electron beam evaporation technique are shown in Fig. 1 (a). The analysis of the crystal structure revealed that the particles were found to be well identified as crystallized cubic ITO [JCPDS-391058]. Different peaks in the XRD pattern, which appeared at 2θ values 30.34, 35.31 and 50.13, were due to reflections from (222), (400) and (441) planes of

Conclusions

ITO thin films with different compositions (Sn = 0, 5,10,15,20 at.wt %) were deposited by an electron beam evaporation technique. The effect of increasing Sn concentration on the structural and optical properties of the films was investigated. As evidenced from the X-ray diffraction analysis, the crystallite size was found to decrease with Sn doping. SEM observations also indicate a smooth surface morphology with small grains for the films studied. This result may be particularly useful for gas

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Structural and optical properties of indium tin oxide (ITO) thin films with different compositions prepared by electron beam evaporation

Abstract

Introduction

Section snippets

Preparation of source materials

Results and discussion

Conclusions

J Solid State Chem

Appl Surf Sci

Mater Lett

Thin Solid Films

Sens Actuators B

Surf Coat Technol

Mater Chem Phys

Physica E

Thin Solid Films

Thin Solid Films

Thin Solid Films

J Phys Chem Solids

Mater Lett