Aluminum-doped Zinc Oxide Films Research Articles

Chitosan-gated low-voltage transparent indium-free aluminum-doped zinc oxide thin-film transistors

Low-voltage transparent indium-free aluminum-doped zinc oxide (AZO) thin-film transistors (TFTs) are demonstrated by using chitosan polymer electrolyte as the gate dielectric. Chitosan with a large specific capacitance (0.4 μF/cm2) is obtained possibly due to the strong electric-double-layer (EDL) effect through the mobile-proton hopping mechanism. Herein, low-cost indium-free AZO film is developed for replacing the traditional ITO/IZO electrodes. A simple method is developed to fabricate all of the channel and source/drain electrodes during one-step sputter process by using such a low-cost indium-free AZO film. The optimized TFTs with 30 nm AZO thickness shows the best performance with a low operation voltage of 1.5 V, a large on-off ratio of 105, and a field-effect mobility of 8.3 cm2/Vs, respectively. The chitosan-gated AZO TFTs may provide a good candidate for the applications of next-generation transparent flexible low-cost portable electronics.

Plasmonic behaviors of metallic AZO thin film and AZO nanodisk array.

Aluminum-doped zinc oxide (AZO) is well known as transparent conducting material for electro-optical devices, but is rarely used as plasmonic material, particularly on the localized surface plasmon resonance (LSPR) behavior of AZO nanostructure and its plasmonic devices. In this study, we systematically investigate the plasmonic behaviors of AZO thin films and patterned AZO nanostructures with various structural dimensions under different annealing treatments. We find that AZO film can possess highly-tunable, metal-like, and low-loss plasmonic property and the LSPR characteristic of AZO nanostructure is observed in the near-infrared (NIR) region under proper annealing conditions. Finally, environmental index sensing is performed to demonstrate the capability of AZO nanostructure for optical sensing application. High index sensitivity of 873 nm per refractive index unit (RIU) variation is obtained in experiment.

Tunneling Characteristics Depending on Schottky Barriers and Diffusion Current in SiOC.

To obtain a diffusion current in SiOC, the aluminum doped zinc oxide films were deposited on SiOC/Si wafer by a RF magnetron sputtering. All the X-ray patterns of the SiOC films showed amorphous phases. The level of binding energy of Si atoms will lead to an additional potential modulation by long range Coulombic and covalent interactions with oxygen ions. The growth of the AZO film was affected by the characteristics of SiOC, resulting in similar trends in XPS spectra and a shift to higher AZO lattice d values than the original AZO d values in XRD analyses. The charges trapped by the defects at the interlayer between AZO and SiOC films induced the decreased mobility of carriers. In the absence of trap charges, AZO grown on SiOC film such as the sample prepared at O2 = 25 or 30 sccm, which has low charge carrier concentration and high mobility, showed high mobility in an ambipolar characteristic of oxide semiconductor due to the tunneling effect and diffusion current. The structural matching of an interface between AZO and amorphous SiOC enhanced the height of Schottky Barrier (SB), and then the mobility was increased by the tunneling effect from band to band through the high SB.

The influence of target erosion grade in the optoelectronic properties of AZO coatings growth by magnetron sputtering

Aluminum-doped zinc oxide (AZO) transparent conductor coating has emerged as promising substitute to tin-doped indium oxide (ITO) as electrode in optoelectronic applications such as photovoltaics or light emitting diodes (LEDs). Besides its high transmission in the visible spectral region and low resistivity, AZO presents a main advantage over other candidates such as graphene, carbon nanotubes or silver nanowires; it can be deposited using the technology industrially implemented to manufacture ITO layers, the magnetron sputtering (MS). This is a productive, reliable and green manufacturing technique. But to guarantee the robustness, reproducibility and reliability of the process there are still some issues to be addressed, such as the effect and control of the target state. In this paper a thorough study of the influence of the target erosion grade in developed coatings has been performed. AZO films have been deposited from a ceramic target by RF MS. Structure, optical transmittance and electrical properties of the produced coatings have been analyzed as function of the target erosion grade. No noticeable differences have been found neither in optoelectronic properties nor in the structure of the coatings, indicating that the RF MS is a stable and consistent process through the whole life of the target.

Enhanced electrical and optical properties of room temperature deposited Aluminium doped Zinc Oxide (AZO) thin films by excimer laser annealing

High quality transparent conductive oxides (TCOs) often require a high thermal budget fabrication process. In this study, Excimer Laser Annealing (ELA) at a wavelength of 248nm has been explored as a processing mechanism to facilitate low thermal budget fabrication of high quality aluminium doped zinc oxide (AZO) thin films. 180nm thick AZO films were prepared by radio frequency magnetron sputtering at room temperature on fused silica substrates. The effects of the applied RF power and the sputtering pressure on the outcome of ELA at different laser energy densities and number of pulses have been investigated. AZO films deposited with no intentional heating at 180W, and at 2mTorr of 0.2% oxygen in argon were selected as the optimum as-deposited films in this work, with a resistivity of 1×10−3Ω.cm, and an average visible transmission of 85%. ELA was found to result in noticeably reduced resistivity of 5×10−4 Ω.cm, and enhancing the average visible transmission to 90% when AZO is processed with 5 pulses at 125mJ/cm2. Therefore, the combination of RF magnetron sputtering and ELA, both low thermal budget and scalable techniques, can provide a viable fabrication route of high quality AZO films for use as transparent electrodes.

Effect of Aluminum doping on the properties of magnetron sputtered ZnO films

Aluminum doped zinc oxide (AZO) thin films were coated on the glass substrate by diffusing the Al in to the sputtered ZnO thin films. Initially, zinc oxide (ZnO) thin films were deposited on glass substrates using reactive dc magnetron sputtering (DCMS) technique followed by Aluminum (Al) were deposited over the ZnO films by vacuum evaporation technique. The prepared samples were annealed at 400 °C for 3h in vacuum in order to diffuse Al uniformly in to ZnO matrix. The influence of Al doping over the micro-structure, surface morphology, optical and electrical properties of the films were studied by X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM), UV-Visible spectroscopy and four point probe (FPP) method respectively. Resistivity of the AZO films decreased with increase in the Al concentration and the film deposited with high Al concentration offered low electrical resistivity of ∼28 Ω.cm and optical bandgap of 3.21 eV.

The Influence of Oxygen Pressure on ZnO:Al Thin Films Properties Grown by Layer by Layer Growth Method at Magnetron Sputtering

The influence of oxygen pressure in the deposition chamber on the structure, morphology, optical and electrical properties of aluminum doped ZnO films deposited by a layer by layer growth method in magnetron sputtering on glass substrates was studied. The effect of the application of the traditional one-step approach and our proposed layer by layer growth method in magnetron sputtering on the properties of doped by aluminum ZnO films was analyzed. It is found that with decreasing oxygen pressure in the deposition chamber improves the structure, increases transmittance in the visible spectrum of radiation and decreases resistivity of ZnO:Al films. It is shown that the application of layer by layer growth method in magnetron sputtering allows to grow the transparent conductive ZnO:Al films with higher performance parameters, compared with the films which condensed by traditional approach in magnetron sputtering. The layer by layer growth method allows to grown ZnO:Al films with electrical resistance at 6.1·10-4 Ohm·cm and transmission in the visible light of 95%, which is promising for their aplication in photovoltaic devices.

High Performance AZO Thin Films Deposited by RF Magnetron Sputtering at Low Temperature

Transparent conductive aluminum-doped zinc oxide (AZO) films had been prepared on glass substrates by radio frequency magnetron sputtering with different sputtering powers using an ultra- high density ceramic target at 100°C Crystallinity levels, microstructures, optical and electrical properties of these thin films were systematically investigated by the scanning electron microscope, X-ray diffraction diffractometer, UV-visible spectrophotometer and four-point probe method. All the AZO thin films exhibited preferential orientation along the (0 0 2) plane with hexagonal wurtzite structure. The average transmittance of the thin films is over 86%, the highest transmittance 88.97% and the lowest resistivity 3x10-4Ω·cm are simultaneously obtained at 200W. With the sputtering power increase, the grain growth was deteriorated and the resistivity increased from 3x10-4Ω·cm to 2.5x10- 3Ω·cm. The band gaps of AZO films range from 3.81eV to 3.89eV. This article discussed some important patents related to the methods of depositing AZO thin films. Keywords: AZO thin films, electrical properties, low temperature, magnetron sputtering, microstructure, optical properties.

Aluminum-doped zinc oxide thin film as seeds layer effects on the alignment of zinc oxide nanorods synthesized in the chemical bath deposition

Aluminum-doped zinc oxide thin films with different thicknesses were deposited using a radio frequency sputtering system. As thickness increased, crystallinity improved and resistivity decreased. The optical transmittance of the films was over 80%. Well-aligned zinc oxide nanorods were synthesized on aluminum doped zinc oxide thin films in the chemical bath deposition. It was found that the morphological, structural, and optical properties of zinc oxide nanorods were significantly influenced by thicknesses of aluminum-doped zinc oxide films. Zinc oxide nanorods exhibited better crystallinity as film thickness increased. High transmittance of over 65% was obtained from zinc oxide nanorods. Vertical alignment of zinc oxide nanorods was dependent on growth direction of aluminum-doped zinc oxide films.

Preparation of aluminum doped zinc oxide films with low resistivity and outstanding transparency by a sol–gel method for potential applications in perovskite solar cell

Highly transparent and conductive aluminum doped zinc oxide (AZO) films were prepared by sol–gel method on the glass substrates. The effects of doping concentration, annealing temperature and facing direction during annealing on the structural, electrical and optical properties of AZO films were studied by performing a series of characterizations including X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, UV–vis spectrophotometry, four-point probe method and Hall effect measurement system. The results showed that the AZO films were wurtzite crystallized with c-axis preferred orientation. A minimum resistivity of 1.8 × 10−3 Ω cm and a transmittance above 90% were obtained for the film doped with 1.5 at.% aluminum, annealed at 510 °C and faced-down in the oven, which was among the best performance of the currently reported works based on sol–gel process. Moreover, energy level analysis revealed that the AZO film has a work function of 4.3 eV, exhibiting great potential in perovskite solar cell applications.

Al-doped ZnO thin films deposited by confocal sputtering as electrodes in ZnO-based thin-film transistors

Aluminum doped zinc oxide (AZO) films were deposited by confocal RF magnetron sputtering at different substrate temperatures. AZO films with a transparency up to 90% in the visible spectrum were obtained. Polycrystalline AZO films, with planes in the (002) and (103) orientations of the zincblende (hexagonal) structure, were obtained at substrate temperatures higher than 60°C. The best AZO films, obtained at 75°C, showed an electrical resistivity of 8.8×10−4Ω-cm, with a carrier concentration of 4×1020cm−3 and mobility of 20cm2/V-s. These are appropriate values for solar cell applications. In addition, ZnO-based thin-film transistors (TFTs) were fabricated for evaluating the behavior of the AZO films as source and drain contacts on the transistors. The field effect mobility and the threshold voltage of the fabricated devices were 20cm2/V-s and 7V, respectively. The TFTs showed an Ion/Ioff ratio of up to 9 orders of magnitude. A specific contact resistance of approximately 0.06Ω-cm2 was determined for the AZO/ZnO interface. This result corresponds to the first report of the ZnO/AZO specific contact resistance obtained with a maximum processing temperature of 100°C. Therefore, this TFT technology is fully compatible with flexible substrates and can be used for transparent and large area electronics.

Low-temperature, solution-processed aluminum-doped zinc oxide as electron transport layer for stable efficient polymer solar cells

A simple low-temperature solution-processed zinc oxide (ZnO) and aluminum-doped ZnO (AZO) were synthesized and investigated as an electron transport layer (ETL) for inverted polymer solar cells. A solar cell with a blend of poly(4,8-bis-alkyloxy-benzo[1,2-b:4,5-b′] dithiophene-alt-alkylcarbonyl-thieno [3,4-b] thiophene) and (6,6)-phenyl-C71-butyric acid methyl ester as an active layer and AZO as ETL demonstrates a high power conversion efficiency (PCE) of 7.36% under the illumination of AM 1.5G, 100mW/cm2. Compared to the cells with ZnO ETL (PCE of 6.85%), the PCE is improved by 7.45% with the introduction of an AZO layer. The improved PCE is ascribed to the enhanced short circuit current density, which results from the electron transport property of the AZO layer. Moreover, AZO is a more stable interfacial layer than ZnO. The PCE of the solar cells with AZO as ETL retain 85% of their original value after storage for 120days, superior to the 39% of cells with ZnO ETL. The results above indicate that a simple low-temperature solution-processed AZO film is an efficient and economical ETL for high-performance inverted polymer solar cells. Due to its environmental friendliness, good electrical properties, and simple preparation approach, AZO has the potential to be applied in high-performance, large-scale industrialization of solar cells and other electronic devices.

In situ Al-doped ZnO films by atomic layer deposition with an interrupted flow

In situ aluminum-doped ZnO (AZO) films were grown on glass substrates by atomic layer deposition (ALD) with an interrupted flow at temperatures in range 200–280 °C; the optimal temperature, 260 °C, depended on the electrical properties. To assess the effect of the ratio of pulses of diethylzinc (DEZn) and trimethylaluminium (TMA) on the structural, optical and electrical properties, we grew AZO films with various pulse ratios of DEZn:TMA in a range from 3:1 to 10:1 at 260 °C. These properties and the content of Al were investigated with X-ray diffraction, X-ray reflectivity (XRR), a high-resolution transmission electron microscope (HRTEM), a secondary-ion mass spectrometer (SIMS), transmission spectra, Hall measurements and X-ray photoelectron spectra (XPS). The electrical resistivity was least, 5.7 × 10−4 Ω cm, for ALD-AZO films with pulse ratio 6:1; the carrier mobility was 8.80 cm2 V−1 s−1 and optical transmittance up to 94%. The epitaxial AZO films grown in situ also on m-plane sapphire exhibited the two-fold symmetry of ZnO (110) in the orthorhombic crystal system. All results show that a novel in situ doping method with an interrupted flow controls the Al content of AZO films more easily, and is more usefully applicable for a structure with a large aspect ratio for an advanced photoelectric device.

Comparative study of structural and visible luminescence properties of AZO thin film deposited on GaAs and porous GaAs substrates

The objective of this study is to investigate the effect of the pre-treatment of GaAs substrate by making it porous on structural and visible luminescence properties of aluminum - doped zinc oxide (AZO) thin films deposited by the use of a radio frequency magnetron sputtering. Porous GaAs has been prepared by anodic etching of n+-type (100) GaAs wafers in a HF:C2H5OH:HCl:H2O2:H2O electrolyte. Structural and visible luminescence properties of deposited films have been investigated by means of x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and room temperature photoluminescence (PL) spectroscopy. XRD patterns of deposited films show predominant orientation along (100) crystal plane of hexagonal ZnO. A diffraction peak due to cubic zinc arsenide (Zn3As2) is only observed in the XRD pattern of the film deposited on porous GaAs. XPS results further confirm that the AZO film deposited on porous GaAs is doped with arsenic (As). This is attributed to the enhancement of As diffusion into the film due to nanostructure nature of porous GaAs. The use of porous GaAs as a substrate also leads to an enhancement of the visible PL intensity of the deposited film. This is found to have a strong relationship with the change in the content of zinc and oxygen related defects in the film as it is revealed from peak analysis of XPS and PL spectra.

Transparent conductive ZnO layers on polymer substrates: Thin film deposition and application in organic solar cells

Aluminum doped ZnO (AZO) and pure ZnO thin films are grown on polymer substrates by pulsed-laser deposition and the optical, electrical, and structural film properties are investigated. Laser fluence, substrate temperature, and oxygen pressure are varied to obtain transparent, conductive, and stoichiometric AZO layers on polyethylene terephthalate (PET) that are free of cracks. At low fluence (1J/cm2) and low pressure (10−3mbar), AZO/PET samples of high optical transmission in the visible range, low electrical sheet resistance, and high figure of merit (FOM) are produced. AZO films on fluorinated ethylene propylene have low FOM. The AZO films on PET substrates are used as electron transport layer in inverted organic solar cell devices employing P3HT:PCBM as photovoltaic polymer-fullerene bulk heterojunction.

Study of Plasma Properties for the Low‐Temperature Deposition of Highly Conductive Aluminum Doped ZnO Film Using ICP Assisted DC Magnetron Sputtering

This work reports an investigation of the Al‐doped ZnO (AZO) film deposition process using ICP assisted DC magnetron plasmas, at different working pressures. The mechanism of plasma formation and plasma properties are analyzed by various diagnostic tools. Data show that there is the presence of low‐frequency oscillations in the range of 3–8 MHz, which are expected to be caused by high‐frequency waves. It is seen that ICP source is quite capable of producing the high‐energy warm electrons that are expected to be produced by the Landau damping. The plasma properties and their role on the electrical and structural properties of the AZO film are carefully studied. This work also reports the deposition of high conductive AZO films with resistivity as low as ∼ 6.8 × 10−4 cm using low‐temperature process.

Electrical and optical properties of nanosized films of doped zinc and indium oxides deposited by RF magnetron sputtering at room temperature

We have compared the electrical and optical properties of nanosized films of tin-doped indium oxide (ITO) and aluminum-doped zinc oxide (AZO) deposited on silicon and glass substrates by low-power RF magnetron sputtering at room temperature. Dependences of the resistivity and optical transmission coefficient of deposited oxides on the RF radiation power are presented. The plots of resistivity versus RF sputtering power for both oxides are nonlinear, with a minimum in the region of 50 W. For AZO, this minimum has been observed for the first time. The minimum value of resistivity for AZO is about 2.9 × 10−3 Ω cm and that for ITO is about 5.4 × 10−4 Ω cm. Both oxides are characterized by high optical transmission coefficients, close to 90% in a wavelength range of 500–1000 nm.

Role of the dopant aluminum for the growth of sputtered ZnO:Al investigated by means of a seed layer concept

This work elucidates the effect of the dopant aluminum on the growth of magnetron-sputtered aluminum-doped zinc oxide (ZnO:Al) films by means of a seed layer concept. Thin (<100 nm), highly doped seed layers and subsequently grown thick (∼800 nm), lowly doped bulk films were deposited using a ZnO:Al2O3 target with 2 wt. % and 1 wt. % Al2O3, respectively. We investigated the effect of bulk and seed layer deposition temperature as well as seed layer thickness on electrical, optical, and structural properties of ZnO:Al films. A reduction of deposition temperature by 100 °C was achieved without deteriorating conductivity, transparency, and etching morphology which renders these low-temperature films applicable as light-scattering front contact for thin-film silicon solar cells. Lowly doped bulk layers on highly doped seed layers showed smaller grains and lower surface roughness than their counterpart without seed layer. We attributed this observation to the beneficial role of the dopant aluminum that induces an enhanced surface diffusion length via a surfactant effect. The enhanced surface diffusion length promotes 2D-growth of the highly doped seed layer, which is then adopted by the subsequently grown and lowly doped bulk layer. Furthermore, we explained the seed layer induced increase of tensile stress on the basis of the grain boundary relaxation model. The model relates the grain size reduction to the tensile stress increase within the ZnO:Al films. Finally, temperature-dependent conductivity measurements, optical fits, and etching characteristics revealed that seed layers reduced grain boundary scattering. Thus, seed layers induced optimized grain boundary morphology with the result of a higher charge carrier mobility and more suitable etching characteristics. It is particularly compelling that we observed smaller grains to correlate with an enhanced charge carrier mobility. A seed layer thickness of 5 nm was sufficient to induce the beneficial effects.

Highly textured conductive and transparent ZnO films for HIT solar cell applications

In this paper, aluminium-doped zinc oxide (AZO) thin-films were fabricated on both glass and silicon substrates by radio-frequency magnetron sputtering at various working pressures of 0.15–0.46 Pa. The effect of working pressure on the structural, electrical, and optical properties of the deposited AZO films was carefully studied. The lower working pressure is more favourable to large grain size, smooth surface, low electrical resistivity, and moderate optical transparency. For the AZO films deposited at lower working pressures, the larger grain size can be ascribed to the higher kinetic energy of the sputtered particles while the lower electrical resistivity is strongly related to both the presence of fewer grain boundaries due to the larger grain size and more activated amount of dopants in the films. We then applied AZO films deposited at 0.15 Pa to Al/AZO/n-a-Si : H/i-a-Si : H/p-c-Si/Al heterojunction Si solar cells with intrinsic thin layer (HIT) solar cells and achieved highly textured surfaces via acid etching. The HIT solar cells after acid texturing showed a higher external quantum efficiency (EQE) value than those with smooth AZO films mainly in the wavelength region from 300 to 500 nm, leading to an obvious increase in the conversion efficiency from 14.1% to 14.7%.

Using the acetylacetonates of zinc and aluminium for the Metalorganic Chemical Vapour Deposition of aluminium doped zinc oxide films

Metalorganic Chemical Vapour Deposition is a promising method for the growth of thin aluminium doped zinc oxide films (ZnO:Al), a material with potential application as transparent conducting oxide (TCO), e.g. for the use as front electrode in solar cells. For the low-cost deposition, the choice of the precursors is extremely important. Here we present the deposition of quite homogeneous films from the acetylacetonates of zinc and aluminium that are rather cheap, commercially available and easy to handle. A user-made CVD-reactor activating the deposition process by the light of halogen lamps was used for film deposition. Well-ordered films with an aluminium content between 0 and 8% were grown on borosilicate glass and Si(100). On both types of substrate, the films are crystalline and show a preferred orientation along the (002)-direction. The 0.3 to 0.5μm thick films are highly transparent in the visible region. The best films show a low electric resistivity between 2.4 and 8mΩcm.