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 III-V based detector chips with narrow bandgap thin films compatible with 8- and 12-inch full-wafer processing.
Traditionally, low energy band gap III-V semiconductor layers are used to create shortwave infrared (SWIR) sensor arrays. Because they need to be grown by high-temperature epitaxy (typically on wafer sizes of only 3-4 inches), the starting material has orders-of-magnitude higher cost compared to standard silicon wafers. Moreover, flip-chip hybridization (usually die-to-die) is required, which further increases the cost. The most compelling solution from a cost perspective is to monolithically integrate an absorber layer directly on top of the silicon-based readout integrated circuit (ROIC).
Image sensors with a thin film photodetector active layer integrated on 300mm CMOS wafers have already been demonstrated with solution-processed Pb-based Colloidal Quantum Dot (CQD) films. The goal of this thesis subject is to develop a new photodiode architecture based on a heterojunction between silicon and III-V QD materials, working in the SWIR wavelength range from 1 µm to 2.5 µm. This heterojunction is expected to lead to image sensors with lower noise and better reliability compared to traditional above-integrated-circuit integration for QD-based photodiodes.
This PhD project will employ a stepwise approach to integrate a photodiode stack on top of a Si-based ROIC. III-V CQD materials with tunable absorption from 1-2.5 μm will be prepared by collaborators or purchased commercially.
The first phase will involve developing basic test devices to understand the band alignment between Si and CQDs. The heterojunction performance will then be optimized in a second phase, by improving the passivation of both the Si and the QD side of the junction, via chemical modification, to improve photodiode performance metrics. The reliability of the heterojunction under stress will also be assessed and optimized.
The PhD project will be dedicated to the technological fabrication of the devices and the opto-electronic characterizations and optimizations of the photodiodes in close collaboration with IRIG colleagues and STMicroelectronics.

Expected profile
A Master 2 in (electrical engineering, nanoscience or materials chemistry) with significant experimentation and lab work as well as lots of teamwork.

Supervision:​
Peter Reiss from CEA-IRIG. Jonathan Steckel and Andras Pattantyus-Abraham 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 above-integrated-circuit integration approach and the presence of regulated elements like Pb. A silicon-QD heterojunction involving Pb-free III-V QDs is needed to enable new applications through easier integration, reduced noise and better reliability. The basic properties of Si/Pb-free QD heterojunctions will be investigated and functioning photodiodes will be fabricated. The interface passivation will be improved through chemical modification of the Si and the QD, such that photodiode performance and reliability is enhanced.