Impurity States And Pressure Effect In Strained GaN/Al_xGa_1-xN Heterojunctions Under Electric Fields

In recent years, the physical properties of quantum heterostructures composed of the group-III nitrides semiconductors with wide-band-gaps, such as A1N, GaN, and InN, and their ternary compounds, are widely studied due to development of their promising application in the fabrication of optoelectronic devices, particularly for high-brightness blue/green lightemitting diodes (LED's) and laser diodes (LD's). The investigation in the strained semiconductor heterostructure materials with wide-band gaps has an important practical significance to improve their physical properties and to speed up theoretical development, and can provide a guidance and new ideas for the design of semiconductor devices.A simplified coherent potential approximation is developed to calculate physical parameters of ternary mixed crystals and a modified Lee-Low-Pines transformation is applied to deal with the electron-phonon interaction. In this thesis, a variational method is applied to investigate binding energies of donor impurity states in strained realistic heterojunctions of nitride semiconductors under external electric fields, and the influence from the interaction between electrons and phonons, as well as the effects of hydrostatic pressure and screening, is also studied. The thesis is organized as three main parts. Firstly, the binding energies of impurity states in strained zinc-blende GaN/AlxGa1-xN heterojunctions with GaN substrates and with [001] and [111] orientations are respectively calculated by considering the abrupt change of potential barriers for ideal interfaces. Compared with a wurtzite structure, a nitride semiconductor with zinc-blende structure has advantages of high symmetry, low phonon scattering ratio, as well as the easy cleavage, so that the further study on this structure has an impotant significancy. The result indicates that the binding energies of impurity states nearly linearly increase with pressure. The Stark effect on the binding energies as a function of electric fields shows a blue or red shift for different impurity positions, respectively. For different crystal growth directions, binding energies of impurity states have significantly different sensitivities for the electric field, pressure, and change of Al composition.Based on the investigation about zinc-bende nitride heterojunctions in the first part, the second part of the thesis expands the investigation to deal with the impurity states in free strained wurtzite nitride heterostructures. A theoretical calculation is performed about the quantum confined Stark effect and pressure effect on the donor impurity states in [0001]-oriented free strained wurtzite GaN/AlxGa1-xN heterojunctions. Furthermore, under the random-phase-approximation, the influence of dielectric screening on the impurity states is also taken into account. The binding energies of impurities in a wurtzite heterojunction system increase about 40% than that in a zinc-blende structure. The Stark energy red shift increases dramatically due to the influence from hydrostatic pressure on the effective mass of an electron and dielectric constants of materials. The influence from pressure on the shift is more obvious with increase of electric field and impurity position. When the screening effect is taken into account, the binding energies of impurity states decrease dramatically. For a given impurity position, the change of screened binding energies with the increase of the hydrostatic pressure is more sensitive than those without the screening effect and it decreases dramatically Stark blue(red) shift increasing with an electric field.In the third part of the thesis, the binding energies of bound polarons near the interface of a strained wurtzite heterojunction and quantum confined Stark effect under an external electric field are studied. A modified LLP variational method is used to deal with the interactions of electron-phonon and impurity-phonon. Considering the influences from the half-space phonon modes and the interface-optical phonon modes, the binding energies bound polarons decrease dramatically. The contribution from the half-space phonon modes on the binding energies is three times larger than that from the interface optical phonon modes. The contributions from the LO-like phonon mode and one branch of IF phonon modes with a higher frequency play an important role for the binding energy. With different impurity positions, the Stark blue shift decreases obviously but the change of the Stark red shift has a certain relationship with the electric field by considering the phonon effect.