Browsing by Author "Wu, Yung-Chi"

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Characteristics of exciton-polaritons in ZnO-based hybrid microcavities
(2011-02-28)
Chen, Jun-Rong
;
Lu, Tien-Chang
;
Wu, Yung-Chi
;
Lin, Shiang-Chi
;
Hsieh, Wen-Feng
Wide bandgap semiconductors are promising materials for the development of polariton-based optoelectronic devices operating at room temperature (RT). We report the characteristics of ZnO-based microcavities (MCs) in the strong coupling regime at RT with a vacuum Rabi splitting of 72 meV. The impact of scattering states of excitons on polariton dispersion is investigated. Only the lower polariton branches (LPBs) can be clearly observed in ZnO MCs since the large vacuum Rabi splitting pushes the upper polariton branches (UPBs) into the scattering absorption states in the ZnO bulk active region. In addition, we systematically investigate the polariton relaxation bottleneck in bulk ZnO-based MCs. Angle-resolved photoluminescence measurements are performed from 100 to 300 K for different cavity-exciton detunings. A clear polariton relaxation bottleneck is observed at low temperature and large negative cavity detuning conditions. The bottleneck is suppressed with increasing temperature and decreasing detuning, due to more efficient phonon-assisted relaxation and a longer radiative lifetime of the polaritons. (C) 2011 Optical Society of America
Exciton Localization Behaviors of Basal Stacking Faults in a-Plane AlGaN Alloys
(2011)
Huang, Huei-Min
;
Wu, Yung-Chi
;
Lu, Tien-Chang
;
光電工程學系
;
Department of Photonics
We study the basal plane stacking faults (BSFs) related optical properties in a-plane AlGaN alloys with different Al composition ranging from 0 to 0.28. The low-temperature photoluminescence (PL) spectra for AlGaN show two dominant peaks attributed to the emission of near band edge and BSFs-bound excitons, respectively. The PL integrated intensity ratio of the BSFs to NBE is found to correlate to the density of BSFs observed by the transmission electron microscopy. Finally, the exciton localization behaviors of BSFs in a-plane AlGaN alloys is observed and discussed in this study. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3561422] All rights reserved.
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.
Exciton-polaritons study in ZnO-based hybrid microcavities
(2012)
Lan, Yu-Pin
;
Lai, Ying-Yu
;
Huang, Si-Wei
;
Chen, Jun-Rong
;
Wu, Yung-Chi
The characteristics of exciton-polaritons in ZnO-based microcavities (MCs) are demonstrated with a large vacuum Rabi splitting due to large exciton binding energy and oscillator strength. The lower polariton branches (LPBs) can be clearly observed. For low temperature and large negative detuning conditions, a clear polariton relaxation bottleneck in bulk ZnO-based MCs has been observed in angle-resolved photoluminescence measurements from 100 to 353 K at different cavity-exciton detunings. The bottleneck is strongly suppressed with increasing the temperature and pumping power and reducing detuning. This observed results supposed to be due to more efficient phonon-assisted relaxation and a longer radiative lifetime of the polaritons. In addition, the linewidth broadening, blue-shift of the emission peak, and polarization of polariton lasing from below threshold to up threshold are also discussed.
GaN-based microcavity polariton light emitting diodes
(2012)
Lu, Tien-Chang
;
Lai, Ying-Yu
;
Huang, Si-Wei
;
Chen, Jun-Rong
;
Wu, Yung-Chi
Here we report the first realization of a current injection microcavity GaN exciton-polariton light emitting diode (LED) operating under room temperature (RT). The hybrid microcavity structure consists of InGaN/GaN quantum wells sandwiched between bottom epitaxial DBR and top dielectric DBR. The anti-crossing behavior of polariton LED denotes a clear signature of the strong interaction between excitons and cavity photons.
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.
High Reflectance Patterned Substrate Designed for Optical Devices
(2015)
Lan, Yu-Pin
;
Wu, Yung-Chi
;
Lin, Da-Wei
;
Hong, Kuo-Bin
;
Lee, Chia-Yu
This article proposed an approach by using patterned sapphire substrate to act as a 2-dimensional distributed Bragg reflector (DBR) mirror to increase the output power of LEDs.
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)
Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature
(2009-02-09)
Chen, Jun-Rong
;
Lu, Tien-Chang
;
Wu, Yung-Chi
;
Lin, Shiang-Chi
;
Liu, Wei-Rein
Wide-band gap ZnO semiconductors are attractive materials for the investigation of microcavity exciton polaritons due to the large exciton binding energy and oscillator strength. We report the growth and characterization of bulk ZnO-based hybrid microcavity. The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based hybrid microcavity structure, which consists of 30 pair epitaxially grown AlN/AlGaN distributed Bragg reflector (DBR) on the bottom side of the 3/2 lambda thick ZnO cavity and 9 pair SiO(2)/HfO(2) DBR as the top mirror. The cavity quality factor is about 221. The experimental results show good agreement with theoretically calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be about 58 meV in the ZnO-based hybrid microcavity.
Large Vacuum Rabi Splitting in ZnO-Based Microcavities
(2009)
Chen, Jun-Rong
;
Lu, Tien-Chang
;
Wu, Yung-Chi
;
Lin, Shiang-Chi
;
Liu, Wei-Rein
We report the growth and characterization of bulk ZnO-based hybrid microcavity (MC). The phenomenon of strong exciton-photon coupling at room temperature has been observed in the ZnO-based MC. The cavity quality factor is 221. The experimental results show good agreement with calculated exciton-polariton dispersion curves based on transfer matrix method. From the theoretical and experimental exciton-polariton dispersion curves with two different cavity-exciton detuning values, the large vacuum Rabi splitting is estimated to be 58 meV in the ZnO-based MC.
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.
Observation of Polariton Dispersions for ZnO Microcavities in Strong Couple Regime
(2010)
Chen, Jun-Rong
;
Lu, Tien-Chang
;
Wu, Yung-Chi
;
Lin, Shiang-Chi
;
Liu, Wei-Rein
Characteristics of ZnO-based MCs with vacuum Rabi splitting of 72 meV are demonstrated. It is found that only the LPBs can be observed in ZnO MCs since the Rabi splitting pushes UPBs into scattering absorption.
Peculiar near-band-edge emission of polarization-dependent XEOL from a non-polar a-plane ZnO wafer
(2018-02-05)
Lin, Bi-Hsuan
;
Wu, Yung-Chi
;
Chen, Huang-Yeh
;
Tseng, Shao-Chin
;
Wu, Jian-Xing
Polarization-dependent hard X-ray excited optical luminescence (XEOL) was used to study not only the optical properties but also the crystallographic orientations of a non-polar a-plane ZnO wafer. In addition to a positive-edge jump and extra oscillations in the near-band-edge (NBE) XEOL yield, we observed a blue shift of the NBE emission peak that follows the polarization-dependent X-ray absorption near-edge structure (XANES) as the X-ray energy is tuned across the Zn K-edge. This NBE blue shift is caused by the larger X-ray absorption, generating higher free carriers to reduce the exciton-LO phonon coupling, which causes a decrease in the exciton activation energy. The extra oscillations in XANES and XEOL as the polarization is set parallel to the c-axis is attributed to simultaneous excitations of the Zn 4p - O 2p pi-bond along the c-axis and the bilayer sigma-bond, whereas only the sigma-bond is excited when the polarization is perpendicular to the c-axis. The polarization-dependent XEOL spectra can be used to determine the crystallographic orientations. (c) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Polarization-dependent XEOL: Comparison of peculiar near-band-edge emission of non-polar alpha-plane GaN and ZnO wafers
(2019-03-04)
Lin, Bi-Hsuan
;
Wu, Yung-Chi
;
Lee, Jyh-Fu
;
Tang, Mau-Tsu
;
Hsieh, Wen-Feng
Polarization-dependent hard X-ray excited optical luminescence (XEOL) spectroscopy was used to study the properties of near-band-edge (NBE) emission of non-polar alpha-GaN and alpha-ZnO wafers. We found similar behaviors of alpha-GaN with alpha-ZnO that include the positive intensity jump and the blue shift of the NBE emission peak in the XEOL spectrum following the polarization-dependent X-ray absorption near-edge structure (XANES) as the X-ray energy is tuned across the Ga or Zn K-edge. Furthermore, as the X-ray energy is set above the K-edges, different oscillations of XEOL, also following the XANES, were observed depending upon the directions of the excitation X-ray electric field. However, the blue shift of alpha-ZnO is about two times larger than that of alpha-GaN that results from the larger polar field in alpha-ZnO than that in alpha-GaN. For both alpha-GaN and alpha-ZnO, the above K-edge excited oscillations in XANES and XEOL spectra, where the polarization is set parallel to the pi-axis, are attributed to simultaneous excitations of the p-bond along the pi-axis and in-plane sigma-bonds, whereas only the in-plane sigma-bonds are excited for the polarization perpendicular to the pi-axis. Therefore, these polarization dependent oscillation features of XEOL yields that follow the fluorescence yields can be used as an alternative way to determine the crystallographic orientations. Published under license by AIP Publishing.
Purely Sidewall InGaN/GaN Core-Shell Nanorod Green Light-Emitting Diodes
(2015)
Lin, Da-Wei
;
Wu, Yung-Chi
;
Kuo, Hao-Chung
;
Chi, Gou-Chung
;
Hsu, Lung-Hsing
A novel purely sidewall InGaN/GaN core-shell nanorod green light-emitting diode (LED) has been demonstrated by 3D dielectric passivation and selective epitaxial growth technologies. The LED device exhibits unprecedented stable emission wavelength and low efficiency droop.
Room temperature polariton lasing vs. photon lasing in a ZnO-based hybrid microcavity
(2012-02-27)
Lu, Tien-Chang
;
Lai, Ying-Yu
;
Lan, Yu-Pin
;
Huang, Si-Wei
;
Chen, Jun-Rong
We report on the room temperature polariton lasing and photon lasing in a ZnO-based hybrid microcavity under optical pumping. A series of experimental studies of the polariton lasing (exciton-photon detunings of delta = -119meV) in the strong-coupling regime are discussed and compared to a photon lasing (delta = -45meV) in the weak-coupling regime obtained in the same structure. The measured threshold power density (31.8kW/cm(2)) of polariton lasing is one order of magnitude lower than that of the photon lasing (318.2kW/cm(2)). In addition, the comparison between polariton lasing and photon lasing is done in terms of the linewidth broadening, blue-shift of the emission peak, and polarization. (C) 2012 Optical Society of America
The dominant effect of non-centrosymmetric displacement on the crystal-field energy splitting in the strained a-plane ZnO epi-films on r-plane sapphires
(2017-06-28)
Wu, Yung-Chi
;
Liu, Wei-Rein
;
Chen, Hou-Ren
;
Hsu, Chia-Hung
;
Hsieh, Wen-Feng
The polarization-dependent photoluminescence (PL) of the a-plane ZnO (a-ZnO) films grown on r-plane sapphire substrates shows that the crystal-field energy splitting (Delta(CF)) becomes larger with increasing film thickness from 24 nm to 291 nm. In the use of the nonlinear fitting of the stress-strain tensor to the X-ray diffraction data, we found crystal symmetry breaking from wurtzite to monoclinic in these films and determined the corresponding lattice constants and the relaxed lattice constants. With the available lattice variables, we further used the tight-binding method to calculate the band energies and compared with the PL spectra. The results confirm that the crystal deformation and the non-centrosymmetric displacement had opposite effects on the Delta(CF) at the Gamma point and the displacement induced by the strain is the dominant effect on Delta(CF).
The optical property of vertical external cavity surface-emitting laser
(2015)
Wu, Yung-Chi
;
Lin, Da-Wei
;
Hsieh, Wen-Feng
;
Lan, Yu-Pin
;
光電工程學系
Under passive Q-switched laser pumping, a vertical-external-cavity surface-emitting laser consisted of a semiconductor gain medium with a high reflectivity dielectric distributed Bragg reflection and an external mirror can achieve ultra-low threshold lasing at room temperature.