Infrared Photodetectors Based on HgTe Quantum Dots: Effect of Surface Chemistry and Coupling with Plasmonic Structures on Their Performance

Abstract

Mercury telluride (HgTe) colloidal quantum dots (CQDs) are distinguished as the only material capable of continuous tunability across the full infrared spectrum, from the visible into the terahertz ranges (1–100 μm). This remarkable feature originates from the unique band structure of HgTe CQDs, having a zero bandgap as bulk material. Alongside its intrinsic air stability and efficient charge transport, this property of HgTe CQDs has driven comprehensive research and considerable technological progress over the last 25 years. As a result, HgTe CQDs now hold the leading role as an active material for advanced photonic and optoelectronic systems operating in the short- and mid-infrared spectral ranges (SWIR and MWIR, respectively). This thesis critically surveys the state-of-the-art in HgTe CQDs, including advances in synthesis, electronic structure modelling, and charge carrier dynamics, in its first chapter. Special attention is given to the integration of HgTe CQDs into functional photonic architectures. The second chapter includes detailed description of the experimental methods and characterization techniques.

The environment surrounding CQDs determines their charge carrier dynamics, influencing both their optical properties and the performance of devices based thereon. Typically for many CQDs, they are stabilized in solutions or thin films with long-chain organic ligands, favoring colloidal stability and high emission efficiency. Conversely, device fabrication requires ligand exchange to short-chain molecules to promote electrical conductivity and close packing of the CQDs. However, this exchange process can introduce surface defects and traps, which in turn enhance non-radiative processes and limit the device performance. Thus, the third chapter of this thesis reveals a systematic study of charge carrier dynamics in HgTe CCQDs across different environments: colloidal solutions with long-chain ligands, thin films derived from these solutions, and films processed with short-chain ligands optimized for photodetection application. By combination of advanced ultrafast spectroscopy and electrical transport measurements, we found an extremely low threshold for Auger non-radiative relaxation when transitioning to short-chain ligands, which inevitably leads to close packing of the CQDs and emergence of some traps on their surface. The interplay among the Auger process, the so-called phonon bottleneck and excitonic recombination is shown to substantially impact the device performance, with responsivity dropping by several orders of magnitude as excitation flux increased at room temperature.

To partially address those issues, we advanced the synthetic procedure enabling direct synthesis of colloidally stable HgTe CQDs capped with short-chain conductive ligands. The approach described in the fourth chapter eliminates the need for post-synthetic ligand exchange, and thereby the CQD surface remains intact. These improvements resulted in an order-of-magnitude reduction in the surface trap density as compared to HgTe CQDs subjected to ligand exchange. Additionally, modulation of the cation-to-anion ratio and incorporation of cyclic thiol ligands enabled controlled n- and p-type self-doping, finely tuning the Fermi level of the CQD films. These optimizations yielded HgTe CQD-based photodiodes with outstanding performance at telecom wavelength.

Concurrently to the alteration of surface chemistry of HgTe CQDs, we also focused on alternative ways to enhance their emission performance. Recent developments in the field of nanophotonics have introduced periodic resonant nanostructures that support bound states in the continuum (BICs), which theoretically can eliminate radiative losses and yield infinite quality factors (Q). Imperfections turn these into quasi-BIC (qBIC) modes, but even so, their high Q-factors and strong local field enhancements are highly advantageous. Chapter five introduces plasmonic metasurfaces composed of regularly patterned nanobumps using direct femtosecond laser writing on thin gold films, which is a scalable and versatile approach. These metasurfaces were coupled with HgTe CQDs, resulting in up to twelve-fold photoluminescence enhancement in the 900–1700 nm window under optimal conditions. By adjusting the metasurface geometry, selective spectral enhancement was achieved, in terms of both emission shaping and its directionality. Taking advantage from simplicity and scalability of the laser patterning, we integrated these plasmonic metasurfaces into field-effect transistor (FET) devices, by using HgTe CQDs as the active layer and imprinting qBIC arrays over gold electrodes. These devices operated at 200 K and 5 V bias and achieved high specific detectivities of ~8.7 × 10¹¹ Jones in the qBIC region while exhibiting rapid response times, highlighting their potential for SWIR photodetectors.

In summary, this thesis presents a comprehensive study of HgTe CQD synthesis, surface and electronic structure engineering, investigation of charge carrier dynamics and final integration of this promising IR material into photonic devices. Employing large-scalable fabrication methods and refining both material properties and device architectures, our work established practical pathways towards high-performance HgTe CQD-based emitters and photodetectors.

Date of Award	25 Aug 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Andrey ROGACH (Supervisor)