Hybrid integration of crystalline oxides for silicon photonics
Functional oxides, epitaxy, silicon photonics, nonlinear optics
Starting February 2020.
Silicon photonics, i.e. the use of Si for integrated circuits, has emerged industrially more than a decade ago and is now a well-established technology. Currently, the main application addressed is data-communication, providing optical transceiver cables for datacentres. For future communication networks, new challenges have to be considered in terms of speed, power consumption, flexibility, and reliability. Thus, radically new solutions are required. At C2N, we are currently exploring a new paradigm for advanced photonic circuits based on the hybrid integration of crystalline oxides in the silicon photonics platform for the telecom wavelength range (1.3µm-1.55µm). The core concept is to exploit the giant nonlinear optical coefficients and rare-earth doping (e.g. Erbium) of functional oxides to realize groundbreaking devices such as a multi-wavelength amplified emitter for Si photonics.
This internship will explore the properties of rare-earth doped oxide thin films, and in particular the tuning of their oxygen content. We focus on yttria-stabilized zirconia (YSZ) as oxide matrix, having recently obtained first results in terms of light amplification and nonlinear effects with YSZ waveguides. The work will take place in the framework of a national research project, with available expertise on materials design, thin film growth, photonic devices, and advanced optical characterizations.
Epitaxial growth of oxide thin films by pulsed laser deposition (PLD), photoluminescence, micro-fabrication processes in cleanroom.
Gifted and willing for experimental physics, well-disposed towards infrared electromagnetic waves, with a background in materials science and/or solid state physics.
Possibility for a doctoral thesis
Yes, funded by French ANR (FOIST project, 2019-2023).
Electrical Transport at III-V on Si hybrid interfaces: Simulation, Fabrication and Measurements
Silicon will be the future of integrated photonics. III-V materials should be associated to Si to provide efficient emission or amplification in the 1.55µm Telecom domain as well as low-cost light-weight photovoltaic devices. C2N has developed bonding techniques that demonstrate electrical conduction at the hybrid interface [1-3].
The internship aims at studying the electrical transport at the hybrid interface according to the bonding conditions, the material at the interface and the doping of the layers. A home-made simulation tool based on ab-initio band and transport calculation will be used , as well as commercial tools. The optimized transport configuration will then be checked for photonic relevance in a hybrid laser.
C2N has large clean room facilities where hybrid interfaces will be fabricated and measured.
The student intern may acquire knowledge in device simulation, clean-room nano-fabrication techniques and experimental characterization.
 A.Talneau et al., Appl. Phys. Lett.,103,081901 (2013)
 A.Talneau et al., Microelec. Engineering, 192, (2018)
 A.Talneau et al., Materiau 2018, Strasbourg (2018)
 M. Pala and D. Esseni, Phys. Rev. B 97, 125310 (2018)
Optimization of Schottky diodes for rectifier and frequency doubler in the THz frequency range
There is a lot of interest in the Terahertz (THz) frequency range. Very high-frequency optoelectronic and nano-electronic components are exhibiting performance in progress and gradually bridge the gap between electronics and optics. Various applications are emerging in this frequency range such as telecommunications, spectroscopy (chemistry, physics and astronomy), medical imaging, and security or defense. Our team’s research focuses on the design and optimization of new detectors and sources that would be compact, electrically tunable, low-power, low-cost, achievable, and the most effective possible.
For THz sources, we want to use frequency multipliers (i.e. a circuit allowing to obtain in output a multiple frequency of the input frequency), and for detectors we rely on rectifying circuits that provide a continuous response (or BF) to an HF or THz signal. These two types of circuits use Schottky diodes. The objective of internship work is to optimize Schottky GaAs diodes at THz frequencies for these two specific applications. Both circuit architectures require matching cells. A nonlinear electrical model of the Schottky diode is required for the harmonic balance method in the Keysight ADS environment.
The proposed work is a continuation of previous work lead in the team. A Monte Carlo code (1D physical modeling) allows to model the core of the diode by including all the important physical effects, a commercial software named Silvaco allow to access to the 2D or 3D parasitic elements, and an efficient nonlinear electrical model has been validated. Finally, simulations under Keysight ADS have already been undertaken for the two types of functions.
Six physical and geometric parameters of the diode largely control its operation at THz. These are the thicknesses and the doping levels of the so-called N and N+ zones, the diode area and the Schottky barrier height at the M/SC interface. With hypotheses and simplifications we will try to optimize the diode according to only 3 parameters: the thickness and doping level of the N zone (ND, LD) and the area (S) of the diode (which depends closely on the excitation power).
Organization of work
• Modeling Schottky diode parasitic resistors and capacitances with Silvaco numerical tools,
• Generation of the nonlinear model of these two parasitic elements,
• Refining of the Schottky Diode Impact Ionization Electrical Model,
• Optimization of the 3 parameters ND, LD and S for rectification application,
• Optimization of the 3 parameters ND, LD and S for the frequency doubler.
Depending on the student’s training and affinity, the work will include varying doses of physics and HF/THz circuit design elements under Keysight ADS. This work can be continued in thesis.
Electronic structure of bulk semiconductors, alloys and heterostructures based on InSb including strains
The electronic structure of materials is the key point to understand their behavior. Its calculation requires successive approximations because computing the full Hamiltonian of the “complete” crystal is out of range, even for most efficient computers.
The electronic structure gives access to various parameters such as: dielectric function, piezoelectric tensor, inter-atomic force constant for the dispersion of phonons, and it appears to be crucial in various tools devoted to transport modeling (Monte Carlo, NEGF…).
Several approaches for band structure calculations are available, such as the ab-initio method and semi-empirical ones. The later need adjustment parameters and provide satisfactory results (up to 5-6 eV from both sides of the forbidden bandgap). Most of the time, it is enough, but these semi empirical models are not always easy to obtain due to the lack of measurement results. Then Ab-initio calculations are mandatory.
We propose an internship based largely on the density functional theory (DFT) to access to the electronic structure. For this, the candidate will use either the Quantum Espresso or Abinit softwares, to calculate first the electronic structure of a little known bulk materials such as InSb or ternary alloys as In1-xAlxSb useful for the realization of high-performance IR photo-detectors. The very strong spin-orbit coupling of InSb makes a little more difficult the calculations of the electronic structure. Then, the strain effects in InSb and in In1-xAlxSb alloys will be evaluated. Thirdly, the heterojunction InSb/In1-xAlxSb will be calculated.
The results using various functionals of exchange and correlation (GGA and hybrids) will be confronted to G0W0 when it should be relevant. The advantage of the GGA functional or hybrids is the calculation speed but in this case the energy gap is, most of the time, underestimated.
The band structures calculated in the frame of the DFT method will allow to improve the calculation performed by k.p multiband method and by empirical pseudopotentials approach (EPM). These last tools are well suitable when millions of calculations have to be generated as it is the case for Monte-Carlo simulators because Hamiltonian’ sizes of k.p and EPM are far smaller than within the DFT approach.
This work will be done at C2N with Anne-Sophie Grimault and Frédéric Aniel in collaboration with colleagues of the LSI (Ecole Polytechnique) : Jelena Sjakste and Nathalie VAST.
Artificial quantum materials with photons
Quantum systems containing many interacting particles are particularly hard to comprehend: when the number of particles increases, the size of the Hilbert space diverges and calculations based on classical computers become intractable. One way to learn about quantum many-body systems is to realize experiments in a well-controlled environment, as originally proposed by Richard Feynman. Pioneering experiments have been realized using cold atoms, trapped ions or superconducting circuits. A promising approach nowadays is to use photons in order to implement photonic quantum materials. In this case, quantum properties are directly imprinted on a light field, which is a great advantage as it provides optical access to the observables of the system. Moreover, photonic quantum materials may be useful for realizing practical devices (quantum light sources for instance).
Our group at C2N has developed a unique expertise in designing photonic materials using lattices of coupled microcavities. For example, we have recently realized the first topological laser in a 1D lattice, and explored exploring Dirac physics in 2D honeycomb lattices.
To progress toward the implementation of multi-photon quantum phases, we need to create strong photon-photon interactions. To do so, we mix the cavity photons with electronic excitations (excitons) created in quantum wells located in the cavity. The resulting exciton-photon state, named cavity polariton, shows significant interactions which have allowed demonstrating many fascinating such superfluidity of light.
The challenge we propose now is to engineer interactions that are strong enough (at the single photon level) to enter the quantum regime. The work will start with the development and characterization of novel active materials based on coupled quantum wells, which are expected to give rise to much stronger interactions. Photon-photon interactions will be characterized by low temperature spectroscopy and by photon correlations. The smoking gun evidence for the quantum regime will be the demonstration of single photon emission in a single lattice site. We will then build larger and larger lattices that will enable implementing and studying many-body Hamiltonians of increasing complexity.
We are looking for a candidate with skills and interest in experimental work, as well as solid knowledge in quantum optics and solid state physics. The work will be mainly experimental (low-temperature quantum optical spectroscopy), and the student will be introduced to sample processing in unique technological environment offered by the C2N clean room. Finally, this research being part of a large international collaboration, the student will directly interact with scientists from other laboratories and particularly with theoreticians.
Mechanical friction between perfectly flat 2D materials
Interfacial adhesion and friction are of special interest for the development of microelectromechanical systems. The frictional force between two perfectly flat materials is highly dependent on the orientation between the layers. The clearest example of this is superlubricity, a phenomenon proposed theoretically in [Hirano et al., PRB 41, 11837 (1990)], which consists in an ultralow friction state between graphite layers in incommensurate states. The lattice mismatch created by the incommensurate lattices results in the cancellation of one of the channels of energy dissipation, preventing the collective stick-slip motion of atoms in contact and as a consequence the kinetic friction force is vanishingly small. Other dissipative processes, such as electronic and phononic friction, persist and therefore the net frictional force will not be zero. Nevertheless, the reduction of the friction is expected to be of orders of magnitude. This phenomenon has been measured experimentally [Dienwiebel et al., PRL 92, 126101 (2004), Koren et al., Science 348, 6235 (2015)] showing a frictional force between layers within the sensitivity of the instrument (or virtually zero) and a high angular dependence of the friction force with the orientation between layers. This dependence showed an increase of orders of magnitude in the friction close to perfect alignment of the layers, with the expected 60° periodicity characteristic of the honeycomb lattice structure. Interestingly, these highly sensitive friction measurements have not shown any indication of other commensurate states, probably because the remaining friction forces (e.g. phononic and electronic friction) are much larger than the friction generated by these commensurate states, or because the resolution of the instrument was not good enough.
Superlubricity has not been measured in other material, nor in combination of different materials. The question of the existence of superlubricity between two layers with different lattice constants remains to be investigated. We propose to combine recent developments in the fabrication of van der Waals heterostructure with advance atomic force microscope techniques to investigate the frictional forces between perfectly flat 2D materials with different lattice constants. This thesis project will focus on the understanding of: the existence of superlubricity in incommensurate materials, scaling of the friction between perfectly flat surfaces (e.g., area versus edge scaling), role of phononic and electronic friction and interlayer spacing effects.
Reconfigurable holograms with nanometric resolution
The internship mission is a scientific study about the design of a spatial light modulator with very high resolution (pixel pitch<1 μm).
Based on C2N research work, the new architecture is based on plasmonic particles used as "optical tweezers" that can trap beads that modulate the phase and/or amplitude of the incident light wave. The main objective of the internship will be to study the feasibility of this concept through a literature review, analytical calculations and electromagnetic simulations. The internship will take place at C2N at Palaiseau (91).
(in french) Hologrammes reconfigurables à résolution nanométrique
Le sujet du stage est l’exploration d’une nouvelle voie technologique pour la conception d’un modulateur spatial de lumière à très haute
résolution (<1 μm). Basée sur les travaux de recherche du C2N, il s’agit d’une architecture à partir de particules plasmoniques utilisées comme « pinces optiques » qui pourront piéger des billes modulant la phase et/ou amplitude de l’onde lumineuse incidente. L’objectif principal du stage sera d’étudier la faisabilité de ce concept au moyen d’une étude bibliographique, de calculs analytiques et de simulations électromagnétiques. Le stage se déroulera au C2N, et pourra être poursuivi par une thèse.
All-Dielectric Metamaterials for Metadevices : Negative Index and Near-Zero Index Materials
see attached file
Probing valley Hall currents in graphene
The first isolation of graphene, one atom thick layer of carbon atoms arrange in hexagons, in 2005 attracted a lot of attention given its remarkable characteristics: high carriers mobility, ambipolar behavior, great mechanical strength. Not long after this, other 2D layered materials were found to have also remarkable characteristics and host a large number of phenomena of condensed matter physics. A great technological achievement of our days is the stack of these 2D layers to form van der Waals (vdW) heterostructures [Nat. Nanotech 5, 722] and therefore combine the properties of each material to design materials with new properties [Nature 499, 419].
Contrary to conventional 2D materials, grown by molecular beam epitaxy, the layered vdW structures can be make of any combination of 2D materials, given that there are not restrictions of lattice parameter. Other degree of freedom of these heterostructures is the relative angular alignment between its layers, which can dramatically alter its fundamental properties. A remarkable example of this, predicted for certain vdW heterostructures, is the emergence of phases of matter where charge carriers flow without dissipation. In heterostructures of graphene crystallographically aligned with boron nitride (insulator isomorph to graphene), such a phase has been predicted. However, due to the scarcity of experimental tools to control the layer alignment the observations available remain inconclusive [Science 346, 448; Science adv. 4, eaaq0194].
In this experimental internship (with possibility to be extended to a PhD thesis), we propose to use a new technique to control the angular alignment between layers in a vdW heterostructure [Science 361, 690] to investigate the generation and control of electrical signals related to this phases of matter in graphene. The successful candidates will participate actively in sample fabrication (assembly of vdW heterostructures, angular control of layers using an AFM, e-beam lithography) and electronic transport measurements at low temperatures. Experience in micro and nano fabrication process is not required since the students will have the necessary training at the lab.