Fengshan Zhang

A new method to control nano-porous structure of sol-gel-derived silica films and their properties

Abstract To control structure of nano-porous silica films effectively, a base/acid two-step catalytic sol-gel process was investigated. The nano-porous structure and IR absorption of the thin films were characterized by means of FE-SEM, AFM, SEM, FTIR, and ellipsometry, respectively. The experimental results have shown that the base/acid two-step catalysis can adjust the refractive index of the films from 1.18 to 1.41 rapidly and continuously. With a change of the experimental conditions, the frequency shift of the ω 4 (TO 3 ) peak of FTIR absorption to a lower wave number indicates a decrease in the average Si-O-Si bridging angle. A reduction of FWHM of the ω 4 peak reveals a narrow distribution of the Si-O-Si bridging angle. These results suggest that influence of the experimental conditions on the FTIR absorption of the nano-porous silica films is attributed to a change of silica particle shape and state of aggregation of the particles in the films.

Strengthening mechanism of porous silica films derived by two-step catalysis

A scratch-resistant improvement of sol-gel derived nano-porous silica films by a base/acid two-step catalysis is reported. The silica films were prepared with the two-step catalytic sol-gel process and a dip-coating method, and then post annealed in air and a mixed gas atmosphere. The films were characterized with FE-SEM, AFM, TEM, XPS, FTIR, an ellipsometer, and an abrasion test. With the post annealing, the frequency shift of the FTIR absorption ω4(TO3) peak of the films to a lower wavenumber results from a decrease in the average Si-O-Si bridging angle. An increase in the full-width at half-maximum of the ω4 peak is ascribed to a wide distribution of the bridging angle because of formation of strained Si-O-Si bonds caused by the post annealing. XPS analysis shows that the effect of the mixed gas annealing on the binding energy and full-width at half-maximum (FWHM) for Si (2p) and O (1s) peaks is attributed to the further condensation reaction of the films resulting from the disappearance of the OH groups. The abrasion and adhesion tests indicate that the two-step catalysis obviously improves the scratch resistance and adhesion of the films, and that the mixed gas treatment further strengthens the silica network. The increase in strength is attributed to more Si-O-Si bond cross-linkages between silica particles formed by the two-step catalysis and the mixed gas treatment.

The determination of infrared optical constants of rare earth fluorides by classical Lorentz oscillator model

Rare earth fluoride (REF3 ,R E= La, Ce, Pr, Sm, Er, Yb, Y) films were deposited on Ge(1 1 1) and silicon wafers in order to determine optical constants from the near infrared up to the high frequency tail of the reststrahlen band. The FT-IR transmission spectrum and the reflection spectrum were used to examine the infrared optical properties of the fluorides. The optical constants of the films in the infrared spectrum from 2 to 20 µm were calculated using the classical Lorentz oscillator model by fitting the transmission spectrum. The reflective spectrum of the fluorides on silicon was used to demonstrate the absorption difference in this region. It is found that the extinction coefficients of rare earth fluorides films with P nma structure (REF3 ,R E= Y, Yb, Er) are larger than the other fluorides films with P ¯ 3c1 structure (REF3 ,R E= La, Ce, Pr, Sm) from 15 to 20 µm. (Some figures in this article are in colour only in the electronic version)

Identification of degradation mechanisms of blue InGaN/GaN laser diodes

A comprehensive analysis of the degradation mechanism of blue InGaN/GaN laser diodes (LDs) is carried out by investigating the electrical and optical characteristics. The increase in the leakage current as well as decrease in the slope efficiency is observed. The luminescence properties of the active region at different aging stages are studied by means of cathodoluminescence. Significant degradation of the active region is observed on the room temperature cathodoluminescence while the low temperature cathodoluminescence shows almost no degradation, indicating that the degradation of the LDs is due to generation of low temperature frozen point defects. Furthermore, the generation of the defects follows a kinetic mechanism enhanced by electron-hole non-radiative recombination which explains the acceleration of time degradation in our LDs.

In0.25Ga0.75As growth on low-temperature thin buffer layers formed on GaAs (001) substrate

High-quality In0.25Ga0.75As films were grown on low-temperature (LT) ultra-thin GaAs buffer layers formed on GaAs (0 0 1) substrate by molecular beam epitaxy. The epilayers were studied by atomic force microscopy (AFM), photo luminescence (PL) and double crystal X-ray diffraction (DCXRD), All the measurements indicated that LT thin buffer layer technique is a simple but powerful growth technique for heteroepitaxy

Non-polarizing broadband dichroic mirror

A non-polarizing broadband dichroic mirror with high reflectance from 400 to 600 nm and high transmittance from 650 to 1000 nm was designed with a simple structure. The newly designed mirror showed good tolerance for angle, thickness and refractive index errors. Then the dichroic mirror was fabricated by electron-beam evaporation with ion-beam-assisted deposition (IBAD). The experimental spectral performances showed a good agreement with the theoretical curves.

LOW FREQUENCY ACOUSTIC RESPONSE OF SURFACE CRACKS BY ATOM FORCE ACOUSTIC MICROSCOPY

Surface crack of CeF3 films generated by thermal stress were characterized by scanning electron microscopy and atom force acoustic microscopy (AFAM). Low frequency (8–18 kHz) acoustic response of films and cracks was measured by AFAM. The low frequency acoustic response is similar to what had been got at several MHz or even higher frequency. It was found that surface elastic properties of CeF3 films can be easily qualitatively measured by low frequency AFAM.

UNDERLAYER ROUGHNESS INFLUENCE ON THE PROPERTIES OF Ag THIN FILM

The effects of the roughness of ZnS underlayer on the microstructure, optical, and electrical properties of nanometer Ag thin film have been investigated in this paper. Nanometer Ag thin films in glass/ZnS/7.5 nm Ag/30 nm ZnS stacks have been deposited and analyzed. In the stacks, the underlayers of ZnS have been sputtered with various thicknesses to generate various surface roughnesses. The X-ray diffraction (XRD) has been used to study the crystal structure of Ag films. The surface topography and the roughness of ZnS underlayer have been analyzed by atomic force microscopy. The sheet resistant will become larger as the increasing of the roughness. The optical constants can be derived by fitting the transmission and reflectance spectrum. From optical constants comparison of Ag films, with the surface of the stack becoming rougher, it was found that the refractive index will increase but the extinction coefficient will decrease.

Control of polarization for a broadband dichroic filter at 45° incidence

A method for the control of polarization for a broadband dichroic filter was reported and some design examples were elaborated. This method could be applied over a wide range of wavelengths and a wide range of polarizations in the transmission region. A nonpolarizing broadband dichroic filter and a broadband dichroic filter with certain polarization were designed and fabricated by electron beam evaporation with ion beam assisted deposition. The experimental spectral performances showed good agreement with their theoretical curves. In addition, the application of the method was discussed.

Influence on optical and electrical properties of silver layer by adjoining Ta2O5 layers

Two kinds of structural film stacks, Ta2O5\Ag and Ta2O5\Ag\Ta2O5, were made for investigating the influence on the optical and electrical properties of ultra-thin Ag film by e adjoining Ta2O5 layers. Different samples were prepared by changing the deposition condition of Ta2O5 with different Argon pressure and sputter power. All samples have a uniform 6nm thickness of Ag layer controlled by deposition time. Optical and electrical measurements were carried out on samples and from the transmission and reflectance spectrum the optical constants can be derived. For Ta2O5\Ag film stacks, from measurement we found that the optical and electrical properties of 6nm Ag film strongly depend on the deposition condition of Ta2O5 layers. The sheet resistance is changed from 12Ω to 31Ω and reflectance is changed from 21% to 46% at 2500nm wavelength, which indicate the difference in Ag layer structure. But for the Ta2O5\Ag\Ta2O5 stacks, the changed deposition condition of the upper Ta2O5 layers just bring slightly shift of the optical and electrical property.