OSE Dissertation Defense by Arman Rashidi on Carrier Dynamics in High-Speed III-nitride Light-Emitting Diodes for Visible-Light Communication
Posted: October 27, 2018
Date: Friday, November 02, 2018
Time: 2:00 PM
Location: CHTM, Room 103
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III-nitride light-emitting diodes (LEDs) are now ubiquitous in solid-state lighting (SSL) systems. Despite the significant advancement of III-nitride LEDs, the origins of fundamental challenges such as efficiency droop, thermal droop, and green gap are not completely understood. In addition, emerging applications such as micro-pixel LED displays and visible-light communication (VLC) require efficient LEDs capable of high-speed modulation. Studies of carrier dynamics are essential to better understand the fundamental efficiency challenges and enable the design of high-efficiency, high-speed LEDs. Among approaches to characterize the carrier dynamics in LEDs, electrically injected methods are preferred over optically pumped methods to capture the carrier dynamics under typical operating conditions.
In this dissertation, a comprehensive method is developed to study carrier dynamics in electrically injected LEDs using small-signal RF measurements and differential rate equation analysis. This method represents an entirely new approach to characterizing the classic DC properties of III-nitride LEDs and enables simultaneous determination of the dynamic properties. This method enabled the determination of the injection efficiency, carrier density in the active region, modulation bandwidth and differential carrier lifetime, carrier escape lifetime, radiative and non-radiative recombination rates, and RC time constant in LEDs. Understanding the above properties under electrical injection provides valuable information about the origins of fundamental efficiency challenges and aids in the co-optimization of modulation bandwidth and efficiency. For instance, study of different lifetimes enabled the design of GHz-bandwidth LEDs operating at low current densities for VLC applications. The impact of crystal orientation on the modulation bandwidth was studied and the trade-off between bandwidth and efficiency was investigated for GaN micro-LEDs. The described method was used to experimentally determine the injection efficiency in III-nitride LEDs and exclude injection efficiency as a potential origin of efficiency droop. Moreover, the origins of efficiency and thermal droop in III-nitride LEDs were investigated by analyzing the carrier density and radiative efficiency as a function of current density and temperature under isothermal conditions. The isothermal condition was enabled by a pulsed-RF method, which was developed for the first time. This approach is expected to accelerate the development of LEDs tailored for micro-pixel displays and VLC.