Browsing by Author "Tseng, Y. C."

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Control of metamorphic buffer structure and device performance of InxGa1-xAs epitaxial layers fabricated by metal organic chemical vapor deposition
(2014-12-05)
Nguyen, H. Q.
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Yu, H. W.
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Luc, Q. H.
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Tang, Y. Z.
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Phan, V. T. H.
Using a step-graded (SG) buffer structure via metal-organic chemical vapor deposition, we demonstrate a high suitability of In0.5Ga0.5As epitaxial layers on a GaAs substrate for electronic device application. Taking advantage of the technique\'s precise control, we were able to increase the number of SG layers to achieve a fairly low dislocation density (similar to 10(6)cm(-2)), while keeping each individual SG layer slightly exceeding the critical thickness (similar to 80 nm) for strain relaxation. This met the demanded but contradictory requirements, and even offered excellent scalability by lowering the whole buffer structure down to 2.3 mu m. This scalability overwhelmingly excels the forefront studies. The effects of the SG misfit strain on the crystal quality and surface morphology of In0.5Ga0.5As epitaxial layers were carefully investigated, and were correlated to threading dislocation (TD) blocking mechanisms. From microstructural analyses, TDs can be blocked effectively through self-annihilation reactions, or hindered randomly by misfit dislocation mechanisms. Growth conditions for avoiding phase separation were also explored and identified. The buffer-improved, high-quality In0.5Ga0.5As epitaxial layers enabled a high-performance, metal-oxide-semiconductor capacitor on a GaAs substrate. The devices displayed remarkable capacitance-voltage responses with small frequency dispersion. A promising interface trap density of 3 x 10(12) eV(-1) cm(-2) in a conductance test was also obtained. These electrical performances are competitive to those using lattice-coherent but pricey InGaAs/InP systems.
Electronic contribution to the enhancement of the ferromagnetic ordering temperature by Si substitution in Gd-5(SixGe1-x)(4)
(2013-08-29)
Tseng, Y. C.
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Paudyal, D.
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Mudryk, Ya
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Pecharsky, V. K.
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Gschneidner, K. A., Jr.
By using a combination of x-ray spectroscopic and diffraction measurements at high pressures together with density functional theory (DFT) calculations, we show that the increase in Curie temperature T-C, induced by Si substitutions in Gd-5(SixGe1-x)(4) giant magnetocaloric materials is predominately electronically driven as opposed to lattice driven. Whereas, a lattice contraction with applied pressure increases the strength of exchange (magnetic) interactions between Gd spins, leading to a modest increase in T-C at a rate of 1.2 K/angstrom(3), much larger enhancements in T-C are obtained with Si doping for the same volume reduction (13.5 K/angstrom(3)), indicating that volume (lattice) effects are secondary. Similarly, an orthorhombic [O(II)] to monoclinic (M) structural phase transition is observed to take place with applied pressure in the paramagnetic state of a Gd-5(Si0.125Ge0.875)(4) sample at room temperature at a much smaller volume than needed to drive the same structural transition with Si doping, indicating that, even in the absence of magnetic order, electronic effects with Si doping dominate the energetics of structural transformations over lattice (volume) effects. DFT calculations show that the electronic mechanism behind this effect is a stronger Si 3p-Gd 5d than Ge 4p-Gd 5d hybridization, a critical ingredient in mediating indirect exchange interactions between localized Gd 4f spins. The results highlight the strong sensitivity of the magnetic ordering temperature to the nature of p-d hybridization, opening opportunities for tailoring the magnetocaloric properties of these compounds by substituting other p and rare-earth elements at the Si/Ge and Gd sites, respectively.
High-power structured laser modes: direct generation of a vortex array
(2020-07-15)
Chen, Y. F.
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Ke, H. T.
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Tseng, Y. C.
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Hsieh, M. X.
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Tung, J. C.
The frequency degeneracy induced by the astigmatism in a nearly hemispherical cavity is originally exploited to generate vortex array laser modes with the output power up to 300 mW. The inhomogeneous Helmholtz equation is employed to derive the wave function for manifesting the characteristics of the lasing modes. The theoretical wave function explicitly reveals the role of the Gouy phase in the formation of vortex arrays. Numerical analyses are further performed to confirm that the thermal lensing effect in the laser crystal assists the lasing transverse order to increase with increasing pump power. It is believed that the high efficiency enables the present laser modes to be useful in the applications of structured vortex beams. (C) 2020 Optical Society of America
High-power structured laser modes: manifestation of quantum Green's function
(2020-08-15)
Chen, Y. F.
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Tseng, Y. C.
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Ke, H. T.
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Hsieh, M. X.
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Tung, J. C.
The distributions of resonant frequencies in an astigmatic cavity are theoretically confirmed to be analogously equivalent to the quantum energy structures of two-dimensional commensurate harmonic oscillators. In the first part [Opt. Lett. 45, 4096 (2020)] of this two-part series study, the lasing modes were verified to reveal a variety of vortex array structures. Here, in the second part of this two-part series study, the lasing modes are confirmed to agree very well with the quantum Green's functions that correspond to a bundle of Lissajous figures in the high-order regime. (C) 2020 Optical Society of America
Magnetocaloric Properties of Melt-Spun Fe-Ni-Mn-Ga Ribbons
(2014-01-01)
Shih, C. W.
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Zhao, X. G.
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Chang, H. W.
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Tseng, Y. C.
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Chang, W. C.
The effects of Ni substitution for Fe on phase constitutions, Curie temperature T-C, and magnetocaloric properties of melt-spun Fe50-xNixMn25Ga25 (x =0, 1, 3, 5 and 7) ribbons have been investigated. X-ray diffraction results show that the main phase in the Fe50-xNixMn25Ga25 (x = 0-7) alloy changed with the increase of Ni content from FCC structure for x = 0 into B2 structure for x = 1-7. Besides, the magnetic phase exhibits phase transition of ferromagnetic into paramagnetic state with increasing temperature for the samples with B2-type structure. The Curie temperature T-C of these ribbons varies in the temperature range of 232-257 K. The peak values of the maximal magnetic entropy change, -Delta S-M(max), are about 1.4-1.6 Jkg/K for Ni-substituted ribbons at a maximum applied field of 30 kOe. On the other hand, the relatively broader temperature range at the half maximum of Delta S-M peak (similar to 90 K), low-cost and nontoxic elements make Fe-Ni-Mn-Ga-based ribbons the promising candidates for magnetic refrigeration applications close to room temperature.
Tunable complex magnetic states of epitaxial core-shell metal oxide nanocrystals fabricated by the phase decomposition method
(2016-07-13)
Chang, S. J.
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Lam, T. N.
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Yang, C. Y.
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Chen, Y. L.
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Chu, Y. H.
We report on the successful fabrication of epitaxial-discrete CoxFe3-xO4/CoO magnetic nanostructures on a SrTiO3 substrate as well as the results of a thorough investigation of the magnetic cross-reactions of the two phases in the vicinity of the epitaxial junction. These nanostructures were originally prepared as Fe3O4-CoO core-shell structures through the phase decomposition of bismuth perovskite precursors by pulsed-laser deposition. An antiphase boundary emerged during the structural/electronic transition from the CoO core to the Co1-xFe2+XO4 shell; this then developed into a ferrimagnetic/antiferromagnetic interface. Uncompensated spins (UCS) arose from the CoxFe3-xO4/CoO interface as a result of strong ferrimagnetic-antiferromagnetic interactions. A notable exchange bias as well as a significant exchange enhancement was observed owing to the UCS, which had a locking effect because of the decoupling of the Co1-xFe2+XO4/CoO reversal from the antiphase boundary. Control of the precursor ratio allowed for the fine-tuning of the Co1-xFe2+XO4 phase and the associated locking behaviors. This, in turn, allowed the anisotropy and coercivity of the nanostructures to be manipulated. Thus, we were able to create and thoroughly understand a complex epitaxial configuration with tunable structural and magnetic properties. This study should open new opportunities with regard to current magnetic oxide technology, which requires novel methods for pursuing extremity of controllable properties over an atomic landscape.