Characterization and modelling of the steady-state and transient plasmonic properties of heavily-doped Ge (Orale)

Abstract : In the last decade, heavily-doped semiconductors have been proposed as low-loss and tunable plasmonic materials for the mid-infrared spectral region. In this framework, we investigate heavily-doped germanium films epitaxially grown on silicon wafers by low-energy plasma-enhanced chemical vapor deposition. Germanium uniquely displays both high mobility (both electron and hole mobilities are higher than those of silicon by factors of 3 and 4) and a non-polar lattice. In terms of losses, there is a possibility that the combination of the impurity field screening by highly mobile charge carriers and the deformation scattering potential, which is less efficient than direct coupling to polar phonons in scattering electron fields, might lead to improved performances of plasmonic resonators. Moreover, advanced large-scale fabrication technologies developed in semiconductor foundries may be employed to obtain almost ideal geometries. In this work we present both a full infrared spectroscopy study of the dielectric constant of heavily-doped (both n-type and p-type) Ge and we employ it to describe the transient plasmonic behavior obtained in nominally intrinsic Ge antennas after all-optical carrier excitation with near-infrared pulses. Preliminary results demonstrate that both the electron and the hole populations need to be taken into account to properly describe the transient behavior. We also experimentally characterize the steady-state frequency-dependent scattering rate of electron-doped Ge. We derive the free carrier concentration (activated dopants) from the Hall coefficient and obtain the incorporated donor density from secondary ion mass spectroscopy. We then compare the data with first-principle scattering cross-sections for electrons possessing energies comparable with those associated with the plasmon field frequency.