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ABSTRACT: GaN-based metal-semiconductor-metal ultraviolet photodetectors (PDs) prepared on a patterned sapphire substrate (PSS) and a conventional flat sapphire substrate were both fabricated and characterized. It was found that we can reduce dark leakage current and enhance by about two orders of magnitude by using a PSS. The internal gain of the PDs prepared on a PSS was also much smaller.
IEEE Photonics Technology Letters 12/2008; · 2.19 Impact Factor
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ABSTRACT: We demonstrate a high-speed GaN-based green light-emitting diode for plastic optical fiber (POF) communication applications. By using a combination of n-type doping and undoped In<sub>x</sub>Ga<sub>1-x</sub>N/GaN based multiple quantum wells (MQWs), and a 76-mum-diameter current-confined aperture structure, we can obtain an extremely high electrical-to-optical (E-O) 3 dB bandwidth (~330 MHz), which is limited by the spontaneous recombination lifetime of the MQWs. A reasonable coupled power (-264 muW) can be simultaneously achieved for a 2 mm in diameter POF with a 0.5 numerical aperture (NA).
IEEE Electron Device Letters 03/2008; · 2.85 Impact Factor
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Advanced Functional Materials 11/2007; 17(18):3773 - 3780. · 10.18 Impact Factor
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J.-W. Shi,
H.-Y. Huang,
C.-K. Wang,
J.-K. Sheu,
W.-C. Lai,
Y.-S. Wu, C.-H. Chen,
J.-T. Chu,
H.-C. Kuo,
Wei-Ping Lin,
Tsung-Hsun Yang,
J.-I. Chyi
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ABSTRACT: We demonstrate a GaN-based phosphor-free near-white-light light-emitting-diode (LED) structure that operates in the visible wavelengths and offers broadening and flattening optical bandwidth performance. The incorporation of GaN-based dual wavelengths (blue and green) multiple-quantum-wells with a transverse p-n junction produces a device which can directly generate stable and near visible white-light emissions. The shape of the optical spectra (440-560 nm) are invariable from low to very high levels of bias currents. The problems of nonuniform carrier distribution and bias dependent electroluminescence spectra that occur in traditional phosphor-free white-light or near-white-light LEDs (with vertical p-n junctions) are eliminated by the demonstrated structure
IEEE Photonics Technology Letters 01/2007; · 2.19 Impact Factor
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ABSTRACT: We demonstrate a high-speed GaN-based light-emitting diode at a wavelength of around 500 nm for the application to plastic optical fiber communication. By use of the n-type doping in the GaN barrier layers of the In<sub>x</sub>Ga<sub>1-x</sub>N-GaN-based multiple-quantum-well (MQW), superior performance of modulation-speed (120 versus 40 MHz) and output power to the undoped control under the same bias current has been observed. According to the measured electrical-to-optical bandwidths and extracted RC-limited bandwidths of both devices, the superior speed performance can be attributed to higher electron/hole radiative recombination rate in the n-doped MQW than that of undoped MQW
IEEE Photonics Technology Letters 09/2006; · 2.19 Impact Factor
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ABSTRACT: In this paper, the optical properties of a novel organic, 2,8-di(t-butyl)-5,11-di[4(t-butyl) phenyl]-6,12-diphenylnaphthacene (tetra(t-butyl)rubrene) have been investigated. Our results show that there are two peaks in the photoluminescence (PL) spectra of tetra(t-butyl)rubrene (TBRb) which are also confirmed in the electroluminescence (EL) spectra. Photo-quenching of the PL intensity is observed when the irradiation time increases. It is shown that oxidation is the dominant reason for photo-quenching. The absolute refractive index and absorption coefficient have also been determined and the results correlate well with the PL results. The results show that TBRb can be a good dopant to achieve the Frster energy transfer and to assist light emission. The optical properties of TBRb are similar to those of rubrene; however, the PL of TBRb is much stronger than that of rubrene. Finally, although crystalline organics have been commonly reported by heating the sample, we report crystallization of TBRb at low temperature
Applied Physics A 07/2005; 81(3):517-521. · 1.63 Impact Factor
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ABSTRACT: Indium-tin-oxide (ITO), Au, Ni, and Pt layers were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation. With proper annealing, it was found that we could improve the optical properties of the ITO layers and achieve a maximum transmittance of 98% at 360 nm. GaN-based MSM UV sensors with ITO, Au, Ni, and Pt as contact electrodes were also fabricated. It was found that we could achieve a maximum 0.12 A photocurrent and a photocurrent to dark current contrast higher than five orders of magnitude for the 600°C-annealed ITO/n-GaN MSM UV sensor at a 5-V bias voltage. We also found that the maximum responsivity at 345 nm was 7.2 A/W and 0.9 A/W when the 600°C-annealed ITO/n-GaN MSM UV sensor was biased at 5 V and 0.5 V, respectively. These values were much larger than those observed from other metal/n-GaN MSM UV sensors. However, the existence of photoconductive gain in the 600°C-annealed ITO/n-GaN MSM UV sensor also results in a slower operation speed and a smaller 3-dB bandwidth as compared with the metal/n-GaN MSM UV sensors.
IEEE Sensors Journal 09/2002; · 1.52 Impact Factor
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ABSTRACT: A detailed study on the effects of Si-doping in the GaN barrier
layers of InGaN-GaN multiquantum well (MQW) light-emitting diodes (LEDs)
has been performed. Compared with unintentionally doped samples, X-ray
diffraction results indicate that Si-doping in barrier layers can
improve the crystal and interfacial qualities of the InGaN-GaN MQW LEDs.
It was also found that the forward voltage is 3.5 and 4.52 V, the 20-mA
luminous intensity is 36.1 and 25.1 mcd for LEDs with a Si-doped barrier
and an unintentionally doped barrier, respectively. These results
suggests that one can significantly improve the performance of InGaN-GaN
MQW LEDs by introducing Si doping in the GaN barrier layers
IEEE Journal of Quantum Electronics 06/2002; · 1.88 Impact Factor
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ABSTRACT: Mg-doped GaN epitaxial layers were annealed in pure O/sub 2/ and pure N/sub 2/. It was found that we could achieve a low-resistive p-type GaN by pure O/sub 2/ annealing at a temperature as low as 400/spl deg/C. With a 500/spl deg/C annealing temperature, it was found that the forward voltage and dynamic resistance of the InGaN/GaN light emitting diode (LED) annealed in pure O/sub 2/ were both smaller than those values observed from InGaN/GaN LED annealed in pure N/sub 2/. It was also found that an incomplete activation of Mg will result in a shorter LED lifetime.
IEEE Electron Device Letters 06/2002; · 2.85 Impact Factor
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ABSTRACT: Si and Zn codoped In/sub x/Ga/sub 1-x/N-GaN multiple-quantum-well (MQW) light-emitting diode (LED) structures were grown by metal-organic vapor phase epitaxy (MOVPE). It was found that we can observe a broad long-wavelength donor-acceptor (D-A) pair related emission at 500 nm/spl sim/560 nm. White light can thus be achieved by the combination of such a long-wavelength D-A pair emission with the InGaN bandedge related blue emission. It was also found that the electroluminescence (EL) spectra of such Si and Zn codoped InGaN-GaN MQW LEDs are very similar to those measured from phosphor-converted white LEDs. That is, we can achieve white light emission without the use of phosphor by properly adjusting the indium composition and the concentrations of the codoped Si and Zn atoms in the active well layers and the amount of injection current.
IEEE Photonics Technology Letters 05/2002; · 2.19 Impact Factor
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ABSTRACT: In this work, we have applied the so called charge asymmetric resonance tunneling (CART) structure to nitride-based green light emitting diode (LED). From our CART LED, we observed an abrupt turn-on voltage near 2.2 V, and the forward voltage is around 3.2 V at 20 mA injection current. At 20 mA, the output power, and external quantum efficiency of the CART LED are about 4 mW, and 6.25%, respectively. The high brightness and efficiency green LED can be obtained by using the CART structure.
IEEE Electron Device Letters 04/2002; · 2.85 Impact Factor
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ABSTRACT: Distributed Bragg reflector (DBR) and charge asymmetric resonance
tunneling (CART) structures were applied to nitride-based green
light-emitting diodes (LEDs) to enhance their output efficiency It was
found that we can reduce the forward voltage at 20 mA from 3.7 to 3.2 V
with the inclusion of CART structure. It was also found that the
electroluminescence peak wavelength of the CART LED is less sensitive to
the amount of injection current. The output power and external quantum
efficiency of the CART LED with DBR structure measured at 20 mA can
reach 7.2 mW and 11.25%, respectively
IEEE Journal of Selected Topics in Quantum Electronics 04/2002; · 3.78 Impact Factor
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ABSTRACT: Indium-tin-oxide (ITO) layers were deposited onto n-GaN films and/or glass substrates by electron-beam evaporation. With proper annealing, we found that we could improve the optical properties of the ITO layers and achieve a maximum transmittance of 98% at 360 nm. GaN-based metal-semiconductor-metal (MSM) photodetectors with ITO transparent contacts were also fabricated. A maximum 0.12-A photocurrent with a photocurrent to dark current contrast higher than five orders of magnitude during ultraviolet irradiation were obtained for a photodetector annealed at 600/spl deg/C. We also found that the maximum photo responsivity at 345 nm is 7.2 and 0.9 A/W when the detector is biased at 5 and 0.5 V, respectively.
IEEE Photonics Technology Letters 09/2001; · 2.19 Impact Factor
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ABSTRACT: This letter deals with the low‐frequency noise in an InGaAs(P)/InP double heterojunction bipolar transistor at room temperature. The recombination is mainly responsible for the noise. The current dependence of the base noise with floating collector was of the form I B <sup>3</sup> and the shot noise of base current corresponding to 3.2×10<sup>-24</sup> A<sup>2</sup>/Hz for f=10 Hz. The current dependence of the collector noise with high frequency short circuited was of the form I c <sup>1.55</sup> and the shot noise of collector current corresponding to 3.2×10<sup>-24</sup> A<sup>2</sup>/Hz for f=10 Hz.
Applied Physics Letters 10/1992; · 3.84 Impact Factor
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ABSTRACT: InGaAs(P)/InP double heterojunction bipolar transistors have been successfully fabricated by inserting an n‐type InGaAsP (E g =0.95 eV) quaternary (0.1 μm, undoped) layer on either side of the base by liquid‐phase epitaxy (LPE). As we know, it is the first time to grow this structure by LPE. These devices have been fabricated using a non‐self‐aligned technology. In this case it can improve the common‐emitter current/voltage (I C /V CE ) characteristics. Small signal current gain h fe about 100 and dc current gain h FE about 80 at I C =38 mA can be obtained. The ideality factor of emitter‐base junction is 1.43.
Journal of Applied Physics 08/1990; · 2.17 Impact Factor
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ABSTRACT: The nonlinear absorption of five metal TMPPs (TMPP: tetrakis-(3,4,5-trimethoxyphenyl)porphyrin) doped in boric acid glass are measured with the linear polarized nanosecond laser pulses at different wavelengths by Z-scan; the two-photon absorption (TPA) is dominant in the near infrared, while the characteristic of saturation absorption (SA) is observed close to the Q(0,0) band of porphyrin. The symmetry allowed two-photon π*←π transitions are suggested to be and , with the cross-sections δ ranging from 25×10−50 to . We analyze the property of SA with a four-level system, and find that the magnitudes of excited-state absorptivity and saturation intensity are not affected by changing the central metal ion in these experiments.
Chemical Physics.
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ABSTRACT: High-performance, blue micro-size InGaN light emitting diodes (LEDs) with diameters of 3 to 20 m have been fabricated. An ion implantation technique and a 12 micron electro-ridge were used to simplify fabrication processes. The 3 to 20µm LEDs that exhibited a large emission photon blue shift (87.5meV ~52.9meV) were observed in electro-luminescence (EL) spectra. Under an increased injection current, the quantum wells become populated with charge carriers, which screened the internal piezoelectric field and caused the energy blue shift of EL eventually. A high injection current caused a high junction temperature that narrowed the band gap (red shift). The size dependent energy shift is largely owing to the competition between the blue and the red shifts. At a bias voltage of 8.96V (which is 140% of the turn on voltage, 6.4V), the 10 µm device exhibited an injection current of 7.9mA. This value exceeds that in literature, i.e., 4mA at a bias voltage of 14V (which is 140% of the turn on voltage, 10V). This phenomenon may be owing to that the ion implantation and electro-ridge designs herein involved a lower series resistance. The external quantum efficiencies (E.Q.E.) of the micro-size LEDs herein were all 0.4%~3.3%, which is better than the values reported in literature, which were ranged between 0.004% and 1.29% for an individual LED and an array LED, respectively. The E.Q.E. of the 15µm device at maximum injection current had the optimum value yet obtained for micro-size LEDs. The dependence of the blue shift and the E.Q.E. on the size warrants further study.
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ABSTRACT: The characterization of fluence-dependent nonlinear absorption and the excited state absorption cross-sections of the zinc complexes of tetrabenzoporphyrin with meso-substituents of phenyl and thienyl (Ia and Ib), and the zinc and lead complexes of porphyrin with meso-substituent of 3,4,5-trimethoxyphenyl (IIa and IIb) are determined with the Z-scan method. The results from Ia and Ib are comparable with the literature report. The lead porphyrin of IIb gives the largest triplet excited state absorption cross-section among these compounds, and shows very strong optical limiting power as a result. Whereas we obtain the smallest σexT=2.5×10−17 cm2 for the zinc porphyrin bearing four electron withdrawing groups of benzoquinone (compound III), which exhibits unique linear absorption property at 532 nm. This result could be due to the rapid decay of its lowest triplet state to yield both long-lived triplet biradicals and S0 states.
Chemical Physics. 262:467-473.
