Bound Polaron, Exciton And Strain Effects In Nitride Semiconductor Quantum Dots

For the groupâ…¢-â…¤nitride semiconductors represented by AlN, GaN and InN show the polarization effects, uniaxial anisotropy and other special characteristic in light-emitting aspect and low-dimensional structures, the bound polaron problem with and without electric field in wurtzite GaN/AlxGa1-xN ellipsoidal quantum dots (QDs) is studied firstly in this thesis by using variational method in the framework of effective mass approximation. Considering the anisotropy effects in a wurtzite structure, the transverse optical-like (TO-like) and longitudinal optical-like (LO-like) phonon modes under the bulk phonon approximation are adopted. Secondly, the exciton problem in strained wurtzite cylindrical QDs and strained zinc-blende ellipsoidal QDs under hydrostatic pressure are studied, respectively. Finally, the bound exciton state in ellipsoidal QDs is further studied.The numerical results show that the contribution from TO-like phonons in the wurtzite structure to the binding energy is positive, but the contribution from LO-like phonons is negative and plays a dominant role, the influence of total phonons obviously lower the binding energy of the impurity state. In addition, it is also found that the interaction between the impurity and phonons is stronger than the one between the electron and phonons. When the external electric field is applied, the electric filed obviously induces the energy shift for the bound polaron and reduces the influence of phonons on the binding energy, and the influence of electric field is more obvious for a larger quantum dot (QD). Finally, we also find that the influence of phonons is dependent strongly upon the dot shape and the anisotropy effects in wurtzite structures.Secondly, by studing the exciton problem in strained wurtzite cylindrical QDs and strained zinc-blende ellipsoidal QDs under hydrostatic pressure, respectively, we find that the strain effects raise slightly the binding energy of exciton state when the QD height is small, but lower significantly the binding energy as the QD height increases due to the piezoelectric and spontaneous polarizations in the wurtzite structure. For the strained zinc-blende ellipsoidal quantum dot, because there is no influence of an electric filed the strain effects always raise slightly the binding energy of exciton states. Moreover, no matter what shape the QD is, the binding energy increases with increasing pressure by considering hydrostatic pressure, and pressure has a more obvious influence on the binding energy of exciton states for the smaller QD sizes.Finally, by studying the positively charged donor ion bound exciton we find that the stability of the three-body system is not only dependent the electron-hole mass ratio, but its best steady state is related to the QD size. The results show that the critical electron-hole mass ratio is between 0.1 and 0.2, moreover, as the symmetry of the QD shape is reduced the best steady state of a bound exciton shifts toward a large QD size.