Complex Optical Index of HgTe Nanocrystal Infrared Thin Films and Its Use for Short Wave Infrared Photodiode Design

The limited investigations of the optical properties of HgTe nanocrystal (NC) thin films have reached a bottleneck for the electromagnetic design of devices. Using broadband ellipsometry, we determine the refractive index (n) and the extinction coefficient (k) for a series of HgTe NC films relevant for infrared sensing applications. Electromagnetic simulations reveal that the n value of HgTe NC thin films can conveniently be approximated by its mean spectral value n=2.35 ± 0.15. We then use this complex optical index to design a diode with (i) a reduced amount of Hg containing material (thin film < 150 nm) and (ii) a thickness of the device better-matched with the carrier diffusion length. We demonstrate that introducing an aluminum grating onto the transparent conductive electrode leads to an enhanced absorption while reinforcing the work-function difference between the two electrodes. Broadband (≈ 1 µm), non-polarized and strong absorption up to 100% is designed. This leads to a responsivity of 0.2 A.W-1 and a detectivity of 2x10 10 Jones for 2 µm cutoff wavelength at room-temperature, while the time response is as short as 110 ns.