Al-doped ZnO Thin Films Research Articles

Suppression of grain boundary contributions on carrier mobility in thin Al-doped ZnO epitaxial films

Realizing very thin transparent conductive (TCO) films is a key aspect of many applications, but often the film quality tends to worsen at very small thicknesses. In this work we demonstrate that Al-doped ZnO (AZO) thin films grown epitaxially on SrTiO3 retain their optimal properties even at thicknesses as low as 30 nm. We deposit by radio frequency magnetron sputtering and investigate the film morphology, structure, crystallinity, electrical and optical properties. We prove that the Hall mobility of epitaxial films is limited mostly by ionized impurity scattering, with a negligible contribution of grain boundary scattering, leading to comparably high mobility in thin epitaxial films. On the contrary, in polycrystalline films the properties of AZO films strongly deteriorate at reduced film thickness, due to grain boundary contributions. The optical carrier density and mobility do not change significantly between epitaxial and polycrystalline films, suggesting that the different electrical properties are indeed mainly due to grain boundary scattering and not to a reduction of carriers and intrinsic mobility. In this way we obtain an epitaxial thin TCO film with the optimal properties of a bulk by a scalable technique, paving the way to applications in plasmonic devices and in thin films solar cells.

Strain-dependent doping and optical absorption in Al-doped graphene-like ZnO monolayer

In this work, we studied strain-dependent doping and optical absorption in Aluminum (Al)-doped graphene-like ZnO (g-AZO) monolayer using hybrid-Density functional theory (DFT) calculations. The effect of biaxial strain (εxy) on the dopant formation energy (Eform), electronic structure, and optical absorption of g-AZO monolayer are investigated in detail. We observed that under application of εxy up to +5%, the value of Eform increases linearly, whereas at εxy > +5%, an inverse sharp parabolic relation with tensile strain is observed. Owing to the lowest value of Eform at εxy = +10%, the highest doping concentration of Al dopants in the g-ZnO monolayer is observed. However, under an εxy up to −10%, the Eform varies linearly with strain, but the change in Eform is not sufficient to induce higher Al doping concentration in the g-ZnO monolayer. Further, the effect of εxy on the optical absorption is studied and it is verified that the highest visible absorption is observed at εxy = +10% (due to the lowest value of Eform). As observed results are well-matched with the reported experimental work on Al-doped ZnO thin films. This work highlights the unique relationship between strain and Eform for g-AZO monolayer for various electronic, optoelectronic, and energy applications.

The investigation of structural, surface topography, and optical behaviors of Al-doped ZnO thin films with annealing temperature deposited by RF magnetron sputtering

The investigation of structural, surface topography, and optical behaviors of Al-doped ZnO thin films with annealing temperature deposited by RF magnetron sputtering

Improvement of the uniformity of structural and electrical properties of transparent conductive Al-doped ZnO thin films by inductively coupled plasma-assisted radio frequency magnetron sputtering

Transparent conductive Al-doped Zinc oxide thin films were deposited on glass substrates by conventional radio frequency magnetron sputtering (C-RFMS) and inductively coupled plasma-assisted radio frequency magnetron sputtering (ICP-RFMS) methods. The structural and electrical properties of films were studied as a function of the radial position (r) of the substrate in both methods. A good thickness uniformity was obtained in ICP-RFMS, while a significant decrease in film thickness was observed at r=30mm (erosion zone) in C-RFMS. The c-axis orientation was achieved in all positions in ICP-RFMS, while crystallinity was degraded at r≥25mm in C-RFMS. Low resistivity ρ of the order of 10−4Ωcm was obtained at all positions in ICP-RFMS, while ρ was rapidly increased at r≥30mm in C-RFMS. In ICP-RFMS, high-energy ion bombardment is suppressed due to the modified plasma density profile and lower space potential. As for transparency, the sample prepared at r=0mm in ICP-RFMS showed an average visible transmittance of 81%, higher than that in C-RFMS. Throughout the study, it was demonstrated that ICP-RFMS method was promising to improve the radial distribution of structural, electrical, and optical properties of Al-doped ZnO thin films.

Effect of Substrate Temperature on Photoelectric Properties of AZO Thin Films

Herein, Al‐doped ZnO (AZO) thin films are prepared on quartz glass substrates by RF magnetron sputtering. The effects of substrate temperature (T s) on the structure, morphology, electrical, and optical properties of AZO films are investigated by X‐ray diffraction, field emission electron microscopy, Hall effect testing instrument, and ultraviolet spectrophotometer, respectively. The results show that the doping of Al element does not change the structure of ZnO films, and all films have typical hexagonal wurtzite structure with preferred c‐axis orientation. The film sample has the strongest (002) diffraction peak and the smallest value of full‐width at half‐maximum at the T s value of 150 °C. With the increase of T s value, the grain size of the film increases first and then decreases. At 150 °C, the film exhibits good electrical conductivity, and its resistivity is 0.6210−2 Ω cm, the carrier concentration is 9.841019 cm−3, and the Hall mobility is 13.66 cm2 V−1 s−1. All samples show high transmittance, and the average transmittance in the stable visible light region is as high as 95% at the T s value of room temperature and 250 °C, and the average transmittance of all samples is above 80%. This provides a method for the application of ZnO transparent conductive films.

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.

Conductivity Control of Al-doped ZnO Thin Films Deposited by Radio Frequency Sputtering

Films made up of Al-doped ZnO (AZO) are the promising transparent conductive oxides. Hall measurements were used to investigate the effects of varying the sputtering parameters on the electrical resistivity, electron concentration and electron mobility of a series of AZO films produced by radio frequency (RF) sputtering. Increases in plasma power, chamber pressure, argon (Ar) flow rate and decreases in substrate temperature lead to the higher electrical resistance. However, all the samples exhibit a pattern in which electrical resistivities decrease with increasing annealing temperature (below 200-400 ºC) and increase (above 200-400 ºC). As the number of electrons increases, the electrical resistance decreases and vice-versa. The carrier compensation effect or excess carrier compensation due to the formation or annihilation of conductive defects is responsible for these shifts in the electrical characteristics. It was also found that the conductivities of the AZO films formed via RF-sputtering dependent on imperfections in structure such as ionized impurities and grain boundaries, as electron mobility is shown to be proportional to the electron concentration.

Light-enhanced electrical behavior of a Au/Al-doped ZnO/p-Si/Al heterostructure: insights from impedance and current-voltage analysis.

In this study, we meticulously deposited an Al-doped ZnO nanoparticle thin film on a p-type silicon substrate using the precise sputtering method. We conducted a comprehensive exploration of the film's structure, morphology, and optical properties. X-ray diffraction (XRD) confirmed its polycrystalline wurtzite configuration with a dominant (002) orientation. High-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) revealed a uniformly textured surface adorned with densely packed nanoparticles. Regarding optical properties, the Al-doped ZnO thin film exhibited exceptional transmittance exceeding 80% across visible and near-infrared spectra. Moving on to electrical characteristics, we assessed the Au/Al-doped ZnO/p-Si/Al heterostructure under dark and illuminated conditions. Through current-voltage (I-V) and impedance measurements, we observed significant improvements in conductivity and performance under illumination. Notably, there was an increase in current conduction and a reduction in impedance, highlighting the advantages of illumination. Collectively, these findings emphasize the promising potential of the Au/Al-doped ZnO/p-Si/Al heterostructure, particularly in the realms of optoelectronic devices and photovoltaics. With its ability to efficiently mobilize charges and adeptly assimilate light, this heterostructure stands as a frontrunner for transformative applications in these technologically vital domains.

Influence of Ar gas pressure on the structural and optical properties and surface topography of Al-doped ZnO thin films sputtered by DC-Magnetron sputtering method

In this work, Aluminum doped Zinc oxide thin films were sputtered on glass substrate by the direct current (DC) magnetron sputtering method. The influence of Ar gas pressure on the structural and optical properties was measured. The optical parameters were calculated by UV–Visible spectroscopy, the nature of transition reveals direct allowed transition for the prepared films. Also, some physical quantities such as the strength of electron–phonon interaction, dissipation factor (tanδ) in the visible region and the lattice dielectric constant were presented for these thin films. The AFM analysis extracts surface parameters of the AZO thin films that help us to quantitively investigate the surface analysis. The band gap energy and transition index without any presumption about transition natural were calculated from Derivation Ineffective Thickness Method (DITM) and it was found that the reaction of Ar gas pressure plays an essential part in controlling the physical quantities of AZO thin films.

Photoelectric characteristics of Al-doped ZnO/p-Si diode prepared by radio frequency magnetron sputtering

Al-doped ZnO (AZO) thin films were deposited on p-type silicon (p-Si) substrates by radio frequency magnetron sputtering technology. The crystal structure, morphology characterization and elemental analysis show that AZO film grows along the c-axis (002) orientation without other impurities. The current–voltage and current-time characteristics under different illumination conditions demonstrate that the Au/AZO/p-Si diode has typical rectification behavior, excellent stability and repeatability. The photocurrent is proportional to the intensity of ultraviolet (UV) irradiation, and the photocurrent reaches 110 μA at a bias voltage of 5 V under 11.75 mW cm−2 UV light irradiation. By calculating the conduction band and valence band offset values of AZO/p-Si heterojunction, the energy band diagrams at different bias states are constructed to explain the photoelectric response behavior. These results will be helpful for the design of high-performance photodiodes.

Understanding of mobility limiting factors in solution grown Al doped ZnO thin film and its low temperature remedy

The solution grown doped ZnO based transparent electrode has shown great potential in future generation flexible and smart devices due to its abundance in earth, low cost, simple and low temperature synthesis process. But solution grown doped ZnO possesses one major drawback, its mobility decreases rapidly with an increase in doping concentration. To eliminate this issue, the understanding of factors that limiting mobility is a prerequisite. But till date, there are very limited resources with detailed understanding of mobility limiting factors in solution grown TCO. Here in this report, with the morphological, optical and electrical investigations, the mobility limiting factor comes out to be surface related property and assigned to be the defects related to surface adsorbed oxygen and oxygen species at the surface. Furthermore, we have modified the surface to remove the surface adsorbed oxygen species by a low temperature (70 °C) simple solution process. Surface modified sample shows more than two orders of improvement in resistivity without any significant change in the transparency in visible range.

Repeated bending durability evaluation of ZnO and Al-doped ZnO thin films grown on cyclo-olefin polymer for flexible oxide device applications

ZnO and Al-doped ZnO (AZO) thin films were fabricated on cyclo-olefin polymer (COP) substrates with different thicknesses for flexible device applications, and their bending strength was evaluated using surface observations and two-terminal resistance measurements. The fabricated films were subjected to repeated bending endurance tests up to 10 000 cycles. As COP substrate thickness was decreased, surface distortion decreased, two-terminal resistance fluctuations decreased, cracks in SiO2 buffer layer suppressed, and bending strength improved. When COP substrate thickness was 50 μm, variation in two-terminal resistance was negligible, and an oxide thin film device with durability of 10 000 cycles was successfully fabricated. Crystallinity evaluation using X-ray diffraction and mechanical characterization using nanoindentation were performed. This study discusses ZnO and AZO thin films for flexible device applications in relation to their surface conditions, electrical properties, crystallinity, and mechanical properties in cyclic bending tests. The results indicate that these thin films are useful materials for flexible devices.

Influence of the Al-Doped ZnO Sputter-Deposition Temperature on Cu(In,Ga)Se2 Solar Cell Performance.

Heterojunction Cu(In,Ga)Se2 (CIGS) solar cells comprise a substrate/Mo/CIGS/CdS/i-ZnO/ZnO:Al. Here, Al-doped zinc oxide (AZO) films were deposited by magnetron sputtering, and the substrate temperature was optimized for CIGS solar cells with two types of CIGS light absorbers with different material properties fabricated by three-stage co-evaporation and two-step metallization followed by sulfurization after selenization (SAS). The microstructure and optoelectronic properties of the AZO thin films fabricated at different substrate temperatures (150–550 °C) were analyzed along with their effects on the CIGS solar cell performance. X-ray diffraction results confirmed that all the deposited AZO films have a hexagonal wurtzite crystal structure regardless of substrate temperature. The optical and electrical properties of the AZO films improved significantly with increasing substrate temperature. Photovoltaic performances of the two types of CIGS solar cells were influenced by changes in the AZO substrate temperature. For the three-stage co-evaporated CIGS cell, as the sputter-deposition temperature of the AZO layer was raised from 150 °C to 550 °C, the efficiencies of CIGS devices decreased monotonically, which suggests the optimum AZO deposition temperature is 150 °C. In contrast, the cell efficiency of CIGS devices fabricated using the two-step SAS-processed CIGS absorbers improved with increasing the AZO deposition temperature from 150 to 350 °C. However, the rise in AZO deposition temperature to 550 °C decreased the cell efficiency, indicating that the optimum AZO deposition temperature was 350 °C. The findings of this study provide insights for the efficient fabrication of CIGS solar cells considering the correlation between CIGS absorber characteristics and AZO layer deposition temperature.

Optimizing the Morphology, Structure and Optoelectronic Properties of Aluminum Doped Zinc Oxide Thin Films Developed via Wet Chemical Synthesis

Transparent conducting oxides (TCOs) are wide band gap semiconductors having found their use in optoelectronics, flexible electronics, flat panel displays, electrochromic windows, transparent heater windows, and many more. Aluminum (Al) doped zinc oxide (AZO) is an important TCO material which is being widely investigated for such applications. Its optoelectronic properties can be tuned by adjusting the Al content. In this work we study the variation patterns of the electrical conductivity and the optical transparency of AZO thin films with altering the Al content between 0 and 8 at%. The AZO thin films were prepared by wet chemical synthesis from its stabilized sol of zinc acetate dihydrate and aluminum nitrate nonahydrate dissolved in an ethanol and methanol mix. The morphological, electrical, and optical characteristics of these films were explored employing optical microscopy, Hall effect measurements, and UV-Vis-NIR spectrophotometry, respectively. We found out that annealing induces cracks into the AZO thin films and can severely degrade its electrical conductivity. Therefore, it’s imperative to control the Al content as well as the film morphology and structure. Before studying the effects of the Al content, the cracks were mitigated by optimizing the deposition and annealing conditions. The films were spin coated from its sol at 3000 RPM for 30 seconds. The films were dried at 100 °C and were subsequently annealed at 450°C. Since annealing induced cracks, therefore three coats were applied and annealed each time to mitigate the number of transverse cracks across the thickness of the film. The crack minimization was also confirmed by the enhancement in electrical conductivity. For the uniform crack-free AZO films, the Al doping was found to significantly modify the electronic behavior of the films. We expect an initial increase in the conductivity up to around 2 at% Al doping beyond which a decrease in conductivity is expected due to Al2O3 formation.

Environmentally Benign Structural, Topographic, and Sensing Properties of Pure and Al-Doped ZnO Thin Films.

In the present research work, Zn1–xAlxO thin films with varying proportions of Al (x = 0.00, 0.01, 0.02, and 0.03) are prepared by a chemical sol–gel spin-coating technique. The crystal structural, morphological, and humidity-sensing properties of the synthesized Zn1–xAlxO thin films, with varying concentrations of Al (x = 0.00, 0.01, 0.02, and 0.03), were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM); a special humidity-controlled chamber was designed for the humidity-sensing studies. In structural and phase analyses, XRD patterns of Zn1–xAlxO thin films show a hexagonal wurtzite crystal structure. The average crystallite sizes of Zn1–xAlxO thin films were calculated and found to be ∼18.00, 22.50, 26.30, and 29.70 nm using the X-ray diffraction (XRD) pattern. The surface morphology of Zn1–xAlxO Al (x = 0.00, 0.01, 0.02, and 0.03) thin films obtained from AFM micrographs analysis indicates the modification of the spherical grains into nanorods, which were distributed throughout the surface of the films. The SEM image of 3 wt % Al-doped ZnO nanomaterials also shows that spherical nanoparticles changed to nanorod-like structures with a high packing density. Furthermore, increasing the Al-doping concentration from 0 to 3 wt % in ZnO NPs shows lower hysteresis loss, less aging effect, and good sensitivity in the range of 9.8–16.5 MΩ/%RH. The sensitivity of the sensing materials increased with increasing Al-doping concentration, which is very useful for humidity sensors.

Surface properties of Al-doped ZnO thin film before and after CF4/Ar plasma etching

Al-doped ZnO (AZO) is considered as an alternative to transparent conductive oxide materials. Patterning and achieving a stable surface are important challenges in the development and optimization of dry etching processes, which must be overcome for the application of AZO in various devices. Therefore, in this study, the etch rate and surface properties of an AZO thin film after plasma etching using the adaptive coupled plasma system were investigated. The fastest etch rate was achieved with a CF4/Ar ratio of 50:50 sccm. Regardless of the ratio of CF4 to Ar, the transmittance of the film in the visible region exceeded 80%. X-ray photoelectron spectroscopy analysis of the AZO thin film confirmed that metal-F bonding persists on the surface after plasma etching. It was also shown that F eliminates O vacancies. Consequently, the work function and bandgap energy increased as the ratio of CF4 increased. This study not only provides information on the effect of plasma on AZO thin film, but identifies the cause of changes in the device characteristics during device fabrication.

Temperature-tuned optical bandgap of Al-doped ZnO spin coated nanostructured thin films

Al-doped ZnO (AZO) nanostructured thin films were produced by spin coating of AZO ink. The structural characteristics were determined by x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD plot showed well-defined and intensive diffraction peaks belonging to hexagonal crystal structure. AZO thin films were observed in the form of nanostructure with size varying generally between 20 and 30 nm in the SEM image. The room temperature bandgap energies of undoped and Al-doped ZnO nanostructured films were obtained as 3.32(7) and 3.35(3) eV, respectively. Temperature-tuned bandgap energy characteristics of AZO films were revealed applying transmission experiments by varying the sample temperature. The temperature-bandgap energy dependency was studied by Varshni and Bose-Einstein expressions and optical parameters of AZO films were revealed.

Effect of Varying Doping Level on Structural, Electrical and Optical Properties of Al Doped ZnO Spin Coated Films

The synthesis and characterization of spin-coated Al-doped ZnO (AZO) thin films with varying Al concentrations (0%, 5%, 10%, 15% and 20%) onto glass substrates have been demonstrated in this paper. The structural, electrical and optical properties of the spin-coated thin films have been investigated by Scanning Electron Microscopy (SEM), Energy Dispersive X-Ray (EDX) analysis, Van Der Pauw method and UV-visible spectroscopy. The EDX study shows well-defined peaks which confirm the presence of only Zn, O and Al and no other impurities in the films. The increase of Al and decrease of Zn weight percentages with increasing doping level confirms the effective substitution of Zn by Al. SEM of the surfaces of the films shows that undoped ZnO films contain particle agglomeration which is reduced with Al doping and the surfaces of the films gradually became more uniform. The thickness of the AZO films varied from 86 to 699 nm with increasing Al doping concentration. The electrical conductivity of the films increased up to ~ 7 × 10-2 (Ω.cm)-1 due to doping with 5% Al concentration. The optical transmittance highly increased above 95% in the visible range with the introduction of Al dopant and it kept rising with the increase of Al concentration. The optical energy band gap of undoped ZnO increased from 3.275eV to 3.342 eV with 5% Al doping.

A novel soft deposition methodology for textured ZnO:Al thin films as efficient transparent conductive oxide layers

In this paper, a novel soft deposition methodology was developed for depositing Al-doped ZnO (AZO) thin films as Transparent Conducting Oxide (TCO) layer. The new proposed methodology could help in reducing the process time and cost that could subsequently be useful for industrial scalability purposes. In general, the effect of substrate distance (dTS), Ar gas flow (FAr), and sputtering power (PW) were analyzed on structural, morphological, optical, and electrical properties. Films exhibiting the best electrical values (3.2 × 10−4 Ω*cm, 21.3 cm2 V−1 s−1, and 9.2 × 1020 cm−3) were deposited at FAr of 5 sccm, PW of 45 W, and room temperature. Additionally, AZO thin films deposited inside the high-density zone (HDZ, dTS < 6 cm) of the plasma sputtering exhibited a textured surface with crater-like morphology, and it was found to be more evident in films deposited at reduced FAr (5 sccm) and low PW (45 W) values. The present methodology could be of great interest for the low-cost production of AZO thin films with a textured surface obtained in a one-step DC sputtering process for modern optoelectronic applications such as flexible solar cells and electroluminescent devices.

Characterization of Pure and Al Doped ZnO Thin Films Prepared by Sol Gel Method for Solar Cell Applications

Characterization of Pure and Al Doped ZnO Thin Films Prepared by Sol Gel Method for Solar Cell Applications