Real-time controlled molecular beam epitaxy for the development of an integrated oxide photonic platform

Scientific field and context:

Silicon-integrated photonic circuits (PICs) are key components of future systems for data communications and quantum computing.¹ However, silicon is inefficient for light emission, absorption, and reconfigurability functions that are essential for PIC operation. Achieving these functionalities therefore requires the integration of additional materials onto silicon. For reconfigurability, ferroelectric BaTiO₃ is an excellent candidate due to its large Pockels coefficients and its ability to be epitaxially grown on silicon using SrTiO₃/Si templates.^2-4 For light emission and absorption, rare-earth ions are particularly attractive, especially for low-noise optical amplifiers and single-photon sources.^5,6 Their performance strongly depends on the host matrix in which they are embedded,⁷ and matrices belonging to the BaTiO₃ family (with the generic formula ABO₃) offer promising opportunities to optimize their optical properties. In this context, we propose to exploit the capabilities of molecular beam epitaxy (MBE), assisted by in situ and real-time characterizations, to fabricate oxide heterostructures combining BaTiO₃ and Er³⁺-doped ABO₃. This approach aims to establish a new photonic integrated circuit platform based on oxide materials epitaxially integrated on silicon via SrTiO₃ templates

Objectives:

Exploiting the potential of functional oxide heterostructures for PICs requires MBE growth control beyond current state of the art. Such control is enabled in the MBE reactor developed at INL, equipped with several real-time monitoring tools including reflection high-energy electron diffraction (RHEED), ellipsometry, curvature measurements, and optical flux monitoring. The first objective of the thesis will be to develop this instrumentation by contributing, together with the team’s technical staff, to the implementation of an interface enabling real-time acquisition and correlation of signals from these monitoring tools. This will also involve exploiting these in situ diagnostics to control the growth of oxide heterostructures. The second objective will be to optimize oxide heterostructures combining ferroelectric BaTiO₃ and Er³⁺-doped ABO₃ for their integration into PICs. For Er³⁺-doped ABO₃ oxides, the first system investigated will be (La,Er)AlO₃, for which promising preliminary results have been obtained in the team. The goal will be to optimize both Er³⁺ incorporation and the host matrix properties in order to (i) achieve highly coherent emission suitable for single-photon sources and (ii) maximize the concentration of optically active Er³⁺ ions for high-performance optical amplifiers. For BaTiO₃, the objective will be to maximize the material tunability by optimizing oxidation during growth and controlling the ferroelectric domain structure. This work will rely on the in situ diagnostics described above as well as on the wide range of ex situ characterization techniques available in the laboratory (X-ray diffraction, XPD, AFM-PFM, electrical measurements, photoluminescence and other advanced spectroscopic characterization tools). Based on these developments, optimized oxide heterostructures will be designed and fabricated with collaborators from the host team (INL ILUM team) and external partners, notably C2N. The most promising structures will then be used to fabricate tunable single-photon sources and optical amplifiers

Scientific challenges:

The main challenges of the project are:

- Developing a methodology for the coupled analysis of multiple in situ measurements in real time, with a common time base, in order to extract relevant physical parameters for understanding the properties of oxide heterostructures and their dependence on growth conditions.

- Achieving emission and absorption performances beyond the current state of the art for Er³⁺ ions embedded in oxide matrices. While these matrices are highly promising, they remain largely unexplored for the integration of Er³⁺ ions.

- Combining Er³⁺-doped oxide thin films with ferroelectric BaTiO₃ in the form of heterostructures, and optimizing their properties for the fabrication of high-performance optical amplifiers and tunable single-photon sources.

Original contributions expected:

The PhD student will be at the heart of the project, and will be more particularly in charge of the following aspects:

-Growth of oxide-based structures by molecular beam epitaxy, and instrumental developments for the implementation of the in-situ and real-time characterization tools

-Structural and chemical characterizations of the epitaxial layers: X-ray diffraction, transmission electron microcopy, XPS, AFM, possible measurement campaigns at SOLEIL and ESRF synchrotrons

-Functional characterizations of the epitaxial layers : photoluminescence, electrical characterizations, etc.

For these activities, the doctoral student will of course take benefit of the expertise and resources available in the laboratory or within the dense collaborative network of the team.

Research program and proposed scientific approach:

The instrumental developments and material investigations will be carried out in parallel, as they are strongly interdependent. Progress in the implementation and coupling of real-time in situ measurements will directly guide the optimization of material growth and properties. The project will be structured in three main phases:

- Optimization of the optical properties of (La,Er)AlO₃, building on promising preliminary results obtained in the laboratory.

- Exploration of other oxide host matrices for Er³⁺, guided by the results obtained and supported by theoretical studies conducted in collaboration with the FOTON laboratory.

- Optimization of heterostructures combining Er³⁺-doped oxides and BaTiO₃, followed by the transfer of the most promising structures to our partners for device fabrication.

External collaboration(s)/partnership(s)

- FOTON (Rennes) for ab initio calculations of Er³⁺ ion solubility in oxide matrices

- IRCP (Paris) for advanced spectroscopy and measurement of Er³⁺ ion optical properties (linewidth, coherence, etc.)

- C2N for the fabrication of devices based on Er³⁺-doped oxide thin films (structures for optical gain measurements and amplifiers)

- GREMAN (Tours, Kévin Nadaud): electrical characterization of structures

- INL ilum team and Nanolyon platform: design, fabrication, and characterization of single-photon sources based on Er³⁺-doped oxide thin films

- Internal collaborations within the INL MFN team hosting the thesis: time-resolved or steady-state photoluminescence measurements for optical characterization of Er³⁺ ions

- For transmission electron microscopy: access to the CLYM (Lyon microscopy platform)

The thesis will take place in the functional materials and nanostructures team at INL

PhD director : Guillaume Saint-Girons, CNRS research director

Co-supervisor : Clarisse Furgeaud, CNRS researcher

Funding : EEA doctoral school, ~2300€ brut/month (~1600€ net/month). Teaching possible at Ecole Centrale de Lyon

We are looking for a holder of a Master 2 degree or equivalent, with skills in materials science and/or optics (optical properties of materials and/or optical systems for instrumentation), motivated by experimental work and able to synthesize results from various characterizations. The mission also requires skills for collaboration and teamwork, writing skills and a certain fluency in English