Abstract
Ultrafast carrier dynamics, including the carrier photoexcitation and relaxation processes, plays an essential role in improving the performance of molybdenum disulfide (MoS2)-based optoelectronic devices. Herein, we investigate the photo-generated carrier dynamics in layered MoS2 crystal using a time-resolved terahertz (THz) spectroscopy. We have analyzed the ultrafast changes of the THz complex photoconductivity deduced from the peak and zero-crossing of THz waveforms. The decay time of the real part of the THz photoconductivity in layered MoS2 crystal is independent with the pump power, while the imaginary part increases with the pump power. We attribute the decay time of the real part to the carrier recombination process via phonon-assistance and the decay time of the imaginary part to the defect-assisted exciton recombination. The peak values of the complex photoconductivity show a trend of saturation with the increase of the pump power because of the many-body effect at high carrier concentration. This work deepens the understanding of the basic ultrafast physical process in MoS2 crystal, which is enlightening for the design of novel optoelectronic devices.
Highlights
Transition metal dichalcogenides (TMDs) are burgeoning layered semiconductors with a chemical formula of MX2 (M represents transition metal elements, including Ti, V, Ta, Mo, W, and Re; X represents chalcogenide atoms, such as S, Se, and Te), in which the van der Waals force connects atomic sheets
Compared with the detection techniques that investigate the photoexcited carrier properties in a static state, such as photocurrent spectroscopy [12], photoluminescence spectroscopy [13], and electroluminescence spectroscopy [14], transient absorption spectroscopy based on the optical pumpprobe technology is indispensable for studying the ultrafast carrier dynamics mechanisms [15]
The saturation pump power Ps of the real and imaginary photoconductivity are 3.6 and 5.7 mW, respectively. These results suggest that both photo-induced carriers and excitons are generated before the pump power of 3.6 mW; the photo-induced carriers are saturated and excitons are continuously generated before 5.7 mW; at last, both the carriers and excitons become saturated due to the possible many-body effect at high carrier concentration [30, 31]
Summary
Transition metal dichalcogenides (TMDs) are burgeoning layered semiconductors with a chemical formula of MX2 (M represents transition metal elements, including Ti, V, Ta, Mo, W, and Re; X represents chalcogenide atoms, such as S, Se, and Te), in which the van der Waals force connects atomic sheets. Sood et al have studied the dynamics of photoexcited carriers in a few-layered MoS2 using OPTP spectroscopy [19] They find that the fast relaxation time occurs due to the capture of electrons and holes by defects, and the slow relaxation time is related to bounded excitons which prevent the defect-assisted Auger recombination. The real part-related time constant τ1 of ∼80 ps is independent of the pump power, while the imaginary part-related time constant τ2 increases from 110 to 260 ps as the pump power increases The former is explained by the phonon-assisted carrier recombination process and the latter is induced by the defect-assisted exciton recombination. With the increase of pump power, the peak values of the real and imaginary parts of the complex conductivity exhibit a trend of saturation, which is attributed to the many-body effect These results deepen the understanding of carrier dynamics in MoS2 crystals. All experiments were measured in a nitrogen environment to avoid THz absorption by atmospheric water vapor
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