Browsing by Author "Fan, Wen-Chung"

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Cathodoluminescence studies of GaAs nano-wires grown on shallow-trench-patterned Si
(2010-11-19)
Lee, Ling
;
Fan, Wen-Chung
;
Ku, Jui-Tai
;
Chang, Wen-Hao
;
Chen, Wei-Kuo
The optical properties of GaAs nano-wires grown on shallow-trench-patterned Si(001) substrates were investigated by cathodoluminescence. The results showed that when the trench width ranges from 80 to 100 nm, the emission efficiency of GaAs can be enhanced and is stronger than that of a homogeneously grown epilayer. The suppression of non-radiative centers is attributed to the trapping of both threading dislocations and planar defects at the trench sidewalls. This approach demonstrates the feasibility of growing nano-scaled GaAs-based optoelectronic devices on Si substrates.
Cathodoluminescence studies of GaAs nano-wires grown on shallow-trench-patterned Si
(2010-11-19)
Lee, Ling
;
Fan, Wen-Chung
;
Ku, Jui-Tai
;
Chang, Wen-Hao
;
Chen, Wei-Kuo
The optical properties of GaAs nano-wires grown on shallow-trench-patterned Si(001) substrates were investigated by cathodoluminescence. The results showed that when the trench width ranges from 80 to 100 nm, the emission efficiency of GaAs can be enhanced and is stronger than that of a homogeneously grown epilayer. The suppression of non-radiative centers is attributed to the trapping of both threading dislocations and planar defects at the trench sidewalls. This approach demonstrates the feasibility of growing nano-scaled GaAs-based optoelectronic devices on Si substrates.
Circular Polarization Dynamics during Magnetic Polaron Formation in Type-II Magnetic Quantum Dots
(2020-06-11)
Barman, Biplob
;
Pientka, James M.
;
Murphy, Joseph R.
;
Cartwright, Alexander N.
;
Chou, Wu-Ching
We used time-resolved photoluminescence spectroscopy to study the circular polarization dynamics of magnetic polarons in type-II (Zn,Mn)Te/ZnSe quantum dots in the presence of an external magnetic field. We investigated the time evolution of the peak energy of the sigma(+) and sigma(-) circularly polarized photoluminescence components and of the circular polarization of the emitted light. We also observed that the value of circular polarization, at long delay times, increases with magnetic field. We found that this system exhibits unexpected characteristics, such as different time scales for the formation of the magnetic polaron, on the one hand, and the evolution of photoluminescence circular polarization, on the other hand. These results are discussed within the framework of a theoretical model developed to describe the dependence of magnetic susceptibility as a function of temperature.
Growth evolution and magneto-optical characteristics of self-assembled ZnTe/ZnMnSe quantum dots
(2013-09-01)
Lee, Ling
;
Fan, Wen-Chung
;
Chien, Kun-Feng
;
Tzou, An-Jye
;
Chou, Wu-Ching
The growth evolution and the magneto-optical characteristics of the self-assembled single-layer ZnTe/ZnMnSe quantum dots grown by molecular beam epitaxy were investigated. As the ZnTe coverage is above 2.0 monolayers, a Stranski-Krastanov growth process is observed with a significantly increased dot density on top of two-dimensional plateaus. As the coverage exceeds 2.7 monolayers, ripened dots dominate instead. Furthermore, a saturated degree of circular polarization of 76% at applied magnetic field of 4 T and the formation of magnetic polarons with a formation time of 7.4 ns were demonstrated. (c) 2012 Elsevier B.V. All rights reserved.
Observation of localized surface plasmons in spatially controlled array structures
(2009-07-29)
Chou, J. W.
;
Lin, Kao-Chin
;
Lee, Yao-Jen
;
Yuan, Chi-Tsu
;
Hsueh, Fu-Kuo
We observed the reflectance spectra of three different nano-scale array structures of Au-coated silicon nanorods. The trends of the reflectance spectra indicate that the localized surface plasmon modes can be spatially controlled by manipulating geometric parameters, namely the lattice constants of the array. In addition, the experimental results were compared with 2D numerical simulations based on the finite element method. Satisfactory agreement between the experimental observations and numerical results was obtained.
Optical and Magnetic Properties of Zn1-xMnxO Grown by Plasma-Assisted Molecular Beam Epitaxy
(2014-06-01)
Chou, Wu Ching
;
Tzou, An-Jye
;
Chen, Hsuan-Shao
;
Chien, Kun-Feng
;
Fan, Wen-Chung
Zn1-xMnxO films (0 <= x <= 0.070) were grown by plasma-assisted molecular beam epitaxy on Si substrates with AlN buffer layers. For low Mn concentration samples, x <= 0.02, photoluminescence spectra show strong near band edge emission. Strong phonon and exciton coupling-induced resonant Raman scattering was observed. Paramagnetic, antiferromagnetic, and ferromagnetic phases were observed. Room temperature ferromagnetism for all samples was attributed to the bound magnetic polaron effect.
Optical Control of Hole Wavefunction in Type-II Magnetic Quantum Dot Structures
(2019-10-24)
Zhang, Peiyao
;
Norden, Tenzin
;
Pientka, James M.
;
Oszwaldowski, Rafal
;
Najafi, Arman
Unlike the extensively studied, spatially direct, quantum dots (QDs) with type-I band alignment where both electrons and holes are confined in the QD, in ZnTe QDs embedded in a (Zn,Mn)Se matrix only the holes are confined in the QDs. This spatially indirect type-II system provides unexplored opportunities to control the magnetic interactions between the hole spins in the nonmagnetic QDs and the spins of the magnetic ions in the matrix. Photoluminescence (PL) was excited either with photons of energy 3.06 eV [above band gap of the (Zn,Mn)Se matrix] or with photons of energy 2.54 eV [below the (Zn,Mn)Se band gap]. In the presence of an external magnetic field, the saturation red shift of the PL peak under 2.54 eV excitation exhibits an up to three-fold increase compared to the shift observed with 3.06 eV excitation. This increase is attributed to multiple hole occupancy of the QDs and the resulting increased penetration of the hole wavefunction tail further into the (Zn,Mn)Se matrix. The proposed model is supported by calculations which include the hole-hole Coulomb interactions as well as the hole-Mn spin exchange interactions.
Optical Studies of GaAs Nanowires Grown on Trenched Si(001) Substrate by Cathodoluminescence
(2012-06-01)
Lee, Ling
;
Chien, Kun-Feng
;
Fan, Wen-Chung
;
Chou, Wu-Ching
;
Ko, Chih-Hsin
"The strains in GaAs nanowires, which were grown from 1700- to 80-nm-wide trenches on the Si(001) wafer with SiO2 masks, were investigated by cathodoluminescence. For 1700- to 500-nm-wide trenches, the in-plane tensile strain at 15 K decreases with the decreasing trench width. The strain increases abruptly when the trench width is 300 nm, and then decreases as the trench width is further decreased. The results revealed that the stress induced by the SiO2 sidewalls dominates when the width is less than the depth of the trench. This approach provides an effective technique to measure the strain of a single nanowire and helps for the demonstration of selectively-grown GaAs with a designed strain. (C) 2012 The Japan Society of Applied Physics"
Origin of localized states in zinc-blende ZnCdSe thin films and the influence on carrier relaxation of self-assembled ZnTe/ZnCdSe quantum dots
(2015-05-25)
Lee, Ling
;
Dai, Yue-Ru
;
Yang, Chu-Shou
;
Fan, Wen-Chung
;
Chou, Wu-Ching
This study discovered the origin of deep level emission in zinc-blende ZnCdSe thin films grown by molecular beam epitaxy, in which a localization behavior was noticed. Pronounced deep level emission observed in films grown under a VI/II ratio of 1.74 (Se-accumulated regime) could be suppressed by a lower VI/II ratio of 1.04 (intermediate regime) and 0.74 (metal-rich regime). Hence the localized states could be correlated to excess selenium accumulated at the growth surface. The localized states also influence the carrier relaxation process of self-assembled ZnTe quantum dots embedded in a ZnCdSe matrix. Once quantum dots surmount the wetting layer, localized electrons in the capping layer dominate the type-II transition and exhibit size-independent lifetimes. (C) 2015 Elsevier B.V. All rights reserved.
Radiative recombination of indirect exciton in type-II ZnSeTe/ZnSe multiple quantum wells
(2011-05-01)
Chia, Chin-Hau
;
Fan, Wen-Chung
;
Lin, Yen-Chen
;
Chou, Wu-Ching
;
電子物理學系
The tunability of the emission energy, oscillator strength and photoluminescence (PL) efficiency by varying the well thickness and excitation density was demonstrated in the ZnSe(0.8)Te(0.2)/ZnSe multiple quantum wells. A significant blueshift about 260 meV of the PL peak energy was observed as the well width decreased from 5 to 1 nm. An extraordinary long lifetime (300 ns) of the recombination for the widest sample was detected. The binding energy of the indirect excitons is determined as 12 meV for the thinnest sample. The reduction of PL efficiency by thermal energy is greatly suppressed by employing a high excitation power. Crown Copyright (C) 2011 Published by Elsevier B.V. All rights reserved.
Scratch Characteristics of ZnMgO Epilayers
(2015-05-01)
Wen, Hua-Chiang
;
Chou, Wu-Ching
;
Chiang, Tun-Yuan
;
Fan, Wen-Chung
;
Lee, Ling
Zn0.75Mg0.25O films were grown via metal organic vapor phase epitaxy on an M-plane sapphire substrate. We used nanoscratch tests to study the abrasive plow of the films; comparable cases of critical pileup were obtained on both sides of each scratch when the ramped load increased from 0 to 250 mu N. The film showed a crack in the bulge edge between the groove at ramped loads of 1000 mu N as well as full plastic deformation. The values of mu were 0.14, 0.33, 0.43, and 0.48 for the ramped loads of 250 mu N and 0.22, 0.26, 0.28, and 0.33 for the ramped load of 1000 mu N. We found that cracking dominated in the case of Zn0.75Mg0.25O films during plowing. Lower values of the coefficient of friction and shallower penetration depths were observed at RT, while higher values were observed in the annealed samples. It is suggested that higher growth temperatures induce lower bonding forces and reduce the shear resistances of Zn-0.75Mg0.25O films.
Using nanoindentation and cathodoluminescence to identify the bundled effect of gallium nitride grown by PA-MBE
(2017-02-05)
Wen, Hua-Chiang
;
Chou, Wu-Ching
;
Chiang, Tun-Yuan
;
Jeng, Yeau-Ren
;
Fan, Wen-Chung
We performed cathodoluminescence (CL) investigations on GaN NCs grown using molecular beam epitaxy. Bundled GaN was used to form NCs that were deposited onto commercial sapphire templates. The structures of the bundled NCs were analyzed using nanoindentation. The samples were grown with a T-G of 700, 800, and 850 degrees C, and the H values were 7.4 +/- 0.2 and 6.4 +/- 0.2 GPa; the E values were 308.4 +/- 4.3, 296.4 +/- 3.8, and 75.5 +/- 1.4 GPa, respectively. It is suspected that the discrepancies of the mechanical parameters obtained by various indentation methods are partially a result of the stress distribution inherent in the different orientations on the sapphire substrate. A scanning electron microscopy (SEM) system equipped with CL allows the direct comparison of SEM images and CL maps, captured from exactly the same area of the samples. In addition to the SEM and CL images, photoluminescence spectroscopy (PL) profiling was obtained by collecting the 20 K PL spectra at the samples. The PL profiling enables the distinguishing of the emissions by the 5 meV of the blueshift from 3.440 to 3.445 eV due to localization effects. The bundled GaN NCs resulted from the (0002) reflection at 34.6 degrees for a wurtzite structure and a raised intensity difference when the TG was increased from 700 to 850 degrees C. The crystal texture of the GaN can influence the mechanical properties at different TG stages. (C) 2016 Elsevier B.V. All rights reserved.
Using nanoindentation and cathodoluminescence to investigate the residual stress of ZnSe
(2015-03-15)
Wen, Hua-Chiang
;
Chou, Wu-Ching
;
Yau, Wei-Hung
;
Fan, Wen-Chung
;
Lee, Ling
In this study we examined the effects of the nanoindentation-induced residual stress of single-crystalline zinc selenide (ZnSe). We employed the nanoindentation technique to evaluate the dislocation mobility of ZnSe at loading ratios of 10 and 2 mN/min, with a holding time of 120 s under a constant load. We visualized the resultant dislocation and microcracks using cathodoluminescence (CL) spectroscopy and mapping to compare the nanoindentation-induced residual stresses of the various ZnSe samples. CL mapping revealed massive dislocation activities during the loading process. The dislocations played roles as nonradiative recombination centers that quenched the local CL intensity. Transmission electron microscopy also revealed the effects of nanoindentation-induced residual stress. To obtain insight into the influence of the residual stress and to determine the dislocation mobilities for ZnSe films, it was essential to monitor the quenching effect of nonradiative recombination centers as a function of CL mapping. (C) 2014 Elsevier B.V. All rights reserved.