Anisotropic crystallographic properties, strain, and their effects on band structure of m -plane GaN on LiAlO2 (100)

The m-plane GaN films grown on LiAlO2(100) by metal-organic chemical vapor deposition exhibit anisotropic crystallographic properties. The Williamson-Hall plots point out they are due to the different tilts and lateral correlation lengths of mosaic blocks parallel and perpendicular to GaN[0001] in the growth plane. The symmetric and asymmetric reciprocal space maps reveal the strain of m-plane GaN to be biaxial in-plane compress epsilon(xx)=-0.79% and epsilon(zz)=-0.14% with an out-of-plane dilatation epsilon(yy)=0.38%. This anisotropic strain further separates the energy levels of top valence band at Gamma point. The energy splitting as 37 meV as well as in-plane polarization anisotropy for transitions are found by the polarized photoluminescence spectra at room temperature. (c) 2008 American Institute of Physics.

Stress analysis of ZrO2/SiO2 multilayers deposited on different substrates with different thickness periods

The effects of repeating thickness periods on stress are studied in ZrO2/SiO2 multilayers deposited by electron-beam evaporation on BK7 glass and fused-silica substrates. The results show that the residual stress is compressive and decreases with an increase of the periods of repeating thickness in the ZrO2/SiO2 multilayers. At the same time, the residual stress in multilayers deposited on BK7 glass is less than that of samples deposited on fused silica. The variation of the microstructure examined by x-ray diffraction shows that microscopic deformation does not correspond to macroscopic stress, which may be due to variation of the interface stress. (c) 2005 Optical Society of America.

Effects of CO2 laser conditioning of the antireflection Y2O3/SiO2 coatings at 351 nm

Y2O3/SiO2 coatings were deposited on fused silica by electron beam evaporation. A continuous wave CO2 laser was used to condition parts of the prepared samples at different scanning speeds in the air. LAMBDA 900 spectrometer was used to investigate the changes of the transmittance and residual reflection spectrum. A Nomarski microscope under dark field was used to examine the changes of the micro defect density. The changes of the surface roughness and the microstructure of the film before and after conditioning were investigated by AFM and X-ray diffraction, respectively. We found that laser-induced damage threshold (LIDT) of the films conditioning at 30 mm/s scanning speed was increased by more than a factor of 3 over the thresholds of the as-deposited films. The conditioning effect was correlated with an irradiation-induced decrease of the defect density and absorption of the films. (c) 2005 Elsevier B.V. All rights reserved.

Influence of different post-treatments on the structure and optical properties of zinc oxide thin films

Zinc oxide (ZnO) films with c-oriented were grown on fused quartz glass substrates at room temperature using dc reactive magnetron sputtering. The as-grown films were annealed at 700 degrees C in air and bombarded by ion beam, respectively. The effects of post-treatments on the structural and optical properties of the ZnO films were investigated by X-ray diffraction (XRD), photoluminescence (PL), optical transmittance and absorption measurements. The XRD spectra indicate that the crystal quality of ZnO films has been improved by both the post-treatments. Compared with the as-grown sample, both annealed and bombarded samples exhibited blueshift in the UV emission peaks, and a strong green emission was found in the annealed ZnO film. In both optical transmittance and absorption spectra, a blueshift of the band-gap edge was observed in the bombarded film, while a redshift was observed in the annealed film. (c) 2004 Elsevier B.V. All rights reserved.

Annealing effects on electron-beam evaporated Al2O3 films

The effects of post-deposited annealing on structure and optical properties of electron-beam evaporated Al2O3 single layers were investigated. The films were annealed in air for 1.5 h at different temperatures from 250 to 400 degrees C. The optical constants and cut-off wavelength were deduced. Microstructure of the samples was characterized by X-ray diffraction (XRD). Profile and surface roughness measurement instrument was used to determine the rms surface roughness. It was found that the cut-off wavelength shifted to short wavelength as the annealing temperature increased and the total optical loss decreased. The film structure remained amorphous even after annealing at 400 degrees C temperature and the samples annealed at higher temperature had the higher rms surface roughness. The decreasing total optical loss with annealing temperature was attributed to the reduction of absorption owing to oxidation of the film by annealing. Guidance to reduce the optical loss of excimer laser mirrors was given. (c) 2004 Elsevier B.V. All rights reserved.

Annealing effects on the optical and structural properties of Al2O3/SiO2 films as UV antireflection coatings on 4H-SiC substrates

Al2O3/SiO2 films have been prepared by electron-beam evaporation as ultraviolet (UV) antireflection coatings on 4H-SiC substrates and annealed at different temperatures. The films were characterized by reflection spectra, ellipsometer system, atomic force microscopy (AFM), X-ray diffraction (XRD) and Xray photoelectron spectroscopy (XPS), respectively. As the annealing temperature increased, the minimum reflectance of the films moved to the shorter wavelength for the variation of refractive indices and the reduction of film thicknesses. The surface grains appeared to get larger in size and the root mean square (RMS) roughness of the annealed films increased with the annealing temperature but was less than that of the as-deposited. The Al2O3/SiO2 films maintained amorphous in microstructure with the increase of the temperature. Meanwhile, the transition and diffusion in film component were found in XPS measurement. These results provided the important references for Al2O3/SiO2 films annealed at reasonable temperatures and prepared as fine anti-reflection coatings on 4H-SiC-based UV optoelectronic devices. (c) 2008 Elsevier B.V. All rights reserved.

Investigation of annealing effects on the laser-induced damage threshold of amorphous Ta2O5 films

Ta2O5 films were deposited using the conventional electron beam evaporation method and then annealed at temperatures in the range 373-673 K. Chemical composition, scattering and absorption were examined by X-ray photoelectron spectroscopy (XPS), total integrated scattering (TIS) measurement and the surface thermal lensing (m) technique, respectively. The laser-induced damage threshold (LIDT) was assessed using the output from an Nd:YAG laser with a pulse length of 12 ns. The results showed that the improvement of the LIDT after annealing was due to the reduced substoichiometric and structural defects present in the film. The LIDT increased slightly below 573K and then increased significantly with increase in annealing temperature, which could be attributed to different dominant defects. Moreover, the root mean square (RMS) roughness and scattering had little effect on the LIDT, while the absorption and the LIDT were in accord with a general relation. (c) 2008 Elsevier Ltd. All rights reserved.

Effects of deposition rates on laser damage threshold of TiO2/SiO2 high reflectors

TiO2 single layers and TiO2/SiO2 high reflectors (HR) are prepared by electron beam evaporation at different TiO2 deposition rates. It is found that the changes of properties of TiO2 films with the increase of rate, such as the increase of refractive index and extinction coefficient and the decrease of physical thickness, lead to the spectrum shift and reflectivity bandwidth broadening of HR together with the increase of absorption and decrease of laser-induced damage threshold. The damages are found of different morphologies: a shallow pit to a seriously delaminated and deep crater, and the different amorphous-to-anatase-to-rutile phase transition processes detected by Raman study. The frequency shift of Raman vibration mode correlates with the strain in. film. Energy dispersive X-ray analysis reveals that impurities and non-stoichiometric defects are two absorption initiations resulting to the laser-induced transformation. (C) 2008 Elsevier B. V. All rights reserved.