Tunability and Losses of Mid-infrared Plasmonics in Heavily Doped Germanium Thin Films

TL;DR

This study investigates heavily doped germanium thin films for mid-infrared plasmonics, demonstrating tunable plasma frequencies and low losses, with potential for room-temperature applications in mid-infrared devices.

Contribution

It provides a comprehensive analysis of tunability and losses in n-type germanium films, revealing weak dependence on defects and temperature, and highlights the potential for underdamped plasmon oscillations.

Findings

Unscreened plasma frequency tunable from 400 to 4800 cm$^{-1}$

Electron scattering rate increases linearly with frequency

Plasmon decay times in the several-picosecond range

Abstract

Heavily-doped semiconductor films are very promising for application in mid-infrared plasmonic devices because the real part of their dielectric function is negative and broadly tunable in this wavelength range. In this work we investigate heavily n-type doped germanium epilayers grown on different substrates, in-situ doped in the $10^{17}$ to $10^{19}$ cm$^{-3}$ range, by infrared spectroscopy, first principle calculations, pump-probe spectroscopy and dc transport measurements to determine the relation between plasma edge and carrier density and to quantify mid-infrared plasmon losses. We demonstrate that the unscreened plasma frequency can be tuned in the 400 - 4800 cm$^{-1}$ range and that the average electron scattering rate, dominated by scattering with optical phonons and charged impurities, increases almost linearly with frequency. We also found weak dependence of losses and tunability on the crystal defect density, on the inactivated dopant density and on the temperature down to 10 K. In films where the plasma was optically activated by pumping in the near-infrared, we found weak but significant dependence of relaxation times on the static doping level of the film. Our results suggest that plasmon decay times in the several-picosecond range can be obtained in n-type germanium thin films grown on silicon substrates hence allowing for underdamped mid-infrared plasma oscillations at room temperature.