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Publications (5)6.89 Total impact

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    ABSTRACT: In this letter, the mechanism for improvement of the dynamic performance of GaN-based light-emitting diodes with an InGaN insertion layer is investigated using the very fast electrical-optical pump-probe technique. Our measurements indicate that, when the bias current is relatively low (100 A/cm<sup>2</sup>), the device with the InGaN insertion layer (device A) exhibits a shorter response time than does the control (device B) without such a layer. However, when the bias current density reaches 0.5 kA/cm<sup>2</sup>, devices A and B exhibit exactly the same response time during operation from room temperature to 200 <sup>°</sup>C. These results indicate that, under low current density (100 A/cm<sup>2</sup>), the piezoelectric (PZ) field inside device A will be stronger, which should result in a lower effective barrier height with a shorter carrier escape time than is the case for device B. On the other hand, under high bias current density, both devices have the same internal response time, which indicates the screening of the PZ field inside due to injected carriers. These dynamic measurement results suggest that the origin of the efficiency droop in our device under low and high bias current densities is carrier leakage and the Auger effect, respectively.
    IEEE Electron Device Letters 06/2011; · 2.79 Impact Factor
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    ABSTRACT: For the first time, the internal carrier dynamic inside GaN-based green light-emitting diodes (LEDs) during operation has been directly observed using the demonstrated electrical-optical pump-probe technique. Short electrical pulses (~100 ps) were pumped into high-speed cascade green LEDs, and the output optical pulses were probed using high-speed photoreceiver circuits. Using such a method, the recombination time constant of the carriers can be directly measured without any assumption about the recombination process. A high-speed cascade LED structure was adopted in the experiments to eliminate the influence of the RC delay time on the measured responses. Our measurement results indicate that both single- and three-LED cascade structures have the same internal response time due to current continuity. Furthermore, based on responses measured under different temperatures (from 25°C to 200°C), the origin of the efficiency droop in GaN-based green LEDs under a high bias current density may be attributed to the strong nonradiative Auger effect rather than device heating or carrier overflow. The demonstrated measurement scheme and high-speed cascade device structure offer a novel and simple way to straightforwardly investigate the internal carrier dynamic inside the active layers of the LED during forward-bias operation.
    IEEE Transactions on Electron Devices 03/2011; · 2.06 Impact Factor
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    ABSTRACT: We demonstrate a novel type of linear cascade green light-emitting diode (LED) arrays as a light source for in-car or harsh environment plastic optical fiber (POF) communications. To further enhance its dynamic and static performance, an InGaN layer is inserted between an n-type GaN cladding layer and InGaN-GaN multiple quantum wells as an efficient current spreading layer. Compared with the control device without that layer, our three-LED cascade array demonstrates a smaller turn-on voltage (9.3 versus 11 V at 20 mA) and a larger output power (25.5 versus 22.5 mW at 180 mA), corresponding to an enhancement of around 31% in wall-plug efficiency. Furthermore, under the constant voltage bias of an in-car battery (12 V), our three-LED array exhibits an electrical-to-optical 3-dB bandwidth (100 versus 40 MHz) performance superior to that of the control device. Even under high-temperature dynamic operation, we observe that the InGaN insertion layer gives strong enhancement of modulation speed with negligible degradation of the output power, unlike the red resonant-cavity LEDs conventionally used for POF. We achieve 200-Mb/s error-free transmission at 200°C which is the highest operation temperature among all the reported high-speed LEDs.
    IEEE Photonics Technology Letters 08/2010; · 2.04 Impact Factor
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    ABSTRACT: We demonstrate cascade green light-emitting-diodes, which greatly release trade-off between output-power and speed and exhibits strong modulation-speed enhancement with negligible output-power degradation from room-temperature to 200°C operation. 200 Mbit/sec error-free transmission at 200°C can be achieved.
    Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC); 04/2010
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    ABSTRACT: In this research, we demonstrate array of transverse-junction (TJ) blue light-emitting-diodes (LEDs), which are specified as a horizontal carrier flow instead of side-by-side injection, with a consequence of InxGa1-xN/GaN multiple-quantumwells (MQWs) as the active region. The demonstrated devices were carried out by the re-growth of n-type GaN on the sidewall of p-type GaN. Regarding the transverse carrier flow of injected carriers, these TJ-LEDs, as compared to the control related to traditional vertical junction structure, can effectively spread injected currents more uniformly, minimize the problem of nonuniform carrier-distribution and current crowding effect, and achieve 35% improvement of power performance.
    Proc SPIE 02/2009;