Made available in DSpace on 2014-12-17T15:14:50Z (GMT). No. of bitstreams: 1 ErlaniaLO.pdf: 3072440 bytes, checksum: c231266f91625a857dc874cab6600510 (MD5) Previous issue date: 2008-12-29 / Conselho Nacional de Desenvolvimento Cient?fico e Tecnol?gico / We have used ab initio calculations to investigate the electronic structure of SiGe based nanocrystals (NC s). This work is divided in three parts. In the first one, we focus the excitonic properties of Si(core)/Ge(shell) and Ge(core)/Si(shell) nanocrystals. We also estimate the changes induced by the effect of strain the electronic structure. We show that Ge/Si (Si/Ge) NC s exhibits type II confinement in the conduction (valence) band. The estimated potential barriers for electrons and holes are 0.16 eV (0.34 eV) and 0.64 eV (0.62 eV) for Si/Ge (Ge/Si) NC s. In contradiction to the expected long recombination lifetimes in type II systems, we found that the recombination lifetime of Ge/Si NC s (τR = 13.39μs) is more than one order of magnitude faster than in Si/Ge NC s (τR = 191.84μs). In the second part, we investigate alloyed Si1−xGex NC s in which Ge atoms are randomly positioned. We show that the optical gaps and electron-hole binding energies decrease linearly with x, while the exciton exchange energy increases with x due to the increase of the spatial extent of the electron and hole wave functions. This also increases the electron-hole wave functions overlap, leading to recombination lifetimes that are very sensitive to the Ge content. Finally, we investigate the radiative transitions in Pand B-doped Si nanocrystals. Our NC sizes range between 1.4 and 1.8 nm of diameters. Using a three-levels model, we show that the radiative lifetimes and oscillator strengths of the transitions between the conduction and the impurity bands, as well as the transitions between the impurity and the valence bands are strongly affected by the impurity position. On the other hand, the direct conduction-to-valence band decay is practically unchanged due to the presence of the impurity / Neste trabalho, utilizamos c?lculos ab-initios para investigar a estrutura eletr?nica de nanocristais (NC s) baseado em SiGe. Para tal, dividimos o mesmo em tr?s partes. Na primeira parte, investigamos as propriedades excit?nicas dos NC s de SiGe ordenados em estruturas do tipo core-shell ( Si[core]/Ge[shell ] e Ge[core]/Si[shell ]). Tamb?m estimamos as modifica??es induzidas pelo efeito do strain na estrutura eletr?nica. Com isso, mostramos que NC s Ge/Si (Si/Ge) exibe confinamento tipo II na banda de condu??o (val?ncia). A barreira de potencial estimada para os el?trons e os buracos s?o 0,16 eV (0,34 eV) e 0,64 eV (0,62 eV) para NC s de Si/Ge (Ge/Si). Em contradi??o com o esperado longo tempo de vida da recombina??o em sistemas do tipo II, verificamos que o tempo de vida da recombina??o nos NC s Ge/Si (τR = 13.39μs) ? mais de uma ordem de grandeza mais r?pido do que nos NC s de Si/Ge (τR = 191.84μs). Na segunda parte, investigamos NC s de Si1−xGex em que ?tomos de Ge s?o aleatoriamente posicionados. Desta forma, verificamos que o gap ?ptico e a energia de liga??o do par el?tron-buraco diminue linearmente com x, enquanto a energia de troca do exciton aumenta com x, devido o aumento da extens?o espacial da fun??o de onda dos el?trons e dos buracos. Isso tamb?m aumenta a superposi??o da fun??o de onda el?tron-buraco, fazendo com que o tempo de vida da recombina??o seja muito sens?vel ? fra??o molar x Ge. Finalmente, investigamos as transi??es radiativas nos NC?s de Si dopados com e Boro (B) e F?sforo (P). O tamanho dos nossos NC s variam entre 1.4 nm e 1.8 nm. Usando um modelo de tr?s n?veis, mostramos que o tempo de vida radiativo e a for?ca de oscilador entre a transi??o da banda de condu??o e o n?vel da impureza, como tamb?m a transi??o entre o n?vel da impureza e a banda de val?ncia s?o afetados pela posi??o da impureza no nanocristal de Si. Por outro lado, o decaimento direto da banda de condu??o para a de val?ncia n?o muda com presen?a da impureza