Aluminum-doped Zinc Oxide Thin Films Research Articles

The effect of substrate roughness on the properties of RF sputtered AZO thin film

Abstract

Enhanced Sol-Gel Route to Obtain a Highly Transparent and Conductive Aluminum-Doped Zinc Oxide Thin Film.

The electrical and optical properties of sol–gel derived aluminum-doped zinc oxide thin films containing 2 at.% Al were investigated considering the modifying effects of (1) increasing the sol H2O content and (2) a thermal treatment procedure with a high-temperature approach followed by an additional heat-treatment step under a reducing atmosphere. According to the results obtained via the TG-DTA analysis, FT-IR spectroscopy, X-ray diffraction technique, and four-point probe resistivity measurements, it is argued that in the modified sample, the sol hydrolysis, decomposition of the deposited gel, and crystallization of grains result in grains of larger crystallite size in the range of 20 to 30 nm and a stronger c-axis preferred orientation with slightly less microstrain. The obtained morphology and grain-boundary characteristics result in improved conductivity considering the resistivity value below 6 mΩ·cm. A detailed investigation of the samples’ optical properties, in terms of analyzing their absorption and dispersion behaviors through UV-Vis-NIR spectroscopy, support our reasoning for the increase of the mobility, and to a lesser extent the concentration of charge carriers, while causing only a slight degradation of optical transmittance down to nearly 80%. Hence, an enhanced performance as a transparent conducting film is claimed for the modified sample by comparing the figure-of-merit values.

Evaluation of structural properties of AZO conductive films modification using low-temperature ultraviolet laser annealing

Abstract In this study, an ultraviolet (355 nm) laser processing system was developed to anneal aluminum-doped zinc oxide (AZO) thin films at room temperature in an air atmosphere; in this system, two key parameters, laser fluence and annealing speed, were varied. The structural properties of the films were thoroughly examined using field emission scanning electron microscopy, atomic force microscopy, and X-ray diffraction (XRD). The results showed that the laser fluence not only influenced the structural properties of the films, but also improved the crystallinity of the films after the laser annealing process, with minimal changes in the thickness of the films and the concentration of the elements in the films. The root mean square surface roughness (R rms ) of the films gradually increased as the laser fluence increased. Moreover, according to the XRD pattern of the films, the intensity of the main peak corresponding to the (002) direction increased as the laser fluence increased. The average crystallite size (20 nm) of the annealed films, determined using the Scherrer equation, was smaller than that of the as-deposited thin film (22 nm) due to the low temperature effect in the laser annealing process.

Impact of deposition methods and doping on structural, optical and electrical properties of ZnO-Al thin films

In this work, thin films of aluminum doped ZnO (AZO) were deposited on ultrasonically cleaned glass substrates by sol-gel process using dip and spin coating techniques. For this purpose, Zinc acetate dihydrate, aluminum nitrate nonahydrate, ethanol and mono ethanolamine were employed as precursor, dopant, solvent and stabilizer, respectively. X-ray diffraction, UV–vis, photoluminescence, 4-point probe and Van der pauw techniques were investigated for the characterization of the prepared AZO thin films. X-ray-analysis revealed that all the prepared films have hexagonal wurtzite structure with a relative preferential orientation along the c-axis and the lattice parameters are close to the standard values reported in literature. UV–vis spectroscopy showed that the average value of the films’ transmittance in the visible region is found to be around 85% and the gap ranges in the interval [3.15 eV–3.30 eV]. The photoluminescence spectrum only showed the UV peak while the broad band of the visible region was completely vanished. The electrical measurements indicate that sol-gel methods provide relatively high resistivities compared to those obtained with physical vapor deposition (PVD) techniques.

Optical and electrical properties of Al doped ZnO thin film with preferred orientation in situ grown at room temperature

Abstract To optimize the process and obtain highly conducting and transparent Aluminum-doped zinc oxide (AZO) thin films, AZO films were deposited on glass substrates at room temperature by Radio-frequency (RF) magnetron sputtering with various Argon flow rates. The influences of Argon flow rate on structure, morphology, optical, electrical and photoluminescence properties of AZO films were investigated by varying the Argon flow rate from 36 to 68 sccm. The best quality AZO film with resistivity 1.39 × 10−3 Ω cm, sheet resistance 8.2 Ω/sq and 84.2% average visible transmittance was prepared at 44 sccm for 30 min. Also, the self-heating effect of target was investigated by preparing AZO films for 10 min and 20 min at 44 sccm, 180 W and 1.0 Pa. The influence of increasing structural quality actually affected by Argon flow rate was more prominent on carrier concentration than mobility. The schematic illustration of microstructural evolution was proposed. The average growth rate of around 60 nm/min demonstrated the self-heating effect of target was weak and could be ignored.

Optical, Structural and Electrical Properties of Aluminum Doped Zinc Oxide Thin Films by MOCVD Technique

Thin films of aluminum-doped ZnO (AZO) were deposited by metal organic chemical vapor deposition from a single solid source precursor, aluminum zinc acetylacetonate. The AZO thin films were deposited on a sodalime glass substrate at different substrate temperatures ranging from 390°C to 450°C. Results were characterized using Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM) with energy dispersive x-ray (EDS) facility attached to it, x-ray diffraction (XRD), UV–Visible spectrometry and a four point probe method (with an applied magnetic field). While RBS determined the thickness and elemental composition of the films, the presence of the expected elements in the films was also corroborated by EDS. The average thickness of the thin films was determined to be 133 nm. SEM micrographs revealed that the films were composed of continuously and uniformly distributed crystalline grains without any cracks. XRD analysis showed that the films were polycrystalline with a structure belonging to ZnO hexagonal wurtzite and grain size ranging from 27.50 nm to 41.30 nm. The optical transmittance of AZO films deposited at different temperatures varied from 75% to 95% in the range of wavelength 400–700 nm, with an optical band gap energy value between 3.27 eV and 3.30 eV. The electrical characterization of AZO films deposited at different substrate temperatures revealed resistivity values between 0.07 Ω cm and 0.01 Ω cm. The mobility was not the same for all the films, so also carrier concentration. The physico-chemical characterization showed that the AZO film deposited at temperature 420°C possessed the best properties among the three samples.

Near-infrared reflective characteristics of aluminum-doped ZnO thin films fabricated by <i>off-axis</i> sputtering

Near-infrared reflective characteristics of aluminum-doped ZnO thin films fabricated by <i>off-axis</i> sputtering

Aluminum-doped zinc oxide thin films deposited by electrospray method

Aluminum-doped ZnO (AZO) thin films are deposited by economically-versatile fabrication method of electrospraying onto two types of substrates – glass and silicon wafer, at substrate temperature of 300 °C, voltage of 18 kV and emitter to collector distance of 6 cm, Zinc acetate dehydrate dissolved in a mixture of deionized water, ethanol and acetic acid is used for deposition of undoped ZnO film, while for Al-doping one two precursor materials are chosen - aluminum chloride and aluminum nitrate dissolved in ethanol. The doping levels are 5 and 10 wt% and the concentration dependence of films properties is investigated. All films exhibit polycrystalline structure with decreasing crystallites size from 21 nm for undoped films to 19 nm and 17 nm for AZO films doped with 5 and 10 wt% Al, respectively. The shift of the XRD peaks of doped samples towards higher diffraction angles reveals successful incorporation of Al into ZnO lattice. The optical properties are investigated by ellipsometric and photoluminescence measurements. A decrease of refractive index from 1.89 for undoped sample to 1.84 and 1.83 for 5 and 10% AZO films, respectively and an increase of optical band gap from 3.30 eV to 3.40 eV are observed. A substantial decrease of sheet resistance is measured for AZO films: 86 Ω/□ and 61 Ω/□ for 5 wt% and 10% doped samples, respectively. Simultaneously, the transmittance of the films is kept high. The average values in the visible range for films with thickness of 200 nm are 86% for undoped ones and 84% and 80% for 5 and 10% Al-doped films, respectively. Thus, the applicability of the studied films as transparent conductive electrodes is demonstrated.

Ellipsometric study on optical properties of hydrogen plasma-treated aluminum-doped ZnO thin film

Aluminum-doped zinc oxide (AZO) thin films were prepared by radio frequency (RF) sputtering at room temperature, and then post-treated by hydrogen (H2) plasma at different durations. After H2 plasma treatment under the condition of 10 W, 200 °C and 3.0 Hours, the resistivity showed a dramatically decrease from 1.6 Ω cm to 3.4 × 10−3 Ω cm, while the transmittance at the wavelength of 550 nm was improved from 90.5% to 96.0%. The optical constants of H2 plasma-treated AZO thin films were detailed characterized by a varied angle spectroscopic ellipsometer. The results show that the refractive index n decreases in the entire measured wavelength range of 350–1100 nm, while the extinction coefficient k decreases in the short wavelength range and changes negligibly at the long wavelength range. These results can provide guidelines for the design and optimization of AZO thin film-based optoelectronic applications.

All-Transparent Zinc Oxide-Based Phototransistor by Mist Atmospheric Pressure Chemical Vapor Deposition

This letter demonstrates an all-transparent ZnO-based phototransistor on which fluorine-doped tin oxide, magnesium oxide, zinc oxide, and aluminum-doped zinc oxide thin films are deposited by mist atmospheric pressure chemical vapor deposition. This phototransistor is designed for ultraviolet detection and can be illuminated from either the front or backside. By comparing the device characteristics, the backside illumination exhibits better performance. The phototransistor is able to be operated in high ultraviolet-to-visible rejection ratio/fast response mode. In this mode, the ultraviolet-to-visible rejection ratio reaches 2915 and the rising/decay time of 5.35/6.8 s are achieved. The second mode is high responsivity/detectivity mode. In this mode, responsivity of 2520 A/W, photoconductive gain of 8682, and detectivity of 1.57 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> cm-Hz-W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are obtained. It is also found that the operation mode is controlled by the gate bias.

Plasma-Assisted Chemical Vapor Synthesis of Aluminum-Doped Zinc Oxide Nanopowder and Synthesis of AZO Films for Optoelectronic Applications

Transparent conducting oxide aluminum-doped zinc oxide (AZO) nanoparticles were synthesized by a plasma-assisted chemical vapor synthesis route using zinc nitrate and aluminum nitrate as the precursors. The injected precursors vaporized in the plasma flame, followed by vapor-phase reaction and subsequent quenching of the vaporized precursors, producing nanosized AZO powder. The amount of aluminum nitrate was varied to obtain samples with 2 at.%, 4 at.% and 8 at.% Al, designated as AZO1, AZO2 and AZO3, respectively. The XRD patterns of the AZO1 and AZO2 nanoparticles indicated the presence of a wurtzite structure without any alumina peaks except in the AZO3 sample, and scanning electron microscopy micrographs revealed spherical particles. The magnetization measurements revealed a ferromagnetic behavior at room temperature in the AZO1 sample, and the ferromagnetic order is decreased in the high field region with an increase in the Al doping amount. AZO thin films were deposited on glass substrates by spin-coating a dispersion of nanoparticles. All the AZO films had a hexagonal wurtzite structure and exhibited a c-axis preferred orientation perpendicular to the substrate. The Hall effect measurements yielded a minimum resistivity of 9.9 × 10−4 Ω cm for the AZO2 film and optical transmission of 80% for both the AZO1 and AZO2 films. However, with 8 at.% Al in the AZO3 film, deterioration in crystallinity, electrical and optical properties were observed. Post-annealing of the AZO1 film in H2 atmosphere caused a significant decrease in resistivity from 1.2 × 10−3 Ω cm to 8.7 × 10−4 Ω cm. The optical band gap energies of the AZO films were determined from the transmission spectra. The blue shift in the band gap from 3.2 eV to 3.28 eV, observed with an increase in Al doping, was interpreted by the Burstein–Moss effect. The photoluminescence spectra of the AZO films revealed a UV near-band edge emission and a green emission peak.

UV-enhanced Ozone Sensing Response of Al-doped Zinc Oxide Nanocrystals at Room Temperature

In this work, we focus on ozone (O3) detection at room temperature in the main objective to be well suited for most of flexible substrates. Aluminum doped zinc oxide (AZO) nanoparticles were deposited in thin films to be used as ozone gas sensors. It was demonstrated that the nanoparticles had good sensitivity to O3 at low temperature (≤ 125°C) when the sensor was exposed to UV illumination. This study provides an easy and efficient way to obtain O3 gas sensors at temperature from 25°C up to 125°C by a simple and versatile solution-process. Electrical resistance measurements at 25°C under UV irradiation showed that AZO thin films were sensitive even at a low ozone concentration (35 ppb).

Experimental and theoretical investigations on temperature and voltage dependence of an Au/AZO thin-film Schottky diode

In this work, aluminum-doped zinc oxide (AZO) thin films were deposited by DC sputtering on a glass substrate at a typical sputtering power. It was observed that the AZO structure changes on the ZnO crystalline structure. The measurement of the transparency spectrum on the AZO films shows an optical bandgap about 4.3 eV, which the Al doping into ZnO structure pronounced width localized states by Urbach energy that was 0.42 eV. Moreover, the experimental results on the electrical properties of AZO/Au thin film were evaluated at different temperatures by four-point probe. Also, a simulation analysis on the electrical parameters of the Schottky Au/AZO junction has been done. Our calculation is based on the thermionic emission theory, in which by fitting the numerical results with the experimental current–voltage measurements, the barrier height, ideality factor and saturation current have been obtained. Our result confirmed excellent rectifying characteristics. The extracted result may be useful in designing nanoelectronic devices.

Structural and electrical properties of Sprayed ZnO:Al thin films

Aluminium doped zinc oxide (AZO) thin films were prepared on glass substrates by spray pyrolysis technique. The Al concentration in the starting solution was varied from (0 to 5) wt %. XRD revealed that both pure and AZO thin films were polycrystalline with hexagonal structure and exhibited (002) preferential orientation. AZO film with minimum electrical resistivity 8.61×10 ³⁻ Ω.cm were obtained at a doping ratio of 2 wt. %.

Aluminum doped zinc oxide (AZO) coated optical fiber LMR refractometers—An experimental demonstration

This work presents the experimental demonstration of lossy mode resonance generation by means of aluminum doped zinc oxide thin-films. The use of such material may allow to optimize the performance of LMR-based sensors, obtaining good sensitivity at lower costs than other overlays, such as those including indium. The refractometric response of the fabricated devices is explored in different spectral regions. One refractometer working in the near infrared region was fabricated, obtaining a sensitivity of 2280 nm/RIU. A second refractometer working in the visible light spectrum was also fabricated. This second device allows to observe rough refractive index variations with the naked eye as a change of the color of the light propagating through the fiber, simplifying the setup needed for its use.

Determination of the barrier height of Pt-Ir Schottky nano-contacts on Al-doped ZnO thin films by conductive Atomic Force Microscopy

By means of the I-V characteristics measured at room temperature, the height of the Schottky barrier established by the conductive Pt-Ir tip of an Atomic Force Microscope on the aluminum doped ZnO thin films were estimated in the range of 0.58-0.64 eV. The ideality factors were in the range of 2.11-1.39, respectively. These values are in accordance with those reported by other authors that measured the height of the Pt Schottky barrier on ZnO by means of several methods. The procedure detailed in this work suggests that the scanning time for obtaining I-V Schottky characteristics is of the order of 2 ms.

Sol-gel derived aluminium doped zinc oxide thin films: A view of aluminium doping effect on physicochemical and NO2 sensing properties

Aluminium doped ZnO (AZO) thin films have been synthesized by using a sol-gel spin coating method. The effect of Al-doping on physicochemical and NO2 sensing properties of the AZO thin films were studied. The structural and compositional analysis confirms the phase formation of ZnO and substitution of aluminium atoms into zinc oxide lattice. The gas sensing performance of AZO thin films investigated at different operational temperatures towards NO2 gas. The NO2 sensing study reveals that the 2 at% AZO film sensor shows excellent gas sensing characteristics at 200 °C.

Utilizing magnetron sputtered AZO-ITO bilayer structure as transparent conducting oxide for improving the performance of flexible CIGS solar cell

Preparing transparent conducting oxide (TCO) layer is one of the critical procedures for the fabrication of CIGS solar cells. Especially conventional fabrication of TCO requires elevated temperature annealing process, which causes fatal effect for flexible CIGS solar cell grown on polyimide (PI) substrate. In this paper, we investigated the preparation method of aluminum-doped zinc oxide (AZO) and tin-doped indium oxide (ITO) bilayer thin films as the window layer by DC magnetron sputtering without annealing process. By varying the thickness of AZO and ITO, the bilayer thin films with high optical transmittance (>81.5% from 400 nm to 1200 nm) were achieved. The formation of ITO grains on AZO layer was analyzed by XRD and AFM. When the AZO thickness reached to 150 nm, an AZO-assisted ITO growth phenomenon was observed in our experiment, which was important for improving the electrical properties of bilayer structure. Compared to single layered (AZO or ITO) structure, the CIGS solar cells using bilayer structure enhanced photoelectric conversion efficiency (∼above 11%) and long-term stability.

Vapor textured aluminum-doped zinc oxide on cellophane paper for flexible thin film solar cells

Paper-based flexible thin film solar cells are promising power sources for the emerging portable and wearable electronics, for which high conversion efficiency and flexibility are demanded. Herein, flexible thin film silicon solar cells using textured aluminum-doped zinc oxide (AZO) for light scattering, and ultrathin silver electrodes combined with cellophane paper substrates, were demonstrated. A universal route to form textured AZO thin film by chemical etching with hydrochloric acid vapor was proposed. The AZO layers textured in this way exhibited similar light scattering properties as compared with state-of-the-art AZO thin films. Applying textured AZO to solar cells on cellophane paper substrates resulted in current density and conversion efficiency increase from 8.7 to 9.5 mA/cm2 and from 4.1% to 5.7%, respectively. Thin film solar cells on cellophane exhibited excellent mechanical flexibility, which maintained 90% of their initial efficiency after bending at a radius of 1 mm for 50 cycles.

Pressure effect on structural and electrical properties of AZO thin films annealed in N2/H2 atmosphere

Transparent and conductive aluminum-doped zinc oxide (AZO) thin films were developed using a sol-gel method and annealed in 95 N2/5 H2 forming gas. The pressure effects of forming gas on the structural and electrical properties, and the underlying mechanisms were investigated. XPS analysis reveals that the fraction of the chemisorbed oxygen in the annealed film in 0.01 MPa 95 N2/5 H2 did not exhibit any apparent change compared to the non-annealed films, while the oxygen vacancy content exhibited a remarkable increase. When pressure goes up to 0.03 MPa, significant changes of both the oxygen vacancy concentration and the chemisorbed oxygen content were observed in the annealed films. At 0.03 MPa, the films displayed a perfect orientation along the c-axis and exhibited the lowest resistivity of 1.6 × 10−3 Ω·cm. Our study shows that it is possible to optimize the oxygen distribution and improve the electrical property of AZO film by adjustment of the forming gas pressure.