Structural and optical characteristics of spin-coated ZnO thin films

Zinc oxide (ZnO) thin films were deposited onto glass substrates by spin-coating method, from a precursor solution containing zinc acetate, ethanol and ammonium hydroxide. After deposition, the films were heated at a temperature of 100 °C in order to remove unwanted materials. Finally, the films were annealed at 500 °C for complete oxidation. X-ray diffraction showed that ZnO films were polycrystalline and have a hexagonal (wurtzite) structure. The crystallites are preferentially oriented with (0 0 2) planes parallel to the substrate surface. The films have a high transparency (more than 75%) in the spectral range from 450 nm to 1300 nm. The analysis of absorption spectra shows the direct nature of band-to-band transitions. The optical bandgap energy ranges between 3.15 eV and 3.25 eV.Some correlations between the processing parameters (spinning speed, temperature of post deposition heat treatment) and structure and optical characteristics of the respective thin films were established.     

Deposition and properties of B-N codoped p-type ZnO thin films by RF magnetron sputtering

B-N codoped p-type ZnO thin films have been realized by radio frequency (rf) magnetron sputtering using a mixture of argon and oxygen as sputtering gas. Types of conduction and electrical properties in codoped ZnO films were found to be dependent on oxygen partial pressure ratios in the sputtering gas mixture. When oxygen partial pressure ratio was 70%, the codoped ZnO film showed p-type conduction and had the best electrical properties. Additionally, the p-ZnO/n-Si heterojunction showed a clear p-n diode characteristic. XRD results indicate that the B-N codoped ZnO film prepared in 70% oxygen partial pressure ratio has high crystal quality with (0 0 2) preferential orientation. Meanwhile, the B-N codoped ZnO film has high optical quality and displays the stronger near band edge (NBE) emission in the temperature-dependent photoluminescence spectrum, the acceptor energy level was estimated to be located at 125 meV above the valence band.     

Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films

In this paper, the experimental results regarding some structural, electrical and optical properties of ZnO thin films prepared by thermal oxidation of metallic Zn thin films are presented.Zn thin films (d=200–400 nm) were deposited by thermal evaporation under vacuum, onto unheated glass substrates, using the quasi-closed volume technique. In order to obtain ZnO films, zinc-coated glass substrates were isochronally heated in air in the 300–660 K temperature range, for thermal oxidation.X-ray diffraction (XRD) studies revealed that the ZnO films obtained present a randomly oriented hexagonal nanocrystalline structure. Depending on the heating temperature of the Zn films, the optical transmittance of the ZnO films in the visible wavelength range varied from 85% to 95%. The optical band gap of the ZnO films was found to be about 3.2 eV. By in situ studying of the temperature dependence of the electrical conductivity during the oxidation process, the value of about 2×10⁻² Ω⁻¹ m⁻¹ was found for the conductivity of completely oxidized ZnO films.     

Enhanced antibacterial performance of hybrid semiconductor nanomaterials: ZnO/SnO2 nanocomposite thin films

The nano-sized coupled oxides ZnO/SnO₂ thin films in a molar ratio of 2:1 (Z2S), 1:1 (ZS) and 1:2 (ZS2) were prepared using sol-gel dip coating method and characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV-vis spectroscopy. Escherichia coli (E. coli, ATCC 25922) was selected as a model for the Gram-negative bacteria to evaluate antibacterial property of composite samples compared with single ZnO (Z) and single SnO₂ (S) films. The antibacterial activity has been studied applying the so-called antibacterial drop test under UV illumination. The bactericidal activity was estimated by relative number of bacteria survived calculated from the number of viable cells which form colonies on the nutrient agar plates. The influence of the SnO₂-ZnO nanocomposite composition on the structural features and on the antibacterial properties of the thin films are reported and discussed. It is found that all coatings exhibited a high antibacterial activity. The coupled oxide photocatalyst Z2S has better photocatalytic activity to bacteria inactivation than ZS, ZS2, Z and S films. Furthermore, nanostructured films were active even in the absence of irradiation.     

Effect of energetic electron beam treatment on Ga-doped ZnO thin films

Transparent conductive zinc oxide (ZnO) thin films were synthesized by a sol–gel spin coating method with the addition of Ga(NO₃)₃ in a Zn(CH₃COO)₂ solution and exposed to electron beam treatment. The UV–Vis spectra demonstrated that all of the films had transmittances of over 85% in the visible region. When Ga(NO₃)₃ was added to the ZnO precursor solution, the resistivity of the ZnO thin film decreased and the carrier concentration increased significantly. After electron beam treatment was performed on the 0.4 at.% Ga-doped ZnO film, the optical band gap increased and the resistivity significantly decreased resulting from the increases of the carrier concentration and mobility. By combining Ga doping and electron beam treatment, the resistivity of the ZnO thin film was reduced by a factor of nine hundred.     

Photoluminescence and photoelectrochemical properties of nanocrystalline ZnO thin films synthesized by spray pyrolysis technique

A simple and inexpensive spray pyrolysis technique (SPT) was employed for the synthesis of nanocrystalline zinc oxide (ZnO) thin films onto soda lime glass and tin doped indium oxide (ITO) coated glass substrates at different substrate temperatures ranging from 300 °C to 500 °C. The synthesized films were polycrystalline, with a (0 0 2) preferential growth along c-axis. SEM micrographs revealed the uniform distribution of spherical grains of about 80-90 nm size. The films were transparent with average visible transmittance of 85% having band gap energy 3.25 eV. All the samples exhibit room temperature photoluminescence (PL). A strong ultraviolet (UV) emission at 398 nm with weak green emission centered at 520 nm confirmed the less defect density in the samples. Moreover, the samples are photoelectrochemically active and exhibit the highest photocurrent of 60 μA, a photovoltage of 280 mV and 0.23 fill factor (FF) for the Zn₄₅₀ films in 0.5 M Na₂SO₄ electrolyte, when illuminated under UV light.     

Effect of hydrogen doping in ZnO thin films by pulsed DC magnetron sputtering

This study examined the role of hydrogen impurities in highly oriented ZnO thin films. Hydrogen intentionally incorporated was found to play an important role as a donor in n-type conduction, improving the free carrier concentration. The increase in the conductivity of ZnO thin films was attributed to the two centers assigned to isolated hydrogen atoms in the anti-bonding sites as well as bond-centered interstitial hydrogen located between the Zn–O bonds and Zn vacancy passivated by one or two hydrogen atoms. Micro Raman spectroscopy showed two additional modes at approximately 501 and 573 cm⁻¹. These two peaks were attributed to damage to the crystal lattice, which could be explained by the optical-phonon branch at the zone boundary and host lattice defects, such as vacancy clusters, respectively.     

Superhydrophobic and transparent ZnO thin films synthesized by spray pyrolysis technique

Superhydrophobic and transparent zinc oxide (ZnO) thin films were deposited by a simple and cost effective spray pyrolysis technique (SPT) onto the glass substrates at 723 K from an aqueous zinc acetate precursor solution. The solution concentration was varied from 0.1 to 0.4 M and its effect on structural, morphological, wetting and optical properties of ZnO thin films was studied. The synthesized films were found to be polycrystalline, with preferential growth along c-axis. A slight improvement in the crystallite size and texture coefficient is observed as the concentration of the solution is increased. SEM micrographs show the uniform distribution of spherical grains of about 60-80 nm grain size. The films were specular and highly transparent with average transmittance of about 85%. The spectrum shows sharp absorption band edge at 381 nm, corresponding to optical gap of 3.25 eV. The samples of texture coefficient less than 90% and roughness less than 75 nm are hydrophobic and above these values they become superhydrophobic in nature. The hydrophobicity coupled with high transmittance is of great importance in commercial application such as transparent self-cleaning surfaces, anti-fog, anti-snow, fluid microchips and microreactors.     

Growth and characteristics of hydrogenated In-doped ZnO thin films by pulsed DC magnetron sputtering

We investigated the role of hydrogen impurities in highly oriented In-doped ZnO (IZO:H) films. The conductivity of ZnO:H films exhibit small variation despite the increase of hydrogen ratio. The small variation of the carrier concentration in IZO:H films can be explained by the reduction of the oxygen deficiency for the charge neutrality and the increase of Vzn-H bonding for partially charge compensation in the films. The additional mode at 573 cm⁻¹ is interpreted as vacancy clusters. The discrepancy between the increase of vacancy clusters (573 cm⁻¹) and small variation of carrier concentration is attributed to the different physical characteristics of the IZO:H films due to the hydrogen existence between bulk and surface. The measured FT-IR peak at 3500 cm⁻¹ exhibits typical characteristic of O-H bonding.     

Characterisation of porous doped ZnO thin films deposited by spray pyrolysis technique

Al or Sn doped ZnO films were deposited by spray pyrolysis using aqueous solutions. The films were deposited on either indium tin oxide coated or bare glass substrates. ZnCl₂, AlCl₃ and SnCl₂ were used as precursors. The effect of ZnCl₂ molar concentration (0.1-0.3 M) and doping percentage (2-4% AlCl₃ or SnCl₂) have been investigated. The main goal of this work being to grow porous ZnO thin films, small temperature substrates (200-300 °C) have been used during the spray pyrolysis deposition. It is shown that, if the X-ray diffraction patterns correspond to ZnO, the films deposited onto bare glass substrate are only partly crystallized while those deposited onto ITO coated glass substrate exhibit better crystallization. The homogeneity of the films decreases when the molar concentration of the precursor increases, while the grain size and the porosity decrease when the Al doping increases. The optical study shows that band tails are present in the absorption spectrum of the films deposited onto bare glass substrate, which is typical of disordered materials. Even after annealing 4 h at 400 °C, the longitudinal resistivity of the films is quite high. This result is attributed to the grain boundary effect and the porosity of the films. Effectively, the presence of an important reflection in the IR region in samples annealed testifies of a high free-carriers density in the ZnO crystallites. Finally it is shown that when deposited in the same electrochemical conditions, the transmission of a polymer film onto the rough sprayed ZnO is smaller than that onto smooth sputtered ZnO.     

Structural and morphological study of ZnO thin films electrodeposited on n-type silicon

In this work, we report on the electrodeposition of ZnO thin films on n-Si (1 0 0) and glass substrates. The influence of the deposition time on the morphology of ZnO thin films was investigated. The ZnO thin films were characterized by X-ray diffraction (XRD), energy dispersive X-ray (EDS) and scanning electron microscopy (SEM). The results show a variation of ZnO texture from main (0 0 2) at 10 min to totally (1 0 1) at 15 min deposition time. The photoluminescence (PL) studies show that both UV (∼382 nm) and blue (∼432 nm) luminescences are the main emissions for the electrodeposited ZnO films. In addition, the film grown at 15 min indicates an evident decrease of the yellow-green (∼520 nm) emission band comparing with that of 10 min. Finally, transmittance spectra show a high transmission value up to 85% in the visible wavelength range. Such results would be very interesting for solar cells applications.     

Excimer laser processing of ZnO thin films prepared by the sol-gel process

ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a sol-gel prepared with anhydrous zinc acetate [Zn(C₂H₃O₂)₂], monoethanolamine [H₂NC₂H₄OH] and isopropanol. The deposited films were dried at 50 and 300 °C. X-ray analysis showed that the films were amorphous. Laser annealing was performed using an excimer laser. The laser pulse repetition rate was 25 Hz with a pulse energy of 5.9 mJ, giving a fluence of 225 mJ cm⁻² on the ZnO film. Typically, five laser pulses per unit area of the film were used. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. The thin films had a transparency of greater than 70% in the visible region. The optical band-gap energy was 3.454 eV. Scanning electron microscopy and profilometry analysis highlighted the change in morphology that occurred as a result of laser processing. This comparative study shows that our sol-gel processing route differs significantly from ZnO sol-gel films prepared by conventional furnace annealing which requires temperatures above 450 °C for the formation of crystalline ZnO.     

Synthesis and characterization of ZnO nano/microfibers thin films by catalyst free solution route

Zinc oxide (ZnO) nano/microfibrous thin films were successfully synthesized by a catalyst free solution route on glass and Si substrates. X-ray diffraction study revealed the formation of ZnO nanofibers of hexagonal crystalline structure. The texture coefficient of different planes varied with annealing temperature and that of the (0 0 2) plane was the highest for films annealed at temperature 873 K. Scanning electron micrograph showed the well formation of ZnO nano/microfibers with an average diameter 500 nm and having an average aspect ratio 150. UV–Vis–NIR spectroscopic study for the films deposited on glass substrates showed the high transmittance in the visible and near-infrared region. It was also observed that the band gap energy decreased as the films were annealed at higher temperature. The band gap energies of nanostructured ZnO thin films were determined to be in the range 3.03–3.61 eV. The photoluminescence study showed an UV emission peak at 397 nm, a visible blue–green emission peak at 468 nm and a green emission peak at 495 nm. Field emission properties of nanofiber ZnO thin film showed considerably low turn-on field around 1.4 V/μm. The emission current was as high as 70 μA at the field of 3.6 V/μm.     

Effect of precursors on structure, optical and electrical properties of chemically deposited nanocrystalline ZnO thin films

Nanocrystalline ZnO thin films were chemically deposited on glass substrates using two different precursors namely, zinc sulphate and zinc nitrate. XRD studies confirm that the films are polycrystalline zinc oxide having hexagonal wurtzite structure with crystallite size in the range 25-33 nm. The surface morphology of film prepared using zinc sulphate exhibits agglomeration of small grains throughout the surface with no visible holes or faulty zones, while the film prepared using zinc nitrate shows a porous structure consisting of grains with different sizes separated by empty spaces. The film prepared using zinc sulphate shows higher reflectance due to its larger refractive index which is related to the packing density of grains in the film. Further, the film prepared using zinc sulphate is found to have normal dispersion for the wavelength range 550-750 nm, whereas the film prepared using zinc nitrate has normal dispersion for the wavelength range 450-750 nm. The direct optical band gaps in the two films are estimated to be 3.01 eV and 3.00 eV, respectively. The change in film resistance with temperature has been explained on the basis of two competing processes, viz. thermal excitation of electrons and atmospheric oxygen adsorption, occurring simultaneously. The activation energies of the films in two different regions indicate the presence of two energy levels - one deep and one shallow near the bottom of the conduction band in the bandgap.     

Study of structural, optical and electrical properties of ZnO and SnO2 thin films

The investigation of structure, optical and electrical properties of tin and zinc oxide films on glass substrates by using magnetron sputtering are carried out. X-ray data show the formation of textured tin oxides film during deposition and its transformation to SnO₂ polycrystalline film at low temperature (200 C) if the concentration of oxygen in the chamber is high (O₂ — 100%, Ar — 0%). Optimal conditions of SnO₂ polycrystalline film deposition (pressure of Ar–O₂ mixture in chamber — 2.7 Pa, concentration of O₂ — 10%) are determined. Low resistivity of as-deposited ZnO film and increasing ZnO crystallite sizes and phase volume at temperatures higher than the melting point of Zn (419.5 C) are explained by formation of conductive Zn and ZnO particle chains and their destruction, respectively.     

Study on the ferromagnetism in Co and N doped ZnO thin films

Present investigation reports the structural, optical and magnetic properties of co-doping of Co and N ions in ZnO samples, prepared by two distinct methods. In the first method, samples are synthesized by Sol–gel technique in which the Co and N are co-doped simultaneously during the growth process itself. In the second case, N ions are implanted in the Co doped ZnO thin films grown by Pulsed Laser Deposition (PLD). Structural studies showed that the nitrogen implantation on Co doped ZnO samples developed compressive stress in the films. X-ray photoelectron spectroscopy confirmed the doping of Co and N in ZnO matrix. In the Resonant Raman scattering multiple LO phonons up to fifth order are observed in the (Co, N) co-doped ZnO. Photoluminescence spectra showed that there is reduction in the bandgap due to the presence of Co in the lattice and also the presence of Zn vacancies in the films. All samples showed ferromagnetic behavior at room temperature. The magnetic moment observed in the implanted films is found to be varied with the different dosages of the implanted N ions. First principle calculations have been carried out to study the possible magnetic interaction in the co-doped system. Present study shows that the ferromagnetic interaction is due to the hybridization between N 2p and Co 3d states in the (Co, N) co-doped ZnO and is very sensitive to the geometrical configurations of dopants and the vacancy in the ZnO host lattice.     

Preparation and characterization of n-type conductive (Al, Co) co-doped ZnO thin films deposited by sputtering from aerogel nanopowders

Highly transparent, n-type conducting ZnO thin films were obtained by low temperature magnetron sputtering of (Co, Al) co-doped ZnO nanocrystalline aerogels. The nanoparticles of ∼30 nm size were synthesized by a sol-gel method using supercritical drying in ethyl alcohol. The structural, optical and electrical properties of the films were investigated. The ZnO films were polycrystalline textured, preferentially oriented with the (0 0 2) crystallographic direction normal to the film plane. The films show within the visible wavelength region an optical transmittance of more than 90% and a low electrical resistivity of 3.5 × 10⁻⁴ Ω cm at room temperature.     

Photoluminescence study of pure and Li-doped ZnO thin films grown by sol-gel technique

The effect of annealing atmosphere, temperature and aging on the photoluminescence of pure and Li-doped ZnO thin films has been investigated. Annealing the pure ZnO in N₂ and He above 800 °C results in green emission centered at ca. 500 nm; however annealing in air red-shifts the green emission to 527 nm. The visible emission of the Li-doped ZnO is found to be largely dependent on the annealing atmosphere. Warm-white photoluminescence with a broad emission band covering nearly the whole visible spectrum is obtained for the Li-doped ZnO films annealed in helium. The substitutional and interstitial extrinsic point defects created by lithium doping may mediate the relative concentration of the intrinsic defects and thereby tune the intrinsic-defect-related visible emission. The enhanced intensity ratio of near-band-edge ultraviolet emission to deep-level visible emission with aging time may be ascribed to both in-diffusion of oxygen from air and self-diffusion of oxygen interstitials to heal the oxygen vacancies during the aging process.     

Effects of growth duration on the structural and optical properties of ZnO nanorods grown on seed-layer ZnO/polyethylene terephthalate substrates

Well-aligned single crystalline zinc oxide (ZnO) nanorods were successfully grown, by hydrothermal synthesis at a low temperature, on flexible polyethylene terephthalate (PET) substrates with a seed layer. Photoluminescence (PL), field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) measurements were used to analyze the optical and structural properties of ZnO nanorods grown for various durations from 0.5 h to 10 h. Regular and well-aligned ZnO nanorods with diameters ranging from 62 nm to 127 nm and lengths from 0.3 μm to 1.65 μm were formed after almost 5 h of growth. The growth rate of ZnO grown on PET substrates is lower than that grown on Si (1 0 0) substrates. Enlarged TEM images show that the tips of the ZnO nanorods grown for 6 h have a round shape, whereas the tips grown for 10 h are sharpened. The crystal properties of ZnO nanorods can be tuned by using the growth duration as a growth condition. The XRD and PL results indicate that the structural and optical properties of the ZnO nanorods are most improved after 5 h and 6 h of growth, respectively.