Nanocrystalline ZnO Films Research Articles

Transparent Zinc Oxide Memristor Structures: Magnetron Sputtering of Thin Films, Resistive Switching Investigation, and Crossbar Array Fabrication

This paper presents the results of experimental studies of the influence of high-frequency magnetron sputtering power on the structural and electrophysical properties of nanocrystalline ZnO films. It is shown that at a magnetron sputtering power of 75 W in an argon atmosphere at room temperature, ZnO films have a relatively smooth surface and a uniform nanocrystalline structure. Based on the results obtained, the formation and study of resistive switching of transparent ITO/ZnO/ITO memristor structures as well as a crossbar array based on them were performed. It is demonstrated that memristor structures based on ZnO films obtained at a magnetron sputtering power of 75 W exhibit stable resistive switching for 1000 cycles between high resistance states (HRS = 537.4 ± 26.7 Ω) and low resistance states (LRS = 291.4 ± 38.5 Ω), while the resistance ratio in HRS/LRS is ~1.8. On the basis of the experimental findings, we carried out mathematical modeling of the resistive switching of this structure, and it demonstrated that the regions with an increase in the electric field strength along the edge of the upper electrode become the main sources of oxygen vacancy generation in ZnO film. A crossbar array of 16 transparent ITO/ZnO/ITO memristor structures was also fabricated, demonstrating 20,000 resistive switching cycles between LRS = 13.8 ± 1.4 kΩ and HRS = 34.8 ± 2.6 kΩ for all devices, with a resistance ratio of HRS/LRS of ~2.5. The obtained results can be used in the development of technological processes for the manufacturing of transparent memristor crossbars for neuromorphic structures of machine vision, robotics, and artificial intelligence systems.

Photoluminescence spectra of nanocrystalline ZnO films obtained by magnetron deposition technique

Photoluminescence spectra of nanocrystalline ZnO films obtained by magnetron deposition technique

Fundamental insight into the formation of the zinc oxide crystal structure

Zinc oxide shows unique properties which are evident in a wide range of applications: as a transparent conducting oxide, wide-bandgap semiconductor, and piezoelectric device. The starting point for understanding the origin of these properties are the subtle details of the crystal structure of ZnO, and thus elucidating its formation process is essential. The in-situ characterization of films deposited at various temperatures provides an important contribution in this regard, especially since this study reports on ZnO film deposition below room temperature down to -240 °C. Systematic investigations on nanocrystalline ZnO films as function of the deposition temperature reveal structural disorders caused by the irregular occupation of oxygen tetrahedra forming dioxygen species in zinc oxide. Three distinct material ranges are identified in the range of deposition temperatures between -240 and 300 °C. The most surprising observations are the segregation of zinc next to ZnO particles for films deposited at room temperature and the disappearance of the Raman bands of the ZnO lattice for those deposited above 100 °C. On both ends of the investigated deposition temperature scale transparent colorless films are obtained, which form a random frozen solid at low temperatures as well as a highly disordered film at high temperatures. The deposits at -80 °C are yellow in color, indicating the presence of superoxide ions. This wide variety in the properties of ZnO is enabled by the high flexibility of the wurtzite structure, which tolerates huge distance variations. This observation and the results presented open up important insights into the behavior of zinc oxide.

Highly selective H2S sensor realized via the facile synthesis of N-doped ZnO nanocrystalline films

Highly selective H2S sensor realized via the facile synthesis of N-doped ZnO nanocrystalline films

Impact of post-annealing temperature on optical and surface properties of tellurium doped ZnO nanocrystalline films

Investigated in this work is the effect of post-annealing temperature on ZnO nanocrystalline thin films doped with 5 at.% tellurium. The spin coating method was used to deposit films on the microscopic glass substrates. XRD, AFM, and UV-spectro-photometry were used to characterize the films structure, surface roughness and optical properties. The XRD spectra showed that the nanocrystalline films are of monocrystalline nature. AFM has confirmed the nanocrystalline character of tellurium-doped ZnO. The transmission of exposed films has been decreased with the increase of annealing temperature. The average transmission of all the films has been revealed to be higher than 80%. The optical band gap varies slightly with post-annealing temperature.

Sensitization of ZnO Photoconductivity in the Visible Range by Colloidal Cesium Lead Halide Nanocrystals

In this work, colloidal perovskite nanocrystals (PNCs) are used to sensitize the photoconductivity of nanocrystalline ZnO films in the visible range. Nanocrystalline ZnO with a crystallite size of 12-16 nm was synthesized by precipitation of a zinc basic carbonate from an aqueous solution, followed by annealing at 300 °C. Perovskite oleic acid- and oleylamine-capped CsPbBr3, CsPb(Cl/Br)3 and CsPb(Br/I)3 PNCs with a size of 6-13 nm were synthesized by a hot injection method at 170 °C in 1-octadecene. Photoconductive nanocomposites were prepared by applying a hexane sol of PNCs to a thick (100 μm) polycrystalline conductive ZnO layer. The spectral dependence of the photoconductivity, the dependence of the photoconductivity on irradiation, and the relaxation of the photoconductivity of the obtained nanocomposites have been studied. Sensitization of ZnO by CsPbBr3 and CsPb(Cl/Br)3 PNCs leads to enhanced photoconductivity in the visible range, the maximum of which is observed at 460 and 500 nm, respectively; close to the absorption maximum of PNCs. Nanocomposites ZnO/CsPb(Br/I)3 turned out to be practically not photosensitive when irradiated with light in the visible range. The data obtained are discussed in terms of the position of the energy levels of ZnO and PNCs and the probable PNCs photodegradation. The structure, morphology, composition, and optical properties of the synthesized nanocrystals have also been studied by XRD, TEM, and XPS. The results can be applied to the creation of artificial neuromorphic systems in the visible optical range.

Investigation of dilute aluminum doped zinc oxide thin films: structural and morphological properties for varies applicationss

To form a better view of the influences of Al in ZnO, the crystalline structure parameters and morphological (via SEM) of Zn1-xAlxO (x=0.0, 0.02, 0.04, 0.06, 0.08 and 0.10) thin films organized onto glass substrates using by spin coating technique. The effects of Al doping on the structural ZnO nano crystalline films are investigated using (XRD), (EDAX) and (SEM). Rietveld refinement was used to examine the XRD patterns of Zn1- xAlxO thin films. This was done using Fullprof software. The XRD results showed that Al ions successfully replaced the Zn2+ lattice sites without any major change in the structure after Zn2+ substitution, and their crystallinity decreased with increasing Al doping content. Also, The XRD analysis confirmed the hexagonal structure. Lattice constant, Cell volume, Atomic Packing Fraction and surface density have been calculated. The microstructural parameters, crystallite size and lattice strain of Zn1-xAlxO thin films were calculated. The changes in microstructural parameters were discussed as dependent Al concentration. The Zn–O bond lengths and bond angle of Zn1-xAlxO were determined and have changed.

Nanoscale-Resistive Switching in Forming-Free Zinc Oxide Memristive Structures.

This article presents the results of experimental studies of the impact of electrode material and the effect of nanoscale film thickness on the resistive switching in forming-free nanocrystalline ZnO films grown by pulsed laser deposition. It was demonstrated that the nanocrystalline ZnO film with TiN, Pt, ZnO:In, and ZnO:Pd bottom electrodes exhibits a nonlinear bipolar effect of forming-free resistive switching. The sample with Pt showed the highest resistance values RHRS and RLRS and the highest value of Uset = 2.7 ± 0.4 V. The samples with the ZnO:In and ZnO:Pd bottom electrode showed the lowest Uset and Ures values. An increase in the number of laser pulses from 1000 to 5000 was shown to lead to an increase in the thickness of the nanocrystalline ZnO film from 7.2 ± 2.5 nm to 53.6 ± 18.3 nm. The dependence of electrophysical parameters (electron concentration, electron mobility, and resistivity) on the thickness of the forming-free nanocrystalline ZnO film for the TiN/ZnO/W structure was investigated. The endurance test and homogeneity test for TiN/ZnO/W structures were performed. The structure Al2O3/TiN/ZnO/W with a nanocrystalline ZnO thickness 41.2 ± 9.7 nm was shown to be preferable for the manufacture of ReRAM and memristive neuromorphic systems due to the highest value of RHRS/RLRS = 2307.8 ± 166.4 and low values of Uset = 1.9 ± 0.2 V and Ures = −1.3 ± 0.5 V. It was demonstrated that the use of the TiN top electrode in the Al2O3/TiN/ZnO memristor structure allowed for the reduction in Uset and Ures and the increase in the RHRS/RLRS ratio. The results obtained can be used in the manufacturing of resistive-switching nanoscale devices for neuromorphic computing based on the forming-free nanocrystalline ZnO oxide films.

Enhanced corrosion resistance of adhesive/galvanised steel interfaces by nanocrystalline ZnO thin film deposition and molecular adhesion promoting films

ABSTRACT In this study, nanocrystalline ZnO (NC ZnO) films are considered as interface layers for polymer/metal bonds consisting of hot-dip galvanised steel (HDG) and epoxy resin. NC ZnO films with a thickness of less than one micrometre were deposited on HDG through hydrothermal deposition from aqueous Zn nitrate/hexamethylenetetramine electrolytes. Furthermore, 3-aminopropylphosphonic acid (APPA) and 3-aminopropyltriethoxysilane (γ-APS) were considered for molecular adhesion-promoting layers. 90°-peel tests of water-saturated epoxy films confirm the improvement of bond strength under humid environments. Electrochemical properties of the thin film coatings analysed by current density potential curves indicate effective inhibition of oxygen reduction and metal alloy dissolution. Scanning Kelvin probe (SKP) measurements confirm that the corrosive delamination of the polymer from the metal substrate is significantly slowed down by the considered surface modifications. The combination of NC ZnO films with APPA as an adhesion promoting molecule showed the best performances.

ZnO thin films containing aliovalent ions for NO2 gas sensor activated by visible light

In this paper we report on the combined visible-light (red λ = 630 nm; green λ = 570 nm; purple-blue λ = 430 nm) and mild thermal-activation modes (25 °C–100 °C) towards ppb-level NO2 detection of ZnO nanocrystalline film containing trivalent (Al-Ga) and tetravalent (Si-Ge) ions. Both trivalent and tetravalent doping trigger visible light fast response/recovery behavior as respect to dark conditions. Remarkably Purple-blue light (λ = 430 nm) combined with thermal activation at 75 °C OT results in best performances in terms of variation of sensor resistance as compared to that in dry air, topping maximum gas relative response RR= RGas/RAir ≈ 20 for the 5% Si doped ZnO at 400 ppb NO2. Aliovalent doping of ZnO is obtained using a colloidal heat-up synthesis yielding highly transparent in the visible range degenerately doped n-type ZnO semiconductors films which were characterized by SEM, XRD, XPS techniques to assess morphology, crystal phases and concentration of surface oxygen vacancies, respectively. Optical characterization of the doped films showed a distinctive localized surface plasmon resonance (LSPR) peak in the near infrared region, deriving from the free charge carrier. Aluminum doping is the most effective to narrow the band gap of the doped ZnO films down to 3.22 eV. The influence of the relative humidity (RH) on the gas sensing performances is investigated and for 40% RH a slight decrease of the NO2 relative response was observed. A consistent model explaining the gas sensing mechanism under dark and light conditions and the influence of RH is presented.

Correlating the properties of RF sputtered ZnO nanocrystalline films deposited using sintered and powder targets

ZnO thin films were deposited by RF magnetron sputtering using commercial sintered and in-house made powder targets. The structural, optical, chemical composition and surface morphology of the ZnO thin films were characterized by X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Field Emission Scanning Electron Microscopy (FESEM), X-ray Photoelectron Spectroscopy (XPS), UV–Visible spectroscopy, Photoluminescence (PL) and Raman Spectroscopy. XRD data established that both films were c-axis oriented and the appearance of E2H mode in Raman spectra confirmed the hexagonal wurtzite structure of the films. AFM micrographs revealed the homogeneity and nanocrystalline nature of the films. XPS and EDX studies confirmed that the films contained only Zn and O elements. Both the films showed superior optical properties and > 90% transmittance in the visible region. The results conclude that powder targets can be used to achieve complex/mixed metal oxide films having optical/electronic quality thereby overcoming the limitation of choice as in the case of commercial targets.

Resistive switching control in forming-free nanocrystalline zinc oxide films

The influence of control parameters on the resistive switching effect in forming-free nanocrystalline zinc oxide films was studied. It was shown, resistive switching from HRS to LRS was observed at (+3.3±0.4) V, and from LRS to HRS at (-2.8±0.6) V. Changing the triangular sweep voltage signal phase by 90 degrees leads to a decrease R HRS from (52.7±5.2) kΩ to (38.3±20.2) kΩ, to an increase R LRS from (3.3±2.2) kΩ to (4.5±3.1) kΩ, and to a decrease R HRS /R LRS ratio from 17 to 9. Experimental results obtained showed, that an increase in amplitude of the sweep voltage U A from 2V to 6V and sweep time t A from 1s to 5s leads to a decrease in the R HRS /R LRS ratio from (25.1±2.4) to (9.3±0.6) and from (23.2±1.8) to (10.4±0.8), respectively. The results can be useful for the development of technological fundamentals of new-generation micro- and nanoelectronics elements manufacturing, including ReRAM elements for neuromorphic structures based on forming-free ZnO nanocrystalline films.

Preparation of ZnO layer for solar cell application

Abstract Zinc Oxide has important role in solar cell as it may enhance the absorption of light into the device and improve the electrical transportation. In the present work, we have used drop casting approach to fabricate the nanocrystalline ZnO film on a glass substrate. Zinc acetate dehydrate and ethanol were used as a starting material and solvent, respectively. The zinc oxide solution was prepared by using a magnetic hot plate with a magnetic stirrer. X-Ray Diffraction (XRD) and UV–Visible were employed to characterize structure and optical properties of the as-deposited film. The XRD results confirm the nanocrystalline structure of the deposited ZnO films. From the UV–visible analysis it was observed that the optical transmittance form the ZnO film is around 80% in the wave length range from 400 nm − 1100 nm and it exhibited the energy band gap to be 3.10 eV.

Controlling of c-axis position of ZnO nano-crystalline films deposited at various substrate temperature by ultrasonic spray method

In this paper, ZnO thin films were elaborated onto hot glass substrates by ultrasonic spray pyrolysis technique. The solution contains zinc acetate dehydrate mixed with methanol with solution concentration equals 0.1 mol/L. The effect of the substrate temperature on the properties of ZnO was investigated. All films deposited were characterized by various techniques such as X-ray diffraction to examine the films structure and UV-Visible spectroscopy to determine the optical proprieties. The obtained results show that films are polycrystalline at hexagonal shape type wurtzite in structure. The crystallite size is varied between 10.36 and 71.1 nm, the optical transmission leads to 90% in visible rang, the optical band gap is varied from 3.22 to 3.28 eV and the electrical resistivity is found between 1.42 Ω cm and 17.60 Ω cm. The elaborated films have UV emission located at 380 nm.

Physical Investigations of (Co, Mn) Co-Doped ZnO Nanocrystalline Films.

Undoped as well as (Co, Mn) co-doped Zinc oxides have been effectively developed on glass substrates, taking advantage of the spray pyrolysis procedure. The X-ray diffraction XRD as well as X-ray photoelectron spectroscopy (XPS) measurements have recognized a pure hexagonal wurtzite form of ZnO, and no other collateral phases such as MnO2 or CoO2 have been observed as a result of doping. The calculated values of the texture coefficient (TC) were between 0.15 and 5.14, indicating a dominant orientation along the (002) plane. The crystallite size (D) varies with the (Co, Mn) contents. The dislocation density (δ) as well as the residual microstrains increased after Co and Mn doping. Furthermore, the surface morphology of the films has been affected significantly by the Co and Mn incorporation, as shown by the scanning electron microscopy (SEM) investigation. The study of the optical properties exhibits a red shift of the band gap energy (Eg) with the (Co, Mn) co-doping. The magnetic measurements have shown that the undoped and (Co, Mn) co-doped ZnO thin films displayed room-temperature ferromagnetism (RTFM).

Energy level gradient trapping based on different work functions of ZnO enhancing response and stablity for lateral photodetectors

Abstract The slow response speed and instability of the lateral photodetectors (L-PDs), often caused by the persistence of the excess minority carriers in the long channel after the termination of the illumination (called Persistent Photoconductivity effects) are the main factors that hinder the practical application of this type of device. In this research, two types of ZnO with different work functions, including ZnO nanoparticles (ZnO-NPs) doped in bulk heterojunction (BHJ) and thermal annealed ZnO nanocrystalline film (ZnO–NF), are incorporated into the L-PDs to construct an energy gradient, which could facilitate the quick transfer of the minority carriers and restrict their release into the active layer. The structure of L-PDs are optimized as quartz/ZnO–NF/PffBT4T-2OD:PC61BM:ZnO-NPs (component ratio = 5:6:1)/Au–Au electrodes. Compared with the control device, the test results show that the optimized L-PDs exhibit higher stability with much lower fall time (3.7 ms, boosted 200-fold) and gentler rise slope of dark current versus time (Kdark) (8.3 × 10−6 nA/s, reduced four orders of magnitude). Meanwhile, the very low dark current (

Synthesis and Memristor Effect of a Forming-Free ZnO Nanocrystalline Films.

We experimentally investigated the effect of post-growth annealing on the morphological, structural, and electrophysical parameters of nanocrystalline ZnO films fabricated by pulsed laser deposition. The influence of post-growth annealing modes on the electroforming voltage and the resistive switching effect in ZnO nanocrystalline films is investigated. We demonstrated that nanocrystalline zinc oxide films, fabricated at certain regimes, show the electroforming-free resistive switching. It was shown, that the forming-free nanocrystalline ZnO film demonstrated a resistive switching effect and switched at a voltage 1.9 ± 0.2 V from 62.42 ± 6.47 (RHRS) to 0.83 ± 0.06 kΩ (RLRS). The influence of ZnO surface morphology on the resistive switching effect is experimentally investigated. It was shown, that the ZnO nanocrystalline film exhibits a stable resistive switching effect, which is weakly dependent on its nanoscale structure. The influence of technological parameters on the resistive switching effect in a forming-free ZnO nanocrystalline film is investigated. The results can be used for fabrication of new-generation micro- and nanoelectronics elements, including random resistive memory (ReRAM) elements for neuromorphic structures based on forming-free ZnO nanocrystalline films.

Surface polarity engineering of ZnO layer for improved photoluminescence of CsPbBr3 quantum dot films

The surface polarity of nanocrystalline ZnO film, a typical electron transport material in optoelectronics, is chemically modified using self-assembled monolayer (SAM) of phenethyl trichlorosilane (PETS). The surface treatment is proved to generate a hydrophobic surface by removing the hydroxyl groups on the oxide layer, which reduces the emission quenching of the subsequent CsPbBr3 quantum dot film. In addition, the surface polarity was engineered with varying PETS concentration, thereby contribute to an increased photoluminescence quantum yield (PLQY) by up to 50%. Meanwhile, the thermal stability of photoluminescence was enhanced showing a wide-range temperature tolerance up to 140 °C.

Effect of anionic fluorine incorporation on structural, optical and electrical properties of ZnO nanocrystalline films

ZnO and ZnO:F (fluorine doped Zinc oxide) films have been deposited on glass substrates at 325 ± 5 °C through ultrasonic spray pyrolysis method (USP). X-ray diffractometer (XRD) has been utilized to determine the crystal structural and some structural parameters such as lattice constants, texture coefficients TC (hkl), crystallite size and dislocation densities of all the ZnO films. The XRD results have revealed that all the ZnO films have the hexagonal wurtzite structure and F incorporation shows no significant effect on the crystal structure. X-ray photoelectron spectroscopy (XPS) measurements have confirmed the presence of fluorine atoms in the ZnO lattice and also the existence of Zn–F bonding states. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements have been carried out to investigate the effect of F doping on the surface morphology of the ZnO films. The optical properties of the films have been analyzed by utilizing transmittance, absorbance and reflectance measurements. Some optical parameters such as extinction coefficient (k), refractive index (n) and optical conductivity have been determined depending on the change in F incorporation. Increasing the F doping initially led to a decrease in the optical band gap, but later increased. Electrical resistivity and carrier concentration values of the deposited films have been determined with Hall effect measurement system and it has been detected that 6% F-doped ZnO film has the minimum resistivity and maximum carrier concentration.

Erbium-ion implantation of single- and nano-crystalline ZnO

This paper reports on the results of Er+ ion implantation into various ZnO structures – standard single crystal c-plane (0001) ZnO, nanostructured thin films and nanorods. Er+ ions were implanted using an ion implantation energy of 400 keV and implantation fluences in the range of 5 × 1014 to 5 × 1015ions/cm2. Er concentration depth profiles and the degree of crystal damage were measured using Rutherford backscattering spectrometry (RBS) and RBS/channelling (RBS/C). Additionally, Raman spectroscopy was used to analyse structural modifications of the prepared samples. The main focus was placed on the luminescence properties of various ZnO structures. The results showed that the characteristic bands of ZnO, i.e. near-band-edge (NBE) luminescence and deep-level emission (DLE) – that can be influenced by the excitation wavelength – appeared in the spectra of single crystals and nanorods. The characteristic luminescence bands of erbium ions in the NIR region appeared in non-annealed ZnO single-crystal samples and nano-crystalline films.