Thin ZnO Layer Research Articles

Resonant Tunneling Induced Enhancement of Electron Field Emission by Ultra-Thin Coatings

The emission of electrons from the surface of a material into vacuum depends strongly on the material’s work function, temperature, and the intensity of electric field. The combined effects of these give rise to a multitude of related phenomena, including Fowler-Nordheim tunneling and Schottky emission, which, in turn, enable several families of devices, ranging from vacuum tubes, to Schottky diodes, and thermionic energy converters. More recently, nanomembrane-based detectors have found applications in high-resolution mass spectrometry measurements in proteomics. Progress in all the aforementioned applications critically depends on discovering materials with effective low surface work functions. We show that a few atomic layer deposition (ALD) cycles of zinc oxide onto suspended diamond nanomembranes, strongly reduces the threshold voltage for the onset of electron field emission which is captured by resonant tunneling from the ZnO layer. Solving the Schroedinger equation, we obtain an electrical field- and thickness-dependent population of the lowest few subbands in the thin ZnO layer, which results in a minimum in the threshold voltage at a thickness of 1.08 nm being in agreement with the experimentally determined value. We conclude that resonant tunneling enables cost-effective ALD coatings that lower the effective work function and enhance field emission from the device.

A fiber optic refractive index sensor with a high index ZnO overlay

An intrinsic fiber optic sensor is developed by inserting a section of no-core fiber between two multimode fibers and is applied for refractive index sensing. The sensing principle is based on modulation of evanescent waves, and hence intensity of transmitted light. The sensitivity of the sensor is found to increase by coating the sensing region with a thin layer of high index ZnO. The number of ZnO layers is also optimized and a high sensitivity of 804.57%/RIU is obtained for nine layers of ZnO-coated sensor, with a fast response time of 5 s. Apart from high sensitivity and a fast response time, the experimental investigations also indicate that the proposed sensor produces fairly accurate, stable and repeatable results, with negligible cross sensitivity to environmental temperature.

Charge transport studies on chemically grown manganite based heterostructures

In this communication, we have successfully fabricated mixed valent La0.7Ca0.3MnO3 (LCMO) manganite based (i) ZnO/LCMO/LAO and (ii) LMO/LCMO/LAO (LMO: LaMnO3–d thin layer; LAO: LaAlO3 substrate) thin film heterostructures using chemical solution deposition (CSD) method. 100 nm LCMO layer was initially grown on single crystalline (100) LAO substrate followed by the growth of 50 nm ZnO and LMO layers separately on the two different heterostructures. In the present study, upper layers of ZnO and LMO were intentionally prepared at 700 °C for 12 h under air environment, thereby some naturally created oxygen vacancies are expected to be present in their lattices. Presence of oxygen vacancies makes ZnO and LMO layers as n–type oxides in the heterostructures. Temperature dependent current–voltage (I–V) characteristics and interface resistivity (under different applied electric fields across interface only) were carried out to understand their charge transport behavior. A strong effect of electric field on the resistivity behavior has been observed due to a reasonable electrically polarizable (active) nature of ZnO and LMO thin layers. Zener double exchange (ZDE) polynomial law has been employed to understand various scattering processes as source of resistivity across, both, ZnO/LCMO and LMO/LCMO interfaces. Transport properties and charge conduction mechanisms have been discussed and compared for both the interfaces in the context of interface state and barrier between electrically active layer and LCMO film. Also, power consumption criteria have been discussed in detail for the presently studied heterostructures for their practical device applications such as field effect devices, memory devices, read–write head devices or any other spintronic devices.

Fast operation of a WO3-based solid-state electrochromic transistor

Electrochromic transistors (ECTs) have attracted attention as advanced memory technology because one can use both electrochromism and switching of electrical conductivity in a nonvolatile manner. Although several solid-state ECTs have been proposed so far, their operating speed is still slow (operating time >1 min) as compared to liquid-based ECTs (∼20 s) due to their asymmetric gate-source electrode configuration. Here we demonstrate a fast operation of a solid-state ECT. We fabricated a solid-state ECT with three terminal gate-source-drain electrodes using an amorphous WO3 film as the electrochromic material and amorphous TaOx as the solid electrolyte. By the insertion of a thin ZnO layer between the source and drain electrodes to achieve pseudo symmetric gate-source electrode configuration, we greatly reduced the operation time to less than 1 s at ±3 V application while keeping the on-to-off ratio of ∼30. The present approach is effective to improve the operating speed of ECTs and may be practically used in advanced memory technologies.

Characterization of performance parameters of organic solar cells with a buffer ZnO layer**Invited talk at 9th International Workshop on Advanced Materials Science and Nanotechnology (IWAMSN2018), 7-11 November 2018, Ninh Binh City, Vietnam

By embedding a thin ZnO layer sandwiched between the hole transport and photoactive layers, organic solar cells (OSC) based on poly(3-hexylthiophene) (P3HT) were prepared by spincoating. UV–vis spectra of the composite films showed that ZnO exhibited a suitable buffer layer that could block holes movement throughout the heterojunction of ITO/ZnO. The enhancement in the fill factor (FF) of the buffer-OSC (BOSC) is attributed to the presence of nanoheterojunctions of ZnO/PCBM and ZnO/ITO. For the normal temperature, the increase of the open-circuit potential and short-circuit current resulted in an overall increase of the energy conversion efficiency. Comparing to OSCs without buffer layer (WOSC), the laminar structure of ITO/ZnO/P3HT/PCBM/Li/Al cells possess a much larger photovoltaic energy conversion efficiency, namely 2.12% (for BOSC) compared to 1.75% (for WOSC).

WO3/BiVO4 Type-II Heterojunction Arrays Decorated with Oxygen-Deficient ZnO Passivation Layer: A Highly Efficient and Stable Photoanode

In the present work, we report a ternary WO3/BiVO4/ZnO photoanode with boosted PEC efficiency and stability toward highly efficient water splitting. The type-II WO3/BiVO4 heterojunction arrays are firstly prepared by hydrothermal growth of WO3 nanoplate arrays onto the substrates of fluorine-doped tin oxide (FTO)-coated glass, followed by spin-coating of BiVO4 layers onto the WO3 nanoplate surfaces. After that, thin ZnO layers are further introduced onto the WO3/BiVO4 heterojunction arrays via atomic layer deposition (ALD), leading to the construction of ternary WO3/BiVO4/ZnO photoanodes. It is verified that the ZnO thin layer in the WO3/BiVO4/ZnO photoanode contains abundant oxygen vacancies, which could act as an effective passivation layer to enhance the charge separation and surface water oxidation kinetics of photogenerated carriers. The as-prepared WO3/BiVO4/ZnO photoanode produces a photocurrent of 2.96 mA cm-2 under simulated sunlight with an incident photon-to-current conversion efficiency (IPCE) of ∼72.8% at 380 nm at a potential of 1.23 V versus RHE without cocatalysts, both of which are comparable to the state-of-the-art WO3/BiVO4 counterparts. Moreover, the photocurrent of the WO3/BiVO4/ZnO photoanode shows only 9% decay after 6 h, suggesting its high photoelectrochemical (PEC) stability.

Fabrication and comparison of dye-sensitized solar cells by using TiO2 and ZnO as photo electrode

Five natural dyes were extracted from the peels of Malus domestica, Punica granatum, Phoenix dactylifera, Raphanus sativus and Solanum melongena by maceration in methanol for application in dye-sensitized solar cells. Their photophysical, electrochemical and photovoltaic properties were investigated. Thin layers of nano materials ZnO and TiO2 were deposited on FTO glasses by doctor’s blade technique. These films were dipped in the dye solution bath for twenty-four hours. The results verified that Solanum melongena dye has better efficiency 1.10% as compared to other dyes. We also investigated that TiO2 is a better photoelectrode as compared to ZnO. By using Solanum melongena andTiO2 nanoparticles we found 1.10% efficiency and 0.67% by using ZnO as photoelectrode. The results indicate that TiO2 is a better photoelectrode as compare to ZnO. Lower efficiency of the natural dyes as compared to synthetic dyes may be due to lower active contents for DSSCs.

EFFECT OF SEEDING FILM TYPE ON MORPHOLOGY AND ELECTRICAL PROPERTIES OF Ga-DOPED ZnO NANOSTRUCTURES GROWN BY HYDROTHERMAL PROCESS

In the present work, we are interested in modifying the structures and electrical properties of Ga-doped ZnO (GZO) nanostructures by incorporation of Ga and F elements into ZnO seeding films. The ZnO, Ga-doped ZnO and F-doped ZnO thin film layers were grown by sol–gel dip coating onto glass substrates. The GZO nanostructures were grown by hydrothermal method starting from zinc nitrate and gallium (III) nitrate hydrate with a crystal growth assistance using the different types of seeding film layers. The X-ray diffractometer (XRD) and field emission scanning electron microscope (FE-SEM) are observe for the crystal structures and surface morphologies of GZO nanostructures. The changes in electrical resistance of the GZO nanostructures due to the different seeding layers were analyzed by the four-point probe technique. It is shown that the structural and electrical properties are found to be affected by the types of seeding layers.

Identification of the Au/ZnO interface as the specific active site for the selective oxidation of the secondary alcohol group in glycerol

In this work, a catalyst containing an Au/ZnO interface was synthesized using controlled thermal treatment to generate a strong metal–support interaction between Au and a thin amorphous ZnO layer, and was used for the aqueous oxidation of glycerol under base-free conditions. We provide evidence that the Au/ZnO interface is the specific active site for selective catalytic oxidation of the secondary alcohol group in glycerol, without affecting the primary alcohol groups, thus affording dihydroxyacetone with high selectivity. Characterization of the active catalyst suggests that ZnO species migrate to the surface of Au particles during thermal pretreatment in air. An Au–O–Zn interface is then formed, along with an abundance of oxygen vacancies. The resulting interfacial sites can specifically adsorb and activate the secondary alcohol group. The study provides more general insight into the role of Au-containing interfaces as the specific active sites in selective oxidation reactions.

Enhanced light absorption in simulations of ultra-thin ZnO layers structured by a SiO2 photonic glass

Hierarchically organized nanostructures are often employed to improve the energy conversion efficiency of photovoltaic and photoelectrochemical cells. Ultra-thin semiconductors can improve the internal carrier collection yield in materials with poor carrier lifetimes by reducing the characteristic length scales of collection. However, reducing the dimension of the light absorber requires strategies to increase absorption and the overall photogeneration when the material is to be used in broadband solar energy conversion applications. Here, we explore a strategy for improving light absorption in nanometer-scale, ultra-thin film ZnO layers by integrating them into a SiO2 colloidal crystal-based photonic glass. We use three-dimensional finite-difference electromagnetic simulations to study the local and total absorption improvements on composite films of close-packed, randomized colloidal structures coated with a thin layer of ZnO. These simulations show that the near band-gap absorption in the ZnO coating is dependent on the degree of vacancies in the colloidal crystal that templates the photonic glass. With these results, we show that disordered, defective colloidal composites can potentially be used to fabricate nanostructured photoelectrodes based on ultra-thin semiconductor layers with improved light absorption characteristics.

Electrical properties of Au–Cu/ZnO/p-Si diode fabricated by atomic layer deposition

The electrical properties of the Au–Cu/ZnO/p-Si diode were investigated with the temperature dependent current–voltage measurements in a wide temperature range from 220 to 360 K with 20 K steps and also frequency dependent capacitance–voltage and conductance–voltage measurements using admittance spectroscopy by changing frequency from 1 to 1000 kHz. The ZnO thin film layer was deposited by atomic layer deposition technique (ALD) to obtain homogenous interface layer and this layer surface was characterized with AFM analyses. Assuming thermionic emission (TE) model, diode parameters such as barrier height and ideality factor were determined for the Au–Cu/ZnO/p-Si diodes and the strong temperature dependence of these values were modeled by TE model with modified by Gaussian distribution of the barrier height. Therefore, the observed non-ideal current behavior was discussed on the basis of barrier inhomogeneity and the presence of native oxide interfacial layer in the diode. The mean barrier height was found as 1.38 eV with 0.18 standard deviation, and the Richardson constant modified by this model was obtained as 28 A/cm2k2 close to the theoretical value. Additionally, the distribution profile of the interface states and series resistance value were evaluated by the capacitance and conductance characteristics. It could be seen from the results that both of them strongly depends on the device frequency. As a result of these works, fabricated Au–Cu/ZnO/p-Si diodes may be used for next technological application in wide range temperature in industry.

Indium phosphide based solar cell using ultra-thin ZnO as an electron selective layer

According to the Shockley–Queisser limit, the maximum achievable efficiency for a single junction solar cell is ~33.2% which corresponds to a bandgap (Eg) of 1.35 eV (InP). However, the maximum reported efficiency for InP solar cells remain at 24.2% ± 0.5%, that is >25% below the standard Shockley–Queisser limit. Through a wide range of simulations, we propose a new device structure, ITO/ ZnO/i-InP/p+ InP (p-i-ZnO-ITO) which might be able to fill this efficiency gap. Our simulation shows that the use of a thin ZnO layer improves passivation of the underlying i-InP layer and provides electron selectivity leading to significantly higher efficiency when compared to their n+/i/p+ homojunction counterpart. As a proof-of-concept, we fabricated ITO/ZnO/i-InP solar cell on a p+ InP substrate and achieved an open-circuit voltage (Voc) and efficiency as high as 819 mV and 18.12%, respectively, along with ~90% internal quantum efficiency. The entire device fabrication process consists of four simple steps which are highly controllable and reproducible. This work lays the foundation for a new generation of thin film InP solar cells based solely on carrier selective heterojunctions without the requirement of extrinsic doping and can be particularly useful when p- and n-doping are challenging as in the case of III–V nanostructures.

Investigation on Structural and Optical Properties of ZnO Film Prepared by Simple Wet Chemical Method

ZnO thin layer has a broad potential application in electronic and optoelectronic devices. In this study, vertically align ZnO layers were deposited on ITO glass using wet chemistry method. The seed layers were prepared using electrodeposition technique at 3°C. The growing process was carried out using chemical bath deposition at 90°C. To improve the structural properties, two different hydrothermal treatment variations were applied separately. From the experiment, it is shown that the hydrothermal process using N 2 gas has given better results, compared to the conventional treatment.

Spray-Pyrolyzed ZnO as Electron Selective Contact for Long-Term Stable Planar CH3NH3PbI3 Perovskite Solar Cells

Electron selective contacts (ESCs) play an important role in the performance of perovskite solar cells (PSCs). ZnO has attracted important attention as a good material for ESCs because of its matched energy levels with those of perovskite, its high transmittance in the visible region, and its high electron mobility. Here we reported the use of ZnO thin layers prepared by spray pyrolysis as ESC for PSCs. Our ZnO based planar CH3NH3PbI3 (MAPbI3) devices not only were stable in a humidity of 35% but also improved the performance even after more than 1 month of preparation, due to an increase of charge transfer at the ZnO interface as it has been characterized by impedance spectroscopy. The formation of ZnO depends on preparation conditions such as gas flow, zinc acetate solution concentrations, and substrate temperatures, all of which have an effect on the performance of stability of MAPbI3 solar cells. Also, the low hysteresis reported for these samples was discussed in this study. We have also observed tha...

Manipulation of optical and electrical properties of ZnO thin films via embedded nano structure

The semiconductor ZnO has gained great interest in the past decades because of its wide bandgap and large exciton binding energy. Especially, the doping and embedded metal nano-particles methods could improve the conductivity of ZnO thin films, which made it possible to fabricate transparent conducting oxides based on ZnO. In the present study, Al-doped ZnO thin layer was employed to manipulate the electrical and optical properties. The band engineering in ZnO makes it possible to manipulate the absorption, emission, reflection spectral of ZnO via the embedded quantum well layer. The ZnO thin films with excellent performance in electrical conductivity and optical transmittance are promising to be applied in the devices including the lager flat screen, touch panel, portable digital device and smart windows. In the present study, we summarized the recent progress in ZnO with embedded metal nanoparticles and application in TCOs.

Performance evaluation of free-silicon organic-inorganic hybrid (SiO2-TiO2-PVP) thin films as a gate dielectric

An approach to achieve the transparent thin film transistors (TTFTs) using transparent electrodes such as ITO or FTO for gate, source and drain electrodes (Free-silicon gate electrode). On the other hand, the high-k transparent metal oxides can be used for this purpose in gate dielectric by a low temperature sol-gel technique. Furthermore, not only use of high-k organic materials in gate dielectric reduces the leakage current but also it improves the device flexibility. The organic-inorganic hybrid composites of SiO2/TiO2-PVP (STP) with different TiO2 ratios are deposited on PET-ITO substrates (Free-silicon gate electrode) as gate dielectric. The thermal behavior and structural characterization of STP films are done by TGA, XRD and XPS analyses. The AFM analyze of STP films shows that they have very smooth surface. A transparent thin layer of ZnO is used as a transistor channel by sputtering technique. Evaluation on electrical performance of OFETs exhibits that they have high mobility, low threshold voltage and acceptable Ion/Ioff ratio.

Effect of gate dielectric on the performance of ZnO based thin film transistor

The paper report the fabrication and characterization of two different sets of bottom gate top contact ZnO thin film transistors (TFTs) using SiO2 and Al2O3 dielectric layers in an attempt to compare and contrast the effect of gate dielectrics on performance of the two devices. ZnO thin film and dielectric layers were deposited at room temperature by radio frequency (RF) sputtering method. The results were validated by those obtained using commercial software tool ATLAS and an analytical model reported by others. The ZnO TFT with Al2O3 dielectric exhibited superior electrical performance as compared to the ZnO TFT with SiO2 dielectric. The study also revealed that the performance of ZnO/Al2O3 based TFTs are far more consistent and reliable as compared to their ZnO/SiO2 counterpart.

TiN/Al2O3/ZnO gate stack engineering for top-gate thin film transistors by combination of post oxidation and annealing

Control of fabrication processes for a gate stack structure with a ZnO thin channel layer and an Al2O3 gate insulator has been examined for enhancing the performance of a top-gate ZnO thin film transistor (TFT). The Al2O3/ZnO interface and the ZnO layer are defective just after the Al2O3 layer formation by atomic layer deposition. Post treatments such as plasma oxidation, annealing after the Al2O3 deposition, and gate metal formation (PMA) are promising to improve the interfacial and channel layer qualities drastically. Post-plasma oxidation effectively reduces the interfacial defect density and eliminates Fermi level pinning at the Al2O3/ZnO interface, which is essential for improving the cut-off of the drain current of TFTs. A thermal effect of post-Al2O3 deposition annealing at 350 °C can improve the crystalline quality of the ZnO layer, enhancing the mobility. On the other hand, impacts of post-Al2O3 deposition annealing and PMA need to be optimized because the annealing can also accompany the increase in the shallow-level defect density and the resulting electron concentration, in addition to the reduction in the deep-level defect density. The development of the interfacial control technique has realized the excellent TFT performance with a large ON/OFF ratio, steep subthreshold characteristics, and high field-effect mobility.

The effect of thickness on the optical, structural and electrical properties of ZnO thin film deposited on n-type Si

In this study, we investigated the effect of thickness on the optical, structural and electrical properties of ZnO thin film deposited on n-type Si wafer. Three groups of transparent conductive ZnO layers were successively deposited by Radio Frequency (RF) magnetron sputtering. Structural properties of ZnO thin film layers were examined by using X-ray diffraction (XRD). Particle sizes of ZnO samples were investigated by the Atomic Force Microscope (AFM) measurements, and the results compared with XRD measurements. The optical characteristics of thin films were obtained via using UV–Vis spectrometer. It was tried to determine the effect of film thickness on electrical and optical properties. Moreover, the devices with ZnO interfacial layer were fabricated using the deposited ZnO films having thickness of 5, 50, and 250 nm. The electrical characteristics of the devices were investigated by capacitance/conductance-voltage (C/G-V) measurements. It was seen that both the band gap and capacitance values decrease as the ZnO film thickness increases.

In-situ grown nanoporous Zn-Cu catalysts on brass foils for enhanced electrochemical reduction of carbon dioxide

In-situ grown nanoporous Zn-Cu catalysts were prepared by simply annealing a commercial brass foil at 500 °C in air, followed by electrochemical reduction. During the annealing process, Zn preferentially melted and migrated out of the framework of the alloy to form a thin layer of ZnO on its surface. Subsequent electroreduction created nanoporous Zn-enriched surface. The Zn concentration increased from 36% to 50% by 10 min, to 81% by 3 h, and to 87% by 12 h annealing treatment while the average pore size decreased from 290 nm to 120 nm as the annealing time increased from 1 h to 12 h. Faradaic efficiency of CO2 reduction to CO and HCOOH was enhanced by nearly 4 and 6 times, respectively, as compared to untreated brass foils. The nanoporous Zn-Cu catalyst presented a stable ratio of CO/H2 and a steady working current density in a continuous electrolysis of 18 h in 0.5 M KHCO3 solution.