Vibrational Modes as the Origin of Dielectric Loss at 0.27-100 THz in 𝑎-Si C:H

Low-loss deposited dielectrics are beneficial for the advancement of superconducting integrated circuits for astronomy. In the microwave band (approximately 1–10 GHz) the dielectric loss at cryogenic temperatures and low electric field strengths is dominated by two-level systems. However, the origin of the loss in the millimeter-submillimeter band (approximately 0.1–1 THz) is not understood. We measured the loss of hydrogenated-amorphous-Si⁢C films in the 0.27–100-THz range using superconducting-microstrip resonators and Fourier-transform spectroscopy. The agreement between the loss data and a Maxwell-Helmholtz-Drude dispersion model suggests that vibrational modes above 10 THz dominate the loss in hydrogenated amorphous Si⁢C above 200 GHz.