| Summary: | 碩士 === 國立臺灣大學 === 電子工程學研究所 === 93 === In this thesis, we report our studies on the effects of quantum-dot (QD) stack number, coupled QD structure, and the lattice mismatch of InGaP cladding layers on the performances of QD lasers. In the portion of QD stack number, it is found that though the lasers with 10 QD stacks have the highest saturation gain, the accumulated strain is too large to be accommodated by the lattice, which leads very large threshold current density in laser performances. A 3-mm-long as-cleaved device with 5 QD stacks lases at 1293 nm and shows the best performances. In terms of the transparency current density per QD layer, the device demonstrates a value of only 6 A/cm2 which is within the range of the calculated results and justifies the good material quality in our QD lasers. Lasers with 3 QD stacks, however, have lower gain and therefore worse performances than the lasers with 5 QD stacks.
In the study of InAs coupled-QD structure, we found that the coupled QDs have lower density than the normal QDs. However, the lower density does not significantly deteriorate the laser performances. A 2-mm-long laser with coupled-QD demonstrates a very low threshold current density of 133A/cm2, which is just slightly lower than the laser with normal QD. In addition, the coupled-QD laser also demonstrates longer emission wavelength than its controlled device.
In the final research subject, we investigate the effect of the lattice mismatch in the InGaP cladding layers on the laser performances. Samples with lattice-matched, compressive strained and tensile strained InGaP layers were grown and compared. We found that the lattice-mismatch InGaP layers will result in wavy strip structure or even hatches. QDs deposited on top of these layers will be separated into two groups. Though the lasers with lattice-mismatch cladding layers still have low threshold current density, the gain is too small to support ground state lasing.
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