Mun-Bo Shim

Highly Stretchable Resistive Pressure Sensors Using a Conductive Elastomeric Composite on a Micropyramid Array

A stretchable resistive pressure sensor is achieved by coating a compressible substrate with a highly stretchable electrode. The substrate contains an array of microscale pyramidal features, and the electrode comprises a polymer composite. When the pressure-induced geometrical change experienced by the electrode is maximized at 40% elongation, a sensitivity of 10.3 kPa(-1) is achieved.

Cost- and time-effective three-dimensional bone-shape reconstruction from X-ray images.

Three‐dimensional (3D) bone shapes need to be created for visualization and pre‐operative surgery planning. Conventionally such shape data is extracted from volumetric data sets, obtained by three‐dimensional sensors, such as computerized tomography (CT) and magnetic resonance imaging (MRI). This conventional method is highly labor intensive and time consuming.

Three-dimensional shape reconstruction of abdominal aortic aneurysm

This paper presents an effective computational technique for reconstructing a three-dimensional shape of an abdominal aortic aneurysm (AAA), from a limited number of computed tomography (CT) images. The three-dimensional template geometry of a healthy abdominal aorta is used as a priori knowledge, and the template geometry is deformed by extended free-form deformation (EFFD), to generate a patient-specific AAA geometry. A two-step optimization scheme is devised to find an optimal set of EFFD parameters that match the cross-section of a deformed template with an AAA contour shown in a CT image. The geometric continuity of a deformed model is maintained by raising the order of the polynomial function used in EFFD. Experimental results show that the proposed method creates the three-dimensional shape of AAA suitable for structural finite element analysis and computational fluid dynamics for medical diagnosis.

Quantum efficiency affected by localized carrier distribution near the V-defect in GaN based quantum well

It is known that due to the formation of in-plane local energy barrier, V-defects can screen the carriers which non-radiatively recombine in threading dislocations (TDs) and hence, enhance the internal quantum efficiency in GaN based light-emitting diodes. By a theoretical modeling capable of describing the inhomogeneous carrier distribution near the V-defect in GaN based quantum wells, we show that the efficient suppression of non-radiative (NR) recombination via TD requires the local energy barrier height of V-defect larger than ∼80 meV. The NR process in TD combined with V-defect influences the quantum efficiency mainly in the low injection current density regime suitably described by the linear dependence of carrier density. We provide a simple phenomenological expression for the NR recombination rate based on the model result.

A Novel Growth Method of Freestanding GaN Using In situ Removal of Si Substrate in Hydride Vapor Phase Epitaxy

We demonstrate the freestanding GaN of 2 in. diameter and 400 µm thickness grown from Si substrate by hydride vapor phase epitaxy. To prevent the formation of cracks in the GaN layer during cooling, the Si substrate was removed at high temperature, which successfully suppressed the tensile stress evolution in GaN. The freestanding GaN exhibited high quality with FWHM of 65 arcsec in (0002) X-ray rocking curve and etch pit density of less than 1×106/cm2. It may be possible to fabricate high-quality freestanding GaN substrates of over 8 in. diameter using this method.

Partial strain relaxation effects on polarization anisotropy of semipolar (112¯2) InGaN/GaN quantum well structures

Partial strain relaxation effects on polarization anisotropy of semipolar (112¯2) InGaN/GaN quantum well (QW) structures were investigated using the multiband effective-mass theory. In the case of strain relaxation of ϵx′x′ along x′-axis, the polarization ratio gradually decreases with increasing strain relaxation. Also, with the strain relaxation by the same amount, the variation of the polarization ratio along x′-axis is shown to be much larger than that along y′-axis. However, the polarization switching is not observed even at a high In composition of 0.4 due to a small strain component (ϵx′x′0) with no strain relaxation. On the other hand, in the case of strain relaxation of ϵy′y′ along y′-axis, the polarization switching is observed, and the optical anisotropy is found to change from positive to negative with increasing strain relaxation. Also, the absolute value of the polarization ratio gradually decreases with increasing carrier density. However, the polarization switching due to the carrier densi...

Piezoelectricity in PbZrxTi1-xO3 Studied by Density-Functional Perturbation Theory Supercell Calculations

First-principles calculations of the piezoresponses (e33) of PbZrxTi1-xO3 (PZT) with various crystalline phases and Zr/Ti ratios (composition: B-site cations) are performed at room temperature using density-functional perturbation theory (DFPT) to understand the experimental observation that large piezoresponses occur at the morphotropic phase boundary (MPB). The results of ab initio calculations indicate that PZT (Zr/Ti = 52/48) with a tetragonal (P4MM) phase shows the lowest formation energy (stable) and the highest piezoelectric coefficient, e33, in the MPB region. The Z*/Δe ratio (where Z* is the Born effective charge and Δe is the variation in the internal strain) is computed, and we find that laterally bonded oxygen atoms are the main contributors to e33 in the above structure owing to the low Δe3 caused by perturbation.

Quaternary AlInGaN/InGaN quantum well on vicinal c-plane substrate for high emission intensity of green wavelengths

Electronic and optical properties of non-trivial semipolar AlInGaN/InGaN quantum well (QW) structures are investigated by using the multiband effective-mass theory and non-Markovian optical model. On vicinal c-plane GaN substrate miscut by a small angle (θ < 40°) from c-plane, the AlInGaN/InGaN system is shown to have ∼3 times larger spontaneous emission peak intensity than the conventional InGaN/GaN system at green wavelength. It is attributed to much larger optical matrix element of the quaternary AlInGaN/InGaN system, derived from the reduction of internal electric field induced by polarizations. This effect exceeds the performance-degrading factor of smaller quasi-Fermi-level separation for the quaternary AlInGaN/InGaN system than that for the conventional InGaN/GaN system. Results indicate that the use of quaternary III-nitride QWs on vicinal substrates may be beneficial in improving the performance of optical devices emitting green light.

Optimal patterns for sequentially multiple focusing in high intensity focused ultrasound and their application to thermal dose

The purpose of this study is to propose a new method for multiple-focus generation to shorten overall treatment time as well as to avoid the formation of high intensity regions outside the target volume. A numerical simulation of the acoustic fields produced by a 1017-element spherical-section ultrasound phased array transducer operating at a frequency of 1.0MHz with 16 cm radius of curvature is performed for the proposed multiple-focus generation. The total foci are partitioned into the several patterns because multiple focusing generally gives rise to the grating lobes outside of the three dimensional region of interest even if applying the optimization of intensity gain in determining the phases and amplitudes of the excitation source vector. The optimization problem is repeatedly formulated in term of the focal points until the multiple-focus patterns cover all the focal points. Genetic algorithm is used for selecting the patterns without the grating lobes. The obtained set of multiple-focus patterns ...

A sparse-element phased array system based on sequentially multiple focusing for the treatment of large tumors

The purpose of this study is the optimal design on High Intensity Focused Ultrasound transducer architecture with sparse-element to reduce the number of RF driving channels required for practical implementation for cost- and time-effective HIFU treatment of tumors. For rapid treatment of large-sized tumor while protecting healthy tissues, volumetric ablation by multiple focusing is one of the promising solutions, which does not require mechanical or electrical beam steering to move focus within a focal area. In the previous study, a numerical optimization with a 1017-element spherical-section ultrasound phased array transducer operating at a frequency of 1.0MHz with 16 cm radius of curvature was performed for the generation of multiple foci. The ultimate design of the current study is the sparse array so as to reduce the number of driving channels and cut down the cost. It is necessary that this design with less number of elements and driving channels shows similar sonication effect with the full array. The distribution of the reduced number of elements in the sparse array can be optimized by the incorporation of genetic algorithms to achieve acceptable properties for the radiation pattern. The simulation results of the optimized sparse array showed that the suggested distributional pattern of phased array achieved a well-defined focal zone with multiple foci at the aimed area. In conclusion, a sparse array can benefit from the reduction of the number of driving channels to achieve similar sonication effect with the full array.