Characterization of transparent and conductive electrodes of indium tin oxide thin films by sequential reactive evaporation

doi:10.1016/S0040-6090(99)00182-0

Thin Solid Films

Volume 349, Issues 1–2, 30 July 1999, Pages 71-77

https://doi.org/10.1016/S0040-6090(99)00182-0 Get rights and content

Abstract

ITO thin films have been deposited onto glass substrates by sequential reactive evaporation as transparent and conductive electrodes for devices. The method has the advantage of low enough temperatures (≤200°C) for the processes of preparation and post-annealing, accurate control of single layer thickness, simplicity and low cost. ITO films with electrical resistivity of 10⁻² Ω cm and optical transparency greater than 90% have been obtained. The structural, compositional, electrical, optical properties of the films depend on the annealing time. The stability of the electrical and optical properties of the electrodes has been investigated showing the feasibility of producing high quality ITO films by this method.

Introduction

Indium tin oxide (ITO) films have been widely studied over the last 30 years since they were found to exhibit electrically conductive and optically transparent characteristics. Therefore, ITO films are extensively used as coating-electrodes in optoelectronic devices [1], electroluminescent devices [2], photovoltaic cells [3], [4], [5], electrochromic devices [1], liquid crystal displays [1], [6], energy-efficient windows [7], storage-type cathode ray tubes [1].

Among the various techniques reported for producing these films, including rf or dc sputtering [8], [9], [10], [11], [12], [13] from an ITO target or an In–Sn alloy target, chemical vapour deposition [14], laser ablation [15], evaporation [16], [17], [18], the thermal vacuum sequential reactive evaporation has been chosen as the method among those cited providing low enough temperatures of the preparation process and post-annealing. In fact, since the ITO films should be used as transparent and conductive electrodes deposited as top-layer onto an electroluminescent multilayer-device prepared by quasi-rheotaxy method [19], [20] at temperatures not exceeding 200°C, this application requires that the ITO films must be prepared at temperatures as low as possible in order to prevent damage to the electroluminescent device. The sequential reactive evaporation consists of indium and tin layers sequentially deposited in oxygen ambient onto substrates held at room temperature and then annealed in air at low temperatures (less than 200°C) in order to enhance the optical transparency and to arrange the electrical conductivity of ITO films. This deposition technique permits an accurate control of the thickness of each metal layer and of the stoichiometry of ITO films with the advantages of simplicity and low cost [16].

In the paper, we report on the structural, compositional, electrical and optical properties of the ITO films prepared by sequential reactive evaporating and successively annealing at 190°C. The influence of the annealing time on the electrical and optical stability of ITO films is also investigated.

Section snippets

Experimental details

ITO films were deposited onto glass substrates (Corning 7059) by sequential reactive evaporation at room temperature. The deposition chamber was equipped with a double-stage vacuum system based on a turbomolecular pump (Turbovac 150) baked by a rotary pump (Trivac D 16 B) able to evacuate down to a final pressure of 1×10⁻⁵ mbar. A tungsten (W) boat and a tantalum (Ta) boat, distanced by 2 cm and located at different heights, were separately loaded with indium (99.999% purity) and tin (99.999%

Structural properties

Fig. 1shows the X-ray diffraction (XRD) patterns obtained with the conventional 2θ/θ scanning mode from the as-deposited (curve a) and annealed at different times (curves b, c, d) ITO films. As far as the as-deposited film is concerned, the relative pattern exhibits at low angle a broad peak from the glass substrate and a thinner peak at about 2θ=33° corresponding to the (101) reflection of In tetragonal phase [22]. There is no evidence of the Sn layer in the spectrum. This could be caused by

Conclusions

ITO thin films have been deposited onto glass substrates by sequential reactive evaporation at room temperature and post-annealing at low enough temperature (190°C). ITO films are found to exhibit conductive and transparent characteristics with electrical resistivity of 10⁻² Ω cm and optical transparency greater than 90% in the visible region. Our best results both for the time stability of the electrical resistivity and for its lowest value have been obtained for the ITO films air-annealed at

Acknowledgements

The authors are indebted to G. Cassano, G. Pace, A. Sacchetti for their helpful technical assistance. This work was financially supported by Italian Ministry of University and Scientific and Technological Research under the framework Piano Nazionale di Ricerca sui Materiali Innovativi Avanzati - TEMA 16, by contract Sviluppo di materiali fotoattivi e relative tecnologie di film sottile.

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Performance optimization of nanometer-thin indium-tin oxide transistors by two-step air annealing
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In this study, the influence of air annealing on nanometer-thin indium tin oxide (ITO) transistors was investigated. Compared with unannealed devices, the field-effect mobility and subthreshold swing of ultrathin ITO transistors with pre-annealing, post-annealing, and two-step annealing were greatly improved. Under a pre-annealing temperature at 200 °C and post-annealing temperature at 100 °C, the ultra-thin ITO transistors showed field-effect mobility of 34.2 cm²/Vs, steep subthreshold swing of 1.95 V/decade, and high I_on/I_off ratio of 2.46 $\times$ 10⁷. X-ray photoelectron spectroscopy revealed that the performance changes could be ascribed to the changes in the oxygen vacancies in the ITO channel under different annealing conditions. The dependence of subthreshold swing on the annealing process may be related to the absorption and release of water molecules from the ultrathin ITO surface. Overall, these results emphasize the importance of the annealing procedures for nanometer-thin ITO transistors.
Cathodoluminescence emission study of nanocrystalline indium oxide films deposited by spray pyrolysis
2007, Thin Solid Films
The results of analysis of In₂O₃ film cathodoluminescence (CL) spectra are presented in this paper. In₂O₃ films, aimed for gas sensor application, were deposited by spray pyrolysis from 0.2 M InCl₃–water solutions. The influence of grain size (10–60 nm), film thickness (20–400 nm), pyrolysis temperature (T_pyr = 400–520 °C), and annealing in the air or nitrogen atmospheres (T_an = 600–1100 °C) on CL emission of In₂O₃ is discussed.
CL spectra of as-deposited In₂O₃ films were characterized by a broad band centered at λ ∼ 570–600 nm. The annealing of studied films leads to a considerable increase of CL intensity. High annealing temperature of In₂O₃ films (T_an > 850 °C) is being accompanied by the appearance of additional bands centered at λ ∼ 400, 550, and 650 nm, which are peculiar to single-crystalline In₂O₃ nanobelts, or nanowires with perfect crystal structure. It was concluded that the improvement of crystal structure and the decrease of the concentration of oxygen vacancies are the main factors determining the change of CL spectra of In₂O₃ films and the appearance of edge luminescence.
Characteristics of ITO films fabricated on glass substrates by high intensity pulsed ion beam method
2007, Journal of Non-Crystalline Solids
Transparent conductive oxides such as indium tin oxide (ITO) are interesting materials due to their wide-band gaps, high visible light transmittance, high infrared reflectance, high electrical conductivity, hardness and chemical inertness. ITO films were fabricated on soda lime glass substrates by using high-intensity pulsed ion beam (HIPIB) technique. The as-deposited films comprised of partially crystallized In₂O₃ and after annealing at 500 °C for 1 h the film changed to polycrystalline phase. After annealing carrier concentration and Hall mobility increased while specific resistance and sheet resistance decreased quickly; and this trend was also observed when film thickness increased up to 300 nm for the post-annealed samples. Further increase in thickness of the film changed the electrical properties slightly. Atomic force microscopy (AFM) revealed that roughness decreased after 500 °C annealing for 1 h in air, except for the film of 65 nm thick. The thickness of the film which relates to the carrier concentration and mobility, degree of crystallization, size of the grain, and connections among grains in film are main factors to determine film’s electrical properties.
Novel type of indium oxide thin films sputtering for opto-electronic applications
2007, Applied Surface Science
Transparent conductive oxide (TCO) thin films play a significant role in recent optical technologies. Displays of various types, photovoltaic systems, and opto-electronic devices use these films as transparent signal electrodes. They are used as heating surfaces and active control layers. Oxides of TCO materials such as: tin, indium, zinc, cadmium, titanium and the like, exhibit their properties. However, indium oxide and indium oxide doped with tin (ITO) coatings are the most used in this technology.
In this work, we present conductive transparent indium oxide thin films which were prepared using a novel triode sputtering method. A pure In₂O₃ target of 2 in. in diameter was used in a laboratory triode sputtering system. This system provided plane plasma discharge at a relatively low pressure 0.5–5 mTorr of pure argon. The substrate temperature was varied during the experiments from room temperature up to 200 °C. The films were deposited on glass, silicon, and flexible polyimide substrates. The films were characterized for optical and electrical properties and compared with the indium oxide films deposited by magnetron sputtering.
An endothelial cell compatible biosensor fabricated using optically thin indium tin oxide silicon nitride electrodes
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This study describes the fabrication and performance of an endothelial cell compatible, optically thin, indium tin oxide (ITO) microimpedance biosensor. The biosensor was constructed by sputtering a thin insulating layer of silicon nitride (Si₃N₄) onto a 100 nm thick ITO layer. Indium tin oxide electrodes were formed by chemically etching 250 or 500 μm diameter holes through the Si₃N₄ insulating layer. The exposed ITO electrode was electrically connected to an ITO counter electrode, approximately 2 cm² in area, via a 400 μL well containing cell culture media. A lock-in amplifier circuit monitored the impedance of porcine pulmonary artery endothelial cells (PPAECs) cultivated on the electrodes as a function of frequency, between 10 and 100 kHz, and as a function of time, at 5.62 kHz. The ITO–Si₃N₄ microelectrodes provided consistent and repeatable impedance measurements to the attachment and spreading of PPAECs. In addition, the ITO–Si₃N₄ electrodes were recyclable, robust, resistant to ethanol sterilization, and had a high optical transmittance. Most importantly, the ITO–Si₃N₄ electrodes allowed optical access for dynamic cellular attachment imaging. The 5.62 kHz time dependent cellular impedance response to the drug Cytochalasin D further demonstrated the feasibility of using this electrode configuration for dynamic cellular impedance studies.
The properties of tin-doped indium oxide films prepared by pulsed magnetron sputtering from powder targets
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Citation Excerpt :
This approach produced coatings with properties comparable to ITO coatings produced by other deposition techniques [7–16].
Mixed indium oxide and tin oxide powder targets have been used to prepare transparent semi-conductive films on glass by pulsed magnetron sputtering. The influences of dopant concentration in the target and oxygen concentration in the plasma were investigated. Also, the properties of the coatings were compared before and after annealing. The morphological structure, crystallinity, optical properties and electrical properties of the films were investigated using a range of techniques, including SEM, XRD, spectrophotometry, four-point probe and van der Pauw method. The main preferred orientations of the coatings are (222) and (400). The transparent lines of coatings deposited with additional oxygen in the plasma showed blue shifts after annealing, with the average transmittance within the visible range being in the range 80–85%. Also, coating resistivities of the order of 10⁻⁴ Ω cm were obtained. The results to date demonstrate that the pulsed magnetron sputtering of Sn-doped In₂O₃ films from powder targets is a versatile, novel technique for the production of high quality transparent conductive oxide (TCO) materials. Due to the flexibility of this technique, preliminary experiments of multi-component tin-doped indium oxide (ITO) (5 at.% Sn)/ZnO (3 at.% Al) coatings are also included here. The incorporation of Al-doped ZnO resulted in an increase in transmission, but also a reduction in the conductivity of these coatings.

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Characterization of transparent and conductive electrodes of indium tin oxide thin films by sequential reactive evaporation

Abstract

Introduction

Section snippets

Experimental details

Structural properties

Conclusions

Acknowledgements

J. Lumin.

Vacuum

Thin Solid Films

Thin Solid Films

Thin Solid Films

Thin Solid Films

Thin Solid Films

Thin Solid Films

Solid State Electron.

Thin Solid Films

J. Cryst. Growth.

J. Cryst. Growth