Confinamento quântico em hetero-estruturas semicondutoras de baixa dimensionalidade

Abstract

Semiconductor materials are responsible for large development in the electronic industry and appearence of new technologies. The concept of heterostructure gave a large impulse to the solid-state physics. It is impossible to imagine modern solid-state physics without semiconductor heterostructures. The physics of semiconductors is nowadays concentrated in the study of the so-called low dimension systems: quantum wells, wires, dots and rings, which are the subject of research of two-thirds of the semiconductors physics community. In this work, we investigate the confinement of carriers and excitons in low dimensional heterostructure; quantum well, dots and ring. Starting with the study of the excitonic properties of Si/Si1−xGex, we consider two possibilities for the band alignment: type-I, where charge carries, electrons and holes, are confined in the same material, and type-II, where these carriers are spatially separated, in different materials. We use an Hamiltonian that, in effective mass approximation, takes into account the existence of non-abrupt interfaces in the system. In type-I, we observed that the exciton energy is increasing when considering applied electric field. In the type-II systems, application of magnetic field affect more the electron confinement than the hole. We investigate some phenomena in quantum rings, such as impurities, geometric effects, roughness and double rings. We calculate the energy levels of the electrons in quantum rings considering a perpendicular magnetic field, taking into account a realistic model, which consists of rings with finite barrier and potential, not limited to small perturbation. When considering the presence of impurity in the quantum ring, there is a breaking of symmetry in the system and, consequently, Aharanov-Bohm (AB) fluctuations are vanish. However, for two impurities, fluctuations are AB recovered if z1 = z2, in the case of positive impurities and for negative impurities fluctuations are recovered independent of positions of impurities. The existence of interfaces roughness is responsible for a considerable shift in the energy carriers. Moreover, the degeneration points of transition in the angular moment AB are raised when the rough surfaces are considered, and in special cases, oscillations in fundamental state are suppressed. Theoretical study of carriers energy in type-I and type-II quantum dots is performed, and also in double quantum dots InGaAs/GaAs, analyzing the effect of distance between the dots, considering two types of coupling: lateral and vertical. The Schödinger equation in three dimensions, in the effective mass approximation, is solved for electrons and holes using a time evolution method of the wave function. We have observed that the curves of Stark shift from binding energy and total exciton in Si/Si0.85Ge0.15 type-I quantum dots are asymmetric, because of the existence of an intrinsic electric dipole in these systems. However, when considering the effect of the magnetic field parallel to the plane, Stark shift becomes more symmetric. For double dots, we see that electron confinement energies in coupled laterally quantum dots, when considering the same radius for both dots degenerate Abstract x as the distance between the dots increases. However, when the radii of the dots are different, their energy are not significantly changed. In the case of vertical coupling, the behavior is similar to the dots side by side. For radii equal in both quantum dots, the pair of states becomes degenerated as distance between the dots increases, which is not the case when considering of dots with different radii.