Li-doped ZnO Thin Films Research Articles

Sol–Gel Synthesis of ZnO:Li Thin Films: Impact of Annealing on Structural and Optical Properties

A sol–gel deposition approach was applied for obtaining nanostructured Li-doped ZnO thin films. ZnO:Li films were successfully spin-coated on quartz and silicon substrates. The evolution of their structural, vibrational, and optical properties with annealing temperature (300–600 °C) was studied by X-ray diffraction (XRD), Fourier Transform Infrared (FTIR), UV-VIS spectroscopic, and field emission scanning electron microscopic (FESEM) characterization techniques. It was found that lithium doping maintains the wurtzite arrangement of ZnO, with increasing crystallite sizes when increasing the annealing temperature. Analysis of the FTIR spectra revealed a broad main absorption band (around 404 cm−1) for Li-doped films, implying the inclusion of Li into the ZnO lattice. The ZnO:Li films were transparent, with slightly decreased transmittance after the use of higher annealing temperatures. The porous network of undoped ZnO films was transformed to a denser, grained, packed structure, induced by lithium doping.

Enhanced Hall Mobility and d0 Ferromagnetism in Li-Doped ZnO Thin Films Prepared by Aerosol-Assisted CVD

Enhanced Hall Mobility and d0 Ferromagnetism in Li-Doped ZnO Thin Films Prepared by Aerosol-Assisted CVD

Lithium doping effect on microstructural and electrical properties of zinc oxide thin film grown by metal-organic chemical vapor deposition

In this study, the undoped and Li-doped ZnO thin films were grown on Si(100) substrate using metal-organic chemical vapor deposition (MOCVD). Zinc acetylacetonate hydrate and lithium acetylacetonate solution were used as ZnO thin film precursor and Li dopant source. The effect of lithium doping on microstructural was characterized using a scanning electron microscope (SEM) and X-ray diffractometer (XRD). XRD diffractogram analysis shows that undoped and Li-doped ZnO thin films have polycrystalline hexagonal wurtzite structures with preferred peak crystal orientation (103). Li doping slightly changes the lattice parameters and cell volume of ZnO thin films through the increase of crystallite size and slightly affects the surface morphology of ZnO thin films. Current-voltage (I-V) measurement and four-point probe method were used to measure the electrical properties of lithium doped ZnO thin films. The electrical conductivity of ZnO thin films increases as Li doping is given compared to undoped films. These results are also supported by the I-V curve of Li-doped ZnO thin films by having a higher slope, indicating improvement in electrical properties.

Continuous Tuning of the Fermi Level in Disorder-Engineered Amorphous Films of Li-Doped ZnO for Thermoelectric Applications.

Amorphous metal-oxide semiconductors can be readily prepared by a solution process at low temperatures, and their energy band structures and carrier concentrations can be controlled based on the oxide composition or the addition of dopants in the design of thermoelectric (TE) materials. However, research on the correlation between the charge transport and TE performance of amorphous metal-oxide semiconductors is still in its infancy. Herein, we present the energy-dependent TE performance characteristics of Li-doped ZnO thin films with different doping levels and charge carrier concentrations. Thin films were prepared by the solution process, and the Li doping level was controlled by the Li precursor concentration added to a Zn precursor solution. Subsequently, a field-effect-modulated Seebeck coefficient measurement device was built to study the energy-dependent TE performance. Notably, the higher ratio of interstitial Li (Liinter) and oxygen vacancies (Ova) in the Li-ZnO device indicates an improved n-type TE performance. To investigate more thoroughly the charge transport phenomena, the localized density of states (DOS) was derived from the temperature-dependent transfer curve; the higher ratio of interstitial Li (Liinter) and oxygen vacancy (Ova) induces a reduction in the localized DOS and lowers the degree of disorder in their DOS. The determined energy-dependent TE characteristics can be used as guidance for the design of efficient TE devices with amorphous metal-oxide semiconductors.

Photo-induced electronic transition and effect of thickness on the resistivity of Li-doped ZnO thin films

Zn1−xLixO (x = 0, 0.005, 0.010 and 0.015) thin films were grown on single-crystal sapphire (Al2O3) substrates (001) by pulsed laser deposition method. We prepared two sets of thin films. In set-1, the doping of Li was varied. A fractional Li-doping causes a systematic rise in the temperature-dependent resistivity of the films. Further, the ultraviolet (UV) light (365 nm) exposure on these films causes a variety of effects. We observed a UV light-induced metal-semiconductor transition in otherwise semiconducting films. The transition manifests in a temperature range above 200 K to 300 K. In set-2, thin films of maximum doping (1.5%) were prepared by varying the thickness. The structural study revealed that the films are crystalline and oriented towards c-axis of wurtzite ZnO. The temperature-dependent resistivity measurements show the increase in resistivity with an increase in the thickness of the films.

Fabrication and Characteristic of a Double Piezoelectric Layer Acceleration Sensor Based on Li-Doped ZnO Thin Film

In this paper, a double piezoelectric layer acceleration sensor based on Li-doped ZnO (LZO) thin film is presented. It is constituted by Pt/LZO/Pt/LZO/Pt/Ti functional layers and a Si cantilever beam with a proof mass. The LZO thin films were prepared by radio frequency (RF) magnetron sputtering. The composition, chemical structure, surface morphology, and thickness of the LZO thin film were analyzed. In order to study the effect of double piezoelectric layers on the sensitivity of the acceleration sensor, we designed two structural models (single and double piezoelectric layers) and fabricated them by using micro-electro-mechanical system (MEMS) technology. The test results show that the resonance frequency of the acceleration sensor was 1363 Hz. The sensitivity of the double piezoelectric layer was 33.1 mV/g, which is higher than the 26.1 mV/g of single piezoelectric layer sensitivity, both at a resonance frequency of 1363 Hz.

Resistive Switching Characteristics of Li-Doped ZnO Thin Films Based on Magnetron Sputtering.

A kind of devices Pt/Ag/ZnO:Li/Pt/Ti with high resistive switching behaviors were prepared on a SiO2/Si substrate by using magnetron sputtering method and mask technology, composed of a bottom electrode (BE) of Pt/Ti, a resistive switching layer of ZnO:Li thin film and a top electrode (TE) of Pt/Ag. To determine the crystal lattice structure and the Li-doped concentration in the resulted ZnO thin films, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) tests were carried out. Resistive switching behaviors of the devices with different thicknesses of Li-doped ZnO thin films were studied at different set and reset voltages based on analog and digital resistive switching characteristics. At room temperature, the fabricated devices represent stable bipolar resistive switching behaviors with a low set voltage, a high switching current ratio and a long retention up to 104 s. In addition, the device can sustain an excellent endurance more than 103 cycles at an applied pulse voltage. The mechanism on how the thicknesses of the Li-doped ZnO thin films affect the resistive switching behaviors was investigated by installing conduction mechanism models. This study provides a new strategy for fabricating the resistive random access memory (ReRAM) device used in practice.

Fabrication and Characterization of the Li-Doped ZnO Thin Films Piezoelectric Energy Harvester with Multi-Resonant Frequencies.

A novel piezoelectric energy harvester with multi-resonant frequencies based on Li-doped ZnO (LZO) thin films is proposed in this paper, consisting of an elastic element with three (or more) different length cantilever beam arrays and a piezoelectric structure (Al/Li-doped ZnO/Pt/Ti). The LZO thin films of piezoelectric structure were prepared on Pt/Ti/SiO2/Si by using a radio frequency (RF) magnetron sputtering method under certain process conditions. When the LZO thin films were deposited with an LZO target concentration of 5 wt%, the piezoelectric coefficient d33 was 9.86 pm/V. Based on this, the energy harvester chips were fabricated on a <100> silicon substrate using micro-electromechanical systems (MEMS) technology, and its performance can be measured by fixing it to a printed circuit board (PCB) test substrate. The experimental results show that, when exerting an external vibration acceleration of 2.2 g and a vibration frequency of 999 Hz, the energy harvester can achieve a big load voltage of 1.02 V at a load resistance of 600 kΩ, and a high load power of 2.3 µW at a load resistance of 200 kΩ.

Effects of lithium doping on: microstructure, morphology, nanomechanical properties and corrosion behaviour of ZnO thin films grown by spray pyrolysis technique

Li-doped ZnO thin films were prepared on glass substrate by a chemical spray pyrolysis method, in the temperature of 460 °C. The effects of Li content on the microstructural, morphological and mechanical characteristics of the doped (ZnO:Li) thin films were also examined. The XRD study showed a sharp preferred c-axis orientation and showed that (ZnO:Li) films have a wurtzite structure and grow principally along the c-axis orientation with a preferred orientation (002). The film morphology was examined by (AFM) and (SEM). Results of SEM observations showed that sprayed thin films, exhibited uniform and harmonious texture. Furthermore, ZnO:Li thin films revealed uniform and spherical shaped crystallites with an approximate medium size of 200 nm. AFM characterization demonstrated an amelioration of the surface roughness of the ZnO:Li thin films. The mechanical characteristics of ZnO:Li thin films have been investigated by the nano-indentation experiment. It has been found that the addition of lithium enhances the hardness and Young’s modulus. On the other hand, the corrosion behaviour of Li-doped thin films is examined in chloride solutions. The electrochemical experiments confirmed that the lithium doping could ameliorate the anti-corrosion performance.

Study of defects in Li-doped ZnO thin films

Li-doped ZnO films were deposited on glass substrate by sol-gel spin coating technique. In order to investigate the effect of Li doping on ZnO thin films properties, we have varied the Li doping ratio in the range of 0–10%. The structural and optical properties of samples have been investigated by X-ray diffraction and UV–Vis spectroscopy. XRD analysis reveals that doped and undoped films have an hexagonal Wurtzite structure. The optical measurements indicate that Li doping causes the red shift of the absorption edge. Films photoluminescence spectra do not contain any defects peak related to Li atoms. Being too close to the valence band, the Li defect level is buried in the band tail width states; this explains the difficulty of its detection. However, the effect of Li doping, manifests through its oxygen vacancy defects promotion to the detriment of oxygen interstitial oxygen defect.

Fabrication and characteristic of force sensor based on piezoelectric effect of Li-doped ZnO thin films

In this paper, a force sensor based on piezoelectric effect of Li-doped ZnO (LZO) thin films was presented, which constituted by Pt/LZO/Pt/Ti functional layers and Si cantilever beam. The chips were designed and fabricated by micro electro-mechanical system (MEMS) technology on silicon wafer with [Formula: see text] orientation. In this sandwich structure, the top electrode (TE) was Pt and bottom electrode (BE) was Pt/Ti, LZO piezoelectric thin films were prepared by radio frequency (RF) magnetron sputtering method. The microstructure and morphology were analyzed through X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM), analysis results shows that the LZO thin films with highly c-axis orientation and uniform grain size distribution under sputtering power of 220 W. The experimental results show, when external force loaded on the tip of the beam, the output voltage [Formula: see text] was 280.3 mV at external force of 5 N, the sensitivity of the proposed sensor was 46.1 mV/N in the range of 1–5 N.

Fabrication of Fast-response Ultraviolet Light Sensors with LZO Thin Films using Sol-gel Spin-coating Method

The Li-doped ZnO (LZO) thin films with various Li concentrations (0, 1, 3, 5, 7, and 9 at.%) were deposited on quartz substrates by sol-gel spin-coating method. The effects of Li doping on the structural and optical properties of the thin films were investigated. Undoped ZnO thin film and LZO thin films had three different diffraction peaks at 31.5 °, 34.2 °, and 36.0 °corresponding to the ZnO (100), (002), and (101) planes, respectively. The c-axis orientation of the LZO thin films increased with increasing Li doping concentration, owing to the high diffusivity of the Li dopant. A broad red emission was observed by photoluminescence of the undoped ZnO thin film, whereas LZO thin films had broad green emissions. The transmittance and optical band gap blue-shifted with increasing Li doping concentration. In terms of photoresponse, all LZO thin films had greater rise and decay rates than the undoped ZnO thin film and the photosensitivity increased with increasing Li doping concentration.

The Effect of the Buffer Layer on the Structural, Optical, and Ultraviolet Photoresponse of the Li-Doped ZnO Thin Films

The Effect of the Buffer Layer on the Structural, Optical, and Ultraviolet Photoresponse of the Li-Doped ZnO Thin Films

Ultraviolet Photoresponse Properties of Li-Doped ZnO Thin Films Prepared by Sol–Gel Spin-Coating Method

Ultraviolet Photoresponse Properties of Li-Doped ZnO Thin Films Prepared by Sol–Gel Spin-Coating Method

Structural, morphological, optical and photodetector properties of sprayed Li-doped ZnO thin films

Lithium-doped zinc oxide thin films (ZnO:Li) at different percentages (0–4%) were deposited on glass substrates at 460 °C using the spray pyrolysis technique. The effect of lithium content on the structural, morphological and optical properties of ZnO:Li thin films was investigated. First, X-ray diffraction study revealed that undoped and Li-doped ZnO thin films crystallize in hexagonal wurtzite structure with a preferred orientation of the crystallites along (002) direction. The scanning electron microscopy (SEM) showed a clear hexagonal-shaped granular onto the surface of such films. Indeed, the averaged grain size, visualized by SEM, varied from 100 nm, for undoped ZnO, to 200 nm for ZnO:Li 4%. Second, the optical analysis by means of the transmittance and the reflectance measurements revealed that in UV–Vis domain, the average transmittance of the ZnO:Li thin films was 75%, while the average reflectance was <35% in the visible range. Moreover, the band gap energy, E g, decreased from 3.285 to 3.264 eV as Li content increases. On the other hand, the photoluminescence spectra revealed emission with multiple peaks and exhibited especially two emission bands in UV and visible regions. Finally, the ZnO photodetectors showed superior performance in the view of photocurrent.

Li induced enhancement in c-axis orientation and its effect on structural, optical, and electrical properties of ZnO thin films

Un-doped and Li-doped ZnO thin films with high c-texture have been deposited on glass substrate using sol–gel spin coating method. The effect of Li dopant in concentration range 1–5 at.% on the structural, optical, morphological, and electrical properties of ZnO thin films were analyzed using x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), UV–VIS spectrophotometer, field emission scanning electron microscopy (FESEM), and two point probe method, respectively. XRD analysis reveals that all thin films possess hexagonal wurtzite structure and (0 0 2) preferred orientation. X-ray diffraction intensity and crystallite size appertaining to (0 0 2) plane increases with the increase in Li concentration. FESEM micrographs shows that morphology of all thin films consist of spherical and non-spherical shaped grains. Surface roughness of the films decreases on increasing Li dopant concentration. Average transmittance in visible region is 90.6 ± 0.5 % for all Li-doped ZnO thin films. Blue shift, i.e. widening of optical band gap (Eg) has been observed on doping with Li. Urbach energy (Eu) was found to decrease on Li incorporation. Both Eg and Eu correlate well with variation in c-texture and crystallite size. The average refractive index of ZnO thin films in the visible region is decreased on doping with Li. Electrical conductivity of ZnO thin films is enhanced on increase in Li dopant concentration. The improvement in electrical conductivity also correlates well with increase in c-texture and crystallite size. FTIR spectra portray characteristic absorption bands centered at 901, 760, 602, and 568 cm−1 pertaining to metal–oxygen bonds.

Synthesis of lithium doped zinc oxide by sol gel

Li-doped ZnO thin films were prepared by sol-gel method and deposed on glass substrate using spin coating technique. The effects of Li on structural and optical properties were investigated. The X-ray diffraction (XRD) analysis reveals that Li incorporation leads to the great improvement of the crystalline quality of ZnO thin films. Scanning electron microscopy (SEM) images showed that nanowires are aligned nearly perpendicular to the substrate plane and are affected significantly by Li incorporation. The optical transmission of the films was higher than 80% in the visible region. It is found that the optical gap and the refractive index remain practically constant.

Defect assisted saturable absorption characteristics in Al and Li doped ZnO thin films

The influence of different doping ratios of Al and Li on the nonlinear optical properties, namely, a two-photon absorption and a nonlinear refraction using single beam Z-scan technique, of nano-crystalline ZnO thin films has been investigated in the present study. A sol-gel spin-coated pure ZnO, Al-doped ZnO (AZO), and Li-doped ZnO (LZO) thin films have been prepared. The stoichiometric deviations induced by the occupancy of Al3+ and Li+ ions at the interstitial sites injects the compressive stress in the AZO and LZO thin films, respectively, while the extended defect states below the conduction band leads to a redshift of energy band gap in the corresponding films as compared to pure ZnO thin film. Switching from an induced absorption in ZnO and 1 at. wt. % doped AZO and LZO films to a saturable absorption (SA) in 2 at. wt. % doped AZO and LZO films has been observed, and it is attributed to the saturation of a linear absorption of the defect states. The closed aperture Z-scan technique revealed the self-focusing (a positive nonlinear refractive index) in all the films, which emerge out of the thermo-optical effects due to the continuous illumination of laser pulses. A higher third-order nonlinear optical susceptibility χ(3) of the order 10−3 esu has been observed in all the films.

Efficient multi-coloured Li-doped ZnO thin films fabricated by spray pyrolysis

Undoped and Li-doped ZnO films with bright visible luminescence have been fabricated by the spray pyrolysis technique at 400 °C. The pyrolytic films exhibit multi-coloured emissions of yellow, green and blue, which can be tuned by varying the Li concentration. Simulation of the cathodoluminescence spectra from the Li-doped films using the Huang-Rhys model reveals the energies of the luminescence centres and their electron-phonon coupling strength. These centres are attributable to either VZn or LiZn acceptor states. This work presents a practical route to fabricate inexpensive multi-coloured light emitting coatings on any substrate.

Doping effect investigation of Li-doped nanostructured ZnO thin films prepared by sol–gel process

In this work, we report the effects of Li doping concentration (5, 10 and 15 at.%) on structural, morphological, electrical and optical properties of Li doped ZnO films. We used X-ray diffraction (XRD), scanning electronic microscopy (SEM), Hall-Effect measurements, UV–Vis–NIR and photoluminescence (PL) spectroscopies. XRD patterns showed that the deposited films had a hexagonal-wurtzite crystal structure with preferred orientation along the c-axis. SEM micrographs indicated that grain dispersion in the films were homogeneous. Hall-effect measurements revealed that ZnO thin films doped with 5, 10 and 15 at.% of Li have p-type conduction. The UV–Vis–NIR spectroscopy analyses demonstrated that all thin films were highly transparent in the visible region and the optical transmittance was dependent on the Li doping content. Room temperature PL spectra put into evidence that all Li-doped ZnO thin films emitted a UV-blue light and the emission intensities were found to be affected by Li concentration.