Charge control and energy level engineering in quantum-dot laser active regions
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The research presented in this dissertation focuses on the improvement of the operating characteristics of GaAs-based InAs and InGaAs quantum dot lasers. The close spacing of discrete hole levels in quantum dots is recognized as one of the main limiting factors in laser performance. The close spacing of levels results in thermal broadening of the hole population, which lowers the optical gain and reduces the differential gain. Two methods are used to enhance laser operation. First, the carrier energy level separation is increased by engineering quantum dot shape and composition. The wide separation of the energy states reduces thermal broadening of carrier populations and is experimentally shown to reduce temperature sensitivity of laser threshold. Second, modulation p-doping of the active region is introduced to compensate for the thermal excitation of holes into higher energy levels. A quasiequilibrium model that includes multiple discrete energy levels and the energy levels of the wetting layer is used to show that active region gain can be controlled by charge carriers built into the QD discrete levels. The results of the calculations demonstrate that with moderate amounts of p-doping used to introduce excess holes in the quantum dots, the active region gain, differential gain, and consequently laser frequency response and To can be dramatically improved. The experimental evidence of the enhancement of gain and temperature performance of p-doped QD lasers is presented. Low-threshold p-doped lasers with high power output and characteristic temperature as high as 213K in CW and 232K in pulsed operation are demonstrated.