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.
    IEEE Photonics Journal 10/2012; 4(5):1520-1529. DOI:10.1109/JPHOT.2012.2210867 · 2.21 Impact Factor
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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.
    IEEE Photonics Journal 10/2012; 4(5):1870-1880. DOI:10.1109/JPHOT.2012.2217947 · 2.21 Impact Factor
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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.
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