Aluminum-doped Zinc Oxide Thin Films Research Articles

Aluminum's role in tailoring the characteristics of room temperature co-sputtered gadolinium and aluminum-doped ZnO thin films

This study investigated the characteristics of gadolinium (Gd) and aluminum (Al) co-doped ZnO synthesized by co-sputtering at varying concentrations of Al. FESEM images revealed a morphology containing spherical-hexagonal nanostructures of different sizes based on the amount of Al. The EDS spectra clearly demonstrated the presence of Al, Gd, Zn, and O, confirming the successful incorporation of Gd and Al ions into the ZnO lattice in agreement with XRD profiles, not detecting any secondary phases. The photoluminescence study revealed that doping-induced defect states (oxygen vacancies, zinc vacancies, and zinc interstitial) were responsible for the many characteristic peaks of deep level emission observed in the photoluminescence spectra. Higher Al doping induced changes in magnetic characteristics as reflected in the greater root mean square value (δfrms), and the root mean square phase shift value (Φrms) in MFM analysis, whereas the magnetic correlation length (L) decreases. The incorporation of Al at higher concentrations into Gd-doped ZnO resulted in improved magnetic behavior suggesting the exchange interaction between Gd and Al ions mediated by oxygen vacancies. These results confirm that sufficient Al doping can efficiently improve the properties and magnetic behavior of ZnO-based DMS systems, which could potentially pave the way towards the realization of spin and/or optical based electronic devices.

Influence of power ramps on the physical properties of AZO thin films deposited at room temperature by RF magnetron sputtering technique

Abstract Aluminum-doped zinc oxide (AZO) thin films were deposited on glass substrates at room temperature by RF sputtering technique. Power ramps between 125 and 105 W were applied with a step of 4 W by intervals of 15, 7.5 and 1.8 min, for 180 min at 1.60 Pa. In this study, we investigated the structural, morphological, electrical, and optical properties of AZO films. X-ray Diffraction analysis showed that the films have a wurtzite-type hexagonal crystalline structure with a preferential crystallographic orientation (002) normal to the c axis. The average transmittance is greater than 76% for the wavelength range in the visible spectrum. The bandgap values were found between 3.32 and 4.01 eV, and refractive index was 1.79–2.60. Atomic force microscope measurements show homogeneous films with a roughness between 17–22 nm. A minimum resistivity value of 2.0 × 10−3 Ω cm was obtained for the film by using a power ramp of 4 W/1.8 min.

Enhancement of photodetector performance of aluminum-doped zinc oxide thin films fabricated via SILAR method: Structural, optical, and electrical analysis

This study presents a comprehensive analysis of the structural, optical, electrical, and photosensing characteristics of aluminum-doped zinc oxide (Al:ZnO) thin films, fabricated via successive ionic layer adsorption and reaction (SILAR) techniques. The Raman and X-ray diffraction (XRD) analyses validated the hexagonal wurtzite structure and revealed that Al doping enhances crystallinity and crystal growth of ZnO. The average crystallite size increased from 21.6 nm in undoped sample to 23.8 nm in 5.0 wt% Al-doped sample, while strain effects decreased from 4.67 × 10-3 to 4.36 × 10-3. Scanning electron microscopy (SEM) images revealed improved film morphology and more defined geometries with increased Al doping, while energy-dispersive X-ray analysis (EDAX) confirmed successful Al integration into the ZnO lattice. Optical analyses indicate a narrowing of the ZnO bandgap to 2.81 eV in 5.0 wt% Al:ZnO from 3.02 eV in the undoped variant, while photoluminescence (PL) peak positions remains unchanged. Time-resolved fluorescence (TRF) indicated a reduction in electron lifetime with higher Al doping. Furthermore, Hall effect assessments reveal a decrease in carrier mobility and electrical resistivity, alongside a rise in carrier concentration with higher Al content. The photodetection efficacy was substantially improved with Al doping, particularly at 2.5 wt% Al, achieving a responsivity of 42.5 × 10-2 A/W, external quantum efficiency of 99.3 %, and detectivity of 1.8 × 1011 Jones. These findings exhibit that Al-doping substantially enhances the photodetection properties of ZnO thin films.

Effect of Al doping on structural and optical properties of atomic layer deposited ZnO thin films

In this work, zinc oxide (ZnO) and aluminum-doped ZnO (AZO) thin films with varying Al contents were grown via atomic layer deposition on Si and glass substrates. The structural and optical properties of ZnO and AZO thin films were investigated using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), photoluminescence (PL), and ultraviolet-visible spectroscopy. While undoped ZnO had a wurtzite crystal structure, Al2O3 layers altered the crystal structure depending on the Al content. With the increasing number of Al2O3 monolayers, the γ- Al2O3 structure became more prominent, and ZnO peaks were completely suppressed when the Zn ratio reached 10:1 and beyond. SEM images revealed that the thin films homogeneously covered the entire substrate surface. EDS mapping showed a uniform distribution of Zn, Al, and O in the structure. According to the XPS results, the thin films displayed significant peak intensities corresponding to the main components Zn, O, and Al. Zinc atoms were present only in their oxidized state, not as interstitial atoms. The O 1 s peak could be deconvoluted into three components: the oxygen-zinc bond, the oxygen-aluminum bond, and chemisorbed or OH species. In the AZO samples, a direct correlation was observed between the intensity of the Al 2p peak and the number of Al2O3 layers. Additionally, the calculated elemental ratio of Al increased from 2.582 to 23.955 at.% with more Al2O3 layers. Optical measurements revealed that ZnO and AZO thin films exhibit high transmittance in the visible region. Moreover, introducing Al2O3 layers into the supercycle led to an increasing trend in transmittance. Another improvement in the optical properties was observed as a blue shift in the absorption edge with Al doping, indicating band gap widening. The optical band gap of the ZnO thin films increased from 3.2 to 3.5 eV with Al doping. The origin and concentration of defects were evaluated by deconvoluting the PL spectra. A comparative investigation of the PL spectrum and XPS results revealed variations in defect concentration with deposition parameters and defect formation mechanism was discussed.

Effect of deposition regime transition on the properties of Al:ZnO transparent conducting oxide layer by radio frequency magnetron sputtering system

High-quality zinc oxide thin films doped with aluminium adatoms have effectively been fabricated on pristine soda-lime silica glass substrates via radio frequency magnetron sputtering system. The deposition temperature was varied to explore the impact of deposition regime transition of as-deposited Al:ZnO thin films on their performance as transparent conducting oxide layers. In particular, the depositions were conducted at room temperature, 100 °C, 200 °C, and 300 °C, allowing for a comprehensive assessment of the resulting films. The Raman spectra depicted the modulation of Raman bands in correlation with the deposition regime transition, illustrating the impact of thermal induction on various properties of the as-deposited aluminium-doped zinc oxide thin films. Atomic force microscopy reveals the transformation from nearly spherical to elongated shape structure was obtained as the deposition process shifted from kinetic limited to thermodynamic limited regimes. The phase analysis and grazing incident of x-ray diffractometer disclose a single crystal orientation has been achieved at thermodynamic limited regime. However, two different crystal planes were predominant comparing between the surface and structural of as-deposited aluminium-doped zinc oxide thin films. It is also evident that a highly transparent with low lattice strain and better carrier concentrations of as-deposited aluminium-doped zinc oxide thin films were realized at thermodynamic limited regimes.

Aluminum-doped zinc oxide thickness controllable wavelengths in visible light and high responsivity devices using interrupted flow atomic layer deposition

In this study, we develop a highly sensitive visible light photodetector that utilizes a thin-film structure composed of low-cost aluminum-doped zinc oxide (AZO) and n-type silicon. The AZO thickness can be adequately controlled to fit the different wavelengths of interest for photodetectors in the visible light range using interrupted flow atomic layer deposition (ALD). This in situ aluminum doping method ensures a uniform aluminum distribution within the AZO thin films and effectively increases the internal film reflections and photoresponsivity. The Schottky interface with n-type silicon is created by degenerated AZO due to the lower Fermi level, and visible light can effectively penetrate the underlying depletion zone. Optical simulation of the high conductivity of AZO indicated that the optimal thickness was 54.6, 65.8, and 91.7 nm for devices illuminated with 450 nm blue, 525 nm green and 700 nm red light, respectively. Hall effect measurements confirmed that the AZO film can achieve a low resistivity of 5 × 10–4 Ω-cm and high carrier concentration of 3 × 1020 cm−3 at a suitable precursor ratio. Additionally, AZO films offer multifunctionality by providing optical antireflective properties and forming Schottky junctions with n-type silicon to enable photoelectric conversion. This multifunctional role of AZO was experimentally validated through electrical, optical, and optical-to-electrical experiments, which showed that the optimized device can reach an optical responsivity of approximately 10.7 AW−1 at specific visible light wavelengths. The significant photoelectrical conversion efficiency and simple thin-film structure design facilitate future applications in light intensity measurement, such as in colorimetry or fluorometry.

The Impact of Temperature and Power Variation on the Optical, Wettability, and Anti-Icing Characteristics of AZO Coatings

The structural, wettability, and optical characteristics of aluminum-doped zinc oxide (AZO) thin films were studied with the objective of understanding the impact of deposition power and deposition temperature. Thin films were deposited using a radio frequency (RF) magnetron sputtering technique. The power output of the RF was augmented from 200 to 260 W, and the temperature was increased from 50 to 200 °C, which led to the development of a (002) peak for zinc oxide. The study of film thickness was carried out using the Swanepoel envelope method from data obtained through the UV-Vis spectrum. An increase in surface roughness value was shown to be connected with fluctuations in temperature as well as increases in deposition power. The findings revealed that as deposition power and temperature increased, the value of optical transmittance decreased, ranging from 70% to 90% based on the deposition parameters within the range of wavelengths that extend from 300 to 800 nm. The wettability properties of the samples were studied, and the maximum contact angle achieved was 110°. A Peltier apparatus was utilised in order to investigate the anti-icing capabilities, which revealed that the icing process was slowed down 3.38-fold. This work extends the understanding of the hydrophobicity and anti-icing capabilities of AZO thin films, specifically increasing both attributes which provide feasible options for purposes requiring resistance to ice.

MORPHOLOGICAL, OPTICAL PROPERTIES AND ELECTRONIC STRUCTURE OF ZnO THIN FILMS DOPED WITH ALUMINUM

This paper presents the results of investigation of aluminum-doped zinc oxide thin films obtained by radio-frequency magnetron deposition on a glass substrate. It was shown by energy dispersion X-ray spectroscopy that the average aluminum content in the thin films was 0.2–0.8 at.%. An increase in the average grain size with increasing aluminum concentration was shown, and column-like growth of the films was established using microscopic images. Investigation of the electronic structure showed a redistribution of intensity in the O 1s spectrum with aluminum doping. The valence band spectra were also obtained, and theoretical calculations were performed within the framework of density functional theory to better understanding the results. Using a spectrophotometer, the transparency of the films obtained after aluminum doping was shown to increase. With the help of the Tauc method, the band gap was determined to be 3.41 eV, while DFT calculations showed a band gap of 1.25 eV.

A Comprehensive Investigation of the Mechanical and Tribological Properties of AZO Transparent Conducting Oxide Thin Films Deposited by Medium Frequency Magnetron Sputtering.

This paper presents a detailed analysis of aluminium-doped zinc oxide (AZO) thin films and considers them a promising alternative to indium tin oxide in transparent electrodes. The study focusses on critical properties of AZO, including optical, electrical, and mechanical properties, with potential applications in displays, photovoltaic cells, and protective coatings. The deposited AZO thin films are characterised by excellent optical and electrical parameters, with transparency in the visible light range exceeding 80% and resistivity of 10-3 Ω·cm, which gives a high value of figure of merit of 63. Structural analysis confirms the nanocrystalline nature of as-deposited AZO thin films, featuring hexagonal ZnO, orthorhombic Al2O3, and cubic Al2ZnO4 phases. The study includes nanoindentation measurements, which reveal exceptional hardness (11.4 GPa) and reduced elastic modulus (98 GPa), exceeding typical values reported in the literature, highlighting their protective potential. Abrasion tests have shown extraordinary scratch resistance due to the lack of impact on topography and surface roughness up to 10,000 cycles. This comprehensive study demonstrated that as-deposited AZO thin films are multifunctional materials with exceptional optical, electrical, and mechanical properties. The findings open up possibilities for a variety of applications, especially in protective coatings, where the combination of hardness, scratch resistance, and transparency is both rare and valuable.

Multifunctional Al-doped ZnO thin films for vertically aligned liquid crystal devices

The integration of ITO-free transparent conductive layers remains a big challenge for development of next generation technologies. Here, we demonstrate the feasibility of transparent and conductive Aluminum-doped Zinc Oxide (AZO) thin films to operate simultaneously as electrode and alignment layer in Liquid Crystal (LC) device configuration. Controlling the growth process of AZO thin films by Atomic Layer Deposition (ALD) technique results of predominantly (100) crystallographic oriented AZO with very high transparency and excellent conductivity. The exceptionally low surface free energy of (100) oriented AZO, along with the topological modification following the mechanical rubbing treatment were used to elucidate the mechanism of LC molecules alignment on the surface of AZO film. The uniform vertical orientation of the nematic LC director was confirmed by polarized optical microscopy and pretilt angle measurements. The competitive electro-optical performance in terms of phase modulation, threshold and saturation voltages and contrast ratio of the assembled LC device reveals the great potential of AZO thin films as transparent electrode and alignment layer for future LC display applications with versatile functionality.

Effects of Cyclic Bending Parameters on Aluminum-Doped Zinc Oxide Thin Films for Flexible Device Applications

Effects of Cyclic Bending Parameters on Aluminum-Doped Zinc Oxide Thin Films for Flexible Device Applications

Progress in the Synthesis and Application of Transparent Conducting Film of AZO (ZnO:Al).

Due to the excellent performance and low cost of the new aluminum-doped zinc oxide (AZO) film, it is expected to replace the mature indium-doped tin oxide (ITO) film. The research status and progress of AZO transparent conductive films are summarized in this review. Moreover, the structure, optoelectronic properties, and conductive mechanism of AZO thin films are also detailed. The thin films' main preparation processes and the advantages and disadvantages of each process method are mainly discussed, and their application fields are expounded. AZO thin films with multicomponent composite structures are one of the promising development directions in transparent conductive oxide (TCO) thin films. The development of various preparation processes has promoted the production and application of thin films on a broad scale. Finally, some improvement schemes have been proposed to improve the comprehensive performance of the film. The industrialization prospects of the AZO film, as well as its great development potential in the digital world, are discussed.

High figure-of-merit in Al-doped ZnO thin films grown by ALD through the Al content adjustment

The development of transparent conductive materials is of great interest for a wide variety of semiconducting devices in the fields of optoelectronics and photovoltaics. Here, we optimize the growth and properties of aluminum-doped ZnO (AZO) thin films as an eco-friendly transparent electrode by using atomic layer deposition (ALD) with diethylzinc (DEZn), water vapor and trimethylaluminium (TMA) as zinc, oxygen, and aluminum chemical precursors, respectively. The cycle ratio of DEZn to TMA using ALD is adjusted to control the Al atomic concentration over a broad range of 0.3 - 6.7% while optimizing the structural, optical, and electrical properties of AZO thin films. By incorporating a sufficient amount of Al dopants around 1.1%, we obtain a high quality AZO thin film exhibiting a low electrical resistivity of 6.3 × 10−4 Ω·cm. This AZO thin film grown on quartz substrate also demonstrates an average optical transmittance in the visible range above 85% as well as a high figure-of-merit of 6.4 × 10−3 Ω−1, while maintaining its highly c-axis oriented structure. The present optical and electrical properties result in the fabrication of AZO thin films by ALD with a high figure-of-merit that is highly suitable for many semiconducting devices.

Ultralow Thermal Conductivity and Improved Thermoelectric Properties of Al‐Doped ZnO by In Situ O2 Plasma Treatment

The thriving of Internet‐of‐Things and integrated wireless sensor networks has brought an unprecedented demand for sustainable micro‐Watt‐scale power supplies. Development of high‐performing micro‐thermoelectric generator (μ‐TEG) that can convert waste thermal energy into electricity and provide sustainable micro‐Watt‐scale power is therefore extremely timely and important. Herein, a significant advance in the development of earth‐abundant, nontoxic thermoelectric materials of aluminum‐doped zinc oxide (AZO) is presented. Through nanostructure engineering using a novel in situ O2 plasma treatment, AZO films are demonstrated with ultralow thermal conductivity of 0.16 W m−1 K−1 which is the lowest reported in the literature. This nanostructured film yields a power factor of 294 μW m−1 K−2 at 563 K and has resulted in a state‐of‐the‐art ZT of 0.11 at room temperature and 0.72 at 563 K for AZO thin films. Furthermore, the fabrication and testing of a prototype lateral μ‐TEG are reported based on the AZO thin film which achieves a power output of 1.08 nW with an applied temperature difference of 16.9 °C.

High optoelectronic quality of AZO films grown by RF-magnetron sputtering for organic electronics applications

Aluminum-doped zinc oxide thin films, known by the acronym AZO, were grown by radio-frequency magnetron sputtering method (rf-magnetron sputtering) onto glass substrate at room temperature and without posterior heat treatment. The impact on the structural, electrical, and optical properties of the AZO films was studied as a function of the following deposition parameters: working pressure, rf-power and thickness. Our films showed low electrical resistivity and high transmittance in the visible region comparable to commercial indium tin oxide (ITO) films. We obtained an optimized AZO film with an electrical resistivity of 4.90 × 10−4 Ωcm and presented optical transmittance strikingly high for such a good conductor, with about 98% at 580 nm and an average optical transmittance of about 92% in the visible region. We also built and characterized an organic light-emitting diode (OLED) using the optimized AZO film as a transparent electrode. The AZO-based OLED showed characteristics comparable to a reference ITO-based device, indicating that AZO films have optoelectronic properties good enough to be used in organic electronics. In addition, the results suggest that they are suitable to be employed as transparent conductors in flexible polymeric substrates since their synthesis was performed without intentional heating.

AZO work function enhanced by oxygen plasma immersion ion implantation

The surface of aluminum-doped ZnO (AZO) films was modified using the oxygen plasma immersion ion implantation method, which is also called O-PIII. The modified AZO films were analyzed by an X-ray photoelectron spectroscopy system (XPS), X-ray diffraction (XRD), a UV–visible spectrophotometer, atomic force microscopy (AFM), and a Kelvin probe instrument (KP). The results showed that O-PIII modification can further enhance the work function (WF) of AZO based on oxygen plasma (O-ICP) treatment. The optical characteristics of AZO thin films are nearly entirely preserved, and the surface morphology is enhanced. The variation in WF was strongly linked to the surface stoichiometry. The reduction of oxygen vacancies should be attributed to the high WF. The WF of the modified AZO films decreased rapidly with exposure time in the air promptly following O-ICP and O-PIII modification. Nonetheless, the O-PIII treatment had a longer-lasting effect.

The Effect of Post Deposition Treatment on Properties of ALD Al-Doped ZnO Films

In this paper, aluminum-doped zinc oxide (ZnO:Al or AZO) thin films are grown using atomic layer deposition (ALD) and the influence of postdeposition UV-ozone and thermal annealing treatments on the films' properties are investigated. X-ray diffraction (XRD) revealed a polycrystalline wurtzite structure with a preferable (100) orientation. The crystal size increase after the thermal annealing is observed while UV-ozone exposure led to no significant change in crystallinity. The results of the X-ray photoelectron spectroscopy (XPS) analyses show that a higher amount of oxygen vacancies exists in the ZnO:Al after UV-ozone treatment, and that the ZnO:Al, after annealing, has a lower amount of oxygen vacancies. Important and practical applications of ZnO:Al (such as transparent conductive oxide layer) were found, and its electrical and optical properties demonstrate high tunability after postdeposition treatment, particularly after UV-Ozone exposure, offers a noninvasive and easy way to lower the sheet resistance values. At the same time, UV-Ozone treatment did not cause any significant changes to the polycrystalline structure, surface morphology, or optical properties of the AZO films.

Spray-Deposited Aluminum-Doped Zinc Oxide as an Efficient Electron Transport Layer for Inverted Organic Solar Cells

Spray-deposited thin films of zinc oxide (ZnO) and aluminum-doped zinc oxide (Al-ZnO) are characterized in detail to get insight into the role of a dopant in the matrix. ZnO and Al-ZnO are implemented as electron transport layers (ETLs) in inverted organic solar cells (IOSCs) with PTB7-Th as a donor and IEICO-4F as a nonfullerene acceptor, forming the bulk heterojunction (BHJ) photoactive layer. Organic solar cells (OSCs) based on the ZnO ETL exhibit a short-circuit current density (JSC) of 24.46 mA/cm2 and an open-circuit voltage (VOC) of 0.68 V, yielding a power conversion efficiency (PCE) of 9.3%. A solar cell based on the Al-ZnO ETL yields a higher JSC of 25.16 mA/cm2 and a VOC of 0.71 V, resulting in a PCE of 10.5%, which indicates that Al doping improves the device performance. Time-delayed collection field (TDCF) measurements yielded field-independent charge generation for both devices. Furthermore, steady-state photoluminescence (PL), time-resolved PL, and transient absorption measurements confirm reduction in the number of defect states in Al-ZnO thin films compared to ZnO thin films and efficient charge transfer, yielding an overall improved IOSC device performance.

Al-Doped ZnO Thin Films with 80% Average Transmittance and 32 Ohms per Square Sheet Resistance: A Genuine Alternative to Commercial High-Performance Indium Tin Oxide

In this study, a low-sophistication low-cost spray pyrolysis system built by undergraduate students is used to grow aluminum-doped zinc oxide thin films (ZnO:Al). The pyrolysis system was able to grow polycrystalline ZnO:Al with a hexagonal wurtzite structure preferentially oriented on the c-axis, corresponding to a hexagonal wurtzite structure, and exceptional reproducibility. The ZnO:Al films were studied as transparent conductive oxides (TCOs). Our best ZnO:Al TCO are found to exhibit an 80% average transmittance in the visible range of the electromagnetic spectrum, a sheet resistance of 32 Ω/□, and an optical bandgap of 3.38 eV. After an extensive optical and nanostructural characterization, we determined that the TCOs used are only 4% less efficient than the best ZnO:Al TCOs reported in the literature. This latter, without neglecting that literature-ZnO:Al TCOs, have been grown by sophisticated deposition techniques such as magnetron sputtering. Consequently, we estimate that our ZnO:Al TCOs can be considered an authentic alternative to high-performance aluminum-doped zinc oxide or indium tin oxide TCOs grown through more sophisticated equipment.

Excitation of Epsilon‐Near‐Zero Mode in Optical Fiber

AbstractExperimental excitation of a highly confined epsilon‐near‐zero (ENZ) mode in a side‐polished optical fiber coated with a deep subwavelength thick layer of aluminum‐doped zinc oxide (AZO) is reported. The uniform AZO layer on the fiber is fabricated by atomic layer deposition technique and optimized to exhibit close‐to‐zero permittivity at the near‐infrared wavelength. Highly polarization‐ and wavelength‐dependent transmission with strong resonance strength up to 25 dB is observed in a 30‐nm ENZ‐coated fiber that is 17 mm long. Different from the excitation of the ENZ mode in a planar conducting oxide thin film, the hybrid ENZ mode can be excited via direct phase matching between the fundamental mode of the fiber and the ENZ mode supported by the AZO thin film. The hybrid ENZ mode in the fiber exhibits a relatively long propagation/light–matter interaction length which is a few orders of magnitude longer than those on the planar ENZ substrates. It is further shown that the hybrid resonance in the ENZ fiber can be actively tuned through the refractive index of surrounding medium and the large ENZ's nonlinearity. These ENZ‐optical fibers serve as emerging in‐fiber optical devices, such as advanced in‐fiber ultrafast optical switches/modulators, mode‐locked fiber lasers, and in‐fiber optical gas/biomolecule sensors.