Study On The Electronic Structures Of Low-Dimensional Semiconductors And The Tunable Terahertz Sources

In this report, we investigated the conduction band structures of several types of semiconductor low-dimensional semiconductors subjected to external electric and magnetic fields. Our theoretical and computational results can be used to design tunable THz emitters. The main results of this report are listed below.1. The excitonic spectrum of a symmetric triple-coupled-quantum-well containing a diluted magnetic semiconductor layer under magnetic fields is studied. The numerical results show that the magnetic field can be used to adjust the excitonic spectra and the coupling behaviors of the quantum wells effectively. At low magnetic field, the effects of magnetic-field-induced confining potential on the main features of the excitonic spectra are negligible.2. The excitonic absorption of a nanoring double-quantum well (NDQW) under radial and lateral electric fields is studied theoretically. The variations of excitonic spectrum with different radial electric fields are similar to those of square coupled-double quantum well. For the lateral electric field, the confining effects become weak, which lead to the broadening of the excitonic peaks, and the decrease of the excitonic absorption intensity. Meanwhile, the lateral electric fields destroy the transition selective rule. The little absorption peaks due to the transitions between the discrete states induced by the periodic boundary conditions are broadened into continuum absorption band.3. By using plane-wave method, the conduction band structures of n doped narrow ZnSe/(Zn,Cd,Mn)Se and wide GaAs/(Al,Ga)As quantum wells under in-plane magnetic fields are explored theoretically. Due to the giant Zeeman splitting, in narrow ZnSe/(Zn,Cd,Mn)Se quantum well, the two-dimensional electron gas is fully spin-polarized when the in-plane magnetic field is about several Tesla. Meanwhile, the effects of the in-plane magnetic field on the energy dispersion are negligible.4. The conduction band structure of an n-doped wide GaAs/(Al,Ga)As quantum well subjected to in-plane magnetic fields is calculated. The lowest two sublevels are anti-crossing under in-plane magnetic field. This leads to the appearance of the negative effective mass region on the energy dispersion curve. The effects of doping concentration, magnetic field strength and the electric field parallel to the growth direction on the negative effective mass region are discussed. The transport behaviors of the two-dimensional electron gas in n doped wide are investigated with Monte-Carlo simulation. The possible current oscillations in the above system are discussed.