Ming-Lun Lee

Publications (2)6.16 Total impact

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

  Jin-Wei Shi, H.-W. Huang, F.-M. Kuo, W.-C. Lai, Ming-Lun Lee, Jinn-Kong Sheu
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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; · 2.32 Impact Factor
• Source

  Available from: Jinn-Kong Sheu

  Article: Nonalloyed Cr/Au-based Ohmic contacts to n-GaN

  Ming-Lun Lee, Jinn-Kong Sheu, C. C. Hu
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  ABSTRACT: Nonalloyed Cr/Au-based metal contacts to n-GaN have been demonstrated. The deposited Au/Cr/n-GaN contacts exhibited a specific contact resistance (ρc) of approximately 5.6×10−5 Ω cm2. Although the nonalloyed Ti/Al-based contacts to n-GaN can also exhibit a comparable ρc value, their thermal stability is inferior to the Cr/Au-based contacts. This could be attributed to the fact that Al tends to ball up during thermal annealing. Thus, the surface morphology of most of the annealed Ti/Al-based contacts was quite rough, and the contacts became rectified when they were annealed at a temperature below 700 °C. However, the annealed Cr/Au-based contacts exhibited an Ohmic characteristic and had a smooth surface when annealing temperatures did not exceed 700 °C. In addition, the thermal stability could be further improved by inserting a Pt layer between the Cr and Au layers. This scheme could prevent the diffusion of Au into the Cr layer, thus preventing Au from reaching the Cr/GaN interface where it could form a possible Ga–Au phase, which would degrade the Ohmic contacts.
  Applied Physics Letters 10/2007; 91(18):182106-182106-3. · 3.84 Impact Factor