Browsing by Author "Chi, G. C."

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Characterization of 380 nm UV-LEDs grown on free-standing GaN by atmospheric-pressure metal-organic chemical vapor deposition
(2014-01-01)
Shieh, C. Y.
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Li, Z. Y.
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Kuo, H. C.
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Chang, J. Y.
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Chi, G. C.
We reported the defects and optical characterizations of the ultraviolet light-emitting diodes grown on free-standing GaN substrate (FS-GaN) and sapphire. Cross-sectional transmission electron microscopy (TEM) images showed that the total defect densities of grown UV LEDs on FS-GaN and sapphire including edge, screw and mixed type were 3.6x10(6) cm(-2) and 5.5x10(8) cm(-2). When substrate of UV LEDs was changed from sapphire to FS-GaN, it can be clearly found that the crystallography of GaN epilayers was drastically different from that GaN epilayers on sapphire. Besides, the microstructures or indium clustering can be not observed at UV LEDs on FS-GaN from TEM measurement. The internal quantum efficiency of UVLEDs on FS-GaN and sapphire were 34.8 % and 39.4 % respectively, which attributed to indium clustering in multi-layers quantum wells (MQWs) of UV LEDs on sapphire. The relationship between indium-clustering and efficiency droop were investigated by temperature-dependent electroluminescence (TDEL) measurements.
Chemical vapour deposition growth and Raman characterization of graphene layers and carbon nanotubes
(2016-01-01)
Lai, Y-C
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Rafailov, P. M.
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Vlaikova, E.
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Marinova, V.
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Lin, S. H.
Single-layer graphene films were grown by chemical vapour deposition (CVD) on Cu foil. The CVD process was complemented by plasma enhancement to grow also vertically aligned multiwalled carbon nanotubes using Ni nanoparticles as catalyst. The obtained samples were characterized by Raman spectroscopy analysis. Nature of defects in the samples and optimal growth conditions leading to achieve high quality of graphene and carbon nanotubes are discussed.
Chemical vapour deposition growth of graphene layers on metal substrates
(2014-01-01)
Lai, Y-C
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Yu, S-C
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Rafailov, P. M.
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Vlaikova, E.
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Valkov, S.
Graphene layers were grown by chemical vapour deposition (CVD) on Si wafers covered by a SiO2 substrate layer and a Ni interlayer, and on copper and nickel foil. The obtained graphene layers were characterized by Raman spectroscopy. The films grown on SiO2/Ni substrate and Ni foil comprise mainly multilayer defect-rich graphene, while those on Cu foil exhibit the spectroscopic fingerprint of relatively defect-free single-layer graphene due to the low carbon solubility in copper and the suitably chosen substrate position in a quasi-closed volume. Optimal growth conditions and the nature of defects in the layers are discussed.
Density-dependent energy relaxation of hot electrons in InN epilayers
(2009-01-01)
Yang, M. D.
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Liu, Y. W.
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Shen, J. L.
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Chen, C. W.
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Chi, G. C.
This work investigates the dependence of the hot-electron energy relaxation in InN epilayers on electron density. From the high-energy tail of photoluminescence, the electron temperature of the hot electrons was determined. Acoustic phonons have an important role in the energy relaxation of the hot electrons. The density-dependent electron energy loss rate in InN can be explained by a combination of longitudinal optical and acoustic phonon emissions. A slowing of energy loss rate at high electron densities was observed and attributed to piezoelectric coupling to acoustic phonons. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3056383]
Determination of the microstructure of Eu-treated ZnO nanowires by x-ray absorption
(2010-02-08)
Huang, W. L.
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Labis, J.
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Ray, S. C.
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Liang, Y. R.
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Pao, C. W.
X-ray absorption near-edge structure (XANES), extended x-ray absorption fine structures (EXAFS), and photoluminescence measurements were used to elucidate the microstructural and photoluminescence properties of ZnO nanowires (ZnO-NWs) that had been treated with Eu by thermal diffusion. The O K- and Eu L(3)-edge XANES and EXAFS spectra at the Zn K- and Eu L(3)-edge verified the formation of Eu(2)O(3)-like layer on the surface of ZnO-NWs. X-ray diffraction, XANES and EXAFS measurements consistently suggest the lack of substitutional doping of Eu ions at the Zn ion sites in the interior of ZnO-NWs. The clear sharp and intense emission bands in the range 610-630 nm of Eu-treated ZnO-NWs originated from the intra-4f transition of Eu ions in the Eu(2)O(3)-like surface layer.
Dip Pen Nanolithography of Conductive Silver Traces
(2010-06-03)
Hung, Sheng-Chun
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Nafday, Omkar A.
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Haaheim, Jason R.
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Ren, Fan
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Chi, G. C.
We report the first demonstration of sub mu m, sub-50-mu Omega.cm conductive traces directly written by Dip Pen Nanolithography (DPN). We achieved sub mu m Ag lines with 28.8 mu Omega.cm average resistivity after direct-write printing from a silver nanoparticle-based ink suspension and annealing at 150 degrees C for 10 min. This compares to Ag bulk resistivity of 1.63 mu Omega.cm, where the difference is within the range of previously reported variations in conductivity of Ag-based inks due to annealing conditions and larger width scales. We leveraged DPN's ability to directly place materials at specific locations in order to fabricate and characterize these conductive silver (Ag) traces on electrode patterns and multiple substrates (SiO(2), Kapton, mica). The low viscosity of the AgNP ink solution allowed write speeds up to 1600 mu m/s, almost 4 orders of magnitude higher than typical thiol-on-gold DPN writing speeds. This direct-write methodology paves the way for site-specific deposition of metallic materials for use in applications such as circuit repair, sensor element functionalization, failure analysis, gas sensing, and printable electronics.
Effects of rapid thermal annealing on the structural properties of TiO(2) nanotubes
(2011-10-15)
Lin, J. Y.
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Chou, Y. T.
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Shen, J. L.
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Yang, M. D.
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Wu, C. H.
The structural properties of TiO(2) nanotubes with rapid thermal annealing (RTA) and traditional thermal annealing in O(2) were studied by X-ray diffraction (XRD) and Raman scattering measurements. From analyzing the line width of XRD and the correlation length of the Raman peak, we demonstrate that RTA can be an effective tool for amorphous-anatase transformation in TiO(2) nanotubes. The Raman peak redshifts and reduces its line width after thermal annealing and RTA, which may involves the reduction of oxygen-related defects. (C) 2011 Elsevier B. V. All rights reserved.
Effects of rapid thermal annealing on the structural properties of TiO2 nanotubes
(2011-10-15)
Lin, J. Y.
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Chou, Y. T.
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Shen, J. L.
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Yang, M. D.
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Wu, C. H.
The structural properties of TiO2 nanotubes with rapid thermal annealing (RTA) and traditional thermal annealing in O-2 were studied by X-ray diffraction (XRD) and Raman scattering measurements. From analyzing the line width of XRD and the correlation length of the Raman peak, we demonstrate that RTA can be an effective tool for amorphous-anatase transformation in TiO2 nanotubes. The Raman peak redshifts and reduces its line width after thermal annealing and RTA, which may involves the reduction of oxygen-related defects. (C) 2011 Elsevier B. V. All rights reserved.
Efficiency and Droop Improvement in GaN-Based High-Voltage Light-Emitting Diodes
(2011-08-01)
Wang, C. H.
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Lin, D. W.
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Lee, C. Y.
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Tsai, M. A.
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Chen, G. L.
The efficiency and electrical characteristics of GaN-based high-voltage light-emitting diodes (HV-LEDs) are investigated in detail. The spatial distribution of light output and simulation results showed that 100-V HV-LED with smaller microchips had superior current spreading. As a result, under 1-W operation, the luminous efficiency of 100-V HV-LED with smaller microchips was enhanced by 7.8% compared to that of 50-V HV-LED, while the efficiency droop behaviors were reduced from 28% in 50-V HV-LED to 25.8% in 100-V HV-LED. Moreover, smaller microchips exhibited lower series resistance and forward voltage, leading to higher wall-plug efficiency.
Efficiency Improvement of GaN-Based Light-Emitting Diode Prepared on GaN Nanorod Template
(2010-02-15)
Kuo, C. H.
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Chang, L. C.
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Kuo, C. W.
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Chi, G. C.
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光電工程學系
We demonstrated the formation of GaN-based nanorod (NR) structure by using self-assemble Ni nanoislands as the etching mask. It was found that crystal quality of the GaN epilayer prepared on an NR GaN template was significantly better than that prepared with a conventional low-temperature GaN nucleation layer. With the NR GaN template, it was found that 20-mA light-emitting diode (LED) output power can be enhanced by 39.8%, as compared to the conventional LED.
High sensitivity carbon monoxide sensors made by zinc oxide modified gated GaN/AlGaN high electron mobility transistors under room temperature
(2011-05-30)
Hung, S. C.
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Chen, C. W.
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Shieh, C. Y.
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Chi, G. C.
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Fan, R.
AlGaN/GaN high electron mobility transistors (HEMTs) with zinc oxide (ZnO) nanowires modified gate exhibit significant changes in channel conductance upon expose to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition (CVD) with perfect crystal quality will attach CO molecules and release electrons, which will lead to a change in surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm in the open cavity with continuous gas flow using a 50 x 50 mu m(2) gate sensing area. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3596440]
Hybrid White Light-emitting Diodes by Organic-Inorganic materials
(2015)
Chen, K. J.
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Lai, Y. C.
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Lin, B. C.
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Lin, C. C.
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Chiu, S. H.
This study demonstrated the high uniformity hybrid white light-emitting didoes with polyfluoren (PFO) polymer and quantum dot (QD) with different CCT range from 3000K to 9000K.
Influence of free-standing GaN substrate on ultraviolet light-emitting-diodes by atmospheric-pressure metal-organic chemical vapor deposition
(2013-01-01)
Shieh, C. Y.
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Li, Z. Y.
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Chiu, C. H.
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Tu, P. M.
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Kuo, H. C.
We reported the influence of free-standing (FS) GaN substrate on ultraviolet light-emitting-diodes (UV LEDs) by atmospheric-pressure metal-organic chemical vapor deposition (APMOCVD). The Raman spectrum shows the in-plane compressive stress of the GaN epitaxial structures grown on FS GaN substrate. Besides, the Raman spectrum reveals the relation between the crystal quality and the carrier localization degree in multi-quantum wells (MQWs). High resolution X-ray diffraction (HRXRD) analysis results show that the In0 025Ga0 975N/Al0 08Ga0 92N MQWs grown on FS GaN substrate has higher indium mole fraction than sapphire at the same growth conditions. The higher indium incorporation is corresponding with the red-shift 6 nm (387 nm) of the room temperature photoluminescence (PL) peak. The full widths at half maximum (FWHM) of omega-scan rocking curve in (002) and (102) reflectance on FS GaN substrate (83 arcsec and 77 arcsec) are narrower than UV LEDs grown on sapphire (288 arcsec and 446 arcsec). This superior quality may attribute to homoepitaxial growth structure and better strain relaxation in the FS GaN substrate. An anomalous temperature behavior of PL in UV LEDs designated as an S-shaped peak position dependence and W-shaped linewidth dependence indicate that exciton/carrier motion occurs via photon-assisted tunneling through localized states, what results in incomplete thermalization of localized excitons at low temperature. The Gaussian broadening parameters of carrier localization is about 16.98 meV from the temperature dependent photoluminescence (TDPL) measurement. The saturation temperature from the TDPL linewidth of UV LEDs on FS GaN substrate at about 175 K represents a crossover from a nonthermalized to thermalized energy distribution of excitons.
Investigation of arsenic-doped ZnO thin films grown on Si substrate by atmospheric-pressure metal-organic chemical vapor deposition
(2012-06-01)
Hung, S. C.
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Wang, K. J.
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Lan, S. M.
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Yang, T. N.
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Uen, W. Y.
Zinc oxide (ZnO) thin film was grown on semi-insulating Si substrate using arsine (AsH3) as precursor by atmospheric-pressure metal-organic chemical vapor deposition (AP-MOCVD). In recently reported results, the physical mechanisms for As-doped ZnO thin films are explained as As substitution for oxygen (AsO) or As substitution for Zn and As combined with two Zn vacancies (AsZn2VZn). In this study, we control the in situ annealing ambient into two environments with various temperatures, which are Zn-rich, using diethylzinc (DEZn) as ambient gas, and O-rich, using water vapor as ambient gas, respectively. This should help to create AsO and AsZn2VZn. The ZnO thin film after in situ thermal annealing with H2O vapor ambient at 550 and 750 degrees C show p-type conductivity with hole concentration of 2.651 x 10(17) and 1.782 x 10(18)?cm-3, Hall mobilities of 10.08 and 5.402?cm2/V?s, and resistivities of 2.368 and 0.6485?O?cm, respectively.
Morphology and optical properties of zinc oxide thin films grown on Si (100) by metal-organic chemical vapor deposition
(2009-01-01)
Lan, S. M.
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Uen, W. Y.
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Chan, C. E.
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Chang, K. J.
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Hung, S. C.
Undoped zinc oxide (ZnO) thin films were deposited on Si (100) substrates by atmospheric pressure metal-organic chemical vapor deposition (AP-MOCVD). After a nucleation layer of ZnO about 500 A... was deposited, top ZnO films with 320-440 nm in thickness were fabricated at temperatures varying from 450 to 600 A degrees C, respectively. X-ray diffraction analyses demonstrate that all the films grown have polycrystalline wurtzite structure. Scanning electron microscopy observations indicate that ZnO films grow with c-axis aligned grains when the deposition temperature is lower than 600 A degrees C. However, 3D microstructures without c-axis alignment were found when the film was deposited at 600 A degrees C. Room temperature photoluminescence (PL) spectra peaking at 3.27-3.29 eV were observed for the ZnO films grown at 450-600 A degrees C. However, only the spectrum from the film grown at 600 A degrees C could present a high intensity ratio of the ultraviolet band to deep-level emissions, which is above 392 and show a minimum full-width at half-maximum of 99 meV for the ultraviolet band. Temperature-dependent PL measurements from 20 to 300 K exhibited the luminescent mechanism of the present ZnO films to be near band excitonic emission.
Nonpolar light emitting diode made by m-plane n-ZnO/p-GaN heterostructure
(2011-12-01)
Chen, C. W.
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Hung, S. C.
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Lee, C. H.
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Tun, C. J.
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Kuo, C. H.
Nonpolar (100) m-plane n-ZnO/p-GaN light-emitting-diodes (LEDs) were grown by chemical vapor deposition on p-GaN templates which was grown by metalorganic chemical vapor deposition on LiAlO(2)(100) substrate. Direct current (DC) electroluminescence (EL) measurements yielded a peak at 458nm. The EL peak position was independent of drive current and a full width of half maximum (FWHM) of 21.8 nm was realized at 20mA. The current-voltage characteristics of these diodes showed a forward voltage (V(f)) of 6V with a series resistance of 2.2 x 10(5) Omega. (C) 2011 Optical Society of America
Optical studies of InN epilayers on Si substrates with different buffer layers
(2007-12-01)
Yang, M. D.
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Shen, J. L.
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Chen, M. C.
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Chiang, C. C.
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Lan, S. M.
We have investigated the photoluminescence (PL) and time-resolved PL from the InN epilayers grown on Si substrates with different buffer layers. The narrowest value of the full width at half maximum of the PL peak is 52 meV with the AlN/AlGaN/GaN triple buffer layer, which is better than previous reports on similar InN epilayers on Si substrates. Based on the emission-energy dependence of the PL decays, the localization energy of carriers is also the least for the InN with a triple buffer layer. According to the x-ray diffraction measurements, we suggest that the reduced lattice mismatch between the InN epilayer and the top buffer layer is responsible for improvement of sample quality using the buffer-layer technique. (c) 2007 American Institute of Physics.
Oxygen sensors made by monolayer graphene
(2012-01-01)
Hung, S. C.
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Chen, C. W.
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Yang, M. D.
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Yeh, C. W.
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Wu, C. H.
The electrical resistivity of monolayer graphene exhibit significant changes upon expose to different concentration of oxygen (O-2) at room temperature. The monolayer graphene, grown by chemical vapor deposition (CVD) with perfect uniformity within 1cmx1cm will attach O-2 molecules which will act as a p-type dopant and enhance the hole conductivity, make a change of resistivity of graphene thin film. We quantified the change of resistivity of graphene versus different O-2 concentration and the detection limit of the simple O-2 sensor was 1.25% in volume ratio.
Oxygen sensors made by monolayer graphene under room temperature
(2011-12-12)
Chen, C. W.
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Hung, S. C.
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Yang, M. D.
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Yeh, C. W.
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Wu, C. H.
The electrical resistivity of monolayer graphene exhibit significant changes upon expose to different concentration of oxygen (O(2)) at room temperature. The monolayer graphene, grown by chemical vapor deposition with perfect uniformity within 1 cm x 1 cm will attach O(2) molecules and enhance the hole conductivity, which will lead to a change of resistivity of graphene thin film. We quantified the change of resistivity of graphene versus different O(2) concentration and the detection limit of the simple O(2) sensor was 1.25% in volume ratio. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3668105]
Performance enhancement of GaN-based flip-chip ultraviolet light-emitting diodes with a RPD AlN nucleation layer on patterned sapphire substrate
(2014-08-01)
Lin, B. C.
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Chiu, C. H.
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Lee, C. Y.
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Han, H. V.
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Tu, P. M.
In this work, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the transmission electron microscopy (TEM) image, it can be observed that the tip and incline portion of the pattern was smooth using the RPD AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 7 x 10(7) cm(-2) to 2.5 x 10(7) cm(-2) at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction. (C) 2014 Optical Society of America