Enhancement of near-band edge photoluminescence of ZnO thin films by employing MgF2 buffer layer

ZnO/MgF2/ZnO sandwich structure films were fabricated. The effects of a buffer layer on structure and optical properties of ZnO films were investigated by X-ray diffraction, photoluminescence, optical transmittance and absorption measurements. Measurement results showed that the buffer layer had the effects of improving the quality of ZnO films and releasing the residual stresses in the films. The near-band edge emissions of ZnO films deposited on the MgF2 buffer layer were significantly enhanced compared with those deposited on bare substrate due to the smaller lattice mismatch between MgF2 and ZnO than that between fused silica and ZnO. (c) 2006 Elsevier B.V. All rights reserved.

Enhancement of near-band-edge photoluminescence of ZnO thin films in sandwich configuration at room temperature

ZnO/ITO/ZnO sandwich structure films were fabricated. The effects of buffer layer on the structure and optical properties of ZnO films were investigated by x-ray diffraction (XRD), photoluminescence, optical transmittance, and absorption measurements. XRD spectra indicate that a buffer layer has the effects of lowering the grain orientation of ZnO films and increasing the residual stresses in the films. The near-band-edge emissions of ZnO films deposited on both single indium tin oxide (ITO) buffer and ITO/ZnO double buffers are significantly enhanced compared with that deposited on a bare substrate due to the quantum confinement effect. (C) 2006 American Institute of Physics.

Influence of buffer layer thickness on the structure and optical properties of ZnO thin films

A series of ZnO thin films were deposited on ZnO buffer layers by DC reactive magnetron sputtering. The buffer layer thickness determination of microstructure and optical properties of ZnO films was investigated by X-ray diffraction (XRD), photoluminescence (PL), optical transmittance and absorption measurements. XRD results revealed that the stress of ZnO thin films varied with the buffer layer thickness. With the increase of buffer layer thickness, the band gap edge shifted toward longer wavelength. The near-band-edge (NBE) emission intensity of ZnO films deposited on ZnO buffer layer also varied with the increase of thickness due to the spatial confinement increasing the Coulomb interaction between electrons and holes. The PL measurement showed that the optimum thickness of the ZnO buffer layer was around 12 nm. (c) 2005 Elsevier B.V. All rights reserved.

The effects of oxygen partial pressure on the optical absorption edge and the UV emission energy of ZnO films

The optical absorption edge and ultraviolet (UV) emission energy of ZnO films deposited by direct current (DC) reactive magnetron sputtering at room temperature have been investigated. With the oxygen ratio increasing, the structure of films changes from zinc and zinc oxide coexisting phase to single-phase ZnO and finally to the highly (002) orientation. Both the grain size and the stress of ZnO film vary with the oxygen partial pressure. Upon increasing the oxygen partial pressure in the growing ambient, the visible emission in the room-temperature photoluminescence spectra was suppressed without sacrificing the band-edge emission intensity in the ultraviolet region. The peaks of photoluminescence spectra were located at 3.06---3.15 eV. From optical transmittance spectra of ZnO films, the optical band gap edge was observed to shift towards shorter wavelength with the increase of oxygen partial pressure.

Influence of ZnS and MgO Shell on the Photoluminescence Properties of ZnO Core/Shell Nanowire

Chinese Academy of Sciences；National Science Fund for Distinguished Young Scholar 60925016；National High Technology Research and Development program of China 2009AA034101；Postdoctoral Foundation 0971050000

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) similar to 500 degrees C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 degrees C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments: the grain size increased and stress relaxed for the films deposited at 200-500 degrees C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E-g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 degrees C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.

Effects of the morphology of ZnO/Ag interface on the surface-plasmon-enhanced emission of ZnO films

The effects of the surface morphology of Ag on the surface-plasmon-enhanced emission of ZnO films have been studied for a ZnO/Ag/Si system by photoluminescence spectroscopy and atomic force microscopy. The results indicate that the enhancement of ZnO ultraviolet emission is dependent on the deposition conditions of the Ag interlayers. By examining the dependence of the enhancement ratio of surface-plasmon-mediated emission on the characteristic parameters of Ag surface morphology, we found that the surface plasmon coupling to light is determined by both the Ag particle size and density.

A simple route of morphology control and structural and optical properties of ZnO grown by metal-organic chemical vapour deposition

Employing the metal-organic chemical vapour deposition (MOCVD) technique, we prepare ZnO samples with different morphologies from the film to nanorods through conveniently changing the bubbled diethylzinc flux (BDF) and the carrier gas flux of oxygen (OCGF). The scanning electron microscope images indicate that small BDF and OCGF induce two-dimensional growth while the large ones avail quasi-one-dimensional growth. X-ray diffraction (XRD) and Raman scattering analyses show that all of the morphology-dependent ZnO samples are of high crystal quality with a c-axis orientation. From the precise shifts of the 2 theta. locations of ZnO (002) face in the XRD patterns and the E-2(high) locations in the Raman spectra, we deduce that the compressive stress forms in the ZnO samples and is strengthened with the increasing BDF and OCGF. Photoluminescence spectroscopy results show all the samples have a sharp ultraviolet luminescent band without any defects-related emission. Upon the experiments a possible growth mechanism is proposed.

Valence band offset of ZnO/4H-SiC heterojunction measured by x-ray photoelectron spectroscopy

The valence band offset (VBO) of the wurtzite ZnO/4H-SiC heterojunction is directly determined to be 1.61 +/- 0.23 eV by x-ray photoelectron spectroscopy. The conduction band offset is deduced to be 1.50 +/- 0.23 eV from the known VBO value, which indicates a type-II band alignment for this heterojunction. The experimental VBO value is confirmed and in good agreement with the calculated value based on the transitive property of heterojunctions between ZnO, SiC, and GaN. (C) 2008 American Institute of Physics.

Stress analysis of ZnO film with a GaN buffer layer on sapphire substrate

A 5.35-mu m-thick ZnO film is grown by chemical vapour deposition technique on a sapphire (0001) substrate with a GaN buffer layer. The surface of the ZnO film is smooth and shows many hexagonal features. The full width at half maximum of ZnO (0002) omega-rocking curve is 161 arcsec, corresponding to a high crystal quality of the ZnO film. From the result of x-ray diffraction theta - 2. scanning, the stress status in ZnO film is tensile, which is supported by Raman scattering measurement. The reason of the tensile stress in the ZnO film is analysed in detail. The lattice mismatch and thermal mismatch are excluded and the reason is attributed to the coalescence of grains or islands during the growth of the ZnO film.

Oxygen pressure dependences of structure and properties of ZnO films deposited on amorphous glass substrates by pulsed laser deposition

ZnO films are prepared on glass substrates by pulsed laser deposition (PLD) at different oxygen pressures, and the effects of oxygen pressure on the structure and optoelectrical properties of as-grown ZnO films are investigated. The results show that the crystallite size and surface roughness of the films increase, but the carrier concentration and optical energy gap E-g decrease with increasing oxygen pressure. Only UV emission is found in the photoluminescence (PL) spectra of all the samples, and its intensity increases with oxygen pressure. Furthermore, there are marked differences in structure and properties between the films grown at low oxygen pressures (0.003 and 0.2 Pa) and the films grown at high oxygen pressures (24 and 150 Pa), which is confirmed by the fact that the crystallite size and UV emission intensity markedly increase, but the carrier concentration markedly decreases as oxygen pressure increases from 0.2 to 24 Pa. These results show that the crystal quality, including the microstructural quality and stoichiometry proportion, of the prepared ZnO films improves as oxygen pressure increases, particularly from 0.2 to 24 Pa.