Browsing by Author "Fan, W. C."

Now showing 1 - 20 of 21

Biexciton emission from sol-gel ZnMgO nanopowders
(2010-05-10)
Chia, C. H.
;
Lai, Y. J.
;
Hsu, W. L.
;
Han, T. C.
;
Chiou, J. W.
We studied the power-dependent photoluminescence of Zn(1-x)Mg(x)O nanopowders grown by sol-gel method, at temperature T = 100 K. At moderate optical pumping intensity, a nonlinear emission band due to the radiative recombination of free biexcitons was detected. We found that the free biexciton binding energies of Zn(1-x)Mg(x)O nanopowder (0.01 <= x <= 0.05) are nearly constant (13.5 +/- 1.5 meV). (C) 2010 American Institute of Physics. [doi:10.1063/1.3428780]
Coherent Aharonov-Bohm oscillations in type-II (Zn,Mn)Te/ZnSe quantum dots
(2008-06-01)
Sellers, I. R.
;
Whiteside, V. R.
;
Govorov, A. O.
;
Fan, W. C.
;
Chou, W-C.
The magneto-photoluminescence of type-II (Zn,Mn)Te quantum dots is presented. As a result of the type-II band alignment, Aharonov-Bohm (AB) oscillations in the photoluminescence intensity are evident. In addition, an interesting interplay between the AB effect and the spin polarization in these diluted magnetic semiconductor quantum dots is observed. The intensity of the AB oscillations increases with both magnetic field and the degree of optical polarization, indicating that the suppression of spin fluctuations improves the coherence of the system.
Conventional vs Unconventional Magnetic Polarons: ZnMnTe/ZnSe and ZnTe/ZnMnSe Quantum Dots
(2014-01-01)
Barman, B.
;
Tsai, Y.
;
Scrace, T.
;
Murphy, J. R.
;
Cartwright, A. N.
We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, E-MP, in these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with E-MP = 35 meV. Furthermore, E-MP has unconventionally weak dependence of on both temperature T and magnetic field B-appl. In contrast, magnetic polarons in sample B show conventional characteristics with E-MP decreasing with increasing temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between the two types of QDs to the difference in the location of the Mn ions in the respective structures.
Coulomb induced recombination dynamics in magnetically doped type-IIb quantum dots
(2019-01-01)
Tarasek, S.
;
Chou, W-C.
;
Fan, W. C.
;
Cartwright, A. N.
;
Thomay, T.
We observe that the wavefunction overlap of the carriers in type-IIb quantum dots can be controlled by magnetic doping and strongly depends on the excitation power density. We study epitaxially grown ZnTe/ZnSe and magnetically doped (Zn,Mn)Te/ZnSe quantum dots that show a fast and slow recombination channel at two different energies. The emission shift of the slow recombination is independent of excitation power density (P-ex), whereas the fast recombination channel exhibits a larger emission shift with increasing P-ex. This blue shift saturates for high P-ex, however at a lower P-ex and smaller maximum shift in the magnetic system compared to the nonmagnetic system. The emission wavelength immediately after pulsed excitation typically changes as a function of charge carrier density due to the spatially indirect nature of type-IIb quantum dots. As carriers increase, the confinement potential of the system is altered due to a Coulomb interaction. The magnetic system exhibits a limited change in the wavefunction overlap of the carriers for high excitation power densities which we attribute to magnetic interactions to the hole wavefunction inside the dot. This would allow for an external manipulation of the magnetic polaron binding energy through varying the excitation power density P-ex in quantum dots.
Efficient energy transfer from InGaN quantum wells to Ag nanoparticles
(2013)
Shu, G. W.
;
Chiu, C. H.
;
Huang, L. T.
;
Lin, T. N.
;
Yang, C. C.
Nonradiative energy transfer from an InGaN quantum well to Ag nanoparticles is unambiguously demonstrated by the time-resolved photoluminescence. The distance dependence of the energy transfer rate is found to be proportional to 1/d(3), in good agreement with the prediction of the dipole interaction calculated from the Joule losses in acceptors. The maximum energy-transfer efficiency of this energy transfer system can be as high as 83%.
Fluorescence signals of quantum dots influenced by spatially controlled array structures
(2009-10-14)
Chou, J. W.
;
Lin, K. C.
;
Tang, Y. T.
;
Hsueh, F. K.
;
Lee, Yao-Jen
Fluorescence signals of quantum dots (QDs) influenced by different array structures of gold-coated silicon nanorods (SiNRs) were investigated via experimental observations and two-dimensional (2D) finite element method (FEM) simulations. On the densest gold-coated SiNRs array structure, the highest QD fluorescence quenching rates were observed and on the sparsest array structure, the highest QD fluorescence enhancement rates were observed. By developing a new technique which obtains the optical image of the array structures without losing information about the QD locations, we were able to further investigate how the QD fluorescence is influenced by spatially controlled array structures.
Growth and optical properties of ZnTe quantum dots on ZnMgSe by molecular beam epitaxy
(2015-09-01)
Fan, W. C.
;
Huang, S. H.
;
Chou, W. C.
;
Tsou, M. H.
;
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.
;
Ku, J. T.
;
Chou, W. C.
;
Chen, W. K.
;
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.
Optical characterization of isoelectronic ZnSe(1-x)O(x) semiconductors
(2011-05-15)
Lin, Y. C.
;
Chung, H. L.
;
Ku, J. T.
;
Chen, C. Y.
;
Chien, K. F.
We have studied the temperature dependence of the band gap and the decay dynamics of isoelectronic ZnSe(1-x)O(x) (x=0-0.053) semiconductors, using photoluminescence (PL) and time-resolved PL spectroscopy. The temperature dependence of the band gap of ZnSe(1-x)O(x) 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.
Optical characterization of isoelectronic ZnSe1-xOx semiconductors
(2011-05-15)
Lin, Y. C.
;
Chung, H. L.
;
Ku, J. T.
;
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.
Photoluminescence studies of type-II diluted magnetic semiconductor ZnMnTe/ZnSe quantum dots
(2006-12-25)
Kuo, M. C.
;
Hsu, J. S.
;
Shen, J. L.
;
Chiu, K. C.
;
Fan, W. C.
Type-II diluted magnetic semiconductor ZnMnTe quantum dots (QDs) in ZnSe matrix grown by molecular beam epitaxy were investigated by conventional and magnetophotoluminescence (PL) spectroscopy. The QD emission exhibits a type-II characteristic in excitation power dependence of PL peak energy. A nonzero circular polarization of PL at the absence of magnetic field was observed. This phenomenon is attributed to the accumulation of interface charges confined in adjacent layers. The magneto-optical measurement demonstrates a magnetic-induced degree of circular polarization in the PL spectra, indicating the Mn incorporation into the QD system. (c) 2006 American Institute of Physics.
Pressure-dependent Raman scattering and photoluminescence of Zn1-xCdxSe epilayers
(2007-04-01)
Lin, Y. C.
;
Chiu, C. H.
;
Fan, W. C.
;
Yang, S. L.
;
Chuu, D. S.
Raman and photoluminescence spectra of cubic Zn1-xCdxSe (0 <= x <= 0.32) epilayers were obtained at high pressure. The impurity mode I observed in the phonon Raman spectra at low temperature confirms the intermediate phonon mode behavior. A split transverse optical phonon mode was found in the down-stroke high-pressure Raman scattering. Additionally, the pressure-dependent longitudinal optical (LO) phonon frequencies and the Gruneisen parameter (gamma(LO)) were obtained by quadratic polynomial fitting. Pressure-driven resonant Raman scattering effect was observed in samples with a high Cd concentration (x >= 0.18). The critical pressure of semiconductor-to-metal phase transition (Pt) decreases as the Cd content increases. As the Cd concentration increases from 0 to 0.32, Pt falls from 13.6 to 9.4 GPa, according to Pt (GPa)=13.6-6.8x-20.3x(2). (c) 2007 American Institute of Physics.
Pressure-induced metallization and resonant Raman scattering in Zn(1-x)Mn(x)Te
(2008-07-01)
Lin, Y. C.
;
Fan, W. C.
;
Chiu, C. H.
;
Ke, F. K.
;
Yang, S. L.
Pressure-induced resonant Raman scattering is adopted to analyze the zone-center optical phonon modes and crystal characteristics of Zn(1-x)Mn(x)Te (0 <= x <= 0.26) thin films. The pressure (Pt) at which the semiconducting undergoes a transition to the metallic phase declines as a function of Mn concentration (x) according to the formula Pt(x)=15.7-25.4x+19.0x(2) (GPa). Pressure-dependent longitudinal and transverse optical phonon frequencies and the calculated mode Gruneisen parameters were adopted to investigate the influence of Mn(2+) ions on the iconicity. The experimental results indicate that the manganese ions tend to increase the iconicity of ZnTe under ambient conditions, whereas an external hydrostatic pressure tends to reduce the iconicity and the bond length of Zn(1-x)Mn(x)Te. (c) 2008 American Institute of Physics.
Raman scattering of longitudinal-optical-phonon-plasmon coupling in Cl-doped ZnSe under high pressure
(2007-12-15)
Lin, Y. C.
;
Chiu, C. H.
;
Fan, W. C.
;
Chia, C. H.
;
Yang, S. L.
The vibrational, electronic, and crystalline properties of n-type chlorine-doped ZnSe (ZnSe:Cl) layers with a carrier concentration from 8.2x10(15) to 1.8x10(18) cm(-3) are studied by Raman spectroscopy. The spectral line shapes of the longitudinal-optical-phonon and plasmon coupling mode are analyzed using the Raman scattering efficiency and the dielectric function to obtain the electron densities and mobility. The splitting of the transverse-optical (TO) phonon and the redshift of the chlorine-related impurity vibration mode are clearly observed when pressure is applied. The semiconductor-to-metal phase transition pressure of ZnSe:Cl layers declines as the carrier concentration increases, indicating that n-type doping reduces crystal stability. Additionally, the pressure-induced weakening of the longitudinal-optical-phonon-plasmon coupling efficiency suggests that pressure tends to degrade the n-type characteristic of ZnSe:Cl because of the emergence of the new deep donorlike state. (c) 2007 American Institute of Physics.
Robust magnetic polarons in type-II (Zn, Mn) Te/ZnSe magnetic quantum dots
(2010-11-18)
Sellers, I. R.
;
Oszwaldowski, R.
;
Whiteside, V. R.
;
Eginligil, M.
;
Petrou, A.
We present a magneto-optical study of magnetic polarons in type-II (Zn, Mn) Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to similar to 150 K, with a formation energy of similar to 40 meV. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field (similar to 2 meV at 8 T) and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperature-dependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.
Robust magnetic polarons in type-II (Zn, Mn) Te/ZnSe magnetic quantum dots
(2010-11-18)
Sellers, I. R.
;
Oszwaldowski, R.
;
Whiteside, V. R.
;
Eginligil, M.
;
Petrou, A.
We present a magneto-optical study of magnetic polarons in type-II (Zn, Mn) Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to similar to 150 K, with a formation energy of similar to 40 meV. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field (similar to 2 meV at 8 T) and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperature-dependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.
Study of extending carrier lifetime in ZnTe quantum dots coupled with ZnCdSe quantum well
(2018-03-01)
Fan, W. C.
;
Chou, W. C.
;
Lee, J. D.
;
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.
The study of self-assembled ZnO nanorods grown on Si(111) by plasma-assisted molecular beam epitaxy
(2013-09-01)
Tzou, A. J.
;
Chien, K. F.
;
Lai, H. Y.
;
Ku, J. T.
;
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.
Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots
(2015-07-24)
Barman, B.
;
Oszwaldowski, R.
;
Schweidenback, L.
;
Russ, A. H.
;
Pientka, J. M.
We used continuous wave photoluminescence (cw-PL) and time-resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn, Mn)Se system the holes are confined in the nonmagnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn, Mn)Se matrix. On the other hand, in the (Zn, Mn)Te/ZnSe system, the holes are confined in the magnetic (Zn, Mn)Te QDs, while the electrons remain in the surrounding nonmagnetic ZnSe matrix. The magnetic polaron formation energies E-MP in both systems were measured from the temporal redshift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the ZnTe/(Zn, Mn)Se system the magnetic polaron shows conventional behavior with E-MP decreasing with increasing temperature T and increasing magnetic field B. In contrast, E-MP in the (Zn, Mn)Te/ZnSe system has unconventional dependence on temperature T and magnetic field B; E-MP is weakly dependent on T as well as on B. We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.
Time-resolved photoluminescence of isoelectronic traps in ZnSe(1-x)Te(x) semiconductor alloys
(2008-12-15)
Lin, Y. C.
;
Chou, W. C.
;
Fan, W. C.
;
Ku, J. T.
;
Ke, F. K.
Kohlrausch's stretched exponential law correlates well with the photoluminescence (PL) decay profiles of ZnSe(1-x)Te(x). As the Te concentration increases, the stretching exponent beta initially declines and then monotonically increases. This result can be understood using the hopping-transport and energy transfer model. The increase in the number of isoelectronic Te localized traps can reduce the PL decay rate and increase the linewidth, whereas the hybridization of the Te localized states with the valence-band edge states causes a reduction in both the lifetime and linewidth.