Pure ZnO Thin Films Research Articles

Yellowish-White Emission from Sol-Gel Mediated Lithium and Aluminium Co-Doped Zinc Oxide Thin Films

Li and Al codoped ZnO (LAZO) thin films have been prepared by sol-gel method. There are two emission bands were observed in the spectra of pure ZnO thin films. One band possessed a sharp peak at round 380 nm locating in the UV range; the other band consists of green luminescence. Yellowish-white emission were observed from the LAZO films, which can be fitted into three bands: a strong yellow emission, a moderate green emission and a weak violet emission centered at 598, 538, and 430 nm, respectively. The luminescence color was seen to be yellowish white due to the recombination for the dominant green and yellow emission.

A highly selective and wide range ammonia sensor—Nanostructured ZnO:Co thin film

Abstract Ammonia sensing characteristics of undoped and cobalt (Co)-doped nanostructured ZnO thin films were investigated. Polycrystalline nature with hexagonal wurtzite structure and high crystalline quality with dominant (0 0 2) plane orientation of Co-doped ZnO film were confirmed by the X-ray diffractogram. Scanning electron micrographs of the undoped film demonstrated the uniform deposition of sphere-shaped grains. But, smaller particles with no clear grain boundaries were observed for Co-doped ZnO thin film. Band gap values were found to be 3.26 eV and 3.22 eV for undoped and Co-doped ZnO thin films. Ammonia sensing characteristics of Co-doped ZnO film at room temperature were investigated in the concentration range of 15–1000 ppm. Variation in the sensing performances of Co-doped and pure ZnO thin films has been analyzed and compared.

Photocatalytic Properties of ZnO and Ag Doped ZnO Thin Films

Pure ZnO thin films and Ag doped ZnO thin films were prepared on quartz substrates by sol-gel process. Structural features and UV absorption spectrum have been studied by XRD and UV-Vis-Nir scanning spectrophotometer. Taking phenol as pollutants, further study of the effect of different annealing temperature and Ag dopant amount of ZnO films on photocatalytic properties was carried out. The results showed that, the optimal annealing temperature on photocatalytic degradation of phenol in this experiment was 300 °C, the best molar ratio of ZnO and Ag was 30:1, which was better than pure ZnO film greatly. Excellent adhesion, recyclable and efficient degradation Ag doped ZnO thin films were found in this experiment.

Structural, Optical and Electrical Properties of Spray Pyrolysed Ti-Doped ZnO Films

Pure ZnO has excellent conductivity as it contains high concentration of native defect (oxygen vacancy or zinc interstitials). At high temperature, pure ZnO thin films are not stable chemically and electrically. The performance of ZnO thin films can be greatly modified and improved by appropriate impurity doping. Modification of ZnO with transient metals (Al, In, Ga, Ti) provides a successful and cost effective alternative. In order to create free electrons and enhance n-type conductivity, two criteria should be satisfied: (i) the doping ion should be smaller than, or equal to, the diameter of the host ion; (ii) the ion of the dopant should have higher valency than that of the host atom. The ionic radius of Ti4+ (0.068 nm) should be smaller than that of Zn2+ (0.074 nm), and hence Ti4+ ions can replace Zn2+ ions at substitutional sites. In the present study, ZnO films doped with small Ti concentration were deposited on glass substrate by spray pyrolysis.Both un-doped and Ti-doped ZnO films were deposited on microscopic glass substrates using spray pyrolysis technique systematically by controlling the deposition parameters. For deposition, 0.1 M of zinc acetylacetonate Zn(AcAc) was dissolved in ethanol and sprayed onto microscopic glass substrates with dimensions of 75 x 25 mm2 at fixed substrate temperature; Ts = 400°C. In order to dope ZnO with Ti, titanyl acetylacetonate Ti(AcAc) solution was added to the starting solution. The solution was sprayed using different doping concentrations ranging from 0.1% to 0.9%.The strongest lines in the entire pattern are the principle lines (100), (002) and (101) of the hexagonal wurzite ZnO, in agreement with JCPDS standard card No. 75-0576. On doping with Ti up to 0.7%, no additional peaks are observed. This is attributed to the fact that the concentration of the dopant Ti is low. When the doping concentration is further increased above 0.7%, it is observed that the crystallinity becomes lower. The crystallite size determined using Scherrer’s relation decreased from 39 nm to 12 nm after doping with Ti. Parameters like dislocation density and strain were also estimated.Composition of the films was studied by EDAX. Peaks corresponding to Ti, Zn and O elements were presented along with Si peaks coming from the glass substrate.The value of the band gap of the films is obtained from Tauc’s plot indicate that the band gap gradually increases with increasing Ti concentration. Similar result is reported in the literature. The band gap value decreases from 3.20 eV to 3.32 eV.Room temperature electrical resistivity of the films doped with different concentrations of Ti was studied. As deposited undoped films exhibited resistivity around 105 ohm cm. After annealing in vacuum, the resistivity decreased by three orders to 100 ohm cm. As the doping concentration increased, the resistivtiy decreases further to 8 ohm cm for 0.9 % Ti doping. Ti4+ ions substituted Zn2+ ions within the crystal lattice induce positive TiZn charges in the material. In order to maintain electrical neutrality, two negative electrons are induced to compensate the excess positive charges. Hence, the resistivity slightly decreases due to the increase of the free electrons in the film. The mobility of the films increases from 10 cm2V-1s-1 to 22 cm2V-1s-1 with increase of Ti concentration. The carrier density increases from 6.24 x 1015 cm-3 to 3.54 x 1016 cm-3. The decrease of resistivity can be attributed to the increase of carrier concentration.Preliminary studies on Dye sensitized solar cells (DSSC), indicated that Ti doped ZnO films exhibited higher photo output compared to undoped ZnO. The results show that the films can be used in DSSC.

Appraisal on Textured Grain Growth and Photoconductivity of ZnO Thin Film SILAR

ZnO thin films were prepared by successive ionic layer adsorption reaction (SILAR) method. The textured grain growth along c-axis in pure ZnO thin films and doped with Sn was studied. The structural analysis of the thin films was done by X-ray diffraction and surface morphology by scanning electron microscopy. Textured grain growth of the samples was measured by comparing the peak intensities. Textured grain growth and photo current in ZnO thin films were found to be enhanced by doping with Sn. ZnO thin film having good crystallinity with preferential (002) orientation is a semiconductor with photonic properties of potential benefit to biophotonics. From energy dispersive X-ray analysis, it is inferred that oxygen vacancy creation is responsible for the enhanced textured grain growth in ZnO thin films.

Structural, Optical, and Electrical Properties of ZnO/Nb/ZnO Multilayer Thin Films

ZnO multilayers and pure ZnO thin films were deposited onto glass using a sputtering system, and were subsequently characterized by X-ray diffractometry and UV-Vis spectroscopy. The resistivity of the samples was measured by the four-probe method. All films exhibited preferential orientation along the c-axis and the peak position (002) shifted to a lower position, indicating a reduction in the unit cell size. The pure ZnO thin film exhibited a maximum transmittance of approximately 98%, which decreased as the Nb layer increased, thus increasing the absorbance of the multilayer thin films. The energy band gap decreased as the thickness of the metal increased which higher value was 3.18 eV. The resistivity had a minimum of 0.1 × 10-4 Ω m.

Selective detection of ammonia using spray pyrolysis deposited pure and nickel doped ZnO thin films

Abstract This research paper reports the deposition of nanostructured pure and Ni-doped ZnO thin films deposited at the substrate temperature of 523 K using simple and economical spray pyrolysis technique and subsequently post annealed at 673 K in air atmosphere for 3 h. Ni-doping greatly affected the crystallographic orientation, surface morphology, roughness and room temperature sensing response. Noticeable change in the crystallite size, transmittance and electrical properties was observed. The room temperature sensing characteristics like selectivity, response recovery studies, range of detection, stability and reproducibility of the undoped and Ni-doped ZnO thin films were investigated. Especially, the sensing elements exhibited an excellent selectivity towards ammonia. A lower detection limit of 5 and 25 ppm was observed for undoped and Ni-doped ZnO thin films respectively. The upper detection range was widened to 1000 ppm for the Ni-doped film.

Structural and optical properties of ZnO–SnO2 mixed thin films deposited by spray pyrolysis

Nanocrystalline ZnO–SnO2 mixed thin films were deposited by the spray pyrolysis technique at various substrate temperatures during deposition. The mixed films were prepared in the range of 20.9at.% to 73.4at.% by altering the Zn/(Sn+Zn) atomic ratio in the starting solution. Morphology, crystal structures, and optical properties of the films were characterized by field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and ultraviolet–visible and photoluminescence (PL) spectroscopy. XRD analysis reveals that the crystallinity of the Sn-rich mixed thin films increases with increasing substrate temperatures. FESEM images show that the grain size of mixed thin films is smaller compared to that of pure ZnO and SnO2 thin films. A drop in the thickness and optical bandgap of the film was observed for films fabricated at high temperatures, which coincided with the increased crystallinity of the films. The average optical transmission of mixed thin films increased from 70% to 95% within the visible range (400–800nm) as the substrate temperature increases. Optical bandgap of the films was determined to be in the range of 3.21–3.96eV. The blue shift in the PL spectra from the films was supported by the fact that grain size of the mixed thin films is much smaller than that of the pure ZnO and SnO2 thin films. Due to the improved transmission and reduced grain size, the ZnO–SnO2 mixed thin films can have potential use in photovoltaic and gas sensing applications.

Microstructural analysis and optical characteristics of Cu-doped ZnO thin films prepared by DC magnetron sputtering

The study of the process interactions near the surface and bulk area of functional metal oxide semiconductor materials plays an important role for nanodevice application. Especially it is necessary to investigate the morphological, optical and physical properties of the synthesized semiconductor oxides modified by different states of dopant elements because this allows making a compound system with unique characteristics and novel properties. In this work the pure and Cu-doped ZnO (CZO) thin films were deposited on a silicon oxide (SiO2/Si) substrate by using a direct current reactive magnetron sputtering technique. Synthesized samples were annealed at 500°C, 700°C, and 850°C temperatures under oxygen atmosphere to obtain a good quality crystal structure. Then microstructural and optical properties of the pure and CZO films are systematically investigated by X-ray diffraction, scanning electronic microscopy, and spectrophotometer. The results indicate that Cu-doped ZnO films show stronger preferred orientation toward the c-axis and lattice mismatch, uniform grain size after doping of Cu. The effect of thermal annealing to morphologic and physical parameters of metal oxide thin films was analyzed as functions of the Cu dopant.

Realization of enhanced room temperature ferromagnetism in pure and V-doped ZnO films on TOP functionalization

Abstract

Fast Response Ultra-violet Photodetectors Based on Sol Gel Derived Ga-doped ZnO

Ultraviolet photoconductivity in Gallium doped ZnO (GZO) thin film synthesized by sol-gel technique is investigated. Response characteristics of Pure ZnO thin film and GZO thin film UV photodetector biased at 5V for UV radiation of λ = 365nm and intensity = 24μwatt/cm2 have been studied. GZO UV photodetector is found to exhibit a high photocoductive gain (K = 9.32×102) with fast rise and fall time (Tr = 3 s and Tf = 9 s) in comparison to pure ZnO thin film based photodetector with K = 4.9×101, Tr = 6 s and Tf = 18 s. Formation of some neutral atoms at the grain boundaries of GZO thin film due to heavy doping of Gallium in ZnO lattice is the key factor for lowering of dark current (Ioff ∼ 0.11μA). Upon UV illumination, the release of trapped electrons from surface defects or adsorbed oxygen results in an enhanced photocurrent (Ion = 0.1mA) and hence higher value of K is obtained for GZO thin film based photodetector.

Nanostructured Metallic Oxides Coating

This project has been focused on structural, morphological, optical and photoluminescence properties of pure ZnO thin film. Here, zinc oxide thin films are grown on glass via facile and low cost sol gel spin coating process @ fixed spinning speed of 1000 RPM. The X-ray patterns, the transmittance, the particle size by the atomic force microscope are investigated. The particle size is equal to 120 nm and the surface roughness is found to be 23.33 nm. The photoluminescence analysis reveals (...)

Influence of Fe dopant concentration and annealing temperature on the structural and optical properties of ZnO thin films deposited by sol–gel method

Nanostructured Fe doped ZnO thin films were deposited onto glass substrates by sol–gel spin coating method. Influence of Fe doping concentration and annealing temperature on the structural, compositional, morphological and optical properties were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–Vis spectroscopy and photoluminescence (PL) measurements. XRD analysis showed that all the films prepared in this work possessed a hexagonal wurtzite structure and were preferentially oriented along the c-axis. Pure ZnO thin films possessed extensive strain, whereas Fe doped films possessed compressive strain. In the doped films, least value of stress and strain was observed in the 0.5 at.% Fe doped thin film, annealed at 873 K. Average crystallite size was not significantly affected by Fe doping, but it increased from 15.57 to 17.79 nm with increase in annealing temperature from 673 to 873 K. Fe ions are present in +3 oxidation state as revealed by XPS analysis of the 0.5 at.% Fe doped film. Surface morphology is greatly affected by changes in Fe doping concentration and annealing temperature which is evident in the SEM images. The increase in optical band gap from 3.21 to 3.25 eV, with increase in dopant concentration was attributed to Moss–Burstein shift. But increase in annealing temperature from 673 to 873 K caused a decrease in band gap from 3.22 to 3.20 eV. PL spectra showed emissions due to excitonic combinations in the UV region and defect related emissions in the visible region in all the investigated films.

Investigation of reduced graphene oxide effects on ultra-violet detection of ZnO thin film

The reduced graphene oxide (rGO) incorporated ZnO thin films were fabricated by dip-coating method. The Raman and FT-IR spectra of 0.075wt% incorporated composite film showed reduction of GO in composite film. The transmittanceProd. Type: FTP spectra have shown that rGO incorporation increase the visible light absorption of ZnO thin film while the calculated band gaps of samples were decreased from 3.28 to 3.25eV by increasing the rGO content. The linear trend of I–V curve suggests an ohmic contact between ZnO and rGO. Besides, it was found that by increasing the rGO content, the electrical resistivity was decreased from 4.32×102Ωcm for pure ZnO film to 2.4×101Ωcm for 0.225wt% rGO incorporated composite film. The composite photodetectors not only possessed a desirable UV photosensitivity, but also the response time of optimum sample containing 0.075wt% rGO was reduced to about one-half of pure ZnO thin film. Also, the calculated signal to noise (SNR) showed that highly conductive rGO in composite thin films facilitate the carrier transportation by removing the trapping centers. The mechanism of photoresponsivity improvement of composite thin films was proposed by carrier transportation process.

Optical and photocatalytic properties of composite TiO2/ZnO thin films

Pure TiO2 and ZnO thin films named Ti100 and Zn100 were prepared via sol–gel technique using Ti alkoxide and Zn acetate as metal precursors and dip coating deposition on quartz substrate. Different vol.% ratios of the initial Ti and Zn solutions were used to obtain composite TiO2/ZnO films nominated as Ti25, Ti50 and Ti75. The XRD patterns of pure TiO2 and ZnO films revealed the formation of anatase TiO2 and wurtzite hexagonal ZnO, respectively. The patterns of the composite film Ti75 were similar to the pure TiO2, while the composite films Ti25 and Ti50 revealed amorphous structure. The XPS analysis proved presence of TiO2 and ZnO oxide phases in the pure and composite films that is in accordance with XRD analysis. The UV–vis transmission spectra of the films showed that absorption edges of the composite films were blue shifted in comparison with the pure films. The largest shift was attained in the Ti25 composite film. The Forouhi–Bloomer (FB) and Tauc–Lorentz (TL) dispersion models were used for description of the complex refractive index of the films and for estimation of their thickness and energy band gap. The refractive index in the films’ transparency region and high energy region increased with the increase of TiO2 phase content. The thickness of the films also increased with the increase of TiO2 content. The pure TiO2 film was approximately 2.5 times thicker (145nm) than the pure ZnO film (60nm). The estimation of the energy gaps showed comparable gap value of ∼3.2eV for the pure oxide films. The largest value of ∼3.8eV was calculated for the Ti25 composite film. A good agreement between the two models was observed in the estimation of gap and thickness values. For pure ZnO film, better accordance between experimental and theoretical transmittance spectra was achieved via FB model simulations in the region above the band gap. The photocatalytic properties of the films in air purification were investigated via standard NO oxidation procedure. The Ti100 and Ti75 films exhibited improved performance in comparison to Ti25 and Zn100 films. The highest photocatalytic efficiency of 1.6×10−2mol/einstein in NO oxidation was recorded for pure TiO2 film. Also, the latter showed the highest hydrophylicity without and under UV illumination.

Structural, optical and electrical properties of ZnO:Al thin films for optoelectronic applications

Undoped and aluminum-doped ZnO thin films are prepared by the sol–gel spin-coating process. Zinc acetate dihydrate, ethanol and mono-ethanolamine are used as precursor, solvent and stabilizer, respectively. The atomic percentage of dopant in solution were [Al/Zn] = 1 %, 2 % and 3 %. The effect of Al doping on the optical and electrical properties of ZnO films was investigated by X-ray diffraction (XRD), Four-Point probe technique and UV–visible spectrophotometery. The results from the X-ray diffraction show that the pure ZnO thin films had a polycrystalline structure of the hexagonal Wurtzite Type. A minimum resistivity of $$3.3 \times 10^{-3} \Omega \cdot \mathrm{cm}$$ was obtained for the film doped with 2 mol % Al. Optical transmissions reveal a good transmittance within the visible wavelength spectrum region for all of the films. The value of the band gap is enhanced from 3.21 eV (undoped ZnO) to 3.273 eV (Al/Zn = 3 %), the increase in the band gap can be explained by the Burstein–Moss effect.

Influence of copper doping on structural, optical and sensing properties of spray deposited zinc oxide thin films

Abstract Ammonia is one of the most hazardous substances and it is extremely toxic if inhaled above the moderate level. Sensing ammonia at low concentration levels and at room temperature is one of the most challenging tasks. In this work, nanostructured undoped and copper doped zinc oxide thin films were deposited using spray pyrolysis technique and their structural, morphological, optical and ammonia vapour sensing properties were investigated. The X-ray diffraction (XRD) patterns confirmed the polycrystalline nature of the films with hexagonal wurtzite structure, Field Emission Scanning Electron Microscopy (FESEM) images revealed the spherical grain morphology. XRD and FESEM data revealed the decrease in grain size with increase in Cu-dopant concentration. Decrease in band gap and transmittance with increase in Cu concentration was observed using UV–vis spectrophotometer. The room temperature (303 K) ammonia sensing response of pure and Cu-doped ZnO thin films was found to be significant. The Cu-doped films we report have much better performance than those reported so far with a faster response and recovery times of 30 and 14 s respectively while sensing 50 ppm of NH 3 .

The Properties of the Al-Doped ZnO Thin Film Prepared by Magnetron Sputtering

Pure and Al-doped ZnO thin films were deposited by radio frequency magnetron sputtering at room temperature on n-(100) Si, fused quartz and Pt/Ti/SiO2/Si substrates. Structural, optical and electrical properties of the prepared thin films had been characterized. The X-ray diffraction results indicated that all the films had (002) preferential orientation. The AFM and SEM images showed that the surface of the films was smooth, crack-free and pore-free, the interface between the film and the substrate was sharp and distinct. From the transmission data the band gap energy Eg of the film increased when doped with Al2O3, and the transparent was still high (above 80%). The current voltage characteristics indicated that doping with Al2O3, the leakage current of ZnO thin films increased rapidly and the films acted as a conductive.

Effect of dopant on microstructure and conductivity of ZnO thin film

Al and Ti doped ZnO thin films have been prepared by direct current magnetron cosputtering and thermal oxidation. The effects of these dopants on the microstructural, morphological and electrical properties of deposited films have been studied. The SEM demonstrated that the undoped ZnO thin film consisted of globular ZnO with particle size of ∼100 nm. Scanning electron microscopy EDX analysis confirmed that the dopant content in the ZnO–Al and ZnO–Ti thin film was 2·78 and 3·75 at-% respectively. The particle in the ZnO–Ti thin film was blocky or granular with thorns and its size was several micrometers, while the particle in ZnO–Al thin film was porous granular and the size was ∼2 μm. Resistance value testing and R/R calculation revealed that pure ZnO thin film had the highest gas sensitivity.

Characteraction of Ag-doped ZnO thin film synthesized by sol–gel method and its using in thin film solar cells

Pure 2% and 4% Ag-doped ZnO thin films have been synthesized on glass substrates by sol–gel method. The structure, morphology and optical properties of the samples have been studied by X-ray diffractometer (XRD), scanning probe microscope, UV–vis spectrophotometer, respectively. The XRD result shows that the pure ZnO has a wurtzite hexagonal structure, no phase segregation is observed. The surface morphology of pure ZnO thin film shows that the grains are growing preferentially along the c-axis orientation perpendicular to the substrates. The transmittance spectra reveal that all samples have high transmittance above 90% in visible region. With Ag doping content increase, a red shift is observed. The performance of Ag-doped ZnO films using in thin film solar cells are simulated. The results show that 4% Ag-doped ZnO thin film can greatly improve the absorption of the cells. Compare to pure ZnO, solar cell's energy conversion efficiency improvement of 2.47% is obtained with 4% Ag doped ZnO thin film.