Al-doped ZnO Films Research Articles

Atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) growth of undoped and aluminium-doped ZnO thin film using hot wall reactor

In this contribution, a hot wall reactor via economic atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD) was adopted for Un-doped and Al-doped Zinc oxide films were deposited on borosilicate glass and silicon substrates. To avoid the use of an expensive vacuum system, all experiments were realized at atmospheric pressure. The chemical reagents used for this experiment are Zinc acetylacetonate (Zn(acac)2) and aluminium acetylacetonates (Al (acac)2) under atmospheric conditions. The obtained films are characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and Uv-–vis spectrometer, respectively. As results, it is found that The un-doped ZnO films are polycrystalline. However, a significant enhancement in the intensity of the relevant (100) reflection is observed when Zinc oxide films are doped with Al. It is also observed that the Al-doped Zinc oxide films present higher transparency in the visible region and resistivities of 2.55 ohm cm and 1.44 ohm cm for un-doped and Al doped films respectively

Effects of substrate temperature on thermal stability of Al-doped ZnO thin films capped by AlOx

Effects of substrate temperature on the thermal stability of Al-doped ZnO (AZO) films have been studied. Degradation of electrical properties of AZO films by annealing under flowing N2 gas depends on their crystallinity controlled by the substrate temperature. A thin AlOx capping layer was employed to passivate the thermal degradation of the AZO layer. A strong correlation between Zn desorption and reduction in carrier concentration was observed. Thermal desorption of Zn was prevented by the AlOx layer, retaining carrier concentration. With the AlOx capping layer, the reduction in Hall mobility was prevented in samples with good c-axis orientation, while the reduction in Hall mobility was still observed in poor c-axis oriented films. However, the reduction was smaller than that in bare AZO films. The dependence of Hall mobility evolution on the substrate temperature, and therefore, on crystallinity, strongly suggests the impact of grain boundary scattering on thermal degradation. An increase in optical mobility, which was evaluated from optical spectra using the Drude model, with annealing temperatures, supports the conclusion that an increase in grain boundary scattering by annealing caused the degradation of Hall mobility. The increase in grain boundary scattering induced by Zn desorption was prevented by the capping layer, while contributions of domain alignment and other segregation of defects to the grain boundary scattering, which depend on the substrate temperature retained, leading to different evolutions of Hall mobility.

Optimization of a Zinc Oxide Photoelectrode for Dye-Sensitized Solar Cells

In this study, the influence of the seed-layer thickness of photoelectrodes on the performance of dye-sensitized solar cells was investigated. Al-doped ZnO films deposited on fluorine-doped tin oxide substrates by using facing targets sputtering systems of various thicknesses were used as seed layers, and ZnO rod array photoelectrodes were synthesized on the AZO seed layers via the hydrothermal process. The crystallinity of the ZnO photoelectrode was improved, and the average diameter of the ZnO rod increased from 100 ± 10 to 250 ± 10 nm as the thickness of the seed layer increased from 200 to 600 nm. The maximum fill factor, open-circuit voltage, current density, and power conversion efficiency of the ZnO rod array photoelectrode with a seed layer of thickness 600 nm were 31.08%, 0.53 V, 5.53 mA/cm2, and 0.91%, respectively.

Arrangement of AZO-CZO n-p heterojunction thin film by electrospinning method; microstructural study and photocatalytic properties

The electrospun n-p heterojunction thin films (TFs) of Al-doped (AZO) and Cu-doped ZnO (CZO) were synthesized, and the photocatalytic properties were investigated with two arrangements of blended single layer (BSL) and individual double layers (IDL). The nanofibrous scaffolds were created by cross-linking the 15–20 nm nanoparticles. The BSL showed a higher c/a ratio of ZnO structure. The BSL had a higher density and hydrophobicity. The photoluminescence studies revealed the extra bands in the BSL film attributing to the donor role of interstitial Al and Cu. The BSL exhibited a higher optical absorption. Mott-Schottky analysis confirmed n and p-type AZO and CZO, respectively. The bandgap was severely influenced by the arrangement of the depletion region, in which the more separate charge paths inside the BSL film reduced the bandgap to 3.08 eV in comparison with 3.13 eV for IDL. The kinetic study showed a better photocatalytic activity for the BSL film.

Optical Characterization of Al-doped ZnO Films via Sol-gel Method Using Spectroscopic Ellipsometry

Background: Al-doped ZnO thin films are considered as a promising alternative to ITO in optoelectronic applications. In this work, Al-doped ZnO thin films were prepared using sol-gel spin coating technique. Experimental: The optical properties of the films such as refractive index, extinction coefficient, dielectric function and the absorption coefficient were examined using spectroscopic ellipsometry method in the wavelength range of 300 to 900 nm. The effect of Al doping on ZnO thin films with different Al concentrations was significant. Tauc relation was used to estimate the optical band gap energy of the films. Results: The calculated values of band gap energy were obtained between 3.10 to 3.25 eV. Also the fraction of voids was calculated using Aspnes theory. Conclusion: The free carrier concentration value was obtained in the order of 1019 cm-3.

Electrical and Optical Properties of Al-Doped ZnO Transparent Conductive Oxide Films Prepared via Radio Frequency Magnetron Co-Sputtering System.

In this study, the optical and electrical properties of a transparent conductive oxide (TCO) film synthesized via the radio frequency (RF) magnetron co-sputtering of Al-doped ZnO (AZO) and ZnO targets on a glass substrate were investigated. In the visible region, the resistivity, transmittance, and carrier concentration of the TCO film are influenced by the ratio of Al doping. The samples were prepared using two targets with the same deposition condition, except several different power levels on an AZO target to obtain different Al compositions in the film. The power range was 100-160 W in 20 W steps on the AZO target with a constant 50 W power level on the ZnO target. The electrical and optical characteristics of the film were measured using several apparatuses. The cross-section of the films was measured with via field emission scanning electron microscopy (FESEM) to determine the thickness of the film. The electrical and optical properties of the AZO films were measured via Hall measurement and UV-visible spectroscopy. The structural characteristics of the AZO films were confirmed by Raman spectroscopy.

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.

Design strategy and interface chemistry of ageing stable AZO films as high quality transparent conducting oxide

In this work, an attempt has been made to produce high quality Al-doped ZnO (AZO) transparent conductors by pulsed direct current (DC) magnetron sputtering under varied process conditions, namely sputtering power, sputtering pressure, doping level, and deposition time. The idea is to develop stable AZO films with excellent electro-optical properties, which can replace indium tin oxide (ITO) as transparent conducting oxide. The films are deposited on glass and polyethylene terephthalate (PET) substrates and characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDS) for material properties. The electronic and optical properties are investigated using UV–Vis-NIR absorption spectroscopy, photoluminescence spectroscopy, four probe resistivity and Hall measurements, which facilitate the design strategy to achieve promising electro-optical properties. AZO films under optimized process conditions possess sheet resistance below 5 Ω/sq. and maximum visible light transmittance close to 90%. The lowest value of electrical resistivity is found to be 2.1 × 10−4 Ω·cm. The combination of low electrical resistivity and high optical transparency leading to the figure of merit of the order of 10−2 Ω−1 is critically dependent on the process conditions of sputtering as well as doping contents. High free carrier concentration in AZO films leads to the Burstein-Moss shift and this observed blue shift of the absorption spectrum as compared to that of ZnO with native defects depends on the doping contents and the process conditions. This work further explores the role of interface chemistry to achieve high stability of AZO films under ambient oxidizing conditions. The adsorption kinetics of cationic rhodamine 6G and anionic methyl orange at AZO surface with varying adsorption sites and ageing studies on the sheet resistance reveal that the excellent stability of AZO films is delicately linked to the process conditions as well as the content of the dopant material used in the preparation of the AZO sputtering targets. The findings of the study confirm that the sheet resistances of selective AZO films undergo no change and fixed at 4.5 Ω/sq. even for ageing period of over six months.

Al/F codoping effect on the structural, electrical, and optical properties of ZnO films grown via atomic layer deposition

Aluminum/fluorine-codoped zinc oxide (AFZO) thin films were prepared on silicon and glass substrates through atomic layer deposition at 150 °C. Their structural, electrical, and optical properties were investigated as functions of the F/Al codoping ratio. The X-ray diffraction analysis revealed that the preferred growth orientation changed depending on the codoping with F, that is, from (0002) for Al-doped ZnO (AZO) films to (1010) for the AFZO ones. The electrical resistivity of the AFZO films (around 5.0 × 10–4 Ω∙cm) was lower than that of the AZO ones; this was mainly attributed to an increase in the n-type carrier concentration (from 4.11 × 1020 cm3 for AZO to 5.86 × 1020 cm3 for AFZO) due to the substitution of the Zn and O sublattice sites by, respectively, the Al and F atoms. The enhanced carrier concentration affected also the optical property of the AFZO films according to the Moss–Burstein shift. The carrier mobility was similarly improved (from 6.96 cm2/V∙s for AZO to 21.2 cm2/V∙s for AFZO) by the passivation of the oxygen vacancies via the F-doping.

Optical and electrical properties of Al-doped ZnO thin films by atomic layer deposition

The present work highlights the low temperature deposition of Al-doped ZnO (AZO) thin films by atomic layer deposition (ALD). The effects of Al doping concentration and substrate temperature on structural, electrical and optical properties of AZO thin films have been systematically studied. X-ray diffraction analysis demonstrates that the preferred orientation of AZO thin films alters from (100) to (002) with the increase of both substrate temperature and Al doping concentration, which may affect the electrical property of the AZO. The resistivity of the AZO reduces with increasing the temperature from 160 to 220 °C and then remains broadly stable (in the range of 220–260 °C), while the resistivity of the AZO thin films exhibits a sharp fall followed by a slow rise with Al concentration increasing from 0 to 3.57 at.% (atom %). The AZO film deposited at substrate temperature of 220 °C and Al content of ~ 2.17 at.% shows the lowest resistivity of 8.33 × 10–4 Ω cm and the high transmittance of ~ 95%, respectively. The work indicates a promising low temperature deposition of AZO thin films as transparent conductive oxide film.

Dopants and induced residual stress-controlled thermoelectric properties of ZnO thin films

We investigate the structure, optical properties, and thermoelectric power in terms of residual stresses induced in the undoped ZnOand X-doped ZnO (X = Al, Ga) films. The results show that the different native seed layers self-grown during the initial growth cause compressive stress for ZnO and Ga-doped ZnO films and tensile stress for Al-doped ZnO films. The band-gap of the investigated films follows the Burstein–Moss (BM) effect. It is noted that the ZnO film with the lowest compressive stress has a better PF value below 275 °C. The GZO film is more stable at higher temperatures; therefore, it has a better PF value above 275 °C. The residual stress induced in the investigated films is demonstrated through the formation of a self-grown seed layer under the bombardment of high-energy sputtered Ga, Al, Zn, and O species onto the substrate and during film growth.

Investigations of the characteristics and chemical bonds of Al and F co-doped ZnO films as a function of F2flow rate

In this study, we investigate the effect of fluorine (F[Formula: see text] flow rate on the crystal characteristics and chemical bonds of Al-Doped ZnO (AFZO) Films. At first, 1 wt% Al2O3was mixed with 99 wt% ZnO to form the Al-doped ZnO ceramic target, the radio frequency (RF) magnetron sputtering method was used to deposit the AFZO films by introducing different flow rates of F2gas using F2/(Ar + F2) mixing atmosphere during the deposition process. F2flow rates (F2/(Ar + F2)) were changed in the range of [Formula: see text]%, and F2ratio was controlled using the mixing gases of (F2(5%) + 95% Ar) and pure Ar. AFZO films were deposited on the EagleXG glass and the films’ thickness was about 330 nm by controlling the deposition time. After AFZO films were deposited, their crystalline structure and electrical characteristics were well investigated. Finally, a comparative study of surfaces of AFZO films was investigated using X-ray photoelectron spectroscopy (XPS) to find the variations of the chemical bonds as the deposition temperature and F2flow rate were changed.

Hybrid GaAs nanowire-polymer device on glass: Al-doped ZnO (AZO) as transparent conductive oxide for nanowire based photovoltaic applications

Al-doped ZnO (AZO) is pursued as an alternative low-cost transparent conductive oxide (TCO) to expensive ITO. Atomic layer deposition grown AZO films showing resistivity of 5 × 10−3 Ωcm and transmittance >85% in the visible region are reported. Au-assisted GaAs nanowires are grown directly on an optimized AZO coated glass and a GaAs nanowire-polymer hybrid device on glass is demonstrated which confirms that the as-grown GaAs nanowires form a perfect ohmic contact to AZO film. The device shows that AZO can be used as transparent electrode as well as low-cost growth platform for GaAs NWs. Finally, a simple device idea is proposed to fabricate optically transparent GaAs nanowire based solar cells on low-cost glass.

Ammonia gas sensing properties of Al doped ZnO thin films

Aluminium (Al)-doped ZnO thin films are synthesised by chemical spray pyrolysis technique and investigated their gas sensing properties. X-ray diffraction analysis of the films revealed the formation of hexagonal-phase (wurtzite) of ZnO with orientation along (002) plane. Interestingly, the lattice parameters of Al-doped ZnO films showed a decreasing trend with Al doping, suggesting incorporation of Al in the crystal lattice of ZnO. A significant change in both the volume of the unit cell and bond length with an increase in Al concentration is observed. The surface morphological analysis suggested the formation of hexagonal-like ZnO, which transform into spherical particles with Al doping. The energy dispersive spectra confirm the existence of Al, Zn and O. The optical absorption analysis showed that the bandgap of ZnO samples decreases from 3.28 eV to 3.21 eV with Al content in ZnO. The ammonia gas sensing properties of ZnO and Al-doped ZnO are studied at 25 ppm concentration of ammonia gas in air at 100 °C temperature, suggesting reasonable gas sensing characteristics.

Zn-Al Layered Double Hydroxide Thin Film Fabricated by the Sputtering Method and Aqueous Solution Treatment

Zn-Al layered double hydroxides (LDHs) were synthesized herein via a simple process. First, Al-doped ZnO film was deposited onto a glass substrate using the facing target sputtering system. Successful synthesis of the Zn–Al LDH was achieved via a treatment process using an aqueous solution which contains NO3− anions. X-ray diffraction analysis confirmed that it was consistent with the previous Zn–Al LDH synthesis experiment data, and the calculated d-value was 9.1 Å. Scanning electron microscopy observations revealed that the as-synthesized sample had a plate-like structure.

Role of defects and microstructure on the electrical properties of solution-processed Al-doped ZnO transparent conducting films

A cheaper, non-vacuum-based routes are required for the large-scale implementation of TCOs in solar cells and LEDs. Generally, solution-based processing routes result in lower transparency and greater resistivity. To achieve electrical conductivity and transparency via solution processing route, greater insight into the effect of processing and microstructure/defects on the electrical and optical properties is needed. In this work, Al-doped ZnO films were deposited on glass substrates by sol–gel spin coating route. The formation of wurtzite structure was confirmed, and the crystallite size of the films was estimated by X-ray diffraction (XRD) pattern analysis. Greater than 85% transparency in the films was obtained in visible and near IR regime as examined by UV–Vis spectroscopy. An increase in the band gap was observed with increasing Al concentration from 0 to 3 at.%. The electrical properties were evaluated by the Hall measurement, and obtained resistivity was in the order of 10–2 Ωcm. The presence of defect states and their co-relation with the electrical properties were investigated by photoluminescence spectroscopy and X-ray photoelectron spectroscopy.

Reduced doping efficiency of aluminium in Al-doped ZnO film: role of excess aluminium and deposition rate

This work addresses the role of excess Al and deposition rate in reducing the doping efficiency of Al+3 cations in Al-doped ZnO (AZO) films. In this regard, AZO films were deposited on the soda lime glass substrates using co-sputtering from ceramic AZO/ZnO and metallic Al targets. Whereas power to ZnO/AZO target was kept constant, power to Al target was varied to see the effect of varying Al concentration (and deposition rate) on its doping efficiency. Here, Al doping efficiency in ZnO is found to increase initially with increasing deposition rate and Al content, beyond which it decreased because of the formation of higher AlxOy in combination with increased trapped Al in the amorphous regions of these films. Moreover, a detailed discussion on the mechanism of the deposition rate and thereby induced variation in Al doping efficiency is discussed and then related to the optoelectronic properties of these films.

Al-Doping of ZnO Thin Films Deposited by Spray Pyrolysis

Undoped and Al-doped zinc oxide (AZO) layers, greatly transparent and with high mobility, have been prepared at low substrate temperature using a chemical reactive spray technique. The effect of aluminum incorporation in the zinc oxide (ZnO) lattice has been characterized by means of X-ray powder diffraction, Raman spectroscopy, electrical and optical measurements. AZO layers reveal a hexagonal wurtzite structure whose lattice parameter decreases with increasing Al content and whose crystal quality decreases for Al content higher than 2%. Low resistivity AZO films have been found for 3% Al content. Additionally, AZO layers exhibit a blue shift of the optical gap with the increase of Al content that is attributed to the Burstein–Moss effect due to the increase of the charge carrier concentration. A density of free electron greater than 2.0 × 1019 cm–3 is obtained for the AZO thin films with 5% Al. Our results encourage the use of AZO films deposited at a low temperature as electrodes and optical windows in photovoltaic devices.

Optimization of Al-doped ZnO films by RF magnetron sputtering at room temperature for Cu (In, Ga) Se2 solar cells

In this work, Al-doped zinc oxide (AZO) thin films were deposited by RF magnetron sputtering with various RF power at room temperature. The effect of RF power on the structural, electrical, and optical properties of AZO thin films were investigated by XRD, SEM, UV–Vis–NIR spectroscopy and Hall measurements. The lowest resistivity of 1.8×10−3 Ω·cm was obtained at the highest RF power of 450 W. The average optical transmittance is about 90% in the visible range and above 80% in the range of 300-2000 nm. CIGS thin-film solar cells were prepared using the AZO films as the windows layer and an efficiency of 15.36% in CIGS solar cell has been achieved.

Influence of annealing duration on structural, optical and electrical properties of AZO thin films grown on corning glass by dc magnetron sputtering

Al doped ZnO (AZO) thin films were deposited using dc magnetron sputtering on corning glass substrate has successfully done. After deposition, the samples were annealed in vacuum ambient at temperature of 300 °C with variation in annealing duration of 0, 30, and 50 minutes, respectively. The structural, optical and electrical properties of these films have been investigated as a function of annealing duration. Thin films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis spectrometer and I-V measurements. AZO thin films without annealing treatment was showed amorphous structure. However, after annealed at 30 and 50 minutes, the films obtained were polycrystalline with a hexagonal wurtzite structure and preferred orientation in the (002) plane. Average optical transmittance of the AZO thin films was over 85%. In Addition, the electrical resistivity is reduced from 1.85 × 106 Ωcm to 1.93 × 102 Ωcm when applying annealing treatment. It’s can be concluded, the annealing treatment in vacuum ambient was improved the crystallinity and simultaneously improved optical and electrical properties of AZO films.