| Summary: | 博士 === 國立交通大學 === 電子研究所 === 99 === In this dissertation, we study two types of semiconductor terahertz source. 1. The group-III acceptors in Ge under uniaxial stress. It can radiate terahertz electromagnetic wave through the electric-dipole transitions from the excited to the ground acceptor states. 2. The parabolic quantum wells laser with heterogeneous cavities. We expect it can be a high-power terahertz emitter.
In the first part of the dissertation, we study stress effect on the electronic structures and the electric-dipole transitions for group-III acceptors in Ge. The calculation is based on the effective mass theory with a semi-empirical impurity potential which considers the q-dependent screening and the central-cell correction. The stress effect on the electronic structures and the electric-dipole transi-tions can be undertood by connecting with the composition of the acceptor states. We find that the binding energies decrease rapidly with the stress in the low-stress region, and for even-parity states they exhibit remarkable asymmetry between the compressive and the tensile stress due to the large difference between the heavy-hole and the light-hole compositions. The coupling between the heavy-hole and the light-hole bands decreases with increasing stress and is almost negligible in the high-stress region (>3kbar). In this case, the acceptor states are almost pure heavy-hole or light-hole states. This causes the appearance of extra degeneracy of the acceptor states and additional selection rules of the electric-dipole transitions. In addition, we study in detail the electronic structures of the low-lying acceptor states and the electric-dipole transitions in the low-stress region. The results are in agreement with the currently available experimental data. However, because of the lack of the experimental data in the high-stress region, a justification is made for the applicability of our calcu-lation for the case of high stress.
In the second part of the dissertation, we propose and demonstrate theoretically a scheme for high-efficiency terahertz lasing from parabolic quantum wells resonant coupled with two different cavities. An in-resonance photonic crystal metal-metal cavity can increase the intersubband radiative transition rates by several orders; an in-resonance Fabry-Perot cavity can increase the interband transition rate. Simultaneous interband and intersubband lasings can significantly reduce the carrier density and heat generation due to nonradiative processes. In this case, the heat generation remains low and constant, independent of the injection current but the terahertz emission power increases linearly with current. With the present scheme, terahertz emission power of 10 W/cm2 can easily be achieved, accompanied by high intersubband quantum efficiency.
|
|---|
