Aluminum-doped Zinc Oxide Films Research Articles

Designing Low-Cost Arduino Powered Spin Coater for Thin Film Deposition

In the present work, we have designed a low-cost spin coater using the Arduino Uno board. The advantage of selecting Ardunio is, it has pulse width modulation (PWM) based pins. Depending on the width of the pulse, the output voltage changes which will intern changes the speed of the DC motor connected to the PWM pin. The thickness of deposited film using spin coater depends on RPM and duration of rotation. The rotation of the substrate during deposition has three stages a gradual increase in RPM, maintaining constant RPM over a while, and a gradual decrease in RPM. All these parameters can be controlled by an Arduino board. An Aluminum doped Zinc Oxide film was deposited on glass substrate using Arduino based spin coater. X-ray diffraction, UV – VIS spectroscopy, and FTIR methods were used as characterization techniques. Hexagonal crystal structure of deposited AZO layer was confirmed by XRD and optical band gap, transparency were calculated by UV-VIS spectroscopy.

Realizing PIT-like transparency via the coupling of plasmonic dipole and ENZ modes.

Plasmon induced transparency (PIT), known as the coupling of plasmon modes in metamaterials, has attracted intensive research interests in photonic applications. In this work, a PIT-like transparency is realized via the strong coupling of plasmonic dipole and epsilon-near-zero (ENZ) mode. Two types of metasurfaces, namely the gold nanoantenna and dolmen-like metasurface, are designed with an integrated ENZ material aluminum doped zinc oxide (AZO) film. Simulations with the finite element method (FEM) demonstrate that single and double transparent windows are achieved respectively. The adjustments of the peak position and transmittance of transparent windows via the structure parameters and the AZO film thickness are further investigated. This work provides an alternative coupling scheme of realizing PIT-like transparency with simple metasurface design, and offers great potential for future metamaterial applications.

Effect of Aluminum Doping Ratios on the Properties of Aluminum-Doped Zinc Oxide Films Deposited by Mist Chemical Vapor Deposition Method Applying for Photocatalysis.

Aluminum-doped zinc oxide film was deposited on a glass substrate by mist chemical vapor deposition method. The influence of different aluminum doping ratios on the structural and optical properties of zinc oxide film was investigated. The XRD results revealed that the diffraction peak of (101) crystal plane was the dominant peak for the deposited AZO films with the Al doping ratios increasing from 1 wt % to 3 wt %. It was found that the variation of AZO film structures was strongly dependent on the Al/Zn ratios. The intertwined nanosheet structures were obtained when Zn/O ratios were greater than Al/O ratios with the deposition temperature of 400 °C. The optical transmittance of all AZO films was greater than 80% in the visible region. The AZO film deposited with Al doping ratio of 2 wt % showed the highest photocatalytic efficiency between the wavelength of 475 nm and 700 nm, with the high first-order reaction rate of 0.004 min−1 under ultraviolet radiation. The mechanism of the AZO film influenced by aluminum doping ratio during mist chemical vapor deposition process was revealed.

Properties of AZO thin films prepared by stationary and rotating RF magnetized plasma sputtering source

The aluminum-doped zinc oxide (AZO) films were deposited with a star-shaped plasma using a high-density radio frequency (RF) magnetized sputtering source to develop well-consistent target utilization. The deposited AZO film characteristics were investigated using stationary and rotational modes with a velocity of 40 rpm, Ar pressure of 1.0 Pa, and RF power of 40 W with a deposition time of 1.0 h. The deposition rate profile of stationary mode with a roughness of ±30.18% was more fluctuating than that of rotational mode with a roughness of ±10.90%. The lowest electrical resistivities of the AZO films were obtained as 3.8 × 10−4 and 3.6 × 10−4 Ω cm at r = 0 mm for the stationary and rotational modes, respectively. According to the AFM and SEM results, the surface profile of the rotational mode grown AZO film was more uniform and smoother than that of the stationary mode. The intensity of x-ray diffraction peaks (002) of the grown AZO film was decreased from the center to the edge of the vessel. Most of the films showed two broad Raman peaks at about 274 and 575 cm−1, representing additional mode and longitudinal optical phonons of the wurtzite ZnO crystal. All the AZO films showed a high transmittance in the visible range, and the average transmittance of the AZO films in the visible range was ∼84%.

Active tuning of the optical response of field-effect-gated transparent conductive oxides

Transparent Conductive Oxides (TCOs) are a class of materials with high optical transparency and electrical conductivity. This combination makes them extremely appealing for solar cells, optoelectronics and infrared-plasmonics applications. In this work, we report the active tuning of the optical response of aluminium-doped ZnO (AZO) films upon electrical gating within a parallel-plate capacitor configuration. We investigated the electrical-bias-dependent optical response of thin AZO films fabricated by pulsed laser deposition by means of spectroscopic ellipsometry (SE). Calculations based on an exponentially-decaying spatial distribution of injected/depleted charge density in the AZO film are in accordance with experimental observations, allowing the extraction of the Debye length.

The thermo-optical and optical properties of thin ZnO and AZO films produced using the atomic layer deposition technology

In this work, the influence of temperature on the optical constants of zinc oxide (ZnO) and aluminum-doped zinc oxide (AZO) thin layers is presented. Thin layers of ZnO and AZO were deposited on silicon using the atomic layer deposition (ALD) method. Optical constants were determined based on ellipsometric measurements in spectral range of 193–1690 nm at temperatures varying from 25° to 300°C. From thermal ellipsometric results, the thermo-optical coefficient (TOC) and thermal expansion coefficient (TEC) have been determined. We have shown that after two cycles of heating and cooling, the thermo-optic processes are reversible. Additionally, complementary studies: XRD, EDS, SEM have been presented. The relationships between the structure of presented films and their optical properties have been discussed. We have shown that the doping of Al to the ZnO matrix changes the sign of the thermo-optical coefficient from negative to positive. The results have been explained based on Prod-Homme theory.

Study on Fabrication and Material Properties of Al Doped ZnO Films Prepared by Atmospheric Atomic Layer Deposition

This paper presents the study on aluminium-doped zinc oxide (AZO) films prepared by atmospheric atomic layer deposition (AALD) using Diethylzinc (DEZ), Zn(C2H5)2, and Trimethylaluminum (TMA), Al(CH3)3 as precursors. The optimal condition for doping was investigated by changing in DEZ/TMA ratio. The crystal structure of fabricated thin films shows the hexagonal wurtzite structure with the orientation along the c-axis. The influence of heat treatment on the grain size, carrier type and concentration of post-fabricated films deposited on the different substrates which are borosilicate glass and sapphire was also analysed. The Hall measurement to determine the carrier type and resistivity at room temperature to 400oC was performed. The measurement results show that as-deposited samples behave as alloy-like property with p-type carriers and high resistivity. However, they turned into n-type nature as expected with the increase in carrier concentration and consequently the marked decrease in electrical resistance when annealed at the higher temperatures that are at 500oC and 900oC (i.e, 773 and 1173 K). In general, the obtained films with optimized experimental conditions of as- and post-fabrication can be used for thermoelectric applications.

Electrical and Optical Properties of Aluminum-Doped Zinc Oxide Films on a Glass Substrate Prepared by a Toroidal Magnetron Sputtering System at Low Temperature

Aluminum-doped zinc oxide (AZO) thin films were prepared on a glass substrate by employing a toroidal magnetron sputtering system to enhance both resistivity and transparency. The glass substrate temperature was sustained below 50 °C during the deposition of 200 nm thick AZO without an additional heating system for the substrate, and no pre- or postannealing process was carried out. This scenario is quite desirable to reduce damage to the thermally sensitive substrate during the conventional AZO thin film process; nonetheless, significant crystallinity was observed in the XRD analysis due to energy transfer by sputtered ions. The electrical resistivity was approximately 8.3×10−4 Ωcm, and the average transmittance was 90.43%, which were achieved under optimized conditions, with the ratio between the O2 and Ar gas flow rates varied under a specific applied DC power.

Establishing a gold nanoparticle quasi-mesh structure in Al-doped zinc oxide film to enhance photoelectric properties

A low-cost chemical reduction method was applied to reduced gold nanoparticles (Au-NPs), which were used in aluminum-doped zinc oxide (Al:ZnO, AZO) film deposited using a magnetron sputtering system. The Au-NPs were mixed under different process parameters to effectively remove organic matter on the Au-NPs and form a three-dimensional quasimesh structure of Au to improve the photoelectric properties of the AZO film. Different levels of sputtering power were applied to prepare a high-quality AZO seed layer, sintering temperature and rotating speed in spin coating were adjusted to improve the effect of Au-NPs on the photoelectric properties of the AZO film. Transmission electron microscope, scanning electron microscope, and X-ray diffractometer were used to analyze the structure and distribution of the Au-NPs, and also crystal characteristics of the AZO film. An ultraviolet–visible–near infrared spectrophotometer and a Hall effect measurement system were implemented to measure the photoelectric properties of the film. According to the results, a quasi-mesh structure with the optimal Au-NPs distribution was established by depositing the AZO seed layer with 150 W of sputtering power, processing it with thermal annealing at 500 °C, spin coating the Au-NPs on the textured surface of the AZO film at 3000 rpm, and sintering the film at 800 °C for 10 min. This structure reduced film resistance to 3.57 × 10−3 Ω-cm, and a maximal transmittance of 84.15% within the visible spectrum, and resulted in a figure of merit of 3.99 × 10−4 Ω−1.

A Study on Formation of Aluminum-Doped Zinc Oxide Films by Pulsed-Direct Current Sputtering for CIGS Solar Cells.

Considering the relationship between thin film thickness of transparent conductive oxide (TCO) materials and the reversed pulse time in pulsed-direct current (DC) sputtering, aluminum-doped zinc oxide (AZO) films were deposited on glass substrates at different reversed pulse times by changing oxygen/argon (O₂/Ar) gas ratios for window layers of large area CuIn1-xGaxSe₂ (CIGS) solar cells. As a result of the reduced sputtering time, the thickness of AZO film was decreased when the reversed pulsed time was increased. The higher resistance and resistivity of the AZO film was obtained at a higher reversed pulse time. From the structural investigations of AZO such as transmittance and X-ray diffraction (XRD), it was possible to observe the relationship between the crystallinity of AZO and transmittance. Even at the short reversed pulse time of 0.5 µs, it can be concluded that the accumulated charges on the AZO target are completely cleared and the AZO layers show the highest figure of merit (FOM) with low sheet resistance and high transmittance.

Impact of Al-doping on structural, electrical, and optical properties of sol-gel dip coated ZnO:Al thin films

Aluminum-doped ZnO (AZO) thin films were coated on glass substrates using the sol-gel dip-coating technique. We investigated the effect of Al-doping level on the surface morphology, crystal structure, atomic bonding, and optical and electrical properties of the AZO films. The Al-doping levels in the sol-gel solution were 0, 0.25, 0.50, 1.0, 1.5, and 3.0% with a Zn precursor concentration of 0.50 M. The results show that the grain boundary increased with the doping level, while the crystallite size decreased from 27.2 to 14.2 nm. The AZO films were subjected to tensile stress. The Al-doping levels ≤ of 1.5% encouraged the intensity of the (002) x-ray diffraction peak. The center of the (002) peak shifted from 34.46° to 34.51°, and that of the Raman mode shifted from 435.0 to 432.4 cm−1. Doping with 1.5% Al resulted in a maximum electron concentration of 4.7 × 1018 cm−3 with a minimum resistivity of 2.6 × 10−1 Ω cm and a mobility of 5.14 cm2/V.s. The Urbach energy increased from 88 to 120 meV with increasing doping level. For the AZO films doped with Al (≤1.5%), the analysis of the UV–vis spectra reveals that the position of the conduction band (CB) minimum of the films shifted from −0.16 to −0.21 eV and shifted outward from the valence band (VB). Further Al doping to 3.0% resulted in a VB shift toward CB. The optical band gap reached a maximum of 3.33 eV at 1.5% Al. The combination of tensile stress and electron density due to Al doping influences the shift in the optical band gap of the AZO films. The crystal structure, atomic bonding, and electronic band structure of ZnO films can be modified by Al doping.

Magnetron co-sputtering optimized aluminum-doped zinc oxide (AZO) film for high-response formaldehyde sensing

It is very important to conduct trace detection of the gas formaldehyde, a harmful gas of volatile organic compounds (VOCs) that threatens human health. In the present work, the aluminum-doped zinc oxide (AZO) films with different aluminum contents were prepared on silicon substrate by magnetron co-sputtering technology. The test results showed that when the sputtering power of ZnO was 150 W and the sputtering power of Al2O3 was 50 W, the AZO thin film sensor had the best response to formaldehyde at an operating temperature of 240 °C. The sensor had a good response to 100 ppb-3 ppm formaldehyde, and its response to 100 ppb formaldehyde at 240 °C was as high as 43%. The sensor could detect as low as 50 ppb to formaldehyde, and it also exhibited good repeatability, long-term stability, as well as good selectivity over other gases (hydrogen, carbon monoxide, ammonia, benzene, isoprene, nitrogen dioxide and acetone). Therefore, this AZO thin-film sensor is expected to be used in indoor formaldehyde monitoring.

Effect of oxygen annealing temperature on properties of spatial atomic layer deposited aluminum-doped zinc oxide films

In this work, spatial atomic layer deposited (sALD) Al-doped ZnO (AZO) films are prepared, followed by an oxygen post annealing process. Effects of annealing temperature on structural, electrical and optical properties are systematically investigated. The experimental results show that the band gap of the films is not much affected by the oxygen annealing temperature. However, the oxygen annealing is able to remove the oxygen vacancies and improve crystalline structure. Annealing at too high temperatures leads to oxygen desorption from the film. The electrical characterization reveals that the resistivity of the sALD AZO films can be tuned in a range of 1.1 × 10−3 to 1.7 × 10−2 Ω-cm via oxygen annealing.

Characteristics of Aluminum-Doped Zinc Oxide Films Grown Using Facing Target Sputtering for Transparent Electrode of Heterojunction Solar Cells.

In general sputtering, material characteristics can be degraded by high-energy particles located inside the plasma owing to the thin film surface. However, facing target sputtering (FTS) can be used to produce high-quality thin films through maximum control over substrate damage and the reduction of layer damage caused by high-energy particles impacting the substrate. Transparent conductive oxides (TCOs) are being applied to a variety of technologies, including displays and solar cells. The typical transparent electrode material is indium tin oxide (ITO), which contains rare and expensive raw materials. Aluminum-doped zinc oxide (AZO) has attracted increasing attention as a substitute to ITO because it is composed of abundantly available resources and is generally inexpensive. In this study, an AZO thin film was prepared using an FTS system for heterojunction solar cells. The effects of the deposition substrate temperature on the resulting electrical conductivity, structural properties, and optical properties of the AZO thin films were examined.

Synthesis of Al-Doped ZnO Films Assisted with Hollow-Cathode Glow Discharge and Their Characterization

Transparent conductive oxides, such as aluminum-doped zinc oxide (AZO), are of substantial importance for use in a broad range of applications because of their excellent optical and electrical properties. AZO film can be deposited by using several conventional techniques, although they suffer from limitations such as long deposition time, high cost, and the requirement for complex deposition equipment. Here, we used hollow-cathode glow discharge, which produces a high-density plasma and achieves high deposition efficiency. Remarkably, instead of metallic target materials, we used Al2O3 and ZnO powders filled in the hollow cathode as a target, thus avoiding the need for specifically designed targets in this technique. The films were deposited using mixtures of argon and oxygen at various ratios (0% to 50% oxygen), to improve the film characteristics. The films deposited under all conditions exhibited hexagonal wurtzite ZnO structure, while the grain size increased with increasing oxygen content. The film was thick and porous when using low oxygen content, but became thin and dense with increasing oxygen content. The optical transmittance was found to be strongly dependent on the processing gases used, with the highest transmittance of 84% being attained when using 25% oxygen gas. The bandgap of the films lay between 3.27 eV and 3.33 eV. The highest carrier concentration and mobility were attained when using 25% oxygen, and the Hall resistivity decreased with increasing oxygen content. Besides the excellent transmittance and electrical properties of the deposited films, it is expected that the results of this study will be useful for solar cells and optoelectronic applications due to the relatively low cost of this technique and the lack of specific target requirements.

Twisted-motion substrate with sustained azimuthal rotation effect on the growth of AZO thin films by rf-sputtering

Nanorods of aluminum doped zinc oxide films (AZO) were grown on borosilicate glass substrates using an rf-sputtering technique combining simultaneously substrate twisting motion frequency with sustained rotation. Twist-motion frequency of 0.35, 0.5, and, 0.75 Hz between 40° to -40° taking the horizontal plane as reference was adjusted. Effects of this twist-motion frequency as a function of the sustained azimuth rotation of the substrate at 15, 30, and 45 rpm were observed. The morphological, optical, and structural properties were compared between films. Well defined vertical nanorods shapes were observed. The thickness of the films is affected by both twist motion and rotation speed, and the average optical transmittance was greater than 80 % which shows that all samples have high transparencies in the visible range. The combination of these growth parameters of twist and rotation of the substrate allowed us to obtain refractive index values between 1.77 and 2.11 at a wavelength of 700 nm and an energy bandgap in the range of 3.47 and 3.63 eV. The crystallite size of the nanorods obtained was between 29 and 40 nm and the films showed a preferential orientation normal at the surface film to direction of the c-crystallographic plane (002). We report compressive stress in our samples.

Aluminum doped zinc oxide deposited by atomic layer deposition and its applications to micro/nano devices

This work reports investigation on the deposition and evaluation of an aluminum-doped zinc oxide (AZO) thin film and its novel applications to micro- and nano-devices. The AZO thin film is deposited successfully by atomic layer deposition (ALD). 50 nm-thick AZO film with high uniformity is checked by scanning electron microscopy. The element composition of the deposited film with various aluminum dopant concentration is analyzed by energy-dispersive X-ray spectroscopy. In addition, a polycrystalline feature of the deposited film is confirmed by selected area electron diffraction and high-resolution transmission electron microscopy. The lowest sheet resistance of the deposited AZO film is found at 0.7 kΩ/□ with the aluminum dopant concentration at 5 at.%. A novel method employed the ALD in combination with the sacrificial silicon structures is proposed which opens the way to create the ultra-high aspect ratio AZO structures. Moreover, based on this finding, three kinds of micro- and nano-devices employing the deposited AZO thin film have been proposed and demonstrated. Firstly, nanowalled micro-hollows with an aspect ratio of 300 and a height of 15 µm are successfully produced. Secondly, micro- and nano-fluidics, including a hollow fluidic channel with a nanowall structure as a resonator and a fluidic capillary window as an optical modulator is proposed and demonstrated. Lastly, nanomechanical resonators consisting of a bridged nanobeam structure and a vertical nanomechanical capacitive resonator are fabricated and evaluated.

Non-polar a-plane oriented ZnO:Al thin films for optoelectronic applications

This paper reports the details of fabrication of aluminum-doped zinc oxide (AZO) thin films on Corning glass substrates by pulsed laser deposition technique at different laser ablation fluences: 8.75, 17.5 and 35 J/cm2. The influence of laser fluence on the structural, optical and electrical properties of the prepared thin films is analyzed and discussed. X-ray diffraction pattern of AZO thin film confirmed a non-polar a-plane oriented crystal growth in the AZO film prepared at higher fluences. Field emission scanning electron microscopic images indicate the dependence of grain size of AZO on the laser fluence. The highest transmittance (>86%) and the lowest resistivity is attained in thin film prepared at 35 J/cm2 fluence. A remarkable switching behavior from p-type to n-type semiconductivity is observed. The achieved optoelectronic properties of non-polar a–plane oriented AZO thin film, suggests the prepared thin film is a promising material in the fabrication of LED device.

Ellipsometric and third-order nonlinear optical studies of pulsed laser deposited aluminium doped zinc oxide thin films

In the present work, an attempt is made to study the optical properties of aluminium doped zinc oxide (AZO) thin films fabricated by pulsed laser deposition (PLD) technique at different laser fluences. The non-polar ‘a-plane’ orientation of the AZO nanostructure is confirmed by x-ray diffraction technique. The linear optical parameters such as refractive index (n), extinction coefficient ( k ), and dielectric constants ( ε ) are studied by spectroscopic nulling ellipsometer. An increase in extinction coefficient with increase in wavelength is observed. This rare phenomenon is observed for the film prepared at highest laser fluence. The nonlinear optical properties of prepared AZO thin films are evaluated by employing an open aperture (OA) and a closed aperture (CA) Z-scan measurements at a continuous 532 nm wavelength laser source. The presence of reverse saturable absorption (RSA) is observed in all the films; which is attributed to the two-photon absorption mechanism. CA measurements revealed, a switching behaviour from self-de-focusing to a self-focusing mechanism. Third-order nonlinear optical susceptibility, χ ( 3 ) values of AZO films are found be in the order of 10 −6 e.s.u . Further, a-plane oriented AZO films showed optical limiting behaviour with promising optical threshold value (2.964 kJ/cm 2 ) .

Improving optical and electrical performances of aluminum-doped zinc oxide thin films with laser-etched grating structures

A laser etching method was performed to achieve the dual purpose of fabricating grating structures and laser annealing on aluminum-doped zinc oxide (AZO) thin films, and thus improve the film photoelectric performances. Different line spacings and laser fluences were adopted to systematically explore the optimal laser etching condition. Too narrow line spacings or too high laser fluences led to light reflections at the grating external surface to cause more light dissipation, and too wide line spacings or too low laser fluences resulted in relatively small total grating lateral areas being detrimental to multiple internal light reflections. Moreover, too narrow line spacings brought about laser-caused lattice disorder and too high laser fluences produced laser-ablated spots or overburned traces. Therefore, using the medium line spacing and laser fluence, e.g. 40 μm and 0.6 J/cm2 in the present work, was more suitable for synchronously realizing grating structure fabrication and laser annealing. The corresponding AZO film exhibited the maximum figure of merit of 2.89 × 10−2 Ω−1, which was 1.6 times that of the untreated AZO film. This study is expected to expand performance improvement methods of TCO films and promote the application of laser-etched grating structures.