Transparent ZnO Research Articles

Fabrication of porous and visible light active ZnO nanorods and ZnO@TiO2 core-shell photocatalysts for self-cleaning applications.

Highly transparent and self-cleaning ZnO nanorods (NRs) and ZnO@TiO2 core-shell (CS) nanoarrays were fabricated using the sol-gel dip-coating technique. TiO2 nanoparticles (NPs) were coated as a shell layer over the hydrothermally grown ZnO NRs. The number of shell layers on the ZnO NRs was varied by modulating the number of dipping cycles from 1 to 3 to optimize their transmittance. The optimized CS nanoarrays with two dipping cycles display a 2% enhancement of optical transmission compared to the ZnO NRs. In addition, superhydrophilicity (contact angle ∼of 12°) stimulates the self-cleaning nature of the thin films. A water contact angle of 12° was noted for the ZnO@TiO2: 2 cycle sample, indicating their superhydrophilic nature. Moreover, the photocatalytic activity of the pristine ZnO NRs and ZnO@TiO2 CS nanoarrays was tested under UV and direct sunlight through the dye degradation of methylene blue (MB). Based upon the TiO2 morphology and accessibility of the ZnO@TiO2 heterojunction interface, CS nanoarrays with two shell layers exhibit the highest degree of dye photodegradation efficiency of 68.72% and 91% under sunlight and UV light irradiation, respectively. The CS nanoarrays demonstrate medium sunlight and excellent UV-light-driven photocatalytic activity. Our findings suggest that the ZnO@TiO2 CS nanoarrays are potential photocatalysts for dye degradation and self-cleaning applications in solar cell coverings.

Behavior of Al Impurity in ZnO Films: Influence of Al‐Level Doping on Structure, X‐Ray Photoelectron Spectroscopy and Transport Properties

Transparent and conductive ZnO:Al thin films have been deposited by a reactive magnetron sputtering using the layer‐by‐layer growth method. The grown Al‐doped ZnO films of about 100 nm thickness on Si and glass substrates have been investigated with respect to the crystalline phase by X‐ray diffraction (XRD), surface morphology by atomic force microscopy (AFM), chemical bonding, and the electronic structure by measuring the X‐ray photoelectron spectroscopy (XPS) core‐level and valence‐band spectra of the ZnO:Al films. The influence of Al content in ZnO films on structure, optical, and XPS spectra, transport parameters n, μ, ρ have been studied. The weak temperature dependence of conductivity in the temperature range 77–300 K suggests that Al3+ is a fully ionized impurity that provides the values of electron concentration in the range 7 × 1019–2.44 × 1020 cm−3. Al electroactivity (EA) in ZnO films is diminishing with Al content increases. A strong decrease in Al EA from 60% to 30% is observed for ZnO:Al films within 0.93–1.22 at% of Al that indicates on the enhancement of concentration of acceptor native and/or neutral complex defects in ZnO.

From transparent to black amorphous zinc oxide thin films through oxygen deficiency control

Despite the fact that zinc oxide is a well-known transparent oxide, several recent studies on “black” ZnO have renewed its potential for photocatalytic applications. We report on the control of oxygen deficiency in ZnO thin films grown at 300 °C on c-cut sapphire single-crystal substrates by pulsed electron beam deposition (PED) through a slight variation of argon pressure in PED. At a pressure of 2 × 10−2 mbar transparent, stoichiometric (ZnO) and crystalline films are obtained, while at 9 × 10−3 mbar black, oxygen-deficient (ZnO0.85) and amorphous films result. Stoichiometry, structural, and optoelectronic properties of transparent and black ZnO thin films were comparatively analyzed as a function of oxygen deficiency. Black ZnO thin films exhibit enhanced absorption in the visible and near-infrared due to oxygen deficiency, thus extending the range of applications of zinc oxide thin films from transparent electronics to solar absorbers and photocatalysis.

Low temperature chemical vapor deposition as a sustainable method to obtain c-oriented and highly UV luminescent ZnO thin films

We report the fabrication of c-oriented and transparent ZnO films with high UV/visible luminescent ratio on glass substrates using a very simple low temperature-atmospheric pressure chemical vapor deposition technique, where zinc acetate (ZA) and water vapor were the precursors. Both the ZA source and the glass substrate are in a same work temperature zone (240 °C) and no vacuum system is necessary. Highly c-oriented ZnO films were obtained with different growth rates (∼2 to 44 nm/min) by varying the source-substrate distance. The samples were characterized by scanning electron microscopy, X-ray diffraction and UV–vis, photoluminescence (PL) and energy-dispersive X-ray spectroscopies. Films grown with the highest growth rate exhibited narrower bandgap, which is correlated with a violet component in the PL spectra.

Hydrogen donor formation in a solution-processed ZnO film by UV light irradiation

A transparent ZnO film was deposited by the spin-spray method at a low temperature of 90 °C. Ultraviolet (UV) light irradiation to the film for 60 min decreased the film resistivity drastically by three orders of magnitude from 55.3 to 4.43 × 10−2 Ω cm. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profile and 1H solid-state nuclear magnetic resonance (NMR) spectroscopy analysis revealed that the as-deposited ZnO film before UV treatment contained a large amount of water in the ZnO crystal lattice, and hydrogen donors, including interstitial hydrogen in bond-centered sites (Hi), substitutional hydrogen on the oxygen lattice site (HO), and three O–H bonds in a zinc vacancy (VZn−H3), were generated in the ZnO film after UV treatment. From these results, the mechanism of the hydrogen donor formation was proposed in which UV irradiation to photocatalytic ZnO decomposed the water in the ZnO crystal lattice to form H+ and OH− ions, which associated with the oxygen and zinc vacancies, resulting in the formation of Hi, HO, and VZn−H3.

(Invited) Vertical Transparent ZnO TFT Based on a Stack of Thin-Films Deposited in Solution

A lot of works are devoted now to integrate driving TFT with OLED, minimizing the area of the pixel. In other part, people try to fabricate devices using low cost technologies as deposition in solution avoiding any photolithography step in the process. Finally, it can be better if the full driving TFT-OLED device is transparent.The present work try to answer simultaneously to these 3 desires. Particularly, the present paper focus on the TFT.Vertical TFT will answer to the first desire to minimize the area of the device. It is based here on a stack of the gate, the insulator, the source, the semiconducting active layer and the drain; meaning 5 films all solution deposited.Thanks to the present available insulator solutions for the deposition, different insulators can be chosen. High k HFO2 insulator is chosen in this work. It is is spincoated and then annealed at 300C during 1 hour. Figure 1 shows the current-voltage curves of ITO/HFO2/AgNWs stack. After rapid increase, the current stabilizes at 10 pAmp at least still a voltage of 10V, meaning a field of 7.7x105 V/cm2 with a thickness of 130 nm. Present HFO2 film is then a good insulator.ZnO is chosen as active layer. It is spin-coated and then annealed at 180C. This temperature was found enough high to lower the conductivity (Figure 2)The source and drain layers are spin-coated using Ag nanowires (AgNWs) solution. As source AgNWs layer is deposited before ZnO layer, it will be annealed at the same temperature 180C. Figure 3 shows that AgNWs film stays enough conducting after such annealing temperature. It can be used as conducting source and drain.Finally, the full stack of 5 films is characterized as a TFT.Figure 4 shows the transfer characteristic of this TFT plotted at a drain voltage of 2.6V. The drain current is modulated by 4 orders from off to on regime.Very simple and low cost fully in solution process is presented in this work. Working transparent TFT is demonstrated. The process shows its high potential in the development of integrated TFT-OLED in small area. It is very promising.Its shows also the need of future hard work in this way to demonstrate fully integrated driving TFT-OLED Figure 1

Effects of Electron Irradiation on the Properties of ZnO/Au/ZnO Films Deposited on Poly-Imide Substrates

Transparent and conductive ZnO 50 nm/Au 8 nm/ZnO 50 nm tri-layer films were deposited on poly-imide films by radio frequency (RF) and direct current (DC) magnetron sputtering at room temperature, and then the effect of electron irradiation on the crystallization, electrical resistivity and optical properties of the films was considered with X-ray diffraction, UV-visible spectrometer, Atomic force microscope and Hall measurement system. All the films were deposited at a fixed sputtering power, Ar gas flow rate, and distance between target and substrate, while the post-deposition electron irradiation energy was varied from 300 to 900 eV. The electron irradiated films exhibited a flatter surface than the as deposited films that were not electron irradiated, and the XRD patterns also revealed that the electron irradiated films had larger grain sizes than that of as deposited films. The films electron irradiated at 900 eV also showed a higher visible transmittance of 79.8% and a lower sheet resistance of 56.0 Ω/□. Post-deposition films electron irradiated at 900 eV showed a higher figure of merit of 1.86×10-3 Ω-1 than that of the as deposited film of 1.29×10-3 Ω-1. The optical band gap was also enhanced by electron irradiation. The films electron irradiated at 900 eV showed a higher optical band gap of 4.07 eV.

Ultraviolet and visible photo-response of transparent conductive Al-doped ZnO (AZO)/n-Silicon isotype heterojunction device

In this work, the electrical and photoresponse measurements of a transparent conductive Al-doped ZnO (AZO)/n-Si heterojunction device were conducted in visible light and UV wavelengths. AZO film was deposited by sputtering onto an n-Si wafer and investigated by means of morphological, chemical and electrical characterizations. The AZO/n-Si rectifying device exhibits an excellent reproducibility without noticeable variations after 90 days of measurements. At self-powered mode, the maximum on/off ratios were determined as 3081 for visible light and 4778 for UV light illumination of 365 nm. The responsivity and detectivity of the AZO/n-Si photodetector were 0.128 A W−1 and 1.05 × 1011 Jones for 365 nm, whereas they were 0.055 A W−1 and 4.60 × 1010 Jones for 395 nm, respectively (at −2.0 V). This study demonstrated that the n-AZO/n-Si isotype heterojunction photodetector was fabricated at low cost and it is a potential candidate in both the visible region and the UV region with a good performance, in contrast to the widely studied pn heterojunctions.

Transparent ZnO resistive switching memory fabricated by neutral oxygen beam treatment

In this work, a Cu/ZnO/ITO resistive random access memory (RRAM) structure in which ZnO films are irradiated with neutral oxygen beams was employed to investigate the effect of neutral oxygen beams as a surface treatment. It was confirmed that the treatment reduced the defect concentration in the sputtered-ZnO film and improved the resistance change characteristics of the device. These results indicate the great potential of neutral oxygen beams in the development of RRAM devices using ZnO films.

Enhancing transparent thermoelectric properties of Sb-doped ZnO thin films via controlled deposition temperature

ZnO is known as a wide-bandgap transparent thermoelectric material. Herein, the electronic transport and temperature-dependent thermoelectric properties of transparent Sb-doped ZnO (SZO) films are reported. The sputtered SZO films prepared at the optimum deposition temperature of 673 K show an n-type semiconducting behavior with the highest thermoelectric power factor of 406 μW/mK2 at 773 K, and a good transmittance ∼82% in the wavelength region of 400–1100 nm. It can well meet the requirements for transparent thermoelectric thin films. Through X-ray diffraction, UV–Vis, and photoluminescence analyses, the Zn2+ substitution by Sb5+ can be elucidated for significantly enhancing the Seebeck coefficient and thermoelectric power factor due to the increase of density-of-state effective mass.

Improved conductivity and flatness of solution-processed transparent multilayer ZnO conductive films by stacking method

ZnO stacked films are prepared by a wet process under various conditions. Al and Ga are co-doped as dopants, i.e., producing AGZO films. The doping density dependence of the film resistivity shows that the best doping condition is 1.0 wt% of Ga-doping density and 0.1 wt% of Al-doping density, where the minimum sheet resistance of 1.2 × 105 Ω/□ and the best orientation of (002) surface are observed. The stacked AGZO films are prepared by two different methods to improve film properties: Method A is repeated provisional annealing, spin coating, and main annealing, where the maximum number of repetitions is three. On the other hand, method B is repeated provisional annealing and spin coating followed by annealing the stacked films only one time. As a result, the AGZO films produced by method B reveal a minimum surface roughness of about 3.2 nm. In addition, with an increase of the stacked layers, the sheet resistance is decreased.

Structural Engineering Effects on Hump Characteristics of ZnO/InSnO Heterojunction Thin-Film Transistors.

Transparent conductive oxides (TCO) have been extensively investigated as channel materials for thin-film transistors (TFTs). In this study, highly transparent and conductive InSnO (ITO) and ZnO films were deposited, and their material properties were studied in detail. Meanwhile, we fabricated ZnO/ITO heterojunction TFTs, and explored the effects of channel structures on the hump characteristics of ZnO/ITO TFTs. We found that Vhump–VON was negatively correlated with the thickness of the bottom ZnO layer (10, 20, 30, and 40 nm), while it was positively correlated with the thickness of the top ITO layer (3, 5, 7, and 9 nm), where Vhump is the gate voltage corresponding to the occurrence of the hump and VON is the turn-on voltage. The results demonstrated that carrier transport forms dual current paths through both the ZnO and ITO layers, synthetically determining the hump characteristics of the ZnO/ITO TFTs. Notably, the hump was effectively eliminated by reducing the ITO thickness to no more than 5 nm. Furthermore, the hump characteristics of the ZnO/ITO TFTs under positive gate-bias stress (PBS) were examined. This work broadens the practical application of TCO and provides a promising method for solving the hump phenomenon of oxide TFTs.

Low-voltage, self-powered and broadband photodetector with Ohmic, transparent and cost-effective AZO electrodes on vertical aligned MoS2 flakes

The transparent Ohmic electrodes are the extreme necessity to develop cutting-edge broadband photodetectors. Conventional metal electrodes reflect incident light and diminish overall performance of photodetector. Here, we report cost-effective, transparent Ohmic Al-doped ZnO (AZO) electrodes to develop self-powered, highly responsive photodetectors from visible to near-infrared regions using vertically-aligned MoS2 (VA- MoS2) flakes. The MoS2 photodetector with AZO electrode (AZO-MoS2) shows a high responsivity of 40.67 A/W at 800 nm at low biasing voltage of 0.5 V. The self-driven behavior is observed in a broad range from 400 to 1100 nm with high responsivity of 38.18 mA/W at 800 nm. Moreover, AZO-MoS2 device has fast rise and decay time. To understand impact of Ohmic AZO electrode in comparison to traditional metal electrodes, we have also investigated conventional Au-MoS2 PDs fabricated under identical dimensions. The AZO-MoS2 PD demonstrated 100 times higher responsivity than conventional Au-MoS2 PD at 800 nm. We have investigated band-alignment between AZO and MoS2 using ultraviolet spectroscopy (UPS) and confirmed ohmic nature between the AZO and MoS2. These results indicate superior performance of AZO-MoS2 PD device in comparison to traditional metal electrodes.

Single-step process for transparent conductive ZnO:Ga films with uniform ultrahigh haze by oblique angle deposition

Increasing the haze of front electrodes of solar cells while retaining high optical transmittance is beneficial for increasing the power conversion efficiency. However, conventional methods of fabricating hazy films require additional etching steps or materials. Moreover, depositing large-area transparent conductive oxides with uniform ultrahigh haze is challenging. Here, we combine the oblique angle deposition and the atmospheric pressure plasma jet to produce GZO with a uniform ultrahigh haze of > 80% (non-uniformity ~1.35%), high transmittance of 88% (referenced to the substrate), and resistivity of 1.96 × 10–3 Ω cm in a single step. We show that the high haze is caused by the “pre-deposition” of adsorbed particles on the bare substrate downstream and that the upstream dummy area should be avoided for high uniformity. Unlike existing methods, our method produces uniform films with ultrahigh haze and good transmittance in a single step without additional etching/ materials or changing parameters during operation.

Highly transparent and conductive annealed ZnO nanoparticles thin film prepared by pulsed laser deposition

Highly transparent and conductive annealed ZnO nanoparticles thin film prepared by pulsed laser deposition

Investigation on Transparent, Conductive ZnO:Al Films Deposited by Atomic Layer Deposition Process.

Transparent electrodes are a core component for transparent electron devices, photoelectric devices, and advanced displays. In this work, we fabricate fully-transparent, highly-conductive Al-doped ZnO (AZO) films using an atomic layer deposition (ALD) system method of repeatedly stacking ZnO and Al2O3 layers. The influences of Al cycle ratio (0, 2, 3, and 4%) on optical property, conductivity, crystallinity, surface morphology, and material components of the AZO films are examined, and current conduction mechanisms of the AZO films are analyzed. We found that Al doping increases electron concentration and optical bandgap width, allowing the AZO films to excellently combine low resistivity with high transmittance. Besides, Al doping induces preferred-growth-orientation transition from (002) to (100), which improves surface property and enhances current conduction across the AZO films. Interestingly, the AZO films with an Al cycle ratio of 3% show preferable film properties. Transparent ZnO thin film transistors (TFTs) with AZO electrodes are fabricated, and the ZnO TFTs exhibit superior transparency and high performance. This work accelerates the practical application of the ALD process in fabricating transparent electrodes.

Impact of the secondary phase ZnS on CZTS performance solar cells using simulation program

In the present work ,Scaps 1d software is used to study the effect of ZnS secondary phase on the performance of solar cells. The standard CZTS-based devices are composed of a multilayer thin film formed on soda-lime glass (SLG), Mo/MoS2/CZTS/Cds/i-ZnO/ZnO Al. a metallic molybdenum (Mo) back electrode, MoS2 built between the absorber and the back contact; a p-CZTS absorber layer; an n-type buffer layer, typically Cds to form the junction of the solar cell, a TCO doped ZnO layer and a transparent ZnO:Al front contact. Ni/Al metal contact grids complete the cell. An ultrathin layer of ZnS is placed in the standard solar cell structure between the Cds buffer layer and the CZTS absorber layer to represent the second phase (SP) above CZTS [1].Cell characteristics such as ƞ, FF, Jsc , and Voc were studied. The formation of ZnS on the surface of CZTS has negative effects on the solar cell parameters where the conversion efficiency ƞ decreases.The impact of this layer on the performance of CZTS solar cells is also investigated through the results obtained from the analysis of the complex impedance (Z*) and the modulus (M*). An equivalent circuit of the solar cell consisting of series and parallel resistors has been evolved to study the electrical behavior around each interface of this structure.This new procedure has been very beneficial for the identity of the most important parameters of the diffusion and recombination process.

Transparent conductive ZnO:Al/Cu mesh composite film and its electric heating performance

Transparent conductive films (TCFs) play an indispensable role in optoelectronic devices because of their high conductivity and high optical transmittance. In order to obtain indium-free transparent conductive films with better performance, we need to improve the conductivity, while not damaging the transmittance. Metal mesh is highly conductive but prone to oxidation and abrasion, while transparent conductive oxide (TCO) is stable but less conductive. Thus, we composite the metal mesh with the stable TCO to achieve complementary advantages. In this work, we fabricate a hexagonal Cu mesh and then cover the Cu mesh with Al-doped ZnO (AZO) film by using lithography and magnetron sputtering. The line width and length of mesh are 15 µm and 150 µm, respectively, which are not visible to the naked eye. The effect of AZO growth temperature on the properties of such AZO/Cu mesh composite film is studied and the optimal temperature is 300 ℃. By designing the mesh and optimizing the process, the transmittance (400–800 nm), sheet resistance and FoM of AZO/ Cu mesh composite film reach 86.4%, 4.9 Ω/sq and 4.73 × 10<sup>–2</sup> Ω<sup>–1</sup>, respectively, thus possessing both transparent and conductive property. Because of its low cost, competitive optoelectronic performance and stability, the potential applications of AZO/Cu mesh composite film in transparent electronics are fantastic. When used as a transparent conductor to connect LED to 3 V DC power, the luminance of LED in series with AZO/Cu mesh composite film is lighter than that of AZO film and Cu mesh. According to the Ohmic heating effect of electric current passing through a conductor, AZO/Cu mesh composite film can be designed as electric heating film. At low voltage safe for human body, AZO/Cu mesh composite film can implement fast, uniform and stabile heat. In the cyclic electric heating test, the AZO/Cu mesh composite film can be heated rapidly to 175 ℃ all the time, showing a fast temperature response and stable cyclic performance. More importantly, the AZO is itself transparent and conductive and prevents the metal from oxidizing effectively, thus ensuring the overall performance and maintaining the electric heating response. The experimental result and simulation application show that the AZO/Cu mesh composite film has a great potential application in transparent and heating film for defogging and defrosting glass.

ZnO–PTFE-based antimicrobial, anti-reflective display coatings and high-sensitivity touch sensors

Transparent ZnO–PTFE composite thin films deposited via RF co-sputtering are demonstrated as antireflective and antimicrobial display coatings and high-sensitivity antimicrobial touch sensor for a wide range of applications in interactive devices for a safer experience.

Comparative study of optical properties of ZnO and Zn0.95La0.05O thin films

In the present work, transparent ZnO and Zn0.95La0.05O films were prepared at a deposition temperature of 400 0C on the glass substrate through the chemical spray pyrolysis technique. The precursor solutions were prepared by using salts such as Zinc acetate dihydrate, Lanthanum chloride heptahydrate, and double distilled water as the solvent. The optical band gap values were observed to be considerably dependent on La ion dopant, which can be explained using Burstein-Moss (B-M) shift. Urbach energy increases from 108.386 meV to 160.295 meV for ZnO and Zn0.95La0.05O films respectively, indicating poor crystallinity of the deposits with La content. According to the photoluminescence (PL) investigation, La ion doping increased the intensity of impurity levels in the bandgap of the host matrix, which can broaden its visible-light sensitivity, making it advantageous for potential useful applications. Hence, these results indicated that lanthanum doping plays a vital role in tuning the optical bandgap and photoluminescence property of the ZnO nanostructure.