Sputter-deposited Indium Tin Oxide Research Articles

Wafer-Scale Characterization of 1692-Pixel-Per-Inch Blue Micro-LED Arrays with an Optimized ITO Layer.

Wafer-scale blue micro-light-emitting diode (micro-LED) arrays were fabricated with a pixel size of 12 μm, a pixel pitch of 15 μm, and a pixel density of 1692 pixels per inch, achieved by optimizing the properties of e-beam-deposited and sputter-deposited indium tin oxide (ITO). Although the sputter-deposited ITO (S-ITO) films exhibited a densely packed morphology and lower resistivity compared to the e-beam-deposited ITO (E-ITO) films, the forward voltage (VF) values of a micro-LED with the S-ITO films were higher than those with the E-ITO films. The VF values for a single pixel and for four pixels with E-ITO films were 2.82 V and 2.83 V, respectively, while the corresponding values for S-ITO films were 3.50 V and 3.52 V. This was attributed to ion bombardment damage and nitrogen vacancies in the p-GaN layer. Surprisingly, the VF variations of a single pixel and of four pixels with the optimized E-ITO spreading layer from five different regions were only 0.09 V and 0.10 V, respectively. This extremely uniform VF variation is suitable for creating micro-LED displays to be used in AR and VR applications, circumventing the bottleneck in the development of long-lifespan and high-brightness organic LED devices for industrial mass production.

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]

Dual mode OPV-OLED device with photovoltaic and light-emitting functionalities

The rapid development of organic optoelectronic devices such as organic photovoltaics (OPVs) and organic light-emitting devices (OLEDs) is largely attributable to their advantageous properties of their large area, ultrathin thickness, flexiblility, transparency, and solution processability. Herein, we fabricate and characterize a dual mode OPV-OLED device with three-terminal structure comprising a polymer-based bulk-heterojunction inverted OPV unit and a top-emission white phosphorescent OLED unit back-to-back connected via intermediate metal alloy electrode. Sputter-deposited indium tin oxide was used as a transparent cathode of the inverted OPV unit, whereas Ag-doped Al served as a common OPV/OLED anode, allowing the decoupling of electricity generation and light mission functions. Notably, the doping of Al by Ag facilitated the reduction of surface roughness, allowing the above electrode to be used as a common anode and dramatically reducing the leakage current. Finally, the top-emission OLED unit featured an ultrathin layer of Ag-doped Mg as a semitransparent cathode. Thus, successful integration of the OPV-OLED elements results in the decoupling of electricity generation and light emission functionalities, achieving a power conversion efficiency of 3.4% and an external quantum efficiency of 9.9%.

Formation of indium–tin oxide ohmic contacts for β-Ga2O3

Sputter-deposited indium–tin oxide (ITO) electrodes became ohmic contacts for unintentionally doped β-Ga2O3(010) substrates with a carrier concentration of 2 × 1017 cm−3 after rapid thermal annealing in a wide range of annealing temperatures of 900–1150 °C. The formation of an ohmic contact is attributed to interdiffusion between ITO and β-Ga2O3, as evidenced by the results of transmission electron microscopy and energy-dispersive X-ray spectroscopy. The interdiffusion decreases the band gap and increases the donor concentration of β-Ga2O3 at the interface, and forms an intermediate semiconductor layer desirable for carrier transport. The ITO ohmic contact is particularly useful for future β-Ga2O3 devices operated at high temperatures.

Influence of Oxygen Concentration on the Performance of Ultra-Thin RF Magnetron Sputter Deposited Indium Tin Oxide Films as a Top Electrode for Photovoltaic Devices.

The opportunity for substantial efficiency enhancements of thin film hydrogenated amorphous silicon (a-Si:H) solar photovoltaic (PV) cells using plasmonic absorbers requires ultra-thin transparent conducting oxide top electrodes with low resistivity and high transmittances in the visible range of the electromagnetic spectrum. Fabricating ultra-thin indium tin oxide (ITO) films (sub-50 nm) using conventional methods has presented a number of challenges; however, a novel method involving chemical shaving of thicker (greater than 80 nm) RF sputter deposited high-quality ITO films has been demonstrated. This study investigates the effect of oxygen concentration on the etch rates of RF sputter deposited ITO films to provide a detailed understanding of the interaction of all critical experimental parameters to help create even thinner layers to allow for more finely tune plasmonic resonances. ITO films were deposited on silicon substrates with a 98-nm, thermally grown oxide using RF magnetron sputtering with oxygen concentrations of 0, 0.4 and 1.0 sccm and annealed at 300 °C air ambient. Then the films were etched using a combination of water and hydrochloric and nitric acids for 1, 3, 5 and 8 min at room temperature. In-between each etching process cycle, the films were characterized by X-ray diffraction, atomic force microscopy, Raman Spectroscopy, 4-point probe (electrical conductivity), and variable angle spectroscopic ellipsometry. All the films were polycrystalline in nature and highly oriented along the (222) reflection. Ultra-thin ITO films with record low resistivity values (as low as 5.83 × 10−4 Ω·cm) were obtained and high optical transparency is exhibited in the 300–1000 nm wavelength region for all the ITO films. The etch rate, preferred crystal lattice growth plane, d-spacing and lattice distortion were also observed to be highly dependent on the nature of growth environment for RF sputter deposited ITO films. The structural, electrical, and optical properties of the ITO films are discussed with respect to the oxygen ambient nature and etching time in detail to provide guidance for plasmonic enhanced a-Si:H solar PV cell fabrication.

Microstructural characterization of high-quality indium tin oxide films deposited by thermionically enhanced magnetron sputtering at low temperature

In this work, high-quality indium tin oxide (ITO) films are deposited by thermionically enhanced magnetron sputtering at a low deposition temperature. Microstructural evolution in the various growth stages is elucidated using material characterization techniques including grazing-incidence X-ray diffractometry and high-resolution transmission electron microscopy.Experimental results reveal that the ITO film is mainly amorphous throughout its thickness when it is deposited at ambient temperature without thermionic enhancement (TE). In contrast, the ITO film that is deposited at ambient temperature with TE has a columnar crystalline structure with grains of 40–80 nm in size. When the deposition temperature is increased to 200 °C with TE, the film exhibits a complete columnar crystalline structure throughout its thickness with grain sizes of 60–100 nm. The sputter-deposited ITO films using TE exhibit a higher average visible transmittance and a lower electrical resistivity than those without using TE because of the enhancement in their crystalline growth by the intensive ion bombardment.

Solution-processed flexible transparent conductors composed of silver nanowire networks embedded in indium tin oxide nanoparticle matrices

Although silver nanowire meshes have already demonstrated sheet resistance and optical transmittance comparable to those of sputter-deposited indium tin oxide thin films, other critical issues including surface morphology, mechanical adhesion and flexibility have to be addressed before widely employing silver nanowire networks as transparent conductors in optoelectronic devices. Here, we demonstrate the efficacy of low temperature solution-processed flexible metal nanowire networks embedded in a conductive metal oxide nanoparticle matrix as transparent conductors, and investigate their microstructural, optoelectronic, and mechanical properties in attempting to resolve nearly all of the technological issues imposed on silver nanowire networks. Surrounding silver nanowires by conductive indium tin oxide nanoparticles offers low wire to wire junction resistance, smooth surface morphology, and excellent mechanical adhesion and flexibility while maintaining the high transmittance and the low sheet resistance. In addition, we discuss the relationship between sheet resistance and transmittance in the silver nanowire composite transparent conductors and their maximum achievable transmittance. Although we have selected silver nanowires and indium tin oxide nanoparticle matrix as demonstration materials, we anticipate that various metal nanowire meshes embedded in various conductive metal oxide nanoparticle matrices can effectively serve as transparent conductors for a wide variety of optoelectronic devices owing to their superior performance, simple, cost-effective, and gentle processing.

Stability of ITO Thin Film on Flexible Substrate Under Thermal Aging and Thermal Cycling Conditions

Indium-tin-oxide (ITO) is a transparent conductive thin film that is widely used as a top conducive layer in photovoltaic solar cells. However, ITO is sensitive to environmental conditions and the electrical conductivity degrades as a consequence of harsh environmental conditions. Furthermore, the thermal expansion coefficient mismatch between the ITO film and the substrate creates stress/strain on the film when the package is subjected to fluctuating temperatures. This could create micro-cracks and consequently damage the film. Therefore, this study was designed to study the effect of the thermal cycling and thermal aging on the ITO thin films to simulate the effect of continuous high temperatures and fluctuating temperatures that may be applied on the thin films during the usage. In this study, two sets of experiments were conducted on a 60Ω/□square sputter-deposited ITO on 127 μm heat stabilized Poly Ethylene Terephthalate (PET) substrate. The first set of experiments contained four samples which were thermally aged at 100°C for 30 days and the other set of experiments contained another four samples which were thermally cycled for 900 cycles. The thermal profile consisted of a high temperature of 100°C, a low temperature of 0°C, dwell time of 10 minutes, and ramp rate of 10°C/min , as depicted in Fig. 1. The initial results showed that the ITO thin film is not stable in the thermal aging experiment and the electrical resistivity gradually increased for all samples until the end of the 30 days. The degradation happened during the thermal cycling as well. However, SEM images show that the morphology of the ITO surface is stable under both conditions. Energy-dispersive X-ray (EDX) spectroscopy analysis showed stability in the ITO thin film in terms of composition. XRD spectra confirmed the improved crystallinity for the thermally aged films, which corresponded to the increased transmission in the visible region.

Effect of heat treatment on the NO2-sensing properties of sputter-deposited indium tin oxide thin films

Transparent conducting indium tin oxide (ITO) films were deposited onto glass substrates by radio-frequency magnetron sputtering at 648 K, under an oxygen partial pressure of 1 Pa. The effect of annealing on the electrical properties of the films was studied. Characterization of the coatings revealed an electrical resistivity below 6.5 × 10− 3 Ω cm. The ITO films deposited at 648 K were amorphous, while the crystallinity improved after annealing at 700 K. The surface morphology examined by scanning electron microscopy appears to be uniform over the entire surface area after annealing. The NO2-sensing properties of the ITO films were investigated and showed sensitivity at concentrations lower than 50 ppm, at a working temperature of 600 K.

Structural, optical, and electrical properties of E-beam and sputter-deposited ITO films for LED applications

The structural, electrical, and optical properties of e-beam and sputter-deposited Indium-Tin-Oxide (ITO) films on sapphire and p-GaN wafers, as well as the post-deposition annealing effect for light-emitting-diode applications, were analyzed and compared. The specific contact resistance of the sputtered ITO film on a p-GaN wafer was found to be slightly higher than that of the e-beam deposited ITO films. However, in general, the electrical, structural and optical properties of the sputtered ITO films that were annealed at comparably lower temperature are better than those of the e-beam deposited ITO films, which is the consequence of a larger average grain size. The sputtered ITO film that was annealed at 500°C demonstrated a resistivity of as low as 3.6 × 10−4 Ω·cm, along with a high transmittance of ∼98% at 450 nm.

Thickness Dependence of Structural, Electrical and Optical Properties of Sputter Deposited Indium Tin Oxide Films

Thickness Dependence of Structural, Electrical and Optical Properties of Sputter Deposited Indium Tin Oxide Films

Effects of Hydrogen in Working Gas on Valence States of Oxygen in Sputter-Deposited Indium Tin Oxide Thin Films

X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy-elastic recoil detection analysis (RBS-ERDA) revealed that hydrogen in working gas for dc-plasma sputter deposition resided in indium tin oxide (ITO) films and generated the O(-) state seen as the suboxide-like O 1s peak in XPS. Growth of the suboxide-like O 1s peak was parallel with an increase of the resided hydrogen quantified by RBS-ERDA. The first-principles band structure calculation revealed that the electronic structure of In(2)O(3) crystal was realized typically for the most conductive as-deposited film grown in the gas containing hydrogen of 1%. The as-deposited film grown in the gas containing hydrogen of more than 1% exhibited rather high density but low mobility of carriers and showed the electronic structure above 4 eV originated from the O(-) state due to the resided hydrogen in addition to that of the most conducting one. Both well preserved In(2)O(3) band structure and proper concentration of the O(2-) vacancy are indispensable for achieving the highest conductivity; however, the O(-) state lowers efficiency of the carrier doping using the O(2-) vacancy in the lattice and increases density of the ionized scattering center for the carriers.

Conductive oxide thin films: Model systems for understanding and controlling surface plasmon resonance

Degeneratively doped conductive oxides represent a unique host for exploring the inter-relationship between the properties of charge carriers and their collective plasmonic response. These materials often lack interband transitions that obfuscate interpretation of spectral response in elemental metals, and unlike metals, the electronic transport properties of conductive oxides are easily tunable. This work explores the process-structure-property relationships that regulate surface plasmon resonance (SPR) in sputter deposited indium tin oxide (ITO) thin films. Film deposition conditions are used to regulate film microstructure and tune the electronic mobility to between 7 and 40 cm2 V−1 s−1. Postdeposition annealing in low oxygen partial pressure atmospheres is used to engineer the ITO defect equilibrium and modulate carrier concentrations to between 1020 and 1021 cm−3. These electronic transport properties are modulated with near independence enabling straightforward interpretation of their influence on the SPR response observed in the infrared reflectivity spectrum. Higher electronic mobilities favor narrower surface plasmon absorption bands, while higher carrier concentrations favor higher absorption band frequencies. A simple free electron model, having only electronic carrier density and electronic mobility as variables, can be used to describe ITO’s dielectric response. Calculations that combine this dielectric function and the Fresnel equations provide simulated reflectivity spectra that match experimental data with remarkable accuracy. Because these spectra use no fitting parameters and are calculated with well-studied material properties, it opens the opportunity for future design of plasmonic response in advanced material systems including degeneratively doped semiconductors, silicides, and nitrides.

Electron-Beam and Sputter-Deposited Indium–Tin Oxide Omnidirectional Reflectors for High-Power Wafer-Bonded AlGaInP Light-Emitting Diodes

Performance variation of high-power wafer-bonded AlGaInP light-emitting diode (LED) with an emission wavelength of induced by E-beam and sputter-deposited indium–tin oxide (ITO) films, one component of triple-layer omnidirectional reflectors (ODRs), has been investigated in detail. The entire ODRs consist of p-GaP, dispersive dot contacts of acting as ohmic contacts, an intermediate low-refractive index ITO, and a silver (Ag) layer. The results show that annealing under nitrogen atmosphere yields a much rougher surface for the E-beam evaporated ITO than that for the magnetron sputtered one, which leads to a lower reflectivity of the ITO/Ag system. A similar resistivity of the two ITO films after annealing confirms no influence on the current-voltage characteristics of the corresponding devices. The smoother surface morphology of the sputter-deposited ITO after annealing enhances the light output power of 18% as compared to the one with the E-beam evaporated ITO at for the ITO ODR-based AlGaInP LEDs.

Interfacial reactions between indium tin oxide and triphenylamine tetramer layers induced by photoirradiation

The effects of photoirradiation on the interfacial chemical reactions between indium tin oxide (ITO) films and layers of triphenylamine tetramer (TPTE) were investigated by using in situ x-ray photoelectron spectroscopy (XPS). Thin TPTE layers deposited onto sputter-deposited ITO films were irradiated with violet light-emitting diodes (peak wavelength: 380 nm). Shifts in the peak positions of spectral components that originated in the organic layer toward the higher binding-energy side were observed in the XPS profiles during the early stages of irradiation. No further peak shifts were observed after additional irradiation. An increase in the ratio of the organic component in the O 1s spectra was also observed during the photoirradiation. The ratio of the organic component increased in proportion to the cube root of the irradiation time. These results suggest that photoirradiation induces an increase in the height of the carrier injection barrier at the interface between TPTE and ITO in the early stages of the irradiation, possibly due to the rapid diffusion controlled formation and growth of an oxidized TPTE layer, which is considered to act as a high resistance layer.

Highly Selective Response of Dopamine Against its Metabolite and Interfering Molecules at Sputtered ITO Electrode Surface

The highly selective detection of dopamine (DA) against 3,4-dihydroxyphenylacetic acid (DOPAC), L-ascorbic acid (AA), and uric acid (UA) was studied using a sputter deposited indium tin oxide electrode. The sensitivity of the electrode surface was more than 100 times higher for DA than for DOPAC, AA and UA as revealed by voltammograms obtained in a phosphate buffer. The DA response is linear from 10−6 to 10−3 M in the presence of 0.1 mM AA, suggesting that high selectivity is achieved in the mixed solution.

Transparent conducting sol–gel ATO coatings for display applications by an improved dip coating technique

Transparent conducting coatings of sol–gel ATO (antimony-doped tin oxide) were used to improve surface smoothness of commercial sputter-deposited ITO (indium tin oxide) coatings for application as display electrodes. In order to overcome the deteriorating evaporation-cooling during dip coating, the coating solution was heated moderately to 25 °C thus providing the substrate with the required heat. This way, the surface roughness of the ITO could be reduced with an only 45 nm thick ATO coating to R pv = 3.8 nm ( R a = 0.4 nm) compared to 31 nm (3.8 nm) for the ITO substrate. Another benefit of such additional coating is the possibility to tailor surface properties of the electrodes in wide ranges. This was used to increase the work function of the ITO substrate from initially 4.3–4.6 eV to about 4.8–5.2 eV by the ATO coating.

Radio frequency magnetron sputter-deposited indium tin oxide for use as a cathode in transparent organic light-emitting diode

Indium tin oxide (ITO) films were prepared by radio frequency magnetron sputtering at room temperature, for use as a cathode in a transparent organic light-emitting diode (TOLED). To minimize damage to the TOLED by the ITO sputtering process, the target-to-substrate distance was increased to 20 cm. An ITO film deposited at the optimum oxygen partial pressure exhibited an electrical resistivity as low as 4.06 × 10 − 4 Ω cm and a high optical transmittance of 91% in the visible range. The film was used as a transparent cathode for a TOLED with structure of an ITO coated glass substrate / Naphthylphenyldiamide (60 nm) / Tris–(8-hydroxyquinoline) aluminum (60 nm) / LiF (1 nm) / Al (2 nm) / Ag (8 nm) / ITO cathode (100 nm). A maximum luminance of 37,000 cd/m 2 was obtained. The device performance was comparable to a conventional OLED.

Semitransparent Organic Photodetectors Utilizing Sputter-Deposited Indium Tin Oxide for Top Contact Electrode

Semitransparent organic photodetectors with the heterostructure of copper phthalocyanine (CuPc) and N,N'-bis(2,5-di-tert-butylphenyl) 3,4,9,10-perylenedicarboximide (BPPC) were fabricated using sputter-deposited indium tin oxide (ITO) for a top contact electrode, and the I–V and frequency characteristics were investigated. To prevent ITO penetration into an organic layer, an amorphous carbon nitride (a-C:N) buffer layer was placed between the organic layer and an ITO electrode. A cutoff frequency of more than 2 MHz was obtained for a semitransparent photodetector by applying a reverse bias voltage of -2 V under visible light illumination. We demonstrated that the fabricated semitransparent organic photodetectors can be applicable to a photoreflector with light-emitting diodes for integrated devices with organic light-emitting devices.

Inverted hybrid organic light-emitting device with polyethylene dioxythiophene-polystyrene sulfonate as an anode buffer layer

In this study we demonstrate inverted organic light-emitting diodes (OLEDs) utilizing highly conductive polyethylene dioxythiophene–polystyrene sulfonate as a buffer layer to a radio-frequency sputter-deposited indium–tin–oxide as the anode. In comparison to an entirely small-molecule-based reference, a reduction in operation voltage of 8.4 V at superior efficiencies of 4.2 cd/A and 1 lm/W obtained at 1.000 cd/m2 was achieved. The inverted cell configuration is desirable for next-generation active-matrix OLED displays.