Browsing by Author "Cheng, S. J."

Now showing 1 - 9 of 9

Anomalous anticrossing of neutral exciton states in GaAs/AlGaAs quantum dots
(2014-03-12)
Kumar, S.
;
Zallo, E.
;
Liao, Y. H.
;
Lin, P. Y.
;
Trotta, R.
We study the effects of heavy hole-light hole (HH-LH) mixing on fine-structure and polarization properties of neutral excitons ( X-0) confined in single GaAs/AlGaAs quantum dots (QDs) under the application of anisotropic biaxial stress. In the large HH-LH mixing regime, these properties are substantially different from the usually observed properties in the case of small or no mixing. By varying the applied stress, the mixing in the initially strain-free QDs changes from similar to 0 to similar to 70% and an anomalous anticrossing of the X-0 bright states is observed. The latter is attributed to stress-induced rotation of the in-plane principal axis of the QD confinement potential. We show that the analysis of free-excitonic emission of bulk GaAs surrounding the QDs not only allows estimation of the stress and mixing in the QDs, but also provides the quantum-confinement-induced HH-LH splitting of the as-grown QDs.
Anomalous diamagnetic shift for negative trions in single semiconductor quantum dots
(2010-03-01)
Fu, Y. J.
;
Lin, S. D.
;
Tsai, M. F.
;
Lin, H.
;
Lin, C. H.
We report on the magnetic response of negative trions X(-) in single self-assembled InAs/GaAs quantum dots. Unlike the conventional quadratic diamagnetic shift for neutral excitons, the observed X(-) diamagnetic shifts are small and nonquadratic. In particular, we also observed a reversal in sign of the conventional diamagnetic shift. A theoretical analysis indicates that such anomalous behaviors for X(-) arise from an apparent change in the electron wave function extent after photon emission due to the strong Coulomb attraction induced by the hole in its initial state. This effect can be very pronounced in small quantum dots, where the electron wave function becomes weakly confined and extended much into the barrier region. When the electrons gradually lose confinement, the magnetic response of X(-) will transit gradually from the usual quadratic diamagnetic shift to a quartic dependence, and finally into a special paramagnetic regime with an overall negative energy shift.
Efficiency enhancement of UV/blue light emitting diodes via nanoscaled epitaxial lateral overgrowth of GaN on a SiO(2) nanorod-array patterned sapphire substrate
(2008-11-15)
Chiu, C. H.
;
Li, Zhen-Yu
;
Chao, C. L.
;
Lo, M. H.
;
Kuo, H. C.
High-quality GaN layer was successfully grown on a SiO(2) nanorod-array patterned sapphire substrate (NAPSS) using metal-organic chemical vapor deposition by a nanoscaled epitaxial lateral overgrowth (NELO) method. From tunneling electron microscope images, well coalescence and turned dislocations are parallel to the surface direction were clearly observed. The Raman shift measurement shows the residual stress in GaN was greatly reduced from 1.359 to 0.652 GPa when compared to a GaN on flat sapphire substrate. Also, a high-efficiency GaN-based light-emitting diodes (LEDs) were demonstrated on the NELO NAPSS GaN, the respective output power and external quantum efficiency of the NELO NAPSS LEDs were estimated to be 22 mW and 40.2% at an injection current of 20 mA, showing an enhancement factor of 52% over conventional LEDs. The significant improvement resulted from both the enhanced light extraction assisted by the NAPSS, and the reduced dislocation densities using the NELO method. (c) 2008 Published by Elsevier B.V.
Efficiency enhancement of UV/blue light emitting diodes via nanoscaled epitaxial lateral overgrowth of GaN on a SiO2 nanorod-array patterned sapphire substrate
(2008-11-15)
Chiu, C. H.
;
Li, Zhen-Yu
;
Chao, C. L.
;
Lo, M. H.
;
Kuo, H. C.
High-quality GaN layer was successfully grown on a SiO2 nanorod-array patterned sapphire substrate (NAPSS) using metal-organic chemical vapor deposition by a nanoscaled epitaxial lateral overgrowth (NELO) method. From tunneling electron microscope images, well coalescence and turned dislocations are parallel to the surface direction were clearly observed. The Raman shift measurement shows the residual stress in GaN was greatly reduced from 1.359 to 0.652 GPa when compared to a GaN on flat sapphire substrate. Also, a high-efficiency GaN-based light-emitting diodes (LEDs) were demonstrated on the NELO NAPSS GaN, the respective output power and external quantum efficiency of the NELO NAPSS LEDs were estimated to be 22 mW and 40.2% at an injection current of 20 mA, showing an enhancement factor of 52% over conventional LEDs. The significant improvement resulted from both the enhanced light extraction assisted by the NAPSS, and the reduced dislocation densities using the NELO method. (c) 2008 Published by Elsevier B.V.
Freestanding high quality GaN substrate by associated GaN nanorods self-separated hydride vapor-phase epitaxy
(2009-08-03)
Chao, C. L.
;
Chiu, C. H.
;
Lee, Y. J.
;
Kuo, H. C.
;
Liu, Po-Chun
This work proposes a method for fabricating 2 in. freestanding GaN substrates of high crystallographic quality and low residual strain. Arrays of GaN nanorods with sidewalls coated with silicon dioxide (SiO(2)) were randomly arranged on the sapphire substrate as a growth template for subsequent hydride vapor-phase epitaxy (HVPE). The passivation of the sidewalls coated with SiO(2) prevents the coalescence of GaN grains in spaces between the rods, causing them to grow preferentially on the top of individual rods. The proposed method significantly improves GaN crystal quality and results in self-separation from the underlying host sapphire substrate due to the relaxation of thermal strains in the HVPE cooling-down process.
Nanoscale epitaxial lateral overgrowth of GaN-based light-emitting diodes on a SiO(2) nanorod-array patterned sapphire template
(2008-08-25)
Chiu, C. H.
;
Yen, H. H.
;
Chao, C. L.
;
Li, Z. Y.
;
Yu, Peichen
High efficiency GaN-based light-emitting diodes (LEDs) are demonstrated by a nanoscale epitaxial lateral overgrowth (NELO) method on a SiO(2) nanorod-array patterned sapphire substrate (NAPSS). The transmission electron microscopy images suggest that the voids between SiO(2) nanorods and the stacking faults introduced during the NELO of GaN can effectively suppress the threading dislocation density. The output power and external quantum efficiency of the fabricated LED were enhanced by 52% and 56%, respectively, compared to those of a conventional LED. The improvements originated from both the enhanced light extraction assisted by the NAPSS and the reduced dislocation densities using the NELO method. (C) 2008 American Institute of Physics.
Optical fine structures of highly quantized InGaAs/GaAs self-assembled quantum dots
(2010-06-30)
Ramirez, H. Y.
;
Lin, C. H.
;
Chao, C. C.
;
Hsu, Y.
;
You, W. T.
A theoretical model for the electron-hole exchange interaction in three-dimensionally (3D) confining semiconductor nanostructures is presented to explain the observed decreasing tendency of the fine-structure splittings (FSSs) of small InGaAs/GaAs self-assembled quantum dots (QDs) with increasing the emission energies. The experimentally revealed FSS reduction is shown to be highly associated with the significant 3D spreading of electronic orbitals and reduced overlap of electron and hole wave functions in small and/or Ga-diffused QDs. The combination of quantum size and Ga-diffusion effects substantially reduces the averaged e-h exchange interaction and leads to the reduced FSSs in the regime of high emission energy.
Single-exciton energy shell structure in InAs/GaAs quantum dots
(2008-12-01)
Awirothananon, S.
;
Raymond, S.
;
Studenikin, S.
;
Vachon, M.
;
Render, W.
The energy shell structure of a single exciton confined in a self-assembled quantum dot (QD), including excited states, is studied in a regime where the direct Coulomb attraction energy is comparable to the kinetic energy of the carriers. This is achieved via magnetophotoluminescence excitation spectroscopy experiments, where a magnetic field applied perpendicular to the plane of the QD is used to reveal the angular-momentum content of energy shells. The absorption spectrum of the QDs is modeled, and comparison with experiment allows us to relate the observed transitions to interband QD bound-state transitions. The blueshift of the absorption peaks compared to the emission peaks is then interpreted in terms of many-body interactions, and we show that for a highly symmetric situation, the observed energy difference gives a direct measurement of the extra exchange energy gained upon addition of an extra exciton in the QD.
Singlet-triplet transitions in highly correlated nanowire quantum dots
(2010-02-01)
Chen, Y. T.
;
Chao, C. C.
;
Huang, S. Y.
;
Tang, C. S.
;
Cheng, S. J.
We consider a quantum dot embedded in a three-dimensional nanowire with tunable aspect ratio a. A configuration interaction theory is developed to calculate the energy spectra of the finite ID quantum dot systems charged with two electrons in the presence of magnetic fields B along the wire axis. Fruitful singlet-triplet transition behaviors are revealed and explained in terms of the competing exchange interaction, correlation interaction, and spin-Zeeman energy. In the high aspect ratio regime, the singlet-triplet transitions are shown designable by tuning the parameters a and B. The transition also manifest the highly correlated nature of long nanowire quantum dots. (C) 2009 Elsevier B.V. All rights reserved.