Duan Xiaofeng

Defect reduction in GaAs/Si film with InAs quantum-dot dislocation filter grown by metalorganic chemical vapor deposition

The growth of GaAs epilayers on silicon substrates with multiple layers of InAs quantum dots (QDs) as dislocation filters by metalorganic chemical vapor deposition (MOCVD) is investigated in detail. The growth conditions of single and multiple layers of QDs used as dislocation filters in GaAs/Si epilayers are optimized. It is found that the insertion of a five-layer InAs QDs into the GaAs buffer layer effectively reduces the dislocation density of GaAs/Si film. Compared with the dislocation density of 5×107 cm−2 in the GaAs/Si sample without QDs, a density of 2×106 cm−2 is achieved in the sample with QD dislocation filters.

Optical Properties of Hexagonal and Cubic ZnS Nanoribbons: Experiment and Theory

Optical properties of hexagonal and cubic ZnS nanoribbons are studied by using valence electron energy loss spectroscopy (VEELS) and ab initio band structure calculations. The peaks in VEELS are assigned to interband transitions by comparing the interband transition strengths with the calculated densities of states. The optical properties are deduced from the experimental VEELS, and the theoretical calculations give consistent results. This combination of experimental and theoretical approaches provides a comprehensive understanding of the optical properties of polytype ZnS.

Defect Reduction in GaAs/Si Films with the a-Si Buffer Layer Grown by Metalorganic Chemical Vapor Deposition*

The growth of GaAs epilayers on silicon substrates with a thin amorphous silicon (a-Si) buffer layer by metalorganic chemical vapor deposition is investigated in detail. Combining with the two-step growth method, the growth conditions of the a-Si buffer layer are optimized for growth of high-quality GaAs/Si epilayers. The a-Si buffer layer exhibits the best effect with thickness of 1.8 nm and growth temperature of 620°C. It is found that the introduction of this a-Si layer on Si substrates effectively reduces the dislocation density in GaAs/Si films. As compared with the dislocation density of 5 × 107 cm−2 in the GaAs/Si sample without the a-Si layer, a density of 3 × 105 cm−2 is achieved in the sample with the a-Si layer, and the defect reduction mechanism is discussed in detail.

Modeling of Fano Resonance in High-Contrast Resonant Grating Structures

A new model is presented for Fano resonance in resonant grating structure based on the temporal coupled mode theory. By using this model, the reflection spectrum can be reproduced with the information of eigenmode of the structure, which can be numerically calculated by the finite element method. Therefore, the eigenmode plays a key role in determining the profile of the line shape of the Fano resonance in the resonant grating structure. When the space of two grating modulations is decreased, the line shape experiences a significant change. Such a drastic change can be attributed to the increase of quality factor of the eigenmodes. Thus, our model not only provides a simple and intuitive understanding on the mechanism of Fano resonance, but it also offers a convenient way to engineer the line shape of the Fano resonance. The proposed model can be used in many applications, such as biosensors, optical filters, and optical switchers.

Centroid registration difference algorithm for detecting spatial moving targets

In order to meet the needs of recognizing the targets under the moving space background by micro spacecraft target detector (MTD). According to the particular characters of the spatial background, this paper proposes an algorithm which combines the image centroid registration with the single difference method, named the Centroid Registration Difference Algorithm. In this algorithm, we labeled only the available bright sign spots and has distilled its centroid in two frames, and then used the image centroid registration to compare the two images and eliminated the same light spots between them, then the remaining bright spots in the present image were our targets. The simulation results indicate that the proposed algorithm has a low computational cost and the effect of the recognition is good.

Fabrication and characterization of novel high-speed InGaAs/InP uni-traveling-carrier photodetector for high responsivity*

A top-illuminated circular mesa uni-traveling-carrier photodetector (UTC-PD) is proposed in this paper. By employing Gaussian graded doping in InGaAs absorption layer and InP depleted layer, the responsivity and high speed response characteristics of the device are optimized simultaneously. The responsivity up to 1.071 A/W (the external quantum efficiency of 86%) is obtained at 1550 nm with a 40-μm diameter device under 10-V reverse bias condition. Meanwhile, the dark current of 7.874 nA and the 3-dB bandwidth of 11 GHz are obtained with the same device at a reverse bias voltage of 3 V.

Optical Characterization of β-FeSi2 Thin Films Prepared by Femtosecond Laser Ablation

Iron disilicide thin films are prepared on fused quartz using femtosecond laser deposition (FsPLD) with a FeSi2 alloy target. X-ray diffraction results indicate the films are single-phase, orthorhombic, β-FeSi2. Field scanning electron microscopy, high resolution transmission electron microscopy, UV–VIS–NIR spectroscopy and Raman microscope are used to characterize the structure, composition, and optical properties of the β-FeSi2 films. Normal incidence spectral transmittance and reflectance data indicate a minimum, direct energy gap of 0.85 eV. The two most intense lines of Raman scattering peaked at 181.3 cm−1 and 235.6 cm−1 for the film on fused quartz, and at 191.2 cm−1 and 243.8 cm−1 for the film on Si (100), are observed.

Structure refinement of the icosahedral AIPdMn quasicrystal using quantitative CBED

The atomic structure of the perfect AIPdMn icosahedral phase has been studied on a single crystal specimen by quantitative convergent beam electron diffraction (QCBED) technique in combination with describing the shape of atonic surface by symme try-adapted series of surface harmonics. The spherical model was used as the starting model for the refinement. By fitting the calculated electron diffraction intensities to the experimental line scan profile, the coefficients in the surface harmonics expansion of the boundaries of atomic surface are refined. The refined parameters show that the fluctuations of the external boundary of atomic surface for Pd atn0 can be as large as 0.2 nm. The boundaries of atomic surfaces for Mn show little fluctuation. In the present model. the number of unphysically short interatonic distances is significantly reduced in comparison with the spherical model.

Giant magneto-impedance of Co50Ni22Ga28 alloy with high chemical ordering

The microstructure Of Co50Ni22Ga28 ribbon with the L-10 structure is examined. The band-like morphology is observed. These bands with the width in a range of 40-200 nm appear along the transverse direction of the ribbon. The giant magnetoimpedance (GMT) effect in this alloy is measured. The results show that Co50Ni22Ga28 exhibits a sharp peak of the GMI effect. The maximum GMI ratio up to 360% is detected. The GMI effect measured versus temperature shows large jumps of the magnetoimpedance amplitude at the reversal martensitic transformation temperature 240 degrees C and Curie temperature 375 degrees C respectively. The jump ratios of the magnetoimpedance amplitude examined at these temperatures are about 5 and 10, respectively.

Defects in GaN films grown on Si(111) substrates by metal-organic chemical vapour deposition

We report the transmission-electron microscopy study of the defects in wurtzitic GaN films grown on Si(111) substrates with AIN buffer layers by the metal-organic chemical vapour deposition method. The In0.1Ga0.9N/GaN multiple quantum well (MQW) reduced the dislocation density by obstructing the mixed and screw dislocations passing through the MQW. No evident reduction of the edge dislocations density by the MQW was observed. It was found that dislocations with screw component can be located at the boundaries of sub-grains slightly in-plane misoriented.