Browsing by Author "Chiang, C. H."

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Analysis of strain relaxation in GaAs/InGaAs/GaAs structures by spectroscopy of relaxation-induced states
(2007-02-01)
Chen, J. F.
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Chiang, C. H.
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Hsieh, P. C.
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Wang, J. S.
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電子物理學系
Strain relaxation in GaAs/In0.2Ga0.8As/GaAs structures is investigated by analyzing relaxation-induced traps. Strain relaxation is shown to cause carrier depletion by the induction of a 0.53 eV trap in the top GaAs layer, a 0.13 eV trap in the InGaAs layer, and a 0.33 eV trap in the neighboring lower GaAs layer. The 0.53 eV trap which exhibits a logarithmic function of transient capacitance is attributed to threading dislocations. The 0.33 eV trap exhibits an exponential transient capacitance, suggesting a GaAs point defect as its origin. Given its activation energy, it is assigned to the EL6 in GaAs, commonly considered to be As-i-V-Ga complexes. This trap and the 0.13 eV trap are regarded as the same, since their energy difference is comparable to the optically determined conduction-band offset. The spatial location of this trap correlates with that of misfit dislocations. Accordingly, the production of this trap is determined from the mechanism of strain relaxation. A likely mode of strain relaxation is deduced from the locations of these traps. (c) 2007 American Institute of Physics.
Bimodel onset strain relaxation in InAs quantum dots with an InGaAs capping layer
(2010-08-30)
Chen, J. F.
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Chen, Ross C. C.
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Chiang, C. H.
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Chen, Y. F.
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Wu, Y. H.
Capping InAs quantum dots (QDs) with an InGaAs layer allows strain relaxation to induce a low-energy electron state below a set of fine dot family states, which is consistent with photoluminescence (PL) spectra. The evolution of InAs thickness suggests a bimodal onset relaxation, i.e., a fine dot family that is strain-relieved by indium outdiffusion from the QDs, as suggested by transmission electron microscopy, and a low-energy dot family that is strain relaxed by the generation of lattice misfits. The indium outdiffusion can explain an abnormal PL blueshift in 70 meV in the fine dot family at onset of strain relaxation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3483757]
Compensation effect and differential capacitance analysis of electronic energy band structure in relaxed InAs quantum dots
(2010-09-15)
Chen, J. F.
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Chen, Ross C. C.
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Chiang, C. H.
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Hsieh, M. C.
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Chang, Y. C.
The use of a differential capacitance technique for analyzing the effect of strain relaxation on the electronic energy band structure in relaxed InAs self-assembled quantum dots (QDs) is presented. Strain relaxation is shown to induce a deep defect state and compensate the ionized impurity in the bottom GaAs layer, leading to a double depletion width and a long emission time. An expression of capacitance at different frequency and voltage is derived for analyzing the experimental data. It has been shown that the relationship between the low-frequency and high-frequency capacitances can be well explained by a Schottky depletion model with a compensated concentration in the bottom GaAs layer. A simple expression is presented to account for the modulation of the free electrons in the top GaAs layer. This capacitance analysis shows a long low-energy tail for the electron ground state, suggesting not very uniform strain relaxation. The results of this study illustrate a carrier compensation effect of the defect state on the electronic energy band structure near the QDs. (C) 2010 American Institute of Physics. [doi:10.1063/1.3467938]
Deep-level emissions in GaAsN/GaAs structures grown by metal organic chemical vapor deposition
(2007-06-15)
Chen, J. F.
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Ke, C. T.
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Hsieh, P. C.
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Chiang, C. H.
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Lee, W. I.
This work presents the deep-level photoluminescence of coherently strained GaAsN/GaAs quantum-well (QW) structures with various GaAsN thicknesses and N contents. A broad deep-level emission at similar to 1.1 eV is observed, whose wavelength is redshifted as the GaAsN thickness increases. Based on its energy separation from the QW emission, this emission is attributed to a transition between the QW electron ground state and a deep level at similar to 0.2 eV above the GaAsN valence-band (VB) edge. This level is shown to be tied to the GaAs band edge. A transition between this level and the GaAs conduction band allows the GaAsN-GaAs band alignment to be evaluated. A type II band lineup is obtained with VB offsets of 0.03 and 0.002 eV for N=0.6% and 1.8%, respectively. The decreased VB offset suggests a transition from type II to type I with increasing N content. Thermal annealing effectively removes this level and improves the QW emission. The concentration of this level is not clearly correlated with N content, suggesting that this level is induced by a low-temperature growth of the GaAsN layer to suppress the composition fluctuation. Given its energy, this level is tentatively assigned to V-Ga. (c) 2007 American Institute of Physics.
Development of a Parallel Explicit Finite-Volume Euler Equation Solver using the Immersed Boundary Method with Hybrid MPI-CUDA Paradigm
(2020-02-01)
Kuo, F. A.
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Chiang, C. H.
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Lo, M. C.
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Wu, J. S.
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機械工程學系
This study proposed the application of a novel immersed boundary method (IBM) for the treatment of irregular geometries using Cartesian computational grids for high speed compressible gas flows modelled using the unsteady Euler equations. Furthermore, the method is accelerated through the use of multiple Graphics Processing Units - specifically using Nvidia's CUDA together with MPI - due to the computationally intensive nature associated with the numerical solution to multi-dimensional continuity equations. Due to the high degree of locality required for efficient multiple GPU computation, the Split Harten-Lax-van-Leer (SHLL) scheme is employed for vector splitting of fluxes across cell interfaces. NVIDIA visual profiler shows that our proposed method having a computational speed of 98.6 GFLOPS and 61% efficiency based on the Roofline analysis that provides the theoretical computing speed of reaching 160 GLOPS with an average 2.225 operations/byte. To demonstrate the validity of the method, results from several benchmark problems covering both subsonic and supersonic flow regimes are presented. Performance testing using 96 GPU devices demonstrates a speed up of 89 times that of a single GPU (i.e. 92% efficiency) for a benchmark problem employing 48 million cells. Discussions regarding communication overhead and parallel efficiency for varying problem sizes are also presented.
Effect of antimony incorporation on the density, shape, and luminescence of InAs quantum dots
(2008-07-15)
Chen, J. F.
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Chiang, C. H.
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Wu, Y. H.
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Chang, L.
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Chi, J. Y.
This work investigates the surfactant effect on exposed and buried InAs quantum dots (QDs) by incorporating Sb into the QD layers with various Sb beam equivalent pressures (BEPs). Secondary ion mass spectroscopy shows the presence of Sb in the exposed and buried QD layers with the Sb intensity in the exposed layer substantially exceeding that in the buried layer. Incorporating Sb can reduce the density of the exposed QDs by more than two orders of magnitude. However, a high Sb BEP yields a surface morphology with a regular periodic structure of ellipsoid terraces. A good room-temperature photoluminescence (PL) at similar to 1600 nm from the exposed QDs is observed, suggesting that the Sb incorporation probably improves the emission efficiency by reducing the surface recombination velocity at the surface of the exposed QDs. Increasing Sb BEP causes a blueshift of the emission from the exposed QDs due to a reduction in the dot height as suggested by atomic force microscopy. Increasing Sb BEP can also blueshift the similar to 1300 nm emission from the buried QDs by decreasing the dot height. However, a high Sb BEP yields a quantum well-like PL feature formed by the clustering of the buried QDs into an undulated planar layer. These results indicate a marked Sb surfactant effect that can be used to control the density, shape, and luminescence of the exposed and buried QDs. (C) 2008 American Institute of Physics.
How do InAs quantum dots relax when the InAs growth thickness exceeds the dislocation-induced critical thickness?
(2012-01-01)
Chen, J. F.
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Lin, Y. C.
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Chiang, C. H.
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Chen, Ross C. C.
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Chen, Y. F.
A simple critical thickness for generating lattice misfits is insufficient to describe the onset strain relaxation in InAs quantum dots (QDs). A predominant dot family is shown to relieve its strain by In/Ga interdiffusion, rather than by lattice misfits, at the onset of strain relaxation. This argument is based on photoluminescence spectra, which show the emergence of a fine blueshifted transition at the onset of strain relaxation, along with a low-energy transition from a dot family degraded by lattice misfits. From the analysis of the temperature-dependent blueshift and energy separation between the ground and excited-state transitions, the blueshift is attributed to In/Ga interdiffusion. Transmission electron microscopy suggests a relaxation-induced indium migration from the interdiffused dot family to the dislocated dot family. Post-growth thermal annealing can further relieve strain by inducing more In/Ga interdiffusion in the interdiffused dot family and more dislocations in the dislocated dot family. This study explains the co-existence of strong carrier confinement in the QDs and enormous misfit-related traps in the capacitance-voltage spectra, and an elongated QD electron-emission time. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3675519]
Improving the photoluminescence properties of self-assembled InAs surface quantum dots by incorporation of antimony
(2011-08-15)
Chiang, C. H.
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Wu, Y. H.
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Hsieh, M. C.
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Yang, C. H.
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Wang, J. F.
This study investigates the effects of surfactant and segregation from InAs surface quantum dots (SQDs) by incorporating antimony (Sb) into the QD layers. The Sb surfactant effect extends planar growth and suppresses dot formation. Incorporating Sb can reduce the density of SQDs by more than two orders of magnitude. Photoluminescence (PL) reveals enhancement in the optical properties of InAs SQDs as the Sb beam equivalent pressure (BEP) increases. This improvement is caused by the segregation of Sb on the surface of SQDs, which reduces non-radiative recombination and suppresses carrier loss. The dark line at the SQDs surface in the transmission electron microscopic image suggests that the incorporated Sb probably segregates close to the surface of the SQDs. These results indicate a marked Sb segregation effect that can be exploited to improve the surface-sensitive properties of SQDs for biological sensing. Crown Copyright (C) 2011 Published by Elsevier B. V. All rights reserved.
Nonpolar a-plane GaN grown on r-plane sapphire using multilayer AlN buffer by metalorganic chemical vapor deposition
(2011-01-15)
Chiang, C. H.
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Chen, K. M.
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Wu, Y. H.
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Yeh, Y. S.
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Lee, W. I.
Mirror-like and pit-free non-polar a-plane (11-20) GaN films are grown on r-plane (1-102) sapphire substrates using metalorganic chemical vapor deposition (MOCVD) with multilayer high-low-high temperature AlN buffer layers. The buffer layer structure and film quality are essential to the growth of a flat, crack-free and pit-free a-plane GaN film. The multilayer AlN buffer structure includes a thin low-temperature- deposited AlN (LT-AlN) layer inserted into the high-temperature-deposited AlN (HT-AlN) layer. The results demonstrate that the multilayer AlN buffer structure can improve the surface morphology of the upper a-plane GaN film. The grown multilayer AlN buffer structure reduced the tensile stress on the AlN buffer layers and increased the compressive stress on the a-plane GaN film. The multilayer AlN buffer structure markedly improves the surface morphology of the a-plane GaN film, as revealed by scanning electron microscopy. The effects of various growth V/III ratios was investigated to obtain a-plane GaN films with better surface morphology. The mean roughness of the surface was 1.02 nm, as revealed by atomic force microscopy. Accordingly, the multilayer AlN buffer structure improves the surface morphology and facilitates the complete coalescence of the a-plane GaN layer. (C) 2010 Elsevier B.V. All rights reserved.
Photoluminescence and Raman studies of GaN films grown by MOCVD
(2009)
Luong Tien Tung
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Lin, K. L.
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Chang, E. Y.
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Huang, W. C.
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Hsiao, Y. L.
The photoluminescence (PL) of high quality GaN epitaxial layer grown by MOCVD was investigated for various excitation power and temperatures from 8.3 to 300K. The PL at 8.3 K and with relatively low excitation power of GaN film grown on c-plane sapphire by using MOCVD shows clearly free-exciton A, B and exciton A bound to neutral donors (D degrees X) at 3.502 eV, 3.509 eV, and 3.496 eV, respectively. The full width at half maximum (FWHM) and binding energy of exciton A of the high quality GaN film were evaluated as small as 3.7 meV and 27.9 +/- 0.5 meV, respectively. In addition, PL and Raman scattering of GaN films grown on r-plane sapphire and (111) Si substrates by using MOCVD were examined. The residual stress effect was detected in all films.
Relaxation-induced lattice misfits and their effects on the emission properties of InAs quantum dots
(2007-09-05)
Chen, J. F.
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Wang, Y. Z.
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Chiang, C. H.
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Hsiao, R. S.
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Wu, Y. H.
Strain relaxation in InAs/InGaAs quantum dots (QDs) is shown to introduce misfits in the QD and neighboring GaAs bottom layer. A capacitance -voltage profiling shows an electron accumulation peak at the QD with a long emission time, followed by additional carrier depletion caused by the misfits in the GaAs bottom layer. The emission-time increase is explained by the suppression of tunneling for the QD excited states due to the additional carrier depletion. As a result, electrons are thermally activated from the QD states to the GaAs conduction band, consistent with observed emission energies of 0.160 and 0.068 eV which are comparable to the confinement energies of the QD electron ground and first-excited states, respectively, relative to the GaAs conduction band. This is in contrast to non-relaxed samples in which emission energy of 60 meV is observed, corresponding to the emission from the QD ground state to the first-excited state.
Structural and optical properties of buried InAs/GaAs quantum dots on GaAsSb buffer layer
(2009-09-21)
Wu, Y. H.
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Chang, Li
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Lin, P. Y.
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Chiang, C. H.
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Chen, J. F.
In this work, we investigated the effect of a 1 nm thick GaAs(94.3)Sb(0.57) buffer layer on structural and optical properties of buried InAs quantum dots (QDs) and wetting layers (WLs) in GaAs using photoluminescence (PL) and transmission electron microscopy (TEM). The density and emission wavelength of the QDs on the buffer were increased due to the size and the shape modification in comparison with those without a buffer. PL analysis of the ground-state (GS) peak of the QDs on the buffer showed a red-shift of 18 meV with an enhanced intensity. In addition, PL and TEM show that the buffer has a weak effect on the WLs and no apparent changes occur for the buffer during QD deposition and the capping process.