Germanium Nanostructures for Efficient Light Harvesting Devices

Abstract

The growing World energy demand is setting new challenges toward the use of alternative and green resources as well as for the development of more efficient and low-power consuming devices. Thanks to their unique optical properties, group IV (such as: Si, Ge, C) nanostructures (NS) show promising applications for cheap multi-junction solar cells and, in general, for efficient energy-tunable light harvesting devices. Among them, Ge reveals interesting optical properties due to its quasi-direct bandgap and larger absorption coefficient that make it intrinsically more suitable than Si for what concerns light harvesting applications. Moreover, the larger exciton Bohr radius of Ge (~24 nm) with respect to Si, gives the chance to easily tune the optical properties of Ge NSs by varying their size. However, the properties of Ge NS depends not only by size through quantum confinement effects, but many other parameters can concur in controlling their optical behavior, especially for what concerns the optical bandgap. Discerning the role of these parameters and controlling their effects in the light absorption process contains not only a fundamental research theme, but represents a key-factor toward the implementation of Ge NS in any type of light harvesting device. For this reason, this thesis reports a detailed study on the synthesis, structural and optical properties of Ge nanostructures (quantum well, QW, or quantum dots, QDs) embedded in a dielectric matrix as well as the investigations of photo-conduction properties in prototypal light harvesting devices employing Ge NSs. Although the optical behavior of a single amorphous Ge QW can be fully modeled within the quantum confinement effect theory, this situation dramatically changes for a 3-dimensional confinement, as in an ensemble of QDs. In this last case, the effects of quantum confinement can be hidden or weakened by other parameters, such as: QD spacing and distribution, type and quality of the hosting matrix and abundance of defects related to the synthesis technique used. For this reason, we will investigate in detail the synthesis and optical properties of Ge QDs embedded in SiO2 or Si3N4 matrices, grown after thermal annealing of Ge-rich films synthesized by co-sputtering deposition; plasma enhanced chemical vapour deposition (PECVD) and ion implantation. We will give evidences of the strength of quantum confinement effects occurring in these systems as well as discerning the contributions coming from other concomitant effects. Finally, we will demonstrate that Ge nanostructures can be effectively used as active absorber and conductive medium in light harvesting devices. In particular, we will report on the spectral response of metal-insulating-semiconductor (MIS) photodetectors employing a single amorphous Ge QW or a packed array of Ge QDs as active light sensitizer and conductive medium. Both types of NSs have a fundamental role on the performances of these prototypal devices, demonstrating the large potentiality of such nanostructures for the development of high efficiency photodetectors and low cost solar cells.