Quantum confinement effects on light absorption in Germanium for solar energy conversion

Abstract

The world demand for energy is continuously increasing with a rate that will soon become unsustainable given the current exploitation of energy sources (such as fossil fuels). In addition, it should be figured out that most of commonly used energy resource are limited and that humankind has liberated a quantity of carbon (as CO2) in the past 250 years that it took our planet about 250 million of years to sequester. In this context, a wide and exciting range of possible solutions to provide enough and cleaner energy is represented by nanotechnologies offering innovative materials with interesting effects exploitable for energy production, distribution and saving. Among other materials, Group-IV semiconductors have been deeply investigated since they allow the fabrication of abundant, non-toxic, mono-elemental nanostructures (as Si quantum dots, C nanotubes, Ge nanowires, et al.) thanks to high purity and mature technology. Moreover, fascinating effects due to quantum confinement in this nanostructures can be effectively exploited for energy production in photovoltaics devices. Among them, Ge reveals interesting optical properties due to its quasi-direct bandgap, higher absorption coefficient and larger exciton Bohr radius with respect to Si, giving the chance to easily tune the optical properties by exploiting quantum confinement effect (QCE). However, the properties of Ge quantum dots (QDs) depends not only on the size as many other parameters can concur in controlling their optical behavior, especially for what concerns the optical bandgap. For this reason, the aim of this thesis is devoted to a detailed investigation of the optical properties of Ge QD, with particular emphasis on the light absorption properties and its modulation by QCE.