Epitaxial Single-Layer MoS on GaN with Enhanced Valley Helicity.

Engineering the substrate of 2D transition metal dichalcogenides can couple the quasiparticle interaction between the 2D material and substrate, providing an additional route to realize conceptual quantum phenomena and novel device functionalities, such as realization of a 12-time increased valley spitting in single-layer WSe through the interfacial magnetic exchange field from a ferromagnetic EuS substrate, and band-to-band tunnel field-effect transistors with a subthreshold swing below 60 mV dec at room temperature based on bilayer n-MoS and heavily doped p-germanium, etc. Here, it is demonstrated that epitaxially grown single-layer MoS on a lattice-matched GaN substrate, possessing a type-I band alignment, exhibits strong substrate-induced interactions. The phonons in GaN quickly dissipate the energy of photogenerated carriers through electron-phonon interaction, resulting in a short exciton lifetime in the MoS /GaN heterostructure. This interaction enables an enhanced valley helicity at room temperature (0.33 ± 0.05) observed in both steady-state and time-resolved circularly polarized photoluminescence measurements. The findings highlight the importance of substrate engineering for modulating the intrinsic valley carriers in ultrathin 2D materials and potentially open new paths for valleytronics and valley-optoelectronic device applications.