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.     

Effect of pulse voltage and aluminum purity on the characteristics of anodic aluminum oxide using hybrid pulse anodization at room temperature

Most of the anodic aluminum oxide (AAO) templates are performed by potentiostatical method at 0-10 °C to inhibit the Joule's heat enhanced dissolution in aluminum oxide for well-ordered cell configuration. In this article, we propose the hybrid pulse anodization (HPA) method with effective suppression of Joule's heat generation to fabricate AAO at room temperature. Effects of purity of aluminum (Al) foils and pulse voltage on the evolution of pore characteristics have been investigated. The AAO morphology is captured by scanning electron microscope and analyzed via gray-scale imaging in order to identify the pore size distribution. The increased applied potential results in the widened pores and non-uniform cell arrangement due to the increased current density and variation. Moreover, low-purity Al foils lead to the reduced AAO distribution uniformity owing to the uneven electric field induced pits on the Al surface for inferior pore arrangement. Extending both the positive and negative pulse period from 1 s to 5 s during HPA can enhance the distribution uniformity of AAO from high-purity Al by up to about 95%. In addition, the relationship between AAO configuration and Al purity and pulse voltage is further discussed and established.     

Highly transparent conductive and near-infrared reflective ZnO:Al thin films

Al-doped ZnO (AZO) thin films have been prepared on glass substrates by pulsed laser deposition. The structural, optical, and electrical properties were strongly dependent on the growth temperatures. The lowest resistivity of 4.5 × 10⁻⁴ Ωcm was obtained at an optimized temperature of 350 °C. The AZO films deposited at 350 °C also had the high optical transmittance above 87% in the visible range and the low transmittance (<15% at 1500 nm) and high reflectance (∼50% at 2000 nm) in the near-IR region. The good IR-reflective properties of ZnO:Al films show that they are promising for near-IR reflecting mirrors and heat reflectors.     

Design and fabrication of thin film optical filters to replace fiber Bragg grating in Yb-doped fiber laser

Thin film optical filters were designed and fabricated on the end-side of the fiber as the end pumping and out coupler devices to replace the fiber Bragg grating in Yb-doped fiber laser system. It was found that the average transmittance of the end pumping device in the laser pumping wavelength (900-985 nm) is around 95.2%, and the average reflectance in the laser irradiation wavelength (1065-1085 nm) is 99.72%. The average reflectance of the out coupler device is 99.7% (900-1035 nm), and the average transmittance is 20% (1065-1085 nm), respectively.     

Degradation in optical reflectance of Al film mirror induced by proton irradiation

Mirror made of Al films can yield high reflectance over a broad wavelength range, and have been widely used in spacecraft optical instruments for high quality optical applications. However, such mirrors might be deteriorated under the irradiation of charged particles in the Earth radiation belt. In order to reveal the deterioration mechanism, the change in optical properties of Al film mirrors induced by proton irradiation with less than 200 keV was studied in a vacuum environment with a heat sink. Experimental results showed that the proton irradiation led to an obvious degradation of spectral reflectance of the Al film mirror within the wavelength range from 250 to 800 nm. The threshold fluence 1 × 10¹⁶ cm⁻² was found, above which the reflectance decreased greatly with increasing proton fluence when the radiation damage primarily occurred in the Al film. According to the experimental results, a formula for the performance evolution of Al film optical mirrors irradiated with protons is proposed.     

Transparent conducting Sc-codoped AZO film prepared from ZnO:Al-Sc by RF-DC sputtering

Al and Sc-codoped zinc oxide (also expressed as Sc-codoped AZO or ZnO:Al-Sc) films were sputtered on STN glass using RF power sources on ZnO and DC power sources on Al-1.7wt.% Sc alloy. X-ray diffraction (XRD) of the codoped films displayed that they are crystalline and textured at (002) and (103). Examination through transmission electron microscopy (TEM) depicted that these films consists of columnar grains. X-ray photoelectron spectroscopy (XPS) analysis of the films indicated that the O1s comprises O(I), O(II), O(III), and O(IV). The component O(I) centered at 530.00 ± 0.15eV was attributable to Sc₂O₃; the O(III) at 531.25 ± 0.20eV was to the oxygen deficient regions within the matrix of ZnO. The transmittance of visible light (i.e., wavelength in the range from 400 to 800nm) for the film was higher than 80%. The electrical resistivity is lower (1.76 < 2.81Ω-cm), the corrosion-resistance in 3.5% NaCl solution is better for the codoped film in comparison with the usual AZO. Heat treatment of the films (at 200-400°C for 1h) improved the optical transmittance, electrical conductivity, and corrosion-resistance in saline solution.     

Optical constants and band edge of amorphous zinc oxide thin films

The optical characteristics of amorphous zinc oxide (a-ZnO) thin films grown by radio frequency reactive magnetron sputtering on various substrates at temperature < 325 K have been investigated in the spectral range 340-1600 nm. The amorphous nature of the a-ZnO films was verified by X-ray diffraction and the optical constants were obtained by analysis of the measured ellipsometric spectra using the Cauchy-Urbach model. Refractive indices and extinction coefficients of the films were determined to be in the range 1.67-1.93 and 3.9 × 10^− 8-0.32, respectively. The band edge of the films on Si (100) and quartz has been determined by spectroscopic ellipsometry (3.39 ± 0.05 eV) and spectrophotometric (3.35 ± 0.05 eV) methods, respectively. From the angle dependence of the p-polarized reflectivity we deduce a Brewster angle of 60.5°. Measurement of the polarized optical properties shows a high transmissivity (81%-99%) and low absorptivity (< 5%) in the visible and near infrared regions at different angles of incidence. Also, we found that there was a higher absorptivity for wavelength < 370 nm. This wavelength, ∼ 370 nm, therefore indicated that the band edge for a-ZnO thin films is about 3.35 eV.     

Etching process optimization using NH4Cl aqueous solution to texture ZnO:Al films for efficient light trapping in flexible thin film solar cells

0.5 μm-thick aluminum-doped zinc oxide (ZnO:Al) films were deposited at 100 °C on polyethylene terephthalate substrates by Radio Frequency magnetron sputtering. The as-deposited films were compact and dense, showing grain sizes of 32.0 ± 6.4 nm and resistivities of (8.5 ± 0.7) × 10^− 4 Ω cm. The average transmittance in the visible wavelength range of the structure ZnO:Al/PET was around 77%. The capability of a novel two-step chemical etching using diluted NH₄Cl aqueous solution to achieve efficient textured surfaces for light trapping was analyzed. The results indicated that both the aqueous solution and the etching method resulted appropriated to obtain etched surfaces with a surface roughness of 32 ± 5 nm, haze factors at 500 nm of 9% and light scattering at angles up to 50°. To validate all these results, a commercially ITO coated PET substrate was used for comparison.     

Sol-gel derived aluminum doped zinc oxide for application as anti-reflection coating in terrestrial silicon solar cells

Sol-gel grown polycrystalline Al doped zinc oxide (AZO) thin films have been deposited on Si wafers, microscopy slide glass and fluorine doped tin oxide coated glass substrates using the spin coating technique. The atomic ratio of Al:Zn in the films is 0.2. From the X-ray diffraction investigations it is found that the preferential growth of (100) reflection peak has taken place in the 450, 550 and 600 °C annealed films. Scanning electron microscopic study has shown that the films contain well-defined grains arranged in a closely packed array. The resistivity of the 500 °C annealed film is measured to be 5 × 10^− 1 Ω cm. The films have exhibited excellent optical transmittance (~ 90%) in the 400-1100 nm wavelength range. Refractive indices (n = 1.9-1.95) of the films on Si wafer are independent of the annealing temperature. Thickness of the films produced at 4000 rpm is in the range of 58-62 nm. The refractive index and thickness of these films are nearly appropriate to cause destructive interference after reflection from front emitters of solar cells. These films have demonstrated a reflectivity value of about 3% at a wavelength of 700 nm. The AZO coated silicon solar cells possess V_oc and I_sc values of 573 mV and 237 mA, respectively.     

Structural and optical characterization of Al-doped ZnO films prepared by thermal oxidation of evaporated Zn/Al multilayered films

Aluminum-doped zinc oxide (ZnO:Al) thin films (t = 68–138 nm) were prepared by thermal oxidation in air flow, at 720 K, of the multilayered metallic Zn/Al thin stacks deposited in vacuum onto glass substrates by physical vapor deposition. The effect of Al content (3.7–8.2 at.%) on the structural (crystallinity, texture, stress, surface morphology) and optical (transmittance, absorbance, energy band gap) characteristics of doped ZnO thin films was investigated. The X-ray diffraction spectra revealed that the Al-doped ZnO films have a hexagonal (wurtzite) structure with preferential orientation with c-axis perpendicular to the substrate surface. A tensile residual stress increasing with Al content was observed. The films showed a high transmittance (about 90%) in the visible and NIR regions. The optical band gap value was found to decrease with Al content from 3.22 eV to 3.18 eV. The results are discussed in correlation with structural characteristics and Al content in the films.     

Thin gold films on SnO2:In: Temperature-dependent effects on the optical properties

Gold films with thicknesses of 5 ± 0.5 nm were sputter deposited onto SnO₂:In-coated glass kept at different temperatures up to 140 °C, and similar films, deposited onto substrates at 25 °C, were annealing post treated at the same temperatures. Nanostructures and optical properties were recorded by scanning electron microscopy and spectrophotometry in the 0.3 to 2.5 μm wavelength range, respectively. Annealing had a minor influence on the optical transmittance despite significant changes in the scale of the nanostructure, whereas deposition onto substrates heated to 140 °C yielded granular films with strong plasmon absorption of luminous radiation. These results are of considerable interest for optical devices with gold films prepared at elevated temperature or operating at such temperature.     

Effects of temperature and voltage mode on nanoporous anodic aluminum oxide films by one-step anodization

Many conventional anodic aluminum oxide (AAO) templates were performed using two-step direct current anodization (DCA) at low temperature (0–5 °C) to avoid dissolution effects. This process is relatively complex. Pulse anodization (PA) by switching between high and low voltages has been used to improve wear resistance and corrosion resistance in barrier type anodic oxidation of aluminum or hard anodization for current nanotechnology. However, there are only few investigations of AAO by hybrid pulse anodization (HPA) with normal-positive and small-negative voltages, especially for the one-step anodization, to shorten the running time. In this article, the effects of temperature and voltage modes (DCA vs. HPA) on one-step anodization have been investigated. The porous AAO films were fabricated using one-step anodization in 0.5 M oxalic acid in different voltage modes including the HPA and DCA and the environment temperature were varied at 5–15 °C. The morphology, pore size and oxide thickness of AAO films were characterized by high resolution field emission scanning electron microscope. The pore size distribution and circularity of AAO films can be quantitatively analyzed by image processing of SEM. The pore distribution uniformity and circularity of AAO by HPA is much better than DCA due to its effective cooling at relatively high temperatures. On the other hand, increasing environment temperature can increase the growth rate and enlarge the pore size of AAO films. The results of one-step anodization by hybrid pulse could promote the AAO quality and provide a simple and convenient fabrication compared to DCA.     

End-closed NiCoFe-B nanotube arrays by electroless method

A novel approach is obtained during the fabrication of NiCoFe-B nanotube arrays via electroless method. Porous anodic aluminum oxide (AAO) templates fabricated by anodization of aluminum foil were sensitized using PdCl₂ solution and immersed into electroless plating baths at room temperature to produce nanotube arrays. Compositional and morphological properties of the nanotube arrays are characterized. Results indicates the formation of end-closed nanotubes with the dimension of 100-130 nm in outside diameter, which is determined by the pore size of the AAO template, and about 15 nm in thickness of tube walls. The possible formation mechanism of end-closed metallic nanotube arrays is discussed.     

Penetrating the oxide barrier in situ and separating freestanding porous anodic alumina films in one step

A simple method for penetrating the barrier layer of an anodic aluminum oxide (AAO) film and for detaching the AAO film from residual Al foil was developed by reversing the bias voltage in situ after the anodization process is completed. With this technique, we have been able to obtain large pieces of free-standing AAO membranes with regular pore sizes of sub-10 nm. By combining Ar ion milling and wetting enhancement processes, Au nanowires were grown in the sub-10 nm pores of the AAO films. Further scaling down of the pore size and extension to the deposition of nanowires and nanotubes of materials other than Au should be possible by further optimizing this procedure.     

Doping optimization and surface modification of aluminum doped zinc oxide films as transparent conductive coating

Aluminum doped zinc oxide (ZnO:Al) films were grown using spray pyrolysis technique. Effect of doping on structural, electrical, optical and morphological properties was studied. Aluminum doping improved the prominence of [002] growth while maintaining the grain size ~ 48 nm. Using an intermediate Al/Zn atomic ratio in precursor (1.5:100), we could achieve a low resistivity ρ ~ 7 × 10^− 4 Ωcm. These films possessed an average visible transmittance ~ 88%, an optical gap ~ 3.7 eV and plasma wavelength at 1.87 μm. A simultaneous use of methanol and iso-propanol in the precursor lead to a moderate surface roughness ~ 12 nm. The films were surface modified using wet chemical etching in diluted hydrochloric acid, for varied time intervals (5 s-15 s) and etchant concentrations (0.125%-1%). The etching experiments could be used to know the building of the film as also to modify the surface for desired optical and morphological properties.     

Optical properties of CdSe and CdO thin films electrochemically prepared

The present work comparatively studies the optical properties of CdO against CdSe samples potentiostatically electrodeposited onto fluorine-doped tin oxide/glasses (FTO). The films were prepared by electrochemical reduction processes in dimethyl sulfoxide (DMSO) solution. The optical properties are studied by transmittance measurements and diffuse reflectance spectra (DRS). Structural properties of the films were also studied by X-ray diffraction spectroscopy (XRD). For CdSe the direct bandgap energy obtained from both transmittance measurements and first derivative peak position of the DRS is 1.69 ± 0.01 eV. In the case of CdO both direct and indirect gaps were found. Careful deconvolution of the spectra allows assigning the direct bandgap energy at 2.52 eV. A first indirect bandgap appears at 2.03 eV, while it is possible to have a second one at 0.46 eV.     

The properties of heterojunction based on CuI/pentacene/Al

To improve the injection of charge carriers from ITO electrode into the molecular semiconductor in the pentacene-based photovoltaic structures, we propose introducing an additional transport copper iodide (CuI) layer with high conductivity. The organic flexible barrier based on ITO/CuI/pentacene/Al was fabricated using flexible polyethyleneterephtalate substrate with conductive ITO layer by vacuum deposition technique. CuI films, annealed at the temperature of 523 K, exhibited optical transmittance ∼80% in the wavelength range 400-900 nm, minimum resistivity about 1.8 × 10³ Ω/sq, and developed surface. They were used for fabrication of photosensitive barrier structure on the basis of pentacene.     

Modification of optical and electrical properties of ITO using a thin Al capping layer

In this study, the work function, transmittance, and resistivity of indium tin oxide (ITO) thin films were successfully modified by depositing an Al capping layer on top of ITO with subsequent thermal annealing. The 5 nm thick Al layer was deposited by a conventional dc magnetron sputtering method and the layer was converted into an aluminum oxinitride by subjecting the sample to rapid thermal annealing (RTA) under a nitrogen atmosphere. The films exhibited a high transmittance of 86% on average within the visible wavelength region with an average resistivity value of 7.9 × 10^− 4 Ω cm. Heat-treating the Al/ITO films via RTA resulted in the decrease of the optical band gap from that of bare ITO. In addition, the films showed red-shift phenomena due to their decreased band gaps when the heat-treatment temperature was increased. The resultant electrical and optical characteristics can be explained by the formation of aluminum oxinitride on the surface of the ITO films. The work function of the heat-treated films increased by up to 0.26 eV from that of a bare ITO film. The increase of the work function predicts the reduction of the hole-injection barrier in organic light-emitting diode (OLED) devices and the eventual use of these films could provide much improved efficiency of devices.     

Influence of gamma irradiation on the refractive index of Fe-doped barium titanate thin films

Polycrystalline Fe-doped barium titanate (Fe-doped BaTiO₃) thin films were grown by thermal decomposition of the precursors deposited from a sol-gel system onto quartz substrates. The changes in the transmittance spectra induced by gamma irradiation on the Fe-doped BaTiO₃ thin films were quantified. The values for the optical energy band gap were in the range of 3.42-3.95 eV depending on the annealing time. The refractive index of the film, as measured in the 350-750 nm wavelength range was in the 2.17-1.88 range for the as prepared film, and this increased to 2.34-1.95 after gamma irradiation at 15 kGy. The extinction coefficient of the film was in the order of 10⁻² and increased after gamma irradiation. We obtained tuneable complex refractive index of the films by exposure to various gamma rays doses.     

Optical properties of CulnSe2 thin films

The optical constants of vacuum-deposited CulnSe₂ films were determined from the measured transmittance and reflectance at normal incidence of light in the wavelength range 500 to 2000 nm. The analysis of the experimental points of the absorption coefficient revealed the existence of two optical transition processes: an allowed direct transition withE _g=1.03±0.01 eV and a forbidden direct transition withE _f=1.254±0.001 eV. The optical constants of the films were independent of the substrate temperature.On leave to the Kingdom of Saudi Arabia.