Indirect excitons for emerging quantum states – IXTASE

IXTASE consortium considers three different kinds of systems hosting IXs fluids: GaN-based heterostructures, two-dimensional TMD-based stacks, TMD monolayers (ML) and bilayers deposited on polar GaN-based substrates.

Molecular beam epitaxy and lithography of GaN/(AlGa)N structures with electrostatic traps for optical injection of IXs is already implemented with IXTASE, while the fabrication of structures for electrical injection of IXs still need to be developed. TMD-based systems are fabricated mainly by mechanical exfoliation, and involve hBN encapsulation technique, contacting and processing the TMD structures into gated devices, with graphene flakes as contacts.

Spectroscopic studies of IX fluids rely on spatially, temporally and polarization-resolved differential reflectivity, µ-Photoluminescence and µ-Electroluminescence, as well as on spatial and temporal interferometry. The parameters, such as IX density, temperature, applied electric bias, IX dipole length, electrostatic trap shape and depth as well as nature and geometry of hosting TMD layers need to be varied to explore IX phases. Diffusion and localization, polarization, spectral energy, line-shape and width, as well as emission coherence should reveal the properties of the IX fluids.

IX fluids in GaN QWs

We demonstrated that exciton density in the trap can be controlled via an external electric bias, which is capable of altering the trap depth. Application of a negative bias deepens the trapping potential but does not lead to any additional accumulation of excitons in the trap. This is due to non-radiative exciton dissociation instigated by the lateral electric field at the electrode edges. By contrast, application of a positive bias washes out the electrode-induced trapping potential. Thus, excitons get released from the trap and recover free propagation in the plane. Provided that non-radiative losses related to electrode edges could be reduced, more complex multi-electrode devices, inspired by GaAs-based technology, but taking advantage of GaN particularities, may be interesting to address in the future.

IX fluids in TMDs

In accordance with the proposed research plan, we have tuned the intralayer and IX excitons into resonance in gated structures (collaborations with Basel University). These results show that the interlayer exciton IX can be tuned in energy below the intralayer exciton, and the consortium has better understood now the nature of the interaction.

Hetero-bilayers made of MoSe2/WSe2 and encapsulated in hBN have been INSP realized, including electrical contacts of bottom/top layers as well as each TMD monolayers. Hence, the group has succeeded in observing inter-layer excitons, with optical properties possibly varied by the applied electrode voltages. In particular, it was shown that the concentration of excess carriers interacting with the exciton fluid is possibly varied electrically. Thus, exciton fluids are accessible in the regime where the concentration of free carriers is minimized. For interlayer excitons experiments have also reported optical properties which are interpreted in the literature as evidence for exciton confinement in the moiré potential spontaneously formed in hetero-bilayers. This evidence includes the emergence of multiple photoluminescence lines that exhibit counter-circular polarizations.

IX fluids in hybrid GaN/AlGaN/TMDs

LPCNO deposited hBN and MoS2 layers of different thickness on the surface of specially designed GaN/AlGaN heterostructure grown in CRHEA. The samples, including those with electrostatic traps obtained by lithography, where studied by optical spectroscopy both in LPCNO (TMD emission) and L2C (GaN IX emission). First evidence of the MoS2 (MoSe2) energy blueshift induced by GaN/AlGaN heterostructure is obtained and tentatively interpreted in terms of the charge transfer from GaN/AlGaN to MoS2(MoSe2). Thick hBN layers are found to increase the surface potential in GaN/AlGaN heterostructures. However, the induced in-plane barrier seems to be too weak for efficient exciton trapping.

IX fluids in GaN QWs

Dramatic effect of the dipole length on the exciton transport has been evidenced in a set of experiments but in order to construct temperature/density/dipole moment phase diagram of IX spatial interferometry experiments and modelling of the experimentally observed IX emission will be realized. On the other hand, in-depth studies of electrically injected IX in p-i-n AlGaN/GaN light-emitting diode structure grown on a GaN bulk substrate by µPL is upcoming.

IX fluids in TMDs

Heterobilayeres with optically created and electrically tunable IXs are promising for studies of IX diffusion and localization. An important challenge is to be met: different twist angles and lattice constants result in periodic variations of the bandgap (moiré potentials). It is important to understand how an IX moves through this irregular landscape, and how the coupling between the layers and hence also the depth of the moiré potentials can be tuned by adding hBN spacer layers of 1ML, 2ML, or 3ML thickness.

IX fluids in hybrid GaN/AlGaN/TMDs

We plan to work in two directions. On one hand, we will continue to explore the effect of various TMDs and graphene on the surface potential in GaN/AlGaN heterostructures, in order to evaluate their usefulness for IX trapping in GaN quantum wells. On the other hand, we’ll address the effect of the IXs in the quantum well on the optical resonances in TMDs.

Publications

1. «Capacitively-coupled and inductively-coupled excitons in bilayer MoS2« arxiv.org/abs/2108.04248

2. «Stacking-dependent exciton multiplicity in WSe2 bilayers« arxiv.org/abs/2112.08994

3. “Exciton spectroscopy and diffusion in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride« arxiv.org/abs/2204.07351

4. «One pot chemical vapor deposition of high optical quality large area monolayer Janus transition metal dichalcogenides« arxiv.org/abs/2205.04751

5. “Second harmonic generation control in twisted bilayers of transition metal dichalcogenides“, Phys. Rev. B 105, 115420 (2022)

6. « Interlayer exciton mediated second harmonic generation in bilayer MoS2”, Nature Communications 12, 6894 (2021)

7. « Mott insulator of strongly interacting two-dimensional semiconductor excitons«, Nature Physics 18, 149 (2022)

8. «Electrical control of excitons in GaN/(Al,Ga)N quantum wells« arxiv.org/abs/2203.13761

9. “Complexity of the dipolar exciton Mott transition in GaN/(AlGa)N nanostructures “, Phys. Rev. B 103, 045308 (2022)

Conferences

1. “Complexity of dipolar exciton Mott transition in GaN/(AlGa)N nanostructures” , International Conference on Optics of Excitons in Confined Systems (OECS 17), Dortmund, September 2012, oral

2. “Electrically-controlled exciton transport and trapping in GaN/(Al,Ga)N quantum wells”, International Conference on Physics of Light-Matter Coupling in Nanostructures, Varadero (Cuba), April 2012, oral

3. “Atomically thin semiconductors for photonics and spintronics “, International Conference on Optics of Excitons in Confined Systems (OECS 17), Dortmund, September 2012, keynote oral

4. “Interlayer exciton mediated second harmonic generation control in transition metal dichalcogenides” MRS Fall Meeting 2021, Boston, US, oral

5. “Engineering Quantum States in Layered Semiconductors” MRS Spring Meeting 2022, Honolulu, US, May 2022, invited oral

This research proposal aims at exploring novel semiconductor nanostructures hosting indirect excitons (IXs) and emerging collective states that IXs may form. We target IX fluids in two types of materials: GaN and Transition Metal Dichalcogenides (TMD). The dipole moment of IXs in these materials can be engineered on an atomic scale, whereas their large binding energy allows for relatively high quantum degeneracy temperature. This enriches the collective phase diagram, as compared to atomic systems and GaAs. IX with tunable dipole moments, adjustable light-matter coupling and transport properties can be engineered with various types of TMDs and in combination with GaN. The consortium represents critical mass of the human potential, expertise in indirect exciton physics, nitrides and TMDs, as well as growth, fabrication and spectroscopy facilities required to be internationally leading and attain the main goal of the project: uncover collective phases by controlling novel IXs species.