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Now showing 1 - 10 of 11

Optical characterization of isoelectronic ZnSe1-xOx semiconductors
(2011-05-15)
Lin, Y. C.
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Chung, H. L.
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Ku, J. T.
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Chen, C. Y.
;
Chien, K. F.
We have studied the temperature dependence of the band gap and the decay dynamics of isoelectronic ZnSe1-xOx (x=0-0.053) semiconductors, using photoluminescence (PL) and time-resolved PL spectroscopy. The temperature dependence of the band gap of ZnSe1-xOx decreases with the increase in 0 concentration. The Kohlrausch law is in good agreement with the O-induced complex decay profiles. As temperature increases, the mechanism of carrier decay undergoes a complicated change from trapped to free excitons. These findings are consistent with the temperature dependence of the stretching exponent beta. (C) 2010 Elsevier B.V. All rights reserved.
Study of extending carrier lifetime in ZnTe quantum dots coupled with ZnCdSe quantum well
(2018-03-01)
Fan, W. C.
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Chou, W. C.
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Lee, J. D.
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Lee, Ling
;
Nguyen Dang Phu
We demonstrated the growth of a self-assembled type-II ZnTe/ZnSe quantum dot (QD) structure coupled with a type-I Zn0.88Cd0.12Se/ZnSe quantum well (QW) on the (001) GaAs substrate by molecular beam epitaxy (MBE). As the spacer thickness is less than 2 nm, the carrier lifetime increasing from 20 ns to nearly 200 ns was successfully achieved. By utilizing the time-resolved photoluminescence (TRPL) and PL with different excitation power, we identify the PL emission from the coupled QDs consisting of two recombination mechanisms. One is the recombination between electrons in ZnSe barrier and holes confined within ZnTe QDs, and the other is between electrons confined in Zn0.88Cd0.12Se QW and holes confined within ZnTe QDs. According to the band diagram and power-dependent PL, both of the two recombinations reveal the type-II transition. In addition, the second recombination mechanism dominates the whole carrier recombination as the spacer thickness is less than 2 nm. A significant extension of carrier lifetime by increasing the electron and hole separation is illustrated in a type-II ZnTe/ZnSe QD structure coupling with a type-I ZnCdSe/ZnSe QW. Current sample structure could be used to increase the quantum efficient of solar cell based on the II-VI compound semiconductors.
Photoluminescence properties of MgxZn1-xO films grown by molecular beam epitaxy
(2017-02-01)
Wu, T. Y.
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Huang, Y. S.
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Hu, S. Y.
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Lee, Y. C.
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Tiong, K. K.
The optical properties of MgxZn1-xO films with x=0.03, 0.06, 0.08, and 0.11 grown by molecular beam epitaxy (MBE) have been studied by temperature-dependent photoluminescence (PL) measurement. It is presented that the full-width at half-maximum (FWHM) of the 12 K PL spectrum of MgZnO films increases with increasing Mg concentration and would deviate significantly from the simulation curve of Schubert model with higher Mg contents. The abnormal broader PL FWHM is inferred from larger compositional fluctuation by incorporating higher Mg contents, which results in larger effect of excitonic localization to induce more significant S-shaped behavior of the PL peak energy with temperature dependence. Additionally, the degree of localization increases as the linear proportion of the PL FWHM, indicating that the excitonic behavior in MgZnO films belong to the strong localization effect.
Photoluminescence of localized excitons in ZnCdO thin films grown by molecular beam epitaxy
(2016-07)
Wu, T. Y.
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Huang, Y. S.
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Hu, S. Y.
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Lee, Y. C.
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Tiong, K. K.
We have investigated the luminescence characteristics of Zn1-xCdxO thin films with different Cd contents grown by molecular beam epitaxy system. The temperature-dependent photoluminescence (PL) and excitation power-dependent PL spectra were measured to clarify the luminescence mechanisms of the Zn1-xCdxO thin films. The peak energy of the Zn1-xCdxO thin films with increasing the Cd concentration is observed as redshift and can be fitted by the quadratic function of alloy content. The broadened full-width at half-maximum (FWHM) estimated from the 15 K PL spectra as a function of Cd content shows a larger deviation between the experimental values and theoretical curve, which indicates that experimental FWHM values are affected not only by alloy compositional disorder but also by localized excitons occupying states in the tail of the density of states. The Urbach energy determined from an analysis of the lineshape of the low-energy side of the PL spectrum and the degree of localization effect estimated from the temperature-induced S-shaped PL peak position described an increasing mean exciton-localization effects in ZnCdO films with increasing the Cd content. In addition, the PL intensity and peak position as a function of excitation power are carried out to clarify the types of radiative recombination and the effects of localized exciton in the ZnCdO films with different Cd contents. (C) 2016 Elsevier Ltd. All rights reserved.
Growth and optical properties of ZnTe quantum dots on ZnMgSe by molecular beam epitaxy
(2015-09-01)
Fan, W. C.
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Huang, S. H.
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Chou, W. C.
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Tsou, M. H.
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Yang, C. S.
Self-assembled type-II ZnTe quantum dots (QDs) were grown on GaAs (0 0 1) substrates with Zn1-xMgxSe (x=0.24 and 0.52) buffer layers by molecular beam epitaxy. The optical properties of ZnTe QDs were investigated by low-temperature photoluminescence (PL) and time-resolved PL. An abrupt variation of the PL peak energy with coverage implies the existence of wetting layer of 3.2 MLs and 40 MLs for the Mg concentration x=0.24 and 0.52, respectively. The thickness of wetting layer is larger than that of Zaire QDs grown on ZnSe buffer layers because the strain between ZaTe and Zn1-xMgxSe is smaller. The non-mono-exponential decay profiles reflect the processes of carrier transfer and recapture. The Kohlrausch\'s stretching exponential well fits the decay profiles of ZaTe/Zn1-xMgxSe QDs. (C) 2015 Elsevier B.V. All rights reserved.
Magneto-optical properties of ZnMnTe/ZnSe quantum dots
(2011-05-15)
Fan, W. C.
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Ku, J. T.
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Chou, W. C.
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Chen, W. K.
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Chang, W. H.
The sigma(+) and sigma(-) circularly polarized time-integrated and time-resolved Photoluminescence (PL) measurements were employed to investigate the carrier spin dynamics of ZnMnTe/ZnSe Quantum Dots (QDs) grown on GaAs substrates by molecular beam epitaxy. The Kohlrausch's stretching exponential function well correlates both the sigma(+) and sigma(-) decay profiles. The measured spin relaxation time is about 23 ns. (C) 2010 Elsevier B.V. All rights reserved.
Carrier Dynamics in Self-Assembled CdTe Stranski-Krastanow Quantum Dots Grown on ZnSe by Molecular Beam Epitaxy
(2008-11-01)
Yang, C. S.
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Chin, K. F.
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Lai, J. Y.
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Luo, C. W.
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Chou, W. C.
Self-assembled CdTe/ZnSe Stranski-Krastanow quantum-dot (QD) structures, which have a CdSe-like precursor-layer (PCL) between CdTe quantum dots and ZnSe matrix, were grown by molecular beam epitaxy. The carrier dynamics of the structures was studied by photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements. The temperature-dependent PL spectra revealed that there is a carrier transformation from small dots to larger dots via the precursor layer. Temperature-dependent PL measurements verify the existence of the QD excited excitonic state in 5.0 mono-layer (ML) coverage. In 0.6 ML coverage, the TRPL spectra exhibited a double-exponential decay process, as the detection energy at PCL emission energy. The faster decay time is corresponded to the carrier lifetime in PCL, the slower decay time exhibits the carrier in small QD. However, in 5.0 ML coverage, a faster and slower decay time represents the carrier lifetime in QD excited state for larger QD and the carrier lifetime in QD ground state for smaller QD.
The study of self-assembled ZnO nanorods grown on Si(111) by plasma-assisted molecular beam epitaxy
(2013-09-01)
Tzou, A. J.
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Chien, K. F.
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Lai, H. Y.
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Ku, J. T.
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Lee, L.
The growth and optical properties of self-assembled ZnO nanorods grown on Si(111) substrate by plasma-assisted molecular beam epitaxy (PA-MBE) were studied. By controlling the Zn/O flux ratio, the growth of ZnO nanorods on Si(111) substrate without catalyst has been achieved. Scanning electron microscopy (SEM) shows the ZnO nanorods with various density and diameter could be controlled. Photoluminescence (PL) measurements exhibit nice optical properties. The sharp near band edge PL emission with full width at half maximum (FWHM) of about 17 meV indicates that the ZnO nanorods could be used as the high efficient photonic devices. Crown Copyright (c) 2013 Published by Elsevier B.V. All rights reserved.
Thermal-activated carrier transfer in ZnCdO thin film grown by plasma-assisted molecular beam epitaxy
(2013-09-01)
Chien, K. F.
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Hsu, W. L.
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Tzou, A. J. .
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Lin, Y. C.
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Chou, W. C.
` The thermal-activated carrier transfer processes in a Zn0.98Cd0.020 thin film grown by plasma-assisted molecular beam epitaxy were investigated using temperature-dependent and time-resolved photoluminescence (PL) spectroscopy. As the temperature increases from 50 to 220 K, the carriers transfer from shallow to deep localized states. Additionally, the carriers escape from the deep localized states above 220 K due to an activation energy of about 19 meV. (c) 2013 Elsevier B.V. All rights reserved.
Optical properties of Zn1-xMnxO thin films grown by molecular beam epitaxy
(2013-09-01)
Chien, K. F.
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Yang, Y. L.
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Tzou, A. J.
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Chou, W. C.
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電子物理學系
Zn(1-x)Mn(x)0 (x=0-0.061) thin films were grown by molecular beam epitaxy (MBE) system. Transmittance shows an increase of the band gap with the increasing Mn concentration. Resonant Raman scattering (RRS) spectra showed 11 longitudinal optical phonon lines for the Zn1-xMnxO samples. For the Zn0.997Mn0.003O sample, circular polarization degree of 9% was observed at magnetic field B=5 T. The dependence of circular polarization rate on the magnetic field intensity exhibits Brillouin type para-magnetism. (c) 2012 Elsevier B.V. All rights reserved.

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