Browsing by Author "Zhang, Xin"
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Item Characterizations of annealed ion implanted silicon carbide materials and devices(University of Delaware, 2005) Zhang, XinSilicon carbide (SiC) materials have unique materials properties, such as high good thermal conductivity high saturated velocity and high breakdown field, which make them attractive many applications in the areas of high temperature, high frequency and high power devices. Despite the superior materials properties of SiC as listed above, it is difficult to fabricate SiC devices with conventional silicon CMOS technology. One of the difficulties is to introduce dopants into SiC. Unlike silicon, the introduction of dopants into SiC was mainly done with ion implantation. The lattice damage and strain caused by ion implantation needs to be removed by high temperature annealing. Therefore, a major part of this thesis was to design and realize a high temperature annealing furnace to remove the lattice damage and strain. Annealed samples were analyzed with high resolution X-Ray diffraction and Fourier Transform Infrared Spectroscopy. Our results suggest that high temperature thermal annealing can remove the strain and damage caused by ion implantation. ☐ Recent advances in impurity doped silicon terahertz emitters motivated us to study the possibility of using nitrogen doped 4H-SiC devices as terahertz emitters. Due to the two non-equivalent donor sites of nitrogen in 4H-SiC, for the first time, strong terahertz emission was observed up to temperatures of 150K. This result suggests that it may be possible to use SiC terahertz emitters for high temperature, high power portable applications.Item Static coded aperture in robotic X-ray tomography systems(Optics Express, 2022-02-22) Mao, Tianyi; Ma, Xu; Cuadros, Angela P.; Dai, XiuBin; Wang, Zhiteng; Zhang, Xin; Zhu, Shujin; Zhu, Jianjian; Arce, Gonzalo R.Coded aperture X-ray computed tomography is a computational imaging technique capable of reconstructing inner structures of an object from a reduced set of X-ray projection measurements. Coded apertures are placed in front of the X-ray sources from different views and thus significantly reduce the radiation dose. This paper introduces coded aperture X-ray computed tomography for robotic X-ray systems which offer positioning flexibility. While single coded-aperture 3D tomography was recently introduced for standard trajectory CT scanning, it is shown that significant gains in imaging performance can be attained by simple modifications in the CT scanning trajectories enabled by emerging dual robotic CT systems. In particular, the subject is fixed on a plane and the CT system uniformly rotates around the r −axis which is misaligned with the coordinate axes. A single stationary coded aperture is placed on front of the robotic X-ray source above the plane and the corresponding X-ray projections are measured by a two-dimensional detector on the second arm of the robotic system. The compressive measurements with misalignment enable the reconstruction of high-resolution three-dimensional volumetric images from the low-resolution coded projections on the detector at a sub-sampling rate. An efficient algorithm is proposed to generate the rotation matrix with two basic sub-matrices and thus the forward model is formulated. The stationary coded aperture is designed based on the Pearson product-moment correlation coefficient analysis and the direct binary search algorithm is used to obtain the optimized coded aperture. Simulations using simulated datasets show significant gains in reconstruction performance compared to conventional coded aperture CT systems.