Imaging in the infrared wavelength range has been essential in scientific, military and surveillance applications. Currently, it could become a crucial enabler of new industries such as autonomous mobility, augmented reality, and biometrics. Extensive deployment of infrared cameras is however prevented by high manufacturing costs. One way to enable monolithic integration is by replacing expensive, small-scale detector chips with narrow bandgap thin films compatible with full-wafer processing. Traditionally, low energy band gap III-V semiconductor layers are used to create shortwave infrared (SWIR) sensor arrays. In this project, we target to replace epitaxially grown materials with colloidal quantum dots (QDs) of III-V semiconductors (InAs, InSb). In addition to the advantage of low-cost and scalable production inherent to colloidal synthesis, III-V QDs have several appealing features for SWIR imaging, such as a high absorption coefficient, size-tunable absorption onset, and solution processability. Due to the narrow band gap of InAs and InSb, it is possible to cover a large spectral range in the SWIR region from around 1 µm to 2 µm. Nonetheless, despite considerable progress in the chemical synthesis of these types of QD materials, several challenges remain to be addressed before their industrial application in optoelectronics. In particular, their surface chemistry needs to be optimized to achieve efficient passivation of electronic defects and to enhance their stability under external stress such as irradiation and elevated temperature.
This PhD project will employ a stepwise approach to develop InAs and InSb QDs of precisely controlled absorption onset, low size distribution and optimised surface passivation enabling stable performance when integrated into imagers working at elevated temperatures up to 150°C.
In the first stage, the colloidal synthesis of InAs, InSb and In(As,Sb) alloy QDs will be optimized to be able to cover the target wavelength range of 1 – 2 µm and obtain narrow size distributions <10%. The method that will be employed to impart thermal stability of solid-state QD films for devices will be the use of surface passivation. One method to be explored is by growing one or more shells of appropriate semiconductor material on the core QDs. The second method will be novel ligand development and implementation in solution and/or the solid-state on its own and combined with shell growth. Photoluminescence spectroscopy is a suitable technique to monitor successful shell growth and will be complemented with other optical and structural studies. The stability of the optical properties under irradiation and thermal stress will be assessed as a function of the nature and thickness of the semiconductor and/or molecular shells and the best candidates will be used for integration in photodiodes. Surface ligand exchange with suitable molecules may be employed and the preparation of QD thin films on appropriate charge transport layers will be developed. The device performance and stability will be monitored and further optimization of the core/shell structure will be implemented based on these results.
The optoelectronic characterizations and optimizations of the photodiodes will be performed in close collaboration with colleagues from CEA-LETI and STMicroelectronics.

Expected profile
A Master 2 in organic chemistry, nanoscience or materials chemistry with significant experimentation and lab work as well as the ability and will to work in an interdisciplinary research team.

Supervision:
​Peter Reiss from CEA-IRIG, Jonathan Steckel and Ajay Singh ​from ST.

Summary:
Colloidal Quantum Dots (QD) are novel building blocks for the fabrication of image sensors with high-performance tunable light detection in the SWIR wavelength range but are currently limited by the low thermal stability under operation and the presence of regulated elements like Pb. In this PhD project, we develop novel building blocks for SWIR optoelectronics based on III-V semiconductor QDs (InAs, InSb). Their synthesis will be optimized to cover a large spectral range of absorption from 1 to 2 µm while maintaining a narrow size distribution. A library of core/shell and core/shell/shell structures will be synthesized by overgrowing the core III-V QDs with appropriate semiconductors and molecules and their stability under irradiation and thermal stress will be assessed. Ultimately the best candidates will be integrated into SWIR photodetector devices and their performance evaluated. ​