Study of co-integrated TeraHertz source arrays in Silicon and III-V photonics technology
TeraHertz (THz) radiation is of growing interest for imaging and spectroscopy in various application fields such as safety, health, environment and industrial control, since in this frequency range many dielectric materials are transparent and many molecules present unique spectral signatures for their identification. However, the limitations of the current sources, required for this active Imaging, hinder its use over long distances or through thick materials.
This thesis proposes to develop a widely tunable THz power source in the form of an array of photoconductor sources excited by photomixing two infrared lasers. The aim is to integrate several dozen or even hundreds of sources on a single component, by co-integrating components made of III-V materials on a silicon photonic substrate, in order to offer an innovative solution to power and tunability problems.
This thesis work, shared between the Bordeaux and Grenoble sites, is positioned in fields with strong industrial potential: integrated photonics and silicon integration technologies. Several items will be addressed , including the study of the architecture of the complete photonic system using simulation tools, the choice of structures and materials, technological development on CEA LETI platforms, and performance characterization. A proof-of-concept with a small number of sources is planned, followed by the design of a large-scale matrix system.
The project represents a major technological challenge, but its success would pave the way for a significant improvement in the penetration capacity of THz radiation, and would also contribute to the broadening of THz application fields.
Epitaxial layer on GaAs or Ge transfer to sapphire or silicate for gravitational waves mirror realization
Gravitational waves were predicted by the theory of general relativity, they are created in the universe by extreme cosmic events. Their measurement on earth in large instruments such as VIRGO in Italy is a challenge in terms of measurement sensitivity. These instruments are large interferometers (several kilometers), and the entire optical chain must minimize noise to be sensitive to very small modifications in space-time. Mirrors are one of the key elements of the optical chain.
In this thesis, we propose to create a new type of mirror making it possible to significantly improve the sensitivity of an interferometer. This mirror is based on a sequence of thin epitaxial layers with variations in optical index between each of them. These thin layers must be on a silica or sapphire base. Such a structure is not achievable by additive manufacturing (ie by depositing the layers on the sapphire or silica substrate), because the thin layers are monocrystalline, and the silica is amorphous when the sapphire has an unsuitable lattice parameter. Only thin layer transfer techniques allow the creation of such a stack.
This thesis will study thin layer transfer technologies to study one or more options permitting the transfer of monocrystalline layers from the donor substrate to the receiver substrate. Each of the necessary steps will be studied, and mechanisms will be proposed to explain the experimental observations. Demonstrators will be produced and their optical performances evaluated to determine if they are in line with the required sensitivity.
Quantum Cascade III-V/Si laser micro-sources
This thesis project focuses on the development of innovative micro-laser sources by combining III-V Quantum Cascade materials with Silicon Photonic Crystals. By integrating these advanced technologies, we aim to create hybrid lasers emitting in the middle infrared. This approach has significant advantages for medium-infrared spectrometry (MIR), a crucial technique for the chemical detection of gaseous, solid and liquid compounds.
The CEA-LETI Optical Sensor Laboratory offers a state-of-the-art research environment, where the candidate will have the opportunity to design, model, manufacture and characterize these devices. This thesis is part of a competitive but promising context, where technological advances could open new perspectives in areas such as "well-being and the environment". For Master 2 students who are passionate about photonics and emerging technologies, this research offers an opportunity to actively contribute to innovation in a growing field.
In situ study of the impact of the electric field on the properties of chalcogenide materials
Chalcogenide materials (PCM, OTS, NL, TE, FESO, etc.) are the basis of the most innovative concepts in microelectronics, from PCM memories to the new neuromorphic and spinorbitronic devices (FESO, SOT-RAM, etc.). Part of their operation relies on out-of-equilibrium physics induced by the electronic excitation resulting from the application of an intense electric field. The aim of this thesis is to measure experimentally on chalcogenide thin films the effects induced by the intense electric field on the atomic structure and electronic properties of the material with femtosecond (fs) time resolution. The 'in-operando' conditions of the devices will be reproduced using a THz fs pulse to generate electric fields of the order of a few MV/cm. The induced changes will then be probed using various in situ diagnostic methods (optical spectroscopy or x-ray diffraction and/or ARPES). The results will be compared with ab initio simulations using a state-of-the-art method developed with the University of Liège. Ultimately, the ability to predict the response of different chalcogenide alloys on time scales fs under extreme field conditions will make it possible to optimise the composition and performance of the materials (e- switch effect, electromigration of species under field conditions, etc.), while providing an understanding of the underlying fundamental mechanisms linking electronic excitation, evolution and the properties of the chalcogenide alloys.
“Remote epitaxy" of Cd(Hg)Te
A new way of considering epitaxy has recently appeared thanks to the development of 2D materials. Whereas conventional epitaxy involving covalent bonds is limited in particular to a lattice parameter matching between the substrate and the epitaxial membrane, it appears that this constraint can be significantly released if the epitaxial growth is done by van der Waals interactions. 2D materials are ideal candidates for this type of growth since their surface does not have hanging bonds.
"Remote epitaxy" is a recent innovative and original approach that consists in cutting the classical covalent epitaxial growth by inserting a sheet of 2D material to allow the transmission of the “crystalline field” between the substrate and the epitaxial layer. The stress in the first epitaxial layers is then significantly reduced with the possibility of easily exfoliate and release (thanks to the low energy interface) the epitaxial membrane from its substrate.
This approach has been successfully used in the case of III-V materials with the intercalation of a graphene sheet. We propose in this thesis to study the “remote epitaxy” of II-VI semiconductors, CdTe and HgCdTe that are at the heart of many applications areas such as infrared detection and imaging, X-ray detection and medical applications or photovoltaic.
Several 2D materials will be studied, either reported or directly grown on the surface of the substrate. Graphene will be transferred by dry-method to generate clean interfaces. Preferably, 2D material will be directly grown on the substrate surface. This study will be done in collaboration with the 2D SPINTEC team.
Study of InP and AsGa wafer bonding mechanisms
Direct bonding consists of bringing sufficiently smooth and clean surfaces into contact, in order to create adhesion between them without adding any external material. This technology presents many advantages for the production of stacked structures for microelectronics and micro-technologies and has given rise to numerous innovations (manufacturing of SOI by SmartCutTM, manufacturing of SmartSiCTM, production of MEMS, wafer level packaging, 3D integration, etc.). Today, the rise of photonic technologies and the development of direct die-to-wafer bonding are paving the way for the integration of materials such as InP and GaAs in the world of silicon. In order to push these developments we wish to study the bonding mechanisms of these materials which have not yet been established in the literature.
The thesis will consist of studying the direct bonding mechanisms of InP and GaAs wafers:
A first part of the study will consist of finely characterizing the surface of these materials during pre-bonding preparations (type of bonds created, type of oxide, quantity of water adsorbed, etc.).
Then the impact of water in the establishment of adhesion will be particularly studied in relation to the mechanisms established for silicon and its oxide. The stress corrosion sensitivity of InP and GaAs surface oxides will be evaluated. Infrared spectroscopy and X-ray reflectivity studies at the synchrotron will support the conclusions.
A final axis will concern the mechanical properties of these materials to better understand their integration within heterostructures. Their ductile-brittle transition will be characterized using bonding on silicon or other substrates (silica, sapphire, etc.).
The candidate will be trained in all clean room technological tools allowing direct bonding (chemical cleaning, CMP polishing, bonding, thermal annealing) and their characterization (infrared spectroscopy, acoustic microscopy, anhydrous bonding energy measurement, X-ray reflectivity, mass spectrometry, etc.).
Qubit conditioning circuit based on Single Electron Transistor electronics
A new research direction has emerged that consists in the design of cryogenics integrated circuits (cryo-CMOS) to address the needs of many scientific experiments in astronomy, in physics of particles or in quantum physics. Nevertheless, the power consumption of such solutions is still high and prevent their use in applications that necessitate the conditioning of large number of devices.
The proposed PhD position intends to explore an alternative path by using Single Electron Transistor (SET) ifor the design of ultra-low noise, ultra-low power consumption conditioning electronics. Indeed, SETs have quantized behaviour at cryogenics temperature that could help in reaching noise and power consumption optimum. As applied case, this thesis aims at providing a conditioning electronics dedicated to silicon qubit in the frame of quantum computing project.
The applicant should have very good knowledge of both semiconductor and quantum physics. Good understanding of analog electronics and signal processing is required as well. The applicant should be creative and should have a strong taste for experiments. Last, the applicant should have good modelling skills and a good knowledge of associated computing language (Pyhton)
Study of emerging materials for Threshold Switching Selector for MRAM technology
The objective of this thesis is to explore novel Threshold Switching Selector (TSS) materials for emerging MRAM (Magnetic Random-Access Memory) technologies. A selector serves as a simple two-terminal device, behaving like a switch or a diode that turns on above a certain voltage and stays off otherwise. When combined to a memory element, it prevents sneak current in non-selected memory cells, enabling denser memories. In addition, TSS aims at replacing the selection transistor and at reducing the number of vias to connect with the CMOS, thus saving power and surface area.
To achieve TSS compatible with MRAM, it is critical to develop new selectors materials that match the characteristics of magnetic tunnel junction (MTJ). For example, Ovonic threshold switch (OTS) used with phase change PC-RAM (in production) has a threshold voltage larger than 2V. This voltage is too high for MTJs that must be operated below 1V to avoid degrading the MgO tunnel barrier.
W color centers for integrated quantum photonics on silicon chips
The integration on a silicon-on-insulator (SOI) chip of all the components necessary for the generation, manipulation and detection of photonic quantum bits is nowadays seen as the most promising route toward scalability for quantum photonic engineering. Until now however, the lack of an “on-demand” source of single photons in silicon has hindered a full exploitation of this strategy.
This PhD project aims to develop such a silicon-based single photon source, integrated into a SOI photonic chip. The source will exploit the spontaneous emission of a single point defect in silicon, the color center W, whose ability to emit single photons has been demonstrated in 2022 by PHELIQS and partners. We will place a single W center at the core of an optical microcavity. Thanks to Purcell-enhancement, a quantum cavity effect, the single photons will all be prepared in the same quantum state, and efficiently funnelled into a waveguide. In order to build such coupled W-cavity systems with a high success rate, we will first develop ordered arrays of isolated W centers by localized ion implantation of SOI wafers. At the end of the project, we will realize a proof-of-principle integrated quantum optics experiment, exploiting W-single photon sources and single photon detectors on the same SOI chip.
The PhD student will be mostly in charge of the study of W centers and cavity effects by advanced optical spectroscopy. He/she will be also involved in technological developments.
Towards sustainable electronics: impact and understanding of the substitution of high GWP gases on plasma etching processes
To address environmental concerns in the microelectronics industry, CEA-LETI is committed to an eco-innovation approach [1]. In this context, the development of eco-responsible processes that reduce PerFluoro-Carbon (PFC) emissions is crucial [2]. Plasma etching processes are a major emitter of PFCs because they traditionally use high GWP gases. The aim of this thesis will be to develop plasma etching processes on 300mm substrates by replacing fluorinated gases such as NF3, SF6 and CF4 with high GWP by F2 (GWP ~0). This work will involve understanding the changes induced in the etching processes for advanced devices. The dual challenge is to reduce the environmental impact while guaranteeing the high performance of these devices.
To achieve this objective, you will implement new F2-based mixtures on targeted applications for etching materials and cleaning reactors. You will provide comparative data on the performance of these new processes compared with reference processes in terms of etching results and the level of abatement of reaction by-products.
You will be based at the CEA-LETI Etching Laboratory. The work, which will be predominantly experimental on CEA-Leti's 300mm platform (see : https://www.youtube.com/watch?v=on1NH08AZfE), will benefit from the state-of-the-art process equipment and characterisation resources of the Nanotechnologies platform. The scientific and industrial interest of the subject guarantees that your work will be showcased in international communications. If you want your research to have an impact on society, apply now!