| Summary: | 博士 === 國立交通大學 === 電子工程系所 === 96 === In this dissertation, the InGaN/GaN laser diode is theoretically studied. We have optimized its active region and the cladding layer composed of a p-type AlGaN/GaN superlattice by studying the spillover effect, the influence of dopants, and the key factor making the vertical resistance of the p-type superlattice large.
The effects of electron spillover from quantum wells on the optical property of InGaN/GaN laser diodes are theoretically studied in detail. Six-band model including strain effect is used to calculate valence band states. Continuous subbands are simulated deliberately by dense discretized subbands for the spillover electrons. The calculation results show obvious differences in the radiative current densities and the gain spectra between the cases with and without considering the spillover effect. We further investigate the spillover effect on the radiative current densities and the spontaneous emission spectra, with variations in the depth and the width of quantum wells, the total loss of the cavity, and the temperature. For shallow wells, the spillover effect is particularly important. It broadens both the gain and the spontaneous emission spectra and hence deteriorates the threshold of laser diodes. Such an effect can be alleviated by employing lasers with a long cavity and a multi-quantum-well active region. The concepts of the electron spillover studied in this work are not only applicable to the nitride lasers, but also to other kinds of quantum-well lasers.
The influences of the modulation-doping in InGaN/GaN laser diodes are also theoretically studied with the effects of electron spillover from quantum wells considered. The calculation results show that the threshold current can be significantly reduced by p-type modulation-doping around the wells but not by n-type doping, supposed that the layers are of a perfect quality and the impurity-induced defects are ignored. Also, the p-type modulation doping can make the threshold current more insensitive to the cavity loss compared with other cases. An optimized threshold current density can be achieved for single-quantum-well lasers by introducing p-type dopants. However, the dopant concentration is high and may be inaccessible. For double-quantum-well lasers an optimized low threshold current can be achieved with a slighter and practicable p-type doping level.
We also study the vertical transport of holes through p-type AlGaN/GaN superlattices with both Ga- and N-face polarities by drift-diffusion, tunneling, and thermionic emission models to find the key factors that dominantly influence the average vertical resistivity at different temperatures. It is shown that although the acceptors in the barriers are easily ionized to give a high spatially averaged density of holes, the barriers themselves are the main obstacle to the transport of holes through the superlattices. In our calculation results, the number of barriers in the superlattices dominantly affects the average vertical resistivity if the barriers are thin enough. So the resistivity can be reduced by decreasing the barrier number for a fixed total length of superlattices. Our results show that about 50% reduction in the resistivity can be excepted when the structure varies from Al0.11Ga0.8N(2 nm)/GaN(2 nm) to Al0.11Ga0.8N(6 nm)/GaN(2 nm).
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