The Electron States In Strained Wurtzite Quantum Wells

In recent years, strained wurtzite semiconductors and their low-dimensional structures are widely used in modern illumination and terahertz communication and become important materials for fabricating a new generation of electronic and opto-electronic devices. The electron states in these systems are closely related to the light-emitting and photovoltaic properties of materials thus become one focus of theoretical and experimental researches. In this thesis, the impurity states, exciton states and optical absorptions of electrons bound by quantum wells transitting from ground states to excited states as well as the effect of phonon scattering in strained wurtzite quantum wells (the keystones are nitrides) are theoretically investigated in order to explore the probability to improve the quantum efficiency of optical devices based on these materials. We found that the electron states in these quantum-well systems exhibit lots of novel characteristics, since the energy-band structures, phonon modes, and dielectric properties of materials are prominently influenced by interfaces, strain and anisotropic. The main contents and results are generalized as follows:(1) The ground-state binding energies of electrons bound to quasi-hydrogen donor impurities in strained wurtzite asymmetric AlxGa1-xN/GaN/AlyGa1-yN single quantum wells and GaN/AlxGa1-xN double quantum wells have been calculated through a variational method. The size effects of impurity state binding energies due to the modulation of barrier widths or/and heights of the single quantum wells are studied. Furthermore, the influence on the impurity state binding energies by adjusting the central barriers in double quantum wells is discussed. It is found from our results that the binding energies are influenced dramatically by the variation of band offsets due to the build-in electrical fields and strains. A mutation of binding energies will occur when the electric potential in one well is lower than that in the other well resulted from the change of middle barriers. For comparison, the variations of ground-state binding energies for these double quantum wells as functions of the ratio of left and right wells and impurity positions have been also discussed when there are and there are not built-in electric fields.(2) At first, the ground-state binding energies of excitons in GaN/AlxGa1-xN strained wurtzite coupled double quantum wells have been caculated with a variational method. The results indicate that the built-in electric fields modulated by well sizes could cause a transformation between direct and indirect excitons. Almost all the excitons in asymmetric double quantum wells are indirect, and the binding energies of indirect ones are very small and insensitive to the change of structural scale. Then, the polaronic effects of exciton states in GaN/InxGa1-xN strained wurtzite are studied using the LLP variational method and self-consistent computation. The results show that the built-in electric fields, the exciton-phonon interaction and the electron-hole plasma collectively weaken the Coulomb coupling between an electron and a hole to reduce the binding energy. Among all the electron-phonon interactions, interface optical phonon modes play a dominant role in determining excitonic binding energy. Not only does the two-dimensional electron-hole gas screen the built-in electric field, but also reduces the polaronic effect to the binding energy. The excitons may collapse in an electron-hole gas of higher densty.(3) The optical absorptions of electronic intersubband transitions in AlxGa1-xN/InyGa1-yN/InzGa1-zN single quantum wells, in ZnO/MgxZn1-xO single quantum wells under external electric fields, in GaN/AlxGa1-xN coupled double quantum wells, and in GaN/AlxGa1-xN step quantum wells implanted an InyGa1-yN nanogroove layer are investigated in turn. The results indicate that the quantum confinement on electons can be strengthened either by increasing the height of one barrier in a single quantum well or by applying an external electric field to restrain the built-in electric field or by using a quantum-well structure with five layers (i.e., double quantum wells or implanting a nanogroove). By choosing suitable structural parameters (size and compostion), external field and stress, the optical absorption will be achieved in the terahertz ranges as required as one can. Besides, the influences from external stress on electron energy levels and absorption spectra are preliminarily studied.(4) The influences from the elastic scattering by various kinds of optical phonon modes on electronic intersubband transitions are studied via the Fermi gold rule. The phonon-assisted transition rates of electrons from the ground-states to the first excited states in GaN/InxGa1-xN quantum wells are calculated. The results show that interface and half-space optical phonons play important roles in the process of1-2transition caused by built-in electric fields which also greatly reduce the transition rate of electrons.The above conclusions are expected to extend and enrich understanding the electron states in strained wurtzite low-dimensional structures and to provide a useful guidance for related opto-electronic experiments and device design.