Conductive Aluminum-doped Zinc Oxide Research Articles

A Novel High-Dynamic a-Si:H Multicolor pin-Detector with ZnO:Al Front and Back Contacts

ABSTRACTConventional silicon sensors provide insufficient capability for color reproduction by producing three color signals (RGB) to fit their pixel sensitivities to that of the human eye. Photo detector devices based on a-SiC:H and a-SiGe:H offer the opportunity to shift spectral sensitivity continuously by varying external bias voltages. Based on the measurement results of Rieve et al. [2] we determined the absorption coefficients of a group of a-SiC:H and a-SiGe:H layers to simulate the penetration depth of photons at different energies into the device structure [3]. Knowing the indices of absorption, refraction and extinction, it is possible to engineer diodes in such a way that accumulations of charge carriers are generated at varying device depths. The use of common chromium (Cr) cathodes avoids a sharp falling edge of the sensitivity towards longer wavelengths. The use of low reflective and highly conductive aluminum-doped zinc oxide (ZnO:Al) as a material for the back contact of the diodes suppresses Fresnel reflections and has been verified experimentally. A dynamic range of more than 100 dB at 1000 lux could be obtained by I-/V- measurements of a-Si:H pin diodes fabricated between two ZnO:Al contacts. Optically the detectors show maxima between 440 nm and 630 nm with reduced Fresnel reflections and a very low leakage current of 10-8A/cm2 at -5 V bias voltage. Present research efforts concentrate on the verification of the optical properties and exact thicknesses of the chosen layers as well as on the development of a front and back illumination device structure which ensures a continuous spectral shift of the photosensitivity. This work requires extensive bandgap engineering and offers good prospects to improve security imaging tools as well as chemical analysis systems.

Novel etching method on high rate ZnO:Al thin films reactively sputtered from dual tube metallic targets for silicon-based solar cells

Highly transparent and conductive aluminum-doped zinc oxide thin films (ZnO:Al) were reactively sputtered from metallic targets at high rate of up to 90 nm m/min. For the application as transparent light scattering front contact in silicon thin film solar cells, a texture etching process is applied. Typically, it is difficult to achieve appropriate etch features in hydrochloric acid as the deposition process must be tuned and the interrelation is not well understood. We thus introduce a novel two-step etching method based on hydrofluoric acid. By tuning the etch parameters we varied the surface morphology and achieved a regular distribution of large craters with the feature size of 1–2 μm in diameter and about 250 nm in depth. Microcrystalline silicon single junction solar cells (μc-Si:H) and amorphous/microcrystalline silicon (a-Si:H/μc-Si:H) tandem solar cells with high efficiency of up to 8.2% and 11.4%, respectively, were achieved with optimized ZnO:Al films as light scattering transparent front contact.

Preparation and Hydrophobic Processing of Aluminum-Doped Zinc Oxide Films

In this research, transparent and conductive aluminum-doped zinc oxide (AZO) films were prepared on glass substrates by metal-organic chemical vapor deposition (MOCVD). A nanostructured hydrophobic layer of fluorocarbon (FC) compounds was formed on the films by low-temperature dielectric barrier discharge plasma enhanced chemical vapor deposition (DBD-PECVD), as hydrophobic processing. Scanning electron microscopy (SEM) and contact angle analyzer were used to characterize and analyze the surface morphology, structure and hydrophobicity of these samples. The results indicate that the hydrophobicity of transparent conductive AZO films was enhanced by the deposition of the FC film.

Transparent and Conductive Aluminum Doped Zinc Oxide Films Sputtered from Aerogel Nanopowders

Transparent and Conductive Aluminum Doped Zinc Oxide Films Sputtered from Aerogel Nanopowders

Impedance Control of Reactive Sputtering Process in Mid-Frequency Mode with Dual Cathodes to Deposit Al-Doped ZnO Films

Aluminum-doped zinc oxide (AZO) films were deposited on glass substrates at 300°C by reactive mid-frequency (mf, 50 kHz) magnetron sputtering using dual magnetron cathodes with aluminum–zinc alloy targets. In order to keep the very high deposition rate stable in the transition region of the reactive sputtering system, the reactive gas (O2) flow was controlled using the “discharge impedance feedback system”, where the discharge current value was used to control the O2 flow. The highest deposition rate for the transparent conductive AZO films achieved by this dual magnetron sputtering (DMS) system was 242 nm/min, which was higher by one order of magnitude than that achieved by the conventional reactive sputtering system. The lowest resistivity of the AZO film obtained by such a high deposition rate was 4.4×10-4 Ωcm. The structure and electrical properties of the films varied systematically by controlling the discharge current in the transition region using this system.

Al-Doped ZnO Films Deposited by Reactive Magnetron Sputtering in Mid-Frequency Mode with Dual Cathodes

Aluminum-doped zinc oxide (AZO) films were deposited on heated (300°C) glass substrates by reactive mid-frequency (mf, 50 kHz) magnetron sputtering using dual magnetron cathodes with aluminum–zinc alloy targets. In order to keep the very high deposition rate, reactive gas control using plasma emission was carried out to stabilize the discharge in the “transition region” of the reactive sputtering system. The highest deposition rate for the transparent conductive AZO films by this dual magnetron sputtering (DMS) system was 290 nm/min, which was higher than that for the conventional reactive sputtering system by one order of magnitude. The lowest resistivity of the AZO film was 3.9×10-4 Ω cm. The structure and electrical properties of the films could be controlled systematically by the reactive gas control system in the transition region.

Preparation and properties of transparent conductive aluminum-doped zinc oxide thin films by sol–gel process

Highly conductive and transparent aluminum-doped zinc oxide thin films have been prepared from the solution of zinc acetate and aluminum nitrate in ethanol by the sol–gel process. The effect of changing the aluminum-to-zinc ratio from 0 to 5 at. % and annealing temperature from 0 °C to 700 °C in air has been investigated. The resistivities of thin films were measured as a function of annealing temperature and also as a function of aluminum dopant concentration in the solution. As-deposited films have high resistivity and high optical transmission. Annealing of the as-deposited films in air leads to a substantial reduction in resistivity. The films have a minimum value of resistivity of 1.5×10−4 Ω cm for 0.8 at. % aluminum-doped zinc oxide and a maximum transmission of about 91% when deposited on glass substrates. X-ray diffraction measurements employing Cu Kα radiation were performed to determine the crystallinity of the ZnO:Al films which showed that the films were polycrystalline with a hexagonal structure when annealed at 500 °C.

The effect of deposition temperature on the properties of Al-doped zinc oxide thin films

Transparent and conductive aluminum-doped zinc oxide films have been prepared by RF reactive magnetron sputtering with different substrate temperatures. The structural characteristics of the films were investigated by the X-ray diffractometry, scanning electron microscopy, atomic force microscopy and transmission electron microscopy, while the electric and optical properties of the films were studied by the Hall measurement and optical spectroscopy, respectively. It has been found that all of the films deposited were flat and smooth with a c-axis preferred orientation perpendicular to the substrate. The lowest resistivity obtained in this study was 4.16×10 −4 Ω cm for the films deposited at the substrate temperature of 250°C. By calculating the mean free path of electrons, ion impurity scattering is considered to be the dominant factor for the decrease of conductivity. Optical transmittance measurement results show a good transparency within the visible wavelength range for the films deposited at substrate temperatures above 200°C.

Effects of aluminum content and substrate temperature on the structural and electrical properties of aluminum-doped ZnO films prepared by ultrasonic spray pyrolysis

Transparent conductive aluminum-doped zinc oxide (AZO) films were prepared by an ultrasonic spray pyrolysis method. A vertical type hot wall furnace was used as a reactor in the deposition system Zinc acetate dissolved in methanol was selected as a precursor. The substrate temperature was varied from 180 °C to 240 °C. Aluminum (Al) was doped into ZnO films by incorporating anhydrous aluminum chloride AlCl3 in the zinc acetate CH3CO2)2Zn solution. The proportion of the Al in the starting solution was varied from 0 wt % to 3.0 wt %. The crystallographic properties and surface morphologies of the films were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The resistivity of the films was measured by the Van der Pauw method, and the mobility and carrier concentration were obtained through Hall effect measurements. Transmittance was measured in the visible region. The effects of substrate temperature and aluminum content in the starting solution on the structural and electrical properties of the AZO films are discussed.

Transparent and conductive aluminum doped zinc oxide films prepared by mid-frequency reactive magnetron sputtering

In this study, aluminum doped zinc oxide films were prepared by reactive mid-frequency (MF) magnetron sputtering (Leybold TwinMagTM) at deposition rate of approx. 9 nm/s and substrate temperature of 100 to 300°C. Process stabilization in the metallic mode was performed by the control of plasma impedance due to adjustment of oxygen flow. Metallic Zn:Al targets with different aluminum content were used. ZnO:Al films prepared by this technique exhibit resistivity of 3.0×10−4 Ω cm at 200°C substrate temperature and 4.8×10−4 Ω cm at 100°C, respectively, for films of approx. 500 nm thickness. The optical, electrical and structural properties of these films were investigated by means of optical spectroscopy (UV-VIS-NIR), X-ray diffraction, atomic force microscopy, Hall mobility and conductivity measurements. Electron probe micro analysis and secondary ion mass spectroscopy were used for chemical characterization.

Sol‐Gel Preparation of Transparent and Conductive Aluminum‐Doped Zinc Oxide Films with Highly Preferential Crystal Orientation

Transparent aluminum‐doped zinc oxide (ZnO) films were prepared via the sol‐gel method on silica‐glass substrates from 2‐methoxyethanol solutions of zinc acetate and aluminum chloride that contained monoethanolamine. Dip coating was conducted at room temperature, with substrate withdrawal rates of 1.2‐7.0 cm/min. After each deposition, the films were heat‐treated in air at 200°‐450°C for 10 min (pre‐heat‐treatment). After six to fourteen layers had been deposited, the films were then subjected to annealing in air at 500°‐800°C for 1 h (the first post‐heat‐treatment), followed by annealing in nitrogen at 500°‐700°C for 15 min to 4 h (the second post‐heat‐treatment). All the films obtained were transparent and showed only an extremely sharp ZnO (002) peak in the X‐ray diffractometry (XRD) patterns. The effects of the aluminum content, the substrate withdrawal speed, and the heat‐treatment conditions on the electrical resistivity of the films were studied. All these factors strongly affected the resistivity. The lowest resistivity value (6.5 10‐3 Omegacm) was achieved in a film that contained 0.5 at.% aluminum, prepared with a low substrate withdrawal speed (1.2 cm/min), and a pre‐heat‐treatment of individual layer at 400°C in air and a post‐heat‐treatment of the entire film at 600°C in air, followed by a post‐heat‐treatment at 600°C in nitrogen. These preparation parameters also affected the degree of crystal orientation, which was revealed by the intensity of the ZnO (002) XRD peak. Higher crystal orientation was effective in reducing the film resistivity, whereas the higher grain‐packing density and possible aluminum segregation were thought to have positive and negative effects, respectively, in reducing the resistivity.

Textured aluminium-doped ZnO thin films prepared by magnetron sputtering

The electrical properties of RF magnetron-sputtered aluminium-doped zinc oxide (AZO) films are studied. It is seen that the properties are closely related to their structural properties and doping incorporation. The highly conductive milky AZO films with a wedge-like surface consist of very small crystal grains. It is interesting to note that texturization is obtained in this case at a film thickness less than . At a substrate temperature of , texturization occurs and the resistivity obtained after hydrogen treatment is . This result is very significant and it may accelerate the application of inexpensive AZO films in hydrogenated amorphous silicon solar cells.