Undoped Zinc Oxide Thin Films Research Articles

Effects of fluorine incorporation on the microstructure and optical properties of ZnO thin films synthesized by Sol-gel technique

Fluorine doped Zinc oxide (ZnO) [FZO] transparent conducting thin films were deposited by sol-gel spin coating method. Zinc acetate and amonium fluoride were used as precursor solution materials. Microstructural, morphologycal, and optical properties of the films were investigated as a function of flourine (F) doping ranging from 0 to 3.5% mole. From x-ray diffraction (XRD) patterns the films exhibited a hexagonal wurtzite structure with preferred c-axis orientation. All the films had a highly preferential c-axis orientation and exhibited (002) preferential growth in all the dopant ratios. Grain sizes of the films were varied in range of 15 to 18 nm. TheSEM images of the films showed well crystallinity with good homogeneity, smooth surface and uniform grains. The average optical transmittance in the entire visible wavelength region was higher than 95% for all F-doped ZnO except undoped ZnO thin film. The band gap energy values were determined as 3.270, 3.291, 3.293 and 3,287 eV, respectively for undoped ZnO, 0.5, 1.5 and 3.5% mole FZO samples. The band gap values increased with increasing the F dopant concentration for all doped samples. These results make FZO thin films an attractive candidate for transparent material applications. Key words: Flourine doped Zinc oxide (ZnO), Sol-gel growth, transparent conducting oxide (TCO), thin films, microstructures.

Growth of Transparent and Conductive Polycrystalline (0001)‐ZnO Films on Glass Substrates Under Low‐Temperature Hydrothermal Conditions

AbstractFlat panel display technology seems to be an ever‐expanding field developing into a multibillion dollar market. A set of technical solutions involve a transparent conducting film (TCF) that is today still dominated by indium‐tin‐oxide (ITO). In a race to find alternatives that would avoid the indium pitfalls, mainly due to its increasing price and limited natural availablity, replacement materials have been extensively investigated. This work demonstrates that by exploiting basic principles of crystal growth in geometrically constrained conditions, zinc oxide (ZnO) could easily be utilized for this purpose. ZnO layers were grown on inexpensive glass substrates via low‐temperature citrate‐assisted hydrothermal (HT) method. It was shown that in the nucleation stage the crystal growth can be efficiently controlled by spatially confined oriented growth (SCOG) mechanism to produce smooth and dense (0001) oriented polycrystalline ZnO films with superb optical properties. Our products show optical transparency of 82% and surprisingly low sheet resistance for undoped ZnO, only in the order of few 100 Ω sq−1. We believe that a very high degree of self‐organization between the ZnO crystals in our polycrystalline films grown under controlled SCOG conditions is main reason for the highest so far reported transparency to conductivity ratio for undoped ZnO thin film ceramics.

RF magnetron sputtering of ZnO and Al-doped ZnO films from ceramic and nanopowder targets: a comparative study

Undoped zinc oxide (ZnO) and aluminium-doped zinc oxide (ZAO) thin films have been prepared on glass substrates by RF magnetron sputtering from ceramic and nanopowder targets at room temperature (RT). The effects of target nature on the properties of the films have been studied. The X-ray diffraction (XRD) patterns show that ZnO and Al-doped ZnO thin films are highly textured along the c-axis perpendicular to the surface of the substrate from either nanopowder or ceramic targets. From the SEM images, it appears that the surface morphologies of ZAO films exhibit difference from that of undoped ZnO films. All films exhibit a transmittance higher than 80% in the visible region. The optical band gap (Eg) of ZnO and ZAO films obtained from nanopowder target is higher than those prepared using ceramic target. In two cases, Al-doping leads to a larger optical band gap (Eg) of the films.

Effect of the nanostructure on room temperature ferromagnetism and resistivity of undoped ZnO thin films grown by chemical vapor deposition

Undoped zinc oxide thin films having various types of morphology and nanostructure were deposited by metal organic chemical vapor deposition (MOCVD) on single-crystalline substrates. Water-assisted MOCVD process at low temperatures (300–500°C) was applied along with conventional MOCVD in oxygen-containing atmosphere at 500 and 600°C. The strong correlation between room-temperature ferromagnetism of the films, their electrical properties and morphology at nano-scale was demonstrated.

Textured surface ZnO:B/(hydrogenated gallium-doped ZnO) and (hydrogenated gallium-doped ZnO)/ZnO:B transparent conductive oxide layers for Si-based thin film solar cells

Textured surface un-doped zinc oxide (ZnO) and boron-doped ZnO (ZnO:B) thin films with pyramid-like grains were directly deposited by metal organic chemical vapor deposition (MOCVD) technique and hydrogenated gallium-doped ZnO (HGZO) thin films with good electrical properties were grown by pulsed direct-current magnetron sputtering. In order to obtain high quality transparent conductive oxides (TCOs) for Si-based thin film solar cells, through combining the advantages of MOCVD–ZnO and sputter-HGZO thin films, bi-layer textured surface glass/ZnO:B/HGZO and glass/HGZO/ZnO:B TCO thin films with milky surface were fabricated and investigated in detail. The main research purposes are to explore appropriate surface morphology and improve the electrical properties of TCO layers. Compared with textured surface un-doped ZnO and ZnO:B thin films, both the bi-layer ZnO/HGZO and ZnO:B/HGZO thin films exhibit high electron mobility. The ZnO:B/HGZO thin films present modified pyramid-like surface morphology with high transparencies (T>85%) and excellent electrical properties (resistivity of ~9.28×10−4Ωcm and mobility of ~28.8cm2/Vs). Meanwhile, the HGZO/ZnO thin films exhibit large grain size (~500nm–1000nm) with preferred growth orientations along (100) and (002) crystallographic directions. Bi-layer HGZO/ZnO and HGZO/ZnO:B thin films exhibit relatively higher diffuse transmittance than that of corresponding ZnO and ZnO:B thin films.

Nanostructure and optical properties of M doped ZnO (M=Ni, Mn) thin films prepared by sol–gel process

The transparent thin films of undoped, Mn-doped, and Ni-doped zinc oxide (ZnO) have been deposited on glass substrates via sol–gel technique using zinc acetate dehydrate, nickel chloride, and manganese chloride as precursors. The structural properties and morphologies of the deposited undoped and doped ZnO thin films have been investigated. X-ray diffraction (XRD) spectra, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS) were used to examine the morphology and microstructure of the thin films. Optical properties of the thin films were determined by photoluminescence (PL) and UV/vis spectroscopy. The analyzed results indicate that the obtained films are of good crystal quality and have smooth surfaces, which have a pure hexagonal wurtzite ZnO structure without any Mn or Ni related phases. The band gap energy was estimated by Tauc's method and found to be 3.28, 3.26, and 3.34 eV for ZnO, Ni-doped ZnO, and Mn-doped ZnO thin films at room temperature, respectively. Room temperature photoluminescence is observed for the ZnO, Ni-doped ZnO, and Mn-doped ZnO thin films.

Studies of electrical and optical properties of Sn doped and undoped ZnO thin films by the spray pyrolysis method

Undoped and tin (Sn) doped zinc oxide (ZnO) thin film have been deposited by spray pyrolysis method of zinc acetate and tin chloride. Toe concentration ratio of[Sn ]/ [Zn] was varied from 0-5 at %. Toe effect of doping Sn on physical and optical properties was studied by different techniques such as X-ray diffraction for structural characterization, SEM for thickness and morphology, UV-Vis spectroscopy for optical properties, Hall effect and photoconductivity for electrical characterization. X-ray diffraction (XRD) patterns show that the films deposited are polycrystalline with (002) plane as the preferential orientation. According to Scherrer's equation, grain size values on the films are found between 30 - 45 nm with spherical shape. Optical transmittance was about 92% in visible range for the optimum film and shows that the band gap decreases from 3 .29 to 3.17 e V.

Photoluminescence, FTIR and X-ray diffraction studies on undoped and Al-doped ZnO thin films grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis

Undoped and aluminum-doped zinc oxide (ZnO) thin films have been grown on polycrystalline α-alumina substrates by ultrasonic spray pyrolysis (USP) technique using zinc acetate dihydrate and aluminum chloride hexahydrate (Al source) dissolved in methanol, ethanol and deionized water. A number of techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, and photoluminescence (PL) were used to characterize the obtained ZnO thin films. It was seen that the orientation changed with increase in substrate temperature. During the ZnO deposition Zn source reacted with polycrystalline α-Al 2O 3 substrate to form an intermediate ZnAl 2O 4 spinel layer. It has been interestingly found that the intensity of green emission at 2.48 eV remarkably increased when the obtained ZnO:Al films were deposited at 380 °C. The FTIR absorbance intensity of spectroscopic band at 447±6 cm −1 is very sensitive to oxygen sublattice disorder resulting from non-stoichiometry, which is consistent with the result of PL characterization.

Transparent and conductive undoped zinc oxide thin films grown by atomic layer deposition

Atomic layer deposition (ALD) was used to fabricate transparent and conductive thin films of ZnO. Two hundred-nano metre thick ZnO films were deposited on glass substrates at low growth temperatures varied between 120 and 240 °C. As zinc and oxygen precursors we used diethylzinc (DEZn) and deionized water, respectively. To find optimal film parameters, the structure, surface morphology, optical and electrical measurements were carried on. The films obtained at 200 °C show the highest carrier concentration (∼1020 cm−3) and the lowest resistivity (2 × 10−3 Ω cm). The films exhibit mobilities up to 37 cm2/Vs that we associate to the process technology used. An important point of our approach was that the films studied were not intentionally doped (with Al or other group III elements) but the high electrical conductivity was achieved by playing with the sample stoichiometry and growth conditions.

Structural and Optical Properties of Cobalt-Doped Zinc Oxide Thin Films Prepared By Spray Pyrolysis Technique

Undoped and Co-doped zinc oxide (CZO) thin films have been prepared by spray pyrolysis technique using solution of zinc acetate and cobalt chloride. The effect of Co dopants on structural and optical properties has been investigated. The films were found to exhibit maximum transmittance (~90%) and low absorbance. The structural properties of the deposited films were examined by x-ray diffraction (XRD). These films, deposited on glass substrates at (400? C), have a polycrystalline texture with a wurtzite hexagonal structure, and the grain size was decreased with increasing Co concentration, and no change was observed in lattice constants while the optical band gap decreased from (3.18-3.02) eV for direct allowed transition. Other parameters such as Texture Coefficient (Tc), dislocation density (?) and number of crystals (M) were also calculated .

Structural and Optical Properties of Cobalt-Doped Zinc Oxide Thin Films Prepared By Spray Pyrolysis Technique

Undoped and Co-doped zinc oxide (CZO) thin films have been prepared by spray pyrolysis technique using solution of zinc acetate and cobalt chloride. The effect of Co dopants on structural and optical properties has been investigated. The films were found to exhibit maximum transmittance (~90%) and low absorbance. The structural properties of the deposited films were examined by x-ray diffraction (XRD). These films, deposited on glass substrates at (400? C), have a polycrystalline texture with a wurtzite hexagonal structure, and the grain size was decreased with increasing Co concentration, and no change was observed in lattice constants while the optical band gap decreased from (3.18-3.02) eV for direct allowed transition. Other parameters such as Texture Coefficient (Tc), dislocation density (?) and number of crystals (M) were also calculated .

Sol–gel synthesis, comparative characterisation, and reliability analyses of undoped and Al-doped zinc oxide thin films

Thin films of sol–gel derived undoped and Al-doped zinc oxide material systems were fabricated for potential use in transparent conducting oxide modules. A comparative characterisation of the functional properties of the films was performed. Undoped zinc oxide thin films were obtained with desired properties. Al doping reduced the average crystallite size, and led to a denser and less porous morphology, and also caused an increase in transparency in the UV region. An improvement in electrical conductivity was achieved upon Al doping. Temperature cycling and accelerated delamination test results demonstrated the high physical reliability of the thin films of the material system obtained in this study.

Annealing effect on properties of transparent and conducting ZnO thin films

This work presents the effect of postdeposition annealing on the structural, electrical and optical properties of undoped ZnO (zinc oxide) thin films, prepared by radio-frequency sputtering method. Two samples, 0.17 and 0.32 µm-thick, were annealed in vacuum from room temperature to 350 °C while another 0.32 µm-thick sample was annealed in air at 300 °C for 1 h. X-ray diffraction analysis revealed that all the films had a c-axis orientation of the wurtzite structure normal to the substrate. Electrical measurements showed that the resistivity of samples annealed in vacuum decreased gradually with the increase of annealing temperature. For the 0.32 µm-thick sample, the gradual decrease of the resistivity was essentially due to a gradual increase in the mobility. On the other hand, the resistivity of the sample annealed in air increased strongly. The average transmission within the visible wavelength region for all films was higher than 80%. The band gap of samples annealed in vacuum increased whereas the band gap of the one annealed in air decreased. The main changes observed in all samples of this study were explained in terms of the effect of oxygen chemisorption and microstructural properties.

Morphology and optical properties of zinc oxide thin films grown on Si (100) by metal-organic chemical vapor deposition

Undoped zinc oxide (ZnO) thin films were deposited on Si (100) substrates by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD). After a nucleation layer of ZnO about 500 Awas depos- ited, top ZnO films with 320-440 nm in thickness were fabricated at temperatures varying from 450 to 600 C, respectively. X-ray diffraction analyses demonstrate that all the films grown have polycrystalline wurtzite structure. Scanning electron microscopy observations indicate that ZnO films grow with c-axis aligned grains when the deposition temperature is lower than 600 C. However, 3D microstructures without c-axis alignment were found when the film was deposited at 600 C. Room temperature photoluminescence (PL) spectra peaking at 3.27-3.29 eV were observed for the ZnO films grown at 450-600 C. However, only the spectrum from the film grown at 600 C could present a high intensity ratio of the ultraviolet band to deep-level emissions, which is above 392 and show a minimum full-width at half-maximum of 99 meV for the ultraviolet band. Temperature-dependent PL measurements from 20 to 300 K exhibited the luminescent mechanism of the present ZnO films to be near band excitonic emission.

The superior performance of the electrochemically grown ZnO thin films as methane sensor

Pd–Ag/ZnO/Zn and Rh/ZnO/Zn MIM (metal–insulator–metal) gas sensors were fabricated using nanoporous ZnO thin films, obtained by an electrochemical deposition method in the absence and presence of UV light. A high-purity Zn anode, a Pt cathode, a calomel reference electrode and a 0.3-M oxalic acid electrolyte were used for deposition. Pd–Ag (26%) and Rh were used separately as the catalytic metal electrodes to fabricate the two different types of MIM configurations. A gas response of the order of 3.85 ± 2, a response time of 5 ± 0.5 s and a recovery time of 16 ± 0.5 s were obtained with the Pd–Ag contact, while the Rh contact showed a response of the order of 4.82 ± 2, a response time of 24 ± 0.5 s and a recovery time of 72 ± 0.5 s, at the optimum temperature of 220 °C, which is the lowest temperature so far reported for metal oxide sensors to sense 1% methane in a N2 carrier gas. The undoped zinc oxide thin films grown by UV-assisted electrochemical anodization of high-purity Zn demonstrated a better performance for methane sensing. The experiments were repeated in synthetic air and a somewhat reduced performance was observed. The selectivity in the presence of hydrogen and the stability of the sensors were studied.

Al-doped zinc oxide thin films for liquid petroleum gas (LPG) sensors

Undoped and Al-doped zinc oxide thin films were prepared by chemical spray pyrolysis technique using Zn(CH3COO)2 as a precursor solution without or with AlCl3 as a doping solution, respectively. The dopant concentration (at% Al to Zn) was varied from 0 to 1.5 at%. Grazing incidence X-ray diffraction (GIXRD) analyses of the films confirm that all the films are of polycrystalline zinc oxide in nature, possessing hexagonal wurtzite structure. The films show the change in preferential orientation depending on the doping concentration of Al. The crystallite size in the 0.5 at% Al-doped film is found to be minimum (∼103 nm) among all the films. It is observed that the resistivity of the Al-doped films decreases with the Al dopant concentration up to 1 at%. At a higher doping concentration of 1.5 at%, the disorder produced in the lattice causes an increase in the resistivity of the film. It is found that compared to the undoped ZnO film, Al-doped films show high response to LPG. Among all the doped films studied, the 0.5 at% Al-doped ZnO film shows the maximum response (∼89%) to 1 vol% of LPG in air at 325 °C. Further, the response and recovery times of the films to LPG become shorter at higher operating temperatures. A possible mechanism of LPG sensing has been explained.

Comparison of the optical properties of undoped and Ga-doped ZnO thin films deposited using RF magnetron sputtering at room temperature

The optical properties of undoped zinc oxide (ZnO) thin films of various thicknesses were compared with those of Ga-doped (GZO) thin films. Transparent, high-quality undoped ZnO and GZO films were deposited successfully using radio-frequency (RF) sputtering at room temperature. The films were polycrystalline with a hexagonal structure and a strongly preferred orientation along the c-axis. The films had an average optical transmission >85% in the visible part of the electromagnetic spectrum. The undoped ZnO thin films were more transparent than the GZO thin films. In the photoluminescence (PL) spectrum, ZnO film has higher quality than GZO as a result of decrease in the green emission intensity.

Structural and optical properties of sputtered ZnO thin films

Zinc oxide (ZnO) and aluminium-doped zinc oxide (ZnO:Al) thin films were prepared by RF diode sputtering at varying deposition conditions. The effects of negative bias voltage and RF power on structural and optical properties were investigated. X-ray diffraction measurements (XRD) confirmed that both un-doped and Al-doped ZnO films are polycrystalline and have hexagonal wurtzite structure. The preferential 〈0 0 1〉 orientation and surface roughness evaluated by AFM measurements showed dependence on applied bias voltage and RF power. The sputtered ZnO and ZnO:Al films had high optical transmittance (>90%) in the wavelength range of 400–800 nm, which was not influenced by bias voltage and RF power. ZnO:Al were conductive and highly transparent. Optical band gap of un-doped and Al-doped ZnO thin films depended on negative bias and RF power and in both cases showed tendency to narrowing.

Peculiarities of nitrogen dioxide detection with sprayed undoped and indium-doped zinc oxide thin films

Nitrogen dioxide (NO2) sensors based on sprayed zinc oxide (ZnO) thin films have been prepared. The effect of the film thickness and the In-doping on the sensor performance (sensor response and resistance) is analyzed. By adding 3 wt.% of indium nitrate to the spraying solution it is possible to enhance the film–gas response to 5 ppm of NO2 at 275 °C. At the same time the film resistance is sensibly reduced. The film crystallographic structure, morphology and additive content are studied by means of X-ray Diffraction, Scanning Electron Microscopy, X-ray Photoelectron Spectroscopy and Atomic Force Microscopy. The possible sensitization mechanism is discussed.

Effect of thermal annealing on ZnO:Al thin films grown by spray pyrolysis

We report the effect of thermal annealing in air on the structural and optical properties of undoped and aluminium-doped (1%–4%) zinc oxide (AZO) thin films , grown by the spray pyrolysis technique on quartz substrates. Films were characterized by X-ray diffraction, low-temperature photoluminescence , electrical resistivity , and Raman spectroscopy after annealing at temperatures between 500 and 900 ∘ C. Annealing in air improves the long-range order crystalline quality of the bulk crystals, but promotes a number of point defects in the surface affecting both the resistivity and the photoluminescence.