Aluminum-doped Zinc Oxide Films Research Articles

Effect of RF power of Al2O3 target on the physical properties of aluminum-doped zinc oxide films

In this report, we investigate the optical and electrical properties of aluminum zinc oxide (AZO) thin films using magnetron sputtering method in preparing the thin films and spectrophotometer to measure the optical characteristics. The X-ray diffraction (XRD) patterns of the films are found to have amorphous structure. The optical band gap Eg, refractive index n, electronic $${\alpha }_{\mathrm{Ep}}$$ polarizability, carrier concentration Nc, plasma frequency $${\omega }_{\mathrm{p}}$$ , electrical resistivity $${\rho }_{\mathrm{s}}$$ , and sheet resistance $${R}_{\mathrm{sh}}$$ are found to decrease with increasing the RF power of Al2O3 target. The refractive index shows a normal dispersion. Urbach energy Eu and figure of merit are found to increase with increasing the RF power of Al2O3 target. Our results manifest that the change of the RF power of the Al2O3 target can improve the AZO thin films, which is appropriate for solar sell applications.

High-density plasma etching characteristics of aluminum-doped zinc oxide thin films in Cl2/Ar plasma

We investigated the etching characteristics of aluminum-doped zinc oxide (AZO) thin films in an adaptively coupled plasma (ACP) system. The dry etching characteristics of AZO films were studied by changing the Cl2/Ar gas mixing ratio, RF power, DC bias voltage. We determined the following optimized process conditions: RF power of 500 W, DC bias voltage of −100 V, process pressure of 15 mTorr. In Cl2/Ar plasma (=50:50%), the maximum etching rate of AZO films is 70.45 nm min−1. The ion composition of Cl2/Ar plasma was determined by optical emission spectrometry (OES). The chemical reactions on the surface of AZO films were analyzed by x-ray photoelectron spectroscopy (XPS).

Investigation of optical properties of aluminum-doped zinc oxide films via flow-limited field-injection electrostatic spraying

Zinc oxide films doped with 0 at. %–5 at. % aluminum are fabricated by flow-limited field-injection electrostatic spraying using a sol–gel processed precursor. X-ray diffraction and Rietveld analyses indicate that the films are highly (002)-oriented with large crystallites and that the lattice constants of the doped-ZnO tend to decrease with Al doping below the values of undoped-ZnO. Optical properties were measured by ellipsometry and analyzed using the Drude model of permittivity. The electron scattering rate is calculated to be minimal at 3 at. % Al, which may indicate a reduction in the ionized impurities due to the lattice strain and the absence of Al clusters, which is enabled by the sol–gel precursor. Insights are offered regarding the effects of Al doping on film density, electron concentration, and background permittivity, which may prove important in tuning the film properties for plasmonic applications.

Influence of Al content and annealing atmosphere on optoelectronic characteristics of Al:ZnO thin films

Aluminum-doped zinc oxide (AZO) was cosputtered from ZnO and Al targets on glass substrates at room temperature. The effect of Al-target power and annealing atmosphere on structural and optoelectronic properties of AZO films was investigated.[Al]/[Zn] ratio increased to 0.258 when the Al-target power increased to 250 W. At a fixed Al-target power of 200 W, the [Al]/[Zn] ratio was 0.104 and the as-deposited AZO film demonstrated a low resistivity of 3.19 × 10−4 Ω-cm and high transmittance of approximately 90% in the visible region. After annealing at 500 °C in forming gas (5% H2 in Ar) atmosphere, the resistivity of the AZO film can be further reduced to 9.38 × 10−5 Ω-cm. Given its low-temperature process and good optoelectronic properties, sputtered AZO films that use ZnO and Al targets have high potential in various optoelectronic devices.

Methanol solvent effect on photosensing performance of AZO thin films grown by nebulizer spray pyrolysis

Dopant concentration variation or solvent change is commonly used for the growth and characterization of doped metallic thin films. In contrast, we have studied the effects of varying methanol solvent volume ratio for aluminum doped zinc oxide (AZO) thin film growth. AZO thin films were coated on glass and ITO substrates by nebulizer spray pyrolysis method. The effect of different solvent concentrations and the water to methanol ratio on the resultant film properties were analyzed. The structural and lattice constants of the prepared films were investigated by x-ray diffraction (XRD). Pure methanol solvent enhances the structural quality of the AZO film, as confirmed from the XRD and x-ray photoelectron spectroscopy analyses. The atomic force microscopy images showed thinner and smooth film surface for the pure methanol solvent condition without water. The optical absorption study showed a high transmittance (∼93%) in the visible region with a bandgap of 3.30 eV, which is close to bulk ZnO (3.36 eV). The photoluminescence spectra showed a powerful UV emission at 390 nm, indicating that the films were of good quality. Ag/AZO/Cu2O/ITO structured photodiodes were fabricated using ITO substrate and their photosensing properties were studied. All the prepared films (S1, S2, S3, S4 and S5) showed an increase in current level for both dark and light conditions, with an increase in methanol solvent ratio. The film S5 (pure methanol solvent) under the dark and light illumination conditions at a bias of 1.0 V, showed current values of ∼6.5 × 10–5 A and ∼1.85 × 10–4 A, respectively.

An insight into the low doping efficiency of Al in sol–gel-derived ZnO:Al films: role of the dopant chemical state

We prepared aluminum-doped zinc oxide (ZnO:Al, AZO) films by a sol–gel method and investigated the effect of doping on the electrical properties of AZO films in the Al doping concentration range of 0.5–3.0 at.%. The negative effect of Al dopants on the carrier mobility counteracted their positive effect on the carrier concentration and the resistivity of the films remained almost constant at the optimum Al doping level of 0.5 at.%. Moreover, the doping efficiency of Al dopant decreased with increasing doping level and the maximum efficiency was not higher than 50% even at the optimum doping concentration. The microstructure and chemical composition of the films were examined by X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy to elucidate the mechanism behind the low doping efficiency of Al dopant in the sol–gel-derived AZO films. The reason was attributed to the passivation of the dopant in two forms, defect complex nAlZn–VZn originating from the dopant–defect reaction in the ZnO grains and aluminum oxide phase segregating at the ZnO grain boundaries.

Improvement in structural, electrical, and optical properties of Al-doped ZnO nanolayers by sodium carbonate prepared via solgel method

The significant improvement in crystallinity, electrical, and optical properties of aluminum-doped zinc oxide (AZO) thin films was attained by Na doping. Both Al and Na contents were introduced into ZnO host via facile and cost-effective solgel method. Ultra-thin films of Na-doped AZO (NAZO), with thickness of about 40 nm, were deposited on glass substrates via spin-coating technique. The obtained results showed low sodium incorporation reduces the crystal strain which in turn leads to increase crystallite size from 31 nm for AZO to 49 nm for 2 at% Na-doped AZO layer. In addition, 2 at.% of Na doping, that was determined as an optimum level of doping concentration, decreased the macroscopic surface roughness (MRs) without notable changes in surface micro-cracks, leading to remarkable enhancement of the layer conductivity (about 54%). On the other hand, high concentration of sodium content (10 at%) increased the crystal strain and reduced the crystallinity order of AZO layer which resulted in high MRs and also high electrical sheet resistance. Optical dispersion curve and real and imaginary parts of dielectric constant were obtained using our precise method. More interestingly, 2 at% of Na doping led to an increase in transparency of AZO layer in the visible range, while its extinction coefficient (light power absorption) was enhanced which can be regarded as advantage for such thin films. Therefore, the optimum Na content can provide higher optical absorption even with higher transparency by reducing the light scattering and reflection. This finding provides a useful procedure to prepare transparent oxide semiconductor layers with noticeable light absorption, which is very interesting for various applications such as solar cells and anti-reflection coatings.

The physical and optical properties of AZO thin films on teflon substrate at low deposition temperature by spray pyrolysis

<span>This paper presents the physical and optical properties of AZO thin films on Teflon substrate at low deposition temperature by spray pyrolysis. In this study, the effect of different process parameters such as spray time and substrate to nozzle distance on the physical and optical characteristic of aluminium doped zinc oxide (AZO) deposited on Teflon substrates was investigated. The AZO thin films were successfully deposited onto Teflon substrate by spray pyrolysis technique at low deposition temperature. The physical analysis by X-ray diffraction (XRD) shows that the deposited Teflon substrate films have a preferred orientation along the direction (100) and (101). Optical measurements were conducted using Jasco/V-670 Ex Uv-Vis-NIR Spectrophotometer model to confirms that in visible ray it is possible to get good reflectance of AZO films with a reflection of 80%. The values of band gaps Eg were calculated from the spectra of UV-Visible reflectance that were vary between 3.06 and 3.14 eV. </span>

Effect of RF sputtering power and vacuum annealing on the properties of AZO thin films prepared from ceramic target in confocal configuration

Aluminum-doped zinc oxide (AZO) thin films are deposited on glass substrate by radio frequency (RF) magnetron sputtering method in confocal configuration at room temperature (RT). Several techniques are used to investigate the effects of sputtering power, from 50 to 300 W, on structural, optical and electrical properties. It is found, from Grazing Incidence X-Ray Diffraction (GIXRD) analysis, that the sputtering power has a great influence on the crystalline quality of AZO films. A preferential orientation along the c-axis is obtained at lower sputtering power of 50 W. XPS analysis confirms the existence of only Zn and Al in the oxidized state in accordance with the XRD results. SEM and AFM observations of AZO films reveal dense and homogenous distribution of small grains on the surface. Analysis by UV–Vis–NIR spectroscopy at RT reveals that the average optical transmittance in the visible range (400–800 nm) of the AZO thin films is higher than 75%. Hall-effect measurements put into evidence that the electrical resistivity of the films increases with the increase of the sputtering power. A low resistivity of 1.65 × 10−3 Ω cm is obtained for the lowest sputtering power (50W). Moreover, AZO thin film deposited at 50 W and annealed for 1h at 400 °C under vacuum shows a lower resistivity of 5.75 × 10−4 Ω cm, an average optical transmission above 86% and a high figure of merit (ФTC) value of 3.4 × 10−3 Ω−1sq which suggests that the fabricated AZO thin films are promising for optoelectronic devices in the UV region.

Enhanced photoelectrochemical performance of Al-doped ZnO thin films prepared by co-spray technique

Aluminium doped zinc oxide (AZO) films were achieved using co-spray method. Different techniques including UV–Visible (UV–Vis), X-ray diffraction (XRD), electrochemical impedance spectroscopy (EIS), Fourier transform infrared (FTIR) and photoelectrochemical (PEC) were used to characterize the obtained films. XRD indicated that all films exhibited single-phase ZnO hexagonal Würtzite structure. AZO films showed a transparency more than 80% in the visible region with direct optical band gap ranging between 3.24 and 3.28 eV. A broad absorption band due to the stretching vibration of Zn–O bond located at 410 cm−1 was observed for all samples. In the high frequency region, all EIS spectra have only one single half-circle and the diameter decreases in the presence of a specific amount of Al (2–4%). The values of the exponent αCPE from EIS results indicated that ZnO:Al films presented a good homogeneity and smoothness. PEC study indicated that the film generates maximum photocurrents up to 1.2 μA/cm2 for a doping level of 2%.

Effect of Thickness and Doping Concentration of Aluminium to the Fabrication of ZnO Photoconductive Sensor

The effect of thickness of aluminium doped zinc oxide (AZO), doping concentration of Aluminium as well as the annealing temperature to the fabrication of AZO photoconductor sensor is the main focused of this research. Silvaco TCAD tool is used to fabricate the AZO photoconductor sensor and to obtain the electrical characteristics of the device by using its simulation software. The device performance was investigated with various thicknesses of AZO film, annealing temperatures and different Al doping concentration. The Al doping was varied 1×1017 cm−3, 1×1019 cm−3 and 1×1021 cm−3, the thickness was varied 0.2 nm, 1.0 nm and 1.2 nm and the temperature was varied from 300°C, 500°C and 700°C. The electrical properties of AZO photodetector were characterized using current voltage (I-V) measurement system respectively. The result shows that by changing the thicknesses of aluminium doped zinc oxide (AZO), and the doping concentration of aluminium in the zinc oxide have greatly improved the electrical characteristics as well as the electrical performances of the photoconductor sensor. Indeed, the post fabrication and simulation process using simulation tools is helpful for the determination of the parameters during the later process of the real device fabrication.

On the Structural, Optical and Electrical Characterization of Zinc Oxide and Aluminium doped Zinc Oxide for Optoelectronic Applications

Zinc Oxide (ZnO) and Aluminium doped Zinc Oxide (AZO) thin films are deposited on the glass slides by sol-gel spin coating technique. Zinc acetate dehydrate, 2 methoxyethanol, and diethanolamine are respectively used as a precursor, solvent, and stabilizer. Aluminium nitrate nonahydrate was used as the dopant source to obtain the atomic percentage of the dopant of 2%, 4%, 6% and 8%. The structural, optical, and electrical properties of the films were investigated using X-ray Diffraction (XRD), UV-visible spectrophotometry, and a Four-point probe technique respectively. The results from structural analyses show that the films are polycrystalline with a hexagonal wurtzite structure and a preferential orientation alongside the \(c\)-axis. The value obtained for the unit cell \(a=3.020\) A and \(c= 5.108\) A are in line with the reported literature. The transmittance of the films was observed within the visible region of the spectrum and the optical bandgap of the un-doped ZnO was established to be around 4.11 eV. However, the optical bandgap of the AZO films (4 and 6 at %) marginally decreases with doping concentration, which may be ascribed to the shrinkage of band effect due to the increase in carrier concentration. The lowest resistivity of \(3.53\times {10}^{-3}\,\Omega\) cm was observed for the doping concentration of 2% of Al. From the results, it was established that as the doping concentration increases, the thicknesses of the thin films were increased. Likewise, the increase in doping leads to a better uniformly distributed absorption spectra of the deposited AZO thin films.

Growth of c-plane and m-plane aluminium-doped zinc oxide thin films: epitaxy on flexible substrates with cubic-structure seeds.

Manufacturing high-quality zinc oxide (ZnO) devices demands control of the orientation of ZnO materials due to the spontaneous and piezoelectric polarity perpendicular to the c-plane. However, flexible electronic and optoelectronic devices are mostly built on polymers or glass substrates which lack suitable epitaxy seeds for the orientation control. Applying cubic-structure seeds, it was possible to fabricate polar c-plane and nonpolar m-plane aluminium-doped zinc oxide (AZO) films epitaxially on flexible Hastelloy substrates through minimizing the lattice mismatch. The growth is predicted of c-plane and m-plane AZO on cubic buffers with lattice parameters of 3.94-4.63 Å and 5.20-5.60 Å, respectively. The ∼80 nm-thick m-plane AZO film has a resistivity of ∼11.43 ± 0.01 × 10-4 Ω cm, while the c-plane AZO film shows a resistivity of ∼2.68 ± 0.02 × 10-4 Ω cm comparable to commercial indium tin oxide films. An abnormally higher carrier concentration in the c-plane than in the m-plane AZO film results from the electrical polarity along the c-axis. The resistivity of the c-plane AZO film drops to the order of 10-5 Ω cm at 500 K owing to the semiconducting behaviour. Epitaxial AZO films with low resistivities and controllable orientations on flexible substrates offer optimal transparent electrodes and epitaxy seeds for high-performance flexible ZnO devices.

Broad Frequency Shift of Parametric Processes in Epsilon-Near-Zero Time-Varying Media

The ultrafast changes of material properties induced by short laser pulses can lead to a frequency shift of reflected and transmitted radiation. Recent reports highlight how such a frequency shift is enhanced in spectral regions where the material features a near-zero real part of the permittivity. Here, we investigate the frequency shift for fields generated by four-wave mixing. In our experiment, we observed a frequency shift of more than 60 nm (compared to the pulse width of ∼40 nm) in the phase conjugated radiation generated by a 500 nm aluminium-doped zinc oxide (AZO) film pumped close to the epsilon-near-zero wavelength. Our results indicate applications of time-varying media for nonlinear optics and frequency conversion.

One-Step Synthesis of Porous Transparent Conductive Oxides by Hierarchical Self-Assembly of Aluminum-Doped ZnO Nanoparticles.

Transparent conductive oxides (TCOs) are highly desirable for numerous applications ranging from photovoltaics to light-emitting diodes and photoelectrochemical devices. Despite progress, it remains challenging to fabricate porous TCOs (pTCOs) that may provide, for instance, a hierarchical nanostructured morphology for the separation of photoexcited hole/electron couples. Here, we present a facile process for the fabrication of porous architectures of aluminum-doped zinc oxide (AZO), a low-cost and earth-abundant transparent conductive oxide. Three-dimensional nanostructured films of AZO with tunable porosities from 10 to 98% were rapidly self-assembled from flame-made nanoparticle aerosols. Successful Al doping was confirmed by X-ray photoemission spectroscopy, high-resolution transmission electron microscopy, elemental mapping, X-ray diffraction, and Fourier transform infrared spectroscopy. An optimal Al-doping level of 1% was found to induce the highest material conductivity, while a higher amount led to partial segregation and formation of aluminum oxide domains. A controllable semiconducting to conducting behavior with a resistivity change of more than 4 orders of magnitudes from about 3 × 102 to 9.4 × 106 Ω cm was observed by increasing the AZO film porosity from 10 to 98%. While the denser AZO morphologies may find immediate application as transparent electrodes, we demonstrate that the ultraporous semiconducting layers have potential as a light-driven gas sensor, showing a high response of 1.92-1 ppm of ethanol at room temperature. We believe that these tunable porous transparent conductive oxides and their scalable fabrication method may provide a highly performing material for future optoelectronic devices.

Influence of Zr50Cu50 thin film metallic glass as buffer layer on the structural and optoelectrical properties of AZO films**Project supported by the National Natural Science Foundation of China (Grant No. 51571085), the Key Science and Technology Program of Henan Province, China (Grant No. 19212210210), the Foundation of Henan Educational Committee, China (Grant No. 13B430019), and the Henan Postdoctoral Science Foundation,

Aluminum-doped ZnO (AZO) thin films with thin film metallic glass of Zr50Cu50 as buffer are prepared on glass substrates by the pulsed laser deposition. The influence of buffer thickness and substrate temperature on structural, optical, and electrical properties of AZO thin film are investigated. Increasing the thickness of buffer layer and substrate temperature can both promote the transformation of AZO from amorphous to crystalline structure, while they show (100) and (002) unique preferential orientations, respectively. After inserting Zr50Cu50 layer between the glass substrate and AZO film, the sheet resistance and visible transmittance decrease, but the infrared transmittance increases. With substrate temperature increasing from 25 °C to 520 °C, the sheet resistance of AZO(100 nm)/Zr50Cu50(4 nm) film first increases and then decreases, and the infrared transmittance is improved. The AZO(100 nm)/Zr50Cu50(4 nm) film deposited at a substrate temperature of 360 °C exhibits a low sheet resistance of 26.7 Ω/□, high transmittance of 82.1% in the visible light region, 81.6% in near-infrared region, and low surface roughness of 0.85 nm, which are useful properties for their potential applications in tandem solar cell and infrared technology.

Aluminum doped zinc oxide (AZO)-based pressure sensor

The feasibility of using aluminum-doped zinc oxide (AZO) as a piezoresistive pressure sensor material is reported. AZO has attracted a great deal of attention in many applications because of its nontoxicity, abundancy and lower cost than other materials such as indium tin oxide (ITO). The AZO films were deposited on polyethylene (PE) substrates by a radio-frequency (rf) magnetron sputtering method and they were used as the diaphragms of the pressure sensors. The piezoresistive sensor was tested for different pressures in vacuum and gage pressure conditions. The response characteristics indicated that resistance increased with the bending of the AZO layer in both compressive and tensile operation modes. The sensor characteristics exhibited that the AZO piezoresistive sensor can be used to measure ambient pressure quantitatively. This investigation indicated that AZO can be used as an alternative material for the fabrication of pressure sensors.

Effects of nitrogen/oxygen on the electrical and optical properties and microstructure of triple layer AZO/Ag/AZO thin films

Aluminum-doped zinc oxide (AZO)/Ag/aluminum-doped zinc oxide (AAA) specimens are prepared by varying O2 and N2 flow rates during the AZO deposition to investigate the gas effects on surface morphology, microstructure, conductivity type and optical and electrical properties. The AZO specimens are found to have decreases in grain size and RMS surface roughness (SRq), and an increase in compressive residual stress when the N2 flow rate increases higher than 2.5 sccm. The addition of O2 can contribute a similar tendency to grain size, SRq and residual stress; the grain size and SRq are always comparatively smaller, while the residual stress is higher than that prepared with N2. The grain size and SRq are obtained to be proportional to the AZO thickness, whereas they are inversely proportional to the compressive residual stress of AZO. Specimen D prepared with (N2: 0, O2: 2.5) sccm and having 60-nm AZO thickness possesses the strongest anti-reflection effect of the AAA structure, and therefore impedes the reflection from the Ag interlayer significantly. P-type conductivity is achieved by introducing the N2 into the AZO layers and (N2: 15, O2: 0) sccm is used to achieved the highest carrier concentration (CC) and mobility (Mb), and thus the lowest resistivity (R) of all specimens in this study. Increasing the O2 flow rate leads to decreases in Mb and CC, but can obtain the highest average transmittance ( T ¯ ) as (N2: 0, O2: 2.5) sccm is applied. The O2 flow rate becomes the dominant factor for the electrical and optical properties and microstructure as AZO films are deposited in the N2 and O2 mixed atmosphere. (N2: 15, O2: 0) sccm is used to achieve the highest CC and the lowest R of the oxide/metal/oxide specimens reported in the literatures.

Introduction of Ag nanoparticles by picosecond LIFT to improve the photoelectric property of AZO films

The photoelectric properties of conductive films are improved by doping Ag on aluminum-doped zinc oxide (AZO) films by laser induced forward transfer (LIFT). Firstly, the picosecond laser induced transfer mechanism of Ag films was revealed by numerical simulation; then, different-thickness Ag films were deposited on the AZO films by picosecond LIFT. When the film thickness is 30 nm and 50 nm, we have successfully obtained some Ag-AZO films with better optoelectronic properties by adjusting the laser parameters.

Interfacial Defect-Mediated Near-Infrared Silicon Photodetection with Metal Oxides.

Due to recent breakthroughs in silicon photonics, sub-band-gap photodetection in silicon (Si) has become vital to the development of next-generation integrated photonic devices for telecommunication systems. In particular, photodetection in Si using complementary metal-oxide semiconductor (CMOS) compatible materials is in high demand for cost-effective integration. Here, we achieve broad-band near-infrared photodetection in Si/metal-oxide Schottky junctions where the photocurrent is generated from interface defects induced by aluminum-doped zinc oxide (AZO) films deposited on a Si substrate. The combination of photoexcited carrier generation from both interface defect states and intrinsic Si bulk defect states contributes to a photoresponse of 1 mA/W at 1325 nm and 0.22 mA/W at 1550 nm with zero-biasing. From a fit to the Fowler equation for photoemission, we quantitatively determine the individual contributions from these effects. Finally, using this analysis, we demonstrate a gold-nanoparticle-coated photodiode that has three distinct photocurrent responses resulting from hot carriers in the gold, interface defects from the AZO, and bulk defects within the Si. The hot carrier response is found to dominate near the band gap of Si, while the interface defects dominate for longer wavelengths.