Performance comparison of Nb and NbN antenna-coupled microbolometers

We report the experimental results of a comparison between free-standing Nb and NbN microbolometer bridges coupled to equiangular spiral antennas on Si substrates. Because of the difference in material resistivity, bolometer resistance and aspect ratio is varied independently. Room-temperature antenna patterns measured at 650 GHz with a backward-wave oscillator are presented, as are I-V curves at T = 300 K and at T = 4 K. At room temperature, zero-bias resistance and specific responsivity are examined, and at 4 K, normal-state resistance and saturation power are studied. Nb devices display significantly lower saturation powers than NbN devices whose dimensions have been adjusted to provide equal resistance. However, for both materials, the inferred thermal conductances are higher than predicted by the Wiedemann-Franz relation, by approximately a factor of similar to 2 for Nb and a factor of similar to 5 for NbN. In general, and especially for the room-temperature responsivity, the substantial spread in parameters from device to device exceeds any systematic difference in performance between the materials.