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Ching-Lien Hsiao,
Jr-Tai Chen, Hsu-Cheng Hsu,
Ying-Chieh Liao,
Po-Han Tseng,
Yen-Ting Chen,
Zhe Chuan Feng,
Li-Wei Tu,
Mitch M. C. Chou,
Li-Chyong Chen,
Kuei-Hsien Chen
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ABSTRACT: Heteroepitaxial growth of m -plane (101_0) InN film on (100)-γ -LiAlO <sub>2</sub> (LAO) substrate has been realized by plasma-assisted molecular-beam epitaxy. Surface treatment of LAO substrate plays an important role in controlling the resultant phase and purity of m -plane InN. X-ray diffraction, reflection high-energy electron diffraction, electron back scatter diffraction, and transmission electron microscopy (TEM) studies revealed formation of pure m -plane InN film using substrate preannealed at 800 ° C but without any nitridation. In contrast, using substrate with nitridation but otherwise identical pretreatment and growth conditions, c -plane (0001) InN columnar structure was grown, instead of m -plane InN film. Structural anisotropy of the m -plane InN epitaxied on LAO is attributed to the I<sub>1</sub> type base-plane stacking faults according to the modified Williamson–Hall and TEM analyses. A rectangular-to-rectangular atomic stacking sequence and a commensurately lattice-matched condition in epitaxial direction of [12_10]<sub> InN </sub>||[001]<sub> LAO </sub> with a small misfit strain of ∼0.2 % are proposed to realize this heteroepitaxy. Angle-dependent polarized UV-Raman spectra showed that all the InN phonon modes follow Raman selection rule well. Strong polarization anisotropy of photoluminescence (PL) emission located at ∼0.63 eV was observed, as evidenced by a high polarization degree of 87% of the m -plane InN determined by infrared polarized PL spectroscopy.
Journal of Applied Physics 05/2010; · 2.17 Impact Factor
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Pai-Chun Wei,
Surojit Chattopadhyay,
Fang-Sheng Lin,
Chih-Ming Hsu,
Shyankay Jou,
Jr-Tai Chen,
Ping-Jung Huang, Hsu-Cheng Hsu,
Han-Chang Shih,
Kuei-Hsien Chen,
Li-Chyong Chen
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ABSTRACT: Photoluminescence (PL) behaviour in InN nanocolumns reveal decreasing, increasing and near invariant peak energies (E(PL)) as a function of temperature. Samples, having E(PL)~0.730 eV at 20 K, showed temperature invariance of E(PL). Samples possessing E(PL) on the lower and higher energy side of 0.730 eV demonstrate a normal redshift and anomalous blueshift, respectively, with increasing temperature. This temperature evolution can be effectively explained on the basis of a competition between a conventional red shift from lattice dilation, dominant for low carrier density sample, on one hand, and a blue shift of the electron and hole quasi Fermi-level separation, dominant for high carrier density samples, on the other.
Optics Express 08/2009; 17(14):11690-7. · 3.59 Impact Factor
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ABSTRACT: Nearly defect-free InN microcrystals grown on Si(111) substrates have been realized by plasma-assisted molecular beam epitaxy. High-resolution transmission electron microscope images reveal that these microcrystals exhibit single-crystalline wurtzite structure. Low temperature photoluminescence (PL) shows a strong emission peak at 0.679 eV with a very narrow linewidth of 17 meV at excitation power density of 3.4 W/cm2. Temperature-dependent PL spectra follow the Varshni equation well, and peak energy blueshifts by ∼ 45 meV from 300 to 15 K. Power-density-dependent PL spectroscopy manifests direct near-band-edge transition. A low carrier density of 3×1017 cm−3 has been estimated from PL empirical relation, which is close to the critical carrier density of the Mott transition of 2×1017 cm−3.
Applied Physics Letters 10/2007; 91(18):181912-181912-3. · 3.84 Impact Factor
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ABSTRACT: Infrared lasing from single-crystalline InN nanobelts grown by metal organic chemical vapor deposition was demonstrated. Transmission electron microscopy studies revealed that the InN nanobelts of rectangular cross section grew along [110] direction and were enclosed by ±(001) and ±(10) planes. The infrared lasing action was observed at 20 K in the InN nanobelts grown on an amorphous silicon nitride coated silicon substrate by continuous wave laser pumping.
Applied Physics Letters 03/2007; 90(12):123109-123109-3. · 3.84 Impact Factor
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ABSTRACT: ZnMgO nanostructures with wurtzite phase were prepared by thermal diffusion of Mg into the ZnO nanowires. As ZnO light-emitting devices have been operated by using ZnMgO layers as energy barrier layers to confine the carriers, it is essential to realize the characterization of ZnMgO particularly. In this work, the Mg content in Zn 1Àx Mg x O alloy determined by X-ray diffraction (XRD) and photoluminescence (PL) shows a good coincidence. The variation of lattice constant and the blueshift of near-band-edge emission indicate that Zn 2+ ions are successfully substituted by Mg 2+ ions in the ZnO lattice. In Raman-scattering studies, the change of E 2 (high) phonon line shape in ZnO:Mg nanostructures reveals the microscopic substitutional disorder. In addition to the host phonons of ZnO, two additional bands around 383 and 510 cm À1 are presumably attributed to the Mg-related vibrational modes.
Journal of Solid State Chemistry. 01/2007; 180:1188-1192.
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ABSTRACT: Upon decreasing size of ZnO quantum dots, the blueshift in bandgap reveals the quantum confinement effect and the diminishing ratio of the second- to first-order resonant Raman intensity reveals reducing the electron-phonon Frohlich interaction
Lasers and Electro-Optics Society, 2006. LEOS 2006. 19th Annual Meeting of the IEEE; 11/2006
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ABSTRACT: We report a simple method for fabricating heterostructured ZnMgO nanowires by annealing the preformed ZnO/MgO core-shell structure. Photoluminescence from the alloy nanowires shows strong near-band-edge (NBE) emission, reflecting good material quality. A blueshift of the NBE emission at room temperature after the annealing treatment is attributed to the diffusion of Mg from the shell into the core ZnO of the nanowires to form a ternary ZnMgO alloy. Band gap engineering and stimulated emissions of ZnMgO nanowires with different Mg doping concentrations are also demonstrated.
Applied Physics Letters 07/2006; 89(1):013101-013101-3. · 3.84 Impact Factor
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ABSTRACT: ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum confinement effect. From the resonant Raman scattering, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Fröhlich interaction.
Applied Physics Letters 07/2006; · 3.84 Impact Factor
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ABSTRACT: Both band gap engineering and spatial confinement of optical phonon were observed depending upon the size of ZnO quantum dots at room temperature. Size-dependent blueshifts of photoluminescence and absorption spectra reveal the quantum confinement effect. The measured Raman spectral shift and asymmetry for the E2(high) mode caused by localization of optical phonons agree well with that calculated by using the modified spatial correlation model.
Applied Physics Letters 06/2006; 88(26):263117-263117-3. · 3.84 Impact Factor
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ABSTRACT: Epitaxial ZnO nanowires and random-growth-oriented nanobelts were grown on c-plane sapphire with and without a pre-coated ZnO epilayer film. On the pre-coated ZnO epilayer, ZnO nanowires are vertically aligned with good in-plane alignment as a result of homoepitaxy, whereas on the bare c-plane sapphire, besides a few nanowires vertically aligned with , the nanowires were properly aligned with three-fold rotation symmetry. The ZnO nanowires are well-defined hexagonal crystals with diameters of 70–500 nm and lengths of up to several micrometres. In the junction regions between the pre-coated epilayer and the bare sapphire surface, however, ZnO nanobelts (nanoribbons) were found. Cathodoluminescence measurements revealed that the emission at 3.26 eV is correlated with free-exciton recombination and the broad green emission at 2.48 eV is attributed to surface defects. The stronger green emission implies that more surface defects exist on the side walls of nanowires and nanobelts. In Raman scattering, the E1(LO) mode is sensitive to the orientation of nanostructure that is consistent with the cathodoluminescence results.
Nanotechnology 02/2006; 17(5):1404. · 3.98 Impact Factor
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ABSTRACT: Epitaxial ZnO nanowires and random-growth-oriented nanobelts were grown on c-plane sapphire with and without a pre-coated ZnO epilayer film. On the pre-coated ZnO epilayer, ZnO nanowires are vertically aligned with good in-plane alignment as a result of homoepitaxy, whereas on the bare c-plane sapphire, besides a few nanowires vertically aligned with [0001] ZnO [0001] Al 2 O 3 , the nanowires were properly aligned with three-fold rotation symmetry. The ZnO nanowires are well-defined hexagonal crystals with diameters of 70–500 nm and lengths of up to several micrometres. In the junction regions between the pre-coated epilayer and the bare sapphire surface, however, ZnO nanobelts (nanoribbons) were found. Cathodoluminescence measurements revealed that the emission at 3.26 eV is correlated with free-exciton recombination and the broad green emission at 2.48 eV is attributed to surface defects. The stronger green emission implies that more surface defects exist on the side walls of nanowires and nanobelts. In Raman scattering, the E1(LO) mode is sensitive to the orientation of nanostructure that is consistent with the cathodoluminescence results.
Nanotechnology 01/2006; 17:1404-1407. · 3.98 Impact Factor
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ABSTRACT: Self-assembled secondary ZnO nanoparticles, recognized with the agglomeration of crystalline subcrystals, are successfully synthesized by a simple sol-gel method. TEM images display that one artificial cluster behaves in a single-crystal-like wurtzite structure because subcrystals coagulate as the same crystal orientation. Moreover, from the resonant Raman scattering, the as-grown sample exhibits phonon red shift; meanwhile, the coupling strength between electron and longitudinal optical phonon, determined by the ratio of second- to first-order Raman scattering cross sections, diminishes compared with the samples after postannealing at 350 and 500 degrees C. The size dependence of electron-phonon coupling is principally as a result of the Fröhlich interaction.
The Journal of Physical Chemistry B 11/2005; 109(39):18385-90. · 3.70 Impact Factor
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ABSTRACT: Photoluminescence and resonant Raman scattering (RRS) were investigated in various sizes of ZnO quantum dots. The size dependent shift of exciton emission reveals quantum confinement effect as changing the band gap. Besides, it was also found the enhancement of RRS intensity and electron-phonon coupling diminishes with decreasing particle’s diameter.
Lasers and Electro-Optics, 2005. CLEO/Pacific Rim 2005. Pacific Rim Conference on; 09/2005
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ABSTRACT: Optical phonon confinement and efficient UV emission of ZnO nanowires were investigated in use of resonant Raman scattering (RRS) and photoluminescence (PL). The high-quality ZnO nanowires with diameters of 80-100 nm and lengths of several micrometers were epitaxially grown through a simple low-pressure vapor-phase deposition method at temperature 550 degrees C on the precoated GaN(0001) buffer layer. The increasing intensity ratio of n-order longitudinal optical (LO) phonon (A(1)(nLO)/E(1)(nLO)) with increasing scattering order in RRS reveals the phonon quantum confinement as shrinking the diameter of ZnO nanowires. The exciton-related recombination near the band-edge transition dominate the UV emissions at room temperature as well as at low temperature that exhibits almost no other nonstoichiometric defects in the ZnO nanowires.
The Journal of Physical Chemistry B 06/2005; 109(18):8749-54. · 3.70 Impact Factor
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ABSTRACT: We report room-temperature ultraviolet stimulated emission and lasing from optically pumped high-quality ZnO nanowires. Emission due to the exciton-exciton scattering process shows apparent stimulated-emission behavior. Several sharp peaks associated with random laser action are seen under high pumping intensity. The mechanism of laser emission is attributed to coherent multiple scattering among the random-growth oriented nanowires. The characteristic cavity length is determined by the Fourier transform of the lasing spectrum.
Journal of Applied Physics 04/2005; · 2.17 Impact Factor
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ABSTRACT: ZnO nanowires have been synthesized on porous silicon substrates with different porosities via the vapour–liquid–solid method. The texture coefficient analysed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. The Raman spectrum reveals the high quality of the ZnO nanowires. From the temperature-dependent photoluminescence spectra, we deduced the activation energies of free and bound excitons.
Nanotechnology 01/2005; 16(2):297. · 3.98 Impact Factor
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ABSTRACT: Vertically well-aligned ZnO nanorods were synthesized without employing any metal catalysts on various substrates including glass, Si(111), 6H-SiC(0001) and sapphire (0001), which were pre-coated with c-oriented ZnO buffer layers, by simple physical vapour deposition. The alignments of the ZnO nanorods on the different substrates depend on the crystallographic alignments of the pre-coated ZnO buffer layers. The ZnO nanorods grown on glass and Si(111) are vertically aligned but randomly oriented in the in-plane direction. In contrast, the vertically aligned ZnO nanorods on 6H-SiC(0001) and sapphire (0001) show an in-plane alignment with azimuthally sixfold symmetry, which indicates the epitaxial relationship between ZnO and the substrate. Similarly, photoluminescence measurements show the distinct appearance of ZnO nanorods on different substrates. Besides the UV band, which was attributed to the recombination of free excitons near the band edge, defect-related visible emissions were also observed for the samples grown on both glass and Si(111) substrates. However, the ZnO nanorods exhibit only strong band edge emission peaks with no noticeable deep level emissions when grown on the 6H-SiC(0001) and sapphire (0001) substrates, which confirms the good crystalline and optical quality of the epitaxial ZnO nanorods.
Nanotechnology 01/2005; 16:2882-2886. · 3.98 Impact Factor
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ABSTRACT: ZnO nanowires have been synthesized on porous silicon substrates with different porosities via the vapour–liquid–solid method. The texture coefficient analysed from the XRD spectra indicates that the nanowires are more highly orientated on the appropriate porosity of porous silicon substrate than on the smooth surface of silicon. The Raman spectrum reveals the high quality of the ZnO nanowires. From the temperature-dependent photoluminescence spectra, we deduced the activation energies of free and bound excitons.
Nanotechnology 01/2005; 16:297-301. · 3.98 Impact Factor
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ABSTRACT: Hexagonal ZnO nanorods have been selectively synthesized via vapor–solid process without gold catalysis on a pre-coated ZnO buffer layer. The presence of nanometer-sized pits or hills on the surface of ZnO buffer layer provides nucleation sites to which the zinc vapor is transferred and condensed. Followed by immediate oxidation the ZnO nanorods were grown on the buffer layer. Contrarily, the SEM images hardly show growth of irregular ZnO nanometer-sized products on the bare sapphire substrate. Besides a strong ultra-violet emission at 3.26 eV observed at room temperature, the coupling strength of the radiative transition to LO-phonon polarization field was deduced in use of the Huang–Rhys factor from low temperature photoluminescence spectra to show that single crystalline ZnO nanorods.
Journal of Crystal Growth 01/2004; 261:520-525. · 1.73 Impact Factor
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ABSTRACT: The influence of annealing on the AgGaS films grown by pulsed laser deposition has been investigated. The X-ray diffraction 2 results show the AgGaS films were found with preferential orientation (1 1 2) normal to the surface and silver droplets were 2 diminished after the post-annealing. Photoluminescence (PL) measurements reveled the exciton energy is slightly red shifted that is possibly due to the thermal strain effect. The binding energy of the shallow donors is ;28 meV determined from temperature dependent PL spectra. In addition, the A-exciton and the ByC-exciton could be observed in the transmittance spectra at room temperature. 2002 Elsevier Science B.V. All rights reserved.
Thin Solid Films 01/2002; 419:237-241. · 1.89 Impact Factor
