Growth and Characterization of a Kind of Nitrogen-Rich Niobium Nitride for Bolometer Applications at Terahertz Frequencies

Niobium is sputtered onto a single crystalline silicon substrate in N₂:Ar=4:1 gas mixture at the total pressure of 2Pa. The temperature coefficient of resistance of the sample is about 0.5% at 300K, and up to 7% at 77K, indicating the possibility of using it to make room-temperature bolometers with performances better than those based on Pt, Bi, or Nb. For a 60-nm-thick sample, the rms surface roughness is 0.45nm over an area of 2μm×2μm. Analyses based on x-ray diffraction and x-ray photoelectronic spectroscopy indicate that the samples are Nb₅N₆ thin films in which there is a combination of Nb³⁺ and Nb⁵⁺, or Nb⁴⁺.