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.

Ultrafast spectroscopy of wide bandgap semiconductor nanostructures

Mehran Shahmohammadi

Group III-nitrides have been considered a promising choice for the realization of optoelectronic devices since 1970. Since the first demonstration of the high-brightness blue light-emitting diodes (LEDs) by Shuji Nakamura and coworkers, the fabrication of highly efficient white LEDs has passed successful developments. A serious physical issue still remained, which prevents their use for high power and highly efficient LEDs: the drop of external quantum efficiency (EQE) of III-nitride LEDs when increasing the driving current, the so-called ''efficiency droop'' problem. In order to have a fast expansion to the lighting market, the cost-per-lumen of packaged LEDs must rapidly decrease. This indeed demands for having LED chips operating with high EQE under the high current operation. Besides the industrial interest of III-nitrides, owing to their large direct bandgap, they feature some interesting optical properties such as large exciton binding energy and large oscillator strength of excitons. However, when the carrier density raised in a semiconductor, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma, that is called the Mott transition. This crossover can drastically affect the optical and electrical characteristics of semiconductors and may, for instance, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. More interestingly, even if biexcitons are frequently seen to dominate the emission of III-nitride heterostructures when the density is raised, no clear experimental report is available on the role of biexcitons in the Mott transition in a two-dimensional (2D) nanostructure. In the first part, the emission properties of high-quality GaN/AlGaN single quantum wells (QWs) at high-carrier densities are examined. They are of crucial importance to provide a deeper insight into the operating conditions of III-nitride based lasers and LEDs, as well as the transition from strong to weak coupling regime of exciton-polaritons in semiconductor microcavities. Employing the same technique then to investigate some InGaN/GaN QWs, the droop signature was investigated comprehensively in both polar and non-polar QWs. Having an accurate estimation of the carrier density in the QWs, the contribution of several non-radiative processes were quantitatively examined. These experiments can indeed provide a deeper insight on the physical phenomena responsible for the efficiency droop in III-nitride LEDs, and can stimulate several theoretical and experimental on this subject. The second part focuses on the transport mechanisms of excitons in ZnO and III-nitride based nano-structures. The transverse movement of donor-bound excitons in a purely bent ZnO microwire as a function of temperature have been investigated, owing to the high spatial, spectral and temporal resolutions of our original time resolved cathodoluminescence system. The movement mechanism was modeled by a hopping process of excitons. Our results pave the way to new experiments allowing to reveal the physics at play at the nanoscale in different materials. However, in case of highly disordered systems, one should take into account more complex considerations, as in the case of InGaN core-shell QW structures studied in the last chapter, the large energy fluctuations prevent excitons to move along the energy gradient at low temperatures.

EPFL2015

Dynamics of quantum dot lasers

Pablo Moreno

The exceptional performance of self-assembled Quantum Dot (QD) materials renders them extremely appealing for their use as optical communications devices. As lasers, they feature reduced and temperature independent threshold current and proper emission wavelength at the fiber telecommunication windows. These characteristics, together with the low linewidth enhancement factor and broad spectrum, make QD materials extremely attractive for application as light emitters or amplifiers. There exist, nevertheless, several unclear issues which prevent QDs from conquering the new generation of optoelectronic devices. Their differential efficiency is lower than expected. The output power of QD lasers is lower than that of their quantum well counterpart. Still, it is their dynamics which has incited the majority of studies. The modulation bandwidth of these devices seems to be limited by the relaxation of carriers from the upper energetic layers to the low levels within the dot. Besides, the electron-hole interaction is widely unknown, the extent of the electron-hole Coulombic attraction is not yet established. Throughout this thesis I present a theoretical and experimental study of the gain and phase dynamics of quantum dot lasers. I explain the appearance of different decay times observed in pump and probe experiments in QD amplifiers as a result of the different electron and hole relaxation times, by means of an electron-hole rate-equation model. The ultrafast hole relaxation first leads to an ultrafast recovery of the gain, which is then followed by electron relaxation and, on the nanosecond timescale, radiative and non-radiative recombinations. The phase dynamics is slower and is affected by thermal redistribution of carriers within the dot. Our results corroborate with spectral measurements of the dephasing and gain in QD amplifiers. Additionally, our work is compared with existing pump and probe results. Exploiting the capacity of QD lasers to emit at two different wavelengths corresponding to the ground state (GS) and excited state (ES), I present a theoretical study of the QD dynamics, based on a linearization of the QD rate-equations. The results predict the existence of single oscillation frequency of GS and ES, meaning that both states are highly coupled. In order to verify our theory, we perform two kinds of experiments. By modulating these lasers at high frequency, we measure separately the dynamics of GS and ES. However, in contradiction to our theory, two different modulation frequencies are found. Additional temporally-resolved measurements of the laser dynamics reveal a surprising effect. By injecting a sub-bandgap pump in an InAs/InGaAs QD laser, the emitted photons are depleted. Through additional transmission and photocurrent measurements, we relate this observation with carrier photoexcitation, which was so far only theoretically addressed. The role of carrier photoexcitation in our experimental laser dynamics is further supported by a rate-equation model. Impelled by this finding, we study the effect of carrier photoexcitation in the static and dynamic characteristics. We find that carrier photoexcitation reduces the efficiency of QD lasers, which is one of the major QD handicaps, and depletes the GS lasing after the ES threshold, as observed experimentally. Moreover, by adding carrier photoexcitation to our linearization of the rate-equations, we find that the theory predicts the appearance of two lasing resonance frequencies, in agreement with our previous experimental results. Additionally, we deal with the improvement of carrier relaxation. In tunnel injection devices, carriers are given an additional path towards the ground state of the dot by growing a quantum well layer close to the QD active plane. Through the quantum-mechanical tunneling effect, carriers relax from the nearby quantum well layer to the QDs, which speeds up relaxation. We aim at the increase of the modulation bandwidth while keeping the good performances quantum dot lasers have exhibited, such as low and temperature insensitive threshold current and proper emission wavelength. In the final part of this work, we present dynamical measurements of 1.5 µm InAs/InP tunnel injection and non-tunnel injection QD lasers, which display remarkable static characteristics. After proving with static measurements that tunnel injection is actually taking place in these structures, we show several dynamic measurements. Pump and probe measurements on QD devices show that the tunnel injection samples exhibit a slightly faster relaxation time than the non-tunnel injection samples used as reference, meaning that relaxation time is improved with tunnel injection. However, by probing the device with an ultrafast pump no improvement of the dynamic characteristics is observed. These results confirm that the laser dynamic properties of InP QD lasers, both standard and tunnel-injection designs, are actually not limited by relaxation of carriers. We point towards the size distribution of these quantum dash-like structures as the limiting factor of the modulation frequency.