Li-doped NiO Thin Films Research Articles

60 MeV Si ion beam irradiation induced modifications in the structural and optical properties of Li doped NiO thin films

Li doped NiO films grown on FTO substrates through spin coating technique were irradiated using 60 MeV Si ions. The films were investigated via several approaches including grazing incidence X-ray diffraction, attenuated total reflectance-Fourier transform infrared spectroscopy, atomic force microscopy, micro-Raman and UV–visible spectroscopy. The crystalline nature of films declines with the initial face-centered cubic structure for 1x1014 ions cm−2 owing to the energy loss of Si ions in a Li doped NiO (Li:NiO) lattice during the generation of defects. The crystallite size of samples was evaluated using Scherrer’s equation. The vibrational modes were investigated via micro-Raman analysis. AFM topography was recorded for pristine and Si ion irradiated films. The ion fluence of Si ion in the used energy range lowers the transmittance Li:NiO films as compared to the pristine.

Fabrication of Li-Doped NiO Thin Films by Ultrasonic Spray Pyrolysis and Its Application in Light-Emitting Diodes

The fabrication of NiO films by different routes is important to extend and improve their applications as hole-transporting layers in organic and inorganic optoelectronic devices. Here, an automated ultrasonic pyrolysis spray method was used to fabricate NiO and Li-doped NiO thin films using nickel acetylacetonate and lithium acetate dihydrate as metal precursor and dimethylformamide as solvent. The effect of the amount of lithium in the precursor solution on the structural, morphological, optical, and electrical properties were studied. XRD results reveal that all the samples are polycrystalline with cubic structure and crystallite sizes in the range of 21 to 25 nm, without any clear trend with the Li doping level. AFM analysis shows that the crystallites form round-shaped aggregates and all the films have low roughness. The optical transmittance of the films reaches values of 60% to 77% with tendency upward as Li content is increased. The electrical study shows that the films are p-type, with the carrier concentration, resistivity, and carrier mobility depending on the lithium doping. NiO:Li (10%) films were successfully incorporated into inorganic light emitting diodes together with Mn-doped ZnS and ZnO:Al films, all deposited on ITO by the same ultrasonic spray pyrolysis technique.

Lithium doping in spray-pyrolyzed NiO thin films: results on their microstructural, optical and electrochromic properties

The Li-doped NiO thin films were prepared onto the plane glass substrates, and the FTO coated glass substrates for electrochromic characterizations, employing the spray-pyrolysis technique. These films were analyzed by a variety of characterization techniques. The structural characterizations reveal that the as-prepared films are of NiO with a cubic structure along with the (2 2 0) plane as the most preferred orientation of crystallite growth. The topographic characterizations show that the surface roughness changes with the varying lithium content. The bandgap energy values are found in the range of 3.40–3.65 eV. Two emission peaks, one corresponding to NBE and another related to the structural defects, are observed in the room-temperature PL spectra of the films. The coloration efficiency of the films improves upon Li doping and is optimum (34.84 cm2/C at 550 nm) for the 3 at.% Li-doped films. The CA analyses indicate that the coloration phenomenon in the films occurs faster in comparison with the bleaching process. The EIS data have been examined based on an equivalent electrical circuit model using fit and simulation analysis.

Preparation and characterization of p- NiO:Li thin films as schottky photodiode

Nickel oxide was prepared using the chemical method. Li-doped NiO thin films with concentrations (0.04) was deposited on glass substrate using drop casting technique with thickness 150nm, and then the annealing at 300 ° C for 60 sec. The effect of the lithium doping on the structural and optical properties of Li: NiO films were examined. The structure of the prepared films were described by x-ray diffraction (XRD) and Atomic Force Microscopy (AFM). The results of X-ray pattern showed the NiO and Li: NiO films to have a polycrystalline structure of a cubic type and preferred orientation (111). (AFM) data indicated that the structure was nanoscale and surface of the films is highly smooth. The study of optical properties included the transmittance spectrum of the prepared films of the wavelengths of (300-900) nm, It was found that the transmittance of pure nickel oxide is 79% in the visible area after doping by Li The transmittance decreases and the absorption increases. The energy gap was also calculated and found to be decreasing by increasing the doping concentration. Other important optical properties of the absorption coefficient and extinction coefficient were calculated. The electrical properties of the prepared films were studied, It was found that the concentration of the carriers increases with increasing doping concentration and mobility decreases. The thin films were deposited on a substrate of FTO to make of schottky diode. The characteristics of I-V, C-V were studied. A solar cell was found the efficiency of the cell was increased by doping.

Epitaxial growth of undoped and Li-doped NiO thin films on α-Al2O3 substrates by mist chemical vapor deposition

Undoped and Li-doped NiO thin films were grown on α-Al2O3 (0 0 0 1) substrates by mist chemical vapor deposition. Both undoped and Li-doped NiO thin films grew bi-epitaxially on the substrates with crystallographic orientation relationships of NiO(1 1 1)[1¯ 1 0] || α-Al2O3(0 0 0 1)[0 1 1¯ 0] and NiO(1 1 1)[1 1¯ 0] || α-Al2O3(0 0 0 1)[0 1 1¯ 0]. In the Li-doped NiO thin film, a periodic structure was observed, in accordance with a mirror-symmetrical oxygen layer on the terraces of the substrate. Both undoped and Li-doped NiO thin films exhibited high transmittance (>80%) in the visible-light region and optical bandgaps of 3.7–3.8 eV. The undoped NiO thin film showed insulating properties and a resistivity of 106 Ω cm or higher. In contrast, the Li-doped NiO thin films had resistivities of 101–105 Ω cm, depending on the Li precursor concentration. Furthermore, they exhibited positive Seebeck coefficients, indicating their p-type conductivity. These results indicate that Li dopants effectively act as acceptors in NiO thin films.

Improving the optical and electrical properties of NiO/n-Si photodiode by Li dopant

In this study, Li-doped NiO solution was produced using a sol–gel spin coating technique. The manufactured solution was deposited onto the n-Si substrate to obtain a homogeneous thin film that could be used for photodiode production. Next, we measured the morphological, electrical, and optical parameters of the manufactured photodiode. First, the morphological properties of the thin film were studied by atomic force microscopy (AFM). Using AFM analysis software, the roughness and grain size of the thin film were estimated as 8.2–10 and 227–239 nm respectively. The optical transparency and the band gap of the Li-doped NiO thin film were studied. The optical measurements of the thin film were taken using a Shimadzu UV–Vis–NIR 3600 spectrophotometer. The transmittance of the thin film was 83.6%, and the band gap was 3.57 eV. The current–voltage (I–V) characteristic of the photodiode was measured in the dark and under various intensities of illumination. We found that the current of the photodiode changed depending on the intensity of illumination. As the intensity of illumination increased, the current also increased, from 6.3 × 10−7 to 3.36 × 10−4 A. These data indicate that the photodiode is sensitive to illumination intensity and, therefore, could be used as an optical sensor. The barrier height and ideality factor of the photodiode were also determined using the thermionic emission model. The barrier height and ideality factor of the Al/n-Si/LiNiO/Au photodiode were 0.81 eV and 3.7 respectively. Also, the capacitance–voltage (C–V), the interface density (Dit), and the serial resistance of the photodiode were found to change with changing frequency. Taken together, these data show that the Li doping ratio improved the optical and electrical properties of NiO. Based on these findings, we propose that Li-doped NiO could be incorporated into optoelectronic devices, such as photodiodes and photosensors.

Electronic Structure and Band Alignment at the NiO and SrTiO3 p-n Heterojunctions.

Understanding the energetics at the interface, including the alignment of valence and conduction bands, built-in potentials, and ionic and electronic reconstructions, is an important challenge in designing oxide interfaces that have controllable multifunctionalities for novel (opto-)electronic devices. In this work, we report detailed investigations on the heterointerface of wide-band-gap p-type NiO and n-type SrTiO3 (STO). We show that despite a large lattice mismatch (∼7%) and dissimilar crystal structure, high-quality NiO and Li-doped NiO (LNO) thin films can be epitaxially grown on STO(001) substrates through a domain-matching epitaxy mechanism. X-ray photoelectron spectroscopy studies indicate that NiO/STO heterojunctions form a type II "staggered" band alignment. In addition, a large built-in potential of up to 0.97 eV was observed at the interface of LNO and Nb-doped STO (NbSTO). The LNO/NbSTO p-n heterojunctions exhibit not only a large rectification ratio of 2 × 103 but also a large ideality factor of 4.3. The NiO/STO p-n heterojunctions have important implications for applications in photocatalysis and photodetectors as the interface provides favorable energetics for facile separation and transport of photogenerated electrons and holes.

Electrical properties of undoped and Li-doped NiO thin films deposited by RF sputtering without intentional heating

The fundamental transmittance and electrical properties of undoped and Li-doped NiO thin films deposited by conventional RF sputtering without intentional heating were evaluated. Both the transmittance and resistivity of undoped and Li-doped NiO decreased with increasing O2 fraction in the sputtering gas, f(O2) = O2/(Ar + O2). The result is attributed to the increase in the concentration of acceptors of Ni vacancies (VNi) under oxygen-rich growth conditions. In addition to VNi, Li atom on the Ni site (LiNi) likely acts as a shallow accepter, which can explain the experimental finding that the carrier concentration of Li-doped NiO was approximately three orders of magnitude higher than that of the undoped case deposited under the same f(O2). The mobility of NiO was remarkably low (around 0.1–1.0 cm2 V−1 s−1) and almost independent of f(O2) or the amount of doping, reflecting the large hole effective mass.

Effect of annealing conditions on the structural, electrical and optical properties of Li-doped NiO thin films

Transparent conductive oxide (TCO) p-type Li-doped NiO thin films were deposited on the (0001) sapphire substrates by magnetron sputtering technique with a high purity NiO:Li2O ceramic target. We systematically investigated the structural, electrical and optical properties of NiO:Li thin films annealed in different conditions. We found that annealing in different conditions greatly affects the physical properties of NiO:Li thin films. Compared with the NiO:Li thin film annealed in oxygen, the hole concentration of NiO:Li thin film annealed in nitrogen at the same processing temperature is obviously lower. Annealed in oxygen at 500 °C, NiO:Li films show excellent crystal quality with single (111) orientation, high hole concentrations. When the annealing temperature increased, the transmittance of NiO:Li thin films become better for wavelength range from ultraviolet (UV) to visible with a significant absorption edge near 350 nm.

Properties of Li-Doped NiO Thin Films Prepared by RF-Magnetron Sputtering.

Li-doped NiO thin films were deposited on glass and c-axis (0001) sapphire single crystal substrates by radio frequency (RF)-niagnetron sputtering. The effects of the type of substrate, substrate temperature and atmosphere on the structural, electrical and optical properties of the NiO thin films were examined. The electrical conductivity of the NiO thin films depends on the type of substrate, substrate temperature and oxygen atmosphere. The electrical conductivity of the thin films on the glass and sapphire substrates was improved by the introduction of oxygen and decreased with increasing substrate temperature. The optical transmittance decreased with the introduction of oxygen and increased with increasing substrate temperature.

Structure and characteristics of Li-doped NiO thin films prepared by using pulsed-laser deposition process.

10 at% Li doped NiO thin films were prepared on glass and sapphire single crystal substrates by using pulsed-laser deposition (PLD) process. The effects of the substrate and the deposition temperature on the crystal structures, electrical, and optical properties of NiO thin films were studied by using XRD, 4 point probe, and UV-VIS spectrometer. The electrical conductivities and the optical properties of NiO thin films depend on the crystallinity of the thin films and substrate temperatures. The Li doped NiO thin film with electrical conductivity of 2.25 S/cm and optical transmittance of 80% was fabricated on the sapphire substrate when processing parameters were optimized.

Structural, optical and electrical properties of undoped and Li-doped NiO thin films prepared by sol–gel spin coating method

Un-doped and lithium doped nickel oxide thin films have been prepared on glass substrates by the sol–gel spin coating technique using nickel acetate and lithium chloride as source materials. The thickness of the films was increased by increasing the number of layers successively deposited on the same substrate. The effect of the number of layers on the structural, optical and electrical properties of NiO thin films was studied. The results of investigations of the samples show that four is the most suitable number of layers of undoped NiO films with high optical transparency. Then, by using these optimized deposition parameters, lithium doped nickel oxide films are prepared. The surface morphology and crystalline structure of the thin films were investigated by atomic force microscopy and X-ray diffraction (XRD), respectively. A modification in the morphology of the films was observed while increasing the Li amount in the solution. XRD patterns show that the films are polycrystalline. The preferred orientations were evaluated from XRD data. The average transmittance of the NiO films in the visible region increases with the increase of Li concentration. The electrical measurements show that the resistance of the films decreases as the lithium content increases.

High conductivity nickel oxide thin films by a facile sol–gel method

Li-doped NiO thin films (Li:NiO) were deposited on glass substrates using a facile sol–gel method. The effects of Li doping concentration on the structural, electrical and optical properties of Li:NiO films were examined. The structural properties of Li:NiO films were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy. XRD revealed the Li:NiO films to have a polycrystalline bunsenite structure. The surface morphology of the Li:NiO films exhibited nanocrystalline grains with a randomly oriented morphology. The resistivity decreased substantially to 1.33kΩcm at 0.11mol%, which is one order of magnitude lower than that of the undoped NiO film. The mean transmittance decreased from 70% to 60% with increasing Li doping concentration from 0.02mol% to 0.19mol%.

Heavy Doping of Li+-ion into NiO Epitaxial Thin Films via Unequilibrium Room-temperature Processing for New Functionalization

AbstractNiO is a typical material for new p-type oxide semiconductors. Conductivity of NiO can be raised with Li+ doping. In case of Li-heavy doping, we can obtain LixNiO2(0.5< × <1.0). Recently the importance of LiNiO2 has been increased as an electrode material for rechargeable lithium cells.In this work, we tried to fabricate a novel NiO material with Li+-heavily doped by applying the pulsed laser-induced room temperature (R.T.) film process. Previously, we have succeeded in the epitaxial growth of various oxide thin films at R.T. such as Sn-doped In2O3 transparent electrodes [1]. Although the many studies have been made on the deposition of NiO epitaxial thin film at low temperatures [2], there are few reports on fabrication and the conductive characteristic for Li-heavily doped NiO epitaxial films. The film deposition at R.T., which is the unequilibrium vapor phase process, is expected to result in different crystal structure and characteristics from the films grown at high-temperatures.A composition-adjusted thin film of LixNi1-xO(0.10< × <0.40) was deposited on a sapphire (α-Al2O3)(0001) or MgO(100) substrates by pulsed laser deposition (PLD) technique in 10−6 Torr of oxygen at R.T. and the high temperatures of 350 and 515°C. Crystalline properties of thin films deposited at R.T. or high temperatures were examined using reflection high energy electron diffraction (RHEED) and X-ray diffraction. For the Li-heavily doped NiO films(x>0.30) grown at R.T., a clear streak RHEED pattern showing epitaxial growth was observed. But the Li-heavily doped NiO films grown at high temperatures, exhibited the ring RHEED pattern, which indicates the policrystal growth of films. Electric conductivity of various Li-doped NiO thin films deposited at R.T. or high temperatures on sapphire (0001) substrates were measured by two-probe method. The interesting results were obtained that conductivity of the film was increased remarkably with an increase of Li-doping for R.T. deposition, but was not changed so much regardless of Li-doping for high-temperature depositions.

Spray deposition and characterization of nanostructured Li doped NiO thin films forapplication in dye-sensitized solar cells

Transparent conducting Li (0–5 wt%) doped NiO thin films with preferentialgrowth along the (111) plane were deposited onto glass substrates bypyrolytic decomposition of nickel nitrate and lithium chloride precursors at500 °C in air. The effect of Li concentration on the structural, optical and transport properties ofNiO thin films was studied by x-ray diffraction (XRD), scanning electron microscopy(SEM), atomic force microscopy (AFM), spectral transmittance, photoluminescence andlinear four-probe resistivity. Activation energies as a function of Li concentration werededuced from the temperature dependent resistivity data measured in the range 300–448 K.The figure of merit was deduced by combining the spectral transmittance and sheetresistance values. The variation in properties of NiO thin film due to Li doping arediscussed based on the above results. A dye-sensitized solar cell has also beenfabricated for the optimized Li doped NiO thin film and the results are presented.

Anisotropic electric conduction derived from self-organized nanogroove array on Li-doped NiO epitaxial film

The self-assembly formation of straight and periodic nanogroove arrays was carried out on the surface of Li-doped NiO thin films by use of atomic steps on the substrate. The nanostructure was formed by annealing Li-doped NiO (111) epitaxial thin film prepared on an atomically stepped α-Al2O3 (0001) substrate via pulsed laser deposition at room temperature. V-shaped nanogrooves, with a depth of ∼20nm and an open-end width of ∼50nm, were observed over the entire substrate and characterized by cross-sectional transmission-electron microscopy. The separation of the aligned nanogrooves was from 80to100nm, comparable to the size of the atomic steps on the substrate. Anisotropic electric conduction was definitely attained for the Li-doped NiO thin film with the nanogroove array. A resistance ratio up to about 100 was obtained for both directions parallel and perpendicular to the nanogrooves. The resultant anisotropy was considered to be mainly caused by anisotropic grain growth due to self-organized reconstruction of the NiO film during annealing.

Combinatorial synthesis of Li-doped NiO thin films and their transparent conducting properties

Transparent and conductive oxide (TCO) thin films of Li-doped NiO were prepared by combinatorial pulsed laser deposition technique. Composition spread approach was applied to optimize the Li concentration for achieving high conductivity and transparency in the film. Conductivity of the composition spread film was found to increase almost linearly from nearly insulating to as high as 1.41 Ω −1 cm −1 with increasing Li content along a 9 mm long film. Optical transparency was found to decrease in an equal proportion across the 40 nm thick film with a slight shift in the absorption edge.

Thin-film Li-doped NiO for thermoelectric hydrogen gas sensor

A hydrogen gas sensor using thermoelectric (TE) Li-doped NiO thin film with platinum catalyst film on the half of its surface was fabricated and this thin-film sensor has strong adhesion on the substrate. When this sensor is exposed to air mixed with the hydrogen gas, catalytic reaction heats up the platinum-coated surface, and then TE voltage builds up along the hot and cold region of the oxide film. The Li-doped NiO thin film was deposited by r.f. sputtering method on three different substrates and followed by thermal annealing. The catalyst Pt thin film is deposited on a half-area of NiO thin film and plays an important role for the sensing properties of this hydrogen sensor. FESEM observations indicated that the thickness of the NiO and Pt thin films for the effective catalytic reaction was approximately 720 and 75 nm, respectively. As-deposited NiO thin film possessed amorphous state and Pt thin film on this NiO film has no catalytic activity. Contrary to the as-deposited sample, thermal annealed NiO thin film has crystallized and Pt thin film on this NiO film shows usual exothermic temperature increase arisen from catalytic reaction. These results enable us to think that the catalytic activity depends mainly on the oxidation state and the crystallization degree of TE materials. In consideration of the above factors, the low resistance crystallized NiO thin film was got through the high-temperature annealing process and by using MgO(1 0 0) as the substrate. The hydrogen sensing measurement shows that the Pt/NiO thin film deposited on MgO(1 0 0) substrate could realize negligible noise level and make clear voltage signal even at room temperature.