Monday, April 18, 2022

Quantum Dot Photodiodes for SWIR Cameras

A research team from Ghent University in Belgium  has published an article titled "Colloidal III–V Quantum Dot Photodiodes for Short-Wave Infrared Photodetection".

Abstract: Short-wave infrared (SWIR) image sensors based on colloidal quantum dots (QDs) are characterized by low cost, small pixel pitch, and spectral tunability. Adoption of QD-SWIR imagers is, however, hampered by a reliance on restricted elements such as Pb and Hg. Here, QD photodiodes, the central element of a QD image sensor, made from non-restricted In(As,P) QDs that operate at wavelengths up to 1400 nm are demonstrated. Three different In(As,P) QD batches that are made using a scalable, one-size-one-batch reaction and feature a band-edge absorption at 1140, 1270, and 1400 nm are implemented. These QDs are post-processed to obtain In(As,P) nanocolloids stabilized by short-chain ligands, from which semiconducting films of n-In(As,P) are formed through spincoating. For all three sizes, sandwiching such films between p-NiO as the hole transport layer and Nb:TiO2 as the electron transport layer yields In(As,P) QD photodiodes that exhibit best internal quantum efficiencies at the QD band gap of 46±5% and are sensitive for SWIR light up to 1400 nm.

a) Normalized absorbance spectra of the three QD batches (red) measured in tetrachloroethylene (TCE) before and (blue) dimethylformamide (DMF) after phase transfer. For each set of spectra, the vertical line indicates the maximum absorbance of the band-edge transition at 1140, 1270, and 1400 nm, respectively. The spectra after ligand exchange have been offset for clarity. b) (top) Photograph of the extraction of QDs from (top phase) octane to (bottom phase) DMF and (bottom) representation of the phase transfer chemistry when using 3-mercapto-1,2-propanediol (MPD) and butylamine (n-BuNH2) as phase transfer agents, indicating several reactions that bring about the replacement of the as-synthesized ligand shell of chloride and oleylamine by deprotonated MPD and n-BuNH2. c) X-ray photoelectron spectra (red) before and (blue) after ligand exchange in different energy ranges, showing the disappearance of chloride, the appearance of sulfide and the preservation of the In:As ratio after ligand exchange.

a) Schematic of the In(As, P) QD field effect transistor, consisting of a spincoated film of ligand exchanged QDs on top of cross-fingered source and drain electrodes and separated from the gate electrode by a thermally grown oxide. b) Transfer characteristics of the field effect transistor at a source–drain voltage of 5 V.

a) (top) Energy level diagram of the In(As,P) QDPD stack used here. The diagram was constructed by combining UPS results for the 1140 In(As,P) QD film and literature data for the contact materials.[39-43] (bottom) Schematic of the QDPD stack. b–d) Dark and photocurrent densities under white-light illumination of In(As,P) QDPDs for specific absorber layers as indicated. e) Photocurrent density as a function of white light illumination power in log–log scale. The reference power is 114.7 mW cm^−2.

a–c) External quantum efficiency spectra for the different In(As,P) QDPDs as indicated, recorded at a reverse bias of −2, −3, and −4 V. The absorbance spectrum of the corresponding In(As,P) QD batch is added in each graph for comparison. d) Normalized transient photocurrent response of the different In(As,P) QDPDs following a 400 µs step illumination. Rise and fall times have been indicated by the dominant fast time constant obtained from a multi-exponential fit of the transient.

Full article (open access):

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