Terahertz spectroscopy of semiconductors and their interfaces

Semiconductors are undisputedly the class of materials that has driven the progress in electronics since the second half of the 20^th century. As such, tools that can assess their electric properties are of primary relevance for developing new technologies. Among these methods, in this thesis, we exploit time-resolved THz spectroscopy (TRTS) to study conductivity in semiconductors. Thanks to THz radiation's low photon energy, it is possible to probe systems where the separation between energy levels is relatively small, such as within energy bands in semiconductors or in excitonic systems, and to retrieve complex-valued conductivity. In this respect, TRTS provide a powerful mean to scrutinize both time evolution of carrier population and charge transport mechanisms in semiconductors.
In the first part of this thesis, we study TRTS carrier dynamics at the interface between Copper-Indium, Gallium Selenide (CIGS) alloys and Cadmium Sulphide (CdS), a heterojunction interface that is relevant for low-cost, thin-film photovoltaics. Contrary to the expectations, better theoretical efficiency predicted in high bandgap CIGS-based devices turn into overall lower device performances when compared with low-gap devices. We demonstrate that the origin of this effect is linked to surface recombination at the CIGS/CdS interface. Recombination at this interface depends on the CIGS energy gap and becomes dominant after a certain Ga content threshold.
In the second part of this thesis, we analyze organic semiconductors. We present an analysis of the conductivity in polyaniline (PANI) thin films doped with a novel dopant: phytic acid. This new combination reveals clear and distinct signatures of band-like transport. Temperature-dependent THz measurements reveal a monotone increase of charge mobility with decreasing temperature and almost invariant scattering rates, links to either reduced effective mass or an improved photogeneration yield. These findings are singular among organic semiconductors and open the path for exploiting phytic acid as a robust dopant in optoelectronic applications.