Ultrafast carrier dynamics in III-nitride nanostructures and LED quantum efficiency

Since the seminal report about the first Candela-€class-brightness InGaN blue light-€emitting diodes (LEDs) by Shuji Nakamura et al. in 1994, III-nitride semiconductors have been one of the most important platforms for optoelectronic devices. The achievement in III-nitride LEDs has been awarded by the physics Nobel Prize in 2014. Despite the success of blue LED technology, the quantum efficiency is still limited as LEDs towards high power, green-red colors, and micrometer dimensions. The interaction of carrier recombination dynamics with alloy disorder, dislocations and point defects has not yet been fully understood, which is essential for tackling these issues. Through the use of nanoscopic and ultrafast spectroscopy techniques, the work of this thesis is dedicated to studying carrier recombination dynamics and its relation to the quantum efficiency of III-nitride LEDs, as summarized as follows:

(1) We directly probe exciton dynamics around an isolated single dislocation, by using time-resolved cathodoluminescence (TR-CL). The exciton diffusion length and effective area of the dislocation are deduced by modeling the CL decays across the dislocation, which reveals the intrinsic properties of dislocations as NRCs. The type of dislocation is confirmed by observing the energy shift of the exciton induced by local strain fields. Meanwhile, we demonstrate that the key role of the underlayer (UL) is to trap non-radiative point defects. Low-kV CL measurements provide strong evidences of annihilation of carriers by point defects at the nanometer scale and the change of diffusion length under different defect densities.

(2) We establish a comprehensive model to describe the carrier dynamics in m-plane InGaN/GaN QWs taking into account the presence of excitons, residual doping, and phase-space filling. Based on picosecond time-resolved photoluminescence (TR-PL) and modeling, we estimate the amount of excitons in QWs at room temperature, and explain the interplay between excitonic population and electron-hole (e-h) plasma. Beyond the e-h plasma scenario, our study provides insight into the physical origins of radiative recombination in LEDs.

(3) By using high injection TR-PL at cryogenic temperatures, we demonstrate evidence that carrier localization in the InGaN/GaN QWs plays a crucial role in intensifying the Auger process, which is linked to the relaxation of momentum conservation by alloy disorder. Furthermore, we study the impact of different densities of point defects on efficiency droop in InGaN/GaN QWs at 300 K. This study suggests an extra efficiency loss, which is likely related to a defect-assisted Auger process. We derive a linear relation between Shockley-Read-Hall (SRH) and defect-assisted Auger recombination, which implies SRH-type defects can be the scattering centers for Auger recombination.

(4) We study highly efficient single InGaN/GaN core-shell microwires, in order to evaluate their potential for micro-/nano- LEDs and mitigating the efficiency droop. We decompose radiative and non-radiative lifetimes of carriers in the sidewall m-plane QW along the wire by spatial mapping of TR-CL from 4 to 300 K, which provides nanoscopic insight on the carrier dynamics in the presence of growth induced gradient of In content. By using high injection TR-PL at low temperature, we observe significant variation of Auger recombination in the active layer, which can be attributed to a different degree of alloy disorder.