A Study On Al_xGa_1-xN/GaN Distributed Bragg Reflectors And Active-Passive Coupled-Double-Microring Silicon Resonators

This disssertation presents the designs, simulations, and analysis on two novel semiconductor device structures: the AlxGa1-xN/GaN tuned distributed Bragg reflector (TDBR) used in GaN based light emmiting diodes (LEDs) and the active-passive coupled-double-microring silicon resonator, both of which are based on the integration of III-V compound semiconductors and silicon.To enhance the omnidirectional reflectance of conventional distributed Bragg reflectors (DBRs) and hence improve their ability of increasing the light extraction efficiency of GaN-based LEDs with silicon substrates, we proposed a novel scheme of TDBRs in which the thickness of every layer is determined by optimization with the maximal omnidirectional reflectance as the optimization aim. The layer thicknesses of a 20.5-pair AlxGa1-xN/GaN TDBR were calculated by the hybrid simulated-annealing genetic algorithm. The calculated results show that the omnidirectional reflectance of the TDBR is much higher than that of the DBR with the same layer number, that the higher Al mole-fraction in AlxGa1-xN alloy is, the greater the potential of the TDBR for improving omnidirectional reflectance, and that the maximal omnidirectional reflectance of the TDBR is tuned to the central wavelength from which the DBR deviates. Conceivably, the application of TDBRs in III-nitride LEDs with Si substrates can effectively reduce optical absorption and hence increase extraction efficiency.The losses and resonant-wavelength deviation are two serious problems counteracting the improvement of microring-resonator performance. This dissertation presents the fundamental properties of microring resonators by the transport equations and finite difference method in time domain (FDTD). The calculated results show that the mutually independent loss/gain and phase tuning can effectively overcome the property determination of microrings caused by the fabrication errors. We proposed a novel scheme of an active-passive coupled-double-microring silicon resonator which consists of a III-V active microring and a silicon-on-insulator (SOI)microring vertically coupled together. The coupling reflective coefficient between the two rings is supposed to approach 1. So they can be considered as one resonator in function. By two different electrical control signals on the two microrings respectively, the loss and the phase can be adjusted respectively and independently. The properties of the coupled-double-ring resonator were investigated. An example of optical switching is presented to illustrate the tuning function of an active-passive coupled-double-microring silicon resonator. The analysis shows that for an active-passive coupled silicon microring resonator, both the exact resonant-wavelength calibration and nice spectral property can be achieved by 1mA-scale and 1μA-scale electrical control signals on the active and the SOI microring, respectively.