| dc.description.abstract | Terahertz (THz) quantum cascade laser (QCL) is the most promising THz light source for the next-generation THz technologies of imaging, spectroscopy, and wireless communication. However, room-temperature operation of THz QCLs, which is necessary for these applications, has yet been demonstrated, and the record value of the highest operation temperature of THz QCLs remains 250K. Thus, this issue has been tackled theoretically and experimentally by many researchers and institutions around the world nowadays. In this thesis, three theoretical research topics related to device design guideline for high-performance devices are presented. (1) development of fast and accurate device simulator, (2) investigation of the influence of impurity doping on device performance, and (3) investigation of the influence of barrier height on device performance.
First, a fast and accurate device simulator is developed based on a rate equation model introduced by Razavipour. This development implements speed-up of the calculation of electron-electron scattering rate, careful selection of a screening effect model, introduction of theoretical models of pure dephasing time, optical linewidth, and a leakage current. In calculation, the developed device simulator can reproduce the device characteristics of previously published devices accurately, and the simulated highest operation temperature of previously published devices are reasonable compared to the experimental values.
Second, the influence of ionized impurities on device performance is investigated based on a two-well resonant-phonon THz QCL designed by Khalatpour et al. Seven doping conditions determined by doping positions and distributions and an undoped condition are simulated over a range of sheet doping density from 1.0×1010 to 1.0×1012cm-2 for a single module. By this simulation, optical linewidth is found to be small in the undoped condition due to absence of ionized-impurity scattering, resulting in the highest optical gain among all doping patterns. Among doped conditions, wide doping in a phonon-well shows the highest optical gain, and this is attributed to mitigation of band-bending effect. Based on these results, a modulation doping scheme is proposed.
Lastly, the influence of barrier height on device performance of two-well resonant-phonon THz QCLs is investigated through two steps of research, and a series of comprehensive research discovers a new device design concept featured by a thick radiation barrier which is around 40Å (Type-B). Furthermore, around 15% of Al-composition is found to provide the optimal barrier height conditions for two-well resonant-phonon structures under both the conventional design (Type-A) and Type-B concepts in device structure exploration. The reason for this feature is clarified by careful analysis of calculation data, and the optimal conditions are found to happen by complex behavior of componential parameters of optical gain, which varys with barrier height. | en |
