Carrier dynamics in quantum dot and GaAs-based quantum dot cascade laser
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Self-organized quantum dots provide unique atomic-like density of states and have important applications in semiconductor lasers. Energy relaxation of charge carriers in quantum dots is important for understanding the physics of devices fabricated from these artificially structured materials. Because the charge carriers relax through discrete energy levels, quantum dots provide a means to study the charge carrier interactions in the semiconductor crystal to unprecedented detail. The physics of the charge carrier relaxation is also expected to be substantially modified from that of the bulk semiconductor because of the modification of the electronic density of states. In this dissertation, we will present our work on the simulation of carrier dynamics in the quantum dot and the application of quantum dots in quantum cascade laser. Based on time-resolved InGaAs quantum dot PL measurement, a fourelectron-level quantum dot energy structure model is built up and rate equations are used to simulate the carrier distribution and relaxation in the quantum dot ensemble. By comparing simulated PL intensity and risetime vs. excitation level curves with the experiment results, we conclude that when carriers are excited into the quantum dots wetting layer, the random carrier capture process is dominated by exciton capture. And the spin blocking effect must be considered to explain relative strong first excited state emission under very low excitation level. Quantum cascade laser using quantum dot as its active region is proposed and studied experimentally. Possible advantages of quantum dot cascade laser over quantum well cascade lasers include lower threshold, high efficiency, vertical cavity surface emitting, etc. InAlAs quantum dot with ground state emission close or shorter than GaAs band edge emission has been developed and used as the active region of GaAs based quantum cascade lasers where lattice matched GaAs/AlGaAs superlattice is used for the carrier selective tunneling. Double plasmon-enhanced and Al-free waveguide is designed to form a thin, low loss and high confining waveguide for the mid-infrared emission. Crystal growth, device processing and characterization at cryogenic temperature have been performed.