Influence of substrate temperature and Cobalt concentration on structural and optical properties of ZnO thin films prepared by Ultrasonic spray technique

Pure and Cobalt doped zinc oxide were deposited on glass substrate by Ultrasonic spray method. Zinc acetate dehydrate, Cobalt chloride, 4-methoxyethanol and monoethanolamine were used as a starting materials, dopant source, solvent and stabilizer, respectively. The ZnO samples and ZnO:Co with Cobalt concentration of 2 wt.% were deposited at 300, 350 and 400 °C. The effects of substrate temperature and presence of Co as doping element on the structural, electrical and optical properties were examined. Both pure and Co doped ZnO samples are (0 0 2) preferentially oriented. The X-ray diffraction results indicate that the samples have polycrystalline nature and hexagonal wurtzite structure with the maximum average crystallite size of ZnO and ZnO:Co were 33.28 and 55.46 nm. An increase in the substrate temperature and presence doping the crystallinity of the thin films increased. The optical transmittance spectra showed transmittance higher than 80% within the visible wavelength region. The band gap energy of the thin films increased after doping from 3.25 to 3.36 eV at 350 °C.     

A comparative study on the nanocrystalline ZnO thin films prepared by ultrasonic spray and sol⿿gel method

Nanocrystalline ZnO thin films are deposited through two different chemical methods: (i) the films prepared by ultrasonic spray with 0.1 M and (ii) dip-coating from zinc acetate complex solutions with 0.5 M, the films obtained at different temperatures. The XRD analyses indicated that ZnO films have nanocrystalline hexagonal structure with (0 0 2) preferential orientation and the maximum crystallite size value of 103 nm measured from the films prepared by dip-coating. UV?vis measurement indicated that all films are transparency in the visible region. The optical band gap increased with decreasing of the Urbach tail energy indicating that the increase in the transition tail width and decrease of the defects, respectively.     

The crystalline structure,conductivity and optical properties of Co-doped ZnO thin films

Transparent conductive Co-doped ZnO thin films were deposited by ultrasonic spray technique. Conditions of preparation have been optimized to get good quality. A set of cobalt (Co)-doped ZnO (between 0 and 3 wt%) thin films were grown on glass substrate at 350 °C. The thin films were annealed at 500 °C for improvement of the physical properties. Nanocrystalline films with hexagonal wurtzite structure and a strong (0 0 2) preferred orientation were obtained. The maximum value of grain size G = 63.99 nm is attained with undoped ZnO film. The optical transmissions spectra showed that both the undoped and doped ZnO films have transparency within the visible wavelength region. The band gap energy decreased after doping from 3.367 to 3.319 eV when Co concentration increased from 0 to 2 wt% with slight increase of electrical conductivity of the films from 7.71 to 8.33 (Ω cm)⁻¹. The best estimated structure, optical and electrical results are achieved in Co-doped ZnO film with 2 wt%.     

Investigation of structural and optical properties of ZnO films co-doped with fluorine and indium

Undoped ZnO film and ZnO films, which are co-doped with F and In (FIZO) at different concentrations, were synthesized by sol–gel technique and the effects of co-doping of F and In on structural and optical properties of ZnO thin films were investigated. The concentration ratio of [F]/[Zn] was altered from 0.25 to 1.75 with 0.50 step at.% mole and [In]/[Zn] was altered from 0.25 to 1.00 with 0.25 step at.% mole. X-ray diffraction analysis indicates that the films have polycrystalline nature and the (0 0 2) preferred orientation is the stronger peak. No extra phases involving zinc, fluorine and indium compounds were observed even at high F and In content. The grain size of undoped ZnO and FIZO thin films varied between 15 and 20 nm with a small fluctuation. From the SEM images, although the undoped ZnO had a smooth and particle-shaped surface, FIZO films had nanofiber-networks shapes over the surface with average size of 500 nm. The surface morphologies and crystallite sizes for the F and In doped films were slightly different from than those of undoped film. From the optical study, a slight shrinkage of band gap was backwardly observed from 3.36 to 3.25 eV with the increasing of F and In content.     

Influence of annealing temperature on structural and optical properties of ZnO: In thin films prepared by ultrasonic spray technique

Transparent conducting indium doped zinc oxide was deposited on glass substrate by ultrasonic spray method. The In doped ZnO samples with indium concentration of 3 wt.% were deposited at 300, 350 and 400 °C with 2 min of deposition time. The effects of substrate temperature and annealing temperature on the structural, electrical and optical properties were examined. The DRX analyses indicated that In doped ZnO films have polycrystalline nature and hexagonal wurtzite structure with (0 0 2) preferential orientation and the maximum average crystallite size of ZnO: In before and annealed at 500 °C were 45.78 and 55.47 nm at a substrate temperature of 350 °C. The crystallinity of the thin films increased by increasing the substrate temperature up 350 °C, the crystallinity improved after annealing temperature at 500 °C. The film annealed at 500 °C and deposited at 350 °C show lower absorption within the visible wavelength region. The band gap energy increased from E_g = 3.25 to 3.36 eV for without annealing and annealed films at 500 °C, respectively, indicating that the increase in the transition tail width. This is due to the increase in the electrical conductivity of the films after annealing temperature.     

Effect of Al incorporation on the structural,morphological, optoelectronic and transport properties of PbS thin films

Doping of PbS thin films with different metal atoms produce considerable changes in structural and material properties that make them useful in the technology of thin film devices. The goal of this work is to study the effects of doping on the structural, morphological, optoelectronic and transport properties of PbS thin films as a function of Al³⁺ concentration. Thin films of pure and Al doped PbS nanoparticles are prepared on soda lime glass substrates by chemical bath deposition technique. The Al content in aqueous solution is varied from 0 to 20 mg. XRD analysis of the films revealed significant enhancement in crystallinity and crystallite size up to an optimum concentration of doping. Films are polycrystalline with crystallite size 19–32 nm, having face centered cubic structure. The optical band gap energy exhibits a decreasing trend and is shifted from 2.41 to 1.34 eV with increasing Al content. The room temperature conductivity of the as-deposited PbS films is in the range of 0.78×10⁻⁸ to 0.67×10⁻⁶(Ω cm)⁻¹ with a maximum for optimum Al content. The Al doped PbS thin film, which we synthesize with optimum Al concentration of 15 mg is found to be a most suitable material for solar control coating applications.     

Dependence of film thickness on the structural and optical properties of ZnO thin films

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10⁻⁴ Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.     

Microstructure and optical properties of nanocrystalline ZnO and ZnO:(Li or Al) thin films

Zinc oxide thin films (ZnO, ZnO:Li, ZnO:Al) were deposited on glass substrates by a sol-gel technique. Zinc acetate, lithium acetate, and aluminum chloride were used as metal ion sources in the precursor solutions. XRD analysis revealed that Li doped and undoped ZnO films formed single phase zincite structure in contrast to Al:ZnO films which did not fully crystallize at the annealing temperature of 550 °C. Crystallized films had a grain size under 50 nm and showed c-axis grain orientation. All films had a very smooth surface with RMS surface roughness values between 0.23 and 0.35 nm. Surface roughness and optical band tail values increased by Al doping. Compared to undoped ZnO films, Li doping slightly increased the optical band gap of the films.     

The effect of heat treatment on the physical properties of sol-gel derived ZnO thin films

Zinc oxide (ZnO) thin films were deposited on microscope glass substrates by sol-gel spin coating method. Zinc acetate (ZnAc) dehydrate was used as the starting salt material source. A homogeneous and stable solution was prepared by dissolving ZnAc in the solution of monoethanolamine (MEA). ZnO thin films were obtained after preheating the spin coated thin films at 250 °C for 5 min after each coating. The films, after the deposition of the eighth layer, were annealed in air at temperatures of 300 °C, 400 °C and 500 °C for 1 h. The effect of thermal annealing in air on the physical properties of the sol-gel derived ZnO thin films are studied. The powder and its thin film were characterized by X-ray diffractometer (XRD) method. XRD analysis revealed that the annealed ZnO thin films consist of single phase ZnO with wurtzite structure (JCPDS 36-1451) and show the c-axis grain orientation. Increasing annealing temperature increased the c-axis orientation and the crystallite size of the film. The annealed films are highly transparent with average transmission exceeding 80% in the visible range (400-700 nm). The measured optical band gap values of the ZnO thin films were between 3.26 eV and 3.28 eV, which were in the range of band gap values of intrinsic ZnO (3.2-3.3 eV). SEM analysis of annealed thin films has shown a completely different surface morphology behavior.     

Sn doping effects on the electro-optical properties of sol gel derived transparent ZnO films

Undoped and tin (Sn) doped ZnO films have been deposited by sol gel spin coating method. The Sn/Zn nominal volume ratio was 1, 3 and 5% in the solution. The effect of Sn incorporation on structural and electro-optical properties of ZnO films was investigated. All the films have polycrystalline structure, with a preferential growth along the ZnO (002) plane. The crystallite size was calculated using a well-known Scherrer's formula and found to be in the range of 26-16 nm. X-ray diffraction patterns of the films showed that Sn incorporation leads to substantial changes in the structural characteristics of ZnO films. The SEM measurements showed that the surface morphology of the films was affected from the Sn incorporation. The highest average optical transmittance value in the visible region was belonging to the undoped ZnO film. The optical band gap and Urbach energy values of these films were determined. The absorption edge shifted to the lower energy depending on the Sn dopant. The shift of absorption edge is associated with shrinkage effect. The electrical conductivity of the ZnO film enhanced with the Sn dopant. From the temperature dependence of conductivity measurements, the activation energy of ZnO film increased with Sn incorporation.     

Influence of aging time of the starting solution on the physical properties of fluorine doped zinc oxide films deposited by a simplified spray pyrolysis technique

Fluorine doped zinc oxide (FZO) films were fabricated from fresh and aged (4, 8, 12 and 16 days) starting solutions using a simplified and low cost spray pyrolysis technique. The X-ray diffraction study showed that the preferential orientation is along the (0 0 2) plane for all the films irrespective of the age of the solution. The crystallite size calculated using the Scherrer’s formula is comparatively smaller only for the film prepared from the starting solution having aging time 4 days which may be due to the efficient incorporation of fluorine atoms into the ZnO lattice. This phenomenon is confirmed by the minimum resistivity value (3.14 × 10⁻² Ω cm) obtained in this particular case. The visible transmittance and the optical band gap values are found to be in the range of 63–83% and 3.20–3.31 eV, respectively. The optical transmittance is found to decrease gradually as the aging time of the solution increases and the optical band gap is found to be slightly higher in the case of the film prepared from the fourth day solution. The scanning electron microscopy results depicted that the microstructure of ZnO:F films are largely influenced by the aging of the starting solution.     

Structural, electrical and optical properties of Gd doped and undoped ZnO:Al (ZAO) thin films prepared by RF magnetron sputtering

The influence of the gadolinium doping on the structural features and opto-electrical properties of ZnO:Al (ZAO) films deposited by radio frequency (RF) magnetron sputtering method onto glass substrates was investigated. X-ray analysis showed that the films were polycrystalline fitting well with a hexagonal wurtzite structure and have preferred orientation in [0 0 2] direction. The Gd doped ZAO film with a thickness of 140 nm showed a high visible region transmittance of 90%. The optical band gap was found to be 3.38 eV for pure ZnO film and 3.58 eV for ZAO films while a drop in optical band gap of ZAO film was observed by Gd doping. The lowest resistivities of 8.4 × 10⁻³ and 10.6 × 10⁻³ Ω cm were observed for Gd doped and undoped ZAO films, respectively, which were deposited at room temperature and annealed at 150 °C.     

Effect of doping concentration on the structural and optical properties of pure and tin doped zinc oxide thin films by nebulizer spray pyrolysis (NSP) technique

Pure and tin doped zinc oxide (Sn:ZnO) thin films were prepared for the first time by NSP technique using aqueous solutions of zinc acetate dehydrate, tin (IV) chloride fendahydrate and methanol. X-ray diffraction patterns confirm that the films are polycrystalline in nature exhibiting hexagonal wurtzite type, with (0 0 2) as preferred orientation. The structural parameters such as lattice constant (‘a’ and ‘c’), crystallite size, dislocation density, micro strain, stress and texture coefficient were calculated from X-ray diffraction studies. Surface morphology was found to be modified with increasing Sn doping concentration. The ZnO films have high transmittance 85% in the visible region, and the transmittance is found to be decreased with the increase of Sn doping concentration. The corresponding optical band gap decreases from 3.25 to 3.08 eV. Room temperature photoluminescence reveals the sharp emission of strong UV peak at 400 nm (3.10 eV) and a strong sharp green luminescence at 528 nm (2.34 eV) in the Sn doped ZnO films. The electrical resistivity is found to be 10⁶ Ω-cm at higher temperature and 10⁵ Ω-cm at lower temperature.     

Effect of thickness on structural, optical and electrical properties of nanostructured ZnO thin films by spray pyrolysis

Transparent conducting zinc oxide thin films were prepared by spray pyrolytic decomposition of zinc acetate onto glass substrates with different thickness. The crystallographic structure of the films was studied by X-ray diffraction (XRD). XRD measurement showed that the films were crystallized in the wurtzite phase type. The grain size, lattice constants and strain in films were calculated. The grain size increases with thickness. The studies on the optical properties show that the direct band gap value increases from 3.15 to 3.24 eV when the thickness varies from 600 to 2350 nm. The temperature dependence of the electrical conductivity during the heat treatment was studied. It was observed that heat treatment improve the electrical conductivity of the ZnO thin films. The conductivity was found to increase with film thickness.     

Growth characteristics and properties of ZnO:Ga thin films prepared by pulsed DC magnetron sputtering

Transparent conductive ZnO:Ga thin films were deposited on Corning 1737 glass substrate by pulsed direct current (DC) magnetron sputtering. The effects of process parameters, namely pulse frequency and film thickness on the structural and optoelectronic properties of ZnO:Ga thin films are evaluated. It shows that highly c-axis (0 0 2) oriented polycrystalline films with good visible transparency and electrical conductivity were prepared at a pulsed frequency of 10 kHz. Increasing the film thickness also enlarged the grain size and carrier mobility which will subsequently lead to the decrease in resistivity. In summary, ZnO:Ga thin film with the lowest electrical resistivity of 2.01 × 10⁻⁴ Ω cm was obtained at a pulse frequency of 10 kHz with 500 nm in thickness. The surface RMS (root mean square) roughness of the film is 2.9 nm with visible transmittance around 86% and optical band gap of 3.83 eV.     

A simple growth route towards ZnO thin films and nanorods

Highly orientated ZnO thin films and the self-organized ZnO nanorods can be easily prepared by a simple chemical vapor deposition method using zinc acetate as a source material at the growth temperature of 180 and 320 °C, respectively. The ZnO thin films deposited on Si (100) substrate have good crystallite quality with the thickness of 490 nm after annealing in oxygen at 800 °C. The ZnO nanorods grown along the [0001] direction have average diameter of 40 nm with length up to 700 nm. The growth mechanism for ZnO nanorods can be explained by a vapor-solid (VS) mechanism. Photoluminescence (PL) properties of ZnO thin films and self-organized nanorods were investigated. The luminescence mechanism for green band emission was attributed to oxygen vacancies and the surface states related to oxygen vacancy played a significant role in PL spectra of ZnO nanorods.     

Microstructural parameters and optical constants of ZnTe thin films with various thicknesses

Different thickness of polycrystalline ZnTe films have been deposited onto glass substrates at room temperature by vacuum evaporation technique. The structural characteristics studied by X-ray diffraction (XRD) showed that the films are polycrystalline and have a zinc blende (cubic) structure. The calculated microstructure parameters revealed that the crystallite size increases and microstrain decreases with increasing film thickness. The transmittance and reflectance have been measured at normal and near normal incidence, respectively, in the spectral range 400-2500 nm. For ZnTe films of different thicknesses, the dependence of absorption coefficient, α on the photon energy showed the occurrence of a direct transition with band gap energy (For ZnTe films of different thicknesses) confirming the independency of deduced energy gap on film thickness. The refractive indices have been evaluated in terms of envelope method, which has been suggested by Swanepoul in the transparent region. The refractive index could be extrapolated by Cauchy dispersion relationship over the whole spectra range, which extended from 400 to 2500 nm. It was observed that the refractive index, n increased upon increasing the film thickness up to 508 nm, lying within the experimental error for further increases in film thickness.     

The effects of solvent nature on spray-deposited ZnO thin film prepared from Zn (CH3COO)2, 2H2O

Transparent conducting zinc oxide was deposited on glass substrate by ultrasonic spray method. The ZnO samples with concentration of 0.1 M were deposited at 300, 350 and 400 °C with 2 min of deposition time. The effects of substrate temperature, ethanol and methanol solution on the structural, electrical and optical properties were examined. The DRX analyses indicated that ZnO films have polycrystalline nature and hexagonal wurtzite structure with (1 0 0) and (0 0 2) preferential orientation corresponding to ZnO films resulting from methanol and ethanol, respectively. The crystallinity of the thin films improved with ethanol solution. All films exhibit an average optical transparency about 80%, in the visible range. The band gap energy of ZnO films obtained with methanol solution higher than of ethanol solution for all the films. The electrical resistivity decrease with ZnO obtained from ethanol indicated; due to the maximum crystallite size retched at this point.     

Green photoluminescence from highly oriented ZnO thin film for photovoltaic application

We report here the fabrication of ZnO nanoparticles embedded on glass substrate by sol–gel and spin coating technique. Transmission electron microscope images revealed that the thin film is composed of ZnO nanoparticles. X-ray diffraction data confirms that the fabricated ZnO nanoparticles have hexagonal unit cell structure. The ZnO nanocrystals of the thin film are oriented along the c-axis of the hexagonal unit cell. UV–vis absorption spectroscopy shows that the absorption occurring at 373 nm in the ZnO thin film. The band gap was calculated from the absorption data and found to be 3.76 eV. This band gap enhancement occurs due to size effect in the nanoscale regime. Room temperature photoluminescence spectrum shows strong green emission at 530 nm owing to the singly ionized oxygen vacancy. This green emission was further investigated by annealing the thin film at different temperature. This singular green emission will be very useful in optoelectronic and nanophotonic devices.