Browsing by Author "Hsieh, W. F."

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Adaptation to the edge of chaos in a self-starting Kerr-lens mode-locked laser
(2009-08-01)
Hsu, C. C.
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Lin, J. H.
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Hsieh, W. F.
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光電工程學系
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Department of Photonics
We experimentally and numerically demonstrated that self-focusing acts as a slow-varying control parameter that suppresses the transient chaos to reach a stable mode-locking (ML) state in a self-starting Kerr-lens mode-locked Ti:sapphire laser without external modulation and feedback control. Based on Fox-Li's approach, including the self-focusing effect, the theoretical simulation reveals that the self-focusing effect is responsible for the self-adaptation. The self-adaptation occurs at the boundary between the chaotic and continuous output regions in which the laser system begins with a transient chaotic state with fractal correlation dimension, and then evolves with reducing dimension into the stable ML state.
Anisotropic strain, magnetic properties, and lattice dynamics in self-assembled multiferroic CoFe2O4-PbTiO3 nanostructures
(2014-04-07)
Tsai, C. Y.
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Chen, H. R.
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Chang, F. C.
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Kuo, H. H.
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Cheng, H. M.
We investigate the anisotropic stress dependent magnetic and phonon behaviors in self-assembled CoFe2O4-PbTiO3 (CFO-PTO) nanostructures deposited on SrRuO3 buffered SrTiO3 substrates of various thickness. The increased vertical compressed stress with increasing thickness enhances the vertical magnetic anisotropy of CFO while gradually reducing the vertical polarization of PTO. By applying the magnetic-field dependent Raman scattering, the CFO-A(1g) and T-2g(1) phonon frequencies shift oppositely because of the magnetostriction. Moreover, the PTO-A(1) mode intensities anomalously enhance and the A(1) and E mode frequencies, respectively, increase and decrease, which prove the existence of the stress-mediated magnetoelectric (ME) effect. This study shows that the ME transmission process is dominated by interfacial stress, which is critical for enhancing the ME efficiency. (C) 2014 AIP Publishing LLC.
Correlation between crystal structure and photoluminescence for epitaxial ZnO on Si(111) using a gamma-Al(2)O(3) buffer layer
(2008-03-21)
Liu, W-R
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Li, Y-H
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Hsieh, W. F.
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Hsu, C-H
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Lee, W. C.
High-quality ZnO epitaxial films were grown by pulsed-laser deposition on Si (1 1 1) substrates with a thin. gamma-Al(2)O(3) buffer layer. The epitaxial. gamma-Al(2)O(3) buffer layer consists of two (1 1 1) oriented domains rotated 60. from each other against the surface normal, which yields the in-plane epitaxial relationship (1 0 0)ZnO
Crystal symmetry breaking of wurtzite to orthorhombic in nonpolar a-ZnO epi-films
(2009)
Kuo, C. C.
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Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
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Hsu, H. C.
Crystal symmetry breaking of wurtzite C(6V) to orthorhombic C(2V) due to in-plane anisotropic strain was investigated for nonpolar a-ZnO epi-films grown on R-sapphire.
Crystal symmetry breaking of wurtzite to orthorhombic in nonpolar a-ZnO epifilms
(2009-07-06)
Kuo, C. C.
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Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
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Hsu, H. C.
Crystal symmetry breaking of wurtzite C-6V to orthorhombic C-2V due to in-plane anisotropic strain was investigated for nonpolar (11 (2) over bar0) ZnO epifilms grown on the R-sapphire. X-ray diffraction results reveal the epilayer is subjected to a compressive strain along the polar c-axis and tensile strains along both a-[11 (2) over bar0] surface normal and in-plane p-[1 (1) over bar 00] axis. The polarized Raman spectra of E-2 modes reveal violation of the C-6V selection rules. Oppositely, the C-2V configuration satisfies the selection rules for the Raman modes. The observed E-1 and E-2 bands in polarized optical reflection and photoluminescence spectra confirm the anisotropic strain causes the structure change to the orthorhombic one. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3159470]
Domain Matching Epitaxial Growth of High-Quality ZnO Film Using a Y(2)O(3) Buffer Layer on Si (111)
(2009-01-01)
Liu, W. -R.
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Li, Y. -H.
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Hsieh, W. F.
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Hsu, C. -H.
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Lee, W. C.
High-quality ZnO epitaxial films have been grown by pulsed-laser deposition on Si (111) substrates using a nanothick high-k oxide Y(2)O(3) buffer layer. Determined by X-ray diffraction and transmission electron microscopy, the epitaxial relationship between ZnO and Y(2)O(3) follows (0001)< 2 (1) over bar(1) over bar0 >(ZnO)parallel to(111)< 10 (1) over bar > Y(2)O(3). ZnO lattice aligns with the hexagonal O sublattice in Y(2)O(3) and the interfacial structure can be well described by domain matching epitaxy with 7 or 8 ZnO {11 (2) over bar0} planes matching 6 or 7 {4 (4) over bar0} planes Of Y(2)O(3) and lead to a significant reduction of residual strain. Superior optical properties were obtained even for ZnO films as thin as 0.21 mu m from photoluminescence results.
Domain Matching Epitaxial Growth of High-Quality ZnO Film Using a Y2O3 Buffer Layer on Si (111)
(2009-01-01)
Liu, W. -R.
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Li, Y. -H.
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Hsieh, W. F.
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Hsu, C. -H.
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Lee, W. C.
High-quality ZnO epitaxial films have been grown by pulsed-laser deposition on Si (111) substrates using a nanothick high-k oxide Y2O3 buffer layer. Determined by X-ray diffraction and transmission electron microscopy, the epitaxial relationship between ZnO and Y2O3 follows (0001)< 2 (1) over bar(1) over bar0 >(ZnO)parallel to(111)< 10 (1) over bar > Y2O3. ZnO lattice aligns with the hexagonal O sublattice in Y2O3 and the interfacial structure can be well described by domain matching epitaxy with 7 or 8 ZnO {11 (2) over bar0} planes matching 6 or 7 {4 (4) over bar0} planes Of Y2O3 and lead to a significant reduction of residual strain. Superior optical properties were obtained even for ZnO films as thin as 0.21 mu m from photoluminescence results.
Effect of geometry on the magnetic properties of CoFe2O4-PbTiO3 multiferroic composites
(2013)
Wang, B. Y.
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Wang, H. T.
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Singh, Shashi B.
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Shao, Y. C.
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Wang, Y. F.
In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element-and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4-PbTiO3 (CFO-PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A-and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L-3,L-2-edge XMCD measurements of the CFO-PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO-PTO composites.
Effect of Threading Dislocations on Local Contacts in Epitaxial ZnO Films
(2010)
Lin, C. Y.
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Liu, W. -R.
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Chang, C. S.
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Hsu, C. -H.
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Hsieh, W. F.
Local conductance of a ZnO epifilm with a columnar-grain structure was studied by conductive-mode atomic force microscopy. The probe-ZnO junction at the grain boundary with high density edge threading dislocations (TDs) behaves as a Schottky contact while the junction at the epitaxial core behaves as an ohmic contact, resulting in the nonuniformity of conductance throughout the film. The calculated Schottky barrier is 0.4 +/- 0.025 eV. The point defects of doubly charged Zn vacancies accumulated at the edge TDs induce local band bending of ZnO, thus contributing to the Schottky nature at the grain boundary.
Effects of ultraviolet treatment on the contact resistivity and electronic transport at the Ti/ZnO interfaces
(2009-07-01)
Lin, Yow-Jon
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Tsai, Chia-Lung
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Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
We report on the effect of ultraviolet (UV) treatment on the specific contact resistance (rho) and electronic transport at the Ti/ZnO interfaces. The experimental results show the same barrier height of Ti/ZnO samples without UV treatment as Ti/ZnO samples with UV treatment and the higher rho of Ti/ZnO samples with UV treatment than Ti/ZnO samples without UV treatment, suggesting the barrier-height independence of rho. Based on the thermionic-emission model and x-ray photoelectron spectroscopy results, we found that the induced decrease in the number of the hydroxides at the surface region of ZnO by UV treatment resulted in decreases in the electron concentration near the surface region and the excess current component related to tunneling, increasing in rho of Ti/ZnO samples.
Fabrication of aligned single wall carbon nanotube absorbers for high power passive mode-locked Nd:GdVO(4) laser
(2012-01-01)
Cheng, S. Y.
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Wang, Y. G.
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Yu, P.
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Cheng, Y. J.
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Tang, J.
Using the vertical evaporation technique we fabricated saturable absorbers by transferring the water-soluble single wall carbon nanotubes (SWCNT) onto a hydrophilic quartz substrate. The fast recovery times of the absorber were measured to be 136 and 790 fs. The modulation depth of the absorber was about 2%. Passive mode-locked Nd:GdVO(4) laser using such an absorber was demonstrated. The continuous wave mode-locked pulses with the pulse duration of 12.4 ps and the repetition of 120 MHz were achieved. The maximum average output power of the mode-locked laser is 2.4 W at the pump power of 13 W. Such kind of absorber has potential to be put into practical use for high power solid-state laser mode locking.
Fabrication of aligned single wall carbon nanotube absorbers for high power passive mode-locked Nd:GdVO4 laser
(2012-01-01)
Cheng, S. Y.
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Wang, Y. G.
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Yu, P.
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Cheng, Y. J.
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Tang, J.
Using the vertical evaporation technique we fabricated saturable absorbers by transferring the water-soluble single wall carbon nanotubes (SWCNT) onto a hydrophilic quartz substrate. The fast recovery times of the absorber were measured to be 136 and 790 fs. The modulation depth of the absorber was about 2%. Passive mode-locked Nd:GdVO4 laser using such an absorber was demonstrated. The continuous wave mode-locked pulses with the pulse duration of 12.4 ps and the repetition of 120 MHz were achieved. The maximum average output power of the mode-locked laser is 2.4 W at the pump power of 13 W. Such kind of absorber has potential to be put into practical use for high power solid-state laser mode locking.
Improvement of Crystalline and Photoluminescence of Atomic Layer Deposited m-Plane ZnO Epitaxial Films by Annealing Treatment
(2012-10-01)
Yang, S.
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Lin, B. H.
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Kuo, C. C.
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Hsu, H. C.
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Liu, W-R
Monocrystalline m-plane ZnO epitaxial films with flat surface morphology were grown on m-plane sapphire by using atomic layer deposition. X-ray diffraction and transmission electron microscopy measurements verify not only the in-plane epitaxial relationship of the as-grown films as (10 (1) over bar0)< 0001 >(ZnO)parallel to(10 (1) over bar0)<(1) over bar2 (1) over bar0 > Al2O3 but also the absence of domains with undesirable orientations, which are generally obtained in the m-plane ZnO films grown by other methods. Experimental results indicate that the basal plane stacking fault (BSF) is the dominant structural defects that contribute to the emission at 3.31 eV in m-plane ZnO films. Exactly how thermal annealing affects the structural and optical properties of ZnO epi-films was also investigated. Additionally, based on time-resolved photoluminescence at 5 K, the decay time of BSF related emission and near-band-edge (NBE) emission were determined. Results of this work further demonstrated that the decay time of NBE emission increases with a higher annealing temperature, accompanied by an improvement in crystal structure.
Influence of the threading dislocations on the electrical properties in epitaxial ZnO thin films
(2006-12-29)
Liu, W. -R.
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Hsieh, W. F.
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Hsu, C. -H.
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Liang, K. S.
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Chien, F. S. -S.
The structural and electrical properties of epitaxial ZnO films grown by pulsed-laser deposition on sapphire (0001) were investigated by high-resolution X-ray diffraction (XRD), atomic force microscopy (AFM), scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM), and transmission electron microscopy (TEM). The results of XRD and AFM revealed that the ZnO films have a columnar-grain structure consisting of epitaxial cores surrounded by annular boundaries. The core and boundary regions exhibited significantly different capacitive responses and field emission current. The results of TEM indicated that the annular boundaries have high-density edge threading dislocations. The shift of flatband voltage and the raise of potential barrier at the boundaries observed by SCM and C-AFM were attributed to the interface trap densities caused by the existence of high-density edge threading dislocations. (c) 2006 Elsevier B.V. All rights reserved.
Optical properties of one- and two-dimensional excitons in m-plane ZnO/MgZnO multiple quantum wells
(2016-03-09)
Chen, H. R.
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Tsai, C. Y.
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Huang, Y. C.
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Kuo, C. C.
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Hsu, H. C.
Five pairs of ZnO/Zn0.9Mg0.1O multiple quantum wells (MQWs) with fixed barrier thickness of 55 nm and three well widths of 4, 8 and 16 nm have been grown on m-plane sapphires by pulsed laser deposition. Due to 2D quantum confinement with decreasing well width, the emission of excitons bound to the basal-plane stacking faults, which are one-dimensionally confined in MQWs, encounters larger blue shift than that of the near-band edge excitons. Furthermore, remarkably reducing coupling of free excitons with A1 longitudinal optical phonons is closely correlated with increasing exciton binding energy but enhanced coupling of E-2-low phonons is a result of increasing interaction with the interface phonons with decreasing well width.
Passive mode-locking in diode-pumped c-cut Nd:LuVO(4) laser with a semiconductor saturable-absorber mirror
(2009-07-01)
Chen, H. R.
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Lin, J. H.
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Song, K. T.
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Lin, K. H.
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Hsieh, W. F.
We demonstrated a diode-pumped passively mode-locked c-cut Nd:LuVO(4) picosecond laser with a semiconductor saturable-absorber mirror (SESAM) at a wavelength of 1067.8 nm. Due to the wide bandwidth of 0.48 nm, stable mode-locking has been generated with a duration as short as 3.7 ps, which is shorter than for the a-cut Nd:LuVO(4) laser. A maximum output power of 1.67 W was achieved to give a highest peak power of 3.47 KW at 18 W absorbed pump power.
Passive mode-locking in diode-pumped c-cut Nd:LuVO4 laser with a semiconductor saturable-absorber mirror
(2009-07-01)
Chen, H. R.
;
Lin, J. H.
;
Song, K. T.
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Lin, K. H.
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Hsieh, W. F.
We demonstrated a diode-pumped passively mode-locked c-cut Nd:LuVO4 picosecond laser with a semiconductor saturable-absorber mirror (SESAM) at a wavelength of 1067.8 nm. Due to the wide bandwidth of 0.48 nm, stable mode-locking has been generated with a duration as short as 3.7 ps, which is shorter than for the a-cut Nd:LuVO4 laser. A maximum output power of 1.67 W was achieved to give a highest peak power of 3.47 KW at 18 W absorbed pump power.
Photoluminescence associated with basal stacking faults in c-plane ZnO epitaxial film grown by atomic layer deposition
(2012-03-05)
Yang, S.
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Kuo, C. C.
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Liu, W. -R.
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Lin, B. H.
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Hsu, H. -C.
Basal plane stacking faults (BSFs) with density of similar to 1 x 10(6) cm(-1) are identified as the dominant defect in the annealed ZnO thin films grown on c-plane sapphire by atomic layer deposition. The dominant peak centered at 3.321 eV in low-temperature photoluminescence measurements is attributed to the emission from the BSFs. The emission mechanism is considered to be the confined indirect excitons in the region of quantum-well-like structure formed by the BSFs. The observed energy shift of 19 meV with respect to the BSF-bounded exciton at low temperature may be caused by the localization effect associated with the coupling between BSF quantum wells. (C) 2012 American Institute of Physics. [http://dx.doi. org/10.1063/1.3692730]
Recombination dynamics of a localized exciton bound at basal stacking faults within the m-(p)lane ZnO film
(2014-07-07)
Yang, S.
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Hsu, H. C.
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Liu, W-R.
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Lin, B. H.
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Kuo, C. C.
We investigated the carrier dynamics near basal stacking faults (BSFs) in m-plane ZnO epitaxial film. The behaviors of the type-II quantum wells related to the BSFs are verified through time-resolved and time-integrated photoluminescence. The decay time of the emission of BSFs is observed to have a higher power law value and longer decay time than the emission of the donor-bound excitons. The spectral-dependent decay times reveal a phenomenon of carriers migrating among band tail states, which are related to the spatial distribution of the type-II quantum wells formed by the BSFs. A high density of excited carriers leads to a band bending effect, which in turn causes a blue-shift of the emission peak of BSFs with a broadened distribution of band tail states. (C) 2014 AIP Publishing LLC.
Single Domain m-Plane ZnO Grown on m-Plane Sapphire by Radio Frequency Magnetron Sputtering
(2012-10-01)
Lin, B. H.
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Liu, W. -R
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Lin, C. Y.
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Hsu, S. T.
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Yang, S.
High-quality m-plane orientated ZnO films have been successfully grown on m-plane sapphire by using radio frequency magnetron sputtering deposition. The introduction of a nanometer-thick, low-temperature-grown ZnO buffer layer effectively eliminates inclusions of other undesirable orientations. The structure characteristics of the ZnO epi-layers were thoroughly studied by synchrotron X-ray scattering and transmission electron microscopy (TEM). The in-plane epitaxial relationship between ZnO and sapphire follows (0002)(ZnO) parallel to (11 (2) over bar0)(sapphire) and (11 (2) over bar0)(ZnO) parallel to (0006)(sapphire) and the ZnO/sapphire interface structure can be described by the domain matching epitaxy along the [11 (2) over bar0](ZnO) direction. The vibrational properties of the films were investigated by polarization dependent micro-Raman spectroscopy. Both XRD and micro-Raman results reveal that the obtained m-ZnO layers are tinder an anisotropic biaxial strain but still retains a hexagonal lattice.