Mono-doped ZnO Research Articles

ZnO, Cu-doped ZnO, Al-doped ZnO and Cu-Al doped ZnO thin films: Advanced micro-morphology, crystalline structures and optical properties

The thin film coatings composed of: undoped ZnO film, ZnO doped with Al, ZnO doped with Cu, and ZnO simultaneously doped with Al and Cu (co-doping) were separately deposited on quartz substrates using RF sputtering method with different targets. The advanced fractal features, crystalline structure and optical properties of sputtered samples were investigated by atomic force microscopy (AFM), X‐ray diffraction (XRD) and UV–vis spectroscopy. Microstructural studies revealed homogeneously granular structure of ZnO layer and axially oriented granular structure of AZO thin film.The transmission spectra of undoped, mono-doped and co-doped ZnO thin films were measured revealing relatively large transmittance of more than 80 % for un-doped and co-doped samples and less than that value for mono-doped thin films in both visible and infrared regions. CAZO thin film was found the most transparent thin film in the visible area being a prerequisite for good TCO. Analysis of absorption coefficients demonstrated that excitonic effects are invisible in mono-doped and co-doped samples. Also, PL spectra show that in these samples there are very high densities of free carriers and presence of impurities, which is important for conductivity of thin films as well as their optical applications. The optical band gap of ZnO thin films decreases by Cu doping from 3.12 eV to 3.09 eV and increases by Al doping to 4.30 eV, but remains exactly between those values in terms of co-doped sample (3.75 eV).

A hybrid density functional study of optical and electronic properties of Al\\Ga-codoped ZnO

The energetic, electronic structures and optical properties of Al-, Ga-doped and Al\\Ga-codoped ZnO systems with the concentration x = y = 3.125% have been investigated using first-principles calculations. The results show that the Al\\Ga-codoped ZnO can be easily prepared at O-rich condition with higher stability than that of Al- and Ga-doped ZnO. Furthermore, the incorporation of Al and Ga into ZnO present a good optical and electronic property with significant improvement for the ZnO co-doped compared to monodoped and pure ZnO. While, the absorbance of Al\\Ga-codoped ZnO becomes lower that the case of pure ZnO in the UV region. The calculations using HSE06 functional underline the impact that inclusion of exact exchange has on the electronic and optical properties.

Response Surface Methodology Optimized Sol–Gel Synthesis of Ag, Mg co-Doped ZnO Nanoparticles with High Photocatalytic Activity

In this paper, pure ZnO, Ag, and Mg mono-doped and co-doped ZnO nanoparticles were synthesized via sol-gel method. The synthesized nanoparticles were characterized using XRD, DRS, BET, TEM, SEM-EDX, and PL techniques. The photocatalytic activity of the prepared photocatalysts was evaluated in degradation Rhodamine B (RhB) dye. Response surface methodology (RSM) was used as a statistical technique for optimizing the effect of synthesis variables on the removal efficiency of RhB. The results predicted from RSM were found to be in good agreement with the experimental results as indicated by correlation coefficient (R2) of 0.9624. Furthermore, statistical tests such as ANOVA table and lack of fit indicated that the model was adequate for representing the experimental data. Ag and Mg co-doped ZnO nanoparticles demonstrated the highest photocatalytic activity in comparison with pure and mono-doped ZnO. Maximum photocatalytic activity of co-doped ZnO nanoparticles was observed for a sample with 3 wt % Ag, 4 wt % Mg calcined at 530°C.

Effects of Eu doping and O vacancy on the magnetic and optical properties of ZnO

We calculated the electronic structure and optical properties of Eu mono-doped ZnO systems with or without O vacancy. We also determined the relative energy of ferromagnetic and antiferromagnetic orders of Eu-double-doped ZnO systems. The double-doped systems possess high Curie temperature and achieve room temperature ferromagnetism. The magnetism in the Eu mono-doped system without O vacancy is caused by the -Eu3+-O2--Eu3+- bound magnetopolaron (BMP) model. The magnetism of Eu mono-doped ZnO systems with O vacancy is more stable than that without O vacancy, and such magnetism is attributed to the -Eu3+-VO++-Eu3+- BMP model. The absorption spectrum for mono-doped systems is red shifted, and this finding confirms that Eu-mono-doped ZnO is a candidate photocatalyst for various applications. Therefore, Eu-double-doped ZnO can be practically used as an unambiguous diluted magnetic semiconductor.

Defect induced room-temperature ferromagnetism and enhanced dielectric property in nanocrystalline ZnO co-doped with Tb and Co

Microstructural, magnetic and dielectric properties of hydrothermally prepared dual ions doped (Tb and Co) zinc oxide (Tb0.05Co0.05Zn0.90O) nanoparticles are investigated in the present work. The polycrystalline single phase of undoped and doped ZnO nanoparticles with hexagonal wurtzite structure is confirmed by X-ray diffraction pattern. Raman and Photoluminescence spectroscopy studies are recorded at room temperature and the measurements ruled out the presence of any impurity phase in both undoped and Tb, Co co-doped ZnO samples. Also, the Photoluminescence emission spectra for the entire samples confirm the presence of various defect centers inside co-doped ZnO wurtzite nanostructure. Magnetic and dielectric properties of co-doped ZnO system are enhanced remarkably as compared to undoped as well as some mono-doped ZnO systems. The room temperature ferromagnetism with high coercive field and magnetization has been observed in the magnetic measurement and the outcome of the measurement will be fruitful for future spintronics device applications. The dielectric behavior has been explained on the basis of Maxwell-Wagner-Sillars polarization phenomenon. High value of dielectric constant was observed due to the presence of grain boundary defects and various others defect centers at the surface of the hexagonal rod like doped ZnO nanostructures.

Formation of p-type ZnO thin film through co-implantation

We present a study on the formation of p-type ZnO thin film through ion implantation. Group V dopants (N, P) with different ionic radii are implanted into chemical vapor deposition grown ZnO thin film on GaN/sapphire substrates prior to thermal activation. It is found that mono-doped ZnO by N+ implantation results in n-type conductivity under thermal activation. Dual-doped ZnO film with a N:P ion implantation dose ratio of 4:1 is found to be p-type under certain thermal activation conditions. Higher p-type activation levels (1019 cm−3) under a wider thermal activation range are found for the N/P dual-doped ZnO film co-implanted by additional oxygen ions. From high resolution x-ray diffraction and x-ray photoelectron spectroscopy it is concluded that the observed p-type conductivities are a result of the promoted formation of PZn–4NO complex defects via the concurrent substitution of nitrogen at oxygen sites and phosphorus at zinc sites. The enhanced solubility and stability of acceptor defects in oxygen co-implanted dual-doped ZnO film are related to the reduction of oxygen vacancy defects at the surface. Our study demonstrates the prospect of the formation of stable p-type ZnO film through co-implantation.

Tailoring of room temperature ferromagnetism and electrical properties in ZnO by Co (3d) and Gd (4f) element co-doping

Nanoparticles of Co2+ and Gd3+ ions co-doped Zinc oxide were prepared by co-precipitation process followed by hydrothermal method. To obtain the desired crystallographic phase and also, to enhance the Oxygen vacancy, the as prepared sample was sintered at 400 °C in vaccum atmosphere. The presence of intrinsic defects and internal stress in the lattice structure of co-doped ZnO has been substantiated by the detail analysis of X-ray diffractograms. Different optical characterizations like Raman spectroscopy and photoluminescence (PL) spectra have been measured. The presence of defects/Oxygen vacancies are also critically analyzed by Raman and PL spectra of co-doped ZnO sample. Magnetic measurement such as magnetization as a function of magnetic field at different low temperatures (300-10 K) and magnetization as a function of temperature (field cooled and zero-field cooled) are measured by SQUID magnetometer. Interestingly, a very strong magnetic interaction and a large value of coercivity even at room temperature have been observed in the co-doped ZnO nanoparticles compared to the mono-doped ZnO nanoparticles. The presence of ferromagnetic ordering and large coercivity were successfully explained by the vacancy assisted bound-magnetic-polaron model and domain wall blowing theory. Negative exchange biased has been observed in our sample implies the formation and exchange interaction at the interface of FM/AFM bilayers. Dielectric measurement suggest that for both 5% Co2+ and Gd3+ ions co-doped ZnO system, the enhanced dielectric data is the contribution of large amount of space charge polarization and the intrinsic defects in the sample. Enhancement of the ac conductivity with the increase of temperature reveals the semiconducting nature of the co-doped ZnO systems.

The effect of boron on the doping efficiency of nitrogen in ZnO

Stable p-type boron–nitrogen codoped ZnO thin films were prepared by plasma-assisted molecular beam epitaxy. The incorporations of boron and nitrogen into ZnO, identified by X-ray diffraction and X-ray photoelectron spectroscopy, led to improvement in p-type conductivity of ZnO with respect to nitrogen monodoped ZnO. It was ascribed to the marked enhancement of nitrogen doping efficiency due to the existence of two types of chemical states of nitrogen atoms: the nitrogen substituting at oxygen site with nearest neighbor of Zinc atom (NO–Zn bonding), and the nitrogen locating at the nearest neighbor of boron atom (NO–B bonding). The strong interaction between the boron and nitrogen makes the complex of BZn-nNO (n = 3 or 4) be a shallow acceptor, which is responsible for the observed p-type behavior.

Lanthanum and zirconium co-doped ZnO nanocomposites: synthesis, characterization and study of photocatalytic activity.

Nanocomposits of zinc oxide co-doped with lanthanum and zirconium were prepared using the modified sol-gel method. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), diffuse reflectance spectroscopy (DRS), and BET surface area measurement. For comparison, the La and Zr mono doped ZnO have also been prepared under the same conditions. The XRD results revealed that all the materials showed a hexagonal wurtzite crystal structure. It was found that the particle size of La-Zr-doped ZnO is much smaller as compared to that of pure ZnO. The effect of operational parameters such as, doping concentration, catalyst loading, pH and initial concentration of methylene blue on the extent of degradation was investigated. The photocatalytic activity of the undoped ZnO, mono-doped and La-Zr-ZnO photocatalysts was evaluated by the photocatalytic degradation of methylene blue in aqueous solution. The presence of lanthanium and/or zirconium causes a red shift in the absorption band of ZnO. The results show that the photocatalytic activity of the La-Zr-ZnO photocatalyst is much higher than that of undoped and mono-doped ZnO, resulting from the La and Zr synergistic effect. The co-operation of the lanthanum and zirconium ion leads to the narrowing of the band gap and greatly improves the photocatalytic activity. The photocatalyst co-doped with lanthanum and zirconium 4 mol% shows the best photoactivity and photodecomposition efficiencies were improved by 92% under UV-Vis irradiation at the end of 30 min, compared with the pure and mono doped samples.

Tailoring the coercivity in ferromagnetic ZnO thin films by 3d and 4f elements codoping

Cluster free, Co (3d) and Eu (4f) doped ZnO thin films were prepared using ion implantation technique accompanied by post annealing treatments. Compared with the mono-doped ZnO thin films, the samples codoped with Co and Eu exhibit a stronger magnetization with a giant coercivity of 1200 Oe at ambient temperature. This was further verified through x-ray magnetic circular dichroism analysis, revealing the exchange interaction between the Co 3d electrons and the localized carriers induced by Eu3+ ions codoping. The insight gained with modulating coercivity in magnetic oxides opens up an avenue for applications requiring non-volatility in spintronic devices.

The synthesis and characterization of Ag–N dual-doped p-type ZnO: experiment and theory

Ag-N dual-doped ZnO films have been fabricated by a chemical bath deposition method. The p-type conductivity of the dual-doped ZnO:(Ag, N) is stable over a long period of time, and the hole concentration in the ZnO:(Ag, N) is much higher than that in mono-doped ZnO:Ag or ZnO:N. We found that this is because AgZn-NO complex acceptors can be formed in ZnO:(Ag, N). First-principles calculations show that the complex acceptors generate a fully occupied band above the valance band maximum, so the acceptor levels become shallower and the hole concentration is increased. Furthermore, the binding energy of the Ag-N complex in ZnO is negative, so ZnO:(Ag, N) can be stable. These results indicate that the Ag-N dual-doping may be expected to be a potential route to achieving high-quality p-type ZnO for use in a variety of devices.

Stable p-type ZnO films dual-doped with silver and nitrogen

Ag–N dual-acceptors doped p-type ZnO films were grown on glass substrates by the sol–gel method. The influence of dual-doping on the structural, electrical, and optical properties of samples was investigated in detail. The p-type conductivity of ZnO:(Ag,N) film is long-time stable. The resistivity of dual-doped ZnO:(Ag,N) film is much lower than that of mono-doped ZnO:Ag and ZnO:N films. ZnO homostructural p–n junction was fabricated by depositing an n-type ZnO layer on a p-type ZnO:(Ag,N) layer. The current–voltage characteristics show typical rectifying behaviors. Moreover, the ZnO:(Ag,N) film exhibits a good c-axis orientation, a high transmittance in the visible region, and a strong ultraviolet emission at room temperature.

Room-temperature ferromagnetism in N+-implanted ZnO:Mn thin films

Mn–N co-doped ZnO films with wurtzite structure were fabricated by RF magnetron sputtering together with the ion-implantation technique. Then a post-annealing at 650°C for 10min in a N2 atmosphere was performed to activate the implanted N+ ions and recover the crystal quality, and a p-type ZnO:Mn–N film with a hole concentration of about 2.1×1016cm−3 was obtained. It is found that the Mn mono-doped ZnO film only exhibits paramagnetic behavior, while after N+-implantation, it shows ferromagnetism at 300K, and the magnetization of the ZnO:Mn–N films can be further enhanced by thermal annealing due to the activation of the N acceptors. Our experimental results confirm that the codoping N acceptors are favorable for ferromagnetic ordering of Mn2+ ions in ZnO, which is consistent with the recent theoretical calculations.

In assisted realization of p-type C-doped ZnO: A first-principles study

Abstract C mono-doped and C–In co-doped ZnO are investigated by the first-principles calculations. It is found that the C mono-doped ZnO is p-type with hole carriers locating nearby valence band maximum. Furthermore, a shallower C acceptor energy level appears in the band gap after incorporating In into C-doped ZnO system. Meantime, compared with C mono-doped ZnO, C–In co-doped ZnO has a lower formation energy, correspondingly a higher chemical stability, and thus to enhance the incorporation efficiency of C. These results suggest that C–In co-doping method provided an efficient technique for realizing p-type ZnO.

Electronic structure and magnetic properties of Cr monodoped and (Cr, Al) codoped ZnO

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of Cr monodoped and (Cr, Al) codoped in ZnO. The results indicate that Cr monodoped in ZnO favors a spin-polarized state with a total magnetic moment of 7.50μ B per supercell and the magnetic moment mainly comes from the unpaired 3d electrons of Cr atoms. In addition, it was found that the ferromagnetic exchange interaction between Cr atoms is short-ranged in Cr monodoped ZnO. Interestingly, the ferromagnetic stability can be enhanced significantly by codoping Al Zn. We think that the enhancement of ferromagnetic stability should be attributed to the additional electrons introduced by Al Zn codoping.

Dual codoping for the fabrication of low resistive p-ZnO

A dual codoping method has been proposed to fabricate low resistive and stable p-ZnO thin films. Both nitrogen (N) and arsenic (As) have been used as acceptors while aluminum (Al) as donor in our dual codoping process. The As–Al–N dual codoped ZnO films have been prepared by RF magnetron sputtering on GaAs substrate using AlN doped ZnO targets (0.5, 1 and 2mol%). In our dual codoping approach, Al and N from target and As from GaAs substrate (back diffusion) take part. X-ray diffraction (XRD), room temperature and low temperature photoluminescence (PL), electron probe micro analysis (EPMA), energy dispersive spectroscopy (EDS), atomic force microscopy (AFM) and Hall effect measurement have been performed to investigate the effect of AlN concentration on the dual codoped ZnO films. All the films (0, 0.5 and 1mol%) showed p-type conductivity except 2mol% AlN doped film. The lowest room temperature resistivity, 8.6×10−2Ωcm has been achieved with a hole concentration of the order, 1020cm−3 for the optimum 1mol% AlN concentration. The observed resistivity is much lower than that of monodoped (As or N) and codoped (AlN or AlAs) ZnO films. The p-type conductivity has been explained by the new complex formation mechanism.

A Codoping Route to Realize Low Resistive and Stable p-Type Conduction in (Li, Ni):ZnO Thin Films Grown by Pulsed Laser Deposition

We report on the growth of Li-Ni codoped p-type ZnO thin films using pulsed laser deposition. Two mole percent Li monodoped ZnO film shows highly insulating behavior. However, a spectacular decrease in electrical resistivity, from 3.6 × 10(3) to 0.15 Ω cm, is observed by incorporating 2 mol % of Ni in the Li-doped ZnO film. Moreover, the activation energy drops to 6 meV from 78 meV with Ni incorporation in Li:ZnO lattice. The codoped [ZnO:(Li, Ni)] thin film shows p-type conduction with room temperature hole concentration of 3.2 × 10(17) cm(-3). Photo-Hall measurements show that the Li-Ni codoped p-ZnO film is highly stable even with UV illumination. XPS measurements reveal that most favorable chemical state of Ni is Ni(3+) in (Li, Ni): ZnO. We argue that these Ni(3+) ions act as reactive donors and increase the Li solubility limit. Codoping of Li, with other transitional metal ions (Mn, Co, etc.) in place of Ni could be the key to realize hole-dominated conductivity in ZnO to envisage ZnO-based homoepitaxial devices.

Influences of unintentionally doped carbon on magnetic properties in Mn–N co-doped ZnO

Carbon impurities are usually unintentionally doped in N-doped ZnO when attempting to realize p-type conductivity by metal-organic chemical vapor deposition. Mn–N co-doping technique, which is developed to realize hole-mediated room temperature ferromagnetism in ZnO, may further enhance the carbon incorporation. In this work, two kinds of Mn–N co-doped samples, grown at low temperature (400 °C) and high temperature (600 °C), respectively, have been compared to study the influences of carbon impurities. In contrast to that found on N mono-doped ZnO, an enhanced incorporation of carbon impurities is observed in the high-temperature-grown Mn–N co-doped sample with the conductivity changed from p to n type. According to X-ray photoelectron spectroscopy measurement, the compensation effect from carbon impurities is applied to elucidate the origin of the conductivity transition. Correspondingly, superconducting quantum interference device measurement certainly shows a much smaller value of the saturation magnetization for the high-temperature-grown sample. A possible effect from carbon related complexes on magnetization is also proposed to explain the decrease of the magnetic moment from the view of weakened spin polarization induced by unintentionally incorporated carbon. This assumption is further supported by the first-principle calculation on the Mn–N co-doped ZnO system with carbon incorporation.

Preparation of Li and Er codoped ZnO thin films and their photoluminescence

Li–Er codoped ZnO thin films have been prepared on Si(100) substrates by pulsed laser deposition (PLD). Both the as-grown and post-annealed films exhibit good crystalline quality with preferred c-axis orientation. After post-annealing at 850 °C, the photoluminescence (PL) related to intra-4 f shell of Er 3+ can be clearly observed. The Li–Er codoped ZnO film shows higher intensity of PL around 1.54 μm than the Er monodoped ZnO film. The behavior is attributed to the lowering of the symmetry of the crystal field around Er 3+ ions by introducing Li + into ZnO lattice, which is also confirmed by Raman scattering spectra.

Temperature-dependent photoluminescence and photoluminescence excitation of aluminum monodoped and aluminum-indium dual-doped ZnO nanorods

The authors report fabrication of aluminum monodoped ZnO (AlZnO) and aluminum-indium dual-doped ZnO (AlInZnO) nanorods arrays. Optical properties of AlZnO and AlInZnO nanorods are studied through temperature-dependent photoluminescence (PL) and PL excitation (PLE). Compared to AlInZnO nanorods, AlZnO nanorods possess better PL properties, as evidenced by a higher ratio of intensity of band-edge emission to green emission at 10 K and a higher PL intensity at room temperature. As supported by x-ray diffraction patterns, AlZnO nanorods also have higher crystallinity than AlInZnO nanorods. Indium doping induces a pronounced donor-acceptor pair transition of ∼3.22 eV at 10 K, the mechanism of which is discussed. Temperature-dependent energies of the A free exciton (FXA) and neutral donor bound exciton (D0X) are analyzed and the Einstein temperature is deduced to be ∼310 K. An activation energy of ∼8 meV is determined from the quenching of D0X as a function of temperature in AlInZnO nanorods. It is interpreted that nonradiative centers caused by indium segregation result in the small activation energy. Moreover, temperature-dependent PLE of AlZnO and AlInZnO nanorods reveals that the donor levels of aluminum and indium are 75 and 102 meV, respectively. Considering that the donor level of Al is shallower than that of In and that the optical and crystal properties of AlZnO nanorods are better than those of AlInZnO nanorods, aluminum is a better n-type dopant than indium for ZnO nanorods.