Radiative transitions in
Germanium based structures
Candidate: Marco Morelli
Advisors: Prof. Giuseppe Grosso, Dr. Michele Virgilio Department of Physics, University of Pisa
Abstract
Crystalline Germanium, as weel as Silicon, is a semiconductor with a diamond-like lattice and an indirect fundamental gap in the electronic spectrum. This implies that direct radiative transitions between the valence band maximum and the conduction band minimum are forbid-den because of total momentum conservation. Despite the fact that in more than 50 years some important progress have been achieved in the employment of Silicon and Germanium as mate-rials for electronics, overcoming the difficoulties connected with the indirect nature of the gap is still an open challenge towards the achievement of an efficient source of light from these materials. In particular, in Germanium at room temperature, the indirect transition is only 136 meV below the direct transition, while this energy distance is much greater in Silicon. Such Ger-manium’s feature, together with a 300 K direct transition at a wavelength of great interest in telecommunications (1.55 µm), has revitalized the interest for Germanium based structures in the latest years.
In this Thesis we present a theoretical study of the optical absorption in Germanium cry-stals. Starting from the microscopic description of electronic and phononic states and from their related materials properties (effective masses, density of states, deformation potentials, etc...), we contribute to the development of a software, in the effective mass formalism, for the com-putation of direct and indirect optical spectra. In particular we analyze the disposition and the electronic filling of various conduction valleys for different temperatures and for different doping and charge injection levels. For this purpose it was necessary to make a self-consistent calcula-tion of the quasi-Fermi levels in the valence and conduccalcula-tion bands.
The software structure was conceived in order to allow the future inclusion of various other relevant mechanism affecting light emission such as temperature, self-absorption, free charge absorption, scattering by phonons and impurities, alloy-caused disorder and interface potentials in nanostructured systems.
