Influence of deposition parameters and heat treatment on the NO2 sensing properties of nanostructured indium tin oxide thin film

Highly oriented and transparent indium tin oxide (ITO) films have been deposited onto glass substrates by radio frequency magnetron sputtering at 648 K, under an oxygen partial pressure of 1 Pa. The effect of the sputtering power and annealing was studied. Transmission was measured with a double beam spectrometer and electrical analysis using four probe and Hall effect setup. Structural characterization of the films was done by X-ray diffraction. 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 optical transmittance of the film was more than 80% in the visible region. 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. At a working temperature of 600 K, the ITO sensor showed high sensitivity to NO₂ gas, at concentrations lower than 50 ppm.     

Transparent thermally stable poly(etherimide) film as flexible substrate for OLEDs

In this work, ITO thin films were deposited onto poly(etherimide) (PEI) substrates at room temperature using r.f. magnetron sputtering and successively they were annealed in the 423-523 K (150-250 °C) temperature range. PEI/ITO substrates were structurally, optically and electrically characterized in order to verify the quality of the deposited ITO films and the PEI thermal stability during the ITO annealing process. A transmittance of about 80% was measured in the visible range. The best electrical properties achieved were: 3.04 × 10^− 4 Ω cm, 12.07 × 10²¹cm²/V.s, 16.8 × 10²¹ cm^− 3, for resistivity, carrier concentration and mobility, respectively. Small molecule Flexible Organic Light Emitting Diodes (FOLED) were then fabricated and characterized onto ITO functionalized PEI substrates. These preliminary results show clearly that PEI can be successfully used as substrate in flexible optoelectronic devices either operating in high temperature or when the process needs high temperatures.     

Effect of deposition parameters and annealing temperature on the structure and properties of Al-doped ZnO thin films

Transparent conductive films of Al-doped ZnO (AZO) were deposited onto inexpensive soda-lime glass substrates by radio frequency (rf) magnetron sputtering using a ZnO target with an Al content of 3 wt%. The Taguchi method with a L₉ orthogonal array, signal-to-noise (S/N) ratio and analysis of variance (ANOVA) were employed to examine the performance characteristics of the coating operations. This study investigated the effect of the deposition parameters (rf power, sputtering pressure, thickness of AZO films, and substrate temperature) on the electrical, structural, morphological and optical properties of AZO films. The grey-based Taguchi method showed the electrical resistivity of AZO films to be about 9.15 × 10⁻³ Ω cm, and the visible range transmittance to be about 89.31%. Additionally, the films were annealed in a vacuum ambient (5.0 × 10⁻⁶ Torr) at temperatures of 400, 450, 500 and 600 °C, for a period of 30 min. It is apparent that the intensity of the X-ray peaks increases with annealing treatment, leading to improved crystallinity of the films. By applying annealing at 500 °C in a vacuum ambient for 30 min, the AZO films show the lowest electrical resistivity of 2.31 × 10⁻³ Ω cm, with about 90% optical transmittance in the visible region and a surface roughness of Ra = 12.25 nm.     

Electrical and optical properties of indium tin oxide films prepared on plastic substrates by radio frequency magnetron sputtering

The influence of deposition power, thickness and oxygen gas flow rate on electrical and optical properties of indium tin oxide (ITO) films deposited on flexible, transparent substrates, such as polycarbonate (PC) and metallocene cyclo-olefin copolymers (mCOC), at room temperature was studied. The ITO films were prepared by radio frequency magnetron sputtering with the target made by sintering a mixture of 90 wt.% of indium oxide (In₂O₃) and 10 wt.% of tin oxide (SnO₂). The results show that (1) average transmission in the visible range (400-700 nm) was about 85%-90%, and (2) ITO films deposited on glass, PC and mCOC at 100 W without supplying additional oxygen gas had optimum resistivity of 6.35 × 10⁻⁴ Ω-cm, 5.86 × 10⁻⁴ Ω-cm and 6.72 × 10⁻⁴ Ω-cm, respectively. In terms of both electrical and optical properties of indium tin oxide films, the optimum thickness was observed to be 150-300 nm.     

Properties of Ce-doped ITO films deposited on polymer substrate by DC magnetron sputtering

Ce-doped indium tin oxide (ITO:Ce) films were deposited on flexible polyimide substrates by DC magnetron sputtering using ITO targets containing various CeO₂ contents (CeO₂ : 0, 0.5, 3.0, 4.0, 6.0 wt.%) at room temperature and post-annealed at 200 °C. The crystallinity of the ITO films decreased with increasing Ce content, and it led to a decrease in surface roughness. In addition, a relatively small change in resistance in dynamic stress mode was obtained for ITO:Ce films even after the annealing at high temperature (200 °C). The minimum resistivity of the amorphous ITO:Ce films was 3.96 × 10^− 4 Ωcm, which was deposited using a 3.0 wt.% CeO₂ doped ITO target. The amorphous ITO:Ce films not only have comparable electrical properties to the polycrystalline films but also have a crystallization temperature > 200 °C. In addition, the amorphous ITO:Ce film showed stable mechanical properties in the bended state.     

Effect of thermal annealing treatment on structural, electrical and optical properties of transparent sol-gel ZnO thin films

Effect of thermal annealing in different ambients on the structural, electrical and optical properties of the sol-gel derived ZnO thin films are studied. XRD results show that the annealed ZnO films with wurtzite structure are randomly oriented. Crystallite size, carrier concentration, resistivity and mobility are found to be dependent on the annealing temperature. The change in carrier concentration is discussed with respect to the removal of adsorbed oxygen from the grain boundaries. The highest carrier concentration and lowest resistivity are 8 × 10¹⁸ cm⁻³ and 2.25 × 10⁻¹ Ω cm, respectively, for the film annealed at 500 °C in vacuum. The annealed films are highly transparent with average transmission exceeding 80% in the wavelength region of 400-800 nm. In all three ambients, the optical band gap value does not change much below 500 °C temperature while above this temperature band gap value decreases for nitrogen and air and increases for vacuum.     

Structural, electrical and optical studies of SILAR deposited cadmium oxide thin films: Annealing effect

Successive ionic layer adsorption and reaction (SILAR) method has been successfully employed for the deposition of cadmium oxide (CdO) thin films. The films were annealed at 623 K for 2 h in an air and changes in the structural, electrical and optical properties were studied. From the X-ray diffraction patterns, it was found that after annealing, H₂O vapors from as-deposited Cd(O₂)_0.88(OH)_0.24 were removed and pure cubic cadmium oxide was obtained. The as-deposited film consists of nanocrystalline grains of average diameter about 20-30 nm with uniform coverage of the substrate surface, whereas for the annealed film randomly oriented morphology with slight increase in the crystallite size has been observed. The electrical resistivity showed the semiconducting nature with room temperature electrical resistivity decreased from 10⁻² to 10⁻³ Ω cm after annealing. The decrease in the band gap energy from 3.3 to 2.7 eV was observed after the annealing.     

Changes in the structural and electrical properties of vacuum post-annealed tungsten- and titanium-doped indium oxide films deposited by radio frequency magnetron sputtering

Tungsten- and titanium-doped indium oxide (IWO and ITiO) films were deposited at room temperature by radio frequency (RF) magnetron sputtering, and vacuum post-annealing was used to improve the electron mobility. With increasing deposition power, the as deposited films showed an increasingly crystalline nature. Compared with ITiO films, IWO films showed crystallinity at lower RF power. IWO films are partially crystallized at 10 W deposition power and become nearly fully crystalline at 20 W. ITiO films are fully crystalline only at 75 W. For this reason, film thickness has a greater impact on the electrical properties of IWO films than ITiO films. Vacuum post-annealing is more effective in improving electron mobility for amorphous than for (partially) crystalline IWO and ITiO films. Changes in the electrical properties of ITiO films can be better controlled as a function of annealing temperature than those of IWO films. Finally, post annealed 308 nm-thick IWO and 325 nm-thick ITiO films have approximately 80% transmittance in visible and near infrared wavelengths (up to 1100 nm), while their sheet resistances decrease to 9.3 and 10 Ω/□, and their electron mobilities are 51 cm²V^− 1 s^− 1 and 50 cm²V^− 1 s^− 1, respectively, making them suitable for use as Transparent Conductive Oxide layers of low bandgap solar cells.     

Fabrication of transparent conductive films with a sandwich structure composed of ITO/Cu/ITO

New transparent conductive films that had a sandwich structure composed of ITO/Cu/ITO multilayer films were prepared by a conventional RF and DC magnetron sputtering process on a polycarbonate substrate without intentional substrate heating. The thickness of each layer in the ITO/Cu/ITO films was kept constant at 50 nm/5 nm/45 nm. The optoelectrical and structural properties of the films were compared with conventional ITO single-layer films and ITO/Cu/ITO multilayered films. Although both films had identical thickness, 100 nm, the ITO/Cu/ITO films showed a lower resistivity, 3.5 × 10⁻⁴ Ω cm. In optical transmittance measurements, however, the ITO single-layer films showed a higher transmittance of 74% in the wavelength range of 300-800 nm. XRD spectra showed that both the ITO and ITO/Cu/ITO films were amorphous. The figure of merit, φ_TC, reached a maximum of 5.2 × 10⁻⁴ Ω⁻¹ for the ITO/Cu/ITO films, which was higher than the φ_TC of the ITO films (1.6 × 10⁻⁴ Ω⁻¹). The φ_TC results suggested that ITO/Cu/ITO films had better optoelectrical properties than conventional ITO single-layer films.     

Optical and electrical properties of heat-resistant Sb-doped Sn1 − xHfxO2 transparent conducting films

To examine variations in the transparent conducting properties after annealing at high temperatures, 300-nm thick Sb-doped Sn_1 − xHf_xO₂ (x = 0.00-0.10) films were deposited onto silica glass substrates by the RF sputtering method and annealed in air up to 1000 °C at 200 °C increments. After annealing, all the Sb-doped SnO₂ films were transparent and electrically conductive, but large cracks, which decreased the electrical conductivity, were generated in several films due to crystallization or the thermal expansion difference between the film and substrate. Only the film deposited at room temperature in an Ar and O₂ mixed atmosphere did not crack after annealing, and its electrical conductivity exceeded 100 S cm^− 1 even after annealing at 1000 °C in air. Hf-doping blue shifted the fundamental absorption edges in the UV region in the Sb-doped Sn_1 − xHf_xO₂ films. Additionally, the optical transmission at 310 nm, T₃₁₀, increased as the Hf concentration increased, whereas the electrical conductivity was inversely proportional to the Hf concentration. On the other hand, thinner films (150-nm thick) with x = 0.00 showed both a high electrical conductivity over 100 S cm^− 1 and a high transparency T₃₁₀ = 65% after high temperature annealing.     

Effects of annealing temperature and heat-treatment duration on electrical properties of sol-gel derived indium-tin-oxide thin films

Transparent indium tin oxide (ITO) thin films have been deposited by the dip-coating process on silica substrates using solutions of 2,4-pentanedione, ethanol, indium and tin salts. The films have been first dried in air at 260 °C for 10 min and then annealed in a reducing atmosphere at different temperatures for various durations. The resistivity of ITO layers was found to decrease with increasing the metal concentration of the starting solution or the annealing temperature. Hence, by adjusting both metal concentration in the coating solution and heat-treatment, resistivities lower than 5 × 10^− 3 Ω cm for an annealing temperature of 550 °C and lower than 2 × 10^− 2 Ω cm for an annealing temperature of 300 °C, were obtained. These results are correlated with the density and the size of ITO grains in the films.     

The properties of radio frequency sputtered transparent and conducting ZnO:F films on polyethylene naphthalate substrate

We report on the properties of ZnO:F films deposited by RF sputtering on polyethylene naphthalate (PEN) substrates and compared them with films deposited on glass. Detailed and systematic investigations of various properties of films were deposited on PEN substrates were carried out as functions of thickness and annealing ambient. The films were deposited at room temperature and annealed at 150 °C in either Ar or 7% H₂/Ar ambients. These films exhibited carrier concentrations between 2 × 10¹⁸/cm³ and 9.5 × 10¹⁹/cm³, mobility between 3 and 11 cm²/V-s, and resistivity between 10^− 1 and 10^− 2 Ω-cm. Hall mobility variation with concentration has been explained assuming ionized impurity and lattice scattering to be the dominant mechanisms. The transmission of the films varied from 68 to 80% with increasing thickness and the absorption edge was limited by the absorption of the PEN substrate. The mechanical flexibility of the films was measured in terms of its critical radius of bending which was determined from the onset of a sharp increase in electrical resistance. The critical radius varied between 6.5 and 17 mm for film thicknesses varying from 20 to 200 nm. The thickness dependence of critical strain and critical radius can be explained by Griffith defect theory.     

Low-temperature processing of transparent conductive indium tin oxide nanocomposites using polyvinyl derivatives

We report on the influence of additives on the electrical, optical, morphological and mechanical properties of transparent conductive indium tin oxide (In₂O₃:Sn; ITO) nanoparticle films by the use of polymers as matrix material. Key issues to fabricate layers suitable for use in electronic device applications are presented. Polyvinyl derivatives polyvinyl acetate, polyvinyl alcohol (PVA) and polyvinyl butyral were applied and their suitability to form transparent conductive ITO nanocomposite coatings at a maximum process temperature of 130 °C was investigated. A low-temperature treatment with UV-light has been developed to provide the possibility of curing ITO thin films deposited on substrates which do not withstand high process temperatures. Compared to best pure ITO layers (0.2 Ω^− 1 cm^− 1), the ITO-PVA nanocomposite coatings show a conductance value of 4.1 Ω^− 1 cm^− 1 and 5.9 Ω^− 1 cm^− 1 after reducing in forming gas. Sheet resistance of ca. 1200 Ω/□ with coexistent transmittance of 85% at 550 nm for a layer thickness of about 1.45 μm was achieved. The conductance enhancement is a consequence of nanoparticulate ITO network densification due to the acting shrinkage forces caused by the polymer matrix during film drying and additionally UV-induced crosslinking of PVA.     

Structural, electrical and optical properties of ITO films with a thin TiO2 seed layer prepared by RF magnetron sputtering

Tin-doped indium oxide (ITO) films were deposited by RF magnetron sputtering on TiO₂-coated glass substrates (the TiO₂ layer is usually called seed layer). The properties of ITO films prepared at a substrate temperature of 300 °C on bare and TiO₂-coated glass substrates have been analyzed by using X-ray diffraction, atomic force microscope, optical and electrical measurements. Comparing with single layer ITO film, the ITO film with a TiO₂ seed layer of 2 nm has a remarkable 41.2% decrease in resistivity and similar optical transmittance. The glass/TiO₂ (2 nm)/ITO film achieved shows a resistivity of 3.37 × 10⁻⁴ Ω cm and an average transmittance of 93.1% in the visible range. The glass/TiO₂ may be a better substrate compared with bare glass for depositing high quality ITO films.     

Thermoelectric properties of nanostructured Al-substituted ZnO thin films

ZnO:Al nano-polycrystalline thin films were deposited by radio-frequency magnetron sputtering on glass substrates. The analysis of the morphology reveals well-connected whiskers with a preferred c-axis orientation perpendicular to the substrate and a dense columnar grain structure. The as-deposited films exhibited a low electrical resistivity of 1 × 10^− 3 Ω cm. Annealing in air produces an increase of the resistivity by more than three orders of magnitude and an increase in the absolute value of the Seebeck coefficient proportional to the resistivity. Annealing of the as-deposited sample in reducing Ar/H₂ atmosphere leads to a decrease in both the resistivity and the absolute value of the Seebeck coefficient. The change in the electrical transport properties is caused by the absorption and desorption of oxygen. Both resistivity and Seebeck coefficient recover to their initial values during annealing of the air-treated sample in reducing Ar/H₂ atmosphere, indicating a reversible process. The analysis by transmission electron microscopy after annealing reveals a stable columnar grain structure with an increase of the grain size. The increase in grain size is larger when the sample is annealed in reducing rather than in oxidising atmosphere. In summary, the reducing Ar/H₂ atmosphere was found to be advantageous for the thermoelectric properties resulting in a maximum power factor of 0.3 mW/K²m at 800 K.     

Effect of vacuum magnetic annealing on the structural and physical properties of the Ni and Al co-doped ZnO films

About 300 nm-thick Zn_0.87Al_0.06Ni_0.07O, Zn_0.83Al_0.06Ni_0.11O and Zn_0.81Al_0.04Ni_0.15O films were deposited on glass substrates at 300 K by co-sputtering ZnO:Al and Ni targets. The films were annealed in vacuum at 673 K for 2 h under a magnetic field of 4.8 × 10⁴ A/m applied along the film plane and then were cooled down to room temperature without magnetic field. All the films have a wurtzite structure and consist of thin columnar grains perpendicular to the substrate. The annealing promotes the (002) orientation growth in the film growing direction for the Zn_0.87Al_0.06Ni_0.07O and Zn_0.83Al_0.06Ni_0.11O films as well as the (100) orientation growth for the Zn_0.81Al_0.04Ni_0.15O film. The annealing results in a slight increase in the grain size. A weak Ni diffraction peak was detected for the annealed films with high Ni content. The annealing enhances the room temperature ferromagnetism of the films. A temperature dependence of magnetization confirms that the Curie temperature is above 400 K for the annealed films. The films magnetically annealed exhibit an anisotropic magnetization behavior. The annealed Zn_0.87Al_0.06Ni_0.07O film has the lowest resistivity (8.73 × 10⁻³ Ω cm), the highest free electron concentration (1.73 × 10²⁰ cm^− 3) and Hall mobility (4.16 cm²V^− 1 s^− 1). A temperature dependence of the resistivity from 50 K to 300 K reveals that the carrier transport mechanism is Mott's variable range hopping in the low temperature range and thermally activated band conduction in the high temperature range.     

Low temperature remote plasma sputtering of indium tin oxide for flexible display applications

Tin doped indium oxide (ITO) has been directly deposited onto a variety of flexible materials by a reactive sputtering technique that utilises a remotely generated, high density plasma. This technique, known as high target utilisation sputtering (HiTUS), allows for the high rate deposition of good quality ITO films onto polymeric materials with no substrate heating or post deposition annealing. Coatings with a resistivity of 3.8 × 10^− 4 Ωcm and an average visible transmission of greater than 90% have been deposited onto PEN and PET substrate materials at a deposition rate of 70 nm/min. The electrical and optical properties are retained when the coatings are flexed through a 1.0 cm bend radius, making them of interest for flexible display applications.     

The influence of annealing temperature on the structural, electrical and optical properties of ion beam sputtered CuInSe2 thin films

CuInSe₂ (CIS) thin films were prepared by ion beam sputtering deposition of copper layer, indium layer and selenium layer on BK7 glass substrates followed by annealing at different temperatures for 1 h in the same vacuum chamber. The influence of annealing temperature (100-400 °C) on the structural, optical and electrical properties of CIS thin films was investigated. X-ray diffraction (XRD) analysis revealed that CIS thin films exhibit chalcopyrite phase and preferential (112) orientation when the annealing temperature is over 300 °C. Both XRD and Raman show that the crystalline quality of CIS thin film and the grain size increase with increasing annealing temperature. The reduction of the stoichiometry deviation during the deposition of CIS thin films is achieved and the elemental composition of Cu, In and Se in the sample annealed at 400 °C is very near to the stoichiometric ratio of 1:1:2. This sample also has an optical energy band gap of about 1.05 eV, a high absorption coefficient of 10⁵ cm⁻¹ and a resistivity of about 0.01 Ω cm.     

The effect of sputtering pressure on electrical, optical and structure properties of indium tin oxide on glass

Indium tin oxide (ITO) thin layers were deposited onto glass substrates by RF magnetron sputtering using different pressures. Subsequently, the films were annealed in a reducing atmosphere at 500 °C for 30 min. Electrical properties were measured by Hall Effect analysis and four-point probe measurements. Optical properties were determined by UV-Vis spectrophotometery. Film structures and compositions were analyzed by X-ray diffractometry and X-ray photoelectron spectroscopy, respectively. The effect of sputter pressure and additional anneals was investigated. The results revealed that the lowest resistivity of 1.69 × 10^− 4 Ω cm was achieved at low pressure (1.2 Pa) and the highest transmittance of ~ 90% was obtained after a second anneal. However, the second anneal decreased the mobility and the conductivity especially for high sputtering pressures. This study also describes the effect of Sn defect clustering on electrical properties of the ITO films.     

Fluorine-doped ZnO transparent conducting thin films prepared by radio frequency magnetron sputtering

Fluorine-doped ZnO transparent conducting thin films were prepared by radio frequency magnetron sputtering at 150 °C on glass substrate. Thermal annealing in vacuum was used to improve the optical and electrical properties of the films. X-ray patterns indicated that (002) preferential growth was observed. The grain size of F-doped ZnO thin films calculated from the full-width at half-maximum of the (002) diffraction lines is in the range of 18-24 nm. The average transmittance in visible region is over 90% for all specimens. The specimen annealed at 400 °C has the lowest resistivity of 1.86 × 10^− 3 Ω cm, the highest mobility of 8.9 cm² V^− 1 s^− 1, the highest carrier concentration of 3.78 × 10²⁰ cm^− 3, and the highest energy band gap of 3.40 eV. The resistivity of F-doped ZnO thin films increases gradually to 4.58 × 10^− 3 Ω cm after annealed at 400 °C for 4 h. The variation of the resistivity is slight.