Investigate electron and hole transport layers for high temperature stability III-V quantum dot photodiode devices

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 currently exhibit undesired degradation under high thermal stress. Thermal degradation can be significantly improved by optimizing the device materials (contacts, hole transport layer (HTL), electron transport layer (ETL) and encapsulation), film thicknesses, and deposition processes used to make quantum film (QF) photodiode devices. As such, a detailed investigation into many different HTL, ETL and top electrode materials will be pursued to find the best candidates to overcome the current limitations. Materials selection and deposition processes for these layers will be chosen and studied among a variety of existing materials developed at LETI. QD films with tunable absorption from 1-2.5 µm will be prepared by STMicroelectronics and CEA-IRIG in collaboration with other partners. The QD patterning step for the fabrication of the devices and the electro-optical testing will be performed internally at LETI with support from STMicroelectronics.

Integrated High Speed optical Link System

Nowadays, billions of digital data are exchanged in the world. Hence, Digital data transfer is a key technical issue within High Performance Computing (HPC) systems. They have to be very fast (faster and faster) and low power (lower and lower). Communication through Optic Fibers, well known for long distance communications, is also well suited for short distance (from centimeter to meter). In this case, the targeted system contains massively parallel optical links based on optical fiber bundle, an array of micro-Leds to send light data to optical fibers, an array of photodetectors to receive light data from the optical links and an embedded processing system (Data modulation, parallelization) to share the input digital data into this massive parallel system.

This study will be based on under development advanced technologies from CEA-Leti (3D stacking Circuits, new micro-Led and photo-diode devices). The PhD student will study the integrated electronic architecture of a such system within this technological environment, which implies compactness, massively parallelism and very promising in terms of performance gain related to the state of the art. High level modelling, pre design, studies on signal integrity and complex modulation will show us the capabilities of such a future optical communication link.

Based in Grenoble, the PhD will be supported in his research work by two teams of experts in integrated circuit design and photonics.

Research environment :

Study of new photodiode architecture for IR imagers

CEA-LETI play a leading role in the development of the HgCdTe material, which now performs to such an extent that it is on board the James Webb Space Telescope (JWST), enabling the observation and study of deep space with a precision unrivalled to date.
However, we believe that it is still possible to take an important step forward in terms of detection performance.
We propose to work on a new photodiode architecture that could further reduce the dark current (and therefore reduce noise and gain sensitivity at low photon flux).
Your role in this thesis will be to contribute to the development of the ultimate photodiode for ultra-high-performance IR detection, and to characterize and simulate the HgCdTe photodiodes manufactured on our technology platform.
This experimental and theoretical work will enable us to propose a physical model of objects manufactured at CEA-Leti, and to determine their sensitivity to technological parameters.
You have a Master's degree in optoelectronics or semiconductor materials physics, and a passion for applied research.
The main technical skills required are: physics of semiconductor components, optoelectronics, data processing, numerical simulations, an interest in experimental work to carry out characterizations, and theoretical work to carry out numerical simulations.

Development of a fully passive plenoptic thermal imaging system

Low-resolution plenoptics is becoming widespread in visible imagers for applications such as autofocus, image post-processing and sometimes depth estimation. Its principle is based on the association of three main constitutive elements, a pixels-sized microlens array, a focal plane array of detectors and reconstruction algorithms.
Through this thesis, we would like to evaluate the possibility of achieving such a plenoptic function in the infrared range for uncooled detector technologies (micro-bolometer)

Within the Optics and PhoTonics Department (DOPT), the Thermal and THz Imaging Laboratory (LI2T) designs, develops, produces and characterizes thermal infrared imager technologies, which are then transferred to an industrial partner. As a doctoral student, your role will consist of:
- Establish preliminary specifications for micro lenses adapted with our detectors and with the application of autofocusing.
- Design and simulate the behavior of this micro-optics and propose original designs in refractive solution or in meta-surface
- Manufacture these micro-optics after evaluating the feasibility of these designs in partnership with the people in charge of manufacturing
- Implement an existing reconstruction algorithm that will be identified in the literature
- Characterize the micro-optics on a dedicated bench
To carry out these missions, you will be integrated into the LI2T laboratory where you will be able to interact with different people in order to familiarize yourself with micro bolometer technologies and where you will have access to the calculation resources of CEA Leti.

High speed III-V on Silicon modulators for optical communications and sensing

Worldwide demand for digital interconnections is driven both by increasing data exchanges and by an increasing number of human and non-human users. Optical communications are commonly used to answer this demand, exploiting the network of optical fibers deployed to interconnect data centers and users alike. The data streams across this planet-scale infrastructure are routed at high-speed by multiple electronic-photonic transducer nodes, located everywhere from inside data centers to inside users’ homes (FTTH). Each node in turn requires multiple high performance optical transceivers, to transmit and receive the information along the network.
Photonic Integrated Circuits (PIC) have been a promising technology for low-power & high-performance optical functions in low-volume systems.The availability of relatively low-cost optical telecommunication devices, such as telecom lasers and optical modulators, has gained the interest of other fields of science and technology, working with infrared light. Photonic circuits can be tuned to function at wavelengths centered on spectral absorption rays of gases of interest (H2O, CO, CO2), for sensing purposes.

III-V Lab and CEA-LETI recently demonstrated selective area growth (SAG) of III-V materials on Silicon, a key process enabling the fabrication of different frequency-tailored quantum wells simultaneously. However, to address future needs, the performances of actual III-V on Silicon modulators need to be improved.
This thesis aims at the development of advanced modulators, in order to demonstrate state of the art transceivers, which could be implemented in the future using the previously developed SAG technology.
Such devices based on III-V waveguides on silicon will be modelled, designed, fabricated and tested towards high-speed light phase and intensity modulation.
This work will require opto-electronic optimizations of the InP-based III-V stack and coupling to the SOI waveguides in order to reach low losses, low absorption, and a higher phaseshift per volt, for high bandwidth capabilities reaching 100 GHz.
High-speed phase modulators will be demonstrated in coherent communication systems, targeting beyond 16QAM modulation schemes for 400 Gbps/lane.
High-precision frequency modulation will be demonstrated in single-sideband modulation (oSSBM), for 100 GHz frequency shifting in sensing applications.

The PhD will be based at CEA-LETI (Grenoble), within the Silicon Photonics Integration Laboratory, in the framework of the R&D Photonics Program existing between CEA and III-V Lab that will co-supervise the thesis work.

Inscription of Optical Waveguide in Silica or Saphirre Optical Fbers and Characterization at High Temperature

Fiber Bragg Gratings are structures, photo-inscribed with femtosecond laser, inside the fiber core and can be used as band pass optical filters (centered around the Bragg wavelength). Bragg wavelengths are easily multiplexed and they give to us the necessary information. Silica-based fiber Bragg gratings are point sensors and can measure temperature up to 1200°C. For higher temperature, sapphire based optical fiber are used, since they can withstand temperature up to 2000°C. However, the sapphire optical fiber is coreless, which leads to an extreme multimodal behavior. Consequently, the measure is less precise and the signal-to-noise ratio is low, compare to a classical silica-based grating. Moreover, each modification of the fiber surface, change the grating spectra.
The thesis objective is the creation of an optical waveguide inside the sapphire fiber, which will leads to less propagation modes inside the fiber, in order to obtain new perspectives for the monitoring in high temperature environments (airplane engines, nuclear reactors, …), which is one of the missions of the DRT/LIST-DIN. To obtain this result, the photo-inscription of a cladding is necessary: the cladding will be ring shape and the internal diameter – i.e. the core – will be few tenths of micrometers. Other techniques are also investigated, such as ion implantation, to create an amorphous sapphire cladding. Then these new structures will be characterized up to 2000°C and under high dynamic pressure (> 10 GPa).

New architectures in inverse geometry for spectral X-ray imaging

Emerging technologies in the field of X-ray sources and detectors open up possibilities for envisioning new breakthrough systems in 3D imaging. In conventional tomography, a large-area detector captures images of an object exposed to X-rays from a point source. The latest generations of medical scanners also incorporate spectrometric semiconductor detectors, providing a real improvement in image quality.
The proposed thesis aims to shift the paradigm by designing a system that combines numerous distributed X-ray sources with a small-sized spectrometric detector. This inverse geometry is innovative in terms of system architectures, allowing relaxation of constraints on sensor dimensions and reduction of certain artifacts.
The thesis work will revolve around the design and simulation of new systems in inverse geometry, along with the development of associated reconstruction algorithms. These algorithms, based on proximal methods and capable of integrating neural networks, must leverage the rich information provided by the spectrometric detector under conditions of sparse acquisition. The student will utilize simulation and reconstruction tools developed within the laboratory and will also have access to experimental resources for validating the developments. Working within a multidisciplinary laboratory with extensive experience in spectrometric detector and X-ray system design, the student will engage in exchanges with external teams, including radiologists, to incorporate a final need into the research.

Enabling patterning fidelity analysis over millimetric distances for curvilinear mask data-preparation : application to photonics devices.

Integrated silicon photonics, which consists of using manufacturing processes from the microelectronics industry to produce photonic components, is considered a critical future technology for very high-speed communications and computing applications.
The creation of silicon photonics devices requires the manipulation of fully curved designs (so-called non-Manhattan). This gives rise to numerous challenges during their manufacturing, and particularly during the design stage of advanced photolithography masks. In order to determine the optimal masks, optical effects compensation (OPC) algorithms are systematically applied. The latter are particularly difficult to implement in the particular case of curvilinear patterns.
The accuracy with which OPC models are able to anticipate pattern printing performance can be assessed using CD-SEM on specific, simple, small structures in a single orientation. However, Photonic devices (for example, waveguides) are continuous, up to a few millimeters long, and cover all orientations in space. A broad and precise metrology of such objects does not exist, making it impossible for OPC engineers to diagnose the quality of the devices produced on the product.
The objective of the thesis is to develop a method for precise, large-scale dimensional measurement of Photonic structures. In particular, we will seek to implement solutions for stitching SEM images and extracting contours, with the development of dedicated metrics. The thesis proposes in particular:
- the study of metrological and scripted solutions to enable characterization by CD-SEM on a large scale, typically by combining several images.

- the implementation of extraction of contours of curved patterns on recombined images.

- the development and implementation of innovative 2D metrics to allow the measurement of curved objects, then their comparison with each other (or with a reference).

- the inclusion of real large-scale contours in optical simulation software (Lumerical, FDTD) to characterize the real performance of the devices.

The thesis will take place for 3 years between the STMicroelectronics site (Crolles) and that of CEA-LETI (Grenoble), in a context of strong collaboration between the teams of the two organizations.

The doctoral student will have access to clean rooms and state-of-the-art industrial and/or research equipment, as well as commercial reference software. You will benefit from all the technical expertise of the supervisory teams at STMicroelectronics and CEA-LETI in photolithography, metrology, image processing and applied IT development (Python).

Etch and integration of phase change materials for reconfigurable photonic

Chalcogenide glasses are materials of interest for many applications: in phase-change memories, for example optical storage (CD-RW, DVD-RAM, Blu-ray Disks) or more recently Storage Class Memory, as a selector in 3D architecture resistive memories (OTS selector) or as an active medium for non-linear optics and reconfigurable photonics. In the latter case, the production of metasurfaces with controllable optical refractive indices and the development of photonic actuators is a subject of renewed interest, given the unique optical properties of these materials.
In this field, CEA-LETI is one of the major international players in the development of thin films of chalcogenide glasses and phase-change materials, as well as in the characterization and understanding of their unusual physical properties. Today, although these materials are well mastered at industrial level for memory applications, it is nevertheless necessary to work on integrating these innovative, high-performance materials into small-scale structures while preserving their unique physical properties. Initial integration studies carried out during a previous thesis showed that the etching and stripping process steps were far from being optimized, even though they are critical if the properties of the material are not to be degraded during the production of photonic structures. Although essential for process control, the etching mechanisms of chalcogenide glasses are relatively poorly described in the literature, with the exception of Ge2Sb2Te5 used in phase-change memories.
This theme is therefore innovative and has real added value. The aim of the thesis is therefore to study and understand the etching mechanisms of GeSbSeTe-based chalcogenide glasses in order to control the profile and size of the transferred structures. The work to be carried out is highly experimental and will take place mainly in the LETI's 300 mm clean room. The candidate will have access to state-of-the-art chalcogenide materials thin films, an industrially designed plasma etching reactor and a wide range of characterization facilities. This thesis will be carried out in collaboration with the LETI advanced materials deposition service and the optoelectronic devices applications department (DOPT).

Contribution of metal-semiconductor interfaces to the operation of the latest generation of infrared photodiodes

This thesis concerns the field of cooled infrared detectors used for astrophysical applications. In this field, the DPFT/SMTP (Infrared Laboratory) of CEA-LETI-MINATEC works closely with Lynred, a world leader in the production of high-performance infrared focal planes. In this context, the infrared laboratory is developing new generations of infrared detectors to meet the needs of future products.
One of the current development axes concerns the quality of the p-type semiconductor metal interface. These developments are driven by the increase in the operating temperature of the detectors, as well as by the very strong performance requirements for space applications.
The challenge of this thesis is to contribute to a better understanding of the chemical species present at the interface of interest as a function of different surface treatment types and to link them to the electrical properties of the contact made.
The candidate will join the infrared laboratory, which includes the entire detector production process. He/she will produce these samples using the technological means available in the LETI clean room, in collaboration with experts in the field. He/she will also have access to the necessary characterization tools (SIMS, XPS, AFM…) available on the nano-characterization platform (PFNC) or in the CEA clean room. Finally, he/she will be involved in the electro-optical characterization of the material, in collaboration with the Cooled Infrared Imaging Laboratory (LIR), which specializes in fine material characterization.