Aluminum-doped Zinc Oxide Films Research Articles

High-performance antireflection nanostructure arrays on aluminum-doped zinc oxide film fabricated with femtosecond laser near-field processing

Transmittance is critical to the performance of optoelectronic devices, and the fabrication of nanostructure arrays with sizes and periods similar to wavelengths on optical surfaces can effectively reduce reflectance and increase transmittance. A method combining a femtosecond laser and the near-field effect of polystyrene (PS) microspheres to prepare integrated high-performance anti-reflective nanostructure arrays on aluminum-doped zinc oxide (AZO) film was proposed in this study. The near-field effect of microspheres with different diameters on a laser light field for several laser wavelengths was simulated with the finite-difference time-domain (FDTD) method. The simulation results showed that a 10-fold improvement of the intensity of a focused laser beam could be obtained with the combination of an 800-nm laser and 1-μm PS microspheres. Nano-craters on the surface of the AZO film were obtained with femtosecond laser irradiation. The depth and the area of these nano craters both increased with the increase of the laser fluences and tended toward saturation when the fluence went from 0.67 to 0.83 J/cm2. Facilitated by the gradient refractive index formed by the nano-craters, the transmittance of the sample treated with 0.83 J/cm2 increased by 8% in the 400–1100 nm band and even by 15% in the 400–600 nm band. The ablation mechanism was revealed by the time-resolved pump-probe technique, which proved that the nano-craters were formed by the phase explosion, and the smooth bottom with fine cracks was induced by the inhomogeneous resolidification of the superheated liquid phase. Our method could endow a photoelectric device with high-performance integrated anti-reflection capability, eliminating the need for additional anti-reflection coating.

In-Operando Optical Spectroscopy of Field-Effect-Gated Al-Doped ZnO

Transparent conductive oxides (TCO) have the unique characteristics of combining optical transparency with high electrical conductivity; such a property makes them uniquely alluring for applications in visible and infrared photonics. One of their most interesting features is the large sensitivity of their optical response to the doping level. We performed the active electrical manipulation of the dielectric properties of aluminum-doped ZnO (AZO), a TCO-based on Earth-abundant elements. We actively tuned the optical and electric performances of AZO films by means of an applied voltage in a parallel-plate capacitor configuration, with SrTiO3 as the dielectric, and monitored the effect of charge injection/depletion by means of in-operando spectroscopic ellipsometry. Calculations of the optical response of the gated system allowed us to extract the spatially resolved variations in the dielectric function of the TCO and infer the injected/depleted charge profile at the interface.

Front SiON/TCO Stacks Development for Double-Side Poly-Si/SiOX Passivated Contacts Solar Cells

This article reports on the use of a front-side SiON/transparent conductive oxide (TCO) bilayer in double-side poly-Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</sub> -based passivated contacts solar cells. This approach presents the advantage of a low indium consumption either by reducing the indium-based TCO thickness or by enhancing its substitution with a zinc-based TCO, such as aluminum-doped zinc oxide (AZO). Thickness optimizations with optical simulations have been performed. An electrical study with a TCO thickness reduced to 20 nm on textured surfaces has shown excellent responses for SiON/TCO stacks, especially regarding the contact resistivity. Finally, the developed SiON/TCO bilayers were integrated in complete solar cells. Interestingly, the substitution of the standard 70-nm-thick ITO layer by a 20-nm-thick ITO film covered by SiON led to an efficiency gain of +0.2% abs. Regarding AZO, the replacement of the standard 70-nm-thick AZO layer by a 20-nm-thick AZO film covered by SiON resulted in a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> gain of +0.7 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . These gains in performances could be raised with further posttreatments. However, the current results already confirm the possibility to optimize thin-poly-Si-based passivated contacts solar cells toward In-free fabrication processes.

Systematic investigations on morphological properties of aluminum-doped zinc oxide transparent electrode prepared from pulsed laser deposition and its electrochromic application

Transparent electrode (TE) is considered as one of the fundamental components in the optoelectronics. The surface of TE layer plays a crucial role in the charge-transport characteristics. Herein, a systematic investigation on the morphological properties of the laser-ablated aluminum-doped zinc oxide (AZO) films as a TE material prepared from various laser fluences is demonstrated. It is revealed that the electrical properties of AZO films are strongly associated with their surface properties rather than that of the bulk film. As the laser fluence increased, the concentration of Al-dopant in AZO films is decreased which directly impacts on the electrical properties. Such vanishment is originated from the bombardment of the incident particles/ions with excessive kinetic energy on film's surface during the deposition. Moreover, the optimized AZO film with low resistivity (1.13 × 10−3 Ω cm) and high optical transmittance (over 90%) achieved from controlling the laser fluence at 0.7 J/cm2 is employed as TE layer in the electrochromic device. Our results highlight that the surface properties of TE layer are very critical for electronic performance.

Improved infrared reflection properties of aluminum-doped zinc oxide thin films depending on sputtering pressure for low emissivity applications

Aluminum-doped zinc oxide (AZO) films were prepared on glass substrates using radio frequency (RF) magnetron sputtering technique. The sputtering pressure was varied between 10 and 80 mTorr for films with a deposition time of 30 min and between 40 and 80 mTorr for films with a deposition time of 180 min. Effects of the sputtering pressure on optical properties, crystal structure, electrical properties, microstructure and infrared reflection of the AZO films were systematically investigated by UV–VIS–NIR spectrophotometry, XRD, Hall measurement, AFM and FTIR, respectively. The prepared AZO film at PAr = 80 mTorr with 180 min deposition time exhibits the lowest electrical resistivity, highest mobility and carrier concentration as well as high average visible transmission in 400–760 nm wavelength range about 82.9%. The average infrared reflection of 63.1% in 8–14 μm waveband makes the film a superior candidate for low-emissivity applications.

Low-voltage driving high-resistance liquid crystal micro-lens with electrically tunable depth of field for the light field imaging system

Light field imaging (LFI) based on Liquid crystal microlens array (LC MLAs) are emerging as a significant area for 3D imaging technology in the field of upcoming Internet of things and artificial intelligence era. However, in scenes of LFI through conventional MLAs, such as biological imaging and medicine imaging, the quality of imaging reconstruction will be severely reduced due to the limited depth of field. Here, we are proposed a low-voltage driving LC MLAs with electrically tunable depth of field (DOF) for the LFI system. An aluminum-doped zinc oxide (AZO) film was deposited on the top of the hole-patterned driven-electrode arrays and used as a high resistance (Hi-R) layer, a uniform gradient electric field was obtained across the sandwiched LC cell. Experimental results confirm that the proposed LC MLAs possess high-quality interference rings and tunable focal length at a lower working voltage. In addition, the focal lengths are tunable from 3.93 to 2.62 mm and the DOF are adjustable from 15.60 to 1.23 mm. The experiments demonstrated that the LFI system based on the proposed structure can clearly capture 3D information of the insets with enlarged depths by changing the working voltage and driving frequency, which indicates that the tunable DOF LC MLAs have a potential application prospects for the biological and medical imaging.

Surface morphological and optical evolution of RF sputtered AZO films for optoelectronic devices

Herein, aluminium-doped zinc oxide (AZO) films were fabricated onto borosilicate glass and fluorine-doped tin oxide (FTO) substrates by RF sputtering. Surface morphology and scaling studies of these films revealed the formation of large grain in AZO films fabricated onto the glass substrates. Several fractal parameters of all samples have been evaluated to understand the deeper morphological insights of observed surfaces. Optical investigations revealed a higher transmittance of ∼93% in the visible region for FTO/AZO films. Estimated values of Urbach energy are found to be 0.20 eV and 0.17 eV for glass/AZO and FTO/AZO films, respectively. Important optical constants such as refractive indices, extinction coefficients, etc have been investigated to explore the optical mechanism of these films. Furthermore, photoluminescence studies suggest the reduction in the deep-level emission defect states for AZO films fabricated onto FTO substrate. These results propose that AZO films may be suitable for use in optoelectronics.

Al L2,3 near edge structure captures the dopant activation and segregation in Al-doped ZnO films

Aluminum-doped zinc oxide (AZO) films have been synthesized using reactive high power impulse magnetron sputtering (HiPIMS) in a wide range of Al contents, 0.6–14.7 at.%. The film structure and microstructure investigated by X-ray diffraction and transmission electron microscopy evolves from nanocrystalline columnar film towards ultrafine nanocrystalline films of wurtzite ZnO structure upon increasing the Al content. Electrical properties measured by 4-point probe and Hall effect setups revealed that effective doping maybe achieved up to 3 at.% Al by using HiPIMS. Most importantly, optical transmittance and electronic structure measurements at the Al L2,3 of electron energy loss near edge structure (ELNES) contain signatures of dopant activation, and dopant segregation that may serve to investigate on the origin of electrical properties degradation and to optimize the electrical properties of AZO films.

Front Transparent Passivation of CIGS-Based Solar Cells via AZO.

We report a novel strategy for the front passivation of solar cells via aluminum-doped zinc oxide (AZO) films in the case of CIGS solar cells, leading to the highest efficiency of 15.07% without alkali metal post treatment and anti−reflective layer. The good passivation of CIGS solar cells via AZO films is attributed to the field passivation simulated by the SCAPS−1D software. The AZO films also exhibit high optical transparency both in visible and near infrared wavelength region, high conductivity, and cost−effective fabrication advantage. Importantly, the AZO films are deposited at room temperature via radio−frequency magnetron sputtering, showing that the AZO films are also applicable to other solar cells such as perovskite solar cells. Our work is of significance for advancing the development of CIGS−based photovoltaics devices by the well front passivation of AZO. The wide application of AZO in other solar cells such as perovskite solar cells and related tandem solar cells may also accelerate the development of these solar cells because of potential passivation of AZO, low deposition temperature, and high optical transparency of AZO.

The development of infrared reflection properties of sputtered aluminum-doped zinc oxide films

Aluminum-doped zinc oxide (AZO) films were prepared on glass substrates using radio frequency (RF) magnetron sputtering technique. The sputtering power ranged from 25 to 100 W. Effects of the sputtering power on optical properties, crystal structure, electrical properties and infrared reflection of the AZO films were systematically investigated by UV–VIS–NIR spectrophotometry, XRD, SEM, XPS, Hall measurement and FTIR, respectively. The results reveal that the grain size is not the only parameter in determining the electrical properties of AZO films, but OII/(OI + OII + OIII) ratio and Al composition are very important. The AZO films deposited at P = 75 W exhibits the lowest electrical resistivity, highest mobility and carrier concentration. The film also has the lowest packing density as well as the highest average visible transmission with a value of 84.9%. The average infrared reflection of 52% makes it a hopeful candidate for low-emissivity applications.

Tailoring the Thickness-Dependent Optical Properties of Conducting Nitrides and Oxides for Epsilon-Near-Zero-Enhanced Photonic Applications.

The unique properties of the emerging photonic materials, conducting nitrides and oxides, especially their tailorability, large damage thresholds, and, importantly, the so-called epsilon-near-zero (ENZ) behavior, have enabled novel photonic phenomena spanning optical circuitry, tunable metasurfaces, and nonlinear optical devices. This work explores direct control of the optical properties of polycrystalline titanium nitride (TiN) and aluminum-doped zinc oxide (AZO) by tailoring the film thickness, and their potential for ENZ-enhanced photonic applications. This study demonstrates that TiN-AZO bilayers support Ferrell-Berreman modes using the thickness-dependent ENZ resonances in the AZO films operating in the telecom wavelengths spanning from 1470 to 1750nm. The bilayer stacks also act as strong light absorbers in the ultraviolet regime using the radiative ENZ modes and the Fabry-Perot modes in the constituent TiN films. The studied Berreman resonators exhibit optically induced reflectance modulation of 15% with picosecond response time. Together with the optical response tailorability of conducting oxides and nitrides, using the field enhancement near the tunable ENZ regime can enable a wide range of nonlinear optical phenomena, including all-optical switching, time refraction, and high-harmonic generation.

ZnO and AZO Film Potentiometric pH Sensors Based on Flexible Printed Circuit Board

In this study, we deposited zinc oxide (ZnO) and aluminum-doped zinc oxide (AZO) on the electroless nickel immersion gold (ENIG) of a flexible printed circuit board (FPCB) as a potentiometric pH sensor. The sensing films of the pH sensor were fabricated by a radio frequency (RF) sputtering system and analyzed by field emission scanning electron microscope (FE-SEM) and X-ray photoelectron spectroscopy (XPS). In the pH 2 to 10 buffer solutions, it was observed that the characteristics of the pH sensor through the voltage–time (V-T) measurement system include average sensitivity and linearity, drift effect, and repeatability. According to the experimental results, the pH sensors in this study could exhibit good characteristics.

Environmental Stability Evaluation of Aluminium Doped Zinc Oxide (AZO) Transparent Electrodes Deposited at Low Temperature for Solar cells

The degradation of transparent electrodes’ electrical conductivity under environmental conditions is considered as a major failure mode for solar cells’ long-term efficiency. In this paper, AZO thin films were subjected to the International Electrotechnical Commission (IEC) 61646 test to examine their environmental stability and suitability as front electrodes for solar cells. To explore the interplay between AZO deposition parameters and environmental stability, AZO films were deposited by radio frequency magnetron sputtering at different parameters and without external heating. The conductivity stability evolution upon the testwas investigated via studying the AZO electrical, structural, and morphological characteristics at different deposition conditions. A direct dependence was identified between the samples’ conductivity degradation rates and the samples’ structural and morphological characteristics including grain size, grain boundary density, surface roughness, and compactness. The samples’ resistivity increases linearly over the test period due to both electron density and mobility degradations. Improved stability was observed for thicker AZO samples (360 nm) originating from enhanced grain size, surface profile, and compactness. These samplesmaintained solar cells' applicable sheet resistance of 21.24 Ω/sq (ρ=7.64×10-4 Ω.cm) following the test. The conducted aging studies demonstrated that manipulating the AZO films growth process via optimizing the deposition parameters is an effective pathway for low-temperature deposited electrodes with enhanced environmental stability

Achieving Scalable Near‐Zero‐Index Materials

Near-zero-index (NZI) materials are becoming increasingly important for photonic designs because they enable new ways to control light–matter interactions at the nanoscale. Many device prototypes that utilize NZI layers are created under conditions that are tool and laboratory specific, making widespread utilization and scalability of NZI materials difficult. Herein, this limitation is circumvented by using transparent conducting oxides (TCOs) produced from scalable commercial sources. The optical response of 49 distinct TCOs with NZI behavior from 12 different suppliers is quantified, including indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), and fluorine-doped tin oxide (FTO). The measurements reveal that the ITO samples have the strongest NZI resonances with many samples exhibiting |n| < 0.6 with resonances occurring between 1150 and 1350 nm. Conversely, the FTO and AZO films present higher values of |n| (ranging from 0.6 to 0.9) at 1500–1900 nm. The optical properties, resistivities, and roughness values for all thin films are reported, creating a useful database for device design. Finally, novel NZI phenomena, such as the strong suppression of non-normal incidence illumination using the data collected from these samples, are demonstrated, opening the door to new opportunities for both research-grade and mass-produced NZI devices.

Preparation and Investigation of Transparent Conducting Aluminium-doped Zinc Oxide Films Prepared by Sol-gel Method for Sensor Application

Aluminium doped Zinc Oxide (AZO) thin films were prepared through chemical route using sol gel method at different annealing temperatures with three different concentrations of Al. It has been observed that on increasing in the annealing temperature, the crystallinity of the films improved. The XRD patters confirms the polycrystalline nature of the samples and found that increasing annealing temperature, increases the crystallinity of the films. The UV-vis spectroscopy analysis shows that, the band-gap decreases in all three concentration of Al ions in ZnO with temperature. The conductivity and the grain size increases with temperature. The band gap corresponding to the absorption decreases with increased annealing temperature.

Atomic Layer Engineering of Aluminum‐Doped Zinc Oxide Films for Efficient and Stable Perovskite Solar Cells

AbstractAtomic layer deposition (ALD) has been considered as an efficient method to deposit high quality and uniform thin films of various electron transport materials for perovskite solar cells (PSCs). Here, the effect of deposition sequence in the ALD process of aluminum‐doped zinc oxide (AZO) films on the performance and stability of PSCs is investigated. Particularly, the surface of AZO film is terminated by diethylzinc (DEZ)/H2O (AZO‐1) or trimethylaluminum (TMA)/H2O pulse (AZO‐2), and investigated with surface‐sensitive X‐ray photoelectron spectroscopy technique. It is observed that AZO‐2 significantly enhances the thermal stability of the upcoming methylammonium lead iodide (MAPbI3) layer and facilitates charge transport at the interface as evidenced by photoluminescence spectroscopes and favorable interfacial band alignment. Finally, planar‐type PSC with AZO‐2 layer exhibits a champion power conversion efficiency of 18.09% with negligible hysteresis and retains 82% of the initial efficiency after aging for 100 h under ambient conditions (relative humidity 40 ± 5%). These results highlight the importance of atomic layer engineering for developing efficient and stable PSCs.

In situ monitoring atomic layer doping processes for Al-doped ZnO layers: Competitive nature of surface reactions between metal precursors

In this work, surface reactions during the atomic layer doping (ALDp) process of aluminum-doped zinc oxide (AZO) films have been studied. Conventional supercycle and alternative quasi-simultaneous codosing methods are analyzed within the 100–200 °C substrate temperature range. Two quasi-simultaneous codosing cases are investigated: (1) diethylzinc (DEZ) followed by trimethylaluminum (TMA) and (ii) TMA followed by DEZ. Quasi-simultaneous codosing experiments featured back-to-back DEZ/TMA or TMA/DEZ precursor and H2O pulses separated by nitrogen (N2) purge cycles. The grown films were characterized via (i) real-time in situ ellipsometry to monitor the individual surface ligand exchange reactions via variations in the film thickness in each half-cycle; (ii) ex situ ellipsometry to determine the film optical constants; (iii) x-ray photoelectron spectroscopy to measure the elemental composition and chemical bonding structure, and (iv) x-ray diffraction to evaluate the crystal properties. The most significant finding of the study is the dominance of TMA over DEZ: for all of the quasi-simultaneous codosing samples, no matter which precursor is pulsed first and whether there is a time delay between TMA and DEZ pulses or not, zinc (Zn) incorporation within the AZO films is substantially lower than aluminum (Al). This result demonstrates the competitive nature of surface reactions between TMA and DEZ, where the winning side is TMA. Al is effectively incorporating into the film while severely limiting Zn-incorporation and even replacing chemisorbed Zn-groups via conversion surface reactions. As a result, the quasi-simultaneous codosing approach for AZO films using DEZ and TMA precursors leads to minimally (less than 2%) Zn-doped Al2O3 films (ZAO), depicting the advantages of controlled ALDp process via the conventional supercycle method.

Aluminum–nitrogen co‐doping improves the blue emission of ZnO films: A combined theoretical–experimental study

AbstractThe investigation on visible emission of ZnO is necessary due its potential application in electrochemical communication, biosensor, and white light diodes, and so forth. Previous report indicates that the visible light of ZnO is related to the defect type and concentration. It can be regulated by various methods. However, quenching of light emission occurs if too many defects are introduced. In this work, combined with theoretical calculation about energy band structure, aluminum–nitrogen co‐doped ZnO is conducive to form p‐type semiconductor, but aluminum doped ZnO films tend to form a deep n‐type semiconductors. Therefore, in the experiment, aluminum–nitrogen co‐doped ZnO films are prepared using radio frequency magnetron sputtering. The substitution of oxygen using nitrogen impurity is expected to introduce more oxygen vacancies and interstitial zinc defects related to visible light emission. At the same time, annealing in vacuum is also a highly non‐equilibrium processes with low oxygen and high zinc contents. Results indicate that combined with aluminum–nitrogen co‐doping and an appropriate annealing temperature, the blue and blue‐green emission of ZnO films can be greatly improved.

Room Temperature Sputtered Aluminum-Doped ZnO Thin Film Transparent Electrode for Application in Solar Cells and for Low-Band-Gap Optoelectronic Devices.

Aluminum-doped zinc oxide (AZO) is a popular, low-cost, nontoxic material that finds application as a transparent conducting electrode in photonic, sensing, and photovoltaic devices. We report the AZO thin films with a high figure of merit on large-area glass substrates by direct current magnetron sputtering without any intentional substrate heating. Furthermore, a simple thermal post-treatment to improve the transmittance of AZO thin film in the infrared region for its application in low-band-gap devices is presented. High optoelectronic properties are obtained by optimizing oxygen content during the sputtering process. The structural, morphological, optoelectrical, and photoluminescence characterization of cold sputtered AZO films is investigated for its latent applications. AZO thin films with an electrical sheet resistance of 8.8 Ω/□ and a visible light transmittance of 78.5% with thickness uniformity above 95% are achieved on 300 mm × 300 mm glass substrate. The AZO film with optimized process conditions is employed as a transparent electrode to fabricate a copper–indium–gallium–selenide-based thin film solar cell, demonstrating 11.8% power conversion efficiency. The AZO film with optimized sputter conditions was post-treated in ambient conditions with an Al blanket to suppress the resistivity by proper organization of the defects due to Al3+ consumption and point defects, resulting in improved transparency (85%) in the infrared region with a sheet resistance of 40 Ω/□. This has great potential for developing scalable and low-cost AZO thin films for transparent electrodes in a wide range of the spectrum.

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.