Optical properties of Fermi polarons in a GaInP/MoSe2 monolayer heterostructure

Engineering optical properties, such as luminescence purity and charge transfer, is crucial for harnessing the application potential of atomically thin transition metal dichalcogenides (TMDCs). While electrostatic gating is widely applied to gain charge control in TMDC monolayers, charge transfer can also be engineered via coupling of TMDC monolayers at semiconductor III/V, organic, or van der Waals interfaces. This confers great advantages, such as ease in implementation and compatibility in device integration. Here, we shed light on the optical properties of many-particle complexes emerging at the GaInP/MoSe2 interface as a highly relevant material combination to manipulate the optical properties of TMDCs in integrated photonic devices. Our study verifies its nature as a type II hetero-interface, which bears the feasibility to display disorder-free photoluminescence. Through optical absorption measurements, we verify that the charged complexes acquire substantial oscillator strength. Furthermore, temperature-dependent photoluminescence, supported by a microscopic theory framework, evidences the suppression of the characteristic carrier recoil effect that was previously observed in the photoluminescence of trions in TMDCs. These phenomena allow us to identify the optical signatures at the TMDC-GaInP interface as Fermi polaron quasiparticle resonances, which are of high importance in researching Bose-Fermi mixtures in condensed matter systems.

💡 Research Summary

In this work the authors investigate the optical and electronic properties of a heterostructure formed by a monolayer of MoSe₂ placed on an epitaxially grown GaInP substrate, a representative III‑V semiconductor. Using low‑temperature scanning tunneling microscopy and differential conductance spectroscopy, they establish that the interface exhibits a type‑II band alignment: the conduction‑band minimum resides in MoSe₂ while the valence‑band maximum is located in GaInP. This alignment drives efficient electron transfer from the substrate into the transition‑metal dichalcogenide, resulting in heavy n‑type doping of the MoSe₂ layer.

Photoluminescence (PL) measurements on three configurations—(i) MoSe₂ on SiO₂, (ii) MoSe₂ directly on GaInP, and (iii) the same GaInP‑supported MoSe₂ capped with a thin hBN layer—reveal striking differences. On SiO₂ the spectrum shows both neutral exciton (X) and trion (X⁻) peaks, with the trion dominating the emission. The trion line exhibits an asymmetric shape caused by the well‑known electron recoil effect. In contrast, on GaInP the neutral exciton is essentially quenched, leaving only a strong trion‑like feature that is well described by a Gaussian profile with a full‑width at half‑maximum (FWHM) of about 8 meV, indicating a dramatic reduction of inhomogeneous broadening. Adding an hBN encapsulation further narrows the line to ≈3.8 meV and transforms the lineshape into a Lorentzian, signifying a highly homogeneous environment.

Statistical analysis of linewidths across many spots confirms that GaInP provides a smoother, less disordered surface than SiO₂, while hBN encapsulation yields the narrowest and most uniform emission. The authors attribute this to the superior crystalline quality of the epitaxial GaInP and its more uniform dielectric constant, which suppresses charge puddling and trap‑induced disorder.

White‑light reflectivity measurements complement the PL data. While SiO₂‑supported MoSe₂/hBN shows a strong exciton absorption but a very weak trion absorption, the GaInP‑supported heterostructure displays two pronounced absorption dips. The lower‑energy dip matches the PL emission and is identified as an attractive Fermi polaron (AP), whereas the higher‑energy dip corresponds to a repulsive Fermi polaron (RP). The substantial oscillator strength of both features demonstrates that the charged complexes are not simple trions but many‑body quasiparticles formed by excitons interacting with a Fermi sea of electrons.

Temperature‑dependent PL experiments, together with a microscopic many‑body theory, reveal that the characteristic carrier‑recoil asymmetry observed for trions on SiO₂ disappears for the GaInP‑supported system. The theory, which treats excitons as impurities immersed in a two‑dimensional electron gas, reproduces the evolution of the RP and AP peaks with temperature for carrier densities ranging from 5 × 10¹⁰ cm⁻² to 7 × 10¹¹ cm⁻². The suppression of recoil is interpreted as a consequence of the high electron density: the exciton is dressed by many electrons, forming a stable polaron whose emission is symmetric and whose energy splitting reflects the underlying attractive or repulsive interaction with the Fermi sea.

Overall, the paper establishes four key findings: (1) a type‑II band alignment enabling efficient charge transfer from GaInP to MoSe₂; (2) the conversion of trion‑like emission into well‑defined attractive and repulsive Fermi polarons with strong optical oscillator strength; (3) the ability to achieve disorder‑free, narrow‑linewidth photoluminescence through the use of a high‑quality III‑V substrate and hBN encapsulation; and (4) the temperature‑controlled suppression of the electron‑recoil effect, confirming the many‑body nature of the observed quasiparticles. These results highlight the potential of III‑V/TMDC hybrid platforms for integrated photonic devices, polaron‑based quantum sensors, and the exploration of Bose‑Fermi mixtures in solid‑state systems.