Lizhi Chen

Theoretical investigation on quantum well lasers with extremely low vertical beam divergence and low threshold current

A specially designed quantum well laser for achieving extremely low vertical beam divergence was reported and theoretically investigated. The laser structure was characterized by two low index layers inserted between the waveguide layers and the cladding layers. The additional layers were intended to achieve wide optical spread in the cladding layers and strong confinement in the active region. This enabled significant reduction of beam divergence with no sacrifice in threshold current density. The numerical results showed that lasers with extremely low vertical beam divergence from 20° down to 11° and threshold current density of less than 131 A/cm2 can be easily achieved by optimization of the structure parameters. Influences of individual key structure parameters on beam divergence and threshold current density are analyzed. Attention is also paid to the minimum cladding layer thicknesses needed to maintain low threshold current densities and low internal loss. The near and far field patterns are given...

Effect of O2 pressure on the synthesis of titanium oxide film by ion beam enhanced deposition

Abstract A series of titanium oxide films have been synthesized on silicon wafers by ion beam enhanced deposition at different O 2 pressures. X-ray Photoelectron Spectroscopy (XPS), X-ray Diffraction (XRD), Glancing Angle Diffraction and Rutherford Backscattering Spectroscopy (RBS) were used to analyze the composition, structure and orientation of the films. From the experimental results, it was found that: (1) the titanium oxide films synthesized at different O 2 pressures exhibit a polycrystal structure with preferred orientation; (2) when O 2 pressure is lower than 8.4×10 −4 Pa, the main composition of the film is TiO whose preferred orientation changed from (220) to (031) with the increase of O 2 pressure; (3) when O 2 pressure is higher than 8.4×10 −4 Pa, rutile-type TiO 2 with (200) preferred orientation was found to be the major composition of the film. The higher the O 2 pressure is, the more stable the composition and preferred orientation become.

Reactive ion beam assisted deposition of a titanium dioxide film on a transparent polyester sheet

In this work, a titanium dioxide film was synthesized on a thin transparent polyester sheet by a reactive ion beam assisted deposition technique at room temperature. In this deposition process, a pure titanium target was sputtered by an argon ion beam under an oxygen atmosphere, and the growing film was concurrently bombarded by another argon ion beam in order to enhance both the adhesion of the film and the reactivity of the depositing titanium species with surrounding oxygen molecules. The microstructure of the as-deposited film was studied by Rutherford backscattering spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analyses. The film was examined to be of rutile structure with a (110) preferential orientation. The optical properties of the film were measured by ultraviolet-visible transmittance spectroscopy and Fourier-transform infrared absorption spectroscopy experiments. It was found that the as-deposited titania film was quite transparent in visible region, while ultraviolet light could be filtered effectively, and infrared transmittance could also be reduced evidently. Our titania-coated polyester sheet should be applicable as a shielding cover on car windscreens or window glasses for the purpose of filtering ultraviolet radiation from the sunlight and partially insulating heat as well.

Chemical composition and structure of titanium oxide films deposited on LTI-carbon by IBED

Abstract Titanium oxide films are deposited on low temperature isotropic carbon (LTI-carbon) by ion beam enhanced deposition (IBED). The chemical composition of the prepared films is analyzed by using X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS). The structure of the prepared films is studied by glancing angle X-ray diffraction (GAXRD) and transmission electronic microscopy (TEM). The surface morphology of the film is observed by means of atomic force microscopy (AFM). The results show that the prepared films are polycrystalline with TiO, Ti 2 O 3 and TiO 2 as coexisting phases. The chemical composition is mainly influenced by O 2 pressure. The higher the O 2 pressure is, the higher the ratio of O/Ti is. The cross-sectional high resolution electron microscopy image show that a ‘titanium oxide/amorphous transitional layer/LTI-carbon’ layered structure has been formed. The structure of the films prepared at 80 keV, is more complex than that of the film prepared at 40 keV, as well as that the surface morphology of the film prepared at 80 keV is rougher.

Aggregation and out diffusion of iron atoms for Fe ion implanted silica films

Iron ions were implanted into a thermally grown silica film with a dose of about 6.5×1016 Fe/cm2 by using a metal vapor vacuum arc ion source at an accelerating voltage of 62 kV. The structure evolution of the film during the postimplantation annealing processes was studied by x-ray diffraction, Rutherford backscattering spectroscopy, x-ray photoelectron spectroscopy, and atomic force microscopy experiments. It was found that the implanted iron atoms diffused out of the film and aggregated with the formation of particle islands on the film surface gradually after annealing. The out diffusion and island formation phenomena are particularly evident when the annealing temperature was elevated up to 800 °C. It was also demonstrated that in the early stage of annealing the growth of iron islands closely depended on the out diffusion of the implanted iron atoms, and both the size and areal density of the islands increased progressively with increasing annealing time. After a certain annealing period, the size o...

Influence of IBED parameters on the composition and structure of titanium oxide films deposited on LTI-carbon

Abstract The chemical composition and structure of titanium oxide films deposited on Low Temperature Isotropic Carbon (LTI-carbon) by ion beam enhanced deposition (IBED) is jointly controlled by the process parameters of IBED. In this paper, the influence of O 2 pressure, substrate temperature and ion beam energy on the composition and structure of the prepared titanium oxide films was studied by means of X-ray Photo-Electron Spectroscopy (XPS), Rutherford Backscattering Spectroscopy (RBS), X-ray Diffraction (XRD) and Glancing Angle X-ray Diffraction (GAXRD). The results show that the prepared films are polycrystalline with TiO, Ti 2 O 3 and TiO 2 as coexisting phases. The influence of O 2 pressure is mainly on the composition of the film. Ion beam energy has a great influence on the structure of the film. Neither the composition nor the structure of the film is sensitive to substrate temperature that ranged from room temperature to 300°C.

Thermal lensing effect in ridge structure InGaN multiple quantum well laser diodes

Time-resolved light-current curves, spectra, and far-field distributions of ridge structure InGaN multiple quantum well laser diodes grown on sapphire substrate are measured with a temporal resolution of 0.1 ns under a pulsed current condition. Results show that the thermal lensing effect clearly improves the confinement of the higher order modes. The thermal lens leads to a lower threshold current for the higher order modes, a higher slope efficiency, and a change in the lasing mode of the device. The threshold current for the higher modes decreases by about 5 mA in every 10 ns in a pulse, and the slope efficiency increases by 7.5 times on the average when higher modes lase. (c) 2006 American Institute of Physics.