Transparent Aluminum-doped Zinc Oxide Research Articles

Optical and electrical properties of Al doped ZnO thin film with preferred orientation in situ grown at room temperature

Abstract To optimize the process and obtain highly conducting and transparent Aluminum-doped zinc oxide (AZO) thin films, AZO films were deposited on glass substrates at room temperature by Radio-frequency (RF) magnetron sputtering with various Argon flow rates. The influences of Argon flow rate on structure, morphology, optical, electrical and photoluminescence properties of AZO films were investigated by varying the Argon flow rate from 36 to 68 sccm. The best quality AZO film with resistivity 1.39 × 10−3 Ω cm, sheet resistance 8.2 Ω/sq and 84.2% average visible transmittance was prepared at 44 sccm for 30 min. Also, the self-heating effect of target was investigated by preparing AZO films for 10 min and 20 min at 44 sccm, 180 W and 1.0 Pa. The influence of increasing structural quality actually affected by Argon flow rate was more prominent on carrier concentration than mobility. The schematic illustration of microstructural evolution was proposed. The average growth rate of around 60 nm/min demonstrated the self-heating effect of target was weak and could be ignored.

Effect of potential voltages on key functional properties of transparent AZO thin films prepared by electrochemical deposition method for optoelectronic applications

Abstract

ITO-free metallization for interdigitated back contact silicon heterojunction solar cells

We report on two different approaches to fabricate interdigitated back contact silicon heterojunction solar cells without using indium tin oxide (ITO). The standard ITO/Ag backend is either modified by replacing ITO with aluminum-doped zinc oxide (AZO) or completely replaced by a sole aluminum (Al) layer. The very transparent AZO enhances the optical properties at the rear side resulting in an increase in short-circuit current density. The efficiency of the AZO cells remains on the level of the ITO ones, as the fill factor drops slightly. On the contrary, the contact resistivity of annealed Al, in comparison to ITO and AZO, to the emitter and BSF layers is much lower, thus the fill factor is increased. Despite lower open circuit voltages, cells with Al achieve efficiencies of up 22 %, a gain of 0.5 %abs compared to the ITO reference.

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.

Impact of self-assembled monolayer assisted surface dipole modulation of PET substrate on the quality of RF-sputtered AZO film

In this study, we fabricated the electron donating/withdrawing group functionalized organosilane self-assembled monolayers (SAMs) on transparent polyethylene terephthalate (PET) flexible substrate followed by the deposition of aluminum doped zinc oxide (AZO) using RF magnetron sputtering at room temperature. The effect of different SAMs on transparent PET substrates and AZO films were studied by contact angle (CA), X-ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM), X-ray diffraction (XRD), Field-Emission scanning electron microscope (FE-SEM), Hall measurement and UV–vis spectroscopy (UV–vis). The results presented that the surface dipole (i.e. electron-donating/withdrawing) of different SAMs functionalized PET substrates affected the quality of the AZO films which deposited on top of them. The crystallinity, the charge mobility, and the carrier concentration of the AZO improved when the film was deposited on the PET functionalized with electron donating group, which was possibly due to favored interaction between electron donating group and Al ions.

Room temperature deposition of homogeneous, highly transparent and conductive Al-doped ZnO films by reactive high power impulse magnetron sputtering

Aluminum doped zinc oxide (AZO) films have been deposited using reactive high power impulse magnetron sputtering (HiPIMS) and reactive direct current (DC) magnetron sputtering from an alloyed target without thermal assistance. These films have been compared in terms of their optical, electrical and structural properties. While both DC and HiPIMS deposited films show comparable transmittance, their electrical properties are significantly improved by the HiPIMS process. The HiPIMS deposited films show a low resistivity down to the order of 10−4Ωcm with a good homogeneity across the substrate, making them potential candidates for electrodes in solar cells. The density of electrons reached up to 11×1020cm−3, making ionized impurities the main scattering defects. This improvement of the film properties can be related to the specific plasma/target interactions in a HiPIMS discharge. This allows the process to take place in the transition mode and to deposit highly conductive, transparent AZO films on large surfaces at low temperature. While the overall oxygen content is above that of stoichiometric ZnO, higher localization of oxygen is found at the interfaces between crystalline domains with substoichiometric composition.

Deposition and Characterization of Al:ZnO Thin Films for Optoelectronic Applications

Transparent aluminum-doped zinc oxide (Al:ZnO) thin films have been successfully synthesized on silicon substrates at room temperature using a sol–gel spin-coating method. The structural and optical properties and surface morphology of the synthesized films were characterized using x-ray diffraction (XRD) analysis, ultraviolet–visible (UV–Vis) spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, micro-Raman spectroscopy, and atomic force microscopy (AFM). The prepared Al:ZnO retained the hexagonal wurtzite structure of ZnO. FTIR and Raman spectra clearly revealed a major peak at 437 cm−1, associated with the ZnO bond. UV–Vis spectra showed that the Al:ZnO films were transparent from the near-ultraviolet to near-infrared region. The effect of film thickness on the physical and optical properties of the Al:ZnO thin films for 2.0 at.% aluminum concentration was investigated. Measurements revealed that the film transparency, optical energy bandgap, Urbach energy, extinction coefficient, and porosity varied with the film thickness. The energy bandgap values for the prepared thin films increased in the range of 3.18 eV to 3.2 eV with increasing film thickness.

Characteristics of atomic layer deposited transparent aluminum-doped zinc oxide thin films at low temperature

Various aluminum-doped zinc oxide (AZO) films were prepared on Si substrate by atomic layer deposition (ALD) at 100 °C. The effect of the composition of AZO films on their electrical, optical characteristics, structural property and surface topography was investigated. The appearance of electrical resistivity shows their semiconducting properties. In most of the visible light band, all the AZO films present transparency of more than 80 %. Al doping suppresses the AZO film crystallization. When the Al doping concentration increases up to 3.95 at%, the AZO film has some small multicrystal grains with random orientation. Al doping improves the roughness of i-ZnO film. The root mean square (RMS) roughness of samples prepared by ALD is much smaller than that prepared by radio-frequency magnetron sputtering reported.

Doping strategies for highly conductive Al-doped ZnO films grown from aqueous solution

Aluminum-doped zinc oxide (AZO) thin films are prepared by a low temperature (100 °C) aqueous solution deposition method with a subsequent UV post-deposition treatment at 140 °C. Film growth is governed by the retrograde solubility of zinc–ammine complexes at basic conditions (pH 11.4). Aluminum was introduced into the film as a dopant by co-precipitation, either using an aluminum metal foil or aluminum nitrate as a precursor. As the presence of Al ions in the solution influences the film morphology as well as the opto-electronic properties, different profiles of the dopant supply were examined and optimized with respect to the optical and electrical properties of the AZO layer. It was found that independent of the chosen aluminum precursor a linearly increasing dopant concentration in the solution is favorable for achieving dense, highly transparent (<8% absorption between 400 and 900 nm) AZO thin films with a lowest resistivity of 3.4 × 10−3 Ω cm.

Effects of the pre-annealing temperature on structural and optical properties of sol–gel deposited aluminium doped zinc oxide

Highly transparent aluminium doped zinc oxide (AZO) thin films were deposited by the sol–gel deposition technique. The effects of various pre-sintering temperatures on the film׳s structural, optical and electrical properties were investigated using scanning electron microscopy, X-ray diffraction spectroscopy, photoluminescence emission measurements, UV–vis spectroscopy and electrical characterization. It was found that the pre-sintering temperature greatly affected the film׳s structural properties and resulted in an almost 100% increase in grain size within the investigated experimental window. Photoluminescence emission and photovoltaic measurements confirmed that the best optoelectronic properties of AZO thin films can be achieved with a pre-sintering temperature of 400°C.

Fabrication and characteristics of high-performance and high-stability aluminum-doped zinc oxide thin-film transistors

Fully transparent aluminum-doped zinc oxide (AZO) thin-film transistors (TFTs) were successfully fabricated on glass substrates at room temperature. Superior properties, such as a high saturation mobility of 59.3 cm2 V−1 s−1, a positive threshold voltage of 1.3 V, a steep subthreshold swing of 122.9 mV/dec, an off-state current on the order of 10−12 A, and an on/off ratio of 2.7 × 108, were obtained. The electrical properties of the AZO TFTs were successively studied within a period of six months. Small property degenerations could be observed from the test results obtained within the study period, which proved the high-performance and high-stability characteristics of AZO TFTs. Furthermore, hysteresis loop scanning of AZO TFTs was performed, and a small hysteresis could be detected in the scanning curves, which suggested the superior properties of a dielectric and a channel-insulator interface. Lastly, we succeeded in manufacturing an organic LED (OLED) flat panel display panel driven by AZO TFTs and obtained an excellent display effect from it. We believe that AZO TFTs are a promising candidate successor to Si-based TFTs in next-generation flat panel displays.

Transparent conducting oxide top contacts for organic electronics

A versatile method for the deposition of transparent conducting oxide (TCO) layers directly onto conjugated polymer thin film substrates is presented. Using pulsed laser deposition (PLD) we identify a narrow window of growth conditions that permit the deposition of highly transparent, low sheet resistance aluminium-doped zinc oxide (AZO) without degradation of the polymer film. Deposition on conjugated polymers mandates the use of low growth temperatures (<200 °C), here we deposit AZO onto poly-3-hexylthiophene (P3HT) thin films at 150 °C, and investigate the microstructural and electrical properties of the AZO as the oxygen pressure in the PLD chamber is varied (5–75 mTorr). The low oxygen pressure conditions previously optimized for AZO deposition on rigid substrates are shown to be unsuitable, resulting in catastrophic damage of the polymer films. By increasing the oxygen pressure, thus reducing the energy of the ablated species, we identify conditions that allow direct deposition of continuous, transparent AZO films without P3HT degradation. We find that uptake of oxygen into the AZO films reduces the intrinsic charge carriers and AZO films with a measured sheet resistance of approximately 500 Ω □−1 can be prepared. To significantly reduce this value we identify a novel process in which AZO is deposited over a range of oxygen pressures – enabling the deposition of highly transparent AZO with sheet resistances below 50 Ω □−1 directly onto P3HT. We propose these low resistivity films are widely applicable as transparent top-contacts in a range of optoelectronic devices and highlight this by demonstrating the operation of a semi-transparent photovoltaic device.

Highly conductive and transparent aluminum-doped zinc oxide thin films deposited on polyethylene terephthalate substrates by pulsed laser deposition

Highly transparent and conductive aluminum-doped zinc oxide thin films were deposited on low-cost flexible polyethylene terephthalate substrates at room temperature using pulsed laser deposition and the effects of oxygen pressure and film thickness on film properties were investigated. It was found that grain sizes play a greater role only at smaller film thicknesses in affecting carrier mobility. Resistivity changes at larger film thickness can be caused by near surface/interface depletion that affected both mobility and carrier concentration. The inherent film transparency did not change and any reduction in the film transmittance is likely related to a thickness dependent attenuation effect. This means that different transparent conducting oxides should each possess an optimum film thickness, whereby optimized zinc oxide is typically about 100nm. A low resistivity of ~6.6×10−4Ωcm with a high normalized transparency index of >0.9 for a 110±10nm thick room-temperature deposited film was obtained, representing one of the best results obtained to date.

Effect of AZO Substrates on Self-Seeded Electrochemical Growth of Vertically Aligned ZnO Nanorod Arrays and Their Optical Properties

We present a single step and an electrochemical synthesis of vertically aligned ZnO nanorod (NR) arrays, directly on transparent aluminium-doped zinc oxide (AZO) electrodes. The NRs grow from mild, aqueous-based solution at low temperature, with no need for catalysts or additional seed layer. The use of textured AZO as substrate allows for highly effective growth of hexagonally faceted, single-crystalline ZnO NRs along the wurtzitec-axis. The matching of the crystal lattices initiates a self-seeding route, thus the inherent growth habit of the AZO crystallites advances the vertical growth and alignment of NRs. Moreover, the thickness-dependant grain size of the AZO layer provides a valuable feature for tuning the diameter of ZnO NRs grown atop. In the absence of any seed mediator, the interfacial quality is expected to improve significantly. This should enhance the thermal and electrical transport throughout the whole nanostructured transparent electrode. The NR growth was investigated under systematic manipulation of the synthesis variables in order to optimize growth conditions for highly aligned, single-crystalline NRs with a large aspect ratio and a good optical quality. The structure and optical property of the AZO/ZnO NR ensembles were characterized by atomic force microscopy, scanning electron microscopy, X-ray diffraction, photoluminescence, and ultraviolet-visible transmission spectroscopy.

Synthesis of ZnO:Al Transparent Conductive Thin Films Using Sol-gel Method

Tin-doped indium oxide (ITO) deposited by vacuum process with excellent optical and electrical properties have been generally used as transparent conductive oxide thin films. In view of high cost and difficulty in recycles for vacuum process, zinc oxide films synthesized by wet chemical process are therefore regarded as a more suitable candidate to replace ITO. In this work, sol-gel method was used to synthesize transparent aluminum-doped zinc oxide (ZnO:Al, AZO) with 450nm thickness for large scale production of CIGS solar cell. The experimental parameters included different solvents, the propylene glycol methyl ether (PGME) to replace the usually used solvent ethylene glycol monomethyl ether (EGME) which is toxic, the amount of Al dopant (ranged from 0 to 2 at%), and various annealing temperatures. The structure, electrical and optical properties of AZO thin films are investigated by a field emission scanning electron microscope (FE-SEM), transmission electron microscope (TEM), X-ray diffractometer (XRD), four-point probe (FPP) and ultraviolet visible light spectroscope (UV-VIS) etc. The experimental results showed that the AZO films were wurtzite crystalline with c-axis preferred orientation and had > 85% transmittance and lower electrical resistivity after annealed in N2 atmosphere. The optimum AZO samples had low electrical resistivity of 1.94¥10-2 Ω-cm, which were synthesized with PGME solvent, 1 at% Al-doping and annealed at 600°C in N2 atmosphere and 500°C in 95 N2/5 H2 atmosphere for one hour, separately.

Compositional study of vacuum annealed Al doped ZnO thin films obtained by RF magnetron sputtering

Aluminum doped zinc oxide (AZO) thin films were obtained by RF magnetron sputtering. The effects of deposition parameters such as power, gas flow conditions, and substrate heating have been studied. Deposited and annealed films were characterized for composition as well as microstructure using x ray photoelectron spectroscopy and x ray diffraction. Films produced were polycrystalline in nature. Surface imaging and roughness studies were carried out using SEM and AFM, respectively. Columnar grain growth was predominantly observed. Optical and electrical properties were evaluated for transparent conducting oxide applications. Processing conditions were optimized to obtain highly transparent AZO films with a low resistivity value of 6.67 × 10−4 Ω cm.

Low-emitting surfaces prepared by applying transparent aluminum-doped zinc oxide coatings via a sol–gel process

Polycrystalline transparent conductive oxide thin films based on aluminum-doped zinc oxide (AZO) have been prepared on soda-lime glass by using an inorganic sol–gel process and the dip-coating technique. The multilayered films were crystallized on the substrate and subsequently annealed in a reducing atmosphere to enhance the number of free charge carriers. Significant characteristics of the functional coatings, such as crystallinity, surface smoothness, layer thickness, electrical conductivity, as well as the optical properties in a large spectral range between 0.25 and 35μm were analyzed. The obtained samples are highly transparent with a visual transmittance above 0.85 and show specific resistivities of up to 1.6·10−3Ω·cm. Applying an optimized AZO-coating with at most 500nm is sufficient to reduce the surface emittance from 0.89 to less than 0.45 in the infrared.

Synthesis of c-axis preferred orientation ZnO:Al transparent conductive thin films using a novel solvent method

Transparent aluminum doped zinc oxide (ZnO:Al, AZO) conducting thin films with a high-preferential c-axis orientation were synthesized using a new sol–gel formula. The films were deposited using a spin-coating route onto borosilicate glass substrates. We used propylene glycol methyl ether (PGME) as the solvent in place of ethylene glycol monomethyl ether (EGME), which is commonly used because it is easier to deposit onto the substrates. PGME is also superior in terms of health and safety. PGME solvent does not need to settle for several days before use and can be spin-coated as soon as the raw material and solvent are mixed. The effects of this novel solvent on the structural, morphological, electrical and optical properties are discussed using XRD, SEM, a four-point probe and UV–VIS spectrophotometry. It was found that the films produced with PGME showed a high-preferential c-axis orientation and compact microstructure in comparison films produced using EGME. The electrical resistivity of AZO thin films produced with PGME solvent was lowered to 3.474 × 10 − 3 Ω cm after annealing in 95 N 2/5H 2 atmosphere. In addition, the optical transmittances of AZO thin films on glass plates were higher than 90% in the visible wavelength region.

Study of transparent conducting ZnO:Al films deposited on organic substrate by reactive magnetron sputtering

A Zn-Al metallic target (Al 2 wt.%)has been used to prepare conductive and transparent aluminium-doped Zinc oxide(ZnOAl) films on PI substrate by direct current reactive magnetron sputtering.The structure, crystallinity, optical properties, electrical properties and adhesion were investigated using a range of techniques, including AFM, XRD, spectrophotometry, four-point probe and adhesion tester.The optimal films were prepared with a substrate temperature of 150°C, O2/Ar ration of 2:38 and sputtering power of 80W.The infrared emission properties of films and the feasibility for military application were also discussed in this paper. All the results to date demonstrate that magnetron sputtering is a cost-effective and easy to fabricating technique.

Effects of sputtering pressure and thickness on properties of ZnO:Al films deposited on polymer substrates

Highly conducting and transparent aluminum doped zinc oxide (ZnO:Al) thin films have been deposited on polyimide substrate by r.f. magnetron sputtering at room temperature. The influence of sputter pressure and thickness on the structural, electrical, and optical properties of ZnO:Al films deposited on polyimide substrate is reported. The crystallinity and degree of orientation was increased by decreasing the sputter pressure. For higher sputtering pressures an increase on the resistivity was observed due to a decrease on the mobility and the carrier concentration. As the film thickness was increased, the crystallite sizes were increased, but the average transmittance in the wavelength range of the visible spectrum was decreased. The electrical performances of the ZnO:Al films deposited on glass substrates are slightly worse than the ones of the films deposited on polyimide substrates with same thickness. The lowest resistivity of 8.6 × 10−4 Ω cm can be obtained for films deposited on glass substrate with the thickness of 800 nm.