Investigation of the Carrier Dynamic in GaN-Based Cascade Green Light-Emitting Diodes Using the Very Fast Electrical–Optical Pump–Probe Technique

Dept. of Electr. Eng., Nat. Central Univ., Jhongli, Taiwan
IEEE Transactions on Electron Devices (Impact Factor: 2.47). 03/2011; 58(2):495 - 500. DOI: 10.1109/TED.2010.2093529
Source: IEEE Xplore


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.

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    • "When the bias current exceeds a certain value (9 10 mA for our case), the PZ field is completely screened, and the effective barrier height in the MQW region thus increases [24], [25]. This should be accompanied by a lower probability of carrier-escape and less output-power degradation under high-temperature operation [24], [25]. Figs. "
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    ABSTRACT: We demonstrate the performance of a novel cyan light-emitting diode (LED) on a patterned sapphire (PS) substrate as a light source for plastic optical fiber (POF) communications with the central wavelength at 500 nm. To further enhance the external quantum efficiency (EQE) and output power of this miniaturized high-speed LED, a LED with a PS substrate is adopted. Furthermore, by greatly reducing the number of active $hbox{In}_{rm x}hbox{Ga}_{1 - {rm x}}hbox{N/GaN}$ multiple quantum wells (MQWs) to four and minimizing the device active area, we can achieve a record-high electrical-to-optical (E–O) bandwidth (as high as 400 MHz) among all the reported high-speed visible LEDs under a very small dc bias current (40 mA). The fiber coupling efficiency has been improved in 4 dB using lens with a 500-$mu hbox{m}$ diameter mounted on the LED chip. Thus, the maximum fiber-coupled power was $-$2.67 dBm at the bias current of 40 mA. The 1.07-Gb/s data transmissions over a 50-m SI-POF fiber have been successfully demonstrated using this device at the bias current of 40 mA.
    Full-text · Article · Oct 2012 · IEEE Photonics Journal
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    • ". A conceptual diagram of the E–O pump-probe measurement setup was given in Fig. 3 [11], [12]. During measurement, the device under testing (DUT) was mounted on a hot plate for temperaturedependent measurement then injected with different dc bias currents. "
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    ABSTRACT: Distinct temperature-dependent dynamic behaviors of GaN-based blue light-emitting diodes (LEDs) are observed by use of the very-fast electrical-optical pump-probe technique. Our static and impulse response measurement results indicate that the behaviors of internal carrier dynamics under different ambient temperatures can be classified into three regimes covering a wide range of bias current densities (20-2000 A/cm2). The first regime is when the bias current density ranges from low to moderate (20-100 A/cm2). The measured external quantum efficiency (EQE) degrades dramatically from 57 to 44%, and the measured waveform and extracted time constants of measured impulse responses are invariable from room temperature (RT) to 200 °C, which indicates that the carrier leakage is not an issue for the observed droop phenomenon. When the bias current density further increases to near 1 kA/cm2, the droop phenomenon are mitigated (44 to 24%). However, a significant shortening of the measured impulse response happens under 200 °C operation due to the device-heating effect. This phenomenon is diminished when the bias current densities are further increased to over 1 kA/cm2, due to the screening of the piezoelectric field. The extracted time constants can also be used to explain the droop phenomenon in GaN LED under high bias currents.
    Full-text · Article · Oct 2012 · IEEE Photonics Journal
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    ABSTRACT: Electrical-optical pump-probe is used to investigate GaN blue LEDs under different temperatures. Measurement result indicates that under moderate current density (∼200A/cm2) piezoelectric field induced carrier-escaping cannot be neglected and is responsible for the observed efficiency-droop.
    No preview · Article · Jan 2011
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