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.     

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.     

ZnMgBeO/Ag/ZnMgBeO transparent multilayer films with UV energy bandgap and very low resistance

The ZnMgBeO/Ag/ZnMgBeO multilayer structures were sputter grown and their electrical and optical properties have been investigated in detail. Results indicated that the ZnMgBeO(30 nm)/Ag(10 nm)/ZnMgBeO(30 nm) optimum structure shows energy bandgap of ~4.5 eV, electrical resistivity of ~6.5×10⁻⁵ Ωcm, and optical transmittance of 78–90% over the visible wavelength range and 74–90% over 300–400 nm range, representing a significant improvement over the previously reported transparent conducting films. High resistivity (~0.12 MΩcm) of the ZnMgBeO layer did not critically affect the conductivity of the multilayer, because the Ag films act as the conducting path. It was also observed that the properties were substantially deteriorated at the Ag thickness of 5 nm, as the Ag film is only partly continuous, resulting in very rough interfaces and surfaces.     

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.     

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.     

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.     

Dispersion and microwave processing of nano-sized ITO powder for the fabrication of transparent conductive oxides

Aqueous dispersions of tin-doped indium oxide (ITO) nanopowder were prepared and the effect of the addition of PEG 400, Tween 80 and β-alanine as dispersants was investigated using zeta potential and particle size distribution measurements. Both PEG 400 and β-alanine were found to produce stable dispersions that were used to deposit ITO thin films on glass substrates by dip and spin coating methods. The ITO thin films were heat-treated using both conventional and microwave heat treatment in order to improve the inter-particle connections and hence the resistivity and transparency of the films. All the films exhibited an average transmittance of >80% over the visible spectrum after being subjected to the heat treatment process. ITO films prepared with no dispersant showed very high resistivity values for both heating methods, however addition of 2 wt% PEG 400 to the dispersion yielded a reduction in the resistivity values to 1.4×10⁻¹ Ω cm and 3.8×10⁻² Ω cm for conventionally and microwave treated films, respectively. The surface morphological studies confirmed that addition of dispersants improved the film uniformity and inter-particle connections of the ITO films considerably.     

Synthesis of high-quality AZO polycrystalline films via target bias radio frequency magnetron sputtering

The deposition rate, transmittance and resistivity of aluminium-doped zinc oxide (AZO) films deposited via radio frequency (r.f.) sputtering change with target thickness. An effective method to control and maintain AZO film properties was developed. The strategy only involved the regulation of target bias voltage of r.f. magnetron sputtering system. The target bias voltage considerably influenced AZO film resistivity. The resistivity of the as-deposited AZO film was 9.82×10⁻⁴ Ω cm with power density of 2.19 W/cm² at target self-bias of −72 V. However, it decreased to 5.98×10⁻⁴ Ω cm when the target bias voltage was increased to −112 V by applying d.c. voltage. Both growth rate and optical band gap of AZO film increased with the absolute value of target bias voltage – growth rate increased from 10.54 nm/min to 25.14 nm/min, and band gap increased from 3.57eV to 3.71 eV when target bias voltage increased from −72 V to −112 V at r.f. power density of 2.19 W/cm². The morphology of AZO films was slightly affected by the target bias voltage. Regulating target bias voltage is an effective method to obtain high-quality AZO thin films deposited via r.f. magnetron sputtering. It is also a good choice to maintain the quality of AZO film in uptime manufacturing deposition.     

Characterization of spray pyrolytically deposited high mobility praseodymium doped CdO thin films

Praseodymium (Pr) doped CdO thin films with high transparency and high mobility were deposited, using a homemade spray pyrolysis setup, on micro-slide glass substrates preheated at 300 °C. Polycrystalline nature and Cd-O bond vibration of deposited films were confirmed by X-ray diffraction, micro-Raman and Fourier transform infrared spectroscopy analyses. The oxidation state of Cd²⁺, O²⁻, and Pr³⁺ was confirmed by X-ray photoelectron spectroscopy analysis. The highest average particle size (92 nm-FESEM) and high RMS (13.48 nm-AFM) values are obtained for 0.50 wt% Pr doped CdO thin film. The optical band gap is varied between 2.38 eV and 2.52 eV, depending on the Pr doping concentration. Photoluminescence spectra revealed that Pr doped CdO thin film exhibits strong green emission at 582 nm. High mobility (82 cm²/V s), high charge carrier concentration (2.19×10²⁰ cm⁻³) and high transmittance (83%) were observed for 0.50 wt% Pr doped CdO film. A high figure of merit (9.79×10⁻³ Ω⁻¹) was obtained for 0.50 wt% Pr doped CdO thin films. The mechanism behind the above results is discussed in detail in this paper.     

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.     

Semiconducting amorphous carbon thin films for transparent conducting electrodes

Semiconducting amorphous carbon thin films were directly grown on SiO₂ substrate by using chemical vapor deposition. Raman spectra and transmission electron microscopy image showed that the a-C films have a short-range ordered amorphous structure. The electrical and optical properties of the a-C thin films were investigated. The films have sheet resistance of 3.7 kΩ/□ and high transmittance of 82%. They exhibit metal-oxide-semiconductor field effect transistor mobility of 10–12 cm² V⁻¹ s⁻¹ at room temperature, which is comparable to previous reported mobility of amorphous carbon. The optical band gap was calculated by Tauc’s relationship and photoluminescence spectra showed that the films are semiconductor with an optical band gap of 1.8 eV. These good physical properties make the a-C films a candidate for the application of transparent conducting electrodes.     

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.     

Preparation and characterization of transparent indium- doped TiO2 films deposited by MOCVD

Titanium dioxide (TiO₂) films doped with different indium (In) concentrations have been prepared on SrTiO₃ (STO) substrates by high vacuum metalorganic chemical vapor deposition (MOCVD). X-ray diffraction (XRD) analyses revealed the TiO₂ films doped with low In concentrations to be [001] oriented anatase phase and the films with high In concentrations to present polycrystalline structures. The 1.8% In-doped TiO₂ film exhibited the best electrical conductivity properties with the lowest resistivity of 8.68×10⁻² Ω cm, a Hall mobility of 10.9 cm² V⁻¹ s⁻¹ and a carrier concentration of 6.5×10¹⁸ cm⁻³. The films showed excellent transparency with average transmittances of over 85% in the visible range.     

Increased CNT growth density with an additional thin Ni layer on the Fe/Al catalyst film

Density of 3 × 10¹¹/cm² and diameter of CNTs of 9–12 nm were successfully controlled by using the multi-layered catalyst film consisting of an additional Ni layer on Fe/Al catalyst film. EDS analysis for the annealed catalyst films revealed that the increase of the density of Fe catalyst particles corresponded with the decrease of Ni in the films, which strongly suggested that the additional thin Ni layer on the Fe/Al multi-layered catalyst films prevented the fine Fe catalyst particles from agglomeration, resulting in the growth of high-density, and uniform diameter of CNTs.     

Deposition of highly oriented lanthanum nickel oxide thin film on silicon wafer by CSD

This paper describes the orientation control and the electrical properties of the chemical solution deposition (CSD) derived LaNiO₃ (LNO) thin film. The LNO precursor solutions were prepared using lanthanum nitrate and nickel acetate as La and Ni source, and ethanol or 2-methoxyethanol and 2-aminoethanol mixed solution as solvents. The LNO films were spin-coated using these precursor solutions and annealed at the temperature from 500 to 700 °C. The resulting LNO film annealed at 700 °C derived from 2-methoxyethanol and 2-aminoethanol mixed solvent exhibited (1 0 0)-orientation, with some surface cracks and pores, and relatively higher resistivity of 2.49 × 10⁻³ Ω cm. The LNO film derived from 2-methoxyethanol and 2-aminoethanol mixed solvent annealed at 700 °C in an oxygen atmosphere showed highly (1 0 0)-orientation, with higher density, a few cracks and pores, and exhibited a good electrical resistivity of 7.27 × 10⁻⁴ Ω cm.     

Structural,optical, and electrical properties of conducting p-type transparent Cu–Cr–O thin films

Cu–Cr–O films were prepared by DC magnetron co-sputtering using Cu and Cr targets on quartz substrates. The films were then annealed at temperatures ranging from 400 °C to 900 °C for 2 h under a controlled Ar atmosphere. The as-deposited and 400 °C-annealed films were amorphous, semi-transparent, and insulated. After annealing at 500 °C, the Cu–Cr–O films contained a mixture of monoclinic CuO and spinel CuCr₂O₄ phases. Annealing at 600 °C led to the formation of delafossite CuCrO₂ phases. When the annealing was further increased to temperatures above 700 °C, the films exhibited a pure delafossite CuCrO₂ phase. The crystallinity and grain size also increased with the annealing temperature. The formation of the delafossite CuCrO₂ phase during post-annealing processing was in good agreement with thermodynamics. The optimum conductivity and transparency were achieved for the film annealed at approximately 700 °C with a figure of merit of 1.51×10⁻⁸ Ω⁻¹ (i.e., electrical resistivity of up to 5.13 Ω-cm and visible light transmittance of up to 58.3%). The lower formation temperature and superior properties of CuCrO₂ found in this study indicated the higher potential of this material for practical applications compared to CuAlO₂.     

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.     

Growth and properties of Cu3SbS4 thin films prepared by a two-stage process for solar cell applications

Cu₃SbS₄ is a promising material for thin film heterojunction solar cells owing to its suitable optical and electrical properties. In this paper, we report the preparation of Cu₃SbS₄ thin films by annealing the Sb₂S₃/CuS stacks, produced by chemical bath deposition, in a graphite box held at different temperatures. The influence of annealing temperature on the growth and properties of these films is investigated. These films are systematically analyzed by evaluating their structural, microstructural, optical and electrical properties using suitable characterization techniques. X-ray diffraction analysis showed that these films exhibit tetragonal crystal structure with the lattice parameters a=0.537 nm and b=1.087 nm. Their crystallite size increases with increasing annealing temperature of the stacks. Raman spectroscopy analysis of these films exhibited modes at 132, 247, 273, 317, 344, 358 and 635 cm⁻¹ due to Cu₃SbS₄ phase. X-ray photoelectron spectroscopy analysis revealed that the films prepared by annealing the stack at 350 °C exhibit a Cu-poor and Sb-rich composition with +1, +5 and −2 oxidation states of Cu, Sb and S, respectively. Morphological studies showed an improvement in the grain size of the films on increasing the annealing temperature. The direct optical band gap of these films was in the range of 0.82–0.85 eV. Hall measurements showed that the films are p-type in nature and their electrical resistivity, hole mobility and hole concentration are in the ranges of 0.14–1.20 Ω-cm, 0.05–2.11 cm² V⁻¹ s⁻¹ and 9.4×10²⁰–1.4×10¹⁹ cm⁻³, respectively. These structural, morphological, optical and electrical properties suggest that Cu₃SbS₄ could be used as an absorber layer for bottom cell in multi-junction solar cells.     

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.