Ultrathin Indium Tin Oxide Films Research Articles

Breaking through the plasma wavelength barrier to extend the transparency range of ultrathin indium tin oxide films into the far infrared

Indium tin oxide (ITO) film, which is the most commonly used transparent conductive film (TCF), has traditionally been believed to be transparent in the visible spectrum but to reflect infrared (IR) light beyond the plasma wavelength (λp). However, our theoretical analysis challenges this notion by demonstrating that an ultrathin ITO TCF that is thinner than the light's penetration depth can overcome the transmission barrier at λp. To validate the theoretical modeling, we have successfully fabricated ultrathin ITO films that, despite having λp ≈ 1 μm, remain transparent from 400 nm to 20 μm. This represents the broadest transparency range ever reported for any In2O3-based TCF. The 10-nm-thick ITO TCFs have high visible transmittance (91.0% at 550 nm), low resistivity (5 × 10−4 Ω cm), and good IR transmittance (averaging 60% over 1.35–18.35 μm). Their IR transparency facilitates radiative cooling of the underlying circuitry. When an operational resistor is enclosed by commercial ITO TCFs that are 140 nm thick, its temperature increases. However, using 10-nm-thick ITO TCFs instead of the commercial ones can completely avoid this temperature rise. Moreover, attaching a silver grid to a 10-nm-thick ITO TCF can reduce the effective sheet resistance to ∼10 Ω/□ at the expense of only ∼3% transmittance. This development paves the way for large-scale applications that require low sheet resistance and far-IR transparency.

Large-scale, high-transparency, ultra-thin ITO membranes with robust conductivity and flexibility

Ultra-thin ITO films exhibit excellent optical properties but poor electrical properties, which greatly limit further applications in the field of optoelectronic devices. In this study, a large-area (4 inches) ITO thin film with a thickness of 10 nm was prepared on a flexible Mica substrate. The film shows low resistivity (1.32×10−4 Ω·cm), high carrier concentration (2.2 × 1021 cm−3), high optical transparency (>98.1%), and an atomic-level flat surface. XPS characterization and first principles calculations prove that the van der Waals interface between ITO and Mica can induce a higher concentration of oxygen vacancies in the ultra ITO thin film, which is the reason for its remarkable conductivity enhancement. In addition, the ITO/Mica flexible film exhibits excellent durability and recyclability, maintaining its photoelectric properties at a high bending radius of 2.5 mm and tens of thousands of bending cycles. This study will provide an important experimental basis for the further development and application of large-area, ultra-thin ITO films in flexible transparent electronic devices.

UWB MIMO optical transparent antenna based on ITO film

An optical transparent antenna using indium tin oxide (ITO) for ultra-wideband (UWB) multiple-input multiple-output (MIMO) applications is proposed. The ultrathin ITO film is coated on a 67×67mm2 glass substrate and over 85% transparency is achieved. The antenna consists of four coplanar waveguide (CPW)-fed antenna elements with rotationally symmetric placement. The adjacent elements are placed orthogonally to produce dual polarisation and reduce coupling. The MIMO antenna operates from 1.5 to 11GHz which covers the UWB applications. The coupling between the elements is less than -23dB. Mean effective gain (MEG), envelope correlation coefficient (ECC) and diversity gain (DG) achieve excellent values which guarantee good MIMO performance. The prototype is verified and the measured results are analysed.

High-stability flexible radio frequency transistor and mixer based on ultrathin indium tin oxide channel

In this work, a high-performance flexible radio frequency transistor using an ultrathin indium tin oxide film channel based on a solution-cast thin polyimide substrate has been demonstrated. The 60 nm short channel transistor shows a record high cut-off frequency of 5 GHz and a maximum oscillation frequency of 11 GHz with high uniformity among 40 devices. The radio frequency characteristics under various bending conditions have been systematically studied under a bending radius of 5 mm for 10 000 times and a bending radius of 1 mm for 1000 times, showing excellent stability with only 20% decrease in the cut-off frequency. Furthermore, a flexible frequency mixer has also been demonstrated at 2.4 GHz with decent conversion gains.

Effects of UV treatment on the properties of ultra-thin indium tin oxide films during growth and after deposition by cavity ring-down spectroscopy

In this work, an ultra-sensitive optical absorption technique based on Cavity Ring-Down Spectroscopy (CRDS) was employed to study the effects of UV treatment on the optical properties of ultra-thin indium tin oxide (ITO) films. The ITO films were submitted to UV treatment either after the deposition process or in-situ during the thin-film growth process. Different flow rates of oxygen in the vacuum chamber during film growth were also investigated. An ITO-coated glass substrate inserted in the CRDS cavity at a Brewster’s angle provided a ring-down time of about 1.6 µs, which enabled measurements of optical absorption loss as small as 3 × 10−6. To compare the effects of the UV film treatment, the CRDS technique was employed to measure the extinction coefficient for samples coated with and without the UV treatment. While the optical absorption data was being collected, the electrical resistivity was also simultaneously monitored. The post-deposition UV treatment was found to improve the optical transparency and the electrical performance of ITO film; the optical extinction coefficient of the ultra-thin ITO film is shown to decrease by about 24%. The in-situ UV treatment during growth is also shown to consistently increase the optical transparency of the ultra-thin ITO films and providing outstanding optical performance especially for high flow rates of oxygen during film growth. The electrical resistivity for oxygen flow rates in the range 0.6 - 1.4 sccm is also improved by the in-situ UV treatment, however it shows a sharp increase for oxygen flow rates beyond 1.4 sccm. The CRDS platform is demonstrated here to provide a highly accurate and sensitive methodology for measurement of minute optical absorption losses in ultra-thin films that typically cannot be precisely measured using other conventional spectrophotometric techniques.

A novel strategy of fabricated flexible ITO electrode by liquid metal ultra-thin oxide film

Flexible transparent conductive films are gaining attention day by day over the last few years due to it is a key component of next generation flexible electronics and optoelectronic devices. Indium tin oxide (ITO) as one of the most widely used transparent conductive material is limited by the traditional deposition approach cannot be achieve ultra-thin, which results in its brittle nature. Herein, a novel strategy for fabricating highly transparent conductive films by liquid metal interface phase separation technique based on low-melting liquid metal of InxSn100-x alloy is reported, during the solid-to-liquid phase transition of alloy, the monolayer of surface oxide film segregated with the bulk phase and was printed on the flexible polyethylene-naphthalate (PEN) substrate under the van der Waals. This novel strategy can directly print the ultra-thin self-oxide with the structure of ITO on PEN substrate, with the resulting of transparency over 97.5% and resistivity as low as 0.21 kΩ cm, providing a new way of low-cost raw ITO material as well as the personalized preparation strategy. The desirable highly transparent conductive films are comparable to recently reported ITO film, together with advantages of pretty steady, make them attractive as various flexible transparent conductive electrodes, for example, an ultra-thin ITO film is developed for luminescent devices.

Large Optical Nonlinearity of Dielectric Nanocavity-Assisted Mie Resonances Strongly Coupled to an Epsilon-near-Zero Mode.

Strong coupling provides a powerful way to modify the nonlinear optical properties of materials. The coupling strength of the state-of-the-art strongly coupled systems is restricted by a weak-field confinement of the cavity, which limits the enhancement of the optical nonlinearity. Here, we investigate a strong coupling between Mie resonant modes of high-index dielectric nanocavities and an epsilon-near-zero mode of an ultrathin indium tin oxide film and obtain an anticrossing splitting of 220 meV. Static nonlinear optical measurements reveal a large enhancement in the intensity-independent effective optical nonlinear coefficients, reaching more than 3 orders of magnitude at the coupled resonance. In addition, we observe a transient response of ∼300 fs for the coupled system. The ultrafast and large optical nonlinear coefficients presented here offer a new route towards strong coupling-assisted high-speed photonics.

Silicon MEMS Nanomechanical Membrane Flexure Sensor With Integrated High Gauge Factor ITO

Introduction of structural modification and novel transduction materials to nanomechanical cantilever (NMC) sensor can bring considerable improvement in sensor performance leading to ultra-sensitive nanomechanical sensor platforms. This work reports development of an optimized and highly sensitive silicon MEMS Nanomechanical Membrane-Flexure (NMF) piezoresistive surface stress sensor. This MEMS structure consists of a circular adsorbate membrane suspended by four inverse trapezoidal flexures. The electromechanical transduction of the sensor is performed by integrating a high gauge factor low temperature sputter deposited Indium Tin Oxide (ITO) thin film as piezoresistor. The ITO thin film was experimentally characterized for extracting its electrical, mechanical, and electromechanical properties to assess its candidature in integrating as strain sensing element with nanomechanical sensors. The gauge factor of ITO thin film was measured using a high precision four-point bending fixture experimental setup. An ultra-thin ITO film deposited at room temperature with no oxygen flow and post annealing treatments exhibited a high negative gauge factor of −430, making it an excellent candidate in nanomechanical sensor platform. The incorporation of sputter deposited ITO thin film as piezoresistive layer obviated the need for doping in silicon which further reduced the fabrication process complexity for NMF sensor. The NMF sensor fabrication following SOI based silicon MEMS process along with device characterization have also been carried out as part of this work. The optimized design of piezoresistive NMF sensor exhibited an improved surface stress sensitivity of nearly 15 times higher than conventional cantilever sensor and thereby opening new possibilities in bio-chemical and environmental sensing applications. [2021-0127]

Improved Electrical and Optical Properties of Ultra-Thin Tin Doped Indium Oxide (ITO) Thin Films by a 3-Dimensionally Confined Magnetron Sputtering Source

The ultra-thin tin doped crystalline indium oxide (ITO) films (≤50 nm) were successfully deposited by a 3-dimensionally confined magnetron sputtering source (L-3DMS) at the temperature lower than 100 °C. The resistivity and the mobility of the ultra-thin ITO films deposited at a low processing temperature were about ~5 × 10−4 Ω · cm and >30 cm2/Vs, respectively, for the thickness of 30 nm. The high quality of the ultra-thin ITO films deposited by L-3DMS is believed to be related to the improved crystallinity with oxygen vacancies of the ITO films by high density plasma and low discharge voltage of the L-3DMS which enables the formation of a crystalline structure a low processing temperature.

Influence of Surface Adsorption on the Electrical Transport Properties of Ultrathin Indium Tin Oxide Films

We systematically investigated the temperature-dependent behaviors of the resistivities ρ and carrier concentrations n of a series of ultrathin indium tin oxide (ITO) films with thickness t ranging from ∼5.3 to ∼35.0 nm grown on yttrium-stabilized ZrO2 substrates. For the t ≳ 17.5 nm films, the resistivity versus temperature can be described by the Bloch–Grüneisen formula, and the carrier concentration almost retains constant over the whole measured temperature range. For the t ≃ 5.3 nm (7.0 nm) film, the carrier concentration increases with increasing temperature above ∼300 K, and the resistivity initially increases with increasing temperature and then decreases with further increasing temperature above ∼330 K (∼355 K). By comparison of the transport properties of ITO films depositing on different substrates and exposing the films on different atmospheres, it is found that the enhancement of carrier concentration and reduction of resistivity at high temperature regime originate from the dissociative adsorption of water vapor on the surface of the ITO films.

Solution-processed ITO thin-film transistors with doping of gallium oxide show high on-off ratios and work at 1 mV drain voltage

Indium tin oxide (ITO) is generally used as an electrode material but has recently been demonstrated to be a competitive candidate for use in semiconductor layers in high-performance thin-film transistors (TFTs), due to its high mobility and strong resistance to wet-etching. Here, we demonstrate TFTs using solution-processed, ultra-thin ITO films with outstanding switching performance. These devices exhibit a mobility of up to 15 cm2 V−1 s−1 and a high on-off ratio of 108. Because the device exhibits significant instability under stress tests, moderate doping with Ga as a dopant is introduced to form Ga-doped ITO TFTs. The resulting device has much enhanced stability, near-zero turn-on voltage, and a high on-off current ratio of 108. Through further involvement of an AlOx dielectric layer, the Ga-doped ITO TFTs exhibit a high apparent mobility of more than 40 cm2 V−1 s−1 and operate at small gate voltages (3 V). Remarkably, the device maintains an on-off ratio of over 104 at drain voltages as small as 1 mV.

New design strategy for broadband perfect absorber by coupling effects between metamaterial and epsilon-near-zero mode

We propose a novel and simple method to achieve perfect light absorption over a broadband by coupling a gap plasmon mode to an epsilon-near-zero (ENZ) mode, which is a new path for tailoring light-matter interactions. Near-perfect light absorption (over 99%) in the wavelength range of 1400 nm–1655 nm can be achieved utilizing a gold-disk periodically patterned metal-dielectric-metal (MDM) device by integrating an ultrathin ITO (indium tin oxide) film. The ENZ mode can be excited effectively by coupling the gap plasmon resonance to conducting ITO (indium tin oxide) nanolayer (ENZ wavelength of the ITO nanofilm close to 1403 nm) under normal incidence. Compared with the device without ITO nanolayer, the device integrating ITO ultrathin film shows wideband perfect light absorption. Besides, the thickness of SiO2 spacer layer is critical for the coupling strength of metamaterial resonator and ENZ mode. To achieve broadband absorption, the coupling strength of the ENZ mode and gap plasmon mode should be between the strong coupling and weak coupling regimes. The method is also effective for the design of multiband optoelectronic devices with simple structure and low cost.

Application of ultra-thin indium–tin–oxide film in liquid crystal lens

The resistance of indium–tin–oxide (ITO) film increases with the decrease in film thickness. An ultra-thin ITO (UTITO) film with a high resistance is realised by adjusting the deposition time during sputtering. The fabricated UTITO film is then placed at the interface between the dielectric and liquid crystal (LC) layers in large-aperture hole-patterned LC lens. With an appropriate frequency and amplitude of applied voltage, the inserted UTITO film effectively spreads the voltage into the centre of the large-aperture hole-patterned LC lens, which assists in rotating the LC molecules therein. Consequently, the addressing voltage and switching time of the LC lens can be remarkably decreased. The LC lens with the insertion of UTITO film obtains a low wavefront error and high focusing quality and image performance. The innovative concept for fabricating the low-voltage LC adaptive lens with a UTITO film is competitive to other methods.

A highly conductive and flexible metal mesh/ultrathin ITO hybrid transparent electrode fabricated using low-temperature crystallization

The low-temperature processing of transparent electrodes on polymer substrates is an increasingly important issue in lightweight and flexible optoelectronic devices and low-temperature processing is essential since most polymer substrates cannot withstand high temperatures. We analyzed the electrical and optical characteristics of indium tin oxide (ITO) films according to the various crystallization conditions and confirmed that the low-temperature crystallized ITO has a rhombohedral structure. The ultrathin ITO films were crystallized at a temperature of 250 °C, which is lower than the glass transition temperature of polyimide substrates, and a hybrid transparent electrode with a metal mesh was formed. Our hybrid transparent electrode had a high transmittance of 77.4% and a low sheet resistance of 6.3 Ω/sq. After 1000 bending cycles at a bending radius of 6 mm, the hybrid transparent electrode maintained a sheet resistance of 6.5 Ω/sq. We applied rhombohedral ITO to the device rather than the commonly used cubic ITO, and as a result, it showed excellent electrical properties and better bending-stress stability.

Study of plasma characteristic and properties of flexible ultra-thin ITO films prepared by large area 3-D confined and planar magnetron sputtering

This work presents the plasma characteristics in a conventional planar magnetron source (PMS) and a large 3-D confined magnetron source (3DMS). We utilize different plasma diagnostic methods to study the plasma characteristics and the favorable plasma condition for the deposition of ultra-thin indium tin oxide (ITO) film near room temperature. Measurements of plasma parameters by Langmuir probe (LP) and energy flux by Calorimetric probe (CP) respectively showed a very high-density plasma and deposited energy flux (EF) on the substrate when operated by the 3DMS. Plasma diagnostic reveals that the plasma density measured in the 3DMS process is nearly one order higher compared to that of the PMS process. We consider the experimental condition of high-density plasma and deposition EF as the favorable process condition for the deposition. In this condition, flexible ultra-thin ITO (UT-ITO) films were deposited by changing the O2 flow rate. The role of negative oxygen ions (NOIs) on the film properties is studied at different O2 flow rates. The condition with the lowest NOI flux produced a UT-ITO film with the lowest resistivity ∼5.0 × 10−4 Ωcm and visible transmittance ∼82% with crystalline microstructure and smooth morphology using 3DMS at low-temperature.

Tailoring Opto-electrical properties of ultra-thin indium tin oxide films via filament doping: Application as a transparent cathode for indoor organic photovoltaics

Either side of organic photovoltaics incorporates a transparent conducting electrode for light entry, which becomes particularly crucial under indoor light conditions with low light intensity. However, since electrical resistivity and optical transparency with respect to film thickness are mutually contradictory, sufficiently thick (>150 nm) electrodes are inevitably required to ensure the appropriate electrical conductivity, at the sacrifice of transmittance. This paper introduces an electric field induced filament doping method to realize ultra-thin indium tin oxide with high conductivity. The proposed method allows for injecting metal dopants (i.e., Ni) into ultra-thin indium tin oxide under an electric field, enabling substantial resistance reduction while retaining high transmittance and low surface roughness. Optimum light absorption and effective carrier transport via filament doping provides improved performance for indoor organic photovoltaics. As a proof-of-principle demonstration, we fabricate poly(3-hexylthiophene):indene-C60 bisadduct based inverted organic photovoltaics with 10 nm Ni-doped indium tin oxide as a transparent cathode, leading to a power conversion efficiency of 14.6 ± 1.8% under the 1000-lux light-emitting diode. This efficiency is 40% higher than that achieved from the device using commercially available 150 nm indium tin oxide.

Electrical and optical properties of sputtered ultra-thin indium tin oxide films using xenon/argon gas

In this work we investigate the optical and electrical properties of sub-50 nm thick indium tin oxide (ITO) deposited by Radio Frequency magnetron sputtering technique. The effect of sputtering gas, oxygen concentration and substrate temperature on these properties was studied. ITO films were deposited on glass and silicon substrates using argon or xenon sputtering gases with oxygen flow rate from 0 to 3 sccm. The deposition temperature was 25 or 250 °C. Sub-50 nm ITO films with resistivity record as $$9.0\times 10^{-4}$$ Ω cm was achieved with xenon-sputtered films deposited at 250 °C. High optical transmission beyond 90% was observed in the range of 400–800 nm corresponding to the optical bandgap in the range of 3.5–3.8 eV.

Factors affecting the properties of highly conductive flexible ultrathin ITO films in confined large area magnetron sputtering in three dimensions

A large area magnetron source with the strongly confined magnetic field from all direction is applied for the deposition of flexible ultrathin ITO (UT-ITO) films of thickness 30 nm at room temperature for their applications as transparent electrodes. The films show a minimum resistivity of ∼5.0 x 10-4 Ωcm and high transmittance >80% at wavelengths of 400-700 nm. Measurements and data reveal that a high plasma density, high energy flux, and a relatively low concentration of negative oxygen ions (NOIs) to the flux of positive ions (PIs) induce lower mechanical stress to the growing films, which enables a lower resistivity and superior crystallinity with the smooth surface. The capability of the magnetron source and the characteristic plasma properties are studied in light of the resulting film properties. The considerably lower resistivity with higher carrier concentration and mobility of the UT-ITO films prepared at a high power density of 3 W/cm2 and a low O2 gas flow can be attributed to the growth of crystallized UT-ITO films, resulting in the combination of the oxygen vacancy and substitution of Sn4+ to In3+ site through the deposition of a high energy flux and a low flux ratio of NOIs to PIs.

Optical and electrical properties of ultra-thin indium tin oxide nanofilms on silicon for infrared photonics

Optical and electrical properties of indium tin oxide (ITO) films on Si substrates in the thickness range from 10 nm to 100 nm were investigated. Spectroscopic ellipsometry was used to obtain the complex permittivity of the ultra-thin films in the spectral range from visible to long-wave infrared. It was found that as the thickness decreases, the Drude component of the electric permittivity becomes vanishingly small, eventually leaving only positive permittivity values. This coincides with an epsilon-near-zero wavelength red-shift from 1.5 to 2.1 um for the films that retain negative permittivities. Hall measurements were conducted to determine that the mobility of the films correspondingly decreased from 35 cm2/Vs to single digits. This decreasing mobility is the result of a non-electrical dead layer that was determined to be ~14 nm thick, which occurs at the ITO/Si interface and is due predominately to interfacial defects. The thickness of this dead layer depends on the deposition process and substrate. The optical and electrical properties of ultra-thin ITO films are useful for the precise design of infrared plasmonic modulators, perfect light absorbers and other photonic devices.

Efficient phase matching algorithm for measurements of ultrathin indium tin oxide film thickness in white light interferometry

A novel method is proposed to measure the thickness of the indium tin oxide (ITO) film, which is less than 20 nm, using valid Fourier’s phase information of white light correlogram and curve matching algorithm. Based on the Fourier transform amplitude information, the valid phase distribution function that contains the thin transparent electrode ITO film thickness information has been successfully extracted. A curve matching algorithm based on standard deviation is employed to accurately calculate the thickness of such thin ITO films. The experimental results show that the thickness values were consistent with that determined using the stylus instruments, indicating that this method can be applied to measure the ITO film thickness ranging from 5 to 100 nm.