| Summary: | 博士 === 國立臺灣大學 === 光電工程學研究所 === 98 === This dissertation studies the carrier dynamics in semiconductor quantum dots by means of numerical simulations and various experimental techniques. Quantum dots exhibit a structure of three dimensional confinements for carriers. This kind of confinement results in the unique density of states and some improved optical properties. Hence, semiconductor devices based on quantum dots have many superior performances and are suitable for using in optical communication.
In the first part of this dissertation, we study the two-state lasing phenomenon in quantum-dot lasers experimentally as well as theoretically. The onset of excited-state lasing prior to ground-state lasing was observed in dynamics measurements under electrical excitation. The explanation for this phenomenon is due to the finite states in quantum-dot lasers. When the optical loss level is close to ground-state saturation gain, the carrier capture time into ground states becomes longer and results in the establishment of excited-state population. We successfully explain the origin of the unique dynamics phenomenon in two-state lasing quantum-dot lasers.
In semiconductors, carrier lifetime is an important parameter determining the device performances. We build up a novel time-resolved system by using a degenerate pump-probe photoluminescence technique, to measure carrier lifetimes of different quantum-dot samples. We demonstrate that this technique can be used in the infrared region with wavelength longer than 1.1 um. In this wavelength region, it is difficult to find high-speed and high sensitivity photon counting devices. We also study the p-doping impact on carrier lifetime in vertically coupled and uncoupled quantum dots by unsing this degenerate pump-probe photoluminescence technique. From the temperature dependent carrier lifetime measurement, we can clearly demonstrate (i) the dominant nonradiative mechanism induced by the extra p-type dopants and (ii) the smaller oscillator strength in the vertically coupled quantum dots.
Finally, we study carrier capture times of vertically coupled and p-doped quantum dots by using a home-made time-resolved up-conversion system. The time resolution of this measurement is 280 fs. Faster capture times in the vertically coupled quantum-dot samples (6.5 ps to 6.7 ps) are observed. It may be a consequence of the enhanced Auger-assisted relaxation mechanism due to high carrier density caused by the thinner spacer layers. However, no capture time difference (8.0 ps) has been observed between the p-doped and undoped quantum-dot samples. Further studies are still needed to reveal the reason for this phenomenon.
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