Internal Transitions Of Quantum Confined Impurities And Applications To THz Emitters

Terahertz (THz) science and technology have attracted much attention on account of the potential applications of THz radiation in many domains recently. Due to the truly discrete electronic spectra of quantum dots (QDs). most of the undesired scattering and relaxation processes are suppressed. The energy levels of QD can be controlled by varying the dimensions and shapes, the QD based terahertz emitters have been a subject of considerable interest. However, controllability of dot dimensions is not mature, QD based devices are subject to large distributions in sizes and hence significant inhomogeneous line broadening which is very severe in THz range.A new way to realize QD using quantum confined impurity atoms is proposed. By investigating the energy states, optical properties and dynamic properties of impurity atoms confined in quantum wells (QWs), it is proved that the quantum confined impurity atoms have the characteristics of QD. The impurity atoms confined in QWs can be thought of as individual single-electron (or single-hole) QDs. The QW systems confining impurity atoms can be grown to high quality by using delta doping and Molecular Beam Epitaxy.A potential model ofδ-doped quantum well is established. The numerical calculations of the influences of delta-doping potential on the energy profile of QW have been performed.The impurity energy spacing in quantum well just falls into the THz range. A new approach is proposed which of achieving THz laser or emission by using transitions between the impurity states in semiconductor QW system.The energy level structure, transitions between the impurity states, and the lifetime of the excited state of beryllium (Be) acceptor atoms confined in GaAs/AlAs QWs are studied both theoretically and experimentally. A Beδ-doped GaAs/AlAs three quantum wells emitter in the terahertz range is fabricated, and electroluminescence is investigated.1. The optical properties of the GaAs/AlAs multiple quantum wells (MQWs) with Be acceptorsδ-doped at the centre of the GaAs wells have been investigated with photoluminescence (PL) and far-infrared (FIR) absorption spectra respectively. The two-hole transition of the acceptor-bound exciton from the ground state, Is. to the excited state,2s, has been clearly observed in PL spectrum. The binding energy and the 1s-2s transition energy of Be in GaAs/AlAs MQWs was measured. Far-infrared absorptions were measured, and three principal absorption lines were observed clearly, which correspond to the transitions of Be from the ground state 1s to the three excited odd-parity states respectively. It is found that the acceptor binding energy and internal transition energies increase with decreasing quantum-well width.2. The influences of ionized impurity delta-layer potential and quantum well width on the energy profile of a QW are calculated by an iterative shooting algorithm under the single-band effective mass and envelop function approximations. As the delta-doping concentration increases, the well depth increases, the energies of HHo and LH0 increase and go to the well bottom, and the energy separation between HHo and LH0 increases. The impacts of ionized impurity delta-layer potential on the impurity state are strong when the well is narrow, the delta-doping concentration is great, and the degree of impurity ionization is high. The energies of the valance subband HHo and LHo increase and go away from the well bottom with decreasing quantum well width for the same delta-doping concentration. The energy separation between HHo and LHo increases also.3. Using a variational method, the binding energy and the internal transition energies of the Be acceptors at the centre of the GaAs/AlAs MQWs have been calculated as a function of the well width under the single-band effective mass and envelop function approximations. The acceptor binding energy increases as the QW width decreases, and has a peak with a maximum at the QW width of 0.7nm. For very narrow wells, the binding energy decreases. The 1s-2s,1s-2px, and 1s-2pz transition energies increase as the QW width decreases. It is found that the theoretical calculations are in good agreement with the experiment results. 4. The dynamics of intra-acceptor hole relaxation in Beδ-doped GaAs/AIAs MQWs with doping at the centre is studie by time-resolved pump-probe spectroscopy using the picosecond free electron laser for infrared experiments. The pump-probe experiments are performed at different temperatures and different pump pulse wavelengths. The 2p excited state lifetime of Be in MQWs decreases with decreasing well width, and is shown to be independent of temperature but strongly dependent on wavelength. The zone-folded acoustic phonon emission and slower decay of the wave functions of impurity states are suggested to account for the reduction of the 2p excited state lifetime in MQWs.5. A Beδ-doped GaAs/AIAs three quantum wells emitter in the terahertz range is fabricated and investigated. The current-voltage characteristics of the device measured at different temperatures demonstrate a strong negative differential resistance and temperature dependence. An electroluminescence peak is observed at low temperature (T=4.5K). The THz emission peak is attributed to the 2p to 1s internal transitions of the Be acceptors in the centre ofδ-doped GaAs quantum well.The result shows the potential of achieving THz laser or emission using transitions between the impurity states in semiconductor QW system.