The role of the Zn buffer layers in the structural and photoluminescence properties of ZnO films on Zn buffer layers deposited by RF magnetron sputtering

Highly c-axis oriented ZnO thin films were grown on Si (100) substrates with Zn buffer layers. Effects of the Zn buffer layer thickness on the structural and optical qualities of ZnO thin films were investigated for the ZnO films with the buffer layers 90, 110, and 130 nm thick using X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analysis techniques. It was confirmed that the quality of a ZnO thin film deposited by RF magnetron sputtering was substantially improved by using a Zn buffer layer. The highest ZnO film quality was obtained with a Zn buffer layer 110 nm thick. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.     

Synthesis of ZnO flowers and their photoluminescence properties

Flower-like ZnO nano/microstructures have been synthesized by thermal treatment of Zn(NH₃)₄²⁺ precursor in aqueous solvent, using ammonia as the structure directing agent. A number of techniques, including X-ray diffraction (XRD), field emission scan electron microscopy (FESEM), transmission electron microscopy (TEM), thermal analysis, and photoluminescence (PL) were used to characterize the obtained ZnO structures. The photoluminescence (PL) measurements indicated that the as-synthesized ZnO structures showed UV (∼375 nm), blue (∼465 nm), and yellow (∼585 nm) emission bands when they were excited by a He-Gd laser using 320 nm as the excitation source. Furthermore, it has been interestingly found that the intensity of light emission at ∼585 nm remarkably decreased when the obtained ZnO nanocrystals were annealed at 600 °C for 3 h in air. The reason might be the possible oxygen vacancies and interstitials in the sample decreased at high temperature.     

Study on field emission and photoluminescence properties of ZnO/graphene hybrids grown on Si substrates

Zinc oxide/graphene (ZnO/G) hybrids are prepared on n-Si (1 0 0) substrates by electrophoretic deposition and magnetron sputtering technique. The crystal structure, morphology and photoluminescence (PL) properties of the ZnO/G hybrids are analyzed via X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and fluorescence–phosphorescence spectrometer, respectively. The results indicate that the crystal quality of ZnO nanostructure deteriorates after depositing graphene buffer layer. Whereas many three dimensional stacking blowballs form in the ZnO/G hybrid, creating a larger surface area than that of ZnO nanostructure. The photoluminescence (PL) spectrum of the ZnO/G hybrid contains multi-peaks, which are consistent with ZnO nanostructure except for two new peaks at 390 and 618 nm. In addition, field emission measurement reveals that E_to and E_thr decrease from 8.01 V μm⁻¹ and 14.90 V μm⁻¹ of the ZnO nanostructure to 2.72 V μm⁻¹ and 7.70 V μm⁻¹ of the ZnO/G hybrid. ZnO/G hybrid is characteristic of having excellent emitting behavior suitable for application in field emission technology.     

AlN codoping and fabrication of ZnO homojunction by RF sputtering

The ZnO homojunction fabricated from undoped and 1 mol% AlN doped (codoped) ZnO targets by RF magnetron sputtering has been reported. The grown films on Si (100) substrate have been characterized by X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), Photoluminescence (PL) and Hall measurements. The increase of d-space value (compared with unstressed bulk) found from XRD for AlN codoped ZnO film supports the formation of p-ZnO due to the N incorporation. The presence of N in the film has been confirmed by EDS and XPS analysis. Further, the p-conductivity in AlN codoped ZnO has been evidenced by low temperature PL (donor-acceptor-pair emission) and room temperature PL (red shift in near-band-edge emission). Hall measurement shows that 1 mol% AlN codoped ZnO has the hole concentration of 3.772 × 10¹⁹ cm⁻³. The fabricated homojunction with 1% AlN doped ZnO (p-type) and undoped ZnO (n-type) exhibits a typical rectification behavior with high breakdown voltage, and rectification ratio, 13.4. The junction parameters such as ideality factor, barrier height and series resistance have also been calculated for the fabricated p-n junction. The energy band diagram has been proposed for the fabricated homojunction.     

Synthesis and characterization of ZnO nanowires for nanosensor applications

In this paper we report the synthesis of ZnO nanowires via chemical vapor deposition (CVD) at 650 °C. It will be shown that these nanowires are suitable for sensing applications. ZnO nanowires were grown with diameters ranging from 50 to 200 nm depending on the substrate position in a CVD synthesis reactor and the growth regimes. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), and Raman spectroscopy (RS) have been used to characterize the ZnO nanowires. To investigate the suitability of the CVD synthesized ZnO nanowires for gas sensing applications, a single ZnO nanowire device (50 nm in diameter) was fabricated using a focused ion beam (FIB). The response to H₂ of a gas nanosensor based on an individual ZnO nanowire is also reported.     

Growth of a-b-axis orientation ZnO films with zinc vacancies by SSCVD

Zinc oxide (ZnO) films were grown on silicon (1 0 0) substrates by single-source chemical vapor deposition (SSCVD). X-ray diffraction (XRD) showed that ZnO thin films have a polycrystalline hexagonal wurtzite structure with (1 0 0) and (1 0 1) orientation, i.e., a-b-axis orientation. Atomic force microscopy (AFM) and scanning electronic microscopy (SEM) showed the films to be of relatively high density with a smooth surface. X-ray photoelectron spectroscopy (XPS) showed that the deposited films were very close to stoichiometry but contained a small number of zinc instead of O vacancies as normally found with ZnO films produced by other methods. These results were also confirmed by photoluminescence (PL) measurements.     

Defect induced room temperature ferromagnetism in well-aligned ZnO nanorods grown on Si (100) substrate

In this work, we report the fabrication of high quality single-crystalline ZnO nanorod arrays which were grown on the silicon (Si) substrate using a microwave assisted solution method. The as grown nanorods were characterized using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), photo-luminescence (PL) and magnetization measurements. The XRD results indicated that the ZnO nanorods are well oriented with the c-axis perpendicular to the substrate and have single phase nature with the wurtzite structure. FE-SEM results showed that the length and diameter of the well aligned rods is about ~ 1 μm and ~ 100 nm respectively, having aspect ratio of 20-30. Room-temperature PL spectrum of the as-grown ZnO nanorods reveals a near-band-edge (NBE) emission peak and defect induced green light emission. The green light emission band at ~ 583 nm might be attributed to surface oxygen vacancies or defects. Magnetization measurements show that the ZnO nanorods exhibit room temperature ferromagnetism which may result due to the presence of defects in the ZnO nanorods.     

Highly luminescent columnar ZnO films grown directly on n-Si and p-Si substrates by low-temperature electrochemical deposition

In this study, nanocolumnar zinc oxide thin films were catalyst-free electrodeposited directly on n-Si and p-Si substrates, what makes an important junction for optoelectronic devices. We demonstrate that ZnO thin films can be grown on Si at low cathodic potential by electrochemical synthesis. The scanning electron microscopy SEM showed that the ZnO thin films consist of nanocolumns with radius of about 150 nm on n-Si and 200 nm on p-Si substrates, possess uniform size distribution and fully covers surfaces. X-ray diffraction (XRD) measurements show that the films are crystalline material and are preferably grown along (0 0 2) direction. The impact of thermal annealing in the temperature range of 150-800 °C on ZnO film properties has been carried out. Low-temperature photoluminescence (PL) spectra of the as-prepared ZnO/Si samples show the extremely high intensity of the near bandgap luminescence along with the absence of visible emission. The optical quality of ZnO thin films was improved after post-deposition thermal treatment at 150 °C and 400 °C in our experiments, however, the luminescence intensity was found to decrease at higher annealing temperatures (800 °C). The obtained results indicate that electrodeposition is an efficient low-temperature technique for the growth of high-quality and crystallographically oriented ZnO thin films on n-Si and p-Si substrates for device applications.     

Effects of sapphire substrates surface treatment on epitaxial ZnO thin films grown by MOCVD

The surface treatment effects of sapphire substrate on the quality of epitaxial ZnO thin films grown by metal-organic chemical vapor deposition (MOCVD) were studied. The sapphire substrates have been investigated by means of atomic force microscopy (AFM) and X-ray diffraction rocking curves (XRCs). The results show that sapphire substrate surfaces have the best-quality by CMP with subsequent chemical etching. The surface treatment effects of sapphire substrate on the ZnO thin films were examined by X-ray diffraction (XRD), atomic force microscopy (AFM) and photoluminescence (PL) measurements. Results show that the intensity of (002) diffraction peak of ZnO thin films on sapphire substrates treated by CMP with subsequent chemical etching is strongest. FWHM of (002) diffraction peak is narrowest and the intensity of UV peak of PL spectrum is strongest, indicating surface treatment on sapphire substrate preparation may improve ZnO thin films crystal quality and photoluminescent property.     

Effect of doping on properties of Zno:Cu and Zno:Ag thin films

ZnO:Cu and ZnS thin films were grown by metal-organic chemical vapour deposition (MOCVD) under atmospheric pressure onto glass substrates. The ZnO:Ag films were fabricated from ZnS films by non-vacuum method that consists of simultaneous oxidation and Ag-doping by the close spaced evaporation (CSE) of silver at the temperature of 500–600 °C. Photo-assisted rapid thermal annealing (PARTA) at ambient air during 10–30 s at the temperature of 700–800 °C was used for the ZnO:Cu films. The samples were studied by X-ray diffraction technique (XRD), atomic force microscopy (AFM), and photoluminescence (PL) measurements. The grain size of ZnO:Cu films increased with an increase of Cu concentration. PL spectra of as-deposited ZnO:Cu films depended on Cu concentration and contained the bands typical for the copper. After PARTA at high temperature the emission maximum shifted towards the short-wave region. During the fabrication of ZnO:Ag films the grain growth process was strongly affected by the Ag loading level. The grain size increased with an increase of Ag concentration and ZnO:Ag films with surface roughness of 8 nm were obtained. Observed 385 nm PL peak for these samples can be attributed to the exciton–exciton emission that proves the high quality of the obtained ZnO:Ag films.     

The characteristics of ZnO deposited on PPC plastic substrate

In this paper, we report the deposition of ZnO thin film on poly propylene carbonate (PPC) plastic substrate by sputtering technique. The structural, optical and electrical properties of the ZnO thin film were investigated. The ZnO thin film deposited on PPC plastic has a smooth surface morphology as revealed by scanning electron microscopy (SEM). X-ray diffraction (XRD) measurement shows that the ZnO thin film has preferential orientation along the c-axis with strong peak observed at 2θ? = 34.25^o, while the photoluminescence (PL) spectrum shows strong UV emission peak at 385 nm. Spectrophotometry measurements reveal that transmission values of the film are low at wavelength shorter than 380 nm. Current-voltage measurements show that the dark- and photocurrents were found to be 6.11 and 89.3 μA, respectively, under dark and illuminated conditions at 5 V.     

Influence of annealing on ZnO thin film grown by plasma-assisted MOCVD

ZnO films were grown on C-plane sapphire substrate by plasma-assisted MOCVD. The films was characterized by XRD, photoluminescence (PL) and the optical transmission spectrum. We found tensile strain in the sample, which had been annealed many times during the growth process, while there is compressive strain in the sample, which was annealed only one time after growth. The PL spectra at room temperature for the sample annealed many times exhibited only one emitting peak at around 380 nm. However, we find Γ₅ and Γ₆ free exciton peaks in the sample annealed only one time after growth. At the same time, the optical transmission indicate that the maximum of the sample's transmission decreases against with the increasing of the c-axis length in ranges from 190 to 900 nm.     

Al-doped ZnO films by pulsed laser deposition at room temperature

Polycrystalline Al-doped ZnO films with good photoluminescence property were successfully deposited on quartz glass substrates by pulsed laser deposition (PLD) at room temperature. The films were obtained by ablating a metallic target (Zn:Al 3 wt%) at various laser energy densities (1.0-2.1 J/cm²) in oxygen atmosphere (9 Pa). The structure of the films was characterized by XRD. Ultraviolet photoluminescence centered at 359-361 nm was observed in the room temperature PL spectra of the Al-doped ZnO films.     

High-quality hexagonal ZnO crystals grown by chemical vapor deposition

High-quality zinc oxide (ZnO) crystals were grown on a (0001) sapphire substrate by chemical vapor deposition at 830 °C under atmospheric pressure. The hexagonal crystals had an average diameter of about 150 μm, and a thickness of about 15 μm, as observed under a polarizing microscope. The large (0002) facet was flat, regular, and neat. In the X-ray diffraction pattern, strong (0002) and weak (0004) peaks indicate that the crystals had a wurtzite structure. The crystalline quality was characterized by Raman scattering, and the E₂(high), E₂(low), and A_l(LO) modes confirm the high quality of the ZnO crystals. Photoluminescence (PL) spectra of the crystals had a strong and sharp ultraviolet emission peak at 379 nm. The PL mechanism was also discussed.     

Synthesis and photoluminescence of Eu-doped ZnO microrods prepared by hydrothermal method

Eu-doped ZnO microrods, with wurtzite structure and [0 0 0 1] growth direction have been successfully synthesized on Si (1 0 0) substrates by a simple hydrothermal method. The samples were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), lifetime decay curves, photoluminescence (PL) and photoluminescence excitation (PLE) spectra, respectively. These results indicate that Eu³⁺ ions are located in the distorted lattice sites near the surface of the ZnO microrods. Additionally, it is also suggested that the surface defects may act as a step in the process of energy transfer from ZnO to Eu³⁺ ions.     

Synthesis, characterization and optical properties of sheet-like ZnO

Sheet-like ZnO with regular hexagon shape and uniform diameter has been successfully synthesized through a two-step method without any metal catalyst. First, the sheet-like ZnO precursor was synthesized in a weak alkaline carbamide environment with stirring in a constant temperature water-bath by the homogeneous precipitation method, then sheet-like ZnO was obtained by calcining at 600 °C for 2 h. The structures and optical properties of sheet-like ZnO have been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) and UV-vis-NIR spectrophotometer. The results reveal that the product is highly crystalline with hexagonal wurtzite phase and has appearance of hexagon at (0 0 0 1) plane. The HRTEM images confirm that the individual sheet-like ZnO is single crystal. The PL spectrum exhibits a narrow ultraviolet emission at 397 nm and a broad visible emission centering at 502 nm. The band gap of sheet-like ZnO is about 3.15 eV.     

Synthetics of ZnO nanostructures by thermal oxidation in water vapor containing environments

Metallic Zn films deposited on glass were wet or dry oxidized at 390 °C in pure N₂ or O₂ to understand the effects of water vapor in different oxygen partial pressure on growth of ZnO nanostructure during thermal oxidation. As-prepared ZnO oxides were characterized by a scanning electron microscope (SEM), an X-ray diffractometer (XRD), and a transmission electron microscope (TEM). Optical and electric properties of these ZnO films were characterized by photoluminescence (PL) and resistance measurements, respectively. It was found that the oxygen partial pressure and water vapor of environment significantly affect the morphologies of ZnO nanostructures. Decreasing oxygen partial pressure in dry oxidation can enhance a green light peak at 500 nm on PL spectra arising from defect-related emission and reduce the resistivity of the oxide films. High H₂O^(g)/O₂ ratio in wet oxidation will significantly increase the intensity of a green light peak and reduce the resistivity of the oxide films. The effect of oxygen partial pressure and H₂O^(g)/O₂ ratio on the PL spectra and resistivity of ZnO films are explained by the theory of defects equilibrium during oxidation.     

Investigation of indium oxide as a self-catalyst in ZnO/ZnInO heterostructure nanowires growth

We investigated the self-catalytic role of indium oxide in the growth process of ZnO/ZnInO heterostructure nanowires on Si(111). The prepared nanowires had hexagonal cross sections and were tapered with tip diameters of 90 ± 5 nm and base diameters of 230 ± 5 nm. Energy dispersive X-ray and field emission Auger spectroscopies indicated that the grown nanowires were heterostructures of ZnO and ZnInO. Analysis of the early growth process revealed that indium may play a self-catalytic role. Therefore, the vapor-liquid-solid mechanism is likely to be responsible for growth of ZnO/ZnInO nanowires. X-ray diffraction and room temperature photoluminescence (PL) data demonstrated that the presence of indium results in a decrease in nanowires' crystallinity. These wires produced a large PL emission peak in the ultraviolet (UV) region and a smaller peak in the green region of the electromagnetic spectrum. The UV peak of the ZnO/ZnInO nanowires is blue-shifted with respect to that of pure ZnO nanowires.     

Photoluminescence of Eu-doped TiO2 thin films prepared by low pressure hot target magnetron sputtering

In this work Eu-doped TiO₂ thin films prepared by reactive magnetron co-sputtering of Ti-Eu metallic target have been studied. The results of photoluminescence (PL) and its correlation with microstructure have been described. Structural properties were examined by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). XRD studies have shown that thin films consisted of TiO₂-anatase and AFM images display their high quality and dense nanocrystalline structure. PL spectra, measured at room temperature, show a dominating strong red luminescence corresponding to ⁵D₀-⁷F₂ transition at ∼ 617 nm and ∼ 623 nm. The evolution of photoluminescence and microstructure of the thin films has been examined as they were additionally annealed in an air ambient.     

The effects of substrate temperature on the structure and properties of ZnO films prepared by pulsed laser deposition

ZnO thin films were prepared by pulsed laser deposition (PLD) on glass substrates with growth temperature from room temperature (RT) to 500 °C. The effects of substrate temperature on the structural and optical properties of ZnO films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission spectra, and RT photoluminescence (PL) measurements. The results showed that crystalline and (0 0 2)-oriented ZnO films were obtained at all substrate temperatures. As the substrate temperature increased from RT to 500 °C, the ratio of grain size in height direction to that in the lateral direction gradually decreased. The same grain size in two directions was obtained at 200 °C, and the size was smallest in all samples, which may result in maximum E_g and E₀ of the films. UV emission was observed only in the films grown at 200 °C, which is probably because the stoichiometry of ZnO films was improved at a suitable substrate temperature. It was suggested that the UV emission might be related to the stoichiometry in the ZnO film rather than the grain size of the thin film.