Indium tin oxide which is commonly as known as ITO is a composed mixture of indium, tin, and oxygen all combined in different proportions. The main composition is dependent upon the quantity of carbon which makes it either a ceramic or an alloy. Indium tin oxide is a great compound and is widely used as transparent conducting oxides due to the characteristics and properties that it possesses. Depending upon this factor, ITO has plenty of applications on the mobile device screens as it has all the features that are necessary to carry out the desired performances.

Introduction

A ternary composition of oxygen, tin, and indium in varying proportions is known as Indium tin oxide (ITO). It can be explained as an alloy or ceramic, the oxygen content determines it. With 74% In, 18% O2, and 8% Sn formulation by weight, typically, Indium tin oxide is encountered as an oxygen-saturated composition. Unsaturated compositions are known as oxygen-deficient ITO as oxygen-saturated compositions are very typical. In thin layers, it is colorless and transparent whereas it goes yellowish to grey in bulk form. It functions as a metal-like mirror in the spectrum’s infrared region.

Most Widely Used Areas

Due to indium tin oxide's optical transparency and electrical conductivity, it is the broadly utilized transparent conducting oxides, along with its easy deposition as a thin film. There needs to be a compromise between transparency and conductivity as the conductivity of the film can be increased by increasing the concentration of charge carriers and increasing the thickness, but it can also decrease its transparency. Most commonly, physical vapor deposition deposits indium tin oxide's thin films on surfaces. A range of sputter deposition techniques or electron beam evaporation is used often.

Material and Properties

Indium tin oxide is tin and indium’s mixed oxide with a 1526–1926 °C (1800–2200 K, 2800–3500 °F) of melting point, the composition mainly determines it. A composition of ca In4Sn is possessed by the most commonly utilized material. Having around 4 eV of a large bandgap, the material is an n-type semiconductor. Indium tin oxide possesses a comparatively high electrical conductivity and is transparent to visible light. More than 80% of optical transmittance can be possessed by a thin film. Indium tin oxide has a low electrical resistivity of ~10−4 Ωcm. These characteristics are used for great benefits in touch-screen applications like mobile phones.

Common Uses

ITO is an optoelectronic material. ITO can be used in both industry and research in a broad range. For many applications, Indium tin oxide can be utilized like architectural windows, supermarket freezer glass doors, thin-film photovoltaics, polymer-based electronics, smart windows, and flat-panel displays. Also, Indium tin oxide's thin films for glass substrates can be beneficial for glass windows for conserving energy. Fully flexible, functional, and electroluminescent lamps can be produced by using Indium tin oxide green tapes. Moreover, indium tin oxide thin films are also utilized for acting as anti-reflective and coatings that are for electroluminescence and liquid crystal displays (LCDs), where thin films are utilized as transparent, conducting electrodes.

Transparent Conductive Coating

Indium tin oxide is usually utilized for making the transparent conductive coating for displays like electronic ink applications, touch panels, plasma displays, OLED displays, and liquid crystal displays. Indium tin oxide's thin films are also utilized in EMI shieldings, antistatic coatings, solar cells, and organic light-emitting diodes. Indium tin oxide is utilized as an anode (hole injection layer) in organic light-emitting diodes. Indium tin oxide films are utilized to defrost aircraft windshields as they are deposited on windshields. Voltage is applied across the film to generate heat.

Optical Coatings

Indium tin oxide is utilized for numerous optical coatings, preferably sodium vapor lamp glasses, and infrared-reflecting coatings (hot mirrors) for automotive. Bragg reflectors for VCSEL lasers, electrowetting on dielectrics, antireflection coatings, and gas sensors are the other included usages. For Low-E window panes, Indium tin oxide is utilized as the IR reflector. In order to increase the blue channel response, indium tin oxide was started to being utilized as a sensor coating in Kodak DCS 520, and then in the later Kodak DCS cameras. At temperatures of 1400 C or more, Indium tin oxide thin film strain gauges can function and they can be utilized in tough environments like rocket engines, jet engines, and gas turbines.

Alternative Synthesis Methods and Alternative Materials

Alternative materials and alternative methods for the preparation of Indium tin oxide are being searched due to indium's limited supply and high cost, the lack of flexibility and the fragility of the ITO layers, and the vacuum being required by the costly layer deposition.

Doped Compounds

We can use alternative materials too. As compared to tin, there are various transition metal dopants in indium oxide specifically molybdenum that gives more conductivity and more electron mobility. The yet-proposed alternative materials are doped binary compounds like indium-doped cadmium oxide and aluminum-doped zinc oxide (AZO). Others are the inorganic alternatives which include gallium, aluminum, or indium-doped zinc oxide (GZO, AZO, or IZO).

Carbon Nanotubes

Another prospective replacement is the carbon nanotube conductive coatings.

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Graphene

Another carbon-based alternative, graphene’s films are flexible and they allow 90% transparency along with a lesser electrical resistance as compared to standard Indium tin oxide. Another potential replacement material is thin metal films. Another hybrid material alternative is being currently tested, it is an electrode that the graphene covers, and it is composed of silver nanowires. Maintaining transparency while being simultaneously flexible and electrically conductive is one of the advantages of such materials.

Conductive Polymers

The development of ICPs (Inherently conductive polymers) is done for some applications of indium tin oxide. For conductivity polymers like PEDOT: PSS, and polyaniline, the conductivity is typically lower as compared to the conductivity for the inorganic materials, however, inorganic materials are cheaper, more flexible, and are more friendly to the environment in manufacturing and processing.

Amorphous Indium–Zinc Oxide

There has been the development of amorphous transparent conducting oxides for improving electrical homogeneity, decrease processing difficulty, and reducing the content of lithium. Amorphous indium-zinc-oxide is a material that maintains short-range order despite the difference in oxygen's ratio to metal atoms between ZnO and In2O3 disrupting crystallization. Indium zinc oxide has similar characteristics as Indium tin oxide.

Even at 500 C, the amorphous structure stays stable, allowing for some considerable processing steps that are usual in organic solar cells. In organic solar cells case, the material's usability can be considerably improved by the enhancement in the homogeneity. A percentage of the area of the cell is rendered unusable because of the poor performance of electrodes in the organic solar cells.

Types of Indium Tin Oxide

Liquid Metal-Based 2D ITO Synthesis

Indium tin alloys of low melting point are used in our process. Same alloys should be capable of depositing 2D Indium tin oxide on wafer scales and being utilized in a liquid metal printing process. Recently, there was the introduction of liquid metals as a reaction medium which helps in forming large-area 2D oxides.

Nanometre thin surface oxides are formed when low-melting-point post-transition metals like Sn, In, and Ga go through self-limiting Cabrera–Mott oxidation in air. There is a minimal adhesion of surface oxide to the parent metal when metal is in the liquid state, which allows a van der Waals transfer technique for being utilized for transferring the grown oxide sheet onto the desired substrate. Oxide dominates the surface oxide when alloys are utilized, providing the greatest reduction in free energy. It results in binary oxide dominating the surface oxide of low-melting-point liquid alloys in most cases. Also, various high-quality 2D metal oxides are created by exploiting this and it can grow on complex low-melting alloy’s surface as it acts as a reaction solvent effectively.

Alloys-an Exception

According to reports, the surface oxide of indium tin alloys is a ternary compound and it possesses both tin and indium, and its indium-tin ratio is similar to the indium-tin ratio of ITO, thus making the indium-tin alloys an exception.

Ternary oxides are formed on these alloys' surface and tin ion's high solubility in lithium oxide is the best explanation for it, and it results in the production of the Indium-tin oxides, which are more thermodynamically favored as compared to pure SnO2 and In2O3. The process that was utilized requires a liquid alloy's small droplet to be placed on the desired substrate, it is then squeezed from the top by a second substrate, therefore, the droplet is spread across the area and covers all of the desired areas. In ambient air, the Cabrera-Mott oxidation process's fast nature guarantees the production of homogeneous surface oxide on expanding of the metal, whereas the parent metal's liquid nature enables the surface oxide to attain conformal contact with the surface, resulting in van der Waals attachment.

Two Substrates

Liquid droplet converts back into the droplets that are spherical shaped because of the liquid metal’s high surface tension when there is a separation between two substrates. A developed cleaning process can be used to remove any leftover metal inclusions, showing highly transparent oxide sheets on various substrates like polymers, wagers, and glasses. LED can be switched on and a gap can be bridged in a circuit due to the highly conductive nature of the deposited transparent oxide sheets.

Bilayer 2D ITO

A focused ion beam (FIB) tool was used to prepare the cross-sections of deposited mono- and bilayer samples utilizing optimized alloy concentration. According to TEM analysis, both of the samples adhered to the surface of the SiO2. A pronounced Van der Waals spacing was displayed by the bilayer sample between the individual layers. Bilayer's overall thickness is in agreement with the AFM measurements. The proposed Cabrera–Mott growth model is strongly supported by the presence of the Van der Waals gap. In that model, each layer grows on the surface of the liquid metal to a set thickness, and desired substance can be reached by adding the extra layers.

Deposition Technique

This technique's low processing temperature isn't enough for inducing recrystallization across the van der Waals gap, however, it can form an indium tin oxide's new form which is a distinct material from conventional bulk Indium tin oxide. When the second layer is deposited, there comes a very large reduction of the sheet resistance, which can be explained by the Van der Waals gap.

Some probable interfacial contaminants, doping effects arising from the SiO2, and substrate's surface roughness (which are not atomically flat but optically smooth) influence the initially deposited monolayer's electronic characteristics as the substrate is in direct contact with that monolayer. The second layer is screened from detrimental substrate effects like these, which results in a considerably enhanced conductivity. There are reports that include the same observations regarding multilayer graphene and the other Van der Waals materials.

Hall Effect Measurements

According to mono- and bilayer samples’ hall effect measurements, 14 cm2V−1 s−1 of mobility was possessed at room temperature by the bilayer sample, with 1.7×1014 cm−2 of 2D carrier density. For 5K and fewer temperatures, the mobility was comparatively high (10 cm2V-1 s-1) according to low-temperature measurements which shows the absence of strong localization. A logarithmic increase with decreasing temperature was displayed by the conductance at temperatures less than 150K.

The behavior is similar to the behavior of the ultrathin Indium Tin oxide films that were reported previously. However, during the Hall measurements, drifting behavior was shown by the monolayer, and that proves it to be highly disordered polycrystalline films, with low mobility, supporting that the substrate heavily impacts the first-deposited layer, while there is an effective screening of the second layer, resulting in the observed considerably enhanced performance.

2D Indium Tin Oxide on Flexible Substrates

The deposition process was compatible with polyimide or other high temperature-resistant polymers because of the liquid alloy’s low melting point. There were fabrications of two-terminal resistive devices and they were subjected to repeated mechanical bending to 2.0, 2.5, and 3 mm. For each radius, 1000 bending cycles were applied to a device in total whereas 3000 mechanical bending cycles were applied on the 2D Indium tin oxide sheet. Also, after 3,000 cycles, the ITO layer’s resistance increases by less than 3.5 percent.

Comparison

A commercial Indium Tin oxide on a PET (polyethylene terephthalate) sample went through the same comparison test. Within five bending cycles to 3mm of radius, a complete failure was displayed by this sample. 2D Indium tin oxide sheets have a lot of applications in flexible optoelectronic devices due to their remarkable electronic characteristics and small radius of curvature, along with their compatibility with roll-to-roll processing.

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Capacitive Touch Screen Panels

Recently, capacitive touch screen panels (TSPs) are gaining a lot of attention because of their intuitive user interfaces for devices from smartphones to tablet PCs and other portable electronic devices. Capacitive touch screen panels are built conventionally with either the two transparent electrodes in series (one layer) or parallel (two layers). Due to remarkable characteristics like transparency and high conductivity, ITO (indium tin oxide) thin film is utilized as a transparent electrode in capacitive TSPs.

Although, a lot of optical loss of almost 16% is generated by the air/ITO/glass/air system as ITO's refractive index is way more than the refractive index of glass substrate because of Fresnel reflectance at the interfaces. In addition, the visibility is degraded by the reflectance difference (R) between the ITO electrode's un-etched and etched regions. Reducing both the R without altering ITO film's other required characteristics and the optical loss at the air/ITO/glass interface would be very desirable. In this paper, the process of minimizing the R and the optical losses are called index matching.

Index Matching

Porous Indium tin oxide was used by Yan et al. for reporting ITO electrode's index matching to a glass substrate for LCD applications. As compared to the sheet resistance of the conventional Indium tin oxide, the sheet resistance of the index-matched Indium tin oxide is way higher because of the porosity.

Here, index-matched indium tin oxide electrodes were fabricated for touch screen panel applications by utilizing two index-matching layers (IMLs) between a glass substrate and the ITO film. The optimum thickness of IMLs was found by using an optical simulation tool for a 30 nm indium tin oxide electrode. After 200 C annealing, an increase of more than 4.3% was obtained with a low sheet resistance of 90. In the visible range, less than 1% of the R is achieved.

Optical Transmittance

They have fabricated Index-matched indium tin oxide electrodes for capacitive touch screen panels for reducing R and improving optical transmittance between the un-etched and etched regions.

Transparent Conductive Oxides

The technological value of transparent conductive oxides (TCOs) is very significant due to their remarkable electronic band structure, making them both transparent and highly conductive in the visible range. TCOs have a current global market of almost 7 billion US dollars across all applications. One of their best application is its usage in touch-responsive screens, light-emitting diodes (LEDs), and in liquid-crystal displays, as transparent electrodes, as they are present in flat-panel monitors and smartphones.

Although, they are utilized in applications like a lab on chip biosensing, plasmonics, electrochromic windows, solar cells, optoelectronic devices, and low-emissivity windows, too.

Binary Oxides

With the dominant compounds being ZnO, SnO2, and In2O3, high-performance TCOs are based on some binary oxides. Material can attain less than 2 orders of magnitude of conductivity like that of metal by heavily doping these oxides. TCO's high transparency results in optical bandgaps that are considerably broader as compared to the fundamental electronic bandgap.

Key TCO

For several decades, there has been industrial usage of Indium Tin Oxide (ITO) as it is a key TCO. ITO has many drawbacks, one of its being its deposition that's commonly done by utilizing physical vapor-based deposition techniques that depend on batch processing and vacuum technology.

The deposited TCOs have 50-500 nm of typical film thickness because of their ceramic nature, these films are not compatible with flexible electronics as they are inherently brittle. In ITO, indium is an expensive element with comparatively low abundance, and in the 21st century, the relatively thick ITO films will be very expensive and unsustainable as global indium reserves are sufficient for supporting the demand well.

Application of 2D ITO in a Capacitive Touch Screen

Two-centimeter-sized printed monolayer ITO sheets were used to develop a prototype transparent capacitive touch screen. Those sheets were deposited on a single glass substrate's two sides.

Gold Electrodes

One electrode was fixed to the backside whereas four gold electrodes were deposited on the touch screen's front side in a square arrangement. A thin PET film physically adhered to the surface and it protected the device's complete front side. There are photos on the device's schematic and photograph. The function of the PET film is both as an insulator and a protective coating. The two Indium Tin Oxide functions as a plate capacitor, with glass substrate as dielectric when they applied an A.C. signal between one of the back and front electrodes.

The device's capacitance changed when a conductive object like a metal pin or finger was in the close vicinity of the device's front side, and it allowed proficient touch detection.

Change in Capacitance

The touched location’s distance from the front electrode determines the magnitude of change in the capacitance. First, for all four electrodes, measure the observed change in capacitance, then triangulate the position, which enables the touch detection with x–y resolution. It depends where the screen was touched as the change in the observed capacitance was highly sensitive to the location, allowing fully functional x-y touch screen operation. When a human finger touched the device, they obtained the same results.

Conclusion

Indium Tin Oxide has various types depending upon the characteristics and properties that it is comprised of. However, one of the major roles of ITO is in the mobile device screens. The conductive layer that if formed is highly protective and exhibits the behaviors that are necessary for smartphones and their touch screens.

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References

https://diamondcoatings.co.uk/indium-tin-oxide-touch-screen/

https://cutt.ly/qJGuBBo

http://diana.korea.ac.kr/xe/html/sci/sci_277.pdf

https://www.azom.com/article.aspx?ArticleID=9634