3 research outputs found
Band Edge Dynamics and Multiexciton Generation in Narrow Band Gap HgTe Nanocrystals
Mercury
chalcogenide nanocrystals and especially HgTe appear as an interesting
platform for the design of low cost mid-infrared (mid-IR) detectors.
Nevertheless, their electronic structure and transport properties
remain poorly understood, and some critical aspects such as the carrier
relaxation dynamics at the band edge have been pushed under the rug.
Some of the previous reports on dynamics are setup-limited, and all
of them have been obtained using photon energy far above the band
edge. These observations raise two main questions: (i) what are the
carrier dynamics at the band edge and (ii) should we expect some additional
effect (multiexciton generation (MEG)) as such narrow band gap materials
are excited far above the band edge? To answer these questions, we
developed a high-bandwidth setup that allows us to understand and
compare the carrier dynamics resonantly pumped at the band edge in
the mid-IR and far above the band edge. We demonstrate that fast (>50
MHz) photoresponse can be obtained even in the mid-IR and that MEG
is occurring in HgTe nanocrystal arrays with a threshold around 3
times the band edge energy. Furthermore, the photoresponse can be
effectively tuned in magnitude and sign using a phototransistor configuration
Intraband Mid-Infrared Transitions in Ag<sub>2</sub>Se Nanocrystals: Potential and Limitations for Hg-Free Low-Cost Photodetection
Infrared photodetection based on
colloidal nanoparticles is a promising
path toward low-cost devices. However, mid-infrared absorption relies
on interband transitions in heavy metal-based materials, which is
a major flaw for the development toward mass market. In the quest
of mercury-free infrared active colloidal materials, we here investigate
Ag<sub>2</sub>Se nanoparticles presenting intraband transition between
3 and 15 μm. With photoemission and infrared spectroscopy, we
are able to propose an electronic spectrum of the material in the
absolute energy scale. We also investigate the origin of doping and
demonstrate that it results from a cation excess under the Ag<sup>+</sup> form. We demonstrate photoconduction into this material under
resonant excitation of the intraband transition. However, performances
are currently quite weak with (i) a slow photoresponse (several seconds)
and (ii) some electrochemical instabilities at room temperature
Short Wave Infrared Devices Based on HgTe Nanocrystals with Air Stable Performances
Colloidal
quantum dots (CQDs) are candidates of interest for the
design of low cost IR detector, especially in the short wave infrared
(SWIR; 0.8–3 μm), where the vicinity of the visible range
makes the high cost of available technologies even more striking.
HgTe nanocrystals are among the most promising candidates to address
SWIR since their spectrum can be tuned all over this range while demonstrating
photoconductive properties. However, several main issues have been
swept under the rug, which prevents further development of active
materials and devices. Here we address two central questions, which
are (i) the stability of the device under ambient air condition and
(ii) the reduction of dark current. Encapsulation of HgTe CQDs is
difficult because of their extreme sensitivity to annealing, we nevertheless
demonstrate an efficient encapsulation method based on a combination
of O<sub>2</sub> and H<sub>2</sub>O repellant layers leading to stability
over >100 days. Finally, we demonstrate that the dark current reduction
can be obtained by switching from a photoconductive geometry to a
photovoltaic (PV) device, which is fabricated using solution and low
temperature based approach. We demonstrate fast photoresponse (>10
kHz) and detectivity enhancement by 1 order of magnitude in the PV
configuration at room temperature. These results pave the way for
narrow bandgap CQD based cost-effective optoelectronic devices in
developing next generation SWIR photonic systems