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PhD : selection by topics

Technological challenges >> Photonics, Imaging and displays
9 proposition(s).

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SOI vertical diodes for LWIR detection

Département d'Optronique (LETI)

Laboratoire d'Imagerie thermique et THz

01-09-2021

SL-DRT-21-0313

patrick.leduc@cea.fr

Photonics, Imaging and displays (.pdf)

Uncooled thermal detectors absorb the infrared flux in wavelength range from 7µm to 14µm. This corresponds to an atmospheric transmission window and to the maximum emission of a blackbody at 300K, which enables to measure temperature variations of less than 100mK. The operating principle of microbolometers is based on the temperature measurement of a suspended membrane absorbing the infrared flux. The thermal transducer is the sensitive element of the microbolomètre, which determines its signal-to-noise ratio and therefore the performance of the bolometric pixel. Most commercial microbolometers use a thermistor with amorphous silicon or vanadium oxide for its high temperature coefficient (TCR = 2-4% / K) and low flicker noise (1/f noise). The thesis proposal deals with a breakthrough technology for microbolometers. Unlike conventional thermistor detectors, the student will examine the use of SOI vertical diodes as transductor. The topic will focus on characterizing and modeling the performance of such a device.

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Design and fabrication of GeSn based components for environmental detection

Département d'Optronique (LETI)

Laboratoire des Capteurs Optiques

01-10-2020

SL-DRT-21-0315

vincent.reboud@cea.fr

Photonics, Imaging and displays (.pdf)

One of the main challenges for silicon photonics is an integrated laser technologically compatible with microelectronic foundries. Traditional, standalone semiconductor lasers use III-V semiconductors that are not accepted in the silicon foundries, contrary to the group-IV semiconductors. CEA Grenoble is among only few labs that already demonstrated mid-infrared optically pumped lasing in group-IV semiconductors, both in Ge and GeSn. With fully relaxed or tensile-strained heterostructures and quantum wells made of silicon-germanium-tin alloys (Si)GeSn, today we target continuous lasing at room temperature and efficient photodetectors in 200 mm wafers. To reach room temerature lasing, we need to improve the optical gain and optimize the carrier confinement. The improvements will require a redesign of the quantum wells and heterojunctions in germanium tin, through playing with atomic compositions and the mechanical strain at the micro-nanoscopic scale. Like the lasers we already demonstrated, the designed (Si)GeSn layers will be epitaxially grown at the main 200 mm CMOS fab facility of CEA Leti, and further fab-processed by the PhD candidate in smaller-scale clean rooms. Laser developments will be used to realize efficient GeSn photodetectors. The objectives of the research will be: (i) to reduce the number of crystalline defects in the GeSn gain regions, (ii) to design efficient (Si)GeSn stacks that confine both electrons and holes, while providing strong optical gain (iii) to apply and control tensile strain in germanium tin layers, (iv) to evaluate the optical gain under optical pumping and electrical injection, at different strains and doping levels, (v) to design and fabricate laser cavities with strong optical confinement (vi) to obtain germanium-based group-IV lasers that are tunable and lase continuously. (vii) to test fabricated devices (light sources and photodetectors) in gas detection cells On a longer term, such lasers will be widely used in ubiquitous miniaturized, low-power devices for optical gas sensing and environmental monitoring. This work will imply contacts with foreign labs working on the same vibrant topic.

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Synthesis and study of chiral organic materials for charge transport in organic semiconductors

Département d'Optronique (LETI)

Laboratoire des Composants Emissifs

01-10-2021

SL-DRT-21-0395

benoit.racine@cea.fr

Photonics, Imaging and displays (.pdf)

The detection and manipulation of the polarized light is very attractive, in particular because of the interest in using circularly polarized light (LCP) in many areas of societal importance such as technologies display, information transmission, cryptography, bio-medical imaging or even the detection of chiral molecules of pharmaceutical interest. Due to their ability to interact specifically with an LCP and to modulate its polarization, chiral molecular materials stand out as an element of choice for exploring these innovative applications and considering new potential in organic electronics. In addition, the specific property of chiral molecules to induce electronic spin selectivity in the conduction of electric current (CISS effect for Chiral Induced Spin Selectivity) also opens up opportunities in the field of organic spintronics. Consequently, the synthesis of innovative pi conjugated chiral semiconductors, presenting an easy modulation of their physicochemical properties and the integration of these materials in optoelectronic devices of the OLEDs, OPDs or OFETs type is of interest both fundamental and 'application. The thesis project will be done in collaboration with a CNRS chemistry laboratory in Rennes, France, and the LCEM laboratory (at CEA / LETI, Grenoble, France) specialized in organic semiconductors. The objectives of the thesis student will be to synthesize new chiral organic charge transporters and to characterize their photophysical (absorption and emission) and opto-electronic properties. The most promising molecules will be integrated into OLEDs and OPDs devices. The photophysical synthesis and characterization part (circular dichroism spectrometer, non-polarized and circularly polarized luminescence spectrometer, PER, etc.) will be carried out at the CNRS chemistry laboratory. The integration of molecules in OLEDs and OPDs devices will be done in the LCEM laboratory where the deposition equipment (PVD chamber for organic materials) and the opto-electronic characterization means (IVL, C (V), TLM, Photocurrent, hall effect, etc.).

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Qualification and quantification of GaN, InGaN and AlGaN surface states :

Département d'Optronique (LETI)

Laboratoire des Composants Emissifs

01-09-2021

SL-DRT-21-0515

David.Vaufrey@cea.fr

Photonics, Imaging and displays (.pdf)

GaN-based µLEDs seem promising for augmented reality (AR) or virtual reality (VR) applications. Indeed, they would make it possible to produce screens with resolutions and luminances that had not yet been achieved. But these µLEDs suffer from a lower efficiency compared to their larger sibling. A commonly accepted explanation for this efficiency degradation lies in the existence of numerous surface defects induced by pixel singularization etching. The smaller the dimensions of the LED, the more important the surface defects play in the electro-optical behavior. Their presence, if they are shallow, can facilitate electrical injection, on the other hand if they are deep, they contribute to the degradation of the electro-optic performances of devices such as LED. This thesis aims to quantify and qualify the surface defects in GaN, InGaN and AlGaN that make up GaN-based µLEDs. The doctoral student will have to carry out by himself all the stages of realization of new components necessary for this study, starting with the design of the photolithography masks, the realization of all the technological steps and finally the electro-optical characterizations such as DLTS (Deep Level Transient Spectroscopy), DLOS (Deep Level Optical Spectroscopy) or photocurrent. At the end, the doctoral student will have to identify the surface defects that are the most limiting for the efficiency of LEDs and the most favorable to the injection of electrical carriers. The thesis will be carried out in close collaboration with Ph Ferrandis (thesis director) from Néel Institute (CNRS), N. Rochat (co-supervisor) from CEA Leti (PFNC Nano Characterization Platform) and David Vaufrey (supervisor) from CEA Leti (LCEM Emissive Device Laboratory). The thesis grant would be fully funded by the CEA Leti in Grenoble for a period of 3 years.

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Study of EBIC and cathodoluminescence applied to narrow gap photodiodes for cooled IR détection

Département d'Optronique (LETI)

Laboratoire d'Imagerie IR

01-10-2021

SL-DRT-21-0626

pierre.bleuet@cea.fr

Photonics, Imaging and displays (.pdf)

Since 40 years, CEA LETI is developing IR detection technologies using narrow gap semi-conductors. This led to the creation of the Lynred Company, formerly Known as Sofradir, now leader in the IR imaging market. In the frame of the collaborative work we have with Lynred for the development of new generations of IR imager, new characterisation needs appear. It addresses different issues, starting from the fine understanding of the photodiode operation when reducing its pixel pitch. It also addresses the understanding of the effects of the metallurgical and technological induced defects on the final IR detection performances. The proposal here is to study the behaviour of narrow gap IR photodiodes when excited with an electron beam within a scanning electron microscope (SEM). A mapping of the electron beam induced current (EBIC) brings important information about charge transport in the narrow gap, whereas the mapping of the associated induced IR luminescence (cathodo-luminescence) carries further and complementary information about radiative recombination of injected charges. This information is particularly interesting when interaction with defects occurs in the structure. In our characterisation group, the EBIC experiment is now operational at cryogenic temperatures. On the other hand, the cathodo-luminescence part of the experiment has to be developed to complete EBIC images. Once operational, the full picture EBIC+cathodo should be investigated using different samples from our fabrication line, focusing on small pixel pitches and high operation temperature structures, making the connection with all the other electro-optical characterisation benches available in our lab.

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Optical optimization of reception components in a 3D FMCW imaging system

Département d'Optronique (LETI)

Laboratoire Architecture Systèmes Photoniques

01-09-2021

SL-DRT-21-0697

laurent.frey@cea.fr

Photonics, Imaging and displays (.pdf)

Capturing distance information from a scene is becoming a major asset for certain new applications. A typical example being facial recognition by a cell phone. Different techniques already exist with varying degrees of advantages and disadvantages. Within LETI / DOPT we are interested in several techniques and in particular that based on optical frequency modulation (FMCW). The suggested thesis is articulated around the optimization of the reception module of a miniaturized 3D FMCW imaging prototype. The work will be carried out on 3 optical components to be designed/ simulated and optimized in order to improve the final 3D imaging system. The candidate should have extensive knowledge in optics / simulation / instrumentation and laser interferometry.

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Reliability of 3D avalanche photodetectors

Département Composants Silicium (LETI)

Laboratoire de Caractérisation et Test Electrique

01-10-2020

SL-DRT-21-0830

jean.coignus@cea.fr

Photonics, Imaging and displays (.pdf)

STMicroelectronics develops various CMOS-based technologies for imaging. The rise and democratization of image sensors is leading to a diversification of technological uses such as high-resolution imagery and telemetry for domestic and automotive use. One of the challenges is to meet market needs and adapt to the competition by constantly improving the performance and reliability of devices. The objective of this thesis is to study and model the reliability of avalanche photodetectors for single photon detection. The principle of this sensor lies in the ability to measure the transit time between an optical source and the detector, from a few centimeters to several tens of meters while being insensitive to the surrounding light. A matrix made up of thousands of pixels makes it possible to restore a faithful 3D image of the target. To date, first tests show that the detector degrades over time, leading to a loss of sensitivity and degradation of measurement precision. Quantifying these effects and understanding these drifts is absolutely necessary to improve the manufacturing process and develop a predictive model of reliability. The thesis will focus equally between the reliability of a single pixel and the reliability of a pixel matrix, in order to approach product reliability. The candidate will rely on a set of characterization and reliability measurement tools, as well as modeling and simulation tools developed at STMicroelectronics.

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Study of the persistence in IR MCT imaging retina for earth observation from space

Département d'Optronique (LETI)

Laboratoire d'Imagerie IR

01-10-2021

SL-DRT-21-0855

nicolas.baier@cea.fr

Photonics, Imaging and displays (.pdf)

IR detection is a major stake for ongoing and future space science missions studying the atmosphere chemistry (for instance MicroCarb mission dealing with the measurement atmosphere CO2 concentration) and more generally for science imaging and sensing (for instance the ARIEL mission aiming at studying exoplanet atmospheres). IR detector is a major performance driver for such instruments. The most widely used technology relies on a sensitive layer made of HgCdTe, a narrow gap semiconductor, suitable for IR photon absorption. This sensitive layer is then hybridized onto a silicon read out integrated circuit (ROIC) for multiplexing and signal conditioning. This particular technology has been developed at CEA LETI then transferred to Lynred for qualification and production. The very high level of measurement precision of science missions requires an in depth study of all sources of detection performance degradation. Among various biases is the persistence phenomena: like the human eye looking at the sun, ghosts of previous images are sometime polluting detected images, thus degrading the instrument performance. This effect is most of the time attributed to the sensitive layer (often associated to charge trapping-detrapping phenomenon) and is difficult to calibrate. Ideally, we look for a way to suppress or at least minimize this effect, playing with fabrication parameters. This requires an important work to improve our understanding of the physical phenomenon involved in persistence. Within previous developments with CNES, LETI has built a measurement bench suitable for such fine characterisations. This work intend to study this effect on identification detectors, with regard to different detector fabrication variations. A second step will be to identify the most efficient technological solutions in collaboration with Lynred Company producing such detectors. Last but not the least the validation of the proposed solution shall be performed.

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Innovative materials for microLEDs, a bright and energy-efficient technology for next-generation displays

Département des Plateformes Technologiques (LETI)

Laboratoire

01-09-2021

SL-DRT-21-0889

philippe.rodriguez@cea.fr

Photonics, Imaging and displays (.pdf)

CONTEXT Many industrial and academic players see microLEDs as the most promising emerging display technology. Allowing to reach very high brightness levels, while keeping a high energy efficiency, microLED displays could bring a new wave of augmented reality devices and revolutionize the way we interact with the digital world. CEA-LETI is a world-leader in microLEDs displays technology, and has partnered with a high-profile technology firm in order to bring these promises into a cutting edge product. ABSTRACT Innovative materials will be selected in order to improve key microLEDs properties. They will be elaborated and their physical, chemical, and opto-electronic properties will be characterized. In-depth scientific analysis will take place in order to correlate micro- and nano-scale properties with the materials behaviour, and the impact of the elaboration conditions on those properties will be studied. The materials will be integrated in the microLEDs, and their impact on the device electro-optical properties will be studied. CANDIDATE In order to be successful, the candidate will need a background knowledge of material science, semiconductor physics, as well as understanding of deposition processes and of materials characterization. During this PhD thesis, the candidate will take part in a state-of-the-art scientific technological environment, and will be able to gather cutting-edge knowledge in the promising field of microLEDs. The candidate will have access to attractive conditions as well as to the numerous advantages given by the CEA.

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