Refractive indices of textured indium tin oxide and zinc oxide thin films

The refractive indices of textured indium tin oxide (ITO) and zinc oxide (ZnO) thin films were measured and compared. The ITO thin film grown on glass and ZnO buffered glass substrates by sputtering showed distinct differences; the refractive index of ITO on glass was about 0.05 higher than that of ITO on ZnO buffered glass in the whole visible spectrum. The ZnO thin film grown on glass and ITO buffered glass substrates by filtered vacuum arc also showed distinct differences; the refractive index of ZnO on glass was higher than that of ZnO on ITO buffered glass in the red and green region, but lower in the blue region. The largest refractive index difference of ZnO on glass and ITO buffered glass was about 0.1 in the visible spectrum. The refractive index variation was correlated with the crystal quality, surface morphology and conductivity of the thin films.     

Eu-doped indium tin oxide thin films fabricated by sol-gel technique

We have fabricated Eu-doped indium tin oxide thin films via the conventional sol-gel technique, and confirmed that the doped Eu atoms were chemically incorporated into the indium tin oxide lattice by substituting the In sites. Optical spectra indicated that the Eu-doped films were free of any impurities leading to additional vibrational effects. Valence states of Eu ions in our Eu-doped indium tin oxide films were discussed in connection with Eu concentration.     

Thickness dependence of structural and optical properties of indium tin oxide nanofiber thin films prepared by electron beam evaporation onto quartz substrates

Indium tin oxide (ITO) thin films, produced by electron beam evaporation technique onto quartz substrates maintained at room temperature, are grown as nanofibers. The dependence of structural and optical properties of ITO thin films on the film thickness (99-662 nm) has been reported. The crystal structure and morphology of the films are investigated by X-ray diffraction and scanning electron microscope techniques, respectively. The particle size is found to increase with increasing film thickness without changing the preferred orientation along (2 2 2) direction. The optical properties of the films are investigated in terms of the measurements of the transmittance and reflectance determined at the normal incidence of the light in the wavelength range (250-2500 nm). The absorption coefficient and refractive index are calculated and the related optical parameters are evaluated. The optical band gap is found to decrease with the increase of the film thickness, whereas the refractive index is found to increase. The optical dielectric constant and the ratio of the free carrier concentration to its effective mass are estimated for the films.     

Indium tin oxide films made from nanoparticles: models for the optical and electrical properties

Porous thin films comprising nanoparticles of In₂O₃:Sn (known as indium tin oxide, ITO) were made by spin coating followed by annealing. The nanoparticles were prepared by a wet chemical technique. The films had a luminous transmittance of ∼90% and an electrical resistivity of ∼10⁻² Ω cm. Spectral transmittance and reflectance were analyzed by first representing the ITO nanoparticles within the Drude theory, with a frequency-dependent scattering time characteristic for ionized impurity scattering, and then applying effective medium theory to account for the porosity. It was found that the individual nanoparticles had a resistivity of ∼2×10⁻⁴ Ω cm, i.e. their electrical properties were comparable to those in the best films made by physical or chemical vapour deposition. Temperature-dependent electrical resistivity data for the films could be reconciled with a model for fluctuation induced tunneling between micrometer-size clusters of internally connected ITO nanoparticles.     

Structural and electrical properties of sputtered indium-zinc oxide thin films

In this study, indium-zinc oxide (IZO) thin films have been prepared at a room temperature, 200 and 300 °C by radio frequency magnetron sputtering from a In₂O₃-12 wt.% ZnO sintered ceramic target, and their dependence of electrical and structural properties on the oxygen content in sputter gas, the substrate temperature and the post-heat treatment was investigated. X-ray diffraction measurements showed that amorphous IZO films were formed at room temperature (RT) regardless of oxygen content in sputter gas, and micro-crystalline and In₂O₃-oriented crystalline films were obtained at 200 and 300 °C, respectively. From the analysis on the electrical and the structural properties of annealed IZO films under Ar atmosphere at 200, 300, 400 and 500 °C, it was shown that oxygen content in sputter gas is a critical parameter that determines the local structure of amorphous IZO film, stability of amorphous phase as well as its eventual crystalline structure, which again decide the electrical properties of the IZO films. As-prepared amorphous IZO film deposited at RT gave specific resistivity as low as 4.48 × 10^− 4 Ω cm, and the highest mobility value amounting to 47 cm²/V s was obtained from amorphous IZO film which was deposited in 0.5% oxygen content in sputter gas and subsequently annealed at 400 °C in Ar atmosphere.     

Electrical and optical properties of nanocrystalline yttrium-doped hafnium oxide thin films

Yttrium-doped hafnium oxide (YDH) films have been produced by sputter-deposition by varying the growth temperature (T_s) from room-temperature (RT) to 400 °C. The electrical and optical properties of YDH films have been investigated. Structural studies indicate that YDH films grown at T_s = RT − 200 °C were amorphous and those grown at 300-400 °C are nanocrystalline. The crystalline YDH films exhibit the high temperature cubic phase of HfO₂. Spectrophotometry analysis indicates that all the YDH films are transparent. The band gap of YDH films was found to be in the range of 6.20-6.28 eV. Frequency variation of frequency dependent resistivity indicates the hopping conduction mechanism operative in YDH films. While the electrical resistivity (ρ_ac) is ~ 1 Ω-m at low frequencies (100 Hz), ρ_ac decreases to ~ 10^− 4 Ω-cm at higher frequencies (1 MHz).     

Effects of thermal treatment on the electrical and optical properties of silver-based indium tin oxide/metal/indium tin oxide structures

In this article, we report the results of the study of thermal treatment effects on the electrical and optical properties of silver-based indium tin oxide/metal/indium tin oxide (IMI) multilayer films. Heat treatment conditions such as temperature and gaseous atmosphere was varied to obtain better electrical and optical properties. We obtained improved electrical properties and observed considerable shift in the transmittance curves after heat treatment. Several analytical tools such as X-ray diffraction, spectroscopic ellipsometer and spectrophotometer were used to explore the causes of the changes in electrical and optical properties. The sheet resistance of the structure was severely influenced by deposition conditions of the indium tin oxide (ITO) layer at the top. Moreover, the shift of optical transmittance could be explained on the basis of the change in refractive indices of ITO layers during heat treatment. The properties of Ag-alloy-based IMI films were compared with those of pure Ag-based ones. Some defects originating from Ag layer corrosion were observed on the surface of ITO-pure Ag–ITO structures, however, their number decreased significantly in the cases of Ag-alloys containing Pd, Au and Cu, though the resistivity values of Ag-alloys were slightly higher than those of silver. Atomic force microscopy measurement results revealed that the surface of the IMI multilayer was so smooth that it meets the required qualifications as the bottom electrode of organic light emitting diodes.     

Highly conducting indium tin oxide (ITO) thin films deposited by pulsed laser ablation

Highly conducting and transparent indium tin oxide (ITO) thin films were prepared on SiO₂ glass and silicon substrates by pulsed laser ablation (PLA) from a 90 wt.% In₂O₃-10 wt.% SnO₂ sintered ceramic target. The growths of ITO films under different oxygen pressures (P_O2) ranging from 1×10⁻⁴–5×10⁻² Torr at low substrate temperatures (T_s) between room temperature (RT) and 200°C were investigated. The opto-electrical properties of the films were found to be strongly dependent on the P_O2 during the film deposition. Under a P_O2 of 1×10⁻² Torr, ITO films with low resistivity of 5.35×10⁻⁴ and 1.75×10⁻⁴ Ω cm were obtained at RT (25°C) and 200°C, respectively. The films exhibited high carrier density and reasonably high Hall mobility at the optimal P_O2 region of 1×10⁻² to 1.5×10⁻² Torr. Optical transmittance in excess of 87% in the visible region of the solar spectrum was displayed by the films deposited at Po₂≥1×10⁻² Torr and it was significantly reduced as the P_O2 decreases.     

Growth and characterization of indium tin oxide thin films deposited on PET substrates

Transparent and conductive indium tin oxide (ITO) thin films were deposited onto polyethylene terephthalate (PET) by d.c. magnetron sputtering as the front and back electrical contact for applications in flexible displays and optoelectronic devices. In addition, ITO powder was used for sputter target in order to reduce the cost and time of the film formation processes. As the sputtering power and pressure increased, the electrical conductivity of ITO films decreased. The films were increasingly dark gray colored as the sputtering power increased, resulting in the loss of transmittance of the films. When the pressure during deposition was higher, however, the optical transmittance improved at visible region of light. ITO films deposited onto PET have shown similar optical transmittance and electrical resistivity, in comparison with films onto glass substrate. High quality films with resistivity as low as 2.5 × 10^− 3 Ω cm and transmittance over 80% have been obtained on to PET substrate by suitably controlling the deposition parameters.     

Extraction of the refractive index profile of thin transparent conductive oxide films from the analysis of reflectance optical spectra

In the direction of growth of fabricated films, the material near the free surface as well as the interface with the substrate exhibits properties which are different from those of the material in the bulk. The resulting spatial inhomogeneity of the refractive index influences positions and values of the extrema of optical spectra. We exploit this to derive the profile of the refractive index by developing a theoretical approach. In the calculations, taking the derived profile into account, we attain a good reproduction of the experimental Transmittance and Reflectance spectra from approximately 1 to 4 eV, the region of relatively weak refractive-index dispersion.     

Ion bombardment-induced enhancement of the properties of indium tin oxide films prepared by plasma-assisted reactive magnetron sputtering

Research on tin doped indium oxide (ITO) has for many years been stimulated by the need to simultaneously optimize the electrical, optical and mechanical properties, and by new challenges related to the deposition of transparent conducting oxides on flexible plastic substrates. In the present work, we investigate the growth and optical, electrical, and mechanical (hardness, elastic modulus and stress) properties of ITO films deposited by plasma assisted reactive magnetron sputtering (PARMS) from an indium-tin alloy target. PARMS achieves an effective control of bombardment by reactive species (e.g., O₂⁺, O⁺) on the surface of the growing film by varying the bias voltage, V_B, induced by a radiofrequency power applied to the substrate. Stress-free films possessing high transparency (> 80% — film on glass) and low resistivity (4 × 10^− 4 Ω cm) can be deposited by PARMS under conditions of intense ion bombardment (≤ 600 eV).     

Deposition by magnetron sputtering and characterization of indium tin oxide thin films

In this work the properties of indium tin oxide (ITO) films deposed on glass substrates by magnetron sputtering technique in the temperature range below 200 °C are studied by various methods. The physical properties of ITO thin films have been investigated using optical transmittance, photoluminescence, atomic force microscopy, ellipsometry, Hall-effect and four point probe methods. It is established that properties of ITO layers depend drastically on the temperature and oxygen partial pressure during the deposition process and exhibit some peculiarities of the surface morphology. It is found that the band gap energy of this material varies in the energy range from 4.1 to 4.4 eV and depends on the growth conditions. It is suggested that local deviations from the stoichiometry and defects are the main physical reasons of Burstein-Moss shift of the optical band gap.     

Influence of thickness and growth temperature on the optical and electrical properties of ZnO thin films

Zinc oxide transparent conductive thin films were prepared using the pulsed laser deposition technique onto Corning glass substrates and the dependences of their optical and electrical properties on the thickness and the growth temperature were investigated. As shown, the films present 90% average transmittance, their energy gap position depending on the film thickness and the growth temperature. An additional absorption band was also observed near 3.44 eV, the position of its maximum also depending on the growth parameters. Finally, the electrical properties of the films were found to be affected mainly by the growth temperature and less by the thickness.     

Improved ITO thin films with a thin ZnO buffer layer by sputtering

In this paper, we report a buffering method of improving the quality of ITO thin films on glass by r.f. magnetron sputtering. By applying a ZnO buffer before the ITO deposition in the same run of sputtering, ITO films showed single (111)-oriented highly textured structure, while ITO films showed mixed-oriented polycrystalline structure on bare glass. A design of experiment was taken out to minimize the resistivity of ITO films in the deposition parameter space (oxygen ratio, total gas pressure, and temperature). Resistance measurements showed that the ITO films with ZnO buffers had a remarkable 50% decrease of resistivity comparing to those without ZnO buffers at optimized deposition condition. Room-temperature Hall effect measurements showed that the decrease in resistivity comes from a large increase of mobility and a slight increase of carrier density after forming gas annealing. The ZnO/glass may be a good alternative substrate to bare glass for producing high quality ITO films for advanced electro-optic applications.     

Investigation on structural, electrical and optical properties of tungsten-doped tin oxide thin films

Tungsten-doped tin oxide (SnO₂:W) transparent conductive films were prepared on quartz substrates by pulsed plasma deposition method with a post-annealing. The structure, chemical states, electrical and optical properties of the films have been investigated with tungsten-doping content and annealing temperature. The lowest resistivity of 6.67 × 10^− 4 Ω cm was obtained, with carrier mobility of 65 cm² V^− 1 s^− 1 and carrier concentration of 1.44 × 10²⁰ cm^− 3 in 3 wt.% tungsten-doping films annealed at 800 °C in air. The average optical transmittance achieves 86% in the visible region, and approximately 85% in near-infrared region, with the optical band gap ranging from 4.05 eV to 4.22 eV.     

Investigation of structural,optical, antibacterial,and dielectric properties of V-doped copper oxide thin films: Comparison with undoped copper oxide thin films

Vanadium doped Copper oxide (CO) thin films were prepared by the sol-gel dip-coating method. The properties of thin films were examined by X-Ray Diffractometer (XRD), UV–Visible-NIR spectrophotometry, and dielectric properties analyzer. The antibacterial and photocatalytic properties were also determined. XRD spectra revealed the dual-phase of copper oxide (cuprite and tenorite) for all percentages of V with no other impurity peak. Tauc's relation is used to probe the optical band gap which is reduced from 1.96 to 1.64 eV with an increase in vanadium doping percentage. The impurity band coalesces with the conduction band of copper oxide to decrease the band gap. Dielectric constant measurements reveal that the Ac conductivity of thin films increases with an increase in V doping percentage.     

Properties of multilayer transparent conducting oxide films

Multilayered transparent conducting oxide (TCO) film structures have been designed and fabricated to achieve both high conductivity and high transmittance in the visible spectrum. Double-layered TCO structures consisting of Sn-doped CdO and Sn-doped CdIn₂O₄, Cd-rich Cd₂SnO₄, or Ga-doped ZnO are discussed. By optimizing the thickness of the individual layers and the doping levels within those layers, an effective conductivity of 20 600 S/cm and an average transmittance larger than 85% in the 400-700 nm range have been achieved for films epitaxially grown on MgO substrates. Bi-layer films consisting of Sn-doped CdO and Ga-doped ZnO have also been deposited on plastic substrates at room temperature with resistivities of ∼1×10⁻⁴ Ω cm and an average transmittance of 80-85% in the visible range. These properties are attractive for future TCO applications.     

Structural, electrical and optical properties of indium tin oxide thin films prepared by RF sputtering using different density ceramic targets

There is an active demand for the commercial indium tin oxide (ITO) target with density above 99% of the theoretical density (TD). Some works found the increase of the target density could lead to a slight decrease of the resistivity of the direct current (DC) sputtered ITO films, however, the possible effect of target density on the radio frequency (RF) sputtered ITO films is not clear. In this paper, ITO targets with different densities are successfully prepared. The structural, electrical and optical properties of the thin films deposited from these targets are studied at the substrate temperature of 750 °C. It is found that the target density has no effect on the above properties and the deposition rate of the RF sputtered ITO thin films, different from the DC sputtered films. So for the RF sputtered process, the target needs not high density so that the used target can be just compacted from the powders without sintering. All the as-prepared ITO films with different densities have a resistivity of 1.56 × 10⁻⁴ Ω cm and a transmittance of ∼87%, which are lower than the ITO films prepared at temperatures lower than 400 °C.     

Investigation of optical and electrical properties of ZnO thin films

Zinc oxide (ZnO) thin films were deposited on Si substrates using various working pressures by magnetron sputter. The resistivity of the deposited ZnO films decreases with working pressure, and the resistivity of 4.3 × 10⁻³ Ω cm can be obtained without post annealing. According to the optical transmittance measurements, the optical transmittance above 90% in the wavelength longer than 430 nm and about 80% in the wavelength of 380 nm can be found. Using time-resolved photoluminescence measurement, the carrier lifetime increases with working pressure due to the reduction of nonradiative recombination rate. The reduction of nonradiative recombination rate is originated from the decrease of oxygen vacancies in the ZnO films deposited at a higher working pressure. This result is verified by the photoluminescence measurements. Besides, by increasing the working pressure, the absorption coefficient was decreased and the associated optical energy gap of ZnO thin films was increased.     

Extraction of optical constants of zinc oxide thin films by ellipsometry with various models

Spectroscopic ellipsometry was used to extract the optical constants of zinc oxide (ZnO) thin films deposited on (100) silicon substrate by filtered cathodic vacuum arc technique. Three dispersion models, namely, Sellmeier dispersion model, Cauchy model and Forouhi–Bloomer model, were evaluated for determining the optical constants of ZnO thin films below the energy band gap. The study shows that the Cauchy model provides the best spectral fittings among these three models. Above the energy band gap, two ellipsometric models, namely, two-phase model and three-phase point-by-point fit, were used. This study reveals that the initial values used in the point-by-point fitting play a critical role. It also shows that the refractive index and the extinction coefficient calculated with the two-phase model can be used as the initial values for the point-by-point fitting. The spectral dependence of the refractive index and extinction coefficient obtained in this work is comparable with the data reported in the literature. In sum, a reliable methodology for determining the optical constants of ZnO thin films in the ultraviolet-visible-near infrared range (2501100 nm) has been developed.