Accelerated Carrier Recombination by Grain Boundary/Edge Defects in MBE Grown Transition Metal Dichalcogenides

TL;DR

This study reveals that grain boundary and edge defects in MBE-grown transition metal dichalcogenides significantly accelerate carrier recombination, impacting their potential for high-speed optoelectronic applications.

Contribution

It demonstrates that grain boundary/edge defects in MBE-grown TMDs serve as efficient recombination centers, drastically increasing carrier recombination rates compared to exfoliated samples.

Findings

Carrier recombination rate in MBE-grown samples is 50 times faster.

Presence of grain boundary/edge defects correlates with increased recombination.

Coherent acoustic phonons observed, indicating strong electron-phonon coupling.

Abstract

Defect-carrier interaction in transition metal dichalcogenides (TMDs) play important roles in carrier relaxation dynamics and carrier transport, which determines the performance of electronic devices. With femtosecond laser time-resolved spectroscopy, we investigated the effect of grain boundary/edge defects on the ultrafast dynamics of photoexcited carrier in MBE grown MoTe2 and MoSe2. We found that, comparing with exfoliated samples, carrier recombination rate in MBE grown samples accelerates by about 50 times. We attribute this striking difference to the existence of abundant grain boundary/edge defects in MBE grown samples, which can serve as effective recombination centers for the photoexcited carriers. We also observed coherent acoustic phonons in both exfoliated and MBE grown MoTe2, indicating strong electron-phonon coupling in this materials. Our measured sound velocity agrees well with previously reported result of theoretical calculation. Our findings provide useful reference for the fundamental parameters: carrier lifetime and sound velocity, reveal the undiscovered carrier recombination effect of grain boundary/edge defects, both of which will facilitate the defect engineering in TMD materials for high speed opto-electronics.