Optoelectronic design on the hot-carrier photodetectors

Conventional photodetectors are mostly propelled by p-n junctions, where the detection wavelength is constrained by the bandgap of semiconductor material. Recently, a meta-linsulator-metal (MIM) configuration is demonstrated to be a promising candidate for the next generation photodetection due to its highly tunable operation wavelength, simple configuration, and possibly low cost. The MIM photodetector is operated under the principle of the hot carriers, which can be excited in the metal layer by the incident photons, and collected by the MIM junction to form measurable electrical current. For hot-electron photodetection, a high and unidirectional photocurrent is strongly desired. This reveals that the total absorption of the entire system is not an ultimate target; nevertheless, a strong and asymmetrical absorption is desired eventually. However, the existing systems can hardly fulfill this stringent requirement since the incident light couples strongly with both metallic layers as well. To satisfy the inherent demands for the hot-carrier photodetection, we have proposed a series of novel configurations such as coaxial Ag/ZnO/Ag nanowires, conformal grating structures, and planar multilayer cavities. Via the finite-element electromagnetic simulation and the probability-based analytical carrier-transport calculation, we explore in detail the optical response and the electrical transport in the devices. Results show that the proposed devices can be designed flexibly and the optimized configurations exhibit superior optical and electrical performance, yielding a strong asymmetrical photoabsorption and a high unidirectional photocurrent. For instance, by asymmetrically aligning the semiconductor barrier relative to the Fermi level of metals and strongly localizing the light energy through plasmonic resonances, the conformal grating system can achieve a strong and highly asymmetrical optical absorption (top metal absorption > 99%) and a three-fold photoresponsivity compared with the conventional grating design; by forming a planar perfect absorber at the targeted wavelength, coaxial nanowire schemes acquire a strong optical absorption and satisfy well the inherent requirement of efficient electron collection, which is predicted to be with an unbiased responsivity of ∼ 200 nA/mW.