Rapid preparation, characterization, and photoluminescence of ZnO films by a novel chemical method

A novel and simple chemical route was developed for the deposition of ZnO film from aqueous solution, integrating the merits of successive ionic layer adsorption and reaction and chemical bath deposition. ZnO thin films on glass and Si(1 0 0) substrates were deposited with the precursor of zinc-ammonia complex. As-deposited ZnO film exhibits good crystallinity with the hexagonal wurtzite crystalline structure and the preferential orientation along (0 0 2) plane. With a dense and continuous appearance, the film is composed of ZnO particles in even size of 200-300 nm. Under the excitation of 340 nm, strong and sharp near band gap emission (∼391 nm) dominates the photoluminescence spectra with several weak emission peaks related to the deep level (∼450-500 nm). In addition, the mechanism for the deposition process of ZnO from aqueous solution was preliminarily discussed.     

ZnO thin films prepared by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on soda lime glass substrates by pulsed laser deposition (PLD) in an oxygen-reactive atmosphere. The structural, optical, and electrical properties of the as-prepared thin films were studied in dependence of substrate temperature and oxygen pressure. High quality polycrystalline ZnO films with hexagonal wurtzite structure were deposited at substrate temperatures of 100 and 300 °C. The RMS roughness of the deposited oxide films was found to be in the range 2-9 nm and was only slightly dependent on substrate temperature and oxygen pressure. Electrical measurements indicated a decrease of film resistivity with the increase of substrate temperature and the decrease of oxygen pressure. The ZnO films exhibited high transmittance of 90% and their energy band gap and thickness were in the range 3.26-3.30 eV and 256-627 nm, respectively.     

Transparent conductive ZnO:B films deposited by magnetron sputtering

This paper focuses on the preparation of boron doped ZnO (ZnO:B) films prepared by nonreactive mid-frequency magnetron sputtering from ceramic target with 2 wt.% doping source. Adjusting power density, ZnO:B film with low resistivity (1.54 × 10^− 3 Ω cm) and high transparency (average transparency from 400 to 1100 nm over 85%) was obtained. Different deposition conditions were introduced as substrate fixed in the target center and hydrogen mediation. Hall mobility increased from 11 to above 26 cm²/V·s, while carrier concentration maintained almost the same, leading to low resistivity of 6.45 × 10^− 4 Ω cm. Transmission spectra of ZnO:B films grown at various growth conditions were determined using a UV-visible-NIR spectrophotometer. An obvious blue-shift of absorption edge was obtained while transmittances between 600 nm and 1100 nm remained almost the same. Optical band baps extracted from transmission spectra showed irregular enhancement due to the Burstein-Moss effect and band gap renormalization. Photoluminescence spectra also showed a gradual increase at UV emission peak due to free exciton transition near band gap. We contributed this enhancement in both optical band gap and UV photoluminescence emission to the lattice structure quality melioration.     

Effect of boron doping on optical properties of sol-gel based nanostructured zinc oxide films on glass

Boron doped zinc oxide thin films (∼80 nm) were deposited onto pure silica glass by sol-gel dip coating technique from the precursor sol/solution of 4.0 wt.% equivalent oxide content. The boron concentration was varied from 0 to 2 at.% w.r.t. Zn using crystalline boric acid. The nanostructured feature of the films was visualized by FESEM images and the largest cluster size of ZnO was found in 1 at.% boron doped film (B1ZO). The presence of mixed crystal phases with hexagonal as major phase was identified from XRD reflections of the films. Particle size, optical band gap, visible specular reflection, room temperature photoluminescence (PL) emissions (3.24-2.28 eV), infra-red (IR) and Raman active longitudinal optical (LO) phonon vibration were found to be dependent on dopant concentration. For the first time, we report the room temperature fine structured PL emissions as phonon replicas originated from the LO phonon (both IR and Raman active) in 1 at.% boron doped zinc oxide film.     

Optical properties of pyrene doped polymer thin films

Thin films of pyrene in polystyrene matrix have been prepared by spin coating technique. The concentration of polystyrene is kept constant to 1 wt.% while that of pyrene dopant varied in the range 2.30×10⁻⁴-2.30×10⁻¹ wt.%. Thickness of the films was found to depend upon concentration of pyrene and varies from 90 to 782 nm. The results of X-ray diffraction analysis reveal the crystalline nature of the films. The optical properties were studied by absorption, excitation and fluorescence spectroscopy. The band gap energy of pyrene in polymer films was calculated from absorption results. A transition from monomer to excimer is observed with thickness variation of the films. The structured part of the spectrum is assigned to the monomer emission while the broad emission band is attributed to well known pyrene excimer-like emission.     

Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.     

Effect of thermal annealing treatment on structural, electrical and optical properties of transparent sol-gel ZnO thin films

Effect of thermal annealing in different ambients on the structural, electrical and optical properties of the sol-gel derived ZnO thin films are studied. XRD results show that the annealed ZnO films with wurtzite structure are randomly oriented. Crystallite size, carrier concentration, resistivity and mobility are found to be dependent on the annealing temperature. The change in carrier concentration is discussed with respect to the removal of adsorbed oxygen from the grain boundaries. The highest carrier concentration and lowest resistivity are 8 × 10¹⁸ cm⁻³ and 2.25 × 10⁻¹ Ω cm, respectively, for the film annealed at 500 °C in vacuum. The annealed films are highly transparent with average transmission exceeding 80% in the wavelength region of 400-800 nm. In all three ambients, the optical band gap value does not change much below 500 °C temperature while above this temperature band gap value decreases for nitrogen and air and increases for vacuum.     

Fabrication of highly oriented (002) ZnO film on glass by sol-gel method

In this study high quality (002) ZnO films were deposited on glass substrate by a sol-gel spin coating process. The as-coated films were post-annealed at different temperatures in air to investigate the effect of annealing temperature in particular. The chemical composition of the precursor sol and the intermediates produced in the films heating process were analyzed by thermo gravimetric analysis/differential thermal analysis (TGA/DTA). The microstructure and its optical properties of ZnO films were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), ultraviolet-visible spectroscopy (UV-Vis) and photoluminescence. TGA/DTA showed that a significant weight loss occurred at around 200-300 °C and the weight stabilized at 300 °C. An extremely sharp (002) diffracted peak in XRD patterns indicated the high preference in crystallinity of these films. FESEM micrographs revealed that the films were filled with particulates with size ranging from 10 to 25 nm as post annealing temperature increased from 400 to 500 °C and turned into porous films at 600 °C. UV-Vis has shown that the films were highly transparent under visible light and had a sharp absorption edge in the ultraviolet region at 380 nm. The measured optical band gap values of the ZnO thin films were around 3.24-3.26 eV. Photoluminescence spectra revealed a strong UV emission centered at about 390 nm corresponding to the near-band-edge emission with a weak defect-related emission at about 520 nm. The intensity of UV emission increased with the annealing temperature. This may be attributed to a higher quality ZnO film while annealed at higher temperature.     

Optical and structural properties of zinc oxide films with different thicknesses prepared by successive ionic layer adsorption and reaction method

In this work, zinc oxide semiconducting films belonging to the II-VI group have been produced by successive ionic layer adsorption and reaction (SILAR) method on glass substrates with 10, 15, 20 and 25 cycles at room temperature. Following the deposition, the samples were dried in air at 400 °C for 1 h. The films were characterized by X-ray diffraction, field emission scanning electron microscopy and optical absorption measurement techniques. The X-ray diffractions of the films showed that they are hexagonal in structure. The crystallite size of ZnO films varied between 34 and 38 nm accordingly with the number of SILAR cycles. The material has exhibited direct band gap transition with the band gap values lying in the range between 3.13 and 3.18 eV. The red shift is observed in the absorption edge as the cycles increased. Transmission of the films decreased from 65 to 40% with increasing the number of cycles.     

Hybrid nanostructures of Au nanocrystals and ZnO nanorods: Layer-by-layer assembly and tunable blue-shift band gap emission

A layer-by-layer assembly technique was developed to synthesize the hybrid nanostructures of Au nanocrystals with diameter of about 5 nm and ZnO nanorods via the electrostatic interaction. In comparison with ZnO nanorods, the Au-ZnO hybrid nanostructures exhibited the broadened and red-shifted surface plasmon band, enhanced band gap emission, and suppressed defect emission due to the strong interfacial coupling between Au and ZnO. Moreover, the band gap emission of the Au-ZnO hybrid nanostructures is controllably blue-shifted with decreasing distance between the Au nanocrystals and ZnO nanorods tuned by the amount of the polyelectrolyte layers due to the exciton and plasmon interactions.     

Effect of thickness on the structural, electrical and optical properties of ZnO films

A series of ZnO films of different thickness have been deposited on glass substrates using sol-gel technique by varying the number of spin coatings and the effect of film thickness on the structural, electrical and optical properties have been investigated. The XRD results indicate that the full width at half maximum (FWHM) of the (0 0 2) diffraction peak and the strain along c-axis are decreased as the film is grown up to a thickness of 300 nm. Above 300 nm, the strain again becomes appreciable. The surface morphology shows that the grains become more uniform and bigger in size as the film thickness increases. Electrical result shows that although ZnO film with thickness of around 260 nm has the highest resistivity but is better for current conduction. The excitonic nature in the absorption spectrum becomes prominent for a film with thickness of around 260 nm. The band gap increases and then decreases as the film grows thicker.     

Synthesis, characterization and optical properties of sheet-like ZnO

Sheet-like ZnO with regular hexagon shape and uniform diameter has been successfully synthesized through a two-step method without any metal catalyst. First, the sheet-like ZnO precursor was synthesized in a weak alkaline carbamide environment with stirring in a constant temperature water-bath by the homogeneous precipitation method, then sheet-like ZnO was obtained by calcining at 600 °C for 2 h. The structures and optical properties of sheet-like ZnO have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and UV-vis-NIR spectrophotometer. The results reveal that the product is highly crystalline with hexagonal wurtzite phase and has appearance of hexagon at (0 0 0 1) plane. The HRTEM images confirm that the individual sheet-like ZnO is single crystal. The PL spectrum exhibits a narrow ultraviolet emission at 397 nm and a broad visible emission centering at 502 nm. The band gap of sheet-like ZnO is about 3.15 eV.     

Solution precursor plasma deposition of nanostructured ZnO coatings

Zinc oxide (ZnO) is a wide band gap semiconducting material that has various applications including optical, electronic, biomedical and corrosion protection. It is usually synthesized via processing routes, such as vapor deposition techniques, sol-gel, spray pyrolysis and thermal spray of pre-synthesized ZnO powders. Cheaper and faster synthesis techniques are of technological importance due to increased demand in alternative energy applications. Here, we report synthesis of nanostructured ZnO coatings directly from a solution precursor in a single step using plasma spray technique. Nanostructured ZnO coatings were deposited from the solution precursor prepared using zinc acetate and water/isopropanol. An axial liquid atomizer was employed in a DC plasma spray torch to create fine droplets of precursor for faster thermal treatment in the plasma plume to form ZnO. Microstructures of coatings revealed ultrafine particulate agglomerates. X-ray diffraction confirmed polycrystalline nature and hexagonal Wurtzite crystal structure of the coatings. Transmission electron microscopy studies showed fine grains in the range of 10-40 nm. Observed optical transmittance (∼65-80%) and reflectivity (∼65-70%) in the visible spectrum, and electrical resistivity (48.5-50.1 mΩ cm) of ZnO coatings are attributed to ultrafine particulate morphology of the coatings.     

Optical and photoconductivity properties of ZnO thin films grown by pulsed filtered cathodic vacuum arc deposition

High quality transparent conductive ZnO thin films with various thicknesses were prepared by pulsed filtered cathodic vacuum arc deposition (PFCVAD) system on glass substrates at room temperature.The high quality of the ZnO thin films was verified by X-ray diffraction and optical measurements. XRD analysis revealed that all films had a strong ZnO (200) peak, indicating c-axis orientation. The ZnO thin films are very transparent (92%) in the near vis regions. For the ZnO thin films deposited at a pressure of 0.086 Pa (6.5 × 10⁻⁴ Torr) optical energy band gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. Urbach tail energy also decreased as the film thickness increased.Spectral dependence of the photoconductivity was obtained from measurements of the samples deposited at various thicknesses. Photoconductivities were observed at energies lower than energy gap which indicates the existence of energy states in the forbidden gap. Photoconductivities of ZnO thin films increase with energy of the light and reach its maximum value at around 2.32 eV. Above this value surface recombination becomes dominant process and reduces the photocurrent. The photoconductivity increases with decreasing the film thickness.     

The growth of nanoscale ZnO films by pulsed-spray evaporation chemical vapor deposition and their structural, electric and optical properties

Great interest in nanoscale thin films (sub-100 nm) has been stimulated by the developing demands of functional devices. In this paper, nanoscale zinc oxide (ZnO) thin films were deposited on glass substrates at 300 °C by pulsed-spray evaporation chemical vapor deposition. Scanning electron micrographs indicate uniform surface morphologies composed of nanometer-sized spherical particles. The growth kinetics and growth mode are studied and the relationship between the film thickness and the electric properties with respect to the growth mode is interpreted. X-ray diffraction shows that all ZnO films grown by this process were crystallized in a hexagonal structure and highly oriented with their c-axes perpendicular to the plane of the substrate. Optical measurements show transparencies above 85% in the visible spectral range for all films. The absorbance in the UV spectral range respects well the Beer-Lambert law, enabling an accurate optical thickness measurement, and the absorption coefficient was measured for a selected wavelength. The measured band gap energies exhibit an almost constant value of 3.41 eV for all films with different thicknesses, which attributed to the thickness-independent crystallite size.     

Non-linear optical and electrical properties of ZnO doped Ni Thin Films obtained using spray ultrasonic technique

In the present study zinc oxide doped Nickel thin films (ZnO:Ni) were deposited on glass substrates using a chemical spray ultrasonic technique. The effect of Ni concentration on the structural, electrical, optical, and non-linear optical (NLO) properties of the ZnO:Ni thin films was investigated. The films were analyzed using X-ray diffraction (XRD), profilometry and optical transmittance. A polycrystalline structure with a preferential growth along the ZnO (002) plane was found, the optical transmittance was found to be higher than 80% and the band gap (E_g) varied from 3.19 to 3.27 eV. The value of the electrical conductivity was found. Moreover, the effective non-linear quadratic and cubic electronic susceptibilities of thin film samples were determined by the SHG and THG techniques, working at 1064 nm.     

Structural and optical characterization of Al-doped ZnO films prepared by thermal oxidation of evaporated Zn/Al multilayered films

Aluminum-doped zinc oxide (ZnO:Al) thin films (t = 68–138 nm) were prepared by thermal oxidation in air flow, at 720 K, of the multilayered metallic Zn/Al thin stacks deposited in vacuum onto glass substrates by physical vapor deposition. The effect of Al content (3.7–8.2 at.%) on the structural (crystallinity, texture, stress, surface morphology) and optical (transmittance, absorbance, energy band gap) characteristics of doped ZnO thin films was investigated. The X-ray diffraction spectra revealed that the Al-doped ZnO films have a hexagonal (wurtzite) structure with preferential orientation with c-axis perpendicular to the substrate surface. A tensile residual stress increasing with Al content was observed. The films showed a high transmittance (about 90%) in the visible and NIR regions. The optical band gap value was found to decrease with Al content from 3.22 eV to 3.18 eV. The results are discussed in correlation with structural characteristics and Al content in the films.     

Growth and characterization of non-polar ZnO thin films by pulsed laser deposition

Non-polar ZnO thin films were fabricated on r-plane sapphire substrates by pulsed laser deposition at various temperatures from 100 to 500 °C. The effects of the substrate temperature on structural, morphological and optical properties of the films were investigated. Based on the X-ray diffraction analysis, the ZnO thin films grown at 300, 400 and 500 °C exhibited the non-polar (a-plane) orientation and those deposited below 300 °C exhibited polar (c-plane) orientation. In the optical properties of non-polar ZnO films, there were two photoluminescence peaks detected. The peaks (near-band edge emission, blue emission) are due to electron transitions from band-to-band and shallow donor level to valence band, respectively.     

Variations of microstructure, conductivity and transparency of Al-doped ZnO thin films prepared by radio frequency magnetron sputtering with target-substrate distances

Al-doped ZnO films were deposited at different target-substrate distances by radio frequency magnetron sputtering. The crystallite size of the films is reduced with increasing the target-substrate distance, but the (002) preferential orientation of ZnO is observed for all the films. It is found that the target-substrate distance has a great influence on the carrier concentration in the films. Reduction of the target-substrate distance is favorable to obtain higher carrier concentrations. The lowest resistivity of 1.1 × 10⁻³ Ω cm is obtained for the film at target-substrate distance of 55 mm. The optical transmittance in the visible range remains higher than 90% for all the films, and the absorption edge shifts towards the shorter wavelength side with decreasing the target-substrate distance. The band gap was widened by 0.11 eV due to the Burstein-Moss (BM) shift from 3.33 eV to 3.44 eV with the reduction of the target-substrate distance from 60 mm to 55 mm.     

A template-free alcoholthermal route to Ti(Sn)-doped ZnO nanorods

Ti(Sn)-doped single-crystalline ZnO nanorods with an average diameter of 20 nm and length up to nearly 1 μm were synthesized by a facile ultrasonic irradiation-assisted alcoholthermal method without involving any templates. Photoluminescence spectra of the Ti-doped ZnO nanorods were measured at room temperature and three emitting bands, being a violet emission at 400-415 nm, a blue band at 450-470 nm and a green band at around 550 nm, were detected. The emission intensities of the Ti-doped ZnO nanorods enhance gradually with increasing the doping concentrations. As to the Sn-doped ZnO nanorods, the green emission shifts to 540 nm and the emission intensities increase first but decrease later with increasing the doping concentrations.