Al-doped ZnO Films Research Articles

Optoelectronic effect of Al-based buffer layers on Al-doped ZnO thin transparent conductive films on flexible substrates

AZO (Al-doped ZnO) film is a transparent conductive film that offers a simple preparation process, low cost, non-toxicity, and excellent stability. It serves as a high-quality material extensively used in liquid crystal displays, solar cells, and other fields. In this study, AZO thin films were prepared on a PET substrate with an Al2O3 buffer layer using the magnetron sputtering method. The deposition parameters were optimized through the Taguchi method and Grey Correlation Analysis. The results demonstrate that the optimal deposition parameters for achieving superior photoelectric properties during the fabrication of AZO films are 100 W (radio frequency power), 1.0 Pa (sputtering pressure), 20 min (coating time), and 20°C (substrate temperature). Furthermore, the incorporation of an Al2O3 buffer layer enhances the quality of AZO film on PET substrate. This investigation successfully deposited uniform and highly efficient AZO thin films onto flexible PET substrates at low temperatures, thereby expanding their potential applications in flexible electronics.

Structural, Morphological, and Optoelectronic Properties of RF Sputtered AZO Thin Films on Glass and Polymer Substrates: A Comparative Study

This paper investigates the thickness-dependent structural, morphological, and optoelectronic properties of Al-doped ZnO (AZO) thin films deposited on glass and flexible polyethylene terephthalate (PET) substrates via confocal magnetron sputtering. The film’s thickness ranged from 50 to 130 nm. X-ray diffraction results show that all AZO films on glass have better structural properties than those on PET. Furthermore, the (002) peak intensity and crystallite size on both substrates improved progressively with thickness. Field emission scanning electron microscopy and atomic force microscopy images revealed that the film morphology and surface roughness are dependent on substrate and thickness. According to the UV–vis-NIR measurement results, the air-referenced transmittance spectra of films on PET were slightly lower than those on glass; however, compared to films on glass, the substrate-referenced transmittance of PET films was higher. Moreover, for both substrates, it is found that the bandgap of fabricated thin films decreases with thickness. Photoluminescence spectra show that for glass and PET substrates, the total luminescence of AZO decreases with increasing film thickness and that green and red emissions are absent from AZO films deposited on PET substrates. AZO films deposited on glass substrates exhibit superior electrical and optoelectronic characteristics.

Saturation degree in dopant monolayers as modulator of Al-doping of ZnO by the Atomic Layer Deposition-supercycle approach

Transparent conductive oxides are trending topic in science and technology due to counterintuitive properties: optical transparency and high electrical conductivity. ZnO is a promising example with optoelectronic properties enhanced by supervalent doping. Al-doped ZnO (AZO) has emerged as leading candidate to replace environmentally hazardous In-doped SnO2. However, an ongoing debate exists regarding whether Al-doping improves its optoelectronic properties by substitutional doping or by promoting active defects. Here, we focused on Al-doping of ZnO by atomic layer deposition (ALD) applying the supercycle approach, showing that, decreasing Zn precursor dosing during dopant cycles, the decreasing saturation degree of Zn-species monolayers leads to morphological and microstructural changes that negatively impact optoelectronic properties, whereas Al content remains invariant. Our results demonstrate that unsaturated surfaces after decreasing Zn precursor dosing play a crucial role in Al incorporation, suggesting that, to maximize the effect of doping, complete oxide substitution reactions rather than those of conventional ALD must control growth, while crystallinity must remain. These findings could impact strategy designing for optimization of optoelectronic properties of AZO films deposited by ALD by inclining the debate towards the hypothesis that electrical properties are determined by Al substitutional doping together with active defects formed due to substitutional doping itself.

Al concentration-dependent electrical modulation of Al-doped ZnO thin film using atomic layer deposition

A series of 150-nm-thick Al-doped ZnO (AZO) thin films with various Al doping concentrations were deposited by atomic layer deposition technique to determine their applicability in transparent electronic devices. For incorporation into transparent electrodes, the electrical properties of AZO films must be modulated to minimize the parasitic effect. The results show that AZO thin films with various Al dopant concentrations had different work-functions and electrical contact properties, while the other physical characteristics were not significantly changed. Hence, AZO thin films can be used as transparent conducting oxide materials and are potential alternatives for the currently used indium-tin-oxide (ITO) thin films.

Investigating the electrical and thermal properties of Cu and Al-doped ZnO thick films using the screen-printing technique for thermal resistance applications

The important properties of prepared Zinc Oxide (ZnO) thick films were investigated. The electrical and thermal properties of ZnO thick films were measured and analyzed using the MATLAB simulation tool. On a glass substrate, thick films of pure ZnO as well as ZnO doped with Cu and Al materials were fabricated using the screen-printing technique. The electrical parameters were calculated using the half-bridge approach to determine the unknown resistance, change in current, and voltage. The half-bridge circuit having a 50 M Ω reference resistance with 30 V excitation voltage. The thermocouple transducer (K type) was used to measure the heating and cooling temperature between 40 °C to 370 °C. The measured maximum TCR values are 0.0095 ppm/°C for pure ZnO film, 0.0068 ppm/°C for Cu-doped ZnO film, and 0.0123 ppm/°C for Al-doped ZnO film. The maximum measured resistance values are 5.9 × 107 Ω for pure ZnO film, 5.9 × 107 Ω for Cu-doped ZnO film, and 5.9 × 107 Ω for Al-doped ZnO film. Using MATLAB simulation, it was possible to observe key factors including the activation energy, unknown resistance, IV characteristics, thermal properties, temperature coefficient resistance (TCR) and resistive linear behavior of the ZnO films.

Effect of optical and electronic structure on the photocatalytic activity of Al doped ZnO ALD thin films on glass fibers

Photocatalytic water treatment can be promising for eliminating toxic pollutants via a more sustainable approach using renewable sources. However, the technique still requires materials design to optimize/maximize the performance of the photocatalytic materials. Immobilizing the thin film photocatalytic materials onto high surface area textile substrates via atomic layer deposition (ALD) can offer a practical design approach. Al-doped ZnO (AZO) ALD films are deposited onto glass fabric substrates, and their photocatalytic performances were investigated in relation to the FESEM, XRD, XPS, UV–Vis, and PL characterizations. The amount of Al and the post-deposition annealing changed the materials structure, thus the photocatalytic activity of the films. Maximum performance was achieved with the 20 % Al-doped ZnO films after a post-deposition thermal annealing step at 450 °C. The sample also showed the highest UV–Vis absorption and PL emission among the samples. However, this sample didn’t show any crystal peaks in the XRD analysis. Furthermore, the charge carrier concentrations and the mobility of the films were investigated via Hall-Effect, which showed that the 20 % AZO sample has very different features with a p-type nature compared to the other AZO films.

Influence of a plasma focus device on the structural and optical properties of highly conductive AZO thin films

In the present work, we deposited Al-doped ZnO (AZO) thin films on microscope glass slides at different pressures (5, 10, 20, and 30 mTorr) using an EAEA-PFD (Plasma Focus Device of the Egyptian Atomic Energy Authority) without any extra heat treatment. Argon gas provided the radiant energy needed to form a crystalline AZO film. This study investigated the deposition and features of AZO thin films. The impact of changes in work pressure on the microstructure, morphology, and optical attributes of AZO films was reported. The results demonstrated that varying the work pressure significantly affects the microstructure, morphology, crystallinity, optical absorbance, and optical energy gap. The microstructure and morphology were evaluated through XRD and scanning electron microscopy (SEM). The optical properties were analyzed using a UVvis double-beam spectrophotometer within a wavelength range from 300 to 1100 nm. XRD patterns indicating a zinc-blende structure and the Williamson–Hall approach were used to estimate the structural parameters. The estimated trend in the particle size of the AZO film demonstrates a decrease from 82 ± 5 nm to 48 ± 5 nm as the pressure increases. The calculated energy gap, Egopt, increases from 3.53 eV to 3.69 eV with increasing work pressure. The conductivities in terms of both optical and electrical decreased with increasing work pressure of the AZO films.

<span>Effect of limited oxygen gas flux on properties of Al-doped ZnO films</span>

In this study, Al-doped ZnO thin films were sputter deposited on glass substrate as a function of limited oxygen gas inlet. The crystal structure, optical, and electrical properties of the films were characterized using X-ray diffraction, scanning electron microscopy, UV-Vis spectroscopy, and Hall measurement. High crystallinity was found in all AZO samples. Surface morphology presented small grain size of ~ 50 – 100 nm, and thickness of AZO thin films were maintained approximately at 280 nm. Average optical transmittance of AZO films was about 90% in the 450 – 600 nm region, and optical band-gap values varied from 3.327 to 3.380 eV. The electrical resistivity of AZO films decreased with an increasing amount of oxygen flux. These AZO films are quite appropriate for the perspective development of UV photodetectors as a central role absorbing components or n-type semiconducting layers.

Fabrication of alcohol sensor using undoped and Al doped ZnO nanostructure film with polymer electrolyte gating

We report the fabrication of two terminal and three terminal gas sensor using Al-doped ZnO nanostructured-films and polymer electrolyte gate dielectric on glass substrate using vacuum free chemical method. The Al doped ZnO films are characterized by UV–vis Spectrometer, SEM, EDX and XRD. The characterization results have revealed the polycrystalline structure of both undoped and doped ZnO; with loosely packed, porous, and spherical granny nanostructure with mean grain size 20–10 nm and bandgap of the films is within the range of 3.12–3.16 eV. The conductivity of the ZnO film is tuned by Al concentration and the maximum value of conductivity was observed in 3 % Al doped ZnO films. Similarly, the best performance index of TFT such as current ON/OFF ratio, high transconductance and low threshold voltage was observed in 3 % Al doping concentration. The ordinary (two-terminal) sensor and three-terminal (FET) sensors' responses towards three different concentrations 50, 250, 500 ppm of ethanol and methanol vapors have been studied. The sensitivity of the film is modulated by Al concentration and higher value of sensitivity was achieved at 3 % Al doped ZnO films. The use of polymer electrolyte enhanced the sensitivity of the device which is more effective in methanol vapor. The Response-Recovery time of the sensor is significantly improved in three terminal devices than the two terminal devices.

Fabrication and analysis of ZnO and Al-doped ZnO films using the SILAR technique

In this study, undoped ZnO and Al-doped ZnO thin films were successfully deposited on ITO substrates at various concentrations using the SILAR method. The deposited thin films underwent characterization via SEM/EDS, XRD, and UV-Vis spectroscopy. XRD analysis revealed that the thin films crystallized at the nanoscale, with crystallization quality varying based on the concentration of Al dopant. SEM/EDS analysis indicated a notable influence of Al doping concentration on the morphology of ZnO thin films. UV-Vis analysis highlighted significant differences in both the optical band gap values and transmittance of the thin films depending on the dopant used.

Underlying mechanism of Al incorporation in sol-gel-based dip-coated ZnO:Al thin films

Al-doped ZnO films were applied to glass substrates using a sol-gel-based dip-coating method. The chemical composition, surface morphology, crystal structure, and optical and electrical properties were examined to elucidate the mechanism underlying doping efficiency. The optimum Al-doping was 1.00 at%. Al doping induced an increase in the crystallite size, light transmission, optical band gap energy, and carrier concentration but decreased the resistivity of the films. Excessive doping level beyond 1.00 at% degraded all the characteristics of the films. The underlying mechanism of the Al dopant atoms in the ZnO lattice is related to the growth units. The addition of aluminum nitrate promoted the formation of Zn(OH)2 growth units in the solution, whereas its further addition resulted in the formation of additional Zn(OH)2, along with the Zn(OH)42−. The deterioration in the characteristics of the films was attributed to the Zn(OH)42− formation. Understanding the relation between the film characteristics and growth-unit formation is a base tool to increase the optimum Al-doping level, improving film application towards high performance of optoelectronic devices.

Study the characterization of ZnO and AZO films prepared by spray pyrolysis and the effect of annealing temperature

Spray-pyrolysis was used to successfully create ZnO and AZO (Al-doped ZnO) thin films on glass substrates at various annealing temperatures. Structure, morphology, and optical characteristics were investigated in relation to doping and annealing temperature using analytical techniques such as FTIR, UV–Vis spectroscopy, XRD, and FE-SEM. The films' singular-phase polycrystalline hexagonal Würtzite structure was revealed by XRD data, and the crystallographic plane for both AZO and ZnO was (0 0 2). Crystallite sizes are increased by raising the annealing temperature and adding Al-doping, which enhances surface morphology. where the diameters of the AZO crystallites expanded from 23.3 to 33.3 nm, whereas the ZnO crystallites grew from 27.8 to 28.5 nm. Scanning electron microscopy's morphology reveals that the surface particles' tiny, round shapes have changed to spherical clusters that resemble cauliflowers with increasing annealing temperatures. Notably, the ZnO film has a high transmittance, whereas the AZO film has a relatively low transmittance. Furthermore, at an annealing temperature of 400 °C, all films show notable absorbance, with the highest absorption seen at 400 and 413 nm for ZnO and AZO, respectively. The bandgap values at various annealing temperatures vary from 3.22 to 3.23 eV and 3.08–3.12 eV for ZnO and AZO, respectively. As the annealing temperature rose, the phenomena showed a drop in value.

Crystal Al-Doped ZnO Thin-Film Preparation by Sputtering Deposition using a Mixed-Powder Target

Al-doped ZnO films were prepared by sputtering deposition using mixtures of Al2O3 and ZnO powder targets. The film quality depended on the conditions of the target powder. The deposition rate, crystallinity, and transparency of films prepared with new, pressed powder targets were higher than those prepared with used, nonpressed powder targets. In addition, we found that pressurizing the new-powder target and increasing the substrate temperature resulted in thin films with high transparency and electrical conductivity

Optical and electrical properties of Al-doped ZnO thin films deposited by sol-gel method

The multilayer (10 layers) Al doped ZnO (AZO) thin films were deposited on glass substrate by sol-gel & dipping method. X-Ray diffraction measurements showed that the AZO films were polycrystalline with a hexagonal wurtzite structure. The morphological properties of the films were analyzed by atomic force microscopy showing continuous and homogeneous film, completely covering the substrates. The thickness, optical constants, optical band gap (Eg) and transmittance (T) of AZO films were assessed by spectroscopic ellipsometry on UV-vis-NIR spectral range. The AZO film has high transmittance above 80% in the visible region and the optical band-gap energy around 3.7 eV. The electrical characteristics regarding conductivity, mobility and carrier concentrations, were measured by Hall Effect measurements (van der Pauw method). The bulk carrier concentration of the AZO film with 10 layers was found to be 1.16x1019 cm-3. The vibrational bands were obtained by Raman analysis. Defects due to oxygen vacancies in the prepared AZO films were evidenced by photoluminescence spectroscopy (PL). The optical and electrical properties of the AZO thin films proved the possibility to be used in optoelectronic applications.

Impact of atomic layer deposition temperature on electrical and optical properties of ZnO:Al films

This work highlights the impact of growth temperature on the electrical and optical properties of Al-doped ZnO (AZO) films deposited by the atomic layer deposition (ALD) technique. The ALD process and super-cycle sequence have been optimized, identifying their influence on film resistivity. By using this optimum ALD procedure, the optical and electrical properties of AZO films have been widely analyzed considering the deposition temperature. Results show promising values with film resistivity in the range of 1 mΩcm and average optical absorption below 2% for 50 nm thick AZO layers. Hall effect, X-ray diffraction and ellipsometry measurements point out that these excellent values are related to their high carrier concentration and mobility, crystalline phase and optical band gap, resulting in ALD AZO films with excellent properties to be applied in photovoltaic devices as transparent conductive oxide electrode.

Temperature shock to manipulate the optical properties of Al-doped ZnO films

We constructed the 2 × 2 × 2 Aluminum-doped Zinc Oxide (AZO) supercrystal cell structure and developed the temperature shock experiment at −40 °C–85 °C. The results showed that the temperature shock can increase the absorption rate of AZO, decrease the transmittance and the band gap of AZO, and change the density of electronic states near the Fermi level. The AZO films were also subjected to temperature shock of −40°C–85 °C, it shows that the transmittance and band gap of AZO films both decreased after temperature shock. The experimental results show that the temperature shock reduce the transmittance and the band gap width of the AZO films. The results are hoped to be helpful to understand the degeneration and improvement of the optical properties of AZO films under temperature shock loading and predict the performance of AZO films.

Modification in structural, electrical and defect luminescence properties of ZnO nanorod films with Cu, Al and double-doping

Zinc oxide nanorods have broad application prospects in the field of photoelectric devices. However, the application as photodetectors in the visible light band is limited by their narrow luminescent spectrum and poor electrical conductivity. In this work, three different zinc oxide nanorod films respectively with Cu, Al and double-doping have been prepared for investigating the modifying effect of their defect luminescence and electric performance. In this paper, Cu doped zinc oxide with rod structure and Al doped zinc oxide with flaky structure are fabricated by hydrothermal method and sol-gel method, respectively. As for the double-doping, Al doped zinc oxide films are grown on the surface of Si by sol-gel method, and Cu doped zinc oxide nanorods are grown on the surface of Al doped zinc oxide films by hydrothermal method. The results show that the Cu doping can improve the number of defects, leading to the electrons transport from VZn to Zni and then releasing the photons. Alternatively, the trivalent aluminum ions exist in zinc oxide nanorods in the form of replacement or gap, increasing the carrier concentration, turn-on voltage and rectification ratio of zinc oxides. As for the Cu–Al double-doping, Al doped zinc oxide films are located between Cu doped zinc oxide nanorods and Si substrate and work as an intermediate layer of current. Trivalent aluminum ions can change the semiconductor properties of zinc oxide and make it have the performance of p-type semiconductor which can improve the electrical properties of zinc oxide semiconductor. Besides, the carrier concentration of Cu–Al double-doping zinc oxide is nearly 3 times higher than that of pure one. Increasing the carrier concentration can increase the current of ZnO–Si heterojunction in the dark, thus improving the photoelectric properties of ZnO nanorods.

The study of optical-electrical properties of ZnO(AZO)/Si heterojunction

In this paper, zinc oxide (ZnO) thin films and Al-doped zinc oxide (AZO) films are prepared. The XRD results show that the addition of Al ions at the middle stage of AZO film (ms-AZO) preparation can increase the grain size and change the preferential growth orientation from (100) crystal plane to (002) crystal plane. The results of UV-VIS spectra show that the addition of Al ions at the ms-AZO preparation can greatly increase the absorption intensity in the visible light region, and the addition of Al ions at the early stage of AZO film (es-AZO) preparation can make the blue shift of the ultraviolet absorption edge. The patterns of I–V curves show that the addition of Al ions can increase the forward current of AZO/Si heterojunction, comparing to that of ZnO/Si heterojunction. Furthermore, the leakage current mechanism may dominate the carrier transport.

Excellent crystalline silicon surface passivation by transparent conductive Al-doped ZnO/ITO stack

The surface passivation materials commonly used in crystalline silicon (c-Si) solar cells are either electrically insulating or opaque to the solar spectrum, which poses the electrical loss or optical loss. Hence it is of great significance to explore functional films that are simultaneously transparent, conductive and passivating for c-Si. In this study, we successfully tune two most well-known transparent conductive oxides, i.e. Al-doped ZnO (AZO) and indium tin oxide (ITO), as excellent passivation layers for c-Si surface. The AZO film is grown by atomic layer deposition and is capped with a magnetron-sputtered ITO film. Dynamic secondary ion mass spectrometry reveals that such stack design can retain higher hydrogen concentration on c-Si surface than a single AZO film during annealing. In addition, the AZO should be highly doped to reduce the energy barrier height around c-Si surface, thereby increasing the charge carrier asymmetry and thus reducing surface recombination. It is also highly crucial to optimize the annealing temperature and interfacial silicon oxide. As such, the effective minority carrier lifetime reaches 2.0 ms and the implied open-circuit voltage approaches 700 mV. Besides, the AZO/ITO stacked films possess negligible optical absorption, low sheet resistance (∼90 Ω/sq), and outstanding antireflection effect on c-Si surface.

Transparent artificial synapses based on Ag/Al-doped ZnO/ITO memristors for bioinspired neuromorphic computing

Today's computing systems to meet the enormous demands of information processing have driven the development of brain-inspired neuromorphic systems that can perform fast and energy-efficient calculations. In this study, Al-doped ZnO (AZO: ZnO to Al2O3 = 98:2 wt%) films with over 95 % light transmission were prepared by RF magnetron sputtering, and a structurally simple Ag/AZO/ITO memristor was fabricated. Moreover, the sustained potentiation/depression behavior of the memristor allows it to mimic artificial synapses that exhibit biological synaptic and neuronal functions such as short-term/long-term plasticity and paired-pulse facilitation.These results will contribute to the research and development of high-performance artificial synapses.