High-sensitivity mid-wave resonant cavity infrared detectors

Resonant cavity infrared detectors (RCIDs) can reduce the noise in sensing a laser signal by strongly suppressing background photocurrent at wavelengths outside the narrow spectral band of interest. We recently reported an RCID with 100-nm-thick InAsSb/InAs absorber, GaAs/AlGaAs bottom mirror, and Ge/SiO₂ top mirror. At T = 300 K, the external quantum efficiency reached 58% atλres ≈ 4.6 μm, with linewidth δλ = 27 nm. The characteristics at 125 K implied a specific detectivity of 5.5 × 10¹² cm Hz½/W, which is more than 3× higher than for a state-of-the-art broadband HgCdTe device operating at that temperature. However, a prominent variation with mesa diameter of the deposited Ge spacer thickness made it difficult to predictably control λres for devices processed with a given diameter. This has been addressed by measuring the reflectivity spectrum following deposition of the spacer, so that thicknesses of the top mirror's SiO₂ and Ge layers could be adjusted appropriately to attain a targeted resonance. This was especially beneficial in matching the λres for a small mesa, needed to minimize the capacitance in high-frequency measurements, to the emission wavelength of a given ewquantum cascade laser.