Aluminum-doped Zinc Oxide Films Research Articles

Evaluation of the structural, optical and electrical properties of AZO thin films prepared by chemical bath deposition for optoelectronics

Aluminum doped zinc oxide (AZO) thin films for electrode applications were deposited on glass substrates using chemical bath deposition (CBD) method. The influence of deposition time on the structural, morphological, and opto-electrical properties of AZO films were investigated. Structural studies confirmed that all the deposited films were hexagonal wurtzite structure with polycrystalline nature and exhibited (002) preferential orientation. There is no other impurity phases were detected for different deposition time. Surface morphological images shows the spherically shaped grains are uniformly arranged on to the entire film surface. The EDS spectrum confirms the presence of Zn, O and Al elements in deposited AZO film. The observed optical transmittance is high (87%) in the visible region, and the calculated band gap value is 3.27 eV. In this study, the transmittance value is decreased with increasing deposition time. The room temperature PL spectrum exposed that AZO thin film deposited at (60 min) has good optical quality with less defect density. The minimum electrical resistivity and maximum carrier concentration values were observed as 8.53 × 10−3(Ω cm) and 3.53 × 1018 cm−3 for 60 min deposited film, respectively. The obtained figure of merit (ϕ) value 3.05 × 10−3(Ω/sq)- 1 is suggested for an optoelectronic device.

Effect of Laser Wavelength on AZO Surface Texturing by Direct Laser Processing / Patterning for Thin-Film Silicon Solar Cells Applications

ABSTRACTSurface texturing of transparent conductive oxides is crucial to improve the fraction of incident light trapped in the absorber layer of thin film silicon based solar cells to improve the device performance. In this work, we fabricate and compare periodic, overlapping, and random surface textures and patterns on aluminium doped zinc oxide (AZO) using direct laser processing. The effects of the used laser wavelength, laser operating frequency, and pulse periodicity on the structural, morphological, and optical response of the AZO films were investigated. By optimizing the laser parameters and the associated process conditions, a drastic increase up to 60% in the transmittance haze over the entire solar was achieved.

Single step route to highly transparent, conductive and hazy aluminium doped zinc oxide films.

Light scattering yet transparent electrodes are important for photovoltaics as they increase device efficiency by prolonging light path lengths. Here, we present a novel single step route to highly textured Al doped ZnO thin films on glass substrates that show a minimum resistivity of ∼3 × 10−3 Ω cm and high visible light transmittance of 83% while still maintaining high haze factor of 63%. Roughness was imparted into the ZnO films during the synthetic process using acetylacetone and deionized water as additives. The highly hazy yet visible and near infrared transparent nature of the conductive ZnO:Al films allow it to be potentially used as an electrode material in amorphous and microcrystalline silicon solar cells.

Influence of Acid and Alkali Etching on Sputtered Aluminium Doped Zinc Oxide Films

In order to increase the efficiency of silicon based Thin Film Solar (TFS) cells, it is necessary to use better light management techniques. Texturization of sputtered aluminium doped zinc oxide (Al:ZnO or AZO) films has opened up a variety of ways for optimization of light trapping schemes. Here, AZO has been etched with acid as well as alkali. A difference in optical and electrical characteristics of acid and alkali etched AZO has been observed. By etching AZO with potassium hydroxide (KOH), 82% transmittance (in the visible range), high rms roughness and ∼13Ω/□ sheet resistance have been achieved.

Optimization of Sputtered AZO Thin Films for Device Application

Transparent conducting aluminium doped zinc oxide (AZO) thin films have attracted much attention for wider application in thin film devices such as thin film transistors, solar cells, gas sensors, piezoelectric transducers, etc. owing to their good electrical and optical properties combined with wide energy gap, non-toxicity, good stability and low cost in comparison to the ITO. The AZO thin films have been deposited by DC magnetron sputtering process using 5N pure zinc oxide target doped with 2 wt% of Al2O3 by varying the sputter parameters such as target power, electrode distance and substrate temperature. The measured thickness of the deposited AZO thin films using stylus profiler is in the range of 190 to 1100 nm. The microstructure of the AZO films has been investigated by X-ray diffraction and Scanning Electron Microscope. The AZO thin films have been optimized to a lowest electrical resistivity of 3.7 X 10- 4 Ω cm measured by four-point probe method, and to an average optical transmittance of 84.1% measured in the wavelength range 300-1100 nm using UV-VIS-NIR spectrophotometer. An attempt has been made through this study to fabricate thin film transistors by incorporating the optimized AZO films and the results are discussed in this paper.

Optical and sensing properties of Al doped ZnO nanocrystalline thin films prepared by spray pyrolysis

Nanocrystalline undoped and aluminum doped ZnO films are deposited by spray pyrolysis method. All film samples show polycrystalline nature and hexagonal wurtzite structure of ZnO. Crystallite size decreases with increase in dopant concentration. The lattice constant-a of undoped ZnO thin film is greater than doped samples whereas c-lattice constant is maximum for 3at.% Al doping. Bond length decreases whereas dislocation density increases in doped samples. Doping decreases the transmission of the films and the optical band gap of ZnO is also reduced from 3.28 to 3.18 eV. SEM shows porous spot-like structures which change to web of needle-like structures when dopant is introduced. Surface morphology of the films further evolves to open ends of tubular structures as the doping is increased further. Undoped ZnO shows maximum sensitivity at 400°C for higher concentration of CO2 whereas aluminium doped samples show maximum sensitivity either at 350°C or below for all CO2 concentrations.

Improved Gas Sensing Performance of ALD AZO 3-D Coated ZnO Nanorods

This paper reports an enhancement on the sensing performance of ZnO nanorod ethanol sensors with a new approach by utilizing nested coatings of Aluminum doped ZnO (AZO) thin films by Atomic Layer Deposition (ALD) technology. ZnO nanorods were grown by the hydrothermal method with the ZnO seed layer synthesized on Silicon wafers by ALD. To enhance the sensing performance of ZnO nanorod ethanol sensors, multiple coated AZO thin film 3-D coatings were deposited on the surface of the intrinsic ZnO nanorods by ALD. To investigate the sensing performance of the ZnO nanorods sensor for the detection of ethanol vapor, a gas sensor testing system was designed and built with a sealed reaction chamber and a temperature controller. The demonstrated sensing performance results include the sensing response comparison between ZnO nanorods before and after ALD coatings with AZO films at different temperatures and with various concentrations of input ethanol vapor. The response times and recovery times of ZnO nanorods before and after ALD coatings with AZO thin films were analyzed to investigate the sensing enhancement. The sensing response improvement peaks at 25°C room temperature with approximately 200% enhancement. However, the sensing response improvement decreases as a function of increasing operating temperature.

Electrical, Optical, and Electrochemical Corrosion Resistance Properties of Aluminum-Doped Zinc Oxide Films Depending on the Hydrogen Content

Electrical, Optical, and Electrochemical Corrosion Resistance Properties of Aluminum-Doped Zinc Oxide Films Depending on the Hydrogen Content

The effect of solvent on Al-doped ZnO thin films deposited via aerosol assisted CVD.

Aluminium-doped zinc oxide (AZO) thin films were deposited via aerosol assisted chemical vapour deposition (AACVD) from zinc acetylacetonate and aluminium chloride at 450 °C. The precursor solutions consisted of methanol in a mixture with one other secondary solvent, including toluene, tetrahydrofuran, n-hexane, cyclohexane, and ethyl acetate. The crystal structures, elemental compositions and surface morphologies of the resulting AZO films were determined, as well as the optoelectronic properties. It was found that the more polar solvents enhanced growth in the (002) plane of the wurtzite crystal structure, and that solutions with low viscosities resulted in superior grain growth. The film deposited from a solution consisting of methanol and ethyl acetate displayed the lowest visible transmittance, due to carbon contamination. However, it also exhibited 60% lower resistivity, in comparison to the film deposited using methanol only. This suggests that optoelectronic properties can be tuned for specific photovoltaic devices.

Investigation of the effects of thickness on physical properties of AZO sol-gel films for photonic device applications

Aluminum doped zinc oxide (AZO) films are synthesized on glass substrates using different number of coatings (6, 12, 18 and 24) by the sol-gel dip-coating technique. The effects of thickness on structural, morphological, electrical optical and waveguiding properties of the prepared films are studied through X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), atomic force microscopy (AFM), Hall effect measurements, UV–Visible–NIR spectrophotometer and m-lines spectroscopy (MLS). XRD analysis shows that all AZO films exhibit the hexagonal wurtzite crystal structure with a preferential growth orientation along the c-axis. The intensity of the (002) diffraction peak and crystallite size tend to increase with increase in the thickness indicating an improvement in the crystallinity of the films. Furthermore, the observation of the high-frequency E2 mode in the Raman spectra supports the hexagonal wurtzite structure of ZnO in the films. SEM and AFM images indicate that microstructure parameters, such as grain size and surface roughness depend sensitively on the thickness of the film. The electrical resistivity and carrier concentration are found to increase and decrease, respectively, as the film thickness increased. Optical transmittance spectra demonstrate that all the films exhibit high average transmittance (86.8–89.1%) in the visible spectral range. Moreover, the band gap energy shows a red-shift with the film thickness. MLS measurements at a 632.8 nm wavelength put into evidence that AZO planar waveguides support single and multi-well confined guided modes for both transverse electric (TE) and transverse magnetic (TM) polarizations. Both TE and TM refractive indexes are found to increase with thickness and reach saturation values when the number of coatings is greater than 18. Moreover, the thickest AZO planar waveguide demonstrates a propagation loss lower than 1 dB/cm.

Transparent and Flexible Capacitors with an Ultrathin Structure by Using Graphene as Bottom Electrodes.

Ultrathin, transparent and flexible capacitors using graphene as the bottom electrodes were directly fabricated on polyethylene naphthalate (PEN) substrates. ZrO2 dielectric films were deposited on the treated surface of graphene by atomic layer deposition (ALD). The deposition process did not introduce any detectible defects in the graphene, as indicated by Raman measurements, guaranteeing the electrical performances of the graphene electrodes. The Aluminum-doped zinc oxide (AZO) films were prepared as the top electrodes using the ALD technique. The capacitors presented a high capacitance density (10.3 fF/μm2 at 10 kHz) and a relatively low leakage current (5.3 × 10−6 A/cm2 at 1 V). Bending tests revealed that the capacitors were able to work normally at an outward bending radius of 10 mm without any deterioration of electrical properties. The capacitors exhibited an average optical transmittance of close to 70% at visible wavelengths. Thus, it opens the door to practical applications in transparent integrated circuits.

Effect of solvents on sol–gel spin-coated nanostructured Al-doped ZnO thin films: a film for key optoelectronic applications

Aluminum-doped zinc oxide (AZO) thin films were deposited by sol–gel spin coating technique onto the glass substrates using different solvents such as 2-methoxyethanol, methanol, ethanol and isopropanol. Prepared films were characterized by XRD, Raman spectrum, SEM, UV–visible spectrophotometer, photoluminescence (PL) and electrical studies. XRD studies showed that all the prepared films are hexagonal wurtzite structure with polycrystalline nature oriented along (002) direction. SEM images showed uniform particles of size around 60 nm distributed regularly on to the entire glass substrate. EDX analysis confirmed the composition of grown AZO film consisting of Al, Zn and O elements. The prepared films showed highest optical transmittance ~ 94% in the visible range and band gap 3.30 eV. PL spectra for all AZO films showed a strong UV emission peak at 387 nm. The AZO films prepared using isopropanol solvent showed high carrier concentration and low resistivity values as 1.72 × 1020 cm−3 and 2.90 × 10−3 Ω cm, respectively, with high figure of merit (ϕ) value 8.42 × 10−3 (Ω/sq)−1.

Low-temperature ultrasonic spray deposited aluminum doped zinc oxide film and its application in flexible Metal-Insulator-Semiconductor diodes

In this work, the fabrication and characterization of fully solution-processed flexible Metal-Insulator-Semiconductor (MIS) diodes are presented. The MIS structure was fabricated using aluminum doped zinc oxide and spin-on glass as semiconductor and insulator, respectively. The maximum temperature used was 200°C. The electrical characteristics of the flexible devices show a good agreement with the typical characteristics of a semiconductor diode even while bent to 5mm tensile radius.

Effect of Al doping on performance of ZnO thin film transistors

Abstract In this work, we investigate the Aluminum-doped Zinc Oxide (AZO) thin films and their feasibility as the active layer for thin film transistors (TFTs). A comparison on performance is made between the AZO TFTs and ZnO TFTs. The electrical properties such as saturation mobility, subthreshold swing, and on-to-off current ratio are improved when AZO is utilized as the active layer. Oxygen component of the thin film materials indicates that Al is the suppressor for oxygen defect in active layer, which improves the subthreshold swing. Moreover, based on band structure analyzation, we observe that the carrier concentration of AZO is higher than ZnO, leading to the enhancement of saturation mobility. The microstructure of the thin films convey that the AZO films exhibit much smaller grain boundaries than ZnO films, which results in the lower off-state current and higher on-to-off current ratio of AZO TFTs. The AZO thin films show huge potential to be the active layer of TFTs.

Nanostructure of Al-ZnO Thin Films on ITO/Glass Substrate Prepared via Sol-Gel Spin Coating Process

Solar energy is probably the most important source of renewable energy available today. ZnO is a potential material for the fabrication solar cell. Zinc oxide (ZnO) nano-structure semiconductors have been recently gained much attention in the electronic and optical device applications. ZnO is a compound semiconductor, which has a high extention band gap (E= 3.37 eV) at room temperature (RT) with a Wurtzite crystal structure. In particular ZnO can be employed as the transparent conducting oxide (TCO) in solar cell applications due to its high productivity, non-toxic, low cost, and excellent electrical conductivity. In this study, aluminum doped ZnO (AZO) polycrystalline films have been fabricated on ITO/substrates via a sol-gel spin-coating method. The quantity of aluminum doping in the solution was 4.0 %. After synthesis, the films were pre-heated at 300°C for 25 min and after that the films were inserted in a Tub-furnace and post-annealing at 450°C to 750°C for 1 h. The microstructural and structural properties of AZO films were studied through X-ray diffraction (XRD), UV-Vis NIR and scanning electron microscope (SEM) analysis. The TCO applications of the transmittance of samples were also examined. The results showed that the annealing temperature does not seriously affect on the transmittance of AZO films over the visible range.

A comparative study on physical properties of Al-doped zinc oxide thin films deposited from zinc acetate and zinc acetylacetonate by spray pyrolysis

Herein we present a comparative study on highly transparent, conductive aluminium-doped ZnO (AZO) thin films deposited by pneumatic spray pyrolysis (PSP) of a zinc acetate based solution and a zinc acetylacetonate based solution on soda lime glass. The structural, optical and electrical properties of the films were studied depending on aluminium content in the precursor solution and substrate temperature during deposition (Ts). The solution used to prepare AZO thin films contained 0.2 M zinc acetate or 0.2 M zinc acetylacetonate and aluminium acetylacetonate between 0–15 at.% [Al]/[Zn]. Substrate temperature was varied from 275 to 450 °C. Transmittance and reflectance spectra were recorded by UV-VIS-NIR spectroscopy. Resistivity, mobility and charge carrier density were determined by four point probe measurements in ambient conditions. According to X-ray diffraction analysis, AZO films are highly c-axis oriented until 2–3 at.% [Al]/[Zn] in the spray solution, depending on zinc precursor. Crystallite size is 25–30 nm for AZO films deposited at Ts 400 °C. All AZO films exhibited high average optical transmittance of 76–94% in the visible spectrum 400–800 nm wavelength range. Eg values ranged from 3.32 in undoped ZnO to 3.58 eV in in highly doped AZO films deposited from zinc acetylacetonate due to the formation of secondary AlOx phases. The lowest resistivities were obtained at Ts 400 °C for films deposited from zinc acetate, 3–7.5 at.% [Al]/[Zn] in solution (0.40 Ωcm) and at Ts 400 °C, 3–7.5 at.% [Al]/[Zn] in solution for films deposited from zinc acetylacetonate (0.47 Ωcm).

Preparation of Aluminum doped Zinc Oxide Thin Films on Glass Substrate by Sparking Process and Their Optical and Electrical Properties

Aluminum doped Zinc Oxide (AZO) nanoparticle thin films were deposited on glass substrates by a double tip sparking process. The effect of Al doped ZnO on electrical and optical transmittance properties were studied. The doping ratios of Al into ZnO were 3, 5, 7, 13 and 22 %. SEM images indicated particle size decreased with increasing the Al content after annealing. Raman spectra results of AZO films associated with the hexagonal structure. AZO films have an average transmittance in visible region at 60 %. The energy gap increased with increasing Al content. The minimum resistivity was found at 5 % of Al doping for AZO film.

Influence of AZO stair-like transparent layers on GaN-based light-emitting diodes

The GaN-based light-emitting diodes (LEDs) with various height ratios of aluminum-doped zinc oxide (AZO) stair-like transparent layers are fabricated and comparatively investigated. The characteristics of the LEDs with conventional plane AZO transparent layer (device A) and AZO stair-like transparent layers having height ratios of 1:1:1 (device B), 1.5:1:0.5 (device C), and 0.5:1:1.5 (device D) are compared. Attributed that the lower resistance is formed in the thinner AZO film of the stair-like structure, the current crowding effect is improved for extending the whole current-spreading area. Experimentally, the forward turn-on voltages of the LEDs are reduced from 3.68 V to 3.42 V as the plane AZO transparent layer is processed to form the stair-like transparent layers with height ratio of 1:1:1. In addition, the light luminous flux, output power, external quantum efficiency, and wall-plug efficiency of the device B are enhanced by 30.5, 12.1, 22.2, and 20.7%, respectively, as compared to the traditional device with plane AZO transparent layer.

Low-temperature spray-coating of high-performing ZnO:Al films for transparent electronics

Ultrasonic spray pyrolysis deposition of ZnO-based materials offers an attractive high-throughput low-cost route towards industrial production of high-quality transparent conductive oxide (TCO) thin-films. In this work, undoped and aluminium-doped ZnO films have been grown employing ultrasonic spray pyrolysis at relatively low-temperate (300°C), followed by a post-annealing treatment. The role of Al concentration in the starting solution, as well as the rapid thermal annealing (RTA) atmosphere, were investigated and correlated to the morphological, structural, electrical and optical properties of the films. The remarkable enhancement of electrical conductivity attained here is mainly ascribed to the combined effects of: (1) homogenous incorporation of Al3+ into the ZnO matrix, which enhances crystal quality providing higher electronic mobility; and (2) the RTA which releases the localized electrons caused by oxygen absorption and thereby increases the free carrier density. Under optimum deposition conditions, a low resistivity and a high optical transmittance around 4×10−3Ωcm and 87%, respectively, were obtained. The application of the RTA post-process after low temperature growth has several advantages relative to the direct growth at high temperature (usually 400–575°C), such as shorter growth time and lower cost associated to the spray pyrolysis equipment requirements and usage. The results suggest that the electrical and optical properties of the ZnO:Al films can be further improved for solar cell applications by controlling the temperature of the post-deposition annealing in reducing atmosphere.

Effect of moving speed during in-line pulsed direct-current magnetron sputtering deposition on the structural and optical properties of Al-doped ZnO films

Aluminum-doped zinc oxide (AZO) films were deposited on glass substrates with pulsed direct current (DC) magnetron sputtering. Deposition was carried out in both static and dynamic modes. The pulsed DC sputtering process provided a stable deposition of AZO films without arcing, resulting in good crystallinity, unlike simple DC magnetron sputtering. The growth characteristics of the films sputtered on various moving speeds of the in-line dynamic mode are investigated and compared to those of the static mode. AZO films deposited at a moving speed of 120cm/min showed a dominant (002) orientation, a smooth surface roughness, and a higher figure of merit (FOM), of which properties are similar to those of the static mode. Comparisons of the characteristics obtained at several moving speeds showed that better crystallinity was obtained at the higher moving speed of 120cm/min. This improvement could be attributed to an averaging effect, which is due to more frequent interruption and initiation of the growth at the higher moving speed. Optical transmission characteristics were compared between the static and dynamic modes, and analyzed in terms of the complex indices of refraction and interference effects of light.