Thèse Étude et Optimisation de Micro-Leds à Nanofils Ingan - Gan pour des Écrans à Haute Résolution et à Faible Consommation d'Énergie H/F - Doctorat.Gouv.Fr

Établissement : Université Paris-Saclay GS Sciences de l'ingénierie et des systèmes École doctorale : Electrical, Optical, Bio-physics and Engineering Laboratoire de recherche : Centre de Nanosciences et de Nanotechnologies Direction de la thèse : Maria TCHERNYCHEVA ORCID 0000000341440793 Début de la thèse : 2026-10-01 Date limite de candidature : 2026-06-22T23:59:59 µ-LEDs offer high brightness and efficiency, but red emission remains inefficient due to the material mismatch and sidewall damage. This PhD will develop monolithic InGaN/GaN nanowire (NW) µ-LEDs grown by PAMBE using selective-area growth (SAG) to control In incorporation, and compare them with hybrid µ-LEDs by phosphor conversion (blue µ-LED with KSiF:Mn red nanophosphor in collaboration with the co-advisor laboratory). Objectives include fabricating tunable NW µ-LEDs, identifying and reducing non-radiative defects, integrating nanophosphors, comparing optical performance, and co-integrating RGB through diameter-dependent In incorporation. In Year 1 SAG and phosphor integration will be optimized and structural/optical characterizations will be performed; in Year 2 red-emitting NW growth, defect mapping, passivation, phosphor integration will be done; in Year 3 device arrays will be tested and EQE for the two types of LEDs will be compared. The expected outcome would be a demonstration of an efficient red NW µ-LEDs and its comarison with more conventional phosphore-converted devices. Micro-light-emitting diodes (µ-LEDs) are rapidly emerging as a key technology for next-generation display and photonic applications, offering exceptional brightness, energy efficiency, and pixel density compared to organic LEDs and LCDs. Their use in flexible and ultra-high-resolution displays, augmented reality systems, and optoelectronic devices has made them one of the most active fields in semiconductor research and applications. For example, InGaN-based blue µ-LEDs exceed the brightness of 10 cd/m².
However, achieving full-color displays with inorganic materials remains a fundamental challenge. Conventional µ-LED fabrication methods involve dry etching of III-V semiconductors, leading to severe sidewall damage and non-radiative recombination, which drastically reduce the external quantum efficiency (EQE). Moreover, the use of heterogeneous material systems (InGaN for blue/green and AlGaInP for red) limits scalability due to non-compatible nanofabrication protocols, mismatched thermal expansion coefficients, poor integration yield, and reduced uniformity.
A promising route to overcome these limitations is the monolithic integration of full-nitride µ-LEDs based on InGaN/GaN materials. While in 2D-based system, high-In compositions (>20%) degrade crystal quality and thus efficiency due to poor In miscibility and lattice mismatch with GaN, axial nanowires (NWs) appear as the promising solution [Phi2017]. NW architectures enable better strain relaxation, and improved light extraction. Importantly, it was demonstrated that the In content and thus the emission color can be controlled by the NW diameter: by increasing the NW diameter in selective area growth (SAG), the In concentration in the active region is increased, shifting the wavelength [Kis2013]. Tuning the emission from blue to red requires a high indium content (>30%), which is possible by using plasma assisted Molecular Beam Epitaxy (MBE) as demonstrated by the host lab [Mor2018], but introduces significant lattice strain, point defects, and efficiency droop. Understanding and controlling these mechanisms is crucial for developing high-performance red µ-LEDs compatible with scalable display technologies.
Another complementary approach, which can be used as a reference, uses blue µ-LEDs combined with red-emitting nanophosphors (such as KSiF:Mn) to achieve color conversion. As demonstrated by the co-advisor team in collaboration with the host laboratory [Gua2019, Kut2023, Abr2024], these systems exhibit high color purity and chemical stability but require optimization of particle size, quantum yield, and dispersion uniformity to ensure efficient color conversion at micron-scale pixel dimensions.
Both approaches (i.e. in-depth research into InGaN NW-based red LEDs for all-InGaN RGB emission and the integration of blue light-excitable red phosphor converters with blue emitting InGaN NW LEDs) will be explored in this PhD project and their efficiency and technological limitations will be assessed. 1) Design and fabricate InGaN/GaN nanowire µ-LEDs with tunable emission (from blue to red) via precise In incorporation control using selective-area growth (SAG) and plasma assisted MBE.
2) Identify and mitigate non-radiative defects that limit quantum efficiency in In-rich red InGaN nanowires using InGaN buffer layer and passivation.
3) Elucidate physical mechanisms linking growth conditions, structural defects, and optical efficiency to guide next-generation µ-LED design.
4) Integrate red-emitting KSiF:Mn nanophosphors (developed and provided by the PhD international co-advisor), synthesized by an eco-friendly microwave-assisted route, ensuring nanoscale homogeneity and high external quantum efficiency.
5) Perform a comparative analysis of direct-emitting red InGaN NW µ-LEDs versus nanophosphor-converted hybrid devices (optical performance, color uniformity, thermal stability). Demonstrate RGB emission.
6) Train the PhD candidate in photonics, material science and transferable skills. The research methodology is based on a feedback loop between the materials synthesis optimization, nanoscale structural and optical analyses, device fabrication and characterization. Two approaches for RGB µ-LEDs will be explored : (i) all-InGaN NW-based µ-LEDs with size-dependent compositional tunning and (ii) the integration of blue light-excitable red phosphor converters with blue emitting InGaN NW µ-LEDs. The (ii) approach relies on the on-going collaboration between the host team and Dr S. Das from NIIST-India (started in 2019 within CEFIPRA indo-french project), the PhD candidate will perform two 6 months stays at NIIST to optimize phosphor synthesis and integration with InGaN µ-LEDs.
The step-by-step methodology addresses the following challenges:
- Growth: selective-area growth (SAG) of InGaN/GaN nanowires by plasma-assisted MBE; control of temperature, Ga/In flux ratio and NW diameter to tune the In incorporation.
- Structural & optical characterization: SEM, TEM/STEM, PL, cathodoluminescence (CL) spectroscopy, high-resolution CL mapping, and EBIC for defect localization.
- Materials engineering: in-situ GaN shell passivation of the surface and low-In buffer layers to trap point defects and reduce Shockley-Read-Hall recombination.
- Nanophosphor integration: optimization of microwave-synthesized red KSiF:Mn for bleu to red color conversion with nanoscale homogeneity and high EQE (work with the co-advisor team, secondments in NIIST-India), integration of the nano-phosphors on µ-LEDs.
- Device testing: fabrication of µ-LED pixels/arrays and electrical/optical characterization (I-V, EL, EQE); comparative analysis of direct vs. phosphor-converted devices.
- Data correlation: link growth parameters structural defects optical/electrical performance to derive design rules.

Background: MSc in electronics or physics or material science
Experience in one of the following : material synthesis techniques or thin-film deposition or device fabrication
At least some experience in optical characterizations (e.g. PL or CL or EL)
Experience in programming or data analysis (e.g. hyperspectral datacube for CL and EBIC) or automation of instruments is considered as a plus for the project
Good level of transferable skills such as Teamwork, Autonomy, Project management, Scientific communication

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