Infrared photoconduction at the diffusion length limit in HgTe nanocrystal arrays

Abstract

Narrow band gap nanocrystals offer an interesting platform for alternative design of low-cost infrared sensors. It has been demonstrated that transport in HgTe nanocrystal arrays occurs between strongly-coupled islands of nanocrystals in which charges are partly delocalized. This, combined with the scaling of the noise with the active volume of the film, make case for device size reduction. Here, with two steps of optical lithography we design a nanotrench which effective channel length corresponds to 5-10 nanocrystals, matching the carrier diffusion length. We demonstrate responsivity as high as 1 kA W^-1, which is 10⁵ times higher than for conventional µm-scale channel length. In this work the associated specific detectivity exceeds 10¹² Jones for 2.5 µm peak detection under 1 V at 200 K and 1 kHz, while the time response is as short as 20 µs, making this performance the highest reported for HgTe NC-based extended short-wave infrared detection.

Fig. 1. Active material and nanotrench device fabrication.

a Infrared absorption spectrum of HgTe nanocrystals (NCs) used in this study. The narrow line at 2900 cm⁻¹ is due to the C–H bond from the capping ligands. b Transmission electron microscopy image of HgTe NCs. The scale bar is 50 nm. The top inset is a high resolution zoom on a HgTe NC. The scale bar is 10 nm. c Sketch of the main step of fabrication leading to nanotrench electrodes: tilted evaporation. Bottom sketch is a scheme of the final device. The red rectangle highlight the nanotrench area. d Scanning electron microscopy image of 85-nm nanogap in between nanotrench electrodes. The scale bar is 200 nm.

Fig. 2. Electrical characterization of the nanotrench electrodes.

a I–V curves for a nanogap functionalized by a HgTe nanocrystal array in the dark and under illumination by a 1.55 µm laser diode. The experiment is conducted at 200 K. b Simulated electric field color map for a 40-nm nanotrench under a 3 V bias. Arrows represent the direction of the electric field. c Transfer curves (channel and gate current as a function of applied gate bias) for an HgTe nanocrystal array deposited on a nanotrench fabricated on a LaF₃ substrate used as a back gate. The measurement is conducted at 200 K.

Fig. 3. Infrared detection using nanotrench device.

a Responsivity and external quantum efficiency (EQE) as a function of the incident power for an array of HgTe nanocrystals (NCs) deposited on nanotrench electrodes (nanotrench spacing: 40 nm) at various temperatures. Applied bias is 1 V, while the illumination is ensured by a 1.55 µm laser diode and modulated at 1 kHz. b Specific detectivity, D* under 1 V as a function of the temperature for various devices: 10 and 20 µm-spaced interdigitated electrodes, e-beam fabricated electrodes with 230, 730, and 1600 nm spacing and nanotrenches electrodes with 40 and 85 nm spacing. For the sake of comparison with state of the art diode we also provide the performance of the photodiode from ref. . c Specific detectivity measured at 200 K as a function of the electrode spacing. An exponential fit which characteristic decay length is 21 nm is also plotted in red. d Photocurrent spectra for an array of HgTe NCs deposited on nanotrench electrodes (electrode spacing: 85 nm) and various applied bias. Measurement is conducted at 200 K.

Fig. 4. Effect of the electric field on spectra and wavefunction overlaps.

a Relative change of the band-edge energy (black) and electron-hole overlap (blue) as a function of the applied electric field. b (resp c, d) Energy profile (black) as a well as electron (ψ_e, blue) and hole (ψ_h, red) wavefunctions under an electric field (F) of 12 kV cm⁻¹, (resp 1300 kV cm⁻¹ and 7000 kV cm⁻¹). The vacuum energy is set to 0 eV. The wavefunctions are shifted to their eigen energy.

Fig. 5. Optical properties of the array of HgTe nanocrystals (NCs) deposited on a nanotrench electrode.

a Absorption map at 2.5 µm of a 300-nm film of NCs on a gold mirror. b. (resp. c) Absorption map at 2.5 µm of a 300-nm film of NCs on a 40-nm nanotrench in TM (resp TE) polarization. TM (resp. TE) polarization correspond to the magnetic (resp. electric) field along the trench. The axes are in nm. d Photocurrent spectrum for a HgTe NC array deposited on a 40-nm nanotrench electrode with and without a polarizer. The bias is 1 V and measurement is conducted at room temperature. e Current as a function of time for a HgTe NC array deposited on a 40-nm nanotrench electrode as the illumination is turned on and off. Pink and green dashed lines correspond to an exponential fit of the rise and the decay as the light is turned on and off. Measured rise and decay time are 22 and 24 µs, respectively. The illumination is ensured by a 1.55 µm laser diode delivering 2 mW of optical power and modulated at 1 kHz. The bias is 1 V.