Aluminum-doped Zinc Oxide Films Research Articles

Transparent conductive oxide-based architectures for the electrical modulation of the optical response: A spectroscopic ellipsometry study

Transparent conductive oxides are a class of materials that combine high optical transparency with high electrical conductivity. This property makes them uniquely appealing as transparent conductive electrodes in solar cells and interesting for optoelectronic and infrared-plasmonic applications. One of the new challenges that researchers and engineers are facing is merging optical and electrical control in a single device for developing next-generation photovoltaic, optoelectronic devices and energy-efficient solid-state lighting. In this work, the authors investigated the possible variations in the dielectric properties of aluminum-doped ZnO (AZO) upon gating by means of spectroscopic ellipsometry (SE). The authors investigated the electrical-bias-dependent optical response of thin AZO films fabricated by magnetron sputtering within a parallel-plane capacitor configuration. The authors address the possibility to control their optical and electric performances by applying bias, monitoring the effect of charge injection/depletion in the AZO layer by means of in operando SE versus applied gate voltage.

Effect of annealing temperature on structural morphological and optical properties of spray pyrolized Al-doped ZnO thin films

Aluminium doped zinc oxide (AZO) thin films were prepared on glass substrate by spray pyrolysis technique and subsequently their structural, morphological and optical properties were investigated and analyzed as a function of annealing temperature. The films were deposited at 350 °C and were annealed at temperature 350, 400, 450 and 500 °C. X-ray diffraction (XRD) analysis confirms that both deposited and annealed films are polycrystalline in nature and have hexagonal wurtzite structure. The crystalline size as well as the crystalline quality of the film was found to increase with the increase of annealing temperature to a certain point. However, the dislocation density and the compressive stress induced in as-deposited AZO films reduce with annealing temperature. SEM image reveals that the deposited film has nanorope like morphology. The average diameter of the rope and the density of the morphology increases with the increase of annealing temperature. As the annealing temperature increases from 350 °C to 450 °C, the average transmittance of the films increases and the band gap value decreases from 3.25 eV to 3.17 eV. In addition, obtained results also show that urbach energy, extinction coefficient, refractive index, optical conductivity has changes noticeably with annealing temperature.

Lossy Mode Resonance Generation on Sputtered Aluminum-Doped Zinc Oxide Thin Films Deposited on Multimode Optical Fiber Structures for Sensing Applications in the 1.55 µm Wavelength Range

In this work, we demonstrated lossy mode resonance (LMR) generation in optical fiber structures based on multimode fibers coated with aluminum-doped zinc oxide (AZO) films. AZO thin films were deposited by using radio frequency magnetron sputtering. In order to exhibit the usefulness of the LMR effect for sensing applications in optical fiber based systems, the deposition conditions of the AZO film coatings were set to obtain the second LMR order within the 1.55 µm wavelength range. An optical transmission configuration setup was used to investigate the LMR effect on fiber structures based on the use of no-core and cladding-removed multimode fibers coated with AZO films synthesized from metallic sputtering targets with different proportions of Zn:Al, 92:8% and 98:2%, at atomic concentrations. The optical and electrical/chemical features of the AZO films were characterized with UV–vis and XPS spectroscopy, respectively. The optical response of the proposed sensing configuration to refractive index (RI) variations was experimentally demonstrated. For the best approach, the sensitivity of wavelength displacement to RI variations on the liquid surrounding media was found to be 1214.7 nm/RIU.

Transparent conductive oxides for the epsilon-near-zero Tamm plasmon polaritons

We demonstrate the possibility of using transparent conducting oxides [aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), indium tin oxide (ITO)] to form Tamm plasmon polaritons in the near-infrared spectral range where the permittivity of oxides is near zero. The spectral properties of the structures are investigated in the framework of the temporal coupled-mode theory and confirmed by the transfer matrix method. It is found that in the critical coupling conditions, the maximal $ Q $Q-factor of a Tamm plasmon polariton is achieved when a photonic crystal is conjugated with the AZO film, while at the conjugation with the ITO films, the broadest spectral line is obtained. The sensitivity of the wavelength and spectral width of the Tamm plasmon polariton to changes in the oxide film thickness, bulk concentration of a dopant, and angle of incidence is demonstrated.

Improving electrical conductivity and its thermal stability of Al-doped ZnO polycrystalline films using ultrathin Al film as a passivation layer

Aluminum-doped ZnO (AZO) has been considered as a prospective material for application as transparent electrodes in solar cells. In this application, an improvement of the Hall mobility is desired to achieve the high conductivity, because an increase in the carrier concentration results in a decrease in transmission at near-infrared wavelengths. However, the achievement of high Hall mobility is still a challenge. Another challenge associated with AZO is the low thermal stability of the electrical properties, which limits the application of AZO. This limitation originates from Zn desorption and migration of oxygen from the environment into the AZO film. It has been shown that using an ultrathin Al capping layer on an AZO film can greatly improve its thermal stability. An improvement in the Hall mobility was obtained for an Al capped AZO film after annealing in N2 gas at 400 °C. The Hall mobility reached 49.1 cm2/V, which is close to the theoretical limit of the mobility for an AZO polycrystalline film, with a moderate carrier concentration of 2.8 × 1020 cm−3, ensuring high transparency in the visible and near-infrared ranges. The obtained results are due to the passivation effect of the AlOx film formed by natural oxidation of Al film. Zn desorption was suppressed and the migration of O from the environment was reduced, leading to an improvement in the thermal stability. Once Zn desorption is prevented, improvement of the crystalline perfection due to thermal annealing can contribute to an improvement in the Hall mobility.

Effect of growth temperature on the key properties of aluminum-doped zinc oxide thin films prepared by atomic layer deposition

Abstract

Power spectral density-based fractal analysis of annealing effect in low cost solution-processed Al-doped ZnO thin films

Thin films of aluminium-doped zinc oxide (Al:ZnO) have been deposited using the sol-gel spin coating method and annealed at 300 °C, 400 °C, and 500 °C. The structural phase purity of the annealed films has been confirmed by the glancing angle x-ray diffraction patterns. The average microcrystalline size and residual microcrystalline stress have been calculated from it. It is observed that the average microcrystalline size increases and the residual microcrystalline compressive stress first increases and then decreases with increasing annealing temperature from 300 °C to 500 °C. The conventional surface roughness calculations and the power spectral density (PSD)-based fractal computations of the AFM micrographs have been carried out. The values of the power-law exponent demonstrate the underlying mechanism of the films’ residual surface growth. All the annealed films have a combined surface transport mechanism of viscous flow and evaporation-condensation. However, evaporation-condensation becomes dominant at the higher annealing temperatures of 400 °C and 500 °C. The spatial and vertical anisotropy of thin film surface topography has been quantified in a scale-invariant and generalized manner with the help of fractal geometrical analysis. The values of the fractal dimensions and Hurst exponents indicate that the vertical roughness of the films increases and the spatial self-similarity decreases with increasing annealing temperature. It is noteworthy that the Hurst exponent can alone depict the complexity of a thin film surface, whereas in conventional techniques at least three parameters are necessary for this. The fractal dimension increases with increasing average grain size and annealing temperature. Consequently, overall surface complexity increases with increasing annealing temperature. Thus, the fractal analysis can find its application in the predictability and assessment of surface topography. Also, various physical properties of films may be correlated with the fractal parameter of thin film surfaces in future works for micro and nano device fabrications.

Li4Ti5O12 coated with ultrathin aluminum-doped zinc oxide films as an anode material for lithium-ion batteries

Aluminum-doped zinc oxide (AZO) films were coated on the surface of Li4Ti5O12 (LTO) particles via atomic layer deposition (ALD) in a fluidized bed reactor. The electrochemical characterization results indicated that the coating of AZO significantly improved the electrochemical performance of LTO between 0.1 V and 3.0 V with a proper coating thickness. Among all AZO-coated LTO samples, 250-10AZO (nine cycles of ZnO ALD followed by one cycle of Al2O3 ALD at a deposition temperature of 250 °C) showed the best performance. After 250 cycles of charge/discharge at a 1 C rate (1 C = 175 mA g−1), the capacity decreased from 203 to 190 mAh g−1 with a capacity retention of ~94% at room temperature and from 224 to 216 mAh g−1 with a capacity retention of ~96% at 55 °C. The AZO coating layer with an appropriate thickness not only increased the capacity of LTO by enhancing conductivity, but also assisted the LTO electrodes to form a beneficial interface layer to protect the LTO from the continuous attack by harmful components in the electrolyte, especially under extreme conditions, such as high temperature and high current rates.

Oxide Heteroepitaxy-Based Flexible Ferroelectric Transistor.

With the rise of Internet of Things, the presence of flexible devices has attracted significant attention owing to design flexibility. A ferroelectric field-effect transistor (FeFET), showing the advantages of high speed, nondestructive readout, and low-power consumption, plays a key role in next-generation technology. However, the performance of these devices is restricted since conventional flexible substrates show poor thermal stability to integrate traditional ferroelectric materials, limiting the compatibility of wearable devices. In this study, we adopt flexible muscovite mica as a substrate due to its good thermal properties and epitaxial integration ability. A flexible FeFET composed of oxide heteroepitaxy on muscovite is realized by combining an aluminum-doped zinc oxide film as the semiconductor channel layer and a Pb(Zr0.7Ti0.3)O3 film as the ferroelectric gate dielectric. The excellent characteristics of the transistor together with superior thermal stability and mechanical flexibility are demonstrated through various mechanical bending and temperature measurements. The on/off current ratio of the FeFET is higher than 103, which based on the field effect in the transfer curve. The smallest bending radius that can be achieved is 5 mm with a cyclability of 300 times and a retention of 100 h. This study opens an avenue to use oxide heteroepitaxy to construct a FeFET for next-generation flexible electronic systems.

Effect of Nitrogen Flow in Hydrogen/Nitrogen Plasma Annealing on Aluminum-Doped Zinc Oxide/Tin-Doped Indium Oxide Bilayer Films Applied in Low Emissivity Glass

Low emissivity glass (low-e glass), which is often used in energy-saving buildings, has high thermal resistance and visible light transmission. Heavily doped wide band gap semiconductors like aluminum-doped zinc oxide (AZO) and tin-doped indium oxide (ITO) have these properties, especially after certain treatment. In our experiments, in-line sputtered AZO and ITO bilayer (AZO/ITO) films on glass substrates were prepared first. The deposition of AZO/ITO films was following by annealing in hydrogen/nitrogen (H2/N2) plasma with different N2 flows. The structure and optical and electrical properties of AZO/ITO films were surveyed. Experiment results indicated that N2 flow in H2/N2 plasma annealing of AZO/ITO films slightly modified the structure and electrical properties of AZO/ITO films. The X-ray diffraction peak corresponding to zinc oxide (002) crystal plane slightly shifted to a higher angle and its full width at half maximum decreased as the N2 flow increased. The electrical resistivity and the emissivity reduced for the plasma annealed AZO/ITO films when the N2 flow was raised. The optimum H2/N2 gas flow was 100/100 for plasma annealed AZO/ITO films in this work for low emissivity application. The emissivity and average visible transmittance for H2/N2 = 100/100 plasma annealed AZO/ITO were 0.07 and 80%, respectively, lying in the range of commercially used low emissivity glass.

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

Abstract

Hierarchically Structured Photoelectrodes Utilizing a Protected Transparent Conducting Oxide Grown Via Atomic Layer Deposition

Hierarchically structured materials are often used to improve light absorption and photocarrier collection in thin-film photoelectrodes. However, the structures require a continuous conductive coating to function as an electrode and maintain electrical contact to the entire, tortuous surface while being transparent to visible light to minimize parasitic absorption. The electrochemically robust transparent conducting oxide (TCO) fluorine-doped tin oxide (FTO) can only be grown by line-of-sight physical deposition methods such as RF sputtering or spray pyrolysis, which in general prevents the deposition of uniform thin coatings of FTO on the interior of a hierarchichally structured scaffold. Other TCOs (tin-doped indium oxide (ITO), aluminum-doped zinc oxide (AZO)) can be grown conformally by surface specific methods such as atomic layer deposition (ALD), but are only stable at near-neutral pH, limiting their use in photoelectrochemical energy conversion. Here, we describe a strategy for improving the stability of ALD-deposited AZO films for anodic and cathodic operation in alkaline and acidic electrolytes. The addition of a conformal, nanoscale TiO2 coating preserves a conductive AZO film for more than 24 hours at pH > 13. This is an improvement of several orders of magnitude compared to the lifetime of pure AZO films, which is also observed in acidic (pH < 1) environments. Although the TiO2 coating is not significantly conductive alone, we observed no decrease in the conductivity of the layered AZO/TiO2 film. The TiO2-coated AZO film was used as an electron collector on hierarchically porous scaffolds formed from close-packed inorganic silica spheres. The improved electrochemical stability will be utilized in the construction of ultra-thin film photoelectrodes in hierarchichally-structured systems with earth-abundant semiconductors, such as cupric oxide (CuO) or hematite (α-Fe2O3).

High-quality-factor flexible and transparent capacitors with Cr–Au nanomeshes as bottom electrodes

Flexible and transparent capacitors have been fabricated on polyethylene naphthalate (PEN) substrates. The Cr–Au nanomeshes, ZrO2, and aluminum-doped zinc oxide (AZO) films were prepared in sequence as bottom electrodes, dielectric layers and top electrodes. A high quality factor of 82 was obtained, nearly 4.9 times larger than that ever reported, which directly indicates much lower power consumption. The flexible capacitors can normally work at a bending radius of 7 mm without any loss of electrical performance, and even maintain electrical characteristics after cyclical bending tests. Moreover, a high capacitance density of 12.3 fF μm−2 and an ultralow leakage current of 1.5 × 10−8 A cm−2 at 1 V have been achieved. Transmittance measurements indicate that the capacitors represent an average optical transmittance of over 50% at a visible region. This is, therefore, a dramatic step toward real applications in flexible and transparent integrated circuits.

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.

Possibilities of Increasing the Usability of Sputtered AZO Films as a Transparent Electrode

Aluminum‐doped zinc oxide (AZO) is a suitable material for use as transparent electrode. Due to its lower price it is applied in the production of silicon solar cells. However, it is difficult to obtain suitable electrical properties at low deposition temperatures. Moreover, the AZO films with low thickness exhibit significantly higher average resistivity, which limits their usability, especially if they are prepared on flexible substrates. A lot of effort is invested to replace indium tin oxide (ITO) to a much greater extent now, since Indium is rare and its price is rising, thus the preparation of low‐thickness AZO films at low temperature is also subject of intense research. The presented paper summarizes the reported results, discusses the causes of the differences between ITO and AZO films, and suggests the direction of research and the potential solution that should lead to increased usability of AZO films and also possible replacement of ITO films.

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.

Thermoelectric properties of Al-doped ZnO composite films with polymer nanoparticles prepared by pulsed laser deposition

The effect of the embedded nanoparticles of polymer poly (methyl methacrylate) known as PMMA on the thermoelectric properties of the films of aluminum-doped ZnO (AZO) was studied. The films were deposited at room temperature using concurrent pulsed laser deposition of AZO and matrix assisted pulsed laser evaporation of PMMA. PMMA nanoparticles were uniformly dispersed in AZO matrix and acted as additional phonon scatterers decreasing room-temperature thermal conductivity. The electrical conductivity of the nanocomposite films increased three times with respect to pure AZO film, more likely due to the conversion of PMMA in amorphous electro-conductive carbon. This could potentially lead to the increase of the thermoelectric effect by a factor between 1.5 and 3 as compared to pure AZO film. Concurrent multi-beam multi-target pulsed laser deposition method of embedding polymer nanoparticles was thus demonstrated to be useful for improving the thermoelectric properties of oxide thin films.

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.

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.

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.