Improvement of optical and electrical properties of ITO thin films by electro-annealing

doi:10.1016/j.vacuum.2015.06.027

Vacuum

Volume 120, Part B, October 2015, Pages 8-13

https://doi.org/10.1016/j.vacuum.2015.06.027 Get rights and content

Highlights

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Electro-annealing in air and vacuum were investigated for crystalline ITO thin films.
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The effect of high electrical current on large area ITO thin films were investigated.
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Electrical, structural and optical properties of ITO thin films were improved by electro-annealing in air and in vacuum.

Abstract

The effect of electro-annealing in vacuum and air on the optical and electrical properties of ITO thin films grown by large area DC magnetron sputtering was investigated. Moreover, the performances of the electro-annealed ITO thin films in vacuum and air were compared. Electro-annealing was performed by applying 0.75, 1.00, 1.25 and 1.50 A constant ac current to the ITO thin films. It was observed that the crystallinity of the films was better for the ITO thin films electro-annealed in vacuum. The changes in sheet resistance of electro-annealed ITO thin films with applied currents were detailed. The transmittance of the films increased for both electro-annealing in vacuum and air. A correlation between band-gap and resistivity for all of the electro-annealed thin films was observed.

Introduction

Transparent conducting oxides (TCOs) have a wide range of applications including transparent electrodes in LCDs, organic light emitting diodes, solar cells, plasma display panels (PDP), transparent heat reflecting windows as well as surface heaters for cameras, lenses, mirrors, car windows, gas sensors and ohmic contacts for surface-emitting diodes [1], [2], [3]. Indium tin oxide (ITO) thin film is one of the most commonly used materials among the TCO thin films due to its relatively low resistivity and high optical transmittance in the visible region of the electromagnetic spectrum [4].

Growth conditions such as oxygen partial pressure, substrate temperature and bias voltage are strongly important parameters for the high electrical conductivity and the transmittance in the visible spectral range of ITO films. Several deposition techniques such as thermal evaporation [5], chemical vapor deposition [6], electron beam evaporation [7], sol–gel, spray pyrolysis [8], pulsed laser deposition [9] and magnetron sputtering [10] have been used for years in order to prepare high quality ITO films. The magnetron sputtering method is the commonly preferred technique due to the reproducible deposition of the films.

Annealing of ITO thin films produced by magnetron sputtering is done to achieve the desired structure and properties of the films for practical applications [10]. The most common method for the annealing of transparent conductive oxides is thermal annealing [11]. As an alternative, recent studies have reported that the performance of deposited ITO films can be improved by electro-annealing which is a process of self-heating by electric current [12], [13], [14]. Electro-annealing offers many advantages, i.e. no external heater is required during annealing and the direct Joule heat generated by applying an electric field to the film leads to a decrement in both impurity generation and heat overloading by the surrounding components [12]. Moreover, electro-annealing is suitable for thermally sensitive substrates [12], because, in the case of electro-annealing, crystallization starts and proceeds faster at lower power levels in contrast to thermal annealing due to efficient energy coupling from Joule heating. Developing a clear understanding about the characteristics of ITO thin films under electrical current flow is crucial to eliminate the life-time effects of the electronic devices. However, there is limited research concerned with the investigation of the influence of electro-annealing on the properties of the ITO thin films by passing electric currents through them in air and vacuum [12], [13], [14]. In the previous studies, Rogozin et al. [12] proposed a method which provides constant power at variable film resistance. They pointed out that electro-annealing in vacuum provides reduction in the thermal budget and a decrease in the kinetic exponent of crystallization in comparison with thermal annealing. Lee et al. [14] observed that optical and electrical properties of the ITO thin films on polyimide substrates can be improved by electro-annealing in air. In a recent study, Pei et al. [13] investigated the effect of electro-annealing in air on the optical and electrical properties of ITO films, and they concluded that the electric power is a decisive factor that determines the performance of the films. We report here a study of the effect of electro-annealing in both air and vacuum on structural, optical and electrical properties of the ITO thin films grown by DC magnetron sputtering using borosilicate glass substrates. In contrast to the previous researches, large area substrates were used to investigate the effect of high electrical current on the ITO thin films in large scale applications.

Section snippets

Experimental

A large area magnetron sputtering coating system was used for the deposition of the ITO thin films [10]. The schematic representation of the large area magnetron sputtering coating system can be seen in Fig. 1. In this system, the movable sample holder is slowly moved underneath the target via a feedthrough system and the film thickness depends on the applied power and angular velocity of the feedthrough mechanism. 3 mm thick and 50 mm × 60 mm width and length borosilicate glasses were used as

Results and discussion

Due to the Joule heating, the temperature of the ITO thin films increases as the electrical current passes through the films. The variation of the temperature versus time of electro-annealed thin films in air and vacuum are shown in Fig. 3. Sample temperature increased during 10 min and then slowly decreased to the room temperature within 20 and 40 min. The highest temperatures for the electro-annealed ITO thin films at the end of the 10 min in air and vacuum were about 98, 124, 229, 327 °C and

Conclusion

Understanding the effect of electric current on ITO thin ﬁlms is essential to increase the life-time of electronic devices. In this study, we analyzed the effect of electro-annealing in air and vacuum on the structural, optical and electrical properties of the crystalline ITO thin films grown by large area DC magnetron sputtering at 250 °C substrate temperature. Both for electro-annealing in air and vacuum, we observed improvement in the structural, optical and electrical properties of the ITO

Acknowledgments

Dr. Basak Bengu is gratefully acknowledged for supplying borosilicate glass and the authors would like to acknowledge the facilities of Applied Quantum Research Center (AQuReC) for the current study.

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Citation Excerpt :
All the curves show similar behavior until 1400 nm and then decreases in the infra-red range until 2500 nm. The slight decrease in transmittance in IR range for as-deposited and annealed coatings may be attributed due to slight variation in stoichiometry, orderedness, residual stresses and defects formed during deposition [11,30,31]. Optical band gap energy has been calculated from the transmittance data using Tauc's plot (eq. (4)).
Indium tin oxide (ITO) coatings have been found useful for diverse applications including electromagnetic interference (EMI) shielding for defense and aerospace purposes, solar cells, gas sensors, etc. However, the reliability, adhesion, transmittance and control of conductivity are some issues that need to be discussed for EMI shielding. Composite multi-layer coating of magnesium di-fluoride (MgF₂) and ITO have been deposited on soda lime glass substrate using electron beam (e-beam) deposition technique at a substrate temperature of 350 °C and annealed in the temperature range of 200–500 °C for 1 h under vacuum. The deposited coatings have been characterized for structural, optical and electrical properties. The deposited coatings were crystalline with a crystallite size in the range of 20–30 nm for ITO and 15–20 nm for MgF₂ coatings. The coating thickness of ITO and MgF₂ determined by Rutherford backscattering technique were found to be 95 nm and 12 nm, respectively. The maximum transmittance in the visible range is ~92% for as-deposited and annealed multi-layer coatings. The band gap has been found in the range of 3.95–4.02 eV, which is suitable for EMI shielding effectiveness. Raman spectroscopy confirms quantum confinement in MgF₂/ITO coatings. The electrical conductivity of as-deposited MgF₂/ITO coating has been found to be 200 (Ω-cm)⁻¹ which increased to a value of 1.002 x 10³ (Ω-cm)⁻¹ for the sample annealed at 500 °C under vacuum. The multi-layer coating annealed at 500 °C show good EMI shielding effectiveness of 50.4 dB without scarifying the transmittance.

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Improvement of optical and electrical properties of ITO thin films by electro-annealing

Highlights

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusion

Acknowledgments

Appl. Surf. Sci.

Thin Solid Films

Thin Solid Films

Appl. Surf. Sci.

Vacuum

Vacuum

Thin Solid Films

Organic electroluminescent diodes

Appl. Phys. Lett.

Efficient inverted polymer solar cells

Appl. Phys. Lett.