Co-doped ZnO Films Research Articles

Effect of Sputtering Pressure on the Properties of Boron and Gallium Co-doped ZnO Thin Films

B and Ga co-doped ZnO films were fabricated by RF magnetron sputtering method. The effects of sputtering pressure on the electrical, optical, structural and morphological properties of the films (BGZO) were investigated. As sputtering pressure increased up to 6mTorr, the film crystallinity was improved. At the sputtering pressure of 6mTorr, the films showed smooth and dense of film surface, lower resistivity and higher Hall mobility. It was also observed that all films showed high transparency in the visible range. The film showed blue shift of optical band gap with increaseing of sputtering pressure.

Ferromagnetic mechanism of (Co, Cu)-codoped ZnO films with different Co concentrations investigated by X-ray photoelectron spectroscopy

Cobalt/copper-codoped ZnO nanoparticles, synthesized with different Co concentrations by a sol–gel method using ethanol as solvent, were studied via XPS. Hexagonal wurtzite structure was found in all samples, with no evidence of any secondary phase. The average crystallite size of the samples was around 20–30nm, altered significantly with increasing Co concentration. Copper ions and Cobalt ions are indeed substituted into the ZnO lattice at the Zn2+ site, as shown by XRD and XPS. Further studies showed dramatic changes of Cu valence from +2 to +1 as the Co concentration level exceeds 1%, accompanied by a blue-shift of the optical bandgap from 3.01 to 3.13eV. Ferromagnetism of the Co-doped Zn0.95Cu0.05O thin films was observed and found to be tunable – a phenomenon associated with the valence state of the Cu ions and the existence of some defects like oxygen vacancies in the films.

Co-doped ZnO dilute magnetic semiconductor thin films by pulsed laser deposition: Excellent transmittance, low resistivity and high mobility

The 120-nm-thick Co-doped ZnO (CZO) dilute magnetic semiconductor films were grown by pulsed laser deposition at various substrate temperatures (Ts) of 100–700 °C. During the film's growth, Ar or O2 or Ar/O2-mixed gas atmosphere was used. Hall measurements indicate the CZO films prepared in Ar atmosphere possess the lower resistivity. Then, the microstructural, optical, electrical and magnetic properties of the CZO films deposited in Ar atmosphere were investigated in detail. For the deposition in Ar atmosphere, the lowest resistivity of 4.30 × 10−2 Ω-cm appeared in the 400 °C-grown CZO film since it had more oxygen vacancies with 0 charge state (VO0). As the Ts was raised to 100–300 °C, the CZO films had low mobilities of 5.1–10.7 cm2/V. This is attributed to the formation of oxygen vacancies with 2+ charge state (VO2+), resulting from the appearance of lattice distortions in these films. A significant relaxation of lattice distortion occurred in the 400 °C-grown CZO film, causing the efficient reduction in the amount of VO2+. Therefore, the 400 °C-grown film had a relatively high mobility of 21.0 cm2/V. With increasing the Ts to 400–700 °C, the transmittances (@450 nm) of 84.1%–95.8% were obtained in the CZO films. Furthermore, the saturation magnetization values of 100, 400 and 700 °C-grown CZO films were 2.74 × 10−5, 3.37 × 10−5 and 5.33 × 10−5 emu, respectively. These results suggest the CZO films are highly potential for spintronic and optoelectronic applications, especially for the 400 °C-grown film.

Fabrication and photoelectric properties of Er3+ and Yb3+ co-doped ZnO films

In this paper, the Er3+ and Yb3+ co-doped ZnO films deposited by a novel thermal decomposition method under different annealing temperature process have been reported. The effects of annealing temperature on the morphology and properties of the films are systematically studied. The resulting spectra demonstrate that the Er3+ and Yb3+ co-doped ZnO films possessed the property of up-conversion, converting IR light into visible light that can be absorbed by amorphous silicon solar cell. After all, inner photoelectric effect of the Er3+ and Yb3+ co-doped ZnO films in the amorphous as a light scattering layer are also found with an infrared 980 nm laser as excitation source.

Effect of hydrogen doping on the properties of Al and F co-doped ZnO films for thin film silicon solar cell applications

Aluminum and fluorine co-doped zinc oxide (AFZO) thin films were prepared in Ar+H2 atmospheres by rf magnetron sputtering at room temperature. The structural, electrical, and optical properties of the prepared films were investigated using X-ray diffraction, scanning electron microscopy, atomic force microscopy, Hall-effect measurement, X-ray photoelectron spectroscopy, and ultraviolet–visible spectrometry, and their dependence on deposition atmosphere (i.e. H2/(H2+Ar) ratio) was studied. The resulting films showed a (0 0 2) diffraction peak, indicating a typical wurtzite structure, and the optimal film crystallinity was obtained with the H2/(H2+Ar) ratio of 3%. The electrical resistivity of AFZO films decreased to 9.16×10−4Ω-cm, which was lower than ZnO:Al and ZnO:F films due to double doping effect of Al and F. The resistivity further decreased to below 5×10−4Ω-cm for the AFZO film with the H2/(H2+Ar) ratio of 3%–5%. All the films regardless of hydrogen content displayed high transmittances (>92%) in the visible wavelength range. Applying the developed AFZO films as front transparent electrodes, amorphous Si thin film solar cells were fabricated and the open-circuit voltage, fill factor, and efficiency of the cell with the hydrogenated AFZO film were improved in contrast to those without the hydrogenated film.

Effect of sol pH on microstructures, optical and magnetic properties of (Co,Fe)-codoped ZnO films synthesized by sol–gel method

Cobalt-iron-codoped ZnO films were synthesized with different precursor sol pHs by a sol–gel method. All the samples possess hexagonal wurtzite structure, with no evidence of secondary phases. The average crystallite size of the samples was in the range of 10–40 nm, accompanying significant differences with the increase of the H+ ion level. The Co/Fe ions are indeed substituted at the Zn2+ site into the ZnO lattice, as shown by XRD and XPS. Strong ultraviolet emission, blue double emission and weak green emission peaks are observed in the photoluminescence spectra of all samples at room temperature. Further studies reveal that the bandgap energy of the ZnO films increases from 3.15 eV (pH = 5) to 3.24 eV (pH = 9). Enhanced room-temperature ferromagnetism is observed for the (Co, Fe)-codoped ZnO films at sol pHs lower than 8. Our work clearly establishes that the origin of room-temperature ferromagnetism in the (Co,Fe)-codoped ZnO films is linked to O vacancies, tunable by the sol pH.

Structural, optical and magnetic properties of pulsed laser deposited Co-doped ZnO films

Zn1−xCoxO films with different Co concentrations (with x=0.00, 0.10, 0.15, and 0.30) were grown by pulsed laser deposition (PLD) technique. The structural and optical properties of the films were investigated by grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy and photoluminescence (PL). The magnetic properties were measured by conventional magnetometry using a SQUID and simulated by ab-initio calculations using Korring–Khon–Rostoker (KKR) method combined with coherent potential approximation (CPA). The effect of Co-doping on the GIXRD and Raman peaks positions, shape and intensity is discussed. PL studies demonstrate that Co-doping induces a decrease of the bandgap energy and quenching of the UV emission. They also suggest the presence of Zn interstitials when x≥0.15. The 10% Co-doped ZnO film shows ferromagnetism at 390K with a spontaneous magnetic moment ≈4×10−5emu and coercive field ≈0.17kOe. The origin of ferromagnetism is explained based on the calculations using KKR method.

The Third-Order Nonlinear Optical Properties in Cobalt-Doped ZnO Films**Supported by National Basic Research Program of China under Grant Nos 2011CB922200 and 2013CB922303.

We quantitatively investigate the third-order optical nonlinear response of Co-doped ZnO thin films prepared by magnetron sputtering using the Z-scan method. The two-photon absorption and optical Kerr effect are revealed to contribute to the third-order nonlinear response of the Co-doped ZnO films. The nonlinear absorption coefficient β is determined to be approximately 8.8 × 10−5 cm/W and the third-order nonlinear susceptibility χ(3) is 2.93 × 10−6 esu. The defect-associated energy levels within the band gap are suggested to be responsible for the enhanced nonlinear response observed in Co-doped ZnO films.

The effects of thickness on properties of B and Ga co-doped ZnO films grown by magnetron sputtering

We investigated the effects of thickness on the electrical, optical, structural and morphological properties of B and Ga co-doped ZnO (BGZO) films grown by radio frequency (RF) magnetron sputtering. All the prepared BGZO films showed preferentially c-axis orientation and structure of hexagonal wurtzite. The results also indicated that with an increase in film thickness, the crystallite sizes of the films were increased and the optical band gap (Eg) was decreased. Below a critical thickness of about 210nm, the thickness of the BGZO films significantly affected the electrical properties of the films. The average transmittance for all the grown films did not change obviously with the thickness.

The effect of growth conditions, point defects and hydrogen on the electronic structure and properties of p-type (Al,N) codoped ZnO: A first principles study

The effects of point defects, hydrogen, and growth conditions on the electronic structure and properties of the (Al,N) codoped p-type ZnO have been investigated using the first principles method. The obtained results showed that the AlZn–NO–VZn complex is a shallow acceptor that can play an important role in achieving the p-type conductivity in the (Al,N) codoped ZnO films. Our results showed also that the electrical conductivity type in the (Al,N) codoped ZnO films strongly depends on the donor/acceptor concentrations ratio. The codoped ZnO films prepared under both Zn-rich and O-rich growth conditions with a donors/acceptors ratio of 1:2 have a p-type conductivity, while those prepared with a ratio of 1:1 cannot be p-type unless if they are prepared under O-rich conditions. The achieved p-type quality depends also on the used nitrogen doping source. To prepare p-type ZnO film of high quality using the (Al,N) codoping method, the use of NO or NO2 is recommended. The presence of donor defects such as oxygen vacancies and hydrogen will significantly affect the electronic properties of the (Al,N) codoped ZnO films, and if the concentration of these defects in the sample is high enough, the material can be easily converted to n-type.

Effect of N and Cu Doping on Structure, Surface Morphology and Photoluminescence Properties of ZnO Thin Films

Thin films of ZnO, ZnO:Cu, ZnO:N and ZnO:(Cu,N) have been deposited on glass substrate at temperature 350ºC by low cost spray pyrolysis (SP) technique at an ambient atmosphere. The X-ray diffraction (XRD) study revealed that the films are of mono-phasic polycrystalline in nature having wurtzite ZnO crystal structure. The preferential orientation of un-doped ZnO films is in the (002) plane which changes to (101) for N and Cu mono-doped and (Cu, N) co-doped ZnO films. Surface morphology studied by scanning electron microscopy (SEM) clearly shows the formation of rock shaped nanostructures and hexagonal crystalline grains. The atomic force microscopy (AFM) study revealed the formation of crystalline grains perpendicular to the surface and reduction of surface roughness with doping. The photoluminescence (PL) spectra of the un-doped and doped ZnO samples show well defined transitions of the near band edge (NBE) and deep level (DLE) emissions emerged from different defect states present in the films.

Effect of Rare-Earth Doping on Structural and Luminescent Properties of Screen-Printed ZnO Films

The effect of Sm3+ and/or Ho3+ doping on structural and luminescent properties of screen-printed ZnO films sintered at 1000°C was investigated by photoluminescence (PL), PL excitation and Raman scattering methods. For all the films, ultraviolet excitonic and visible defect-related PL bands of ZnO were detected. The doping with Ho3+ ions produced an enhancement of PL in ZnO films, the excitonic PL intensity being increased prominently, while the co-doping with Sm3+ and Ho3+ ions resulted in PL decrease in ZnO films. Only for (Sm,Ho)- co-doped ZnO films, the rare-earth PL bands were detected. The reduction of Sm3+ to Sm2+ was observed demonstrating 5D0→7FJ radiative transitions. The mechanism of PL and PL excitation is discussed in terms of the formation of rare-earth complexes as well as energy transfer towards them from ZnO host.

Annealing in tellurium-nitrogen co-doped ZnO films: The roles of intrinsic zinc defects

In this article, the authors have conducted an extensive investigation on the roles of intrinsic zinc defects by annealing of a batch of Te-N co-doped ZnO films. The formation and annihilation of Zn interstitial (Zni) clusters have been found in samples with different annealing temperatures. Electrical and Raman measurements have shown that the Zni clusters are a significant compensation source to holes, and the Te co-doping has a notable effect on suppressing the Zni clusters. Meanwhile, shallow acceptors have been identified in photoluminescence spectra. The NO-Zn-Te complex, zinc vacancy (VZn)-NO complex, and VZn clusters are thought to be the candidates as the shallow acceptors. The evolution of shallow acceptors upon annealing temperature have been also studied. The clustering of VZn at high annealing temperature is proposed to be a possible candidate as a stable acceptor in ZnO.

Effect of magnesium and fluorine dopants on properties of ZnO thin films

Effects of Mg and F codoping on properties of ZnO films were studied in this work. F and Mg codoped ZnO films with a fixed Mg concentration of 20at% and 0–8at% F concentration were deposited on microscope glass substrates by the ultrasonic spray pyrolysis. The morphology of all films showed a spherical grain shape, and F doping decreased roughness with the addition of F concentration as well as decreased grain size with doping higher than 4at% of films. The XRD patterns of all films were identified as a polycrystalline hexagonal wurtzite structure of ZnO with (002) preferred orientation. The band gap of 0at% F doping film was 3.69eV, which decreased to a minimum with 2at% F doping and increased again with higher than 2at% F doping. F doping decreased resistivity to a minimum with addition until 4at% F doping and then increased again with higher than 4at% F doping.

Properties of In–N codoped p-type ZnO nanorods grown through a two-step chemical route

By codoping with a donor–acceptor pair through a two-step chemical method we have succeed to obtain p-type ZnO thin films on glass. Firstly, a thin undoped ZnO seed layer was deposited by sol–gel method followed by the deposition of In–N codoped ZnO film obtained through the hydrothermal technique. The influence of post-deposition annealing temperature (100°C, 300°C and 500°C) on the samples was investigated from a structural, chemical, morphological and optoelectrical point of view. X-ray diffractometry (XRD), infrared ellipsometry and X-ray photoelectron spectroscopy (XPS) analyses have confirmed the codoped nature of the ZnO thin films. The XRD pattern analysis has established the films have wurtzite nanocrystalline structure, the crystallite sizes varying between 10nm and 13nm with the annealing temperature. Continuous and homogenous films with nanorods surface morphology has been obtained, as visualized by scanning electron microscopy measurements. Hall Effect measurements have established that all samples, regardless of annealing temperature, showed p-type conduction due to the successful incorporation of nitrogen in the film, with the highest carrier concentration registered at 500°C. This is in good correlation with the nitrogen content in the films as revealed from XPS. In all samples, the XPS depth profiling has shown a nitrogen gradient with higher elemental concentration at the surface.

磁控溅射法制备钴掺杂氧化锌薄膜室温磁学性质

磁控溅射法制备钴掺杂氧化锌薄膜室温磁学性质

Optical Properties of Al-and Mo-Doped and Codoped ZnO Films Deposited by Sol-Gel Process

Enhancing the optical transmission of ZnO film is an important topic for its application. This work presents the manufacture and potential application of the ZnO materials containing various dopants in recent literatures and patents and then focuses on the comparison of optical properties of Al- and Mo-doped and codoped ZnO films. The films were deposited on quartz glass substrate by a sol-gel process, and characterized by X-ray diffraction, atomic force microscopy, atomic force microscopy, and UV-vis and luminescent spectrophotometries. The Mo-doping was more effective in increasing transmission and widening band gap than the Al-doping, while the codoping was more effective than single dopings. The films also showed strong ultraviolet, violet and bluish violet emission. Keywords: Chemical preparation, codoping, doping, film, photoluminescence, transmittance enhancement, ZnO.

Resistive switching characteristics and conduction mechanisms of nonvolatile memory devices based on Ga and Sn co-doped ZnO films

Nonvolatile memory devices were fabricated utilizing Ga and Sn co-doped ZnO (GZTO) films formed by using a solution process method. X-ray diffraction patterns showed that the crystallinity of the annealed GZTO films was an amorphous phase. X-ray photoelectron spectroscopy spectra of the GZTO films depicted ZnO, GaO, and SnO bonds. Current–voltage measurements on the Al/GZTO/indium-tin-oxide (ITO) devices at 300K showed bipolar resistive switching behaviors. The resistances at both the low resistance state (LRS) and high resistance state (HRS) measured at 0.5V for the devices maintain almost constant without any damage and breakdown above 130s, indicative of the memory stability of the devices. A difference in the resistance between the HRS and the LRS was more than 1 order of the magnitude. The conduction mechanisms of the HRS in the set process for the Al/GZTO/ITO devices were dominated by a space-charge-limited current model.

Effects of N- and N-In doping on ZnO films prepared by using ultrasonic spray pyrolysis

The effects of N-doping, and N-In co-doping on ZnO films were studied by analyzing the structural, electrical, and optical properties of the films prepared by using an ultrasonic spray pyrolysis (USP) method. According to scanning electron microscopy (SEM) data, all films had very complex surface structures. Their polycrystallinity were also proven by using an X-ray diffraction method. The Hall-effect measurement showed that both the undoped and the N-doped ZnO films exhibited n-type conductivity and that the N-In co-doped ZnO film showed p-type conductivity. In the extended X-ray absorption fine structure (EXAFS) analysis, the number of oxygen atoms in the N-In codoped ZnO films was found to be larger than that in the N-doped and the undoped ZnO films. The photoluminescence spectra also showed that the N-In co-doping suppressed the concentration of oxygen vacancies in the ZnO films. Through an effective incorporation of indium atoms, more oxygen atoms seem to have been introduced into the lattice of the N-In co-doped ZnO films.

Fabrication and properties of Al–P codoped p-type zinc oxide films by RF magnetron sputtering

Al–P codoped ZnO (ZnO:(Al, P)) films were grown on quartz substrate by radio frequency magnetron sputtering using mixture of argon and oxygen as sputtering gas. Types of conduction and electrical properties in ZnO:(Al, P) films were found to be dependent on oxygen partial pressure ratios in the sputtering gas mixture. When oxygen partial pressure ratio is 70 %, the codoped ZnO film had the best p-type conduction properties. It has room-temperature resistivity of 1.4 × 10−1 Ω cm, Hall mobility of 13.5 cm2/Vs and carrier concentration of 3.7 × 1018 cm−3. The p-type conduction behavior of the ZnO:(Al, P) film was confirmed by the rectifying I–V characteristics of the p-ZnO:(P, N)/n-ZnO homojunction. The phosphorus-related acceptor energy level was estimated to be located 126 meV above the valence band. Mechanism of the p-type conductivity was discussed in the present work.