Essential Macleod Program (EMP) simulated fabrication of high quality Zn:SnO2/Ag/Zn:SnO2 multilayer transparent conducting electrode on flexible substrates

In the quest of promising Indium free amorphous transparent conducting oxide (TCO), Zn-doped SnO₂/Ag/Zn-doped SnO₂ (OMO) multilayer films were prepared on flexible polyethylene terephthalate (PET) substrates by RF sputtering at room temperature (RT). Growth parameters were optimized by varying sputtering power and working pressure, to have high electrical conductivity and optical transmittance. Optimization of the thickness of each layer was done by Essential Macleod Program (EMP) simulation to get the higher transmission through OMO multilayer. The sheet resistance and transmittance of 3 at% Zn-doped SnO₂ thin film (30 nm) were 2.23 kΩ/□, (ρ ~ 8.92×10⁻³ Ω∙cm) and 81.3% (at λ ~ 550 nm), respectively. By using optimized thicknesses of Zn-doped SnO₂ (30 nm) and Ag (12 nm) and optimized growth condition Zn-doped SnO₂/Ag/Zn-doped SnO₂ multilayer thin films were deposited. The low sheet resistance of 7.2 Ω/□ and high optical transmittance of 85.1% in the 550 nm wavelength region was achieved with 72 nm multilayer film.     

Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Structural and optoelectronic properties of transparent conductive c-axis-oriented ZnO based multilayer thin films with Ru interlayer

ZnO and Ru multilayer thin films are deposited using the sputtering deposition technique at room temperature. The effects of the Ru interlayer thickness and annealing temperature on the properties of multilayer thin films have been studied. An X-ray diffraction study reveals that ZnO layers are highly c-axis-oriented. The use of an Ru interlayer improves the crystalline quality of the subsequently deposited ZnO layers. Moreover, the crystalline quality of the entire structure is further enhanced through thermal annealing in a vacuum. Atomic force microscopy images show that the surface roughness of the multilayer thin films increases with a Ru interlayer thickness greater than 6 nm. The roughness of the film surface increases in correlation with annealing temperatures. This accounts for the decreased optical transmittance of the multilayer thin films annealed at temperatures higher than 450 °C. The electrical resistivity of multilayer thin films decreases with an increase in the metallic interlayer thickness. Thermal annealing at 450 °C causes low resistivity in multilayer thin films. The lowest resistivity reached ～5.4 × 10^?4 Ω cm for multilayer films with a 10-nm-thick Ru interlayer annealed at 450 °C.     

Study of Cu-based Al-doped ZnO multilayer thin films with different annealing conditions

AZO/Cu/AZO multilayer thin films produced under different annealing conditions are studied in this paper, to examine the effects of atmosphere and annealing temperature on their optical and electrical properties. The multilayer thin films are prepared by simultaneous RF magnetron sputtering (for AZO) and DC magnetron sputtering (for Cu). The thin films were annealed in a vacuum or an atmosphere of oxygen at temperatures ranging from 100 to 400 °C in steps of 100 °C for 3 min. High-quality multilayer films (at Cu layer thickness of 15 nm) with resistivity of 1.99×10⁻⁵ Ω-cm and maximum optical transmittance of 76.23% were obtained at 400 °C annealing temperature in a vacuum. These results show the films to be good candidates for use as high quality electrodes in various displays applications.     

Influence of a Ni interlayer on the optical and electrical properties of trilayer GZO/Ni/GZO films

Trilayer GZO/Ni/GZO films were deposited onto polycarbonate (PC) substrates with RF and DC magnetron sputtering, and then the influence of a Ni interlayer on the optical and electrical properties of the films was investigated. A 2-nm-thick Ni interlayer decreased the resistivity to 6.4×10⁻⁴ Ω cm and influenced the optical transmittance.Although optical transmittance deteriorated with Ni insertion, the films showed a relatively high optical transmittance of 74.5% in the visible wavelength region. The figure of merit (FOM) of a GZO single layer film was 1.2×10⁻⁴ Ω⁻¹, while that of the GZO/Ni/GZO films reached a maximum of 8.2×10⁻⁴ Ω⁻¹.Since a higher FOM results in higher quality transparent-conductive oxide (TCO) films, it is concluded that GZO films with a 2 nm Ni interlayer have better optoelectrical performance than single-layer GZO films.     

Improving the electrical properties of niobium-doped titania sputtering targets by sintering in oxygen-deficient atmospheres

Niobium-doped titania (TNO) film can be used as a transparent conductive oxide (TCO) film due to its excellent conductivity and visible transparency. The performances of TNO sputtering targets are thus critical issues in optimizing sputtered films. This study clarifies the influences of inert and reducing atmospheres on the microstructure, densification, crystal structure, and electrical properties of TNO sputtering targets. The results indicate that a sintering atmosphere of 90% Ar–10% H₂ can result in a lower sintered density, larger grain size, and lower resistivity than can an atmosphere of Ar, followed by one of air. Sintering in 90% Ar–10% H₂ or Ar obviously decreases the resistivity of TiO₂, from >10⁸ Ω cm to <10⁻¹ Ω cm, and the TNO target, from >10¹ Ω cm to <10⁻¹ Ω cm. The resistivity of TNO target sintered at 1200 °C in 90% Ar–10% H₂ is as low as 1.8×10⁻² Ω cm.     

Transparent and high infrared reflection film having sandwich structure of SiO2/Al:ZnO/SiO2

New transparent and high infrared reflection films having the sandwich structure of SiO₂/Al:ZnO(AZO)/SiO₂ were deposited on the soda-lime silicate glass at room temperature by radio frequency (R.F.) magnetron sputtering. The optical and electrical properties of SiO₂ (110 nm)/AZO (860 nm)/SiO₂ (110 nm) sandwich films were compared with those of single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. The results show that these sandwich films exhibit high transmittance of over 85% in the visible light range (380–760 nm), and low reflection rate of below 4.5% in the wavelength range of 350–525 nm, which is not shown in the conventional single layer AZO (860 nm) films and double layer SiO₂ (110 nm)/AZO (860 nm) films. Further these sandwich films display a low sheet resistance of 20 Ω/sq by sheet resistance formula and high infrared reflection rate of above 80% in the wavelength range of 15–25 μm. In addition, the infrared reflection property of these sandwich films is determined mainly by the AZO film. The outer SiO₂ film can diminish the interference coloring and increase transparency; the inner SiO₂ film improves the adhesion of the coating to the glass substrate and prevents Ca²⁺ and Na⁺ in the glass substrate from entering the AZO film.     

Structural,optical and electrical properties of low temperature grown undoped and (Al,Ga) co-doped ZnO thin films by spray pyrolysis

Aluminum and gallium co-doped ZnO (AGZO) thin films were grown by simple, flexible and cost-effective spray pyrolysis method on glass substrates at a temperature of 230 °C. Effects of equal co-doping with aluminum (Al) and gallium (Ga) on structural, optical and electrical properties were investigated by X-ray diffraction (XRD), UV–vis–NIR spectrophotometry and Current–Voltage (I–V) measurements, respectively. XRD patterns showed a successful growth with high quality polycrystalline films on glass substrates. The predominant orientation of the films is (002) at dopant concentrations ≤2 at% and (101) at higher dopant concentrations. Incorporation of Al and Ga to the ZnO crystal structure decreased the crystallite size and increased residual stress of the thin films. All films were highly transparent in the visible region with average transmittance of 80%. Increasing doping concentrations increased the optical band gap, from 3.12 to 3.30 eV. A blue shift of the optical band gap was observed from 400 nm to 380 nm with increase in equal co-doping. Co-doping improved the electrical conductivity of ZnO thin films. It has been found from the electrical measurements that films with dopant concentration of 2 at% have lowest resistivity of 1.621×10⁻⁴ Ω cm.     

Electrical and optical properties of diamond-like carbon films deposited by pulsed laser ablation

Pulsed laser ablation of a graphite target was carried out by ArF excimer laser deposition at a laser wavelength of 193 nm and fluences of 10 and 20 J/cm² to produce diamond-like carbon (DLC) films. DLC films were deposited on silicon and quartz substrates under 1 × 10^? 6 Torr pressure at different temperatures from room temperature to 250 °C. The effect of temperature on the electrical and optical properties of the DLC films was studied. Laser Raman Spectroscopy (LRS) showed that the DLC band showed a slight increase to higher frequency with increasing film deposition temperature. Spectroscopic ellipsometry (SE) and ultraviolet–visible absorption spectroscopy showed that the optical band gap of the DLC films was 0.8–2 eV and decreased with increasing substrate temperature. These results were consistent with the electrical resistivity results, which gave values for the films in the range 1.0 × 10⁴–2.8 × 10⁵ Ω cm and which also decreased with deposition temperature. We conclude that at higher substrate deposition temperatures, DLC films show increasing graphitic characteristics yielding lower electrical resistivity and a smaller optical band gap.     

Effect of annealing temperature on structural and optoelectronic properties of γ-CuI thin films prepared by the thermal evaporation method

High quality transparent conducting CuI thin films were deposited at room temperature via thermal evaporation technique followed by post deposition annealing at different temperatures. The samples were characterised by X-ray diffraction (XRD), UV–Vis spectrophotometry, Scanning electron microscopy and I-V measurements. The structural, morphological and optical properties were studied as a function of the annealing temperature from room temperature (RT) to 200 °C. XRD results revealed that the films were polycrystalline with zinc blende structure of cubic phase. Increasing the annealing temperature increased the crystallite size from 33 to 49 nm whilst the dislocation density and lattice strain shifted to lower values. High transmittance of about 70–80% was exhibited by all films in the entire visible spectral range. The as deposited film possesed the lowest resistivity of 3.0×10⁻³ Ω cm.     

Electrical and optical characteristics of transparent conducting Si-doped ZnO/hole-patterned Ag/Si-doped ZnO multilayer films

Hole-patterned Ag layers were first used to form Si-doped ZnO (SZO)/hole-patterned Ag/SZO multilayers and their optical and electrical properties were characterized. Unlike conventional oxide/metal/oxide multilayers, all samples exhibited two characteristic features: (i) a sinusoidal wavelength dependence of the transmittance with double maxima, and (ii) undulation in the visible transmittance, but not in the infrared transmittance. With increasing SZO thickness, the transmittance maxima were red-shifted, and the visible transmittance window widened. The carrier concentration decreased from 7.42×10²² to 2.4×10²² cm⁻³, and the sheet resistances varied from 7 to 10 Ω/sq with increasing SZO thickness. Haacke's figure of merit (FOM) was calculated for the SZO-based multilayer films. The 40 nm-thick SZO multilayers had the highest FOM of 15.9×10^–3 Ω^–1. Finite-difference time-domain (FDTD) simulations were undertaken to interpret the measured transmittance. Based on the FDTD simulations, the undulating transmittance was attributed to surface plasmon-polaritons.     

Room temperature preparation of high performance AZO films by MF sputtering

Aluminum-doped zinc oxide (AZO) thin films have been deposited by MF magnetron sputtering from a ceramic oxide target without heating the substrates. This study has investigated effects of sputtering power on the structural, electrical and optical properties of the AZO films. The films delivered a hexagonal wurtzite structure with (002) preferential orientation and uniform surface morphology with 27–33 nm grain size. The results indicate that residual stress and grain size of the AZO films are dependent on sputtering power. The minimum resistivity of 7.56×10^?4 Ω cm combined with high transmittance of 83% were obtained at deposited power of 1600 W. The films delivered the advantages of a high deposition rate at low substrate temperature and should be suitable for the fabrication of low-cost transparent conductive oxide layer.     

Control of refractive index by annealing to achieve high figure of merit for TiO2/Ag/TiO2 multilayer films

We modified the refractive index (n) of TiO₂ by annealing at various temperatures to obtain a high figure of merit (FOM) for TiO₂/Ag/TiO₂ (45 nm/17 nm/45 nm) multilayer films deposited on glass substrates. Unlike the as-deposited and 300 °C-annealed TiO₂ films, the 600 °C-annealed sample was crystallized in the anatase phase. The as-deposited TiO₂/Ag/as-deposited TiO₂ multilayer film exhibited a transmittance of 94.6% at 550 nm, whereas that of the as-deposited TiO₂/Ag/600 °C-annealed TiO₂ (lower) multilayer film was 96.6%. At 550 nm, n increased from 2.293 to 2.336 with increasing temperature. The carrier concentration, mobility, and sheet resistance varied with increasing annealing temperature. The samples exhibited smooth surfaces with a root-mean-square roughness of 0.37–1.09 nm. The 600 °C-annealed multilayer yielded the highest Haacke's FOM of 193.9×10⁻³ Ω⁻¹.     

Electrical properties of ultrananocrystalline diamond/amorphous carbon nanocomposite films

The electrical surface properties of ultrananocrystalline diamond/amorphous carbon composite films have been investigated by four-point probe I/V and Hall measurements, whereas impedance spectroscopy has been used to establish the electrical bulk properties of the films. It turned out that the surface is p-type conductive with a resistivity of 0.14 Ω cm and a sheet carrier concentration of 7.6 × 10¹³ cm^?2. The bulk resistivity is higher by almost seven orders of magnitude (1.3 × 10⁶ Ω cm). The bulk conduction is thermally activated with an apparent activation energy of 0.17 eV. From Cole–Cole plots of the impedance spectra it can be concluded that there are three different contributions to the bulk conductivity. In order to try to identify these three components contributing to the electrical bulk conduction, Raman spectra have been recorded at five different wavelengths from the IR to UV region. These measurements showed that the UNCD/a-C films consist of at least three components: diamond nanocrystallites, an amorphous carbon matrix, and trans-polyacetylene-like structures probably at the interface between these two.     

The role of substrate temperature on the properties of nanocrystalline Mo doped ZnO thin films by spray pyrolysis

Transparent conducting molybdenum (2 at.%) doped zinc oxide (MZO) films were prepared with various substrate temperatures by spray pyrolysis technique on glass substrates. The effect of substrate temperature on the structural, surface morphological, electrical, optical and photoluminescence properties of these films were studied. The X-ray diffraction analysis revealed that the films are polycrystalline in nature having a wurtzite structure with a preferred grain orientation in the (0 0 2) direction. The average crystallite size of the films increases from 17 nm to 28 nm with the increase of substrate temperature from 573 K to 623 K, thereafter it slightly decreases with further increase of substrate temperature to 723 K. Analysis of structural parameters indicates minimum strain and stress values for films deposited at a substrate temperature of 673 K. From atomic force microscopy (AFM) analysis, it is found that rms roughness of the films deposited at 623 K is a minimum, indicating better optical quality. The scanning electron microscopy (SEM) measurements showed that the surface morphology of the films changes with substrate temperature. Optical parameters such as optical transmittance, reflectance, refractive index, extinction coefficient, dielectric constant and optical band gap have been studied and discussed with respect to substrate temperature. Room temperature photoluminescence (PL) spectra show the deep-level emission in the MZO thin films. The films exhibit a low electrical resistivity of 6.22 × 10^?2 Ω cm with an optical transmittance of 75% in the visible region at a substrate temperature of 623 K.     

Transparent semiconductor zinc oxide thin films deposited on glass substrates by sol–gel process

Transparent semiconductor ZnO thin films were spin-coated onto alkali-free glass substrates by a sol–gel process. The influence of ZnO sols synthesized via different solvents (2-ME, EtOH or IPA) on the surface morphologies, microstructures, optical properties and resistivities of the obtained films were investigated. The as-coated films were annealed in ambient air at 500 °C for 1 h. X-ray diffraction results showed all polycrystalline ZnO thin films to have preferred orientation along the (0 0 2) plane. The surface morphologies, optical transmittances and resistivity values of the sol–gel derived ZnO thin films depended on the solvent used. The ZnO thin films synthesized with IPA as the solvent exhibited the highest average transmittance 92.2%, an RMS roughness of 4.52 nm and a resistivity of 1.5 × 10⁵ Ω cm.     

A one-step method to prepare carbon and dielectric composited films featuring a skeleton–skin structure

A one-step and cost-efficient strategy to prepare carbon and dielectric composites with a skeleton–skin structure by a sol–gel method has been proposed. For the model system in the present work, i.e. C–TiO₂ composite films, the insight of the wet chemical mechanisms in sols was illustrated by the electrical compensation theory. The structure and carbon crystallization evolutions with annealing process were investigated, demonstrating the nano carbon with an average sp² cluster size ranging from 1.1 nm to 1.4 nm. The resultant composite films turn out to be rutile TiO₂ skeleton covered by carbon with intimate interfaces. Benefited from this firm composite structure containing nano carbon, the film possesses tunable and outstanding electrical and optical performance. By appropriate handling annealing temperature, the film could present a high extinction coefficient qualified for solar selective absorber, or show a relatively low extinction coefficient and resistivity (about 10⁻² Ω cm).     

Studies on chemical bath deposited SnS2 films for Cd-free thin film solar cells

Tin disulfide (SnS₂) is a simple binary metal chalcogenide and it has been proposed as a promising buffer material for Cd-free thin film solar cells. The present work explores the deposition of SnS₂ films by a facile chemical bath deposition at different deposition times in the range of 30–120 min. The effect of deposition time on the structural, optical and electrical properties was investigated. The as-grown SnS₂ films showed a hexagonal crystal structure with a high intensity (001) peak at 15.03°. The films showed shuttle shaped grains that were uniformly distributed across the surface of the substrate. The films showed an optical energy band gap in the range of 2.95–2.80 eV. PL spectra showed a strong emission peak in the wavelength range, 410–460 nm with the variation of deposition time. The SnS₂ films prepared at a deposition time of 90 min showed good crystallinity and morphology with low resistivity of 11.2 Ω-cm. A solar cell with device structure of Mo/SnS/SnS₂/i-ZnO/Al: ZnO/Ni/Ag was fabricated. The fabricated solar cell showed an efficiency of 0.91%, which validate the photovoltaic performance of SnS₂ films.     

Highly conducting and transparent antimony doped tin oxide thin films: the role of sputtering power density

Sb doped SnO₂ thin films were deposited on quartz substrates by magnetron sputtering at 600 °C and the effects of sputtering power density on the preferential orientation, structural, surface morphological, optical and electrical properties had been studied. The XRD analyses confirm the formation of cassiterite tetragonal structure and the presence of preferential orientation in (2 1 1) direction for tin oxygen thin films. The dislocation density analyses reveal that the generated defects can be suppressed by the appropriate sputtering power density in the SnO₂ lattice. The studies of surface morphologies show that grain sizes and surface roughness are remarkably affected by the sputtering power density. The resistivity of Sb doped SnO₂ thin films gradually decreases as increasing the sputtering power density, reaches a minimum value of 8.23×10⁻⁴ Ω cm at 7.65/cm² and starts increasing thereafter. The possible mechanisms for the change in resistivity are proposed. The average transmittances are more than 83% in the visible region (380–780 nm) for all the thin films, the optical band gaps are above 4.1 eV. And the mechanisms of the variation of optical properties at different sputtering power densities are addressed.     

Aluminum doped zinc oxide sputtering targets obtained from nanostructured powders: Processing and application

This work reports the production of ceramic targets based on nanostructured Al-doped ZnO (AZO) powders for sputtering applications. The nanostructured powder is obtained by a new patented process based on the detonation of an emulsion containing both Zn and Al metal precursors in the final proportion of 98:2 wt% (ZnO:Al₂O₃), through which the Al contains is highly uniform distributed over ZnO. Due to the nanostructured powder characteristics, the targets can be sintered at substantially lower temperatures (1150–1250 °C) by conventional sintering, contributing to production costs reduction of ceramic targets and consequently the costs of photovoltaic and displays industries. Electrical resistivity values around 3.0–7.0 × 10^?3 Ω cm have been obtained depending on final microstructure of the targets. The electro-optical properties of the films produced at room temperature with thicknesses around 360 nm, besides being highly uniform exhibit a resistivity of about 1 × 10^?3 Ω cm and a transmittance in the visible range above 90%.