Synthesis and characterization of Ni-doped ZnO: A transparent magnetic semiconductor

We report synthesis of a transparent magnetic semiconductor by incorporating Ni in zinc oxide (ZnO) matrix. ZnO and nickel-doped zinc oxide (ZnO:Ni) thin films (∼60 nm) are prepared by fast atom beam (FAB) sputtering. Both undoped and doped films show the presence of ZnO phase only. The Ni concentration (in at%) as determined by energy dispersive X-ray (EDX) technique is ∼12±2%. Magnetisation measurement using a SQUID magnetometer shows that the Ni-doped films are ferromagnetic, having coercivity (H_c) values 192, 310 and 100 Oe and saturation magnetization (M_s) values of 6.22, 5.32 and 4.73 emu/g at 5, 15 and 300 K, respectively. The Ni-doped film is transparent (>80%) across visible wavelength range. Resistivity of the ZnO:Ni film is ∼2.5×10⁻³ Ω cm, which is almost two orders of magnitude lower than the resistivity (∼4.5×10⁻¹ Ω cm) of its undoped counterpart. Impurity d-band splitting is considered to be the cause of increase in conductivity. Interaction between free charges generated by doping and localized d spins of Ni is discussed as the reason for ferromagnetism in the ZnO:Ni film.     

Effect of different dopant elements (Al,Mg and Ni) on microstructural,optical and electrochemical properties of ZnO thin films deposited by spray pyrolysis (SP)

In the present work we studied the influence of the dopant elements and concentration on the microstructural and electrochemical properties of ZnO thin films deposited by spray pyrolysis. Transparent conductive thin films of zinc oxide (ZnO) were prepared by the spray pyrolysis process using an aqueous solution of zinc acetate dehydrate [Zn(CH₃COO)₂·2H₂O] on soda glass substrate heated at 400 ± 5 °C. AlCl₃, MgCl₂ and NiCl₂ were used as dopant. The effect of doping percentage (2–4%) has been investigated. Afterwards the samples were thermally annealed in an ambient air during one hour at 500 °C. X-ray diffraction showed that films have a wurtzite structure with a preferential orientation along the (0 0 2) direction for doped ZnO. The lattice parameters a and c are estimated to be 3.24 and 5.20 ?, respectively. Transmission allowed to estimate the band gaps of ZnO layers. The electrochemical studies revealed that the corrosion resistance of the films depended on the concentration of dopants.     

Growth and characteristics of ZnO nano-aggregates electrodeposited onto p-Si(1 1 1)

In this paper we study nanocrystalline zinc oxide thin films produced by oxidation of electrodeposited zinc nanolayers on a monocrystalline p-Si(1 1 1) substrate.The electrolyte used is ZnCl₂, an aqueous solution of 4 × 10⁻² mol/l concentration. Several deposits were made for various current densities, ranging from 13 mA/cm² to 44 mA/cm², flowing through the solution at room temperature. A parametric study enabled us to assess the effect of the current density on nucleation potential and time as well as zinc films structure. The grazing incidence X-ray diffraction (GIXD) revealed that both Zn and ZnO films are polycrystalline and nanometric. After 1-h oxidation of zinc films at 450 °C in the open air, the structural analyses showed that the obtained ZnO films remained polycrystalline with an average crystal size of about 47 nm and with (1 0 0), (0 0 2) and (1 0 1) as preferential crystallographic orientations.     

Optical properties of ZnO and ZnO:Ce layers grown by spray pyrolysis

In this paper, zinc oxide (ZnO) and cerium-doped zinc oxide (ZnO:Ce) films were deposited by reactive chemical pulverization spray pyrolysis technique using zinc and cerium chlorides as precursors. The effects of Ce concentration on the structural and optical properties of ZnO thin films were investigated in detail. These films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and photoluminescence (PL) measurements. All deposited ZnO layers at the temperature 450 °C are polycrystalline and indicate highly c-axis oriented structure. The dimension of crystallites depends on incorporation of Ce atoms into the ZnO films. The photoluminescence spectra of the films have been studied as a function of the deposition parameters such as doping concentrations and post grows annealing. Photoluminescence spectra were measured at the temperature range from 13 K to 320 K.     

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.     

Particulate assisted growth of ZnO nanorods and microrods by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on the gallium nitride (GaN) and sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) without using any metal catalyst. The experiment was carried out at three different laser wavelengths of Nd:YAG laser (λ = 1064 nm, λ = 532 nm) and KrF excimer laser (λ = 248 nm). The ZnO films grown at λ = 532 nm revealed the presence of ZnO nanorods and microrods. The diameter of the rods varies from 250 nm to 2 μm and the length varies between 9 and 22 μm. The scanning electron microscopy (SEM) images of the rods revealed the absence of frozen balls at the tip of the ZnO rods. The growth of ZnO rods has been explained by vapor-solid (V-S) mechanism. The origin of growth of ZnO rods has been attributed to the ejection of micrometric and sub-micrometric sized particulates from the ZnO target. The ZnO films grown at λ = 1064 nm and λ = 248 nm do not show the rod like morphology. X-ray photoelectron spectroscopy (XPS) has not shown the presence of any impurity except zinc and oxygen.     

Studies of structural evolution and sensing properties of ZnO nanocrystalline films

ZnO nanocrystalline films have been prepared on Si(1 0 0) substrate using direct current (D.C) magnetron sputtering technique at room temperature. The thickness of nanocrystalline films almost linearly increased with deposition duration and the sizes of crystalline grains almost kept unchanged. After deposition, thermal annealing was performed at 800 °C in atmosphere for 2 h in order to improve the qualities of ZnO thin films. Scanning electron microscope (SEM) images showed the surface roughness of the films less than 45 nm. X-ray diffraction (XRD) patterns revealed the slight evolution of the crystal structures. Raman scattering spectra confirmed the data obtained from X-ray diffraction measurements.With these ZnO nanocrystalline films, prototypic gas sensors were fabricated. Both sensitivity and response of the sensors to different gases (H₂ and CH₄) were investigated. A quick response of time, less than 1 second to CH₄ gas sensor has been achieved.     

Conversion of p-type to n-type conductivity in undoped ZnO films by increasing operating temperature

ZnO thin films with the thickness of about 15 nm on (0 0 0 1) sapphire substrates were prepared by pulsed laser deposition. X-ray photoelectron spectroscopy indicated that both as-grown and post-annealed ZnO thin films were oxygen-rich. H₂ sensing measurements of the films indicated that the conductivity type of both the unannealed and annealed ZnO films converted from p-type to n-type in process of increasing the operating temperature. However, the two films showed different conversion temperatures. The origin of the p-type conductivity in the unannealed and annealed ZnO films should be attributed to oxygen related defects and zinc vacancies related defects, respectively. The conversion of the conductivity type was due to the annealing out of the correlated defects. Moreover, p-type ZnO films can work at lower temperature than n-type ZnO films without obvious sensitivity loss.     

1.54 μm emission mechanism of Er-doped zinc oxide thin films

Zinc oxide (ZnO) and Er-doped zinc oxide (ZnO:Er) thin films were formed by pulsed laser deposition, and characterized by photoluminescence (PL) and X-ray diffraction (XRD) in order to clarify the 1.54 μm emission mechanism in the ZnO:Er films. Er ions were excited indirectly by the 325 nm line of a He-Cd laser, and the comparison of the ultraviolet to infrared PL data of ZnO and ZnO:Er films showed that the 1.54 μm emission of Er³⁺ in ZnO:Er film appears at the expense of the band edge emission and the defect emission of ZnO. The crystallinity of the films was varied with the substrate temperature and post-annealing, and it was found that the intensity of the 1.54 μm emission is strongly related with the crystallinity of the films. There are three processes leading to the 1.54 μm emission; absorption of excitation energy by the ZnO host, energy transfer from ZnO to Er ions, and radiative relaxation inside Er ions, and it is suggested that the crystallinity plays an important role in the first two processes.     

Lactic acid aided electrochemical deposition of c-axis preferred orientation of zinc oxide thin films: Structural and morphological features

Compact and homogeneous c-axis preferred orientation of zinc oxide (ZnO) films on indium tin oxide (ITO) coated glass have been prepared electrochemically at −1.2 V vs. Ag|AgCl in a weak acidic condition from 0.06 M Zn(NO₃)₂ with 3 mM lactic acid (LA) added. LA was found having strong influence on the electrodeposition of c-axis preferred orientation of zinc oxide films. Other experimental variables such as deposition temperature, potential, and precursor concentration were also conducted in this article. Among these variables, it was found that precursor concentration of zinc nitrate influenced significantly on growth direction and crystal diameter of zinc oxide. Cyclic voltammetry was used to observe the electrochemistry of the deposition. Crystallinities of the films were examined by X-ray diffractometer. The morphologies of zinc oxide films were observed with a field emitting scanning electron microscope. Optical characteristics of zinc oxide layers were measured with UV-vis spectrophotometer. The band gap of the deposited zinc oxide thin films was evaluated from the Tauc relationship of (αhν)² vs. hν, which was found to be 3.31 eV.     

Fabrication of Sb-doped p-type ZnO thin films by pulsed laser deposition

p-Type ZnO thin films have been realized via monodoping antimony (Sb) acceptor by using pulsed laser deposition. The obtained films with the best electrical properties show a hole concentration in the order of 10¹⁸ cm⁻³ and resistivity in the range of 2-4 Ω cm. X-ray diffraction measurements revealed that all the films possessed a good crystallinity with (0 0 2)-preferred orientation. Guided by X-ray photoemission spectroscopy analysis and a model for large-sized-mismatched group-V dopant in ZnO, an Sb_Zn-2V_Zn complex is believed to be the most possible acceptor in the Sb-doped p-type ZnO thin films.     

Room temperature transparent conducting oxides based on zinc oxide thin films

Doped zinc oxide thin films are grown on glass substrate at room temperature under oxygen atmosphere, using pulsed laser deposition (PLD). O₂ pressure below 1 Pa leads to conductive films. A careful characterization of the film stoichiometry and microstructure using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) concludes on a decrease in crystallinity with Al and Ga additions (≤3%). The progressive loss of the (0 0 2) orientation is associated with a variation of the c parameter value as a function of the film thickness and substrate nature. ZnO:Al and ZnO:Ga thin films show a high optical transmittance (>80%) with an increase in band gap from 3.27 eV (pure ZnO) to 3.88 eV and 3.61 eV for Al and Ga doping, respectively. Optical carrier concentration, optical mobility and optical resistivity are deduced from simulation of the optical data.     

Self-limiting growth of ZnO films on (0 0 0 1) sapphire substrates by atomic layer deposition at low temperatures using diethyl-zinc and nitrous oxide

Atomic layer deposition (ALD) of zinc oxide (ZnO) films on (0 0 0 1) sapphire substrates was conducted at low temperatures by using diethyl-zinc (DEZn) and nitrous oxide (N₂O) as precursors. It was found that a monolayer-by-monolayer growth regime occurred at 300 °C in a range of DEZn flow rates from 5.7 to 8.7 μmol/min. Furthermore, the temperature self-limiting process window for the ALD-grown ZnO films was also observed ranging from 290 to 310 °C. A deposition mechanism is proposed to explain how saturated growth of ZnO is achieved by using DEZn and N₂O. Transmission spectroscopic studies of the ZnO films prepared in the self-limiting regime show that the transmittances of ZnO films are as high as 80% in visible and near infrared spectra. Experimental results indicate that ZnO films with high optical quality can be achieved by ALD at low temperatures using DEZn and N₂O precursors.     

Effects of seed layer on the structure and property of zinc oxide thin films electrochemically deposited on ITO-coated glass

ZnO films with different morphologies were deposited on the ITO-coated glass substrate from zinc nitrate aqueous solution at 65 °C by a seed-layer assisted electrochemical deposition route. The seed layers were pre-deposited galvanostatically at different current densities (i_sl) ranging from −1.30 to −3.0 mA/cm², and the subsequent ZnO films had been done using the potentiostatic technique at the cathode potential of −1.0 V. Densities of nucleation centers in the seed layers varied with increasing the current density, and the ZnO films on them showed variable morphologies and optical properties. The uniform and compact nanocrystalline ZnO film with (0 0 2) preferential orientation was obtained on seed layer that was deposited under the current density (i_sl) of −1.68 mA/cm², which exhibited good optical performances.     

Morphology transition of ZnO films with DMZn flow rate in MOCVD process

We used a metal-organic chemical vapor deposition (MOCVD) method to grow ZnO films on MgAl₂O₄ (1 1 1) substrate, and succeeded in preparing films with microstructures from well-aligned ZnO nanorods to continuous and dense films by adjusting the ratio of the input rates of oxygen and zinc sources (VI/II). At the growth temperature of 350 °C, the ZnO nanorods were formed under a low flow rate of a zinc precursor. On the other hand, continuous and dense ZnO films were formed under a high flow rate of the zinc precursor. There is a transition zone at medium zinc precursor flow rate, where nanorods transform to dense films. We proved that the height of ZnO nanorods and the thickness of ZnO dense films both increase with zinc flow rate, and are consistent with the mass-transport mechanism for ZnO growth. The XRD spectra of the sample in the transition zone show both (0 0 2) and (1 0 1) peaks, where (1 0 1) peaks are formed only in the transition zone. We consider that there are (0 0 2) and (1 0 1) ZnO grains in the early growth stage of dense ZnO films.     

Hydrogen sensors based on ZnO nanoparticles

Hydrogen sensing characteristics of thick films of nanoparticles (∼35 nm diameter) of ZnO, 3% Co doped ZnO, 1% Pt-impregnated ZnO and 1% Pt-impregnated 3% Co-ZnO have been investigated. The last composition exhibits the highest sensitivity for 10-1000 ppm H₂, reaching values upto 1700 as well as good response and recovery times at 125 °C or lower. The sensor is not affected significantly upto 50% relative humidity.     

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-Al₂O₃ substrate to form an intermediate ZnAl₂O₄ 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.     

Structural and electrical properties of zinc oxides thin films prepared by thermal oxidation

We report on zinc oxide (ZnO) thin films (d = 55-120 nm) prepared by thermal oxidation, at 623 K, of metallic zinc films, using a flash-heating method. Zinc films were deposited in vacuum by quasi-closed volume technique onto unheated glass substrates in two arrangements: horizontal and vertical positions relative to incident vapour. Depending on the preparation conditions, both quasi-amorphous and (0 0 2) textured polycrystalline ZnO films were obtained. The surface morphologies were characterized by atomic force microscopy and scanning electron microscopy. By in situ electrical measurements during two heating-cooling cycles up to a temperature of 673 K, an irreversible decrease of electrical conductivity of as flash-oxidized Zn films was revealed. The influence of deposition arrangement and oxidation conditions on the structural, morphological and electrical properties of the ZnO films is discussed.     

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.     

Effects of the oxygen partial pressure and annealing atmospheres on the microstructures and optical properties of Cu-doped ZnO films

Cu-doped zinc oxide (ZnO:Cu) films were deposited on p-Si (1 0 0) substrates at 200 °C under various oxygen partial pressures by using radio frequency reactive magnetron sputtering. The properties of the films were characterized by the X-ray diffraction spectroscopy (XRD), energy dispersive spectrometer, X-ray photoelectron spectroscopy (XPS) and fluorescence spectrophotometer with the emphasis on the evolution of microstructures, element composition, valence state of Cu, optical properties. The results indicated that the properties of ZnO:Cu films were significantly affected by oxygen partial pressures. XRD measurements revealed that the sample prepared at the ratio of O₂:Ar of 15:10 sccm had the best crystal quality among all ZnO:Cu films. XPS analysis results suggested that the valence of Cu in the ZnO films was a mixed state of +1 and +2, and the integrated intensity ratio of Cu²⁺ to Cu⁺ increased with the increment of oxygen partial pressure. The photoluminescence measurements at room temperature revealed a violet, two blue and a green emission. We considered that the origin of green emission came from various oxygen defects when the ZnO:Cu films grew in oxygen poor and enriched environment. Furthermore, the influence of annealing atmosphere on the microstructures and optical properties of ZnO:Cu films were discussed.