COQUADOT

Colloidal quantum dot infrared photodetectors

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 Obiettivo del progetto (Objective)

'Numerous applications extending from military (night vision, surveillance, airborne tracking) to civilian (pharmaceutical and food industry spectroscopy, environmental monitoring, machine and automotive vision, biomedical imaging) are based on infrared photodectors and imaging arrays that detect photons in the Mid Infrared (MWIR) 3 – 5 um and Long wavelength Infrared (LWIR) from 8 - 12 um. Although CdHgTe semiconductor compound offered efficient coverage of the infrared spectrum by varying the stoichiometry the exploitation of this material towards the fabrication of imaging sensors was proved very challenging due to the costly complex growth processes as well as due to inherent spatial non-uniformity issues. Dramatic progress has been made recently in the field of epitaxially grown quantum dots that offer significant advantages of controlled growth as well as normal incidence sensitivity and dramatically lower dark current densities. The main disadvantage of this new approach however lies on the high cost and complexity method of molecular beam epitaxy required to grow the quantum dots as well as the incompatibility with monolithic integration to silicon (CMOS) read-out circuitry. The advent of colloidal quantum dots has been established during the last decade, where the quantum dots can be synthesized in solution phase. Following the bottom-up approach, fabrication of thin films can then take place using room-temperature, low-cost, well-established techniques such as spraycasting and spincoating enabling large-scale manufacturing directly integrated onto CMOS platforms. In this proposal we combine the unique physical properties of quantum dots with the most desired chemical and processing properties of solution-processed materials to develop initially an infrared photodetector and subsequently an infrared imaging array system with high sensitivity and cost that is estimated to be two orders of magnitude lower than current approaches.'

Introduzione (Teaser)

Systems that detect electromagnetic (EM) radiation in the infrared (IR) region are invaluable in many fields. Novel IR photodetectors with significantly enhanced performance should thus have important benefits for EU manufacturers and the economy.

Descrizione progetto (Article)

All objects emit and absorb IR radiation, 'visible' to people as heat. From military surveillance to chemical monitoring, biomedical imaging and machine vision, IR technology is used for a myriad of applications. Enhancing the sensitivity while decreasing the cost will ensure a competitive position for the EU in a large global market sector.

The EU-funded project 'Colloidal quantum dot infrared photodetectors' (COQUADOT) set out to meet the challenge. Its goal was to combine the advanced properties of solution-processed colloidal quantum dots with low-cost, large-scale fabrication methods compatible with monolithic thin-film silicon technology.

Quantum dots are tiny nanocrystals of semiconducting material. COQUADOT scientists explored routes to high-sensitivity optical sensing in the visible and IR regions of the EM spectrum exploiting quantum dots and plasmonics. Plasmonics is a relatively new field. It studies the enhancement in optical near-fields of sub-wavelength when the EM field interacts with conductive electrons at a metal interface or in metallic nanostructures.

Integration of quantum dots with plasmonic structures including novel nano-focusing architectures such as bull's eye gratings offered significant performance enhancement. Novel hybrid photodetectors coupled colloidal quantum dots with graphene and other 2D semiconductors. These led to unprecedented performance (responsivities and sensitivities). The innovative photodetectors were further developed to operate in both the IR and visible regions.

A second line of inquiry investigated a novel approach to overcome the band gap limitations of quantum dot semiconductors. Hot carrier plasmonic-based photodetectors take advantage of energetic electrons generated by the relaxation of plasmonic resonance. The latter is an excitation, a surface phenomenon at certain material interfaces in which photostimulation results in a resonant oscillation of conduction electrons.

The metallic nanostructure was designed such that the spectral responsivity of the photodetector is determined by the geometry and not by the band gap of the semiconductor. The route is expected to lead to new ways of optical sensing and low-cost IR photodetectors.

COQUADOT outcomes make an important contribution to a strong and growing field. Enhanced performance of IR photodetectors will not only enhance penetration of current markets but open the door to new high-tech ones as well.