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Microstructures and native defects are very important for the optical, electrical and magnetic properties of ZnO-based materials. In this study, microstructures of the Cu-doped ZnO (ZnO:Cu) films were changed by applying high magnetic field (HMF) during different growth processes of the as-deposited Zn-Cu films and the subsequent thermal oxidation treatment. The effects of the microstructure evolutions on the defects formation and photoluminescence performance are examined. The results show that the interaction between magnetization and thermal motion promotes (002) preferred orientation of the as-deposited Zn-Cu films. Meanwhile, the variation of the atomic deposition sites due to the HMF leads to the significant differences of surface morphology of the as-deposited films. The application of the HMF during the oxidation affects the adsorption sites of O 2− and leads the surface spherical particles change to rod-like particles. Moreover, the HMF shows more significant effects during the deposition of the Zn-Cu films in enhancing on the amount of the oxygen vacancy, which improves green emission of the ZnO:Cu films. [ABSTRACT FROM AUTHOR]

Sc 2 O 3 modified Bi 2 O 3 -ZnO films varistors with different doping ratios were prepared by Sol-gel method. The result indicated that doping of Sc 2 O 3 significantly enhance the nonlinear current-voltage ( I-V) characteristics of Bi 2 O 3 -ZnO films. The highest non-linearity was obtained for 0.4mol% Sc content with the nonlinear coefficient α = 3.7 and 301 μA in leakage current. The grain size of ZnO phase firstly increased with Sc 2 O 3 doping when the content of Sc 2 O 3 is no more than 0.2 mol%. Upon the much more doping of Sc 2 O 3 , the growth of ZnO crystal was inhibited, leading to smaller grain size. The threshold voltage V T was inversely proportional with grain size of ZnO phase. Doping oxides gathered in the grain boundary significant impact on threshold voltage and leakage current. [ABSTRACT FROM AUTHOR]

The thin films of zinc oxide (ZnO) were grown on sapphire (0 0 1) substrates using pulsed laser deposition (PLD) in O 2 gas ambient and at different growth (substrate) temperatures (200, 300, 400, 500 and 600 °C). The effect of growth temperature on the crystallographic and morphological and photoluminescence properties of the films were investigated using X-ray diffractometer (XRD), field emission scanning electron microscopy (FE-SEM) and photoluminescence spectrometer. All the films reveal a wurtzite ZnO structure. The XRD results are well supported by the surface morphological and photoluminescence studies. The film characteristics were interrelated to their ultraviolet (UV) photo-sensing properties. The UV photodetection properties of the films were investigated at room temperature in metal–semiconductor–metal (MSM) planar configurations and are observed to be strongly driven by the growth temperature dependent crystallographic properties. The dark-current and the photocurrent of the ZnO film-based UV photodetectors are relational to the crystallite/grain size and the quality of ZnO thin films. For the photodetector based on ZnO film with a greater crystallite size, a lower dark current, and a higher photocurrent were achieved under 5 V bias voltage. The time-dependent photoresponse investigations illustrate a highly stable and fast switching UV photoresponse for the photodetector with ZnO film grown at 400°C growth temperature, which exhibits the maximum responsivity of ∼21.65 mA/W upon 2 mW/cm 2 UV light exposure (365 nm), at a bias voltage of 5 V. [ABSTRACT FROM AUTHOR]

Highly-doped ZnO:Al,N films were grown under oxygen-rich conditions on Si substrates by magnetron sputtering using a layer-by-layer growth technique. An investigation of the highly-doped ZnO:Al,N films is attractive for obtaining p -type conductivity in ZnO films as well as for an improvement of performance of ZnO-based ultraviolet (UV) detectors. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), X-ray emission spectroscopy (XES) and Secondary ion mass spectrometry (SIMS) were used for the samples characterization. An effect of high Al and N doping on structure and electronic properties of ZnO films was studied and discussed. [ABSTRACT FROM AUTHOR]

This paper reports on the study of ZnO films grown by chemical spray pyrolysis on glass and Si substrates at temperatures of 300, 350 and 400 °C and on the performances of the corresponding Au/n-ZnO Schottky diodes. XRD measurements have shown that all films are in single phase and have a wurtzite crystal structure. Microstructural properties such as lattice constants ( a = b ≠ c ), unit cell volume, texture, dislocation density, standard deviation have been determined. The crystal size and the microstrain have also been calculated by taking X-ray line broadening profile into account. Besides, the optical features have been investigated by UV–Vis measurements. The optical band gap was found dependent on the temperature of the substrate during film growth and decreased from 3.28 to 3.24 eV in case of films grown at 300 and 400 °C, respectively. Electrical characterizations of Au/n-ZnO Schottky barrier diodes have been investigated using the current-voltage ( I-V ) and the capacitance-voltage ( C-V ) measurements. Results show that the ideality factor firstly increased for the film grown at 350 °C, then decreased for the film grown at 400 °C. Also, barrier height decreased with the increase of substrate temperature. This behaviour has been attributed to the inhomogeneous nature of barrier height, to variation of interface states and to the formation of a new phase on ZnO surface. [ABSTRACT FROM AUTHOR]

This study describes the facile synthesis of ZnO nanolayers by the sol-gel process and evaluates their subsequent surface passivation with ZnS by gas-phase sulfidation. The ZnO nanolayers were formed by a spin-coating technique following calcination in air in the temperature range 500–900 °C to obtain a polycrystalline wurtzite structure of ZnO. The layer thickness was optimized by adjusting the spin-coating speed and concentration of polymer in the starting solution. The continuity and formation of the films was influenced by ZnO crystal size, which was found to increase with increasing calcination temperature. The ZnO nanostructure was covered by a thin ZnS layer in a one-step passivation process carried out at room temperature in the presence of H 2 S. This thin coating of ZnS was found to have a sphalerite structure. The cathodoluminescence intensity of near-band-gap emission of the coated crystal was one order of magnitude greater following the passivation of surface defects. [ABSTRACT FROM AUTHOR]

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 V Zn or Li Zn acceptor states. This work presents a practical route to fabricate inexpensive multi-coloured light emitting coatings on any substrate. [ABSTRACT FROM AUTHOR]

Although one-dimensional ZnO nanodevices have already been widely used in practical applications, there still exists limitation of inevitable growth along polar orientation for wurtzite ZnO. Herein, ZnO films grown along diverse preferred orientations including the non-polar, semi-polar and polar directions could be acquired by a facile and reproducible low-temperature aqueous solution deposition method. The non-epitaxial growth pattern was realized by introducing moderate amount of sodium citrate and ammonia into the non-polar solution without any surfactant or passivated agent. Preferential growth mechanism and surface properties of as-prepared anisotropic ZnO films were investigated. Transmission spectra of ZnO films with anisotropic preferred orientations exhibited an isotropic transmittance. Visible emission spectra indicated multifarious defect types and quantities related to diverse preferred orientations of anisotropic ZnO films. This work could not merely enrich the crystallographic growth of ZnO films along anisotropic orientations, but also benefit for designing ZnO film-based nanodevices with desirable performances from the perspective of regulating anisotropic growth of ZnO. [Display omitted] • Anisotropic growth of ZnO films with diverse preferred orientations are realized. • ZnO films are synthesized by a facile and reproducible aqueous solution method without any surfactant or catalyst. • The mechanism, wettability and optical property originated by anisotropic growth pattern are investigated. • ZnO films grown along semi-polar orientations contribute to the formation of acceptor-type defects. [ABSTRACT FROM AUTHOR]

Although one-dimensional ZnO nanodevices have already been widely used in practical applications, there still exists limitation of inevitable growth along polar orientation for wurtzite ZnO. Herein, ZnO films grown along diverse preferred orientations including the non-polar, semi-polar and polar directions could be acquired by a facile and reproducible low-temperature aqueous solution deposition method. The non-epitaxial growth pattern was realized by introducing moderate amount of sodium citrate and ammonia into the non-polar solution without any surfactant or passivated agent. Preferential growth mechanism and surface properties of as-prepared anisotropic ZnO films were investigated. Transmission spectra of ZnO films with anisotropic preferred orientations exhibited an isotropic transmittance. Visible emission spectra indicated multifarious defect types and quantities related to diverse preferred orientations of anisotropic ZnO films. This work could not merely enrich the crystallographic growth of ZnO films along anisotropic orientations, but also benefit for designing ZnO film-based nanodevices with desirable performances from the perspective of regulating anisotropic growth of ZnO. [Display omitted] • Anisotropic growth of ZnO films with diverse preferred orientations are realized. • ZnO films are synthesized by a facile and reproducible aqueous solution method without any surfactant or catalyst. • The mechanism, wettability and optical property originated by anisotropic growth pattern are investigated. • ZnO films grown along semi-polar orientations contribute to the formation of acceptor-type defects. [ABSTRACT FROM AUTHOR]

We have optimized the photovoltaic properties of active ZnO/p-GaAs heterojunction solar cells by pulsed laser deposition method by varying the oxygen pressure from 0 to 50 mTorr during the fabrication process. We observed the crystallinity and grain size of ZnO enhanced with increasing oxygen pressure from 0 to 30 mTorr. In addition, with this increase of oxygen flux, the intensity of E 1 (LO) modes obtained by Raman measurements declines significantly and almost disappears under an oxygen pressure of 50 mTorr. The change of intensity is assumed to be the change of oxygen vacancy and zinc interstitial concentrations in ZnO films with oxygen pressures. Current-voltage measurements and extractions show that ZnO grown at 30 mTorr displays the best performance with the I SC of 24.3 mA/cm2, the efficiency of 8.746 %, and FF 0 of 68.73 %. The high performance of the heterojunction solar cell grown at the oxygen partial pressure of 30 mTorr might be due to the reduction of oxygen vacancies by increasing oxygen during the deposition. The results reveal the importance of the oxygen processing gas in promoting devices performance. • ZnO films are deposited by PLD on p-GaAs substrates. • Current-voltage measurements and extractions show that ZnO grown at 30 mTorr displays the best performance with the I SC of 24.3 mA/cm2, the efficiency of 8.746 %, and FF 0 of 68.73 %. [ABSTRACT FROM AUTHOR]

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 Zn 2+ 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. [ABSTRACT FROM AUTHOR]

Cu-doped ZnO thin films deposited on glass substrates by a sol–gel method with sol pH ranging from 6 to 10 were investigated. Characterization of the morphology and microstructures by scanning electron microscopy (SEM) and X-ray diffraction (XRD) revealed that the grain size decreases slightly with an increase of the pH value of the precursor sol. ZnO vary in their orientation along the preferred (0 0 2) plane, while the best c -axis orientation occurs in the thin film fabricated from the precursor sol with pH = 8.5. X-ray photoelectron spectroscope (XPS) and XRD results indicate that Cu ions are indeed substituted at the Zn 2+ site into the ZnO lattice. 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 show that adjusting the pH of the precursor sol can well tune the bandgap from 3.19 to 2.42 eV. [ABSTRACT FROM AUTHOR]

Abstract: In this work, ZnO films with high crystal quality were grown by pulsed laser deposition (PLD) on different c-plane AlN/c-sapphire template thereby the thicknesses of AlN buffer layers varied from 150 to 300nm. The properties of ZnO thin films were studied by using high-resolution X-ray diffraction, atomic force microscopy, photoluminescence spectroscopy, and Raman measurement. The comparative investigation results show that inserting an AlN buffer layer is an effective way to improve the crystal quality of ZnO films. Furthermore, the thickness of the AlN buffer layer plays an important role on the quality of ZnO films. The result of (0002) ω-rocking curve with the full width at half maximum (FWHM) of 0.09° indicates that high-quality c-plane ZnO films can be achieved by using a 150nm-thickness AlN/c-sapphire template. In the best knowledge of the authors, this is the minimum value reported at present for ZnO films grown on AlN/c-sapphire templates by PLD. [Copyright &y& Elsevier]

Abstract: Effects of S on solid solubility of Ag and electrical properties of Ag-doped ZnO films are investigated by experimental and first-principles studies. It is found that S alloying in ZnO can increase solid solubility of Ag substituting for Zn (AgZn) due to that Ag binds more easily with S than O, leading to that Ag–S codoped ZnO films gradually transform from n- to p-type conductivity with increasing S content. The low-temperature photoluminescence measurement and first-principles calculation indicate that there are two kinds of AgZn and AgZn–nSO acceptors in the p-type Ag–S codoped ZnO and the AgZn–nSO acceptor has lower formation energy and acceptor transition energy than AgZn. The p-type conduction of the Ag–S codoped ZnO comes dominantly from contribution of the AgZn–nSO acceptor. [Copyright &y& Elsevier]

Abstract: The Schottky contact of Pt/ZnO was formed by depositing ZnO films oriented along c-axis by pulsed-laser deposition on Pt/Ti buffer layer supported by SiO2/Si substrate. Effects of the post-annealing on the crystallinity, uniformity and native defects of ZnO film as well as Schottky contacts of Pt/ZnO films were investigated. Results show that the annealing can improve the crystallinity of ZnO film, suppress the native defects, and enhance the performance of Pt/ZnO Schottky contacts dramatically. The best Schottky diode shows the largest barrier height of 0.8eV with reverse leakage current of 1.5×10−5 A/cm2. The zero-biased photodetector based on the best Schottky diode possesses responsivity of 0.265A/W at 378nm, fast photo-response component with rise time of 10ns and fall time of 17ns. This report demonstrates possibility of ZnO films/Pt hetero-junction with large area Schottky contact, and establishes the potential of this material for use in UV photodetector devices. [Copyright &y& Elsevier]

Abstract: Chemical bath deposition (CBD) is used for depositing different metal chalcogenide and oxide thin films through an ion-by-ion or by adsorption of colloidal particles from the solution on the substrate. In this review, we focused our intension on CBD zinc oxide (ZnO) films synthesis with different proposed deposition mechanisms and their integration into gas sensor and dye-sensitized solar cells. Due to the use of CBD ZnO films in gas sensors first time by our group, we presented film thickness, gas concentration and palladium sensitizer effect on the gas sensing properties in details. Review on CBD grown ZnO implication in dye-sensitized solar cells is also discussed in brief. [Copyright &y& Elsevier]

Abstract: An undoped ZnO thin film with high resistivity was prepared by rf magnetron sputtering on the quartz substrates using a ZnO (99.999% pure) as target and high pure Ar as sputtering gas. Upon annealing in a temperature range from 513 to 923K under 10−3 Pa, the conductive properties of the film change from high resistivity, to n-type, then to p-type and finally to n-type with increasing annealing temperature and the p-type ZnO film was fabricated near 863K reproducibly. Temperature-dependent photoluminescence (PL) of the p-type ZnO shows a dominant PL band at 3.072eV at low temperature, which is related to Zn vacancy (VZn) acceptor. The intensity of the 3.072eV band decreases with increasing temperature, implying increment of amount of the hole induced by VZn. X-ray photoelectron spectroscopy (XPS) and room temperature PL measurements indicate that the undoped ZnO is Zn-rich and has oxygen vacancy (Vo) and interstitial Zn (Zni) donor defects. The amount of Vo and Zni donors changes with the annealing temperature. The intrinsic p-type conduction of the undoped ZnO film is ascribed to that the VZn acceptor concentration can compensate Vo and Zni donor concentration. [Copyright &y& Elsevier]

Abstract: ZnO films prepared by the thermal oxidation of the ZnS films through thermal evaporation are reported. The as-deposited ZnS films have transformed to ZnO films completely at 400°C. The 400–700°C annealed films with a preferential c-axis (002) orientation have a hexagonal wurtzite structure. The band gap of ZnO films shifts towards longer wavelength with the increase of the annealing temperature. The relationship between the band gap energy of ZnO films and the grain size is discussed. The shift of the band gap energy can be ascribed to the quantum confinement effect in nanocrystal ZnO films. The photoluminescence spectra of ZnO films show a dominant ultraviolet emission and no deep level or trap state defect emission in the green region. It confirms the absence of interstitial zinc or oxygen vacancies in ZnO films. These results indicate that ZnO film prepared by this simple thermal oxidation method is a promising candidate for optoelectronic devices and UV laser. [Copyright &y& Elsevier]