Thesis Colloquium: Controlling Defects for Infrared Photodetection

Infrared photodetectors have applications in a variety of fields including Agriculture, Industry, Defense, Communications and Medicine.  As of now, infrared photodetectors based on Bolometers, PbS, HgCdTe, InGaAs, and InSb are most commonly in use. Each of these technologies however has limitations that motivate the discovery of more optimal infrared photodetector materials.

Emerging methods of infrared photodetection thus(1) aim to use colloidal PbS quantum dots, Te, Black Phosphorous, 2D metal chalcogenides and colloidal HgTe based photodetectors. However, each of these materials also has intrinsic limitations that prevent these from surpassing existing technology. For example, black phosphorous is an intrinsically unstable polymorph that degrades over time. Similarly, in case of 2D chalcogenides such as PtSe2, , the band gap decreases rapidly as more layers are added so that thicker films are  metallic. This limits the maximum amount of light that can be absorbed by these materials. Photoresponse in the case of tellurium is similarly limited to extremely thin flakes. Finally, for all existing colloidal quantum dot materials, disorder places limits on carrier collection from thicker films, and thus films thinner than the optical penetration depth are generally employed for  photodetector devices. Such films absorb only a fraction of the incident light.

My thesis discusses approaches towards addressing these issues through controlling and passivating defects in photodetector materials. As an example of control over interfacial defects, we demonstrate epitaxial growth of a heterojunction(2) between two dis-similarly structured semiconductors. The potential use and limitations of this system for photodetection is discussed. We then turn to study a more established material, tellurium, where defect related issues have nonetheless prevented the emergence of a scalable route to high performance photodetectors. We identify the deleterious influence of chain end defects in the material and show that the use oxide to passivate tellurium nanoparticles leads to photoresponse comparable to state-of-the-art tellurium based photodetectors(3) even from an ensemble. While tellurium has significant applications for room temperature infrared photodetectors due to its comparatively wider ~4 micron gap, a narrower band gap material still needs to be identified to allow for detection over a wider range of mid-infrared wavelengths. We therefore worked towards the development of new semiconductors to fill this gap. To this end, we develop a novel chalcogen compound that shows promising photodetector performance (Specific detectivity at 1550 nm ~10^8 Jones) enabling us to perform thermal imaging near room temperature. Further, we also show the possibility of using such a material for other peripheral applications such as deformable electronics.

(1)       Wang, Z.; Gu, Y.; Li, X.; Liu, Y.; Liu, F.; Wu, W. Recent Progress of Quantum Dot Infrared Photodetectors. Adv. Opt. Mater. 2023, 11, 2300970. https://doi.org/10.1002/adom.202300970.