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Observation of anomalous emissions of nonpolar a-plane MgZnO and ZnO epi-films based on XEOL and time-resolved XEOL in hybrid bunch mode
(2020-08-01)
Lin, Bi-Hsuan
;
Wu, Yu-Hao
;
Wu, Yung-Chi
;
Liu, Wei-Rein
;
Lee, Chien-Yu
X-ray excited optical luminescence (XEOL) using an x-ray nanobeam operating in the hybrid bunch mode provides not only a sufficiently high peak power density but also high-quality temporal domain measurements for studying the luminescence dynamics of photonic materials and devices. We used these features to investigate the peculiar emission behavior of the as-grown and rapid thermal annealing (RTA)-treated a-plane MgZnO epi-films, from which an anomalous emission was observed at similar to 325 nm, along with a rapid increase in near-band edge (NBE) emission with increasing x-ray irradiation time. This peculiar emission behavior was also observed in the cathodoluminescence spectra obtained under electron beam excitation. Increases in the NBE emission intensity of the RTA-treated a-plane MgZnO epi-film were observed in terms of both photoluminescence and small anomalous emissions of XEOL. This can be explained by the effective transfer of Mg atoms from interstitial sites to suitable sites as a result of RTA treatment. Based on comparison with an a-plane ZnO epi-film without Mg, we conclude that the anomalous emission peak is caused by Mg-related energy states created by the high-dose x-ray or electron beam irradiation. Furthermore, the rapid increase in NBE emission and the reduction in the long decay lifetime can be attributed to charge transfer from the Mg clusters, which are thus responsible for the anomalous emissions.
Exciton Localization of High-Quality ZnO/MgxZn1-x Multiple Quantum Wells on Si (111) with a Y2O3 Buffer Layer
(2018-08-01)
Liu, Wei-Rein
;
Huang, Wei-Lun
;
Wu, Yung-Chi
;
Lai, Liang-Hsun
;
Hsu, Chia-Hung
We report the structural and optical properties of ten-period ZnO/MgxZn1-x multiple quantum wells (MQWs) prepared on the most widely used semiconductor material, Si. The introduction of a nanometer thick high-k Y2O3 transition layer between Si (111) substrate and a ZnO buffer layer significantly improves the structural perfection of the MQWs grown on top of it. The high structural quality of the ZnO/MgxZn1-xO MQWs is evidenced by the appearance of pronounced high order satellite peaks in X-ray crystal truncation rods; high resolution cross-sectional transmission electron microscopy images also confirmed the regularly arranged well and barrier layers. When the well width is less than similar to 2.7 nm, the quantum-confined Stark effect in MQWs can be negligible. Not only the increasing exciton-binding energy but also reducing exciton-phonon coupling determined in temperature-dependent photoluminescence spectra indicate quantum-size effect. Our results demonstrate that ZnO/MgxZn1-xO MQWs integrated on Si have great potential in UV optoelectronic device applications.
Hard X-ray nanoprobe and time-resolved XEOL to observe increasing luminescence of ZnO and GaN epitaxial structures
(2019-10-21)
Lin, Bi-Hsuan
;
Wu, Yu-Hao
;
Wu, Tai-Sing
;
Wu, Yung-Chi
;
Li, Xiao-Yun
Hard X-ray excited optical luminescence (XEOL) with a nanofocused beam provides both excellent spatial resolution and high enough peak power density, which makes XEOL mapping and luminescence dynamics study achievable. We present here the time and X-ray intensity dependence of the near-band-edge (NBE) luminescence from the nonpolar a-plane MgZnO/ZnO multiple quantum wells (MQWs): the emission intensity increases more than 10 times after high X-ray irradiation. Different from the well-known NBE emission lifetime of ZnO (less than 1 ns), the long decay time gradually decreases from 130 ns to 35 ns with the increasing X-ray irradiation time. We attribute the observed changes in NBE luminescence to the excitation of the Mg-related energy state by the high energy X-ray nanobeam. This suggestion was further confirmed in the XEOL spectra of the Mg-doped and Si-doped c-plane GaN epi-films. Published under license by AIP Publishing.
Capabilities of time-resolved X-ray excited optical luminescence of the Taiwan Photon Source 23A X-ray nanoprobe beamline
(2020-01-01)
Lin, Bi-Hsuan
;
Wu, Yu-Hao
;
Li, Xiao-Yun
;
Hsu, Hsu-Cheng
;
Chiu, Yu-Cheng
Time-resolved X-ray excited optical luminescence (TR-XEOL) was developed successfully for the 23A X-ray nanoprobe beamline located at the Taiwan Photon Source (TPS). The advantages of the TR-XEOL facility include (i) a nano-focused X-ray beam (<60 nm) with excellent spatial resolution and (ii) a streak camera that can simultaneously record the XEOL spectrum and decay time. Three time spans, including normal (30 ps to 2 ns), hybrid (30 ps to 310 ns) and single (30 ps to 1.72 mu s) bunch modes, are available at the TPS, which can fulfil different experimental conditions involving samples with various lifetimes. It is anticipated that TR-XEOL at the TPS X-ray nanoprobe could provide great characterization capabilities for investigating the dynamics of photonic materials.
Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0001)
(2019-12-04)
Liu, Wei-Rein
;
Pao, Chih-Wen
;
Yu, Doris Weijyun
;
Chin, Yi-Ying
;
Wu, Wen-Bin
We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0001) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L-2(,3) edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0005) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L-3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (V-Zn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous V-Zn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density V-Zn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.
Influence of basal stacking faults on luminescence of m-plane Mg0.19Zn0.81O/ZnO quantum wells grown on a two-step MgZnO-buffer layer
(2019-03-01)
Wu, Yung-Chi
;
Chang, Ning-An
;
Yang, Tsung-Han
;
Tsai, Chih-Ya
;
Chen, Hou-Ren
Different well widths of m-plane Mg0.19Zn0.81O/ZnO multiple quantum wells (MQWs) have been fabricated on the two-step pregrown MgZnO-buffered m-plane sapphires by a pulsed laser deposition and have a narrower linewidth on x-ray diffraction than ZnO-buffered MQWs. By using the variation method to calculate the band diagram and by corroborating the results of temperature-dependent photoluminescence, we confirm the binding energies of free exciton and basal-stacking-fault-bound exciton, which is composed of electrons in the barrier and confined hole in the narrow-width MQWs. The coupling of A1(LO) phonon with excitons in m-MQWs shows more sensitivity toward a decrease in well width than E2(low), due to the crystal symmetry and the reduced Bohr radius. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)
Direct Backward Third Harmonic Generation in Nanostructures
(2010-01-01)
Chang, Chieh-Feng
;
Chen, Hsing-Chao
;
Chen, Miin-Jang
;
Liu, Wei-Rein
;
Hsieh, Wen-Feng
We theoretically and experimentally demonstrated that direct backward third harmonic generation (THG) waves can be comparable in magnitude with forward THG waves in nanostructures, such as ZnO thin films and nanoparticles of CdSe and Fe3O4. (C) 2010 Optical Society of America
Study of Nonpolar GaN/ZnO Heterostructures Grown by Molecular Beam Epitaxy
(2013-07-01)
Chang, Chiao-Yun
;
Huang, Huei-Min
;
Lan, Yu-Pin
;
Lu, Tien-Chang
;
Tu, Li-Wei
The growth mechanism and characteristics of the nonpolar GaN/ZnO heterostructure grown on the r-plane sapphire substrate by using molecular beam epitaxy were studied. The crystal interaction between GaN and ZnO epitaxial layers was clarified by using transmission electron microscopy and X-ray diffraction. A new epitaxial relationship of ZnGa2O4 (220)//GaN (10 (13) over bar) in the normal surface direction was obtained in the GaN/ZnO heterostructure. It was believed that the formation of ZnGa2O4 (220) was due to the recrystallizadon of the ZnO layer with Ga atoms, which in turn resulted in the formation of semipolar-oriented GaN. In addition, the main optical transition in the GaN/ZnO heterostructure was attributed to the existence of the interface states and new ZnO:(Ga,N) alloys.
Investigation of Cavity Enhanced XEOL of a Single ZnO Microrod by Using Multifunctional Hard X-ray Nanoprobe
(2019-01-18)
Lin, Bi-Hsuan
;
Li, Xiao-Yun
;
Lin, Dai-Jie
;
Jian, Bo-Lun
;
Hsu, Hsu-Cheng
The multifunctional hard X-ray nanoprobe at Taiwan Photon Source (TPS) exhibits the excellent ability to simultaneously characterize the X-ray absorption, X-ray excited optical luminescence (XEOL) as well as the dynamics of XEOL of materials. Combining the scanning electron microscope (SEM) into the TPS 23A end-station, we can easily and quickly measure the optical properties to map out the morphology of a ZnO microrod. A special phenomenon has been observed that the oscillations in the XEOL associated with the confinement of the optical photons in the single ZnO microrod shows dramatical increase while the X-ray excitation energy is set across the Zn K-edge. Besides having the nano-scale spatial resolution, the synchrotron source also gives a good temporal domain measurement to investigate the luminescence dynamic process. The decay lifetimes of different emission wavelengths and can be simultaneously obtained from the streak image. Besides, SEM can provide the cathodoluminescence (CL) to be a complementary method to analyze the emission properties of materials, we anticipate that the X-ray nanoprobe will open new avenues with great characterization ability for developing nano/microsized optoelectronic devices.
High-power, passively mode-locked Nd:GdVO4 laser using single-walled carbon nanotubes as saturable absorber
(2011-04-01)
Chen, Hou-Ren
;
Wang, Yong-Gang
;
Tsai, Chih-Ya
;
Lin, Kuei-Huei
;
Chang, Teng-Yao
We use a new (to our knowledge) fabrication method of a single-walled carbon nanotube (SWCNT) absorber without polymer to sustain high-power illumination. Using a series of saturable absorbers (SAs) incorporating different amounts of SWCNTs, we demonstrate mode-locking for a Nd:GdVO4 laser in the 1 mu m spectral range. Continuous-wave mode-locking (CWML) pulses with a maximum output power of 3.6 W at 1063 nm and high noise extinction of 61 dB has been achieved to give the highest pulse peak power of 3.6 kW and pulse energy of 30 nJ under 15 W pumping. To our knowledge, this is the highest CWML output power with SWCNT-SAs reported. The measured nonlinear absorption of the SWCNT-SAs shows a modulation depth of similar to 3% with subpicosecond recovery time. (C) 2011 Optical Society of America

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