Aluminum-doped Zinc Oxide Thin Films Research Articles

ZnO:Al thin films from (Al2O3)x(ZnO)(1-x) powder targets by magnetron sputtering

In the present work we prepared Aluminum doped Zinc Oxide (AZO) thin films from powder targets. Various concentrations (W/W percentages) of Al2O3 such as1%, 2%, 3%, 4%, 5%, 6%, 7% and 8% were mixed in ZnO powder and made in the form of a 3 inch disc target. These ceramic targets are sputtered in RF magnetron sputtering unit for the deposition of AZO thin films. Optical and electrical properties are analyzed to get an optimized percentage of mixing for achieving high transparency and low resistivity. At Al2O3 percentage of 3% there is a considerable decrement in the resistivity, and at 7% there is a considerable decrease in the optical transmittance. Mobility and carrier concentration are increasing with Al2O3 percentage. Bandgap of the films is observed to be decreasing with increasing the Al2O3 percentage.

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.

Effects of reduced graphene oxide on nonlinear absorption and optical limiting properties of spin coated aluminium doped zinc oxide thin films

In this work, we investigate the nonlinear absorption and optical limiting properties of reduced graphene oxide (rGO) incorporated aluminum doped zinc oxide (AZO) composite thin films by open aperture z-scan technique using Q-switched Nd:YAG laser at 532 nm. The structural and spectral properties were also systemically analyzed. The composite thin films were synthesized by spin coating technique with online infrared curing facility. Studies on nonlinear optical responses of the investigated samples proved a substantial enhancement in the nonlinear absorption coefficient of AZO:rGO composites compared to AZO thin film. The nonlinear absorption is attributed to the two photon absorption with reverse saturable absorption. The strong nonlinear absorption and nonlinear scattering effects result in the optical limiting property of the composite material. The AZO:rGO thin film exhibits lower optical limiting threshold value (32 MW/cm2) as compared to AZO (59 MW/cm2). Hence it is an excellent optical limiter which explores applications in the field of optoelectronics as protecting material for sensitive photonic devices.

Solution processed aluminum-doped ZnO thin films for transparent heater applications

A study was carried out to tailor the properties of ultrasonic spray pyrolysis deposited Al-doped ZnO (AZO) thin films suitable for transparent heater applications. For this purpose, the influences of solvent/precursor type and molarity, Al-doping concentration, nozzle to substrate distance, solution flow rate, carrier gas flow, substrate temperature, and post-deposition annealing on the microstructure, electrical, optical properties, and heating/deicing behaviors of the films were investigated. The optimized heaters were produced on 50 × 75 mm2 glass substrates at 400 °C with 750 nm thickness and annealed under forming gas for 90 min. This sample revealed a good sheet resistance value of 38.7 Ω/□ and 83% transmittance in the visible region. Heating/deicing tests showed that the maximum achievable surface temperature was 76 °C under an applied potential of 12 V and all ice/water can be removed completely within 250 s. In addition, the AZO heater exhibited a very high thermal resistance value (409 °C cm2/Watts) with stable and reversible heating behavior.

Processing optimization of SiO2-capped aluminum-doped ZnO thin films for transparent heater and near-infrared reflecting applications

In this study, a series of optimization steps were performed in the production of Al-doped ZnO (AZO) thin films to tailor their properties as efficient transparent heaters and for near-infrared (NIR) reflectance. The films were produced on 50 × 75 mm2 glass substrates via magnetron sputtering and capped with a protective SiO2 layer. Processing parameters such as deposition temperature, film thickness, and annealing conditions were all optimized in terms of structure, morphology, optical/electrical properties, and heating/deicing behavior. Electro-thermal characteristics of the films were investigated using a thermal imaging infrared camera under various input voltages. The optimized AZO/SiO2 coatings displayed impressive room-temperature electrical conductivity (σ) of nearly 3774 S/cm with a sheet resistance (Rs) of 3.53 Ω/□, carrier concentration (η) of 1.14 × 1021, and Hall mobility (µ) of 20.48 cm2/Vs. These films exhibited very high optical transmittance (above 96%) in the visible range and reflectance (73% at 2500 nm) in the NIR region. The highest figure of merit (FOM) was achieved as 237 (× 10–3 Ω−1). Deicing tests were performed with samples cooled to − 40 °C and resulted with complete removal of ice/water only within 3 min. In addition, the heater exhibited a high surface temperature of 161 °C (12 V), a good thermal resistance value (219 °C cm2/Watts) with stable and reversible heating behavior. More importantly, these results reveal the potential of optimized AZO/SiO2 coatings as alternatives to transparent tin-doped indium oxide heaters and NIR reflecting mirrors for vehicular applications.

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.

A Brief Study of Physical Properties of Rf Magnetron Sputtered Azo Thin Films

We reported the physical properties of undoped and Aluminium doped ZnO (AZO) thin films that were synthesized by the RF magnetron sputtering method on a quartz substrate. The effect of dopant concentration on structure, morphology, optical and electrical properties of the thin films have been studied systematically by XRD, Raman spectroscopy, AFM, FE-SEM, UV-VIS spectroscopy and I-V measurement (two probe method) respectively. The crystallite growth of the thin films is along c-axis (002) orientations with hexagonal wurtzite structure. The crystallinity is enhanced in the 1%AZO thin films as compared to undoped ZnO. The transition of stress value after introducing the dopant is discussed. These changes are further correlated with the observed morphological changes. The alteration in optical transmission and optical band gap is also discussed extensively. For the AZO (1% and 2%) thin films the n-type conductivity and ohmic nature measured by using Keithley two probe set up. The responsible mechanism for improved conductivity is discussed. In the FTIR spectrum the peaks originated by the tetrahedral coordination of ZnO are observed which further confirms the wurtzite structure of the deposited thin film sample that are recorded by the XRD pattern.

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.

Influence of aluminum doping on microstructure, optical and electrical properties of c axis oriented zinc oxide nano films prepared by nebulizer spray pyrolysis technique

Transparent conducting aluminum-doped ZnO thin films have been prepared by nebulizer spray pyrolysis technique for different doping concentrations on glass substrates. X-ray diffraction patterns reveal that all the films are polycrystalline of hexagonal structure with c–axis orientations. Atomic force microscopy (AFM) images noticed that the films have a uniform surface morphology consist spike shaped particles and low surface roughness. The optical transmittance in visible region was escalated from 85 to 93% and optical band gap of thin films is also improved from 3.27 to 3.37 eV. Photoluminescence spectra show mainly five emissions peaks and a blue shift observed in UV emission peak. Hall Effect measurements show that the prepared films having n-type conductivity with low resistivity and high carrier concentrations.

Air Annealing Effect on Oxygen Vacancy Defects in Al-doped ZnO Films Grown by High-Speed Atmospheric Atomic Layer Deposition.

In this study, aluminum-doped zinc oxide (Al:ZnO) thin films were grown by high-speed atmospheric atomic layer deposition (AALD), and the effects of air annealing on film properties are investigated. The experimental results show that the thermal annealing can significantly reduce the amount of oxygen vacancies defects as evidenced by X-ray photoelectron spectroscopy spectra due to the in-diffusion of oxygen from air to the films. As shown by X-ray diffraction, the annealing repairs the crystalline structure and releases the stress. The absorption coefficient of the films increases with the annealing temperature due to the increased density. The annealing temperature reaching 600 °C leads to relatively significant changes in grain size and band gap. From the results of band gap and Hall-effect measurements, the annealing temperature lower than 600 °C reduces the oxygen vacancies defects acting as shallow donors, while it is suspected that the annealing temperature higher than 600 °C can further remove the oxygen defects introduced mid-gap states.

Influence of linear permittivity on thermally-induced nonlinear optical properties of aluminum-doped zinc oxide thin films: the visible range

In this study, the influence of the linear permittivity on the thermally-induced nonlinear optical properties of aluminum-doped zinc oxide thin films was investigated in the visible range, namely at 405, 532, and 650 nm. The strength of the nonlinear response in the films at these wavelengths represented by the thermal nonlinear index of refraction ( n 2 t h ) and the absorption coefficients (β) magnitudes followed dispersive changes in the films’ permittivity function. These changes were dominantly scattering events within the films, which strongly influenced the nonlinear absorption behavior alternating between saturable absorber and reverse saturable absorber trend. In this research, the results and qualitative description of this relationship are presented.

High-density plasma etching characteristics of aluminum-doped zinc oxide thin films in Cl2/Ar plasma

We investigated the etching characteristics of aluminum-doped zinc oxide (AZO) thin films in an adaptively coupled plasma (ACP) system. The dry etching characteristics of AZO films were studied by changing the Cl2/Ar gas mixing ratio, RF power, DC bias voltage. We determined the following optimized process conditions: RF power of 500 W, DC bias voltage of −100 V, process pressure of 15 mTorr. In Cl2/Ar plasma (=50:50%), the maximum etching rate of AZO films is 70.45 nm min−1. The ion composition of Cl2/Ar plasma was determined by optical emission spectrometry (OES). The chemical reactions on the surface of AZO films were analyzed by x-ray photoelectron spectroscopy (XPS).

Nano-indentation mechanical characterizations of solution processed inorganic metal oxide thin films and influence of grain size

Aluminum-doped zinc oxide thin films were prepared by solution synthesis where molarity concentrations and mixing ratios for oxide and dopant sources were changed while deposition and pre- and post-deposition annealing steps remained the same. Post-deposition high-temperature annealing is known to improve conductivity and transparency but may deteriorate the mechanical characteristics of films. However, we report the successful preparation of thin films with high mechanical hardness (>11 MPa) and reduced modulus (>102 MPa) along with high electro-optical characteristics even after multiple annealing steps. Changes in grain sizes because of changes in molarity concentrations and difference of atomic radii caused mechanical characteristics of films. Both regular and inverse Hall–Petch relations were observed.

Study of a Palladium (Pd)/Aluminum-Doped Zinc Oxide (AZO) Hydrogen Sensor and the Kalman Algorithm for Internet-of-Things (IoT) Application

A palladium (Pd) thin film is decorated on a radio frequency (RF) sputtered aluminum-doped zinc oxide (AZO) thin film to produce a hydrogen sensor. Due to the catalytic activity of the Pd metal, the studied thin film-based device shows remarkably enhanced hydrogen-sensing characteristics. Experimentally, a very high sensing response of $1.12\times 10 ^{4}$ with a response time of 23 s is obtained under 1% H2/air gas at 300 °C. Furthermore, even under an extremely low concentration of 40-ppb H2/air, a sensing response of 0.17 is acquired. The optimal operating temperature of the studied device is 300 °C. A hypothesis is used to interpret the related hydrogen-sensing mechanism of the studied device. A thermodynamic analysis is employed to study the surface coverage of hydrogen molecules on the device’s surface. Furthermore, for the application in wireless transmission of the Internet of Things (IoT), an interesting Kalman algorithm is used to reduce redundant data, save hardware costs, and reduce network congestion. The simulated results show that 93.9% of the redundant data can be removed. The studied device exhibits advantages of a simple structure, easy fabrication, low cost, a widespread sensing range of hydrogen concentration, a very high sensing response, and an extremely low detecting limit, as well as being suitable for IoT application.

UV-response of aluminum-doped zinc oxide transparent films with different microstructures and electrical properties

Pure and aluminum doped-zinc oxide thin films were grown by spray-pyrolysis on glass substrates. The addition of increasing amounts of Al led to significant changes in the microstructure of the films. The texture coefficient of the (002) crystallographic plane was observed to decrease in heavily-doped films. Moreover, films with 10% at. Al showed a rare grain morphology characterized by worm-shaped grains. The presence of Al led to films of lower resistivity when compared to the pure-ZnO film, but the excess of doping agent had an opposed effect. This effect may be due to trap states in the form of ionized impurities and aluminum in interstitial sites. The electrical response of the films to UV light was evaluated, being the undoped films the ones showing the most intense response. The time needed for recovering the original current value after interrupting illumination was also measured and discussed.

Methanol solvent effect on photosensing performance of AZO thin films grown by nebulizer spray pyrolysis

Dopant concentration variation or solvent change is commonly used for the growth and characterization of doped metallic thin films. In contrast, we have studied the effects of varying methanol solvent volume ratio for aluminum doped zinc oxide (AZO) thin film growth. AZO thin films were coated on glass and ITO substrates by nebulizer spray pyrolysis method. The effect of different solvent concentrations and the water to methanol ratio on the resultant film properties were analyzed. The structural and lattice constants of the prepared films were investigated by x-ray diffraction (XRD). Pure methanol solvent enhances the structural quality of the AZO film, as confirmed from the XRD and x-ray photoelectron spectroscopy analyses. The atomic force microscopy images showed thinner and smooth film surface for the pure methanol solvent condition without water. The optical absorption study showed a high transmittance (∼93%) in the visible region with a bandgap of 3.30 eV, which is close to bulk ZnO (3.36 eV). The photoluminescence spectra showed a powerful UV emission at 390 nm, indicating that the films were of good quality. Ag/AZO/Cu2O/ITO structured photodiodes were fabricated using ITO substrate and their photosensing properties were studied. All the prepared films (S1, S2, S3, S4 and S5) showed an increase in current level for both dark and light conditions, with an increase in methanol solvent ratio. The film S5 (pure methanol solvent) under the dark and light illumination conditions at a bias of 1.0 V, showed current values of ∼6.5 × 10–5 A and ∼1.85 × 10–4 A, respectively.

The physical and optical properties of AZO thin films on teflon substrate at low deposition temperature by spray pyrolysis

<span>This paper presents the physical and optical properties of AZO thin films on Teflon substrate at low deposition temperature by spray pyrolysis. In this study, the effect of different process parameters such as spray time and substrate to nozzle distance on the physical and optical characteristic of aluminium doped zinc oxide (AZO) deposited on Teflon substrates was investigated. The AZO thin films were successfully deposited onto Teflon substrate by spray pyrolysis technique at low deposition temperature. The physical analysis by X-ray diffraction (XRD) shows that the deposited Teflon substrate films have a preferred orientation along the direction (100) and (101). Optical measurements were conducted using Jasco/V-670 Ex Uv-Vis-NIR Spectrophotometer model to confirms that in visible ray it is possible to get good reflectance of AZO films with a reflection of 80%. The values of band gaps Eg were calculated from the spectra of UV-Visible reflectance that were vary between 3.06 and 3.14 eV. </span>

Improvement in structural, electrical, and optical properties of Al-doped ZnO nanolayers by sodium carbonate prepared via solgel method

The significant improvement in crystallinity, electrical, and optical properties of aluminum-doped zinc oxide (AZO) thin films was attained by Na doping. Both Al and Na contents were introduced into ZnO host via facile and cost-effective solgel method. Ultra-thin films of Na-doped AZO (NAZO), with thickness of about 40 nm, were deposited on glass substrates via spin-coating technique. The obtained results showed low sodium incorporation reduces the crystal strain which in turn leads to increase crystallite size from 31 nm for AZO to 49 nm for 2 at% Na-doped AZO layer. In addition, 2 at.% of Na doping, that was determined as an optimum level of doping concentration, decreased the macroscopic surface roughness (MRs) without notable changes in surface micro-cracks, leading to remarkable enhancement of the layer conductivity (about 54%). On the other hand, high concentration of sodium content (10 at%) increased the crystal strain and reduced the crystallinity order of AZO layer which resulted in high MRs and also high electrical sheet resistance. Optical dispersion curve and real and imaginary parts of dielectric constant were obtained using our precise method. More interestingly, 2 at% of Na doping led to an increase in transparency of AZO layer in the visible range, while its extinction coefficient (light power absorption) was enhanced which can be regarded as advantage for such thin films. Therefore, the optimum Na content can provide higher optical absorption even with higher transparency by reducing the light scattering and reflection. This finding provides a useful procedure to prepare transparent oxide semiconductor layers with noticeable light absorption, which is very interesting for various applications such as solar cells and anti-reflection coatings.