  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.
