Depth distribution of the strain in the GaN layer with low-temperature AlN interlayer on Si(111) substrate studied by Rutherford backscattering/channeling

The depth distribution of the strain-related tetragonal distortion e(T) in the GaN epilayer with low-temperature AlN interlayer (LT-AlN IL) on Si(111) substrate is investigated by Rutherford backscattering and channeling. The samples with the LT-AlN IL of 8 and 16 nm thickness are studied, which are also compared with the sample without the LT-AlN IL. For the sample with 16-nm-thick LT-AlN IL, it is found that there exists a step-down of e(T) of about 0.1% in the strain distribution. Meanwhile, the angular scan around the normal GaN <0001> axis shows a tilt difference about 0.01degrees between the two parts of GaN separated by the LT-AlN IL, which means that these two GaN layers are partially decoupled by the AlN interlayer. However, for the sample with 8-nm-thick LT-AlN IL, neither step-down of e(T) nor the decoupling phenomenon is found. The 0.01degrees decoupled angle in the sample with 16-nm-thick LT-AlN IL confirms the relaxation of the LT-AlN IL. Thus the step-down of e(T) should result from the compressive strain compensation brought by the relaxed AlN interlayer. It is concluded that the strain compensation effect will occur only when the thickness of the LT-AlN IL is beyond a critical thickness. (C) 2004 American Institute of Physics.

Influence of N doping on the Rashba coefficient, semiconductor-metal transition, and electron effective mass in InSb1-xNx nanowires: Ten-band k center dot p model

The electronic structures of InSb1-xNx nanowires are investigated using the ten-band k center dot p method. It is found that nitrogen increases the Rashba coefficient of the nanowires dramatically. For thick nanowires, the Rashba coefficient may increase by more than 20 times. The semiconductor-metal transition occurs more easily in InSb1-xNx nanowires than in InSb nanowires. The electronic structure of InSb1-xNx nanowires is very different from that of the bulk material. For fixed x the bulk material is a semimetal, while the nanowires are metal-like. In InSb1-xNx bulk material and thick nanowires, an interesting decrease of electron effective mass is observed near k=0 which is induced by the nitrogen, but this phenomenon disappears in thin nanowires.

Structural characterization of AlGaN/GaN superlattices by x-ray diffraction and Rutherford backscattering

We report on structural characterization of AlGaN/GaN superlattices grown on sapphire. The superlattice formation is evidenced by high-resolution x-ray diffraction and transmission electron microscopy. The high resolution x-ray diffraction spectra exhibit a pattern of satellite peaks. The in-plane lattice constants of the superlattices indicate the coherent growth of the AlGaN layer onto GaN. The average At composition in the superlattices is determined to be 0.08 by Rutherford backscattering spectroscopy. The average parallel and perpendicular elastic strains for the SLs are determined to be (e(parallel to)) = +0.25% and (e(perpendicular to)) = -0.17%. (c) 2006 Elsevier Ltd. All rights reserved.

Depth dependence of the tetragonal distortion of a GaN layer on Si(111) studied by Rutherford backscattering/channeling

Rutherford backscattering and channeling have been used to characterize the structure of a GaN layer grown on a Si(111) substrate. The results show that a 1.26 mum GaN epitaxial layer with a rather abrupt interface and a good crystalline quality (chi(min)=3.4%) can be grown on a Si(111) substrate. Using the channeling angular scan around an off-normal <1 (2) over bar 13> axis in the {10 (1) over bar0} plane of the GaN layer, the tetragonal distortion e(T), which is caused by the elastic strain in the epilayer, can be determined. Moreover, the depth dependence of the e(T) can be obtained using this technique. A fully relaxed (e(T)=0) GaN layer for a thickness <2.8 mum is expected. (C) 2002 American Institute of Physics.

Surface acoustic wave velocity and electromechanical coupling coefficient of GaN grown on (0001) sapphire by metal-organic vapour phase epitaxy

High-quality and high-resistivity GaN films were grown on (0001) sapphire face by metal-organic vapour phase epitaxy. To measure the surface acoustic wave properties accurately, we deposited metallized interdigital transducers on the GaN surface. The acoustic surface wave velocity and electromechanical coupling coefficient were measured, respectively, to be 5667 m/s and 1.9% by the pulse method.

Simple reflection technique for measuring the linear electro-optic coefficient of the polymer thin films

The simple reflection technique is usually used to measure the linear electro-optic (EO) coefficient (Pockels coefficient) in the development of EO polymer thin films. But there are some problems in some articles in the determination of the phase shift between the s and p light modes of a laser beam waveguided into the polymer film while a modulating voltage is applied across the electrodes, and different expressions for the linear EO coefficient measured have been given in these articles. In our research, more accurate expression of the linear EO coefficient was deduced by suitable considering the phase shift between the s and p light modes. The linear EO coefficients of several polymer thin films were measured by reflection technique, and the results of the Linear EO coefficient calculated by different expressions were compared. The limit of the simple reflection technique for measuring the linear EO coefficient of the polymer thin films was discussed.

Influence of dispersive coupling coefficient in microring channel drop filters

Wide transmission dips are observed in the through spectra in microring and racetrack channel drop filters by two-dimensional finite-difference time-domain (FDTD) simulation. The transmission spectra, which reflect the coupling efficiency, are also calculated from the FDTD output as the pulse just travels one circle inside the resonator. The results indicate that the dips are caused by the dispersion of the coupling coefficient between the input waveguide and the resonator. In addition, a near-zero channel drop on resonance and a large channel drop off resonance are observed due to the near zero coupling coefficient and a large coupling coefficient, respectively. If the width of the input waveguide is different from that of the ring resonator, the oscillation of the coupling coefficient can be greatly suppressed.

Evaluation of thermal radiation dependent performance of GaSb thermophotovoltaic cell based on an analytical absorption coefficient model

Applying the model dielectric function method, we have expressed the absorption coefficient of GaSb analytically at room temperature relating to the contribution of various critical points of its electronic band structure. The calculated absorption spectrum shows good agreement with the reported experimental data obtained by spectral ellipsometry on nominally undoped sample. Based on this analytical absorption spectrum, we have qualitatively evaluated the response of active absorbing layer structure and its photoelectric conversion properties of GaSb thermophotovoltaic device on the perturbation of external thermal radiation induced by the varying radiator temperature or emissivity. Our calculation has demonstrated that desirable thickness to achieve the maximum conversion efficiency should be decreased with the increment of radiator temperature and the performance degradation brought by any structure deviation from its optimal one would be stronger meanwhile. For the popular radiator temperature, no more than 1500 K in a real solar thermophotovoltaic system, and typical doping profile in GaSb cell, a reasonable absorbing layer structure parameter should be controlled within 100-300 nm for the emitter while 3000-5000 nm for the base.

OXYGEN ANALYSES IN HIGH-TC SUPERCONDUCTING FILMS BY MEV PROTON ELASTIC BACKSCATTERING

A technique for analysis of total oxygen contents in high-T(c) superconducting films is demonstrated. It uses elastic backscattering (EBS) of 1.5-2.5 MeV protons. By comparing the H EBS spectra from substrate materials, the absolute oxygen content in the films can be easily calculated. It is estimated that the analysis can be accurate to better than 5% for YBCO films with thicknesses from several hundred angstroms to several microns. Comparisons with RBS are given and advantages of this technique are shown.

MEV PROTON ELASTIC BACKSCATTERING ANALYSIS OF SILICON-NITRIDE AND OXIDE LAYERS ON SILICON

A simple method for the analysis of concentration ratios N/Si and O/Si in silicon nitride and oxide layers on silicon substrate is presented. 1.95-MeV proton elastic backscattering was used to determine the composition and density. A comparison with 2.1-MeV helium Rutherford backscattering measurements is given. Results are in good agreement with each other. The method is especially useful to analyze samples of 20 000 angstrom or thicker layers. We conclude that these two techniques are complementary for the measurements of samples with different thickness. A brief discussion has been given on results.

A study of the scaling height of the tropospheric aerosol and its extinction coefficient profile

IEECAS SKLLQG

Enhance the optical absorptivity of nanocrystalline TiO2 film with high molar extinction coefficient ruthenium sensitizers for high performance dye-sensitized solar cells

We report two new heteroleptic polypyridyl ruthenium complexes, coded C101 and C102, with high molar extinction coefficients by extending the pi-conjugation of spectator ligands, with a motivation to enhance the optical absorptivity of mesoporous titania film and charge collection yield in a dye-sensitized solar cell. On the basis of this C101 sensitizer, several DSC benchmarks measured under the air mass 1.5 global sunlight have been reached.

Conjugation of Selenophene with Bipyridine for a High Molar Extinction Coefficient Sensitizer in Dye-Sensitized Solar Cells

A high molar extinction coefficient heteroleptic polypyridyl ruthenium sensitizer, featuring a conjugated electron-rich selenophene unit in its ancillary ligand, has been synthesized and demonstrated as an efficient sensitizer in dye-sensitized solar cells. A nanocrystalline titania film stained with this sensitizer shows improved optical absorptivity, which is highly desirable for dye-sensitized solar cells with a thin photoactive layer. With preliminary testing, this sensitizer has already achieved a high efficiency of 10.6% measured under the air mass 1.5 global conditions.