Infrared Bolometers Based on 40-nm-Thick Nano-Thermoelectric Silicon Membranes
State-of the-art infrared photodetectors operating in the mid- and long-wavelength infrared (MWIR and LWIR) are largely dominated by cryogenically cooled quantum sensors when the target is the highest sensitivity and detection speeds. Nano-thermoelectrics provide a route towards competitive uncooled infrared bolometer technology that can obtain high speed and sensitivity, low-power operation, and cost-effectiveness. We demonstrate nano-thermoelectric LWIR bolometers with fast and high-sensitivity response to LWIR around 10 $\mu$m. These devices are based on ultra-thin silicon membranes that utilize the dimensional scaling of silicon nanomembranes in thermoelectric elements and are combined with metallic nanomembranes with subwavelength absorber structures. The fast device performance stems from a low heat capacity design where the thermoelectric beams act both as mechanical supports and transducer elements. Furthermore, by scaling down the thickness of the thermoelectric beams the thermal conductivity is reduced owing to enhanced phonon boundary scattering, resulting in increased sensitivity. The nano-thermoelectric LWIR bolometers are based on 40-nm-thick n- and p-type silicon membranes with LWIR (voltage) responsivities up to 1636 V/W and 1350 V/W and time constants in the range of 300-600 $\mu$s, resulting in specific detectivities up to $1.56\times10^8$ cmHz$^{1/2}$/W. We also investigate the use of a heavily doped N++ substrate to increase optical cavity back reflection, resulting in an increased Si substrate reflectance from 30% to 70%-75% for wavelengths between 8-10 $\mu$m, resulting in an increase in device responsivity by approximately 20%.