Optical properties of high-quality ZnO thin films grown by a sputtering method

Photoluminescence properties of high-quality ZnO thin films prepared by an RF-magnetron sputtering method

The ZnO thin films have been prepared by sol-gel method at different annealing temperatures and aging time. The structural properties of ZnO thin films were investigated by X-ray diffraction (XRD) patterns and atomic force microscope (AFM) images. The results indicated that the film possess a hexagonal wurtzite structure with preferred orientation along the (002) direction. The crystalline quality of films improved with increasing post-annealing temperature, while gradually worsened with prolonging aging time. The optical properties of ZnO thin films were studied by the ultra-violet transmittance (UV-Vis) and photoluminescence (PL) spectra. The variations of UV-Vis transmittance and energy gap accorded well with the tendency presented in XRD patterns and AFM images. The PL spectra appeared the apparent ultraviolet emission and visible emissions. As the annealing temperature increased, the ultraviolet and visible emissions of films enhanced and decreased respectively. However, as the sol aging time prolonged, the PL emission presented an opposite tendency.

In this work, ZnO thin films were deposited on glass substrates using spin coating method. Different annealing temperature from 400 to 550°C significantly distresses the nature of electrical and optical properties of ZnO thin films have been investigated. The effect of annealing temperature on optical and electrical properties of nanostuctured ZnO thin films deposited by spin coating method has been studied. The optical properties were characterized by ultraviolet visible (UV-VIS-NIR) spectrophotometer. The electrical properties were analyzed using I-V measurement (CEP 2000). Gold (Au) was used as a metal contact using electron beam thermal evaporator (ULVAC). The optical transmittance spectrums showed the average transmittance of ZnO thin film with different annealing temperature higher than 90% in visible wavelength region. The optical band properties of the nanostructured ZnO thin films were analyzed by UV Vis and Tauc method was accepted to estimate the optical gap and absorption coefficient. The resistivity of the film decreased as the annealing temperature increased from 400 to 550ºC. The highest conductivity values of ZnO thin film was obtained at 500°C with value 0.000197 Scm-1. Moreover, the higher porosity was found at 400ºC with value 57.5%. The higher porosity of ZnO thin film can be expected the high sensitivity for gas sensor due to the material has a comparatively large surface area.

Formation of high performance nanostructured ZnO thin films as a function of annealing temperature: structural and optical properties

ZnO thin films on glass substrates have been successfully fabricated by sol-gel method for various aging time and annealing temperature. X-ray diffraction (XRD) patterns showed the crystallites at (100), (002) and (101) plane of hexagonal wurtzite structure. Atomic force microscope (AFM) images were used to investigate the surface morphology. The transmittance, energy gap, Urbach energy and photoluminescence (PL) of samples were investigated to explore the effects of annealing temperature on optical properties of ZnO thin films under different sol aging time. The transmittance spectra of thin films aged for 24 hours and 48 hours revealed the enhanced transmittance in visible region as rising annealing temperature, also, the optical band gap of the samples increased and Urbach energy decreased. The photoluminescence (PL) spectra of samples aged for 24 hours and 48 hours were studied and found the increased ultraviolet emission at ~387 nm, and various decreased visible emissions with rising annealing temperature. Nevertheless, the excessive sol aging time for 72 hours deteriorated the optical characteristics of thin films, resulting in that transmittance, energy gap and ultraviolet emission declined, and visible emissions increased with rising annealing temperature. The optical performances of the ZnO films aged for different time have no the same dependence of annealing temperature.

Thin films of undoped ZnO and Cr-doped ZnO were deposited by sol-gel spin coating. The structural and the optical properties of Cr-doped ZnO thin films with different concentrations of Cr (0–2.5 at.% relative to Zn) were studied by using X-ray diffraction (XRD) and photoluminescence (PL). The XRD showed that the thin films grew with a preferred c-axis orientation. No traces of the Cr metal, oxides, or a binary Zn-Cr phase were detected. This means that the addition of Cr into the ZnO matrix preserved the formation of the wurtzite structure of ZnO. However, increasing the amount of Cr dopant in the ZnO thin films caused distortion due to the intercalation of the dopant atoms into the wurtzite structure. The PL spectra showed a blue shift in the near-band-edge emission and a decrease in the deep-level emission upon increasing the amount of Cr doping. The narrowest full width at half maximum (FWHM) was observed in ZnO thin films with 2.5 at.% Cr doping. The FWHM gradually decreased upon further increasing the Cr concentration.

ZnO thin films with ZnO buffer layers were grown by plasma-assisted molecular beam epitaxy (PA-MBE) on p-type Si(100) substrates. Before the growth of the ZnO thin films, the ZnO buffer layers were deposited on the Si substrates for 20 minutes and then annealed at the different substrate temperature ranging from 600 to 800 degrees C in oxygen plasma. The structural and optical properties of the ZnO thin films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and room-temperature (RT) photoluminescence (PL). A narrower full width at half maximum (FWHM) of the XRD spectra for ZnO(002) and a larger grain are observed in the samples with the thermal annealed buffer layers in oxygen plasma, compared to those of the as-grown sample. The surface morphology of the samples is changed from rugged to flat surface. In the PL spectra, near-band edge emission (NBEE) at 3.2 eV (380 nm) and deep-level emission (DLE) around 1.77 to 2.75 eV (700 to 450 nm) are observed. By increasing the annealing temperatures up to 800 degrees C, the PL intensity of the NBEE peak is higher than that of the as-grown sample. These results imply that the structural and optical properties of ZnO thin films are improved by the annealing process.

Effects of buffer layer thickness on properties of ZnO thin films grown on porous silicon by plasma-assisted molecular beam epitaxy

ZnO and Eu-doped ZnO thin films were deposited on quartz substrates by reactive radio-frequency magnetron sputtering from a ZnO and Eu-doped ZnO ceramic target respectively. The properties of thin films were characterized by atom force microscope (AFM), X-ray diffraction (XRD), and photoluminescence spectra (PL). XRD reveals that the prepared thin films possess a single crystalline hexagonal wurtzite crystal structure with preferential c-axis orientation. Under above-bandgap excitation at room temperature, PL shows that ZnO films exhibit strong UV near-band-edge excitonic emission and Eu-doped ZnO thin films present UV emission and red emission from Eu3+ ions, demonstrating efficient energy transfer from the host to Eu3+ ions. The influence of annealing on the structure and optical properties of ZnO and Eu-doped ZnO thin films is studied. The efficient energy transfer from the host to Eu3+ and high luminous efficiency indicates that the prepared Eu-doped ZnO thin films are very promising materials for lighting and flat panel display application.

Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering

Effect of preheating temperature on structural and optical properties of ZnO thin films by sol–gel process

ZnO thin films with different solution concentrations (0.1–0.9 mol/L) were prepared by a simple sol–gel dip-coating technique. X-ray diffraction, ultraviolet–visible spectroscopy, Hall effect measurements and photoluminescence (PL) spectroscopy were employed to investigate the effect of solution concentration on the structural,optical and electrical conductive properties of the ZnO thin films. The results showed that the ZnO thin films preferentially oriented along the (002) direction at higher solution concentration. The careful study of the optical and electrical conductive properties showed that the resistivity decreased monotonously, while the transmittance increased first and then decreased when solution concentrations changed from 0.1 to 0.9 mol/L. Photoluminescence spectra indicated that the defect-related blue emission was increased with the enhancement of solution concentration. The mechanism of the blue emission, and the reasons why high solution concentration was favorable for forming high c-axis oriented ZnO thin films and obtaining low resistivity were also discussed in detail.

Variation of light emitting properties of ZnO thin films depending on post-annealing temperature

In this study, ZnO thin films doped with Al (AZO) and Cu (CZO) were fabricated on a glass substrate via sol-gel spin coating. The influence of 1 atomic % Al and Cu doping on the structure, morphology, as well as optical properties of ZnO thin film was then analyzed with X-ray diffraction (XRD), atomic force microscopy (AFM), and UV-Vis spectroscopy. XRD analysis revealed that all samples possessed hexagonal wurtzite crystal structures with 3 to 4 preferred orientations. Al and Cu doping caused a decrease in crystal size, while the lattice strain (e) and dislocation density (ρ) were increased. AFM indicated that Al and Cu doping reduced the surface roughness of the ZnO thin film. Optical measurement showed that all samples exhibited high transmittance (> 80%) in the visible light region. Transmittance was reduced after doping, while the band gaps for ZnO, AZO, and CZO thin films are 3.26, 3.28, and 3.23 eV. This study showed that an addition of 1 atomic % transition metal (Al and Cu) greatly influences the structure, morphology, and optical properties of ZnO thin film.

Linear and nonlinear optical properties of ZnO thin films deposited by pulsed laser deposition