Co-doped ZnO Films Research Articles

Photocatalytic activity of rare earth elements (Gd and ce) co-doped ZnO nanostructured films

Organic dyes, which are one of the main sources of surface and groundwater pollution, cause health problems as well as environmental hazards. For this reason, ZnO, which stands out among the promising semiconductor photocatalysts in wastewater treatment due to its advantages, was preferred. In particular, Gd–Ce co-doped ZnO nanostructured films, which have been studied for the first time in the literature, were produced by ultrasonic spray pyrolysis (USP) technique and photocatalytic degradation of Methylene blue (MB) dye was carried out. Ultraviolet–Visible Spectrophotometry (UV–Vis) and Field Emission Scanning Electron Microscopy (FESEM) were used to obtain the transmittance and morphology of thin films. According to the FESEM analysis results, morphological changes in the type of nanostructure were observed after the doping. While the photocatalytic degradation percentage of the ZnO film with Gd doping was 65.84 %, when both Gd and Ce were doped, the degradation percentage increased significantly, reaching 91.58 %. Photocatalytic studies revealed that Gd–Ce co-doped ZnO films exhibited better photocatalytic efficiency compared to only Gd doped ZnO films. Therefore, when all the results are evaluated, it is concluded that ZnO:Gd:Ce photocatalysts may have a significant potential application in environmental remediation.

Fabrication of high-performance RT-NH3 gas sensor based on Cu and La co-doped ZnO films through a facile drop-casting method

In this work, undoped ZnO, 2 % Cu-doped ZnO, 2 % La-doped ZnO and 2 % (Cu:La) co-doped ZnO thin films are deposited on glass substrates using the facile drop-casting method, and their physical characteristics were investigated for the gas sensing application. The X-ray diffraction (XRD) patterns confirmed the hexagonal wurtzite structure and increase in crystallinity for the doped samples. Field Emission Scanning Electron Microscopy (FESEM) images revealed a significant change in morphology from the granular particles to densely packed nanoplates-like morphology for (Cu:La) co-doped ZnO. The doped samples exhibited a red shift in absorption edge and shrinkage of the optical bandgap. Multiple emission peaks in photoluminescence (PL) spectra indicated the presence of defect level in the material. It was discovered that Cu-doped and La-doped ZnO thin films had a considerable NH3 sensing response at ambient temperature. The gas response was remarkably enhanced to 341 % for (Cu:La) co-doped ZnO film and a quicker response/recovery time of 80/10 s for 250 ppm NH3. The (Cu:La) co-doped ZnO can serve as a feasible alternative for pure ZnO gas sensors.

Wrinkle type nanostructured of Al-Ce co-doped ZnO thin films for photocatalytic applications

ZnO:Al:Ce thin film photocatalysts were prepared by sol-gel spin coating method for the photocatalytic degradation of Methylene Blue (MB). The effect of Al-Ce co-doping on the structural, surface, optical and photocatalytic properties of ZnO films was analyzed. The crystallite size of the films calculated using the Williamson-Hall method decreased with Al-Ce co-doping. The enhanced photocatalytic activities of ZnO:Al:Ce wrinkle type nanostructured films can be associate with the increased surface area due to the crystallite size reduction by the Al-Ce co-doping. In the morphology analyzes of the films, the wrinkle network structure was observed in the pure ZnO and (6 %) Al-Ce co-doped ZnO films. All ZnO:Al:Ce films exhibited high transparency in the visible light (> 90%). Photoluminescence (PL) spectra depict near band edge (NBE), violet, blue, blue-green and green emission in deposited films. The photocatalytic test shows that the presence of Al-Ce enhances MB degradation efficiency up to 94 %. As a result, it has been observed that the co-existence of Al and Ce has significant effects on both the physical and photocatalytic properties of ZnO films.

Optimization of photoelectric properties of transparent conductive B and Ga co-doped ZnO films for electrochromic applications

To improve the performance of electrochromic devices, it is urgently necessary to develop transparent indium-free conductive electrodes with high conductivity and transparency. In this study, thin films of Ga-doped ZnO (GZO) and B–Ga co-doped ZnO (BGZO) were prepared on glass substrates using the plasma-assisted atomic layer deposition (PEALD) technique, and their structural and optoelectronic properties were characterized. It is found that all thin films were consistent with hexagonal fibrous ZnO structure, and the surface morphology of the membrane was homogenous at nanoscale. UV spectroscopy analysis showed that the doping of B and Ga elements could increase the transparency of ZnO films. The electrical properties can be remarkably enhanced by doping with B and Ga elements. Further more, tungsten oxide (WO3) was deposited on the as-prepared BGZO films using magnetron sputtering method. We activated the sample by applying voltage and observed a color transition from transparent to dark blue. It has a modulation range of 65 % in the 633 nm spectrum and a coloring efficiency of 37.8 cm2C-1. This work opens up remarkable new opportunities for developing high-performance transparent conductive electrode in electrochromic devices.

Improvement in the Stability of Cation (Si)/Anion (F or N) Codoped ZnO Thin-Film Transistors Formed via Atomic Layer Deposition

The characteristics of cation (Si) and anion (F or N) codoped ZnO thin films formed via atomic layer deposition and corresponding device properties were investigated for different anion doping concentrations with a fixed Si doping amount. All films exhibited the (100) primary diffraction peak, indicating a hexagonal wurtzite structure. All codoped ZnO films exhibited the same effect of oxygen vacancy passivation, i.e., reduced oxygen vacancy. However, the electrical and optical characteristics of the films exhibited contrasting tendencies depending on the type of anion element. The optimized device properties, i.e., the subthreshold swing, field-effect mobility, threshold voltage, and bias stability were 6.62 E − 01 V/dec, 5.10 cm2/V·s, -11.16 V, and 6.32 V in the Si/F-19 condition and 5.98 E − 01 V/dec, 16.60 cm2/V·s, 1.07 V, and 0.14 V in the Si/N-19 condition, respectively. These characteristics were attributable to the correlation effects of the reduced interfacial trap density from Si doping and oxygen vacancy passivation from anion doping. Notably, the behavior of each anion was different in terms of the charge carrier concentration. First-principles calculations based on the density functional theory using the Vienna ab initio simulation package (VASP) code were performed to obtain valuable insights into the energy band structure in terms of the performance of the codoped ZnO films.

Cation-cation co-doped ZnO thin films for transparent conductive oxide applications

This paper presents a recent study on the structural and transparent conductive properties of sol–gel spin-coated ZnO films, focusing on the effects of cation-cation co-doping. The co-doping combinations investigated include (Sn-Ga), (Ga-Co), and (Ga-Mg). All the films exhibit a ZnO hexagonal wurtzite crystal structure with varying crystallite sizes. Interestingly, the films deposited on glass substrates show a high degree of texture along the (002) lattice plane. Among the (Ga-Sn) co-doped ZnO films deposited on glass substrates, the film with 1 at% Sn-Ga co-doping demonstrates the most promising results in terms of both transparency and electrical conductivity. On quartz substrates, the (Ga-Co) co-doped ZnO films with a composition of 3 at% Ga and 1 at% Co exhibit significantly enhanced optical transmittance and high electrical conductivity, making them promising candidates for optoelectronic applications. Furthermore, the (Ga-Mg) co-doping results in improved optical transmittance and high electrical conductivity for the ZnO films deposited on glass substrates with a composition of 1 at% Ga and 3 at% Mg. The observed results on cation-cation co-doped ZnO films represent an exciting and novel advancement in the field of TCO materials. The findings contribute significantly to the ongoing progress and understanding of co-doped ZnO films as efficient TCO materials for various optoelectronic applications.

Optimization of the photoelectric performances of Mg and Al co-doped ZnO films by a two-step heat treatment process

Mg and Al co-doped ZnO (AZMO) films were prepared by magnetron co-sputtering to improve the optical and electrical performances of the ZnO-based films simultaneously. A two-step heat treatment process was introduced to obtain high-quality AZMO films. AZMO films were first deposited with substrates heating of 300 °C and then annealed at 500 °C in vacuum. The individual and coupling effects of substrate heating during sputtering deposition and post-annealing treatment on AZMO films were investigated. The results show that substrate heating during sputtering deposition is mainly on increasing the grain size and crystallinity of AZMO films. However, the effects of post-annealing treatment are mainly on making Mg and Al atoms move to the lattice points and ionize to from MgZn and AlZn. Besides, post-annealing treatment can effectively desorb adsorbed oxygen from the surface or grain boundaries of AZMO films and also make lattice oxygen break away from the lattice and change into oxygen vacancy. Compared with one-step heat treatment alone, it is with the two-step heat treatment process that the coupling effects can be exerted by combining the different effects of the two heat treatments, so as to achieve more performances optimization of AZMO films.

Structural, electrical, and optical properties of hydrogen and Cu codoped ZnO films prepared by magnetron sputtering at two substrate temperatures

The Cu-doped ZnO films were prepared by varying H2 flux at substrate temperature (Ts) of 150 and 300 °C, and their structure and electrical-optical properties were investigated by XRD, SEM, AFM, XPS, Raman spectra, resistivity, transmittance spectra and PL spectra. The results indicate that the films prepared at 300 °C have thinner thickness, lower crystallinity, and larger surface roughness than those at 150 °C after introducing H2. By comparison, Zni and Cu contents are higher but VO content is lower in the films at 300 °C. It is found that Raman scattering peaks are mainly related to the type and number of point defects. Compared with films prepared at 150 °C, the films prepared at 300 °C have smaller resistivity but higher electrically conductive stability. The UV emission of films is significantly enhanced at higher H2 fluxes at both Ts. Relatively speaking, the Eg is smaller and UV emission is weaker after introducing H2 at 300 °C. The violet-blue and green-yellow emissions in the films are mainly derived from Zni and VO, respectively. Various possible functions of hydrogen were discussed, and the changes of the structure and optical-electric properties of the films with H2 flux and Ts were explained accordingly.

Control of inhomogeneity and magnetic properties of ZnO:Co films grown by magnetron sputtering using nitrogen

We experimentally report the control of structural inhomogeneity and magnetic properties of Co-doped ZnO films using nitrogen mediated-crystallization. The ZnO:CoN were grown on a silicon substrate at room temperature by RF-magnetron sputtering using nitrogen and followed by a post-annealing treatment for 3 hours at 400 °C, 600 °C, 800 °C and 1000 °C in the air. This method induces changes in inhomogeneity properties comprised by microstructure and stoichiometry of each film, which are confirmed by X-ray diffraction, thermal desorption, and X-ray fluorescence measurements. The difference in inhomogeneity has led to the transformation in the magnetic properties. Films annealed at 400 °C, which showed the highest inhomogeneity, exhibited superparamagnetic-ferromagnetic properties. In contrast, all the other films exhibited diamagnetic properties. Increasing the post-annealing temperature above 400 °C reduces inhomogeneities indicated by improved grain size, decreased impurities, and lattice parameters and stoichiometry of ZnO:CoN films approached those of pure ZnO. Our present results will contribute to control the inhomogeneity of ZnO:Co films to improve magnetic properties at room temperature.

Effect of Al Concentration on Structural, Optical and Electrical Properties of (Gd, Al) Co-Doped ZnO and Its n-ZnO/p-Si (1 0 0) Heterojunction Structures Prepared via Co-Sputtering Method

Heterojunction structures of n-ZnO/p-Si were prepared through the growth of undoped ZnO and (Gd, Al) co-doped ZnO films onto p-type Si (1 0 0) substrates, using a co-sputtering method. The structural and optical properties of the Gd-doped ZnO films were studied as a function of different Al doping concentrations. The X-ray diffraction profiles indicated that the films had a nanocrystalline structure of ZnO with a (0 0 2) preferential orientation. An increase in the Al doping concentration deteriorated the (0 0 2) diffraction peak intensity. The transmittance measurements in the UV-Vis wavelength range indicated that the film's optical gap increased with increase in Al doping concentration. The heterojunction parameters were evaluated using the current-voltage (I-V) characterization carried out of the fabricated n-ZnO/p-Si heterostructure, in dark conditions at room temperature. From these measurements, the n-ZnO-based DMS/p-Si heterojunction diode with the use of (Gd, Al) co-doped ZnO film showed the lowest leakage current of 1.28 × 10-8 A and an ideality factor η of 1.11, close to the ideal diode behavior of η = 1, compared to the n-Gd-doped ZnO/p-Si and n-undoped ZnO/p-Si heterojunction diodes.

Fluorine, chlorine, and gallium co-doped zinc oxide transparent conductive films fabricated using the sol-gel spin method

Transparent conductive films (TCFs) are crucial components of solar cells. In this study, F, Cl, and Ga co-doped ZnO (FCGZO) TCFs were deposited onto a glass substrate using the sol-gel spin-coating method and rapid thermal annealing. The effects of F-doping content on the structural, morphological, electrical, and optical properties of FCGZO films were examined by XRD, TEM, FE-SEM, PL spectroscopy, XPS, Hall effects testing, and UV–vis–NIR spectroscopy. All prepared ZnO films exhibited a hexagonal wurtzite structure and preferentially grew along the c axis perpendicular to the substrate. Changes in the doping concentration of F changed the interplanar crystal spacing and O vacancies in the film. At a doping ratio of 2% (in mole), the F, Cl, and Ga co-doped ZnO film exhibited the best photoelectric performance, with a carrier concentration of 2.62 × 1020 cm−3, mobility of 14.56 cm2/(V·s), and resistivity of 1.64 × 10−3 Ω·cm. The average transmittance (AT) in the 380–1 600 nm region nearly 90% with air as the reference, and the optical band gap was 3.52 eV.

Structural Evolution in an Annealed (Eu, Tb)-Doped ZnO/Si Nanoscale Junction: Implication for Red LED Development

Codoped (Tb,Eu) ZnO films grown by magnetron sputtering on a silicon substrate and annealed up to 1200 °C showed intense photoluminescence (PL) emission from Eu3+ ions. The high-temperature annealing led to diffusion and segregation of rare earth (RE) elements toward the bottom of the film, which induced the formation of nanometric Zn-free inclusions responsible for remarkable PL emission intensity. Combined electron diffraction, chemical contrast imaging, and optical studies of these nanometric phases have been carried out. Large inclusions of zinc silicates and RE silicates a few hundreds of nanometers in size were observed, embedded in a silica phase. The structural determination of these RE-rich inclusions was carried out by combining atomic Z contrast imaging (high-angle annular dark-field imaging) and precession electron diffraction data. Upon annealing at 1200 °C, it appeared that the structure was related to an F-type disilicate structure. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy experiments were carried out to determine the ratios between elements and the oxidation states of the RE elements in the abovementioned inclusions. A (Tb,Eu)2Si2O7 formulation is proposed from dynamical precession electron diffraction tomography refinements, leading to a +III valence state for the RE species in agreement with spectroscopic results. PL modeling is also in good agreement with the experimental data. These results complete those obtained at 1100 °C for which the inclusions were identified as some RE10–x(SiO4)6O2–x oxyapatite structures and pointed out a combined structural and PL properties’ evolution between 1100 and 1200 °C annealing temperatures.

Wafer‐Scale PLD‐Grown High‐κ GCZO Dielectrics for 2D Electronics (Adv. Electron. Mater. 12/2022)

High-κ Gate Dielectrics In article number 2200580, Jing Yu, Wei Han, Francis Chi-Chung Ling, and co-workers prepare large-size (Ga, Cu) co-doped ZnO films with super-high dielectric constant (κ > 50) and good homogeneity by a pulsed laser deposition (PLD) method. The performance of SnS2 field-effect transistor reveals that high-κ Al2O3/GCZO gate dielectric stack is suitable for 2D electronic devices. The success in fabrication of GCZO films not only show higher κ than other conventional dielectrics in terms of compatibility to CMOS processes, but also keep their comparative advantages in the fabrication of high-performance electronic devices over conventional dielectrics.

Magnetic Domain Characterization and Physical Properties of Gd-Doped and (Gd, Al) Co-Doped ZnO Thin Films.

Undoped ZnO, Gd-doped ZnO with various doping concentration (1, 3, 5, and 7 at%), and 3 at% (Gd, Al) co-doped ZnO films were prepared on a glass substrate using the co-reactive sputtering method. The influence of the doping and co-doping process on the films was characterized using X-ray diffraction, FESEM, EDX, MFM, VSM, UV-VIS spectroscopy, and the Hall Effect measurement at room temperature. XRD study confirmed that the Gd and Al ions are incorporated into a ZnO lattice. EDX analysis confirmed the existence of Zn, O, Al, and Gd elements in the prepared Gd-doped ZnO and (Gd, Al) co-doped ZnO films, which suggests the successful doping procedure. All the deposited films obtained maximum optical transmittance above 80%, showing a high transparency of the films in the visible region. The optical band gap was found red-shifted from 3.11 to 3.21 eV with the increase in Gd doping concentration. The increase in band gap energy from 3.14 eV to 3.16 eV was obtained for 3 at% Gd and 3 at% (Gd, Al) co-doped ZnO films. The MFM measurement proved the existence of room-temperature ferromagnetism and spin polarization in Gd and (Gd, Al) co-doped ZnO films. By co-doping with Al, the result obtained from MFM shows the enhancement of magnetic properties, as it exhibited a smaller domain size with a shorter magnetic correlation length L, a larger phase shift Φrms, and the highest value of δfrms compared to the sample with 3 at% Gd incorporated into ZnO. The carrier concentration and electrical conductivity increased with the increase in Gd concentration, whereas the electrical resistivity and hall mobility showed a reverse trend. The similar trend of results obtained for 3 at% (Gd, Al) co-doped ZnO as compared to 3 at% Gd-doped ZnO also indicates greater electrical properties after a shallow donor such as aluminum was incorporated into Gd-doped ZnO thin films. In conclusion, for future applications, one should consider the possible influence of other types of shallow donor incorporation in an attempt to enhance the properties of new types of diluted magnetic semiconductors (DMSs).

Structural, electrical and magnetic properties of Gd-doped and (Al, Gd) codoped ZnO films

Gd-doped and (Al, Gd) codoped ZnO films were prepared by ion implantation and radio frequency magnetron sputtering. The resulting Gd-doped and (Al, Gd) codoped ZnO films were investigated with respect to their structural, electrical and magnetic properties. Room temperature ferromagnetism was obtained for all the doped films and the magnetization shows carrier concentration-dependent behavior. Two different regions can be divided in light of carrier concentration in which the bound magnetic polaron model and carrier-mediated exchange mechanisms produce a marked effect respectively in establishing the long-range ferromagnetic order. The first principle calculations reveal that the magnetic moment of (Al, Gd) codoped ZnO is higher than that of Gd-doped ZnO, and the ferromagnetic states of doped ZnO systems are more stable in the near configuration. Our results confirm that appropriate electron doping can effectively enhance the magnetic moment of the Gd-doped ZnO by controlling its concentrations, which may pave ways for preparing DMS with high magnetic moment using rare earth and group III elements as dopants.

High Dielectric Transparent Film Tailored by Acceptor and Donor Codoping.

High dielectric constant materials are of particular current interests as indispensable components in transistors, capacitors, etc. In this context, there are emerging trends to exploit defect engineering in dielectric ceramics for enhancing the performance. However, demonstrations of similar high dielectric performance in integration-compatible crystalline films are rare. Herein, such a breakthrough via the functionalization of donor-acceptor dipoles by compositional tuning in GaCu codoped ZnO films is reported. The dielectric constant reaches ~200 at 1kHz and the optical transmittance in visible light reaches ~80%. Importantly, by analyzing the impedance spectroscopy data, prominent relaxation mechanisms in correlation with the dipole properties, enabling consistent explanations of the dielectric constant as a function of frequency are discriminated. The atomistic nature of the dipoles is revealed by the systematic X-ray spectroscopy analysis. Spectacularly, similar trends for the dielectric properties are observed, while synthesizing samples by pulsed laser deposition and ion implantation, indicating the general character of the phenomena.

Highly c-axis oriented (Mg, Sn) co-doped ZnO thin films for optoelectronic applications

In this paper, results on the microstructural, optical and electrical characteristics of the (Mg, Sn) co-doped ZnO films as a function of Mg, Sn dopant concentrations have been presented. All the films are highly oriented along the c-axis in the (0 0 2) plane, possessing a hexagonal wurtzite crystal structure of ZnO. Upon doping and codoping, the crystallite size is found to decrease. The root mean square (RMS) roughness is maximum for the undoped ZnO film and decreases upon doping and codoping. The field emission scanning electron micrographs reveal granular surface morphologies of the films. The undoped ZnO films' transmittance is minimal and improves upon doping and codoping with a maximum of ∼95% of transmittance for the Mg:Sn (1:1 at%) co-doped ZnO film in the wavelength range 420–550 nm and beyond 700 nm. The electrical conductivity of the film increases upon doping and codoping with a maximum of 1.38x102 (Ω-cm)−1 for the Mg:Sn (1:1 at%) co-doped ZnO film. It has been established that the Mg:Sn (1:1 at%) co-doped ZnO film is a potential candidate to be used as transparent conductive oxide for optoelectronic applications.

Hidden impurities in transparent conducting oxides: study of vacancies-related defects and impurities in (Cu–Ni) co-doped ZnO films

The effect of hydrogen and nitrogen impurities on the physical properties of transparent conductive oxides is investigated in this study. Therefore, 5 wt.% of copper and 5 wt.% of nickel co-doped zinc oxide ((Cu–Ni)/ZnO) films were prepared using the sol–gel method. The (Cu–Ni)/ZnO films were annealed in an oven at 500 °C for 2 h under air, vacuum, nitrogen, and argon atmospheres. The synthesized zinc hydroxide film was transformed to zinc oxide film during the annealing by evaporating {mathrm{H}}_{2}mathrm{O}. Films annealed under the mentioned atmosphere including as-prepared one were characterized by analyzing with UV–Vis and FTIR spectra in addition to the 2D mapping electrical conductivity of the surface measured by the 4-point probe. The annealed films under air, vacuum, and argon atmospheres led to generate H-related impurities bounded to the oxygen vacancy ({mathrm{H}}_{mathrm{O}}) which they act as shallow donor defects resulting in forming (Cu–Ni)/ZnO films into n-type materials. Whereas, the film annealed under a nitrogen atmosphere has N-related defects bounding to the zinc vacancy ({mathrm{N}}_{mathrm{Zn}}) which they act as shallow acceptor defects resulting in transforming the film from n-type to p-type. These defects affect the optical, electrical, and optoelectronic properties of the (Cu–Ni)/ZnO films.

Tuning of work function of ZnO by doping and co-doping: An investigation using X-ray photoelectron spectroscopy

The shift in the work function of the ZnO thin film upon varying codoping ratio has been investigated by X-ray photoelectron spectroscopy. A notable shift of 0.2 eV in the work function was achieved in the films when the Al:In doping ratio was changed from 0:10 to 10:0. The elemental composition of the dopants obtained from XPS analysis showed the presence of more amount of In dopant than Al in codoped ZnO films. In this way a clear understanding of the variation in electrical properties on dopant ratio is obtained. A decrease in work function was also observed with the increase in free carrier concentration when the codopant ratio is changed. The effects of Burstein moss, band narrowing, and band renormalization observed in the bandgap were explained by an upward and downward shift of valence band maxima of the corresponding thin films. The controllable work function of the codoped ZnO films by varying the doping ratio offers excellent potential advantages in optoelectronic devices.

Anomalous photovoltaic effect with high fill factor in Fe3+-Li+ co-doped ZnO films

Integrating high fill factor (FF) with above-bandgap photovoltage has been of fundamental importance in building high-power conversion efficiency (PCE) devices based on anomalous photovoltaic effect. Herein, the discovery of high performance anomalous photovoltaic effect under white light in designed Zn1–2x(FexLix)O films fabricated by a low-cost sol-gel process is reported, where above-bandgap photovoltage, improved FF, a switchable photoresponse of 34.1 mA/W, and a PCE of 10.3 % are obtained. It is found that the photovoltaic effects are closely related to the ferroelectricity of the co-doped films. A mechanism based on serial connection of poled ferroelectric regions and junctions is proposed to interpret the photovoltaic effects with high FF in Fe3+-Li+ co-doped ZnO films, where the junction is formed between the ferroelectric and weak-ferroelectric regions. This photo-to-electron conversion model suggests a route to design and fabricate cost-efficient solar cell and other optoelectronic devices.