Polycrystalline ZnO Research Articles

Properties of Mg Doped ZnO Films Prepared by Ultrasonic Spray Pyrolysis

This work is focused on the preparation of Mg doped ZnO films by ultrasonic spray pyrolysis and investigated the physical, optical and electrical properties of the films. These films were prepared from 0.02 M of Zn (CH3OO)2.2H2O and MgCl2 was used for doping. The atomic percentage ratio of Mg/Zn was varied from 0 to 9 at.%. The crystalline phase of all films was identified as ZnO polycrystalline structure with preferred orientation along the (002) plane. While, the grain shape of ZnO film was irregular shape, this shape changed to spherical shape with doped Mg concentration and changed again to rod shape with doped Mg concentration to 9 at.%. Although, the Mg dopant affects the shape of ZnO films but don’t affect resistivity of the films. Moreover, the Mg dopant shifts the absorption edge in transmittance spectra of films to lower wavelength and increases band gap of the films.

Low temperature photoluminescence from disordered granular ZnO

In an effort to shed light on the widely discussed issue of defects and photoluminescence (PL) from ZnO material, we have undertaken this in-depth study using low temperature PL and room temperature Raman spectroscopy. Initial defective states of polycrystalline ZnO have been varied by annealing at 500 and 800°C. Further, point defects have been incorporated in the systems by 700keV oxygen ions. 10K PL spectra of all the unirradiated and irradiated samples show dominant donor bound exciton at 3.365eV. Based on the earlier reports, this shallow donor has been assigned as interstitial zinc (IZn) in ZnO. It is found that the PL intensity variation of 3.365eV PL with irradiation is different for 500 and 800°C annealed samples. This donor has been found to be responsible for the red PL (~1.9eV) at cryogenic temperatures (only for the sample irradiated at moderate fluence, 3×1014ions/cm2). Specific defect configuration involving IZn and a zinc vacancy, both stabilized by oxygen vacancies, have been proposed as the origin of ~1.9eV red PL. 500°C annealed sample becomes brown–black after irradiation at highest fluence (3×1016ions/cm2) showing lower resistance compared to the pristine one. The Raman spectrum of this particular sample shows loss of long range crystalline order although IZn related PL emission (at 10K) is highest here. In the light of combined experimental outcome and understanding, an illustration on ion beam irradiation effects in granular ZnO has been presented at the end.

Growth of ZnO nanostructures by femtosecond laser irradiation of polycrystalline targets

The formation of LIPSS upon irradiation with ultrashort laser pulses on the surface of polycrystalline ZnO samples and the potential use of irradiated areas as growth patterns for the production of highly ordered nanostructures upon redeposition have been studied. For this purpose, we have performed different sets of irradiation experiments including static irradiation experiments at low and high repetition rates, as well as scanned beam experiments at high repetition rate, this later in order to generate relatively large template regions for nanostructure growth by redeposition. In all cases, LIPSS formation has been achieved in the ZnO polycrystalline surface. Under appropriate irradiation conditions, the material is redeposited rendering a high density of nanostructures with high aspect ratios and good crystal quality. Given the special luminescent properties and applications of ZnO, particular attention has been paid to the luminescence properties after irradiation and after post-irradiation thermal treatments. The observed evolution has been correlated with evolution of point defects in the treated surfaces. Thermal treatments cause significant changes in both the topography and the cathodoluminescent emission, such as the development of laminar structures, the emergence of nucleation centers and the recovery of ultraviolet emission previously quenched as a consequence of irradiation. Interestingly, LIPSS remain after the luminescent recovery by thermal annealing, opening the possibility to control both luminescence properties and grain size while maintaining an ordered structure with a high effective surface area.

Synthesis of ZnO nanofibers by the electrospinning process

Abstact ZnO nanofibers were fabricated by electrospinning of a solution containing ZnO precursor, polyvinylpyrrolidone and solvent. The polymeric component of nanofibers were completely decomposed, thanks to the calcinations process (from 350 to 650 °C), so to obtain polycrystalline ZnO nanofibers. The crystallinity of ZnO nanofibers improved with higher annealing temperature. The mean diameter of the ZnO nanofibers after an annealing at 550 °C resulted ∼50 nm. Furthermore, we investigated the zinc oxide nanofibers doubling the amount of zinc acetate in the starting solution. The as-spun and annealed ZnO/polyvinylpyrrolidone composite fibers were morphologically and structurally characterized by scanning electron microscopy, X-ray diffraction analyses, and UV/vis optical characterization. The possible application of the synthesized nanofibers as photocatalytic material for water treatment was checked, too. These results are reported and discussed.

Filling the Gaps between Graphene Oxide: A General Strategy toward Nanolayered Oxides

Despite extraordinary developments in the research of 2D inorganic nanomaterials, a scalable and generalized synthetic method toward 2D oxide materials that lack layered lattice structures is still challenging. Herein, an easy and versatile solution‐based route to synthesize oxides with layered nanostructures by combining sol–gel method with graphene oxide (GO) paper templates is reported. GO can stack together to form a paper‐like membrane, the gap between two GO layers provides ideal 2D space to template the growth of oxide nanolayers. By this simple strategy, the gaps are filled successfully with polycrystalline TiO2, ZnO, Fe2O3, and amorphous SiO2 nanolayers with thickness of 1–5 nm. Single or multilayers of the oxide‐based ceramic/glass nanolayers for applications in electronics, catalysts, energy storage, and gas separation can be expected; as an example, it is shown that layered Fe2O3 electrodes exhibit high performance for lithium‐ion battery due to enhanced electrical connections between the 2D nanolayers.

Unexpected ferromagnetism in n-type polycrystalline K-doped ZnO films prepared by RF-magnetron sputtering

The room temperature ferromagnetism in highly c-axis orientated K-doped ZnO polycrystalline films prepared by RF-magnetron sputtering were systematically investigated in this work. The decrease of n-type carriers and the enhanced acceptor-related photoluminescence with the K doping imply the formation of K substitution and Zn vacancy defects. It can be speculated that the room-temperature ferromagnetism observed in K-doped ZnO polycrystalline films is related with the hole. Since the saturation magnetization was positively dependent on the amount of hole defects based on the PL spectra analysis. The ferromagnetism coupling and the room temperature ferromagnetism obtained in K-doped ZnO polycrystalline films thus is originated from p–p coupling of the surrounding O atoms of the K substitution and Zn vacancy defects. The co-existing of both the n-type conductivity and the ferromagnetism induced by localized holes, which is much different from the previous report for epitaxial samples, give new insight into the behavior of d0 ferromagnetism in oxides.

Effect of Sn doping on properties of transparent ZnO thin films prepared by thermal evaporation technique

Sn doping is used as an effective method to increase conductivity and photosensitivity of highly transparent nanostructured ZnO thin films, prepared by multisource vacuum evaporation followed by air annealing. The microstructural characterizations reveal formation of polycrystalline ZnO and ZnO:Sn films, with grain size ∼16–20nm and preferred orientation along (002) plane. Increased electrical conductivity by a factor ∼102 on doping, coupled with the enhancement of transmittance (80–90% in visible range) and photoconductivity lends these wide band gap films (∼3.21eV–3.24eV) application in photovoltaics. Fast response to ethanol (5–7s) indicates suitability of these films in gas sensors.

Role of growth temperature on the structural, optical and electrical properties of ZnO thin films

Present study focuses on the influence of growth temperature on electrical and optical properties of polycrystalline ZnO thin films on sapphire substrates grown by atomic layer deposition technique. Dimethylzinc and de-ionized water were used as precursors by varying the growth temperature ranging from 70 to 130 °C. X-ray diffraction pattern confirms the formation of hexagonal wurtzite phase of ZnO with preferred (103) orientation. AFM results infer the growth of good quality films with root mean square roughness ∼1 nm for all these films having same thickness. The free carrier concentration and mobility of samples is found to increase whereas resistivity decreases with increasing growth temperature. The electrical results are well corroborated on the basis of photoluminescence and X-ray photoelectron spectroscopy. Photoluminescence and Raman measurements show that these ZnO thin films have very low defect concentrations. In particular, this study demonstrates that a device quality ZnO material with improved electrical parameters is obtained at the level of 100 °C which can be used for applications such as Schottky diodes for cross-bar memory applications and in hybrid organic/semiconductor junction devices.

Zinc Oxide Wire-Like Thin Films as Nitrogen Monoxide Gas Sensor

We present an excellent detection for nitrogen monoxide (NO) gas using polycrystalline ZnO wire-like films synthesized via a simple method combined with sputtering of Zn metallic films and subsequent thermal oxidation of the sputtered Zn nanowire films in dry air. Structural and morphological characterization revealed that it would be possible to synthesize polycrystalline hexagonal wurtzite ZnO films of a wire-like nanostructure with widths of 100-150 nm and lengths of several microns by controlling the sputtering conditions. It was found from the gas sensing measurements that the ZnO wire-like thin film gas sensor showed a significantly high response, with a maximum value of 29.2 for 2 ppm NO at , as well as a reversible fast response to NO with a very low detection limit of 50 ppb. In addition, the ZnO wire-like thin film gas sensor also displayed an NO-selective sensing response for NO, , , , and CO gases. Our results illustrate that polycrystalline ZnO wire-like thin films are potential sensing materials for the fabrication of NO-sensitive high-performance gas sensors.

Radiation-Hard ZnO Thin Film Transistors

We report effects for up to 100 Mrad (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) gamma-ray exposure on polycrystalline ZnO thin film transistors (TFTs) deposited by two different techniques. The radiation related TFT changes, either with or without electrical bias during irradiation, are primarily a negative VON shift and a smaller VT shift (ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> ~ - 2.5 V and ΔV <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ~ - 1.5 V for 100 Mrad (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) exposure). Field-effect mobility remains nearly unchanged. Both, VON and VT shifts are nearly completely removed by annealing at 200°C for 1 minute and some recovery is seen even at room temperature. We find that our ZnO TFTs are insensitive to electrical bias during irradiation; that is, unbiased measurements are useful worst case test results. To the best of our knowledge, these are the most radiation-hard thin film transistors reported to date.

Fabrication and Characterization of ZnO Thin Films by Sol-Gel Spin Coating Method for pH Measurement

Transparent semiconductor ZnO thin films deposited on interdigitated electrode (IDE) substrate substrates were obtained by low-cost sol-gel method. The coated ZnO films were annealed in furnace at 500°C for 2 hours. The influence of surface morphologies, crystallization and optical properties was investigated. The structural properties of the annealed ZnO thin films were examined with FESEM and AFM. XRD result shows that all polycrystalline ZnO thin film after annealing have the orientation along the (002) plane. Both FESEM and XRD results revealed that ZnO thin films were composed of hexagonal ZnO crystals in nanoscale dimensions. Moreover, UV-Vis was employed to study the optical properties of the ZnO films. Besides that, the deposited ZnO thin film will further use for pH by I-V curve tracer.

PREPARATION AND CHARACTERIZATION OF TRANSPARENT CONDUCTIVE ZnO THIN FILMS BY DC MAGNETRON SPUTTERING

PREPARATION AND CHARACTERIZATION OF TRANSPARENT CONDUCTIVE ZnO THIN FILMS BY DC MAGNETRON SPUTTERING. Transparent and conductive aluminium-doped zinc oxide thin films have been prepared by dc magnetron sputtering using targets composed of ZnO and Al2O3. Polycrystalline ZnO:Al films were deposited onto a heated glass substrate. The surface morphology and crystalline structure, as well as optical and electrical properties of the deposited films were found to depend directly on substrate temperature. From optical and electrical analysis were observed that the optical transmittance and conductivity of the ZnO:Al transparent conductive oxide films increased when deposition temperature was raised from 200 to 400 oC. Films grown on 300 oC substrates showed a high conductivity value of 0.2 x 102 -1cm-1 and a visible transmission of about 85%. The growth temperatures of 300 oC, aluminium doping levels of 0.9 wt.% were preferable to achieve ZnO:Al films with optical and structural qualities as required for solar cell applications.

AC-magnetic susceptibility of Dy doped ZnO compounds

Dy doped ZnO polycrystalline diluted magnetic semiconductor compounds have been prepared by the so called solid state reaction method. We have studied the M–H and AC magnetic properties of the compounds by using a PPMS magnetometer, and explored the phases and crystal structure by using a X-ray powder diffractometer. The XRD spectra of the compounds show that the substitution of Dy3+ for Zn2+ causes almost no change in the hexagonal wurtzite structure of ZnO, and the Dy3+ ions are successfully substituted into the Zn2+ site of the ZnO matrix. The magnetic measurements, M–H and χ–T, for T in the range from 10 to 300K, show a paramagnetic behavior, including indirect antiferromagnetic couplings between some Dy3+ magnetic moments. Since the Curie–Weiss temperatures, θ, are all negative but decrease in magnitude with increasing Dy concentration. On the other hand, the calculated effective magnetic moments, µeff, per Dy3+ ion slowly increase with increasing Dy concentration, but are all very close to the free ion value of µeff, ~11.0µB. Therefore, the trends of the magnitudes of θs and µeff s indicate that the samples are not only paramagnetic but also have antiferromagnetic couplings due to the complex nature of the compounds. In addition, the thermal variation of average magnetic moment, Peff(T), per Dy3+ ion have been calculated, and have been found to be gradually increasing with increasing temperature and Dy concentration.

A comparison between different ohmic contacts for ZnO thin films

There are several metals that form ohmic contacts for ZnO thin films, such as copper, aluminum and silver. The aim of this work is to make a comparison between these ohmic contacts. To achieve this purpose, polycrystalline ZnO thin films were prepared by the spray pyrolysis technique, and characterized by the I–V measurements at room temperature. Two strips of each metal were thermally evaporated on the surface of the film and measurements were first recorded in the dark and room light, then in the dark before and after annealing for Al, which was found to be the best in the set. Films with aluminum contacts gave the smallest resistivity, best ohmicity and they are slightly affected by light as required. On the other hand, copper was found to be the worst, and films with copper contacts gave the largest resistivity, worst ohmicity and they are the most affected by light. Annealing improved the aluminum contacts due to alloying and doping.

Near-infrared optical constants and optical polarization properties of ZnO thin films

The optical characteristics of ZnO thin films deposited on Si (111) and quartz at temperature <50°C using reactive RF sputtering deposition were examined within the near-infrared (NIR) region. From the X-ray diffraction observations, it was found that the films show a polycrystalline structure. X-ray photoelectron spectroscopy analysis shows the chemical bonding states of zinc and oxygen in the surface of the films and confirms the formation of Zn–O bonds. The field emission scanning electron microscopy and the atomic force microscopy images of the films shown an almost uniform distribution and the surfaces are smooth with grains of the order of 40–60nm. The average surface roughness was in the range of 3.15–24.33nm. Over a wavelength range 700–2500nm, the optical constants of the films were obtained by analysis of the measured ellipsometric spectra using the Cauchy–Urbach model. Refractive indices and extinction coefficients of the ZnO films were determined to be in the range n=1.65–1.73 and κ=0.0060–0.029, respectively. Also, the s- and p-polarized optical properties (transmittance and reflectance) of the films have been measured at different angles of incidence in the UV–Vis–NIR spectral range. In addition, calculations have been carried out in order to determine the optical absorptance by using the conservation of energy. The films reveal a high transmittance (85%–95%), low reflectance (5%–15%), and very low absorptance (<1%) at normal incidence. This means that the sputtered polycrystalline ZnO films could be good candidates for antireflection (AR) optical coatings.

Properties of spray pyrolised ZnO:Sn thin films and their antibacterial activity

Tin doped zinc oxide (ZnO:Sn) thin films were deposited onto glass substrates by the spray pyrolysis technique with the substrate temperature 400°C. The structural, optical, photoluminescence (PL) properties and morphological studies were investigated for the films deposited with various doping concentration (0, 2, 4, 6 and 8at.%) of tin. The XRD results had shown that the films were polycrystalline ZnO with hexagonal wurtzite type structure and the crystallites in the films were oriented along (002) direction. Surface morphology of the films obtained by scanning electron microscope (SEM) exhibited the change in morphology with doping concentration and porous nature for the film with 6at.% of tin. Atomic force microscopy (AFM) revealed nanometer sized particles with decreased surface roughness for Sn doping. Optical analysis exhibited the band gap value of 2.8eV for ZnO:Sn (6at.%) which was lower than the band gap value for undoped ZnO film (3.2eV). The resistivity of 6at.% of Sn doped film was 1.28×102Ωcm with increase in the hall mobility and carrier concentration. The ZnO and Sn doped ZnO thin films exhibited antibacterial activity against Staphylococcus aureus.

ZnO thin films prepared at low annealing temperatures, from a novel, simple sol–gel precursor solution

ZnO thin films show a wide gamut of potential applications, mainly due to their optical and semiconducting properties. ZnO films have been prepared by the sol–gel technique; however, their preparation is rather laborious and the composition of the precursor solutions is relatively complex. In this work, we present the preparation of ZnO thin films by a novel sol–gel method, from a very simple precursor solution, based on zinc acetate and methyl alcohol. Transparent, well-adhered films were obtained on glass slide substrates, in the 100–500 °C annealing temperature range. A very noteworthy feature of the method is that polycrystalline ZnO thin films could be obtained at annealing temperatures as low as 100 °C. The films are transparent (80–90 %) in the visible and near-IR spectral ranges. They were characterized also by micro-Raman spectroscopy and SEM and their thickness was evaluated by profilometer measurements.

Carrier mobility of highly transparent conductive Al-doped ZnO polycrystalline films deposited by radio-frequency, direct-current, and radio-frequency-superimposed direct-current magnetron sputtering: Grain boundary effect and scattering in the grain bulk

The effects of using radio-frequency (RF)-superimposed direct-current (DC) magnetron sputtering deposition on the structural, electrical, and optical properties of aluminum-doped ZnO (AZO)-based highly transparent conducting oxide films have been examined. AZO films were deposited on heated non-alkaline glass substrates (200 °C) using ZnO:Al2O3 (2 wt. % Al2O3) ceramic oxide targets with the total power varied from 150 to 300 W, and at various RF to DC power ratios, AZO films deposited by a mixed approach with the RF to the total power ratio of 0.14 showed the lowest resistivity of 2.47 × 10−4 Ω cm with the highest carrier concentration of 6.88 × 1020 cm−3 and the highest Hall mobility (μH) of 36.8 cm2/Vs together with the maximum value of an average transmittance in the visible spectral range from 400 to 700 nm. From the analysis of optical data based on the simple Drude model combined with the Tauc-Lorentz model and the results of Hall effect measurements, the optical mobility (μopt) was determined. A comparison of μopt with μH clarified the effects of the mixed approach not only on the reduction of the grain boundary contribution to the carrier transport but also on retaining high carrier mobility of in-grains for the AZO films.

Fabrication and opto-electrical properties of amorphous (Zn,B)O thin film by pulsed laser deposition

Growth of amorphous ZnO by B doping and their opto-electrical properties are reported. The B-doped ZnO (ZnO:B) films were grown by pulsed laser deposition using polycrystalline ZnO:B ceramic targets. Although the solubility limit of B in bulk ZnO polycrystal was ³4% ,1 8%-doped ZnO:B showed the shrinkage in the c-axis length. Preferentially (002)-oriented polycrystalline ZnO:B films were grown for the B concentration [B] ¯ 18%; while, amorphous ZnO:B films were obtained for [B] ³26%. It was found that the density of the amorphous ZnO:B film was smaller by 9% than that of crystalline ZnO (5.61g·cm ¹3 ), which is explained mainly by the incorporation of the light B atoms. The optical bandgap of the ZnO:B films increased with [B] and that of the amorphous ZnO:B film was ³3.38eV. The amorphous ZnO:B films have low free electron density of ³10 15 cm ¹3 , suggesting the existence of electron traps. Hall mobility of the amorphous ZnO:B [³1cm 2 (V·s) ¹1 ] was smaller than those of the poly

Synthesis and Sensing Characterization of ZnO Nanofibers Prepared by Electrospinning

In the present work nanofibers has been prepared by electrospinning method. Zinc Oxide nanofibrous thin film were electrospun on ITO substrate from a solution containing Poly Ethylene Oxide (PEO) and Zn(CH3COO)2.2H2O. Calcination process of ZnO/PEO composite at 600oC forms a random network of poly crystalline ZnO nanofiber. Surface morphology of the electrospun ZnO thin film, studied using FE-SEM shows a uniform, continous parallel aligned sharp nanofibers. The crystal structure and orientation of ZnO thin film was determined by X-ray diffraction (XRD). Gas sensing characteristics were studied on a custom made chamber based on two probe resistance measurement method. The film shows a good sensitivity towards organic vapours such as methanol, ethanol, acetone and propanol.