Browsing by Author "Pan, Ching-Ju"

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Acoustic and optical phonon assisted formation of biexcitons
(2007-09-10)
Pan, Ching-Ju
;
Lin, Kuo-Feng
;
Hsieh, Wen-Feng
;
光電工程學系
;
Department of Photonics
Efficient exciton relaxation is required for bounding two cooled excitons to form biexciton. Acoustic and optical phonon scatterings playing key roles in exciton relaxation are responsible for formation of biexcitons at various temperatures. Using ZnO powders, the authors observed a sublinear dependence on excitation power at low temperature, in which the relaxation process involves only emission of acoustic phonons due to the excitons having kinetic energy lower than those of the optical phonons. However, the exponent comes near theoretical value of 2 for participation of optical phonons when the exciton kinetic energy approaches to the energy of the lowest optical phonon.
Calculations of electronic structure and density of states in the wurtzite structure of Zn(1) (-) (x)Mg(x)O alloys using sp(3) semi-empirical tight-binding model
(2009-01-01)
Lin, Kuo-Feng
;
Pan, Ching-Ju
;
Hsieh, Wen-Feng
;
光電工程學系
;
Department of Photonics
Calculating the electronic structure and the density of states in the wurtzite structure of Zn(1) (-) (x)Mg(x)O(ZMO) alloys using sp(3) semi-empirical tight-binding model, we observed increases of both band gap and electron effective mass that agree with the experimental results as increasing Mg composition up to x = 0.3. From the calculated total density of states, the increasing electron effective mass is a result of less orbital overlap of cation sites due to extra density of modes coming from Mg3s and Mg3p orbitals as introducing more Mg composition. Additionally, reducing electronegative characteristic of oxygen was caused by that the O2p was less localized around the oxygen atom.
Raman study of alloy potential fluctuations in MgxZn1-xO nanopowders
(2007-05-09)
Pan, Ching-Ju
;
Lin, Kuo-Feng
;
Hsu, Wei-Tse
;
Hsieh, Wen-Feng
;
光電工程學系
The blueshift of near-band-edge emission and excitonic absorption indicate that Zn2+ ions are successfully substituted by Mg2+ ions in MgxZn1-xO nanopowders for 0 <= x <= 0.14. The changes in Raman spectral linewidth and the asymmetry of the E-2 ( high) mode for various Mg contents can be well described by a modified spatial correlation model that considers the grain size distribution. With increasing Mg concentration, the alloy potential fluctuations lead to a decrease in the grain size, which is induced by the surplus Mg2+ that could form MgO clusters surrounding the crystalline MgZnO.
Reducing exciton-longitudinal optical phonon coupling with increasing Mg incorporation in MgZnO powders
(2007-12-15)
Pan, Ching-Ju
;
Lin, Kuo-Feng
;
Hsu, Wei-Tse
;
Hsieh, Wen-Feng
;
光電工程學系
The coupling between exciton and longitudinal optical (LO) phonon was investigated in the use of temperature-dependent photoluminescence from Mg(x)Zn(1-x)O powders prepared by a sol-gel method in the range of 0 <= x <= 0.05. The exciton binding energy increases to 73 meV for 5 at. % Mg incorporated powders. The strength of exciton-LO phonon coupling was deduced from the energy shift of exciton emission with temperature variation. The increase of the exciton binding energy results from a decrease of the exciton Bohr radius that is responsible for reducing the coupling strength of exciton-LO phonon as increasing Mg content. (c) 2007 American Institute of Physics.
Structural and optical properties of ZnMgO nanostructures formed by Mg in-diffused ZnO nanowires
(2007-04-01)
Pan, Ching-Ju
;
Hsu, Hsu-Cheng
;
Cheng, Hsin-Ming
;
Wu, Chun-Yi
;
Hsieh, Wen-Feng
ZnMgO nanostructures with wurtzite phase were prepared by thermal diffusion of Mg into the ZnO nanowires. As ZnO light-emitting devices have been operated by using ZnMgO layers as energy barrier layers to confine the carriers, it is essential to realize the characterization of ZnMgO particularly. In this work, the Mg content in Zn1-xMgxO alloy determined by X-ray diffraction (XRD) and photoluminescence (PL) shows a good coincidence. The variation of lattice constant and the blueshift of near-band-edge emission indicate that Zn2+ ions are successfully substituted by Mg2+ ions in the ZnO lattice. In Raman-scattering studies, the change of E-2(high) phonon line shape in ZnO:Mg nanostructures reveals the microscopic substitutional disorder. In addition to the host phonons of ZnO, two additional bands around 383 and 510cm(-1) are presumably attributed to the Mg-related vibrational modes. (C) 2007 Elsevier Inc. All rights reserved.