ZnO-based Films Research Articles

Structures and Properties of ZnO-Based Ceramic Films for Low-Voltage Varistors by the R.F. Magnetron Sputtering

ZnO-based ceramic targets with 50mm in diameter and 3mm in depth were prepared by the traditional solid-state sintering process. ZnO-based ceramic films were deposited on Au/Si substrates by R.F. magnetron sputtering using ZnO-based ceramic target. The films have been characterized by X-ray diffraction, energy dispersive X-ray spectroscopy and scanning electron microscopy. The ZnO-based ceramic films, with thickness close to 1 μm, show a good c-axis orientation, corresponding to vertical growth with respect to the substrate. The surface of the films looks very smooth and dense, and the films are the same composition as the ZnO ceramic target composed of Zn, Bi, Sb, Co, Cr and Mn. The ZnO-based ceramic films with nonlinear coefficient of 6.4, nonlinear voltage of 1.6V and the leakage current density 0.3μA/mm2 could be achieved.

Design of ZnO nanostructured films: Characterization and ecotoxicological studies

The potential ecotoxicological impact of ZnO nanostructured films was investigated using common Anabaena flos-aquae, Calothrix pulvinata cyanobacteria and Euglena gracilis euglenoid microalgae. The pre-formed ZnO nanoparticles were synthesized in di(ethylene glycol) medium by forced hydrolysis of ionic Zn 2+ salts. Particle size and shape were controlled by addition of protective agents such as tri- n-octylphosphine oxide and polyoxyethylene stearyl ether. ZnO nanostructured films were directly prepared by spray deposition of pre-formed polyol-based ZnO nanoparticles (ZnO, ZnO-TOPO and ZnO-Brij-76) at 250 °C heated glass substrate. Another sample was prepared from zinc acetate in di(ethylene glycol) diluted in ethanol medium without nanoparticles (ZnO-SOL). In this case, ZnO nanostructured film was formed directly on the glass substrate at the same temperature. Water contact angle on ZnO-based films showed that nanostructured ZnO films containing ZnO nanoparticles prepared without protective agent or with tri- n-octylphosphine oxide present a hydrophobic character while the ones containing nanoparticles prepared using polyoxyethylene stearyl ether or the control sample are hydrophilic. Here we showed that (i) the use of protective agents, (ii) the surface properties of the films and (iii) the nature of the biological system can strongly influence the ecotoxicological studies. Epifluorescence microscopy analyses and Live/Dead tests showed that all films are toxic for Euglena gracilis. In the case of Anabaena flos-aquae and Calothrix pulvinata, tri- n-octylphosphine oxide and polyoxyethylene stearyl ether molecules can prevent ZnO nanoparticle toxicity.

Utilization of Industrial Inkjet Technologies for the Deposition of Conductive Polymers, Functional Oxides and CNTs

ABSTRACTPrinting of functional materials requires reliable deposition processes. This work describes the development of printing processes for selected functional materials utilizing industrial-type inkjet printheads. A well-controlled printing process with fluids containing the conductive polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is presented, allowing linear printing speeds of up to 0.35 m/s in single-pass, and smallest line width of approximately 40 μm when printing 7 pL drop volumes. In addition reliable processes for producing ZnO-based films, which enable novel applications for electronic and UV-sensitive devices, and for printing of conductive carbon nanotube layers are shown.

γ-irradiation effect on transparent conducting ZnO:Ga films

Transparent and conducting pure and Ga-doped ZnO films prepared by e-beam evaporation in vacuum were irradiated at room temperature by Co60 radiation source with the γ-photon average energy of 1.25 MeV and with different doses up to ∼600 kGy. The energy band gap E g, electrical resistivity, carrier density as well as the structure parameters of pure and doped ZnO films versus the impurity content and γ-doses were determined in order to estimate the radiation-induced degradation effect on ZnO-based films used as transparent electrodes for electro-optical device applications.

Growth of ZnO-based Alloy Films and the Schottky Contact

Growth of ZnO-based Alloy Films and the Schottky Contact

Laser-assisted local patterning of ZnO-based spots for mirror-less lasing

ZnO is known as one of the best materials for the implementation of the random lasing effect, associated with mirror-less laser emission in a simultaneously amplifying and highly scattering medium. Normally, the fabrication of this medium requires a rather complicated procedure of deposition and thermal treatment of ZnO-based films on some specific substrates, yielding wurtzite-orientation ZnO nanocrystals. We demonstrate a rapid synthesis of highly efficient ZnO-based random lasing spots on a piece of Zn by employing the phenomenon of laser-induced air breakdown. Being ignited near the surface of a Zn target, plasma of the air breakdown serves as a local reactor to locally transform its properties and thus form a film of well-packed 20–40 nm ZnO nanospheres. Exhibiting extremely high amplification and scattering, this medium is capable of generating the random lasing effect within the exciton-based photoluminescent band.

ZnO-based varistor thick films with high non-linear electrical behavior

Thick films of ZnO-based ceramic varistors prepared by tape casting technique typically show a poor electrical response that still limits their application as protective devices. The excessive volatilization of Bi2O3 during sintering at high temperatures, especially dramatic in the film geometry due to its high area–volume ratio, is found to be the origin of this poor electrical behavior. The problem is overcome by sintering the films in a controlled Bi-rich sealed atmosphere, leading to a high reliability and reproducibility in their non-linear response.

Effect of Sn-doped on microstructural and optical properties of ZnO thin films deposited by sol–gel method

In this study, transparent thin films of Sn-doped ZnO (ZnO:Sn) were deposited onto alkali-free glass substrates by a sol–gel method; the effect of Sn doping on crystallinity, microstructural and optical properties was investigated. The atomic percentages of dopant in ZnO-based sols were Sn/Zn = 0, 1, 2, 3, and 5 at.%. The as-deposited films were pre-heated at 300 °C for 10 min and then annealed in air at 500 °C for 1 h. The results show that Sn-doped ZnO thin films demonstrate obviously improved surface roughness, enhanced transmittance in the 400–600 nm wavelength range and reduced average crystallite size. Among all of the annealed ZnO-based films in this study, films doped with 2 at.% Sn concentration exhibited the best properties, namely an average transmittance of 90%, an RMS roughness value of 1.92 nm and a resistivity of 9.3 × 10 2 Ω-cm.

High quality p-type ZnO films grown by low pressure plasma-assisted MOCVD with N 2O rf plasma doping source

N-doped ZnO films have been grown on (0 0 0 1) sapphire substrates by a novel low-pressure plasma-assisted metalorganic chemical vapor deposition system using N 2O plasma as doping source. X-ray photoelectron spectroscopy analysis confirmed the incorporation of N into the ZnO films. Room temperature p-type conduction was achieved for the N-doped ZnO film at suitable substrate temperatures, with the resistivity of 8.71 Ω cm, hole concentration up to 3.44 × 10 17 cm −3 and mobility of 2.09 cm 2/V s. In the photoluminescence (PL) measurement, a strong near-band-edge emission was observed for both undoped and N-doped films, while the deep-level emission was almost undetectable, which confirmed that the obtained ZnO-based films were well close to stoichiometry and of optically high quality.

Growth of high-quality ZnO-based epitaxial films and their device applications

Growth of high-quality ZnO-based epitaxial films and their device applications

Study on the ZnO-based ceramic films for low-voltage varistors with high stability

ZnO-based ceramic films for low-voltage varistors were fabricated by a novel sol-gel process. The stability of ZnO-based ceramic films has been studied by improving the degradation properties and increasing the microstructure compactness of the films. The degradation of the film varistors was improved via doping with Na+ and heat-treating in oxygen ambient. The microstructure compactness was increased via eliminating the microspores and improving uniformity of the grain size of ZnO by doping with B2O3. The results indicated that ZnO-based ceramic films for low-voltage varistors with high stability can be obtained while the concentration of doped Na+ is 20–40 ppm, B2O3 is 0.25–0.5 mol.% and being heat-treated in oxygen ambient for 2 h.

The influence of post-growth annealing on optical and electrical properties of p-type ZnO films

In this study, p-type ZnO films with excellent electrical properties were prepared by ultrasonic spray pyrolysis (USP) combined with a N–Al co-doping technique. The influence of post-growth annealing conditions, i.e., annealing ambient and temperature, on optical and electrical properties of p-type ZnO films was investigated. The dependence of the intensity of the green emission on the annealing ambient indicates that the green emission might originate from the donor defect, oxygen vacancy. The effect of the annealing temperature on electrical properties of ZnO-based films affirms the prominent action of hydrogen in the conductivity and acceptor doping of ZnO films.

Room-temperature ferromagnetic and ferroelectric behavior in polycrystalline ZnO-based thin films

Polycrystalline ZnO-based thin films with Li and/or Co doping have been prepared by a sol-gel spin-coating method on silicon substrates. Magnetization measurements reveal that Li-doped ZnO film shows paramagnetic behavior. However, the Co-doped ZnO thin films show obvious room-temperature ferromagnetic properties, and ferromagnetic properties can be enhanced by the Li codoping, which may be ascribed to indirect exchange via Li-related defects. All ZnO-based films exhibit ferroelectric behavior, and ferroelectric properties can be tuned by the dopants.

Improved non-linear behaviour of ZnO-based varistor thick films prepared by tape casting and screen printing

Thick films of ZnO-based varistors have been prepared by screen printing and tape casting techniques. The manufacture of varistor thick films by screen printing shows two critical problems; the excessive volatilization of Bi 2O 3 during the sintering step, due to the exaggerated area–volume ratio of these devices and the lack of density of the green compacts, problem inherent to screen printing technology. Tape casting improves compact density and it is also possible to include an additional press step before sintering. An electrical and microstructure comparison between thick films prepared by both technologies has been carried out for a wide range of sintering temperatures. Although far from the electrical response of bulk ceramics, the results obtained show good varistor behavior when sintering the films at low temperatures.

Effect of annealing temperature of a novel Sol–gel process on the electrical properties of low voltage ZnO-based ceramic films

The effect of annealing temperature on the electrical properties of low voltage ZnO-based ceramic films was studied by a novel sol–gel process. The experiment results show that Zn 7Sb 2O 12 and ZnCr 2O 4 phase can form in a lower annealing temperature (550 °C) by the solution doping, and the pyrochlore phase is not detected by X-ray diffractometer (XRD) from 550 °C to 950 °C. Sb 2O 3 phase can change to spinel phase completely; Bi 2O 3 and ZnO may be vaporized when the annealing temperature reaches 750 °C. The ZnO-based ceramic films with nonlinear coefficient of 20, nonlinear voltage of 5 V and the leakage current density of 0.5 μA/mm 2 can be gained at the proper annealing temperature.

The effect of secondary phases on electrical properties of ZnO-based ceramic films prepared by a sol–gel method

The ZnO-based ceramic films doped with Bi2O3, Sb2O3, MnO, Co2O3 and Cr2O3 were prepared for use as film varistors by a sol–gel method. The formation and the changes of the phases in the films doped with different dopants and annealed at different temperatures were investigated via X-ray diffraction analysis. Three secondary phases, i.e., Bi2O3, Zn7Sb2O12 spinel and ZnCr2O4, were detected in the films when the annealing temperature was above 550 °C. The lattice constants of ZnO and Zn7Sb2O12 spinel phase changed with dopants and the annealing temperature, indicating that the diffusion of the ions into the crystals of ZnO and spinel phase had taken placed. The redistribution of the ions changed the constituents of the intergranular phases and the relevant defect species in ZnO grains, and affected intensively the electrical properties of the films, which were used as film varistors. The highest nonlinear coefficient (α) with the lowest leakage current was achieved when the film, which was doped with Bi2O3, Sb2O3, MnO and Cr2O3, was annealed at 750 °C.

Airflow deposition of oxide electroceramic films

Thick films of several oxide ceramic compositions based on lead zirconate titanate (PZT), barium titanate, and zinc oxide were fabricated by deposition from an airflow at room temperature. The as-deposited films showed green densities close to 80% theoretical for all compositions. Unfired, poled PZT-based films provided an audible acoustic response, and some PZT and BaTiO 3 compositions formed translucent layers. X-ray diffraction revealed no preferred crystallographic orientation of the particles in the films. High green densities and increased particle surface energies were thought to operate as drivers for significant reductions in sintering temperature, final grain size, and shrinkage. Films of PZT–PMN (lead magnesium niobate)-Pb 5Ge 3O 11, sintered at 750 °C, had a density of 99.5% theoretical, and relative permittivities greater than 2000. ZnO-based varistor films exhibited higher breakdown fields than equivalent bulk samples prepared by conventional routes.

Analysis of Zn(O,S) films for Cu(In,Ga)Se2 solar cells

Thin-film solar cells based on Cu(In,Ga)Se2 contain a thin buffer layer of CdS in their standard configuration. In order to avoid cadmium in the device for environmental reasons, Cd-free alternatives are investigated. In this thesis, ZnO-based films, containing Mg or S, grown by atomic layer deposition (ALD), are shown to be viable alternatives to CdS. The CdS is an n-type semiconductor, which together with the n-type ZnO top-contact layers form the pn-junction with the p-type Cu(In,Ga)Se2. From device modeling it is known that a buffer layer conduction band (CB) position of 0-0.4 eV above that of the Cu(In,Ga)Se2 layer is consistent with high photovoltaic performance. For the Cu(In,Ga)Se2/ZnO interface this position is measured by photoelectron spectroscopy and optical methods to –0.2 eV, resulting in increased interface recombination. By including sulfur into ZnO, a favorable CB position to Cu(In,Ga)Se2 can be obtained for appropriate sulfur contents, and device efficiencies of up to 16.4% are demonstrated in this work. From theoretical calculations and photoelectron spectroscopy measurements, the shift in the valence and conduction bands of Zn(O,S) are shown to be non-linear with respect to the sulfur content, resulting in a large band gap bowing. ALD is a suitable technique for buffer layer deposition since conformal coverage can be obtained even for very thin films and at low deposition temperatures. However, deposition of Zn(O,S) is shown to deviate from an ideal ALD process with much larger sulfur content in the films than expected from the precursor pulsing ratios and with a clear increase of sulfur towards the Cu(In,Ga)Se2 layer. For (Zn,Mg)O, single-phase ZnO-type films are obtained for Mg/(Zn+Mg) < 0.2. In this region, the band gap increases almost linearly with the Mg content resulting in an improved CB alignment at the heterojunction interface with Cu(In,Ga)Se2 and high device efficiencies of up to 14.1%.

Raman spectroscopy study of ZnO-based ceramic films fabricated by novel sol–gel process

The ZnO-based ceramic films doped with different dopants were prepared by a novel sol–gel process. The phase composition of the films was determined via X-ray diffraction analysis. The influence of the dopants on the residual stress, carrier concentration and the secondary phases was studied by means of Raman spectroscopy. Raman spectra show that the E 2 phonon frequency shifts 3–6 cm −1 to lower wavenumbers, whereas the A 1(LO) mode shifts 3.2–6.1 cm −1 to higher wavenumbers when the films were doped with Bi 2O 3, Sb 2O 3, MnO, Cr 2O 3, Y 2O 3 and Al 2O 3, indicating that both the tensile residual stress and the free carrier concentration were increased with doping. The larger stress is considered to originate from the lattice distortion, which was caused by the substitution of the doping ions for Zn 2+, and the lattice mismatch between the ZnO crystals and the interfacial phases. The secondary phases were affected markedly by both Y 2O 3 and Al 2O 3. The intensity and the position of Raman bands of Zn 7Sb 2O 12 and ZnCr 2O 4 phases changed obviously. The films showed remarkable nonlinear voltage–current characteristics, but the nonlinear coefficient of the films decreased evidently as the addition of Y 2O 3 or Al 2O 3.

Preparation and properties of ZnO-based ceramic films for low-voltage varistors by novel sol-gel process

ZnO-based ceramic films, with thickness close to 2–4 μm, were deposited on Au/Si substrates by novel sol-gel process. The precursors were fabricated by dispersing doped-ZnO ceramic nano-powders into the sols, which were prepared by dissolving zinc acetate dihydrate into 2-methoxyethanol and stabilized by diethanolamine and glacial acetic acid and doped with a concentrated solution of bismuth nitrate, phenylstibonic acid, cobalt nitrate, manganese acetate and chromium nitrate. The doped-ZnO ceramic nano-powders were prepared by sol-gel processing through dried and calcined in air at 150 and 800 °C, respectively. The films were prepared by the spin coating method with the substrate spinning rates of 2000–3000 rpm. After each deposition, the films were heat-treated in air at 350–400 °C for 10 min. After ten to 20 layers, the films were annealed in air at 650–800 °C for 2 h. The films have been characterized by X-ray diffraction and scanning electron microscopy. The nonlinear V–I characteristics of the films for use as varistors were studied by transistor characteristics testing instrument. The nonlinear voltage was 2–10 V, the nonlinear coefficient ( α) was 3–22. The highest nonlinearity coefficient with nonlinear voltage of 4 V could be achieved at 750 °C by the films with a thickness of 2 μm.