Aluminum-doped Zinc Oxide Layer Research Articles

Reversible wettability control of ZnO thin films synthesized by hydrothermal process on different buffer layers

The wetting behaviors of zinc oxide (ZnO) thin films, which were grown on aluminum-doped zinc oxide (AZO) and tin-doped indium oxide (ITO) buffer layers by hydrothermal process, are reported. The ZnO film grown on the AZO layer, which has a morphology containing micro- and nanoscaled features, shows a slight decrease in water contact angle (WCA) after UV illumination. The ZnO film grown on the ITO layer, which is composed of single-scale microrods, exhibits a transition between hydrophobicity and superhydrophilicity under UV exposure for 8h and dark storage for 2 weeks. The wettabilities of the both films are enabled to convert reversibly by alternative heating at 400°C and 100°C. This work may be of huge importance from the viewpoint of self-cleaning and anti-fogging applications based on wettability control of ZnO films.

High-Performance Transparent AZO TFTs Fabricated on Glass Substrate

High-performance transparent low-temperature top-gate type aluminum doped zinc oxide (AZO) thin-film transistors (TFTs) (W/L=100 or 10 μm) are successfully fabricated on glass substrate. All the process temperature is below 100°C. For VG=-2 to 5 V, the TFTs using sputtering deposit AZO layer at room temperature as channel layer exhibits excellent properties, such as a saturation mobility μs of 285 cm2/V·s, a linear field effect mobility μl of 143 cm2/V·s, a threshold voltage Vth of 0.9 V, a steep subthreshold swing of 108 mV/decade, a low off-state current value Ioff of 5×10-13 A, a high ON/OFF ratio of 2×109 and a high transmittance of 82.5%. The results highlight that excellent device performance can be realized in AZO TFTs. Note that this is the best performance of AZO TFT ever reported.

Electrical and Optical Properties of Index-Matched Transparent Conducting Oxide Layers for Liquid Crystal on Si Projection Displays

In this study, the characteristics of several kinds of index-matched transparent conducting oxide layers (TCO) were investigated for applications to liquid crystals on silicon (LCoS) projection displays. The purpose of exploring the characteristics of these materials is to find alternatives to currently used index-matched indium-doped tin oxide (ITO) layers, since index-matched ITO suffers from high production costs and materials shortages. Based on zinc oxide (ZnO) TCO layers, metal dopants such as Al, In, or Ga were added to improve the optical and electrical properties; 15 nm thick ZnO, aluminum-doped ZnO (AZO), indium–gallium-doped ZnO (IGZO), indium-doped ZnO (IZO), and gallium-doped ZnO (GZO) layers were sputtered on glass substrates using radio frequency (RF) and direct current (DC) magnetron sputtering. The measured transparency in the visible radiation range was above 94% for all prepared index-matched TCO layers. Among them, the transparency of AZO layers was the highest, reaching 97.5%. The sheet resistance of the TCO layers was around 100 Ω cm-2, with 82.6 Ω cm-2 being the lowest measured value obtained from a 15 nm thick AZO layer. Furthermore, the sheet resistance uniformity measured by samples with an area of 200×200 mm2 was below 5%. Atomic force microscopy measurement results show that the root-mean-square surface roughness values were lower than 0.01 nm in ZnO and AZO, and 0.128 and 0.261 in IGZO and GZO, respectively. The contact angle, which is another key factor in index-matched TCO-coated substrates, was around 25°, which meets the requirements for LCoS projection display panels. Among the tested TCO layers, AZO exhibited superior characteristics in terms of optical and electrical properties. Therefore, AZO represents an alternative to currently used index-matched ITO layers in LCoS projection displays.

Model for thickness dependence of mobility and concentration in highly conductive zinc oxide

The dependences of the 294 and 10 K mobility μ and volume carrier concentration n on thickness (d=25 to 147 nm) are examined in aluminum-doped zinc oxide (AZO). Two AZO layers are grown at each thickness, one with and one without a 20-nm-thick ZnON buffer layer. Plots of the 10 K sheet concentration ns versus d for buffered (B) and unbuffered (UB) samples give straight lines of similar slope, n=8.36×1020 and 8.32×1020 cm−3, but different x-axis intercepts, δd=−4 and +13 nm, respectively. Plots of ns versus d at 294 K produce substantially the same results. Plots of μ versus d can be well fitted with the equation μ(d)=μ(∞)/[1+d*/(d−δd)], where d* is the thickness for which μ(∞) is reduced by a factor 2. For the B and UB samples, d*=7 and 23 nm, respectively, showing the efficacy of the ZnON buffer. Finally, from n and μ(∞) we can use degenerate electron scattering theory to calculate bulk donor and acceptor concentrations of 1.23×1021 cm−3 and 1.95×1020 cm−3, respectively, and Drude theory to predict a plasmonic resonance at 1.34 μm. The latter is confirmed by reflectance measurements.

Preparation of Naturally Textured Aluminum-Doped Zinc Oxide Films on Flexible Poly(ether sulfone) Substrate by Radio Frequency Magnetron Sputtering

Transparent conducting oxide (TCO) layers of naturally textured aluminum-doped zinc oxide (AZO) were prepared on flexible poly(ether sulfone) (PES) substrates by controlling the radio frequency (RF) magnetron sputtering power without chemical wet etching, and their morphological, optical, and electrical properties were characterized. Layers with crater structures were formed at a single RF power; whereas at high RF power (2000 W), their surfaces resembled that of chemically wet-etched AZO. Layers were also formed by dual RF sputtering powers: sputtering a 0.15-µm-thick layer at high power and a subsequent 0.45-µm-thick layer at low power produced flakes with larger grains or crater-like structures. The measured visible transmittance of all the layers was >85% – sufficient for use in amorphous silicon (a-Si) thin-film solar cells. The measured optical and electrical properties of the naturally textured AZO layers are comparable to those of chemically wet-etched AZO layers, suggesting a possible method for fabricating inexpensive flexible a-Si thin-film solar cells in fewer steps.

Al-doped ZnO thin-film transistor embedded micro-cantilever as a piezoresistive sensor

In this work, an aluminium-doped zinc oxide (AZO) thin film transistor, embedded in a polymer micro-cantilever, is demonstrated for nano-mechanical sensing applications. This device senses the surface stress due to a change in the carrier mobility of the semi-conducting layer. Due to the low Young's modulus and high strain sensitivity of the AZO layer, this micro-cantilever shows a deflection sensitivity of 116 ppm per nanometer of deflection. Also, mechanical characterization of these devices shows that the resonance frequency is in the range of a few tens of kilohertz which is suitable for sensor applications.

Optimization of the absorption efficiency of an amorphous-silicon thin-film tandem solar cell backed by a metallic surface-relief grating

The rigorous coupled-wave approach was used to compute the plane-wave absorptance of a thin-film tandem solar cell with a metallic surface-relief grating as its back reflector. The absorptance is a function of the angle of incidence and the polarization state of incident light; the free-space wavelength; and the period, duty cycle, the corrugation height, and the shape of the unit cell of the surface-relief grating. The solar cell was assumed to be made of hydrogenated amorphous-silicon alloys and the back reflector of bulk aluminum. The incidence and the grating planes were taken to be identical. The AM1.5 solar irradiance spectrum was used for computations in the 400-1100 nm wavelength range. Inspection of parametric plots of the solar-spectrum-integrated (SSI) absorption efficiency and numerical optimization using the differential evolution algorithm were employed to determine the optimal surface-relief grating. For direct insolation, the SSI absorption efficiency is maximizable by appropriate choices of the period, the duty cycle, and the corrugation height, regardless of the shape of the corrugation in each unit cell of the grating. A similar conclusion also holds for diffuse insolation, but the maximum efficiency for diffuse insolation is about 20% smaller than for direct insolation. Although a tin-doped indium-oxide layer at the front and an aluminum-doped zinc-oxide layer between the semiconductor material and the backing metallic layer change the optimal depth of the periodic corrugations, the optimal period of the corrugations does not significantly change.

Chemical speciation at buried interfaces in high-temperature processed polycrystalline silicon thin-film solar cells on ZnO:Al

The combination of polycrystalline silicon (poly-Si) thin films with aluminum doped zinc oxide layers (ZnO:Al) as transparent conductive oxide enables the design of appealing optoelectronic devices at low costs, namely in the field of photovoltaics. The fabrication of both thin-film materials requires high-temperature treatments, which are highly desired for obtaining a high electrical material quality. Annealing procedures are typically applied during crystallization and defect-healing processes for silicon and can boost the carrier mobility and conductivity of ZnO:Al layers. In a combined poly-Si/ZnO:Al layer system, an in-depth knowledge of the interaction of both layers and the control of interface reactions upon thermal treatments is crucial. Therefore, we analyze the influence of rapid thermal treatments up to 1050 °C on solid phase crystallized poly-Si thin-film solar cells on ZnO:Al-coated glass, focusing on chemical interface reactions and modifications of the poly-Si absorber material quality. The presence of a ZnO:Al layer in the solar cell stack was found to limit the poly-Si solar cell performance with open circuit voltages only below 390 mV (compared to 435 mV without ZnO film), even if a silicon nitride (SiN) diffusion barrier was included. A considerable amount of diffused zinc inside the silicon was observed. By grazing-incidence X-ray fluorescence spectrometry, a depth-resolving analysis of the elemental composition close to the poly-Si/(SiN)/ZnO:Al interface was carried out. Temperatures above 1000 °C were found to promote the formation of new chemical compounds within about 10 nm of interface, such as zinc silicates (Zn2SiO4) and aluminium oxide (AlxOy). These results give valuable insights about the temperature-limitations of Si/ZnO thin-film solar cell fabrication and the formation of high-mobility ZnO-layers by thermal anneal.

Overcoming interface losses in organic solar cells by applying low temperature, solution processed aluminum-doped zinc oxide electron extraction layers

Intrinsic zinc oxide (ZnO) is widely used as an electron extraction layer (EEL) for inverted polymer solar cells. Despite the excellent device performance, a major drawback for large area production is its low conductivity. Using microscopic simulations, we derived a technically reasonable threshold value of 10−3 S cm−1 for the conductivity required to overcome transport limitations. For conductivity values typical for ZnO we observed the interface layer thickness restriction at only a few tens of nanometers, either as a fill factor drop due to serial resistance, eventually accompanied by a second diode behavior, or by the need for light soaking. Higher conductive aluminum-doped zinc oxide (AZO), which was introduced earlier, meets the desired conductivity threshold, however, at the cost of high temperature processing. High annealing temperatures (>150 °C) significantly improve the electrical properties of ZnO, but prohibit processing on plastic substrates or organic active layers. Here we report on AZO layers from a sol–gel precursor, which has been already reported to give sufficiently high conductivities at lower processing temperatures (<150 °C). We investigate the influence of different precursor compositions on the electrical properties of the thin films and their performance in inverted poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cells. Low temperature AZO layers with thicknesses up to 680 nm maintained comparable performance to devices with thin AZO layers.

Effect of TCO/μc-Si:H Interface Modification on Hydrogenated Microcrystalline Silicon Thin-Film Solar Cells

The effects of H2plasma exposure on optical, electrical, and structural properties of fluorine-doped tin oxide (FTO) and AZO/FTO substrates have been investigated. With increasing the time of H2-plasma exposure, the hydrogen radical and ions penetrated through the FTO surface to form more suboxides such as SnO and metallic Sn, which was confirmed by the XPS analysis. The Sn reduction on the FTO surface can be effectively eliminated by capping the FTO with a very thin layer of sputtered aluminum-doped zinc oxide (AZO), as confirmed by the XPS analysis. By using the AZO/FTO as front TCO with the subsequent annealing, the p-i-nμc-Si:H cell exhibited a significantly enhancedJSCfrom 15.97 to 19.40 mA/cm2and an increased conversion efficiency from 5.69% to 7.09%. This significant enhancement was ascribed to the effective elimination of the Sn reduction on the FTO surface by the thin AZO layer during the Si-based thin-film deposition with hydrogen-rich plasma exposure. Moreover, the subsequent annealing of the sputtered AZO could lead to less defects as well as a better interface of AZO/FTO.

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 wurtzite c-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.

Power effect of ZnO:Al film as back reflector on the performance of thin-film solar cells

Aluminum-doped zinc oxide (AZO) is attracting interest as a potential transparent conducting oxide material for use in amorphous silicon (a-Si) thin-film solar cells. The absorption loss in the n-a-Si:H/Ag interface of the p-i-n thin film solar is high because the extinction coefficient of the Ag that is used as a back reflector is high. In this work, transparent conducting AZO films with a power in the range 500 W to 900 W prepared under Ar-ambient at a substrate temperature of 25 °C by RF-magnetron in-line sputtering. To minimize the absorption loss at longer wavelengths, an AZO layer we inserted at the n-a-Si:H/Ag interface of a solar cell with a glass/SnO2:F/p-a-SiC:H/buffer-layer/i-a-Si:H/n-a-Si:H/Ag structure and the performance of the cell with AZO/Ag deposited instead of Ag on the back contact, was investigated. The effects of the RF-magnetron sputtering deposition parameters on the optical, electrical and structural properties of the AZO films were analyzed. Optimized AZO films with high transmittance (&amp;gt; 80%) and low resistivity (1.47 × 10−3 Ω-cm) in the wavelength range 400–800 nm were obtained under the sputtering conditions. The p-i-n a-Si solar cell with the AZO/Ag back contact had a conversion efficiency of 9.42%, which was higher than that (7.36%) of the cell without the AZO/Ag back contact. Light-soaking of these cells for 1000 h at 60 °C led to stabilization at 8.32% for the best cell. The relative typical efficiency degradation of about 10% for single-junction cells demonstrates that this treatment does not negatively influence stability.

Structure and morphology of aluminium doped Zinc-oxide layers prepared by atomic layer deposition

The aim of this work is to study the effects of deposition temperature and aluminium incorporation on the crystalline properties, orientation and grain size of atomic layer deposited ZnO layers. X-ray diffraction analysis revealed a change in the dominant crystallite orientation with increasing substrate temperature. The most perfect crystal structure and largest grain size was found at 2at.% aluminium content. Accumulation of compressive strain developed a monotonous increase with the growth temperature. Electric resistivity showed no anisotropy despite the change in the orientation, therefore the dominant conduction mechanism is not grain boundary related.

Broadband wide-angle antireflection enhancement in AZO/Si shell/core subwavelength grating structures with hydrophobic surface for Si-based solar cells

Broadband wide-angle antireflection characteristics of aluminum-doped zinc oxide (AZO)/silicon (Si) shell/core subwavelength grating (SWG) structures with a hydrophobic surface, together with theoretical prediction using a rigorous coupled-wave analysis simulation, were investigated for Si-based solar cells. The AZO films with different thicknesses were deposited on Si SWGs by rf magnetron sputtering method, which forms a shell/core structure. The AZO/Si shell/core SWGs reduced significantly the surface reflection compared to the AZO films/Si substrate. The coverage of AZO films on Si SWGs improved the antireflective property over a wider incident angle. The AZO/Si shell/core SWG structure with a 200 nm-thick AZO layer deposited at an rf power of 200 W exhibited a water contact angle of 123°. This structure also exhibited a low average reflectance of ~2% over a wide wavelength range of 300-2100 nm with a solar weighted reflectance of 2.8%, maintaining a reflectance of < 9.2% at wavelengths of 300-2100 nm up to the incident angle of θ(i) = 70°. The effective electrical properties of AZO films in AZO/Si shell/core SWGs were also analyzed.

The use of aluminium doped ZnO as transparent conductive oxide for CdS/CdTe solar cells

CdTe/CdS and CdTe/ZnO thin film solar cells were grown with a high vacuum evaporation based low temperature process (≤ 420 °C). Aluminium doped zinc oxide (AZO) was used as transparent conducting oxide (TCO) material. AZO exhibited excellent stability during the solar cell processing, and no significant change in electrical conductivity or transparency was observed. The current density loss due to absorption in the 1 μm thick AZO layer with 5 Ω per square sheet resistance was found to be 1.2 mA/cm 2. We investigated the influence of an intrinsic ZnO layer (i:ZnO) in combination with various CdS thicknesses. The i:ZnO layer was found to significantly increase the open circuit voltage of the solar cells with very thin CdS layer. Increasing thickness of the i:ZnO layer leads to UV absorption losses, narrowing of the depletion layer width and hence reduced collection efficiency in the long wavelength (685–830 nm) part. With AZO/i:ZnO bi-layer TCO we could achieve cell efficiencies of 15.6% on glass and 12.4% on the flexible polyimide film.

AZO/ATO double-layered transparent conducting electrode: A thermal stability study

During the fabrication of a thin film photovoltaic device the transparent conducting oxide (TCO), which constitutes an integral part of its basic structure, could be exposed at high temperature processes; in addition, a TCO which could be grown at ambient temperature can be useful in order to minimize possible damages caused to other previously deposited components. Aluminum doped zinc oxide (AZO) is a well known TCO with good properties in terms of light transparency and conductivity, however its electrical properties deteriorate after subsequent high temperature processes in air atmosphere. On the other hand antimony doped tin oxide (ATO) has poorer conductivity than AZO but has good light transparency and better thermal stability. In the present work, ATO layers with a thickness ranging from 30 nm to 210 nm were deposited over a 600 nm thick AZO layer. These AZO/ATO bilayers, which have been developed at ambient temperature by DC magnetron sputtering, were subsequently annealed at high temperatures in order to study their thermal stability. An ATO optimal thickness of 130 nm is found to improve the bilayer thermal stability at temperatures over 350 °C in air atmosphere, keeping good optical properties.

Transparent and conductive ZnO:Al thin films grown by pulsed magnetron sputtering in current or voltage regulation modes

Aluminium-doped zinc oxide (AZO) thin films have been prepared by pulsed magnetron sputtering from a ceramic oxide target in pure argon atmosphere. Sputtering processes were performed in current or voltage regulation modes at different pulsing frequencies up to 200 kHz. Several growth parameters (discharge power, substrate temperature and growth rate) as well as AZO film properties (crystalline structure, optical transmittance and electrical characteristics) have been measured and analysed as a function of the current or voltage applied, the pulsing frequency and the operation time. By adjusting these deposition parameters, AZO layers with resistivity as low as 1.0×10 −3 Ωcm and average visible transmittance above 85% have been obtained at growth rate in the range 70–80 nm/min, at substrate temperatures below 150 °C.

Effects of starting material of aluminum doped zinc oxide underlayer on the electric properties of palladium doped silver film

Crystallographic and electric properties of a palladium doped silver (Ag–Pd) layer formed on an aluminum doped zinc oxide (AZO) film in the multilayer system of Ag–Pd(10.5 nm)/AZO(42 nm)/glass were investigated. All of the films in this study were deposited by dc magnetron sputtering. Two kinds of sputtering targets, aluminum doped zinc metal (AZ) and AZO, were used for the deposition of AZO films. The AZ target was sputtered in an oxygen or carbon dioxide atmosphere whereas the AZO target was sputtered in an argon or argon/oxygen atmosphere. The Ag–Pd layers were deposited under the fixed sputtering condition. From x-ray diffraction patterns it was found that the most developed Ag(111) peak appeared in the case of the Ag–Pd film on the AZO layer deposited from the AZ metal target with oxygen. Atomic force microscope observation revealed that the smallest surface roughness was obtained in this case. Electrical resistivity of the Ag–Pd layer changed by up to 17% depending on deposition conditions and the target materials of the underlayer. The Ag–Pd film deposited on the AZO layer from the AZ target in an oxygen atmosphere also showed the lowest resistivity of 7.1×10−6 Ω cm. The smallest refractive index of Ag–Pd film was shown by the same sample from an analysis using spectroscopic ellipsometry (SE). Deposition of a thin metal (Zn–Al) overlayer on Ag–Pd/AZO/glass increased the overall resistivity of the multilayer. This may imply the diffusion of zinc atoms into the Ag–Pd layer, however, the resistivity recovery was observed after some storage time, which suggests the effect of an electron scattering at the surface.

Optical investigations in a PEM controlled reactive magnetron sputter process for aluminium doped zinc oxide layers using metallic alloy targets

The use of aluminium doped zinc oxide (ZAO) as transparent conductive layer (TCO) has growing importance especially for production of solar modules. A significant cost reduction of TCO thin film production is expected by use of much less expensive metallic alloy targets. ZAO films with good TCO properties can be achieved only in the transition mode of the reactive sputter process. To stabilize the discharge for long stretched magnetron sputter sources on the working point needed a process control system is required. The intensity of zinc emission lines is used as control parameter. The control is performed by adding varying amounts of oxygen gas to the discharge region using the Plasma Emission Monitor PEM ®05. This allows a combination of reproducible TCO film properties with stable high deposition rates. By use of an optical emission spectrometer AOS-4 it was investigated in situ, which emission lines are best suited for the process control. Herefore the chronogram mode of the spectrometer is used, which allows spectral lines to be monitored with a time resolution of a few milliseconds. Furthermore, other spectral line intensities are investigated for use as regulation parameter. A concept for regulating a reactive sputter process by use of several emission lines is discussed. The concept was applied for DC discharge and MF discharge as well. A time-resolved investigation of the interesting spectral lines in MF discharge is carried out with microsecond resolution.