Investigation of Al-doped ZnO (AZO) ceramic targets and thin films

In this work, the ZnO thin film, the Al-doped ZnO (AZO) thin film (0.98M ZnO, 0.02M Al) and the (Al,Co) co-doped ZnO thin film (AZO:Co) (0.95M ZnO, 0.02M Al, 0.03M Co) were deposited on the glass substrate by the Sol–Gel method. We fabricated a sample of the ZnO thin film, a sample of the AZO thin film and three samples of AZO:Co thin films. The spin-coating was used to deposit thin film on the glass substrate. The ZnO and the AZO thin films were annealed at 450[Formula: see text]C while three samples of the AZO:Co thin films were annealed at 300[Formula: see text]C, 450[Formula: see text]C and 600[Formula: see text]C in air for 60 min, respectively. In order to prepare three samples of the AZO:Co thin films, we deposited the (Al,Co) co-doped ZnO on the glass substrate for 20 s then all samples were per-heated at 80[Formula: see text]C for 10 min. we repeated this deposition process five times for each sample. Finally, three samples were annealed at 300[Formula: see text]C, 450[Formula: see text]C and 600[Formula: see text]C in air for 60 min, respectively. The procedure to prepare of the ZnO and AZO thin films was like the AZO:Co thin films except that the annealing temperature was 450[Formula: see text]C. The structural and optical properties of the thin films were investigated by X-ray diffraction technique, UV-Vis spectrophotometer and Field Emission Scanning Electron Microscopy (FESEM). Results indicated that (Al,Co) co-doping in the ZnO thin film improve the optical transmission while changes in the lattice structure is small with respect to the AZO thin film. Also, the AZO:Co thin film which was annealed at 450[Formula: see text]C exhibited simultaneously the high thickness and high optical transmission.

Due to various applications in electronic and optical fields, considerable efforts have been made to develop ZnO based semiconducting materials. ZnO has a wide and direct band gap of 3.37 eV, large exciton binding energy (60 meV), high mechanical and thermal stabilities, and good electrical conductivity. Therefore, ZnO thin films are intensively investigated and applied in solar cells, piezoelectrical devices, gas and biological sensors, catalysts, and high-efficiency short-wavelength optoelectronic nanodevices. To further enhance the properties of ZnO, researchers have been synthesizing ZnO films doped with aluminum or indium. Al doped ZnO (AZO) thin films have been highly developed featuring non-toxicity, low material cost, good thermal stability, high optical transmittance, and good electrical conductivity. [1, 2, 3] Many techniques have been used to deposit AZO thin films currently, such as sol gel processing, magnetron sputtering, pulsed laser deposition (PLD), chemical vapor deposition (CVD), and atomic layer deposition (ALD). Among these film deposition methods, ALD has been extensively investigated for the deposition of thin films of metals, alloys, semiconductors, and oxides. Compared with other techniques, ALD is considered as a novel technique for the deposition of AZO films due to its characteristic feature of self-limiting surface saturating reactions in each ALD cycle. ALD technology consequently and accurately controls the layer thickness with unprecedented atomic level precision, as well as the stoichiometry, composition, uniformity of large areas and maintains excellent conformity over complex surface nanostructures. The other advantage of ALD technology is to introduce each ALD chemical precursor sequentially separated by time to react on the surface of the structure. The concentration of dopant in the ZnO films can be precisely controlled by the number of ALD cycles, so that the physical properties of AZO films, such as carrier density and conductivity, can be adjusted to meet the requirements of different applications. In this study, a Savannah 100 cross-flow thermal ALD reactor was used for the deposition of Al doped ZnO thin films. The precursors of aluminium, zinc and oxygen used in the ALD process for the synthesis of Al doped ZnO thin films were trimethylaluminium (TMA: Al2(CH3)6), diethylzinc (DEZ: (C2H5)2Zn) and DI water. The growth temperature of AZO thin films was maintained at 250 oC. The flow rate of N2, used as a carrier gas, was accurately controlled at 20 sccm. In order to deposit ALD AZO films with various percentages of aluminium, TMA cycles were introduced once in every 30 pulses of DEZ and H2O injection. After deposition, AZO films were annealed in air at 500 oC, 600 oC, and 700 oC for 30 min, respectively. X-ray diffraction (XRD) was performed to characterize the crystal structure of ALD ZnO and AZO films. Field emission scanning electron microscopy (FE-SEM) was used to inspect the morphology of AZO films before and after annealing. The surface roughness was analyzed by atomic force microscopy (AFM). The compositions of AZO thin films were analyzed by energy dispersive X-ray spectroscopy (EDS). The sheet resistivity of AZO films was determined by using four-point probe measurements. Raman spectroscopy and photoluminescence (PL) were used to investigate the properties and detect structures of ALD AZO films. The band gaps of AZO films prepared by the ALD process were analyzed from PL spectroscopy results. The materials property and defect structure of ALD AZO films were investigated by Raman spectroscopy.

We have grown no-doped and Al-doped ZnO thin films with different Al element composition by ultrasonic spray pyrolysis (USP). The deposition by USP was performed with the temperature of 450 °C, ultrasonic frequency of 1.7 MHz, and under atmosphere ambient for 15 minutes on top of Si substrate. Structurally, X-ray diffraction (XRD) pattern of non-doping and Al-doped ZnO (AZO) thin film have polycrystalline hexagonal wurtzite and amorphous. Optical absorbance confirmed that AZO thin films with polycrystalline phase have higher absorbance than amorphous. According to the frequency-dependent Nyquist plot of non-doping and AZO thin films at room temperature, we believed the proper concentration of Al in ZnO thin films can be used to improve electron transfer ability.

Preparation and characterization of La 0.5Sr 0.5CoO 3 /Al-doped ZnO thin film heterojunctions

Influence of oxygen argon ratio on the structural, electrical, optical and thermoelectrical properties of Al-doped ZnO thin films

A series of sol–gel derived Al-doped ZnO (AZO) thin films with rapid thermal annealing process at low temperature were studied to examine the influence of annealing temperature and the Al doping concentration on their microstructure, electrical and optical transport properties. Crystalline AZO thin films were obtained following an annealing process at temperatures between 400 and 600 °C for 10 min in argon gas ambient. AZO thin films with Al doping of 1 at% were oriented more preferentially along the (002) direction, and have larger grain size and lower electrical resistivity, while the highest average optical transmittances of 92% were observed in AZO films with Al doping of 2 at%. With the annealing temperature increasing from 400 to 600 °C, the grain size of AZO films increased, the optical transmittance became higher, and the electrical resistivity decreased to a lowest value of 1.2 × 10−4 Ω cm resulting from the increase of the carrier concentration and the mobility.

Optical and electrical characteristics of Al-doped ZnO thin films prepared by aqueous phase deposition

In this study Al-doped Zinc Oxide (AZO) thin films were successfully deposited on the flexible Polymethyl methacrylate (PMMA) substrate by RF magnetron sputtering. The effects of RF power on the crystal structure, morphology, thickness and optical properties of AZO thin films have been investigated. The AZO thin films were analyzed using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), DEKTAK 3 profilometer, UV–Visible spectroscopy and room temperature photoluminescence (PL) spectroscopy. The XRD patterns show that increase of RF power leads to increase in the predominant direction along (100) and crystal plane of hexagonal ZnO. Moreover, the transmittance of thin films decreased from 76% to 61% and optical bang gap varied among 3.34[Formula: see text]eV to 3.22[Formula: see text]eV with increasing RF power. The PL spectra show excellent light-emitting characteristics: 375[Formula: see text]nm, 428[Formula: see text]nm, 467[Formula: see text]nm and 505[Formula: see text]nm. The results indicate that the peak intensity increases with increasing RF power from 80[Formula: see text]W to 180[Formula: see text]W.

This paper reports characterization of Al-doped ZnO (AZO) thin films deposited on glass substrates at room temperature by a low energy (1.3 kJ) plasma focus device using a ZnO target with an Al content of 3wt%. A particular focus of investigation is on properties relevant to the usage of thin films as hydrogen gas sensors. Indeed, the dependence of angular position of substrate on structural, morphological and gas sensing properties of AZO thin films is investigated. The results obtained from X-ray diffraction (XRD) reveal that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure. Also XRD results indicate strong dependence of crystallinity of deposited thin films on angular position of samples. Scanning electron microscopy and atomic force microscopy results reveal the structure growth and enhancement of surface roughness with decreasing the angle with respect to anode axis. The experiments and measurements involving the AZO deposited thin films towards hydrogen were carried out at different operating temperatures within 150–400 °C for various concentrations of hydrogen in air. The H2 sensing response enhanced with concentration and operating temperature, and reached its maximum at 300 °C to 1000 ppm concentration of hydrogen gas. The results also indicated shorter response times for samples deposited at larger angular positions.

On the properties of aluminium doped zinc oxide thin films deposited on plastic substrates from ceramic targets

Due to the simultaneously superior optical transmittance and low electrical resistivity, transparent conductive electrodes play a significant role in semiconductor electronics. To enhance the electrical properties of these films, one approach is thickness increment which degrades the optical properties. However, a preferred way to optimize both electrical and optical properties of these layers is to introduce a buffer layer. In this work, the effects of buffer layer and film thickness on the structural, electrical, optical and morphological properties of AZO thin films are investigated. Al-doped zinc oxide (AZO) is prepared at various thicknesses of 100 to 300 nm on the bare and 100 nm-thick indium tin oxide (ITO) coated glass substrates by radio frequency sputtering. Results demonstrate that by introducing ITO as a buffer layer, the average values of sheet resistance and strain within the film are decreased (about 76 and 3.3 times lower than films deposited on bare glasses), respectively. Furthermore, the average transmittance of ITO/AZO bilayer is improved nearly 10% regarding single AZO thin film. This indicates that bilayer thin films show better physical properties rather than conventional monolayer thin films. As the AZO film thickness increases, the interplanar spacing, d(002), strain within the film and compressive stress of the film in the hexagonal lattice, decreases indicating the higher yield of AZO crystal. Moreover, with the growth in film thickness, carrier concentration and optical band gap (Eg) of AZO film are increased from 4.62 × 1019 to 8.21 × 1019 cm−3 and from 3.55 to 3.62 eV, respectively due to the Burstein-Moss (BM) effect. The refractive index of AZO thin film is obtained in the range of 2.24–2.26. With the presence of ITO buffer layer, the AZO thin film exhibits a resistivity as low as 6 × 10−4 Ω cm, a sheet resistance of 15 Ω/sq and a high figure of merit (FOM) of 1.19 × 104 (Ω cm)−1 at a film thickness of 300 nm. As a result, the quality of AZO thin films deposited on ITO buffer layer is found to be superior regarding those grown on a bare glass substrate. This study has been performed over these two substrates because of their significant usage in the organic light emitting diodes and photovoltaic applications as an enhanced carrier injecting electrodes.

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.

Transparent and conductive Al-doped ZnO (AZO) thin films were deposited on a glass substrate by direct current sputtering. In a previous study, we noted the influence of time on structural and electrical characteristics of films. In the present paper, the effect of a two-year aging under ambient conditions on the optical properties was investigated. A global improvement of the optical properties of AZO thin films was observed. The optical transmittance spectra revealed a high transmittance more than 90% in the Vis-NIR regions and a high absorption in the ultraviolet range. It is assumed that the crystallinity segregation leads to the decrease of optical scattering. The results from the optical measurements showed a reorganization of the structure leading to the degradation of the structural homogeneity. Nevertheless, the evolution of the figure of merit shows that Al-doped ZnO is a good candidate for the manufacturing and the commercialization of transparent conducting oxide devices.

Al-doped zinc oxide (AZO) thin films are prepared on polycrystalline fluorine-doped tin oxide-coated conducting glass substrates from nitrates baths by the electrodeposition process at 70 °C. The electrochemical, morphological, structural and optical properties of the AZO thin films were investigated in terms of different Al concentration in the starting solution. It was found that the carrier density of AZO thin films varied between −3.11 and −5.56 × 1020 cm−3 when the Al concentration was between 0 and 5 at.%. Atomic force microscopy images reveal that the concentration of Al has a very significant influence on the surface morphology and roughness of thin AZO. X-ray diffraction spectra demonstrate preferential (002) crystallographic orientation having c-axis perpendicular to the surface of the substrate and average crystallites size of the films was about 33–54 nm. With increasing Al doping, AZO films have a strong improved crystalline quality. As compared to pure ZnO, Al-doped ZnO exhibited lower crystallinity and there is a shift in the (002) diffraction peak to higher angles. Due to the doping of Al of any concentration, the films were found to be showing >80 % transparency. As Al concentration increased the optical band gap was also found to be increase from 3.22 to 3.47 eV. The room-temperature photoluminescence spectra indicated that the introduction of Al can improve the intensity of ultraviolet (UV) emission, thus suggesting its greater prospects in UV optoelectronic devices. A detailed comparison and apprehension of electrochemical, optical and structural properties of ZnO and ZnO:Al thin films is done for the determination of optimum concentration of Al doping.

We investigate the effect of the flow rate of oxygen and hydrogen gases on the characteristics of OLEDs (organic light emitting diodes) with Al-doped ZnO (AZO) anodes. The AZO thin films are deposited by radio frequency (RF) magnetron sputtering under different ambient gases (Ar, Ar + O2, and Ar + H2) at 300°C. To investigate the influences of the ambient gases, the flow rate of oxygen and hydrogen in argon was varied from 0.1 sccm to 5 sccm. The AZO thin films were preferentially oriented to the (002) direction regardless of the ambient gase used. The electrical resistivity of the AZO thin films increased with increasing O2 flow rates, whereas the electrical resistivity decreased sharply under an Ar + H2 atmosphere and was nearly the same regardless of the H2 flow rate used. The change of electrical resistivity with changes in the ambient gas composition was mainly interpreted in terms of the charge carrier concentration rather than the charge carrier mobility. All the films showed an average transmittance of over 85% in the visible range. The optical band gap of the AZO films increased with increasing H2 flow rates, whereas the optical band gap of the AZO films deposited in an O2 atmosphere decreased with increasing O2 flow rates. The current density and the luminance of the OLED devices with AZO thin films deposited in 5 sccm of H2 ambient gas were the highest among all the films. The optical band gap energy of AZO thin films plays a major role in OLED device performance, especially the current density and luminance.