Ultrafast Exciton Dynamics in Few-Layer MoTe₂ near the Direct-Indirect Bandgap Transition

Transition metal dichalcogenides (TMDs) are of interest for a variety of material applications ranging from optoelectronics to quantum computing. The 2H semiconducting phase of MoTe₂ is promising as a material for optical devices, such as photodiodes and photovoltaics. The photophysics of such films as their thickness approaches the direct-to-indirect bandgap transition remains underexplored. Leveraging ultrathin film MoTe₂ samples fabricated through a combination of atomic layer deposition (ALD) and chemical vapor deposition (CVD), time-resolved optical spectroscopy was employed to quantify charge carrier kinetics as a function of sample thickness. Samples with thicknesses ranging from several monolayers down to a bilayer were examined. A model mechanism is proposed that includes the fast relaxation and rapid formation of an excitonic state. The excitonic state decays through a combination of thermal relaxation through faster bulk defect trap states and slower surface trap states. The surface trapping decay slowed as the sample thickness increased. The results indicate that both bulk and surface trapping play important roles in carrier lifetimes. Consideration for and control of defect states at interfaces has implications for charge injection and transport across interfaces and is critical for implementing MoTe₂ layers into heterojunction optical devices.