Extension of the wavelength threshold of an infrared detector beyond λt=hc/Δ is demonstrated, without reducing the minimum energy gap (Δ) of the material. Specifically, a photodetector designed with Δ=0.40 eV, and a corresponding λt=3.1 μm, was shown to have an extended threshold of ∼45 μm at 5.3 K, at zero bias. Under negative and positive applied bias, this range was further extended to ∼60 and ∼68 μm, respectively, with the photoresponse becoming stronger at increased biases, but the spectral threshold remained relatively constant. The observed wavelength extension arises from an offset between the two potential barriers in the device. Without the offset, another detector with Δ=0.30 eV showed a photoresponse with the expected wavelength threshold of ∼4 μm.
For the infrared detection in the 3-5 μm range, p-GaAs/Al<sub>x</sub>Ga<sub>1-x</sub>As heterojunction is an attractive material system due to light hole/heavy hole and spin-orbit split-off intra-valance band transitions in this wavelength range. Varying the Al mole fraction (x) provides the tuning for the wavelength threshold, while graded Al<sub>x</sub>Ga<sub>1-x</sub>As potential barriers create an asymmetry to allow a photovoltaic operation. The photovoltaic mode of operation offers the advantage of thermal noise limited performance. In our preliminary work, a 2 – 6 μm photovoltaic detector was studied. Implementation of an additional current blocking barrier improved the specific detectivity (<i>D</i>*) by two orders of magnitude, to 1.9×10<sup>11</sup> Jones at 2.7 μm, at 77K. At zero bias, the resistance-area product (<i>R<sub>0</sub>A</i>) had a value of ~ 7.2×10<sup>8</sup> Ω cm<sup>2</sup>, which is five orders higher in magnitude (with a corresponding reduction of the responsivity by only a factor of ~ 1.5), compared to the <i>R<sub>0</sub>A</i> value without the blocking barrier. A photoresponse was observed up to 130K.
A novel concept utilizing a hot-carrier effect based on carrier interactions is achieved to extend the wavelength of the photodetector’s spectral response. A detector with a designed wavelength threshold (λ<sup><i>t</i></sup>) at 3.1 μm displays two different extended thresholds at different temperatures. A very-long wavelength infrared response up to 55 μm was observed up to 35 K; while a threshold wavelength of 8.9 μm was observed in the temperature range 60K − 90K. Response tuning is implemented via varying the degree of hot-hole injection.