Browsing by Author "Liu, Wei-Rein"

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Acoustic Velocity and Optical Index Birefringence in A-Plane ZnO Thin Films
(2011-02-01)
Chen, Yi-Hsin
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Wen, Yu-Chieh
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Liu, Wei-Rein
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Hsieh, Wen-Feng
;
Sun, Chi-Kuang
The longitudinal acoustic velocity in the direction normal to the c-axis and the birefringence of refractive index of a-plane zinc oxide (ZnO) thin films were measured by ultrafast transient reflection measurement. Through Fourier analysis, the experimental data show that the acoustic velocity is about 6000 m/s and the measured refractive index near the band gap, which is higher than the previously reported results by 20 similar to 30%.
Coherent acoustic phonon oscillation accompanied with backward acoustic pulse below exciton resonance in a ZnO epifilm on oxide-buffered Si(111)
(2016-08-17)
Lin, Ja-Hon
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Shen, Yu-Kai
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Liu, Wei-Rein
;
Lu, Chia-Hui
;
Chen, Yao-Hui
Unlike coherent acoustic phonons (CAPs) generated from heat induced thermal stress by the coated Au film, we demonstrated the oscillation from c-ZnO epitaxial film on oxide buffered Si through a degenerate pump-probe technique. As the excited photon energy was set below the exciton resonance, the electronic stress that resulted from defect resonance was used to induce acoustic wave. The damped oscillation revealed a superposition of a high frequency and long decay CAP signal with a backward propagating acoustic pulse which was generated by the absorption of the penetrated pump beam at the Si surface and selected by the ZnO layer as the acoustic resonator.
Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0001)
(2019-12-04)
Liu, Wei-Rein
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Pao, Chih-Wen
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Yu, Doris Weijyun
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Chin, Yi-Ying
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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.
Direct Backward Third Harmonic Generation in Nanostructures
(2010-01-01)
Chang, Chieh-Feng
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Chen, Hsing-Chao
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Chen, Miin-Jang
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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
Direct backward third-harmonic generation in nanostructures
(2010-03-29)
Chang, Chieh-Feng
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Chen, Hsing-Chao
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Chen, Miin-Jang
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Liu, Wei-Rein
;
Hsieh, Wen-Feng
Direct-backward third harmonic generation (DBTHG) has been regarded as negligible or even inexistent due to the large value of wave-vector mismatch. In the past, BTHG signals were often interpreted as back-reflected or back-scattered forward-THG (FTHG). In this paper, we theoretically and experimentally demonstrate that backward third harmonic waves can be directly generated, and that their magnitude can be comparable with FTHG in nanostructures. Experimental data of DBTHG from ZnO thin films, CdSe quantum dots and Fe(3)O(4) nanoparticles agree well with simulation results based on the Green's function. An integral equation was also derived for fast computation of DBTHG in nano films. Our investigation suggests that DBTHG can be a potentially powerful tool in nano-science research, especially when combined with FTHG measurements. (C) 2010 Optical Society of America
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
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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
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Wu, Yu-Hao
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Wu, Tai-Sing
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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.
Large vacuum Rabi splitting in ZnO-based hybrid microcavities observed at room temperature
(2009-02-09)
Chen, Jun-Rong
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Lu, Tien-Chang
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Wu, Yung-Chi
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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
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Lu, Tien-Chang
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Wu, Yung-Chi
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Lin, Shiang-Chi
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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
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Wu, Yu-Hao
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Wu, Yung-Chi
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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
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Lu, Tien-Chang
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Wu, Yung-Chi
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Lin, Shiang-Chi
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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.
Pump polarization dependent ultrafast carrier dynamics and two-photon absorption in an a-plane ZnO epitaxial film
(2015-10-05)
Lin, Ja-Hon
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Su, Hsing-Jung
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Lu, Chia-Hui
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Chang, Chun-peng
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Liu, Wei-Rein
The ultrafast carrier dynamics of non-polar a-plane ZnO epi-film, with the energy difference between the A- and C-valence bands of about 23 meV, grown on r-plane sapphire were investigated using the reflection type pump-probe technique under two perpendicular polarized pumps. By exciting the electron from A-valence band through pump pokuization perpendicular to the c-axis of a-ZnO (E-pu perpendicular to c), the TDR trace revealed two photon absorption (TPA), hand filling (BF) and bandgap renormalization (BGR) effects that can be reasonably explained by the electron dynamics in the conduction band. By exciting the electron from C-valence band through parallel pump polarization (E-pu parallel to c), only the BF effect was observed in the TDR trace owing to the hole dynamics in the valence hands. The occurrence of TPA was determined by the pump efficiency depending on the energy difference between the pump photon and the intermediate exciton resonance state. (C) 2015 AIP Publishing LLC.
Room temperature excitonic dynamics of non-polar a-plane ZnO epifilms
(2016-09)
Lin, Ja-Hon
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Liu, Wei-Rein
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Lin, Yi-Chin
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Su, Hsing-Jung
;
Chen, Hou-Ren
Pump polarization dependent carrier dynamics, particularly excitonic dynamics, of non-polar a-plane zinc oxide (ZnO) epifilms with two different thicknesses were investigated using time resolved measurements. Unlike the electron and hole dynamics through the above-bandgap excitation, transient differential reflectance (TDR) traces revealed similar trends under two orthogonal pump polarization conditions relative to the c-axis (E-pu +/- c and E-pu vertical bar vertical bar c) of a-ZnO around near-exciton-resonance excitation. By means of a band diagram, the bandgap renormalization (BGR) effect can be reasonably explained by the screening of the Coulomb potential energy due to the accumulation of relaxed free carriers that were initially excited through the absorption of two cascaded pump photons via the excitonic level, a process known as two photon absorption (TPA). Thus, the modulation depths of the TPA around zero time delay, due to simultaneous absorption of one pump and one probe photon via the excitonic level, increased linearly with the pump fluence, proportional to the modulation depth resulting from the BGR effects. (C) 2016 Author(s).
Saturation and beating of acoustic phonon oscillations excited near the exciton resonance of strained polar ZnO/Zn0.8Mg0.2O multiple quantum wells
(2018-01-01)
Liu, Wei-Rein
;
Lin, Ja-Hon
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Chen, Jyun-Sian
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Cheng, Hsin-Ming
;
Li, Sheng-Jie
Saturation and beating of coherent acoustic phonon (CAP) oscillations were observed and attributed to the screening of a built-in electric field with increasing pump power using degenerate pump-probe measurements near the exciton resonance of polar ZnO/Zn0.8Mg0.2O multiple quantum wells (MQWs). After purifying the CAP signals by using an empirical mode decomposition, we found not only that the CAP amplitude follows the trend of the band gap renormalization (BGR) and shows saturation at high pump power, but also that the CAP oscillation period coincides with that of the MQWs, consistent with the XRD and TEM results. An additional low-frequency oscillation modifying the CAP signal is revealed due to the negative change in refractive index caused by BGR as the pump power increases.
Structural and optical characteristics of non-polar ZNO epitaxial films grown by radio-frequency magnetron sputtering
(2018-08-01)
Hsu, Chia-Hung
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Lin, Bi-Hsuan
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Liu, Wei-Rein
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Hsieh, Wen-Fong
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光電工程學系
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
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Liu, Wei-Rein
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Chen, Hou-Ren
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Hsu, Chia-Hung
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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 effect of thermal annealing on the optical and electrical properties of ZnO epitaxial films grown on n-GaAs (001)
(2015-01-01)
Liu, Wei-Rein
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Lin, Bi-Hsuan
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Lin, Chi-Yuan
;
Yang, Song
;
Kuo, Chin-Chia
Wurtzite ZnO epitaxial layers grown on n-type GaAs (001) by pulsed laser deposition (PLD) exhibited n-type conductivity. Post-growth annealing leads the conversion of carrier type from electron to hole, as revealed by Hall effect measurements, although only moderate structural improvement was observed. The carrier type conversion is attributed to thermally activated arsenic diffusion from the substrate, confirmed by secondary ion mass spectrometry and photoluminescence. The surface electrical properties of both the as-deposited n-type and annealed p-type ZnO epitaxial layers were thoroughly characterized by Kelvin force microscopy (KFM) and electrostatic force microscopy (EFM). The results indicated the existence of a high density of surface states close to the ZnO midgap with a density of a few 10(14) cm(-2) eV(-1). The Fermi levels (E-F) of n- and p-type ZnO epitaxial layers were found to be 1.06 eV below the conduction-band minimum (CBM) and 1.612-1.769 eV above the valence-band maximum (VBM), respectively. The small EF difference between the n- and p-type ZnO epitaxial layers implies Fermi level pinning at the surface of both n-and p-type ZnO epitaxial layers.
Thickness effect on ultrafast thermalization of carriers in above-band-gap states in ZnO epitaxial films
(2010-12-15)
Ou, Po-Chi
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Lin, Ja-Hon
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Chang, Chi-An
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Liu, Wei-Rein
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Hsieh, Wen-Feng
Energy-dependent free-carrier dynamics was investigated in 70 nm (thin) and 1 mu m (thick) ZnO epifilms using the optical pump-probe technique. The far-above-band-gap dynamics in the thin epifilm reveals the prolonged relaxation and the slow recovery of renormalized band gap. The band-gap renormalization (BGR) effect is affected by the inefficient carrier-phonon scattering. In addition, the loss of excited carrier density via surface trapping results in an energy-dependent BGR buildup time. However, the far-above-band-gap dynamics in the thick epifilm reveals fast relaxation followed by BGR recovery, which is independent of the photon energy. The near-band-gap dynamics shows an ultrafast carrier thermalization both in the thin and the thick epifilms.
THz Study on the Role of ZnO Crystallinity in ZnO/AgNW/ZnO Composite Electrodes
(2017-07-01)
Ahn, Hyeyoung
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Kao, Ning-Chun
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Liu, Wei-Rein
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Hsieh, Wen-Fung
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光電工程學系
We present the conducting mechanism of silver nanowire (AgNW) networks embedded between zinc oxide (ZnO) bilayers studied by terahertz (THz) reflection spectroscopy. Percolatively connected AgNWs have been considered as one of the most promising alternatives for indium-tin-oxide transparent conductive electrodes (TCEs). Although AgNWs can be spin-coated on nearly any kind of substrate, they leave a large open area between randomly scattered NWs. In ZnO/AgNW/ZnO composite electrodes, the conductive ZnO underlayer has the advantage in charge transport through electrodes and another ZnO layer covering the AgNW/ZnO layer can suppress oxidation of NW surface and improve the thermal stability. The performance of AgNW-based TCEs is highly correlated to the connectivity of NWs at junctions, which can be realized by percolational transition near a critical NW density. Terahertz reflection spectroscopic measurement shows that the photoconductivity of ZnO/AgNW/ZnO composite electrodes is strongly influenced by the ZnO crystallinity. The percolational behavior of the confinement factor in THz spectroscopic analysis reveals that single-crystalline ZnO layers in ZnO/AgNW/ZnO increase the photoconductivity by forming wider effective charge collection area around AgNWs and filling the gap between AgNWs to improve the ohmic contact.
Ultrafast relaxation and absorption saturation at near exciton resonance in a thin ZnO epilayer
(2011-01-01)
Ou, Po-Chi
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Liu, Wei-Rein
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Ton, Ho-Jei
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Lin, Ja-Hon
;
Hsieh, Wen-Feng
We observed ultrafast free exciton thermalization time of 700-900 fs and obtained the magnitude of maximal differential absorption to be 1.8 x 10(4) cm(-1) with the pumping fluence of 10 mu J/cm(2) by measuring transient differential transmission in a thin ZnO epitaxial layer at room temperature. The largest induced transparency occurs near exciton resonance associated with absorption saturation by comparing the excitation from the above band-gap to band-tail states. The pumping dependent transient absorption reveals transition of excitonic relaxation from exciton-phonon scattering to exciton-exciton scattering or to an electron-hole plasma. (C) 2011 American Institute of Physics. [doi:10.1063/1.3525993]