Electro-Optical Characterization of an Amorphous Germanium-Tin (Ge1-XSnx) Microbolometer

Abstract

The utilization of amorphous germanium-tin (Ge1-xSnx) semiconducting thin films as temperature-sensing layers in microbolometers was recently presented and patented. The work in this paper was performed as an extension of the latest study to acquire better Sn concentrations % for microbolometer applications. In this work, Ge1-xSnx thin films with various Sn concentrations %, x, where 0.31 ≤ x ≤ 0.48, were sputter-deposited. The elemental composition of each film was evaluated using energy-dispersive X-ray (EDX) spectroscopy, and the surface morphology was evaluated using atomic force microscopy (AFM), showing average roughness values between ~ 0.2 and 0.8 nm. Measurements of the sheet resistance versus temperature were performed and analyzed, revealing temperature coefficients of resistance, TCRs, ranging from –3.11%/K to –2.52%/K for x ranging from 0.31 to 0.40 above which the Ge1-xSnx thin film was found to exhibit metallic behavior at 0.40 < x ≤ 0.48. Empirical relationships relating the resistivity, TCR, and Sn concentration % of the amorphous Ge1-xSnx thin films were derived. One of the films with a 31% Sn concentration (Ge0.69Sn0.31) was used to fabricate 10×10 μm2 microbolometer prototypes using electron-beam lithography and lift-off techniques, and the microbolometer was fabricated on top of oxidized silicon substrates with no air gap between them. The noise behavior and the maximum detected signal of the fabricated microbolometer were measured. The signal-to-noise ratio, voltage responsivity, and noise equivalent power values of the prototypes were calculated. Finally, the expected performance of a proposed air-bridge microbolometer configuration was calculated.