Growth Of Zinc Oxide Thin Films Research Articles

Micro spot ZnO nanotubes using laser assisted chemical bath deposition: A low-cost approach to UV photodetector fabrication

Due to its outstanding characteristics across a wide variety of applications, such as electrical, photonic, electrochemical, and electromechanical devices, one-dimensional Zinc oxide (ZnO) is one of the most promising nanostructures. Using a continuous wave semiconductor laser with a wavelength of 460 nm and a power of 1 W, functional ZnO nanostructures known as nanotubes (NTs) with high-quality structural, optical, and electrical properties on a glass substrate coated with iron (Fe) to construct a UV photoconductive detector with high responsivity were produced for first time by using a simple and quick laser assisted chemical bath deposition (LACBD). X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), and energy dispersive X-ray (EDX)analyses were used to investigate the evolution of the material's physical and structural characteristics. Our findings revealed that laser-assisted in-situ irradiation of the chemical solution resulted in faster growth of ZnO thin films. To account for the potential of these nanostructures, the effectiveness of ZnO nanotubes (NTs) in ultraviolet (UV) detection has also been investigated. The NT-based photodetector has outstanding stability and responsiveness in both dark and light conditions. Exciton confinement is identified using time-resolved spectroscopy, suggesting that the high surface-to-volume ratio, optical confinement, and high structural quality of the NTs are responsible.

Spectroscopic Ellipsometry Study on Tuning the Electrical and Optical Properties of Zr-Doped ZnO Thin Films Grown by Atomic Layer Deposition.

This work reports the ellipsometry analysis of atomic layer deposition (ALD) films of ZnO doped with Zr to determine parameters like free carrier concentration and mobility. Thin films of zinc oxide (ZnO) and Zr-doped ZnO of thickness ∼100 nm were prepared by atomic layer deposition on sapphire, SiO2/Si(100), and Si(100) substrates. Variable-angle spectroscopic ellipsometry was used to study their optical properties in the 0.5–3.5 eV spectral range. The optical constants were accurately obtained using a model that combines Drude and Tauc–Lorentz oscillators with Bruggeman effective medium approximations, allowing the inclusion of a roughness layer in the optical model. The effect of Zr doping (ca. 1.9–4.4 atom %) was then investigated in both as-prepared samples and samples annealed in the temperature range of 100–300 °C. All of the films exhibited good optical transparency (ca. 70–90% in the visible region). For doping levels below 2.7 atom %, the real part of the dielectric permittivity reveals a semiconductor-to-metal transition in the near-infrared (NIR) region, as the permittivity goes from positive to negative. Besides, the plasma energy increases with increasing Zr concentration, and both resistivity and carrier concentration exhibit slightly parabolic behaviors, with a minimum of ∼1.5 × 10–3 Ω cm and a maximum of 2.4 × 1020 cm–3, respectively, at the same critical Zr concentration (2.7 atom %). In contrast, the carrier mobility decreases rapidly from 76.0 to 19.2 cm2/(V s) with increasing Zr content, while conductivities and carrier mobilities worsen when the annealing temperature increases, probably due to the segregation of ZnO crystals. Finally, the optical band gap is very stable, revealing its interesting independence of substrate composition and annealing temperature, as it collapses to a single master curve when band gap energy is plotted versus free carrier concentration, following the Burstein–Moss effect. Overall, the Zr-doped ZnO films studied here would be a highly desirable system for developing thermally stable transparent conductive oxides (TCOs).

Tuning of defects induced visible photoluminescence by swift heavy ion irradiation and thermal annealing in zinc oxide films

Present study reports the influence of electronic energy deposition on sol-gel grown zinc oxide thin films with different type of microstructures for tailoring the defects induced photoluminescence properties. The microstructure of the films was modified by the controlled thermal annealing and the nature of defects were tuned by swift heavy ions (SHI) irradiations at varying ion fluences of such thin films. It is observed that all the films show two emission bands in the spectral range of 350 nm–1000 nm; (i) a very weak UV-emission band and (ii) a broad visible emission band. The UV-emission band is attributed to the band to band transition while the visible emission to the deep-level defects. The maximum visible emission lies in the green emission region which enhanced with increasing annealing temperature for the samples annealed in controlled Argon gas environment in comparison of samples annealed in an oxygen environment. The intensity of the visible emission band was found to be increased with ion irradiation at low ion fluences while showing reduction at higher ion fluences along with peak broadening. The films annealed at higher annealing temperature exhibit shifting of peak maximum from green region to red region at higher fluences. The peak shifting and peak broadening at higher ion fluences could be attributed to the formation of the high density of complex defects such vacancies cluster and ionized oxygen vacancies in the lattice as tuned under the influence of electronic energy deposition through SHI irradiations. Thus, it is suggested that the intense luminescence induced by the defects could be tuned in controlled manner by selecting the proper irradiations parameters and thus it would be very interesting for the development of the optoelectronic applications.

Synthesis and Characterizations of ZnO Thin Films Grown by Physical Vapor Deposition Technique

In the current study, Zinc oxide (ZnO) thin films have been synthesized over the whole the glass-slide substrate by utilizing the physical vapor deposition (PVD) technique. The Zinc (Zn) seed layer was deposited by heating the high purity Zn powder by using a molybdenum (Mo) boat at 37.503×10-3 Torr vacuum pressure of the PVD chamber. The ZnO thin films were fabricated by oxidation of the Zn seed layer coated glass-slide substrate at 400 °C. The morphological, chemical compositions, crystal quality, structural and optical properties of fabricated ZnO thin film were characterized and studied utilizing several characterization techniques. The results found that the high distribution density, homogenous, uniform, and high-quality ZnO thin film was grown over the entire substrate. The synthesized ZnO thin film with a thickness of 130 nm was grown with high purity and polycrystalline hexagonal-Wurtzite phase of ZnO. The sharp, and dominant diffraction peak was observed at peak position 34.3375 along (002) plane and c-axis. The investigated crystal size, dislocation density, and interplanar spacing were about 13.33 nm, 5.63×10-5 A°, and 2.609 A°, respectively. Also, UV-visible spectroscopy results show the high transmittance and low absorbance in the visible (Vis.) region and were about 90%, and the transmittance decreases sharply near the UV region at a wavelength around 383 nm. Besides, obtained the energy band-gap (Eg) was about 3.24 eV.

Piezoelectric Properties of Zinc Oxide Thin Films Grown by Plasma‐Enhanced Atomic Layer Deposition

Zinc oxide (ZnO) thin films are deposited by plasma‐enhanced atomic layer deposition (PE‐ALD). This deposition method allows depositing stoichiometric and highly resistive ZnO films at room temperature. Despite such important requirements for piezoelectricity being met, not much is known in literature about the piezoelectric properties of ZnO thin films (<70 nm) deposited by PE‐ALD. The films are grown at different substrate temperatures to investigate the effect on crystalline and piezoelectric properties. Films deposited on flexible poly(ethylene terephthalate) (PET) generated a higher piezoelectric current (>1.8 nA) and charge (>80 pC) compared with films deposited on glass (>0.3 nA and >30 pC) due to bending effects of the substrate when mechanically excited. Furthermore, increasing the substrate temperature, during deposition, enhances the growth along the (002) crystallographic orientation, which further strengthens the generated piezoelectric current signal for mechanical excitations along the ZnO film's c‐axis.

Influence of Seed Layer on Surface Morphology of ZnO Thin Films Grown by SILAR Method

Zinc oxide (ZnO) nanostructured seed layer was grown by successive ionic layer adsorption and reaction (SILAR) method on glass substrate. The as-prepared nanostructured seed layer was characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) for its structure and surface morphology. XRD results showed a ZnO seed layer growth oriented along the (100) direction. Study of surface morphology showed ZnO nanocrystal clusters with hexagonal shape. ZnO nanorods (NRs) have been grown over the as-prepared ZnO nanostructured seed layer by immersing seed layer substrate in a chemical bath. It has been found that the morphology of the nanostructured seed layer is a key influencing factor for the growth of vertical ZnO NRs. In our growth method, we were successful in growing vertical NRs with diameter of about 100–240[Formula: see text]nm with perfect hexagonal shape. The transmittance studies were carried out to analyze the optical bandgap of the as-grown ZnO nanorods.

Assessment of structural, morphological and optical properties of ZnO thin films grown by physical and chemical techniques

Zinc oxide (ZnO) is one of the most promising and frequently used semiconductor materials. Zinc oxide (ZnO) thin films are widely used for several applications due to their excellent properties of high electron mobility and good thermal and chemical stability, together with being non-toxic and low cost. The main objective of the present investigation was to identify the modified features of the grown Zinc Oxide thin films on Indium doped Tin Oxide coated conducting glass substrates by employing different physical and chemical growth techniques. The structural analysis confirmed that the both grown thin films preferentially oriented along c-axis with wurtzite crystal structure, however, thin films deposited through aqueous medium holds other planes too. The surface features were examined with scanning electron as well atomic force microscope techniques. The optical properties of the grown thin films will be analyzed through Ultraviolet Visible spectroscopic technique and the results revealed that the deposited films have band gap value within the range between 3.33 and 3.48 eV.

Effects of Grain Size on the UV‐Photoresponse of Zinc Oxide Thin Films Grown by Spray‐Pyrolysis

Zinc oxide films with different average grain size are deposited on glass substrates by spray‐pyrolysis at 425 °C. Samples are characterized by XRD, UV–Vis transmittance, FE‐SEM, and electrical properties measurements. The increase in conductivity observed under 365 nm wavelength illumination is evaluated in terms of grain size. Films with small grains (120 nm) show higher response with respect to films with larger grains (230 and 300 nm). A model considering resistivity and grain size for the prediction of the response of films in terms of UV‐to‐dark conductivity ratio is proposed and validated with experimental results.

Structural, Electrical, and Optical Properties of Bismuth-doped Zinc-oxide Thin Films Grown by Radio-frequency Magnetron Sputtering

The effect of deposition temperature on the structural, optical, and electrical properties of bismuth-doped zinc-oxide (BZO) thin films was studied. The BZO thin films were prepared on sapphire substrates at various deposition temperatures by using radio-frequency magnetron sputtering. X-ray diffraction patterns showed a strong peak due to the (002) planes of Zinc-oxide (ZnO) for all samples, regardless of deposition temperature. As the deposition temperature increased, both the grain size and the film thickness increased. The optical transmittance (~ 80% at ~ 440 nm) increased rapidly with increasing wavelength and approached nearly 99% above the wavelengths of 800 nm. With increasing deposition temperature, both the electron concentration and mobility in the BZO thin films increased whereas the resistivity decreased. These results suggested that the deposition temperature is an important factor in determining the formation of high-quality BZO thin films on sapphire substrates.

Evaluation of the optical characteristics of c-axis oriented zinc oxide thin films grown by sol gel spin coating technique

In this work we have systematically studied the optical characteristics of synthesized wurzite zinc oxide thin films exhibiting (002) orientation. Using sol gel spin coating technique zinc oxide thin films are grown on pre cleaned fused quartz substrates. Structural properties of the films are studied using X-ray diffraction analysis. Micro structural analysis and thickness of the grown samples are analyzed using field emission scanning electron microscopy. With an aim to investigate the optical characteristics of the grown zinc oxide thin films the transmission and reflection spectra are evaluated in the ultraviolet-visible (UV-VIS) range. Using envelope method, the refractive index, extinction coefficient, absorption coefficient, band gap energy and the thickness of the synthesized films are estimated from the recorded UV-VIS spectra. An attempt has also been made to study the influence of crystallographic orientation on the optical characteristics of the grown films.

Photoluminescence of Atomic Layer Deposition Grown ZnO Nanostructures

Atomic Layer Deposition (ALD) technique was used to grow crystalline, nanostructured Zinc Oxide (ZnO) thin films, having very low surface roughness, on silicon (Si) and fused quartz (SiO2) substrates. The film thickness and structural properties were characterized by using X-ray reflectivity and X-ray diffraction techniques. The excitonic light emission of ZnO thin films wasstudied by means of photoluminescence (PL) measurements in the ultraviolet-visible spectral region. Characteristic near band edge emission (NBE) of the ZnO thin films corresponding to free excitonic recombination shows strong signal in the UV range at room temperature. Visible region of the PL spectra of ZnO nanostructures shows that there is no defect related deep level emission (DLE), suggesting the absence of defect states. The results confirm the growth of ZnO thin films of a high quality crystalline structure by ALD technique.

Effect of Nitridation and Pre-Growth Annealing of the Sapphire Substrate on the Quality of Zinc Oxide Thin Films Grown by RF-Magnetron Sputtering

Effect of Nitridation and Pre-Growth Annealing of the Sapphire Substrate on the Quality of Zinc Oxide Thin Films Grown by RF-Magnetron Sputtering

Schottky Diodes on ZnO Thin Films Grown by Plasma-Enhanced Atomic Layer Deposition

Enhancement of the properties of zinc oxide (ZnO)-based Schottky diodes has been explored using a combination of plasma-enhanced atomic layer deposition (PE-ALD) ZnO thin films and silver oxide Schottky contacts deposited by reactive radio-frequency sputtering. The electrical properties of the ZnO thin films were systematically tuned by varying the deposition temperature and oxygen plasma time during PE-ALD to optimize the performance of the diode. Low temperature (80 °C) coupled with relatively long oxygen plasma time (>30 s) PE-ALD is the key to produce ZnO films with net doping concentration lower than 1017 cm−3. Under the optimal deposition conditions identified, the diode shows an ideality factor of 1.33, an effective barrier height of 0.80 eV, and an ON/OFF ratio of ${3.11} \times {10}^{{5}}$ .

Highly textured ZnO thin films grown using sol-gel route for gas sensing application

The crystalline orientation of metal oxide thin films often modulates their structural, optical as well as surface related properties. Viewing in the same line, in the present work we have systematically investigated the structural, micro structural, optical and gas sensing characteristics of highly textured zinc oxide (ZnO) thin films oriented along (002) crystalline planes. The ZnO thin films have been grown on quartz substrates by spin coating the precursor sol prepared through cost effective wet chemical synthesis route. The structural and micro structural characteristics of the grown films are studied using X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) respectively. The optical properties of the synthesized films are investigated by analyzing the ultraviolet-visible (UV–VIS) and photoluminescence (PL) spectroscopy. From the recorded absorption spectra band gap energies of the films are estimated. The gas sensing characteristics of the grown films are investigated upon exposure to various toxic and combustible gases at different operating temperatures. Finally, the effect of crystalline orientation on the structural and optical properties and gas sensing characteristics of the grown ZnO thin films are explored.

Chloride contamination of electrochemically grown zinc oxide thick films

Electrochemically grown zinc oxide thin films have applications in photovoltaics, optoelectronics, and piezoelectrics. Potentiostatic electrodeposition using zinc nitrate typically includes the use of an Ag/AgCl electrode to measure solution potential, which can leak silver chloride into the growth solution. The effects of chloride contamination in the growth solution on the properties of potentiostatically and galvanostatically grown zinc oxide films were investigated. The incorporation of the chloride was particularly notable for galvanostatically grown films deposited for longer than 2 h. The effects of the chloride on the electronic and crystallographic properties of the film were examined with scanning electron microscopy, X-ray diffraction, energy-dispersive spectroscopy, and resistance measurements. Using an Ag/AgCl electrode during the long-term growth of zinc oxide films contaminated the films with chloride, resulting in poor crystallinity, reduced chemical purity, inconsistent electrical conductivity, poor transparency, and a rough film surface.

Effect of substrates and thickness on optical properties in atomic layer deposition grown ZnO thin films

Atomic Layer Deposition technique was used to grow high quality, very low roughness, crystalline, Zinc Oxide (ZnO) thin films on silicon (Si) and fused quartz (SiO2) substrates to study the optical properties. Spectroscopic ellipsometry results of ZnO/Si system, staggered type-II quantum well, demonstrate that there is a significant drop in the magnitudes of both the real and imaginary parts of complex dielectric constants and in near-band gap absorption along with a blue shift of the absorption edge with decreasing film thickness at and below ∼20nm. Conversely, UV–vis absorption spectroscopy of ZnO/SiO2, thin type-I quantum well, consisting of a narrower-band gap semiconductor grown on a wider-band gap (insulator) substrate, shows the similar thickness dependent blue-shift of the absorption edge but with an increase in the magnitude of near-band gap absorption with decreasing film thickness. Thickness dependent blue shift, energy vs. 1/d2, in two different systems, ZnO/Si and ZnO/SiO2, show a difference in their slopes. The observed phenomena can be consistently explained by the corresponding exciton (or carrier/s) deconfinement and confinement effects at the ZnO/Si and ZnO/SiO2 interface respectively, where Tanguy-Elliott amplitude pre-factor plays the key role through the electron-hole overlap factor at the interface.

Influence of solution viscosity on hydrothermally grown ZnO thin films for DSSC applications

Zinc oxide (ZnO) nanowire arrays (NWAs) were grown onto zinc oxide-titanium dioxide (ZnO-TiO2) seeded fluorine doped tin oxide (FTO) conductive substrate by hydrothermal technique. X-ray diffraction (XRD) patterns depict that ZnO thin films are preferentially oriented along the (002) plane with hexagonal wurtzite structure. Viscosity measurements reveal that viscosity of the solutions linearly increases as the concentrations of the polyvinyl alcohol (PVA) increase in the growth solution. Field emission scanning electron microscope (FE-SEM) images show that the NWAs are vertically grown to seeded FTO substrate with hexagonal structure, and the growth of NWAs decreases as the concentration of the PVA increases. Stylus profilometer and atomic force microscopic (AFM) studies predict that the thickness and roughness of the films decrease with increasing the PVA concentrations. The NWAs prepared at 0.1% of PVA exhibits a lower transmittance and higher absorbance than that of the other films. The band gap of the optimized films prepared at 0.0 and 0.1% of PVA is found to be 3.270 and 3.268 eV, respectively. The photo to current conversion efficiency of the DSSC based on photoanodes prepared at 0.0 and 0.1% of PVA exhibits about 0.64 and 0.82%, respectively. Electrochemical impedance spectra reveal that the DSSC based on photoanode prepared at 0.1% of PVA has the highest charge transfer recombination resistance.

Electrodeposition of ZnO thin films on conducting flexible substrates

In this work, we studied the DC electrochemical deposition of zinc oxide (ZnO) thin films on a conducting and flexible substrate, for their application in energy harvesting piezoelectric nanodevices. The deposition process was performed by varying the zinc nitrate concentration (c) in the electrolyte, its temperature (T), and the applied deposition potential (V), and subsequently tracing the influence of such parameters on the morphology (analyzed by scanning electron microscopy), crystallography (X-ray diffraction), and thickness (using the deposition current transient curves) of the ZnO thin films. The variation of the electrodeposition parameters led to the formation of different micro- and nano-structures, such as flat layers, microflowers, nanospheres, webs, and microramifications. Furthermore, the analysis of the deposited charge (by integrating the deposition current transients) illustrated an increase in the deposition rate with the increase of T and c, and a decrease of V. Finally, the maximum ZnO film thickness (~5 \(\upmu {\mathrm{m}}\)) was obtained for T = 80 °C, c = 0.1 M, and V = −1.5 V. This study provides us important tools to tune the electrochemical growth of ZnO thin films.

Importance of substrate rotation speed on the growth of homogeneous ZnO thin films by reactive sputtering

Undoped zinc oxide (ZnO) thin films were reactively deposited on soda-lime glass substrates by radio frequency (rf) sputtering at room temperature. We have studied the importance of substrate rotation speed on the structural, optical and morphological properties. The rotation speeds used were 0, 20, 40, 60, and 80rpm. The XRD measurements confirmed that films were grown with (002) plane preferential orientation. The average transmittance was above 90% in the UV–vis region. The SEM images revealed that films deposited with substrate rotation are more homogeneous than the films deposited without rotation.

Structural and Optical Properties of Nitrogen‐doped Zinc Oxide Thin Films Grown on Muscovite Mica Substrates Using Sol–Gel Process

Nitrogen (N)‐doped zinc oxide (NZO) thin films were grown on muscovite mica substrates by sol–gel spin‐coating and their structural and optical properties were investigated for various N concentrations (0, 3, 6, 9, and 12 at. %). The surface morphologies of all the films exhibited granular structures consisting of spherical nano‐sized crystallites. The (1 0 0) and (0 0 2) diffraction peaks were observed at 0, 3, and 6 at. % N concentration, while the (1 0 0) peak could not be distinguished and the (0 0 2) peak intensities decreased gradually at the higher concentrations. The near‐band‐edge to deep‐level emission intensity ratio increased significantly with increasing N concentration up to 6 at. %. All the films exhibited an average transmittance of more than 75% in the visible region. The optical band gap and Urbach energy determined by the measured transmittance were obviously dependent on the N concentration.