High intrinsic mobility and ultrafast carrier dynamics in multilayer metal-dichalcogenideMoS2

Abstract

The ultimate limitations on carrier mobilities in metal dichalcogenides, and the dynamics associated with carrier relaxation, are unclear. We present measurements of the frequency-dependent conductivity of multilayer dichalcogenide MoS2 by optical-pump terahertz-probe spectroscopy. We find mobilities in this material approaching 4200 cm 2 V −1 s −1 at low temperatures. The temperature dependence of scattering indicates that the mobility, an order of magnitude larger than previously reported for MoS2, is intrinsically limited by acoustic phonon scattering at THz frequencies. Our measurements of carrier relaxation reveal picosecond cooling times followed by recombination lasting tens of nanoseconds and dominated by Auger scattering into defects. Our results provide a useful context in which to understand and evaluate the performance of MoS2-based electronic and optoelectronic devices.Layered two-dimensional transition metal dichalcogenides have recently enjoyed a resurgence of interest from the scientific community both from a new science perspective and also for novel applications [1][2][3][4][5][6][7][8][9][10]. In contrast to graphene, metal dichalcogenides have non-zero bandgaps and are also efficient light emitters, making them attractive for electronics and optoelectronics [1][2][3][4][5][6][7][8][9][10]. The ability to synthesize atomically thin semiconducting crystals and their heterostructures and transfer them to arbitrary substrates has opened the possibility of transparent, flexible electronics and optoelectronics based on these material systems [10][11][12][13][14][15][16][17]. The best reported carrier mobilities in metal dichalcogenides, typically in the few hundred cm 2 V −1 s −1 range for MoS 2 [18,19], are not as large as in graphene. But the reported mobilities are large in comparison to those of organic materials often used for flexible electronics [1]. Physical mechanisms limiting the mobility of monolayer and multilayer MoS 2 field effect transistors have been the subject of several experimental [18][19][20][21][22][23][24] and theoretical [25,26] investigations. Charged impurity scattering, electron-phonon interaction, and screening by the surrounding dielectric environment are all believed to affect the mobility. Due to the challenge of isolating these effects, the intrinsic mobility of MoS 2 , and the ultimate performance of electronic devices, remains unclear. Similarly, carrier intraband and interband scattering and relaxation rates, which determine the performance of almost all proposed and demonstrated electronic and optoelectronic metal dichalcogenide devices, remain poorly understood.In this report, we present optical-pump THz-probe measurements of the time-and frequency-dependent conductivity of multilayer MoS 2 . Previously reported electrical measurements [18,19,21,22] have probed the MoS 2 DC carrier transport, and all-optical measurements [24,27] have probed the exciton temporal dynamics. In contrast, optical-pump THz-probe spectroscopy can measure the time development of the co...

Surface Recombination Limited Lifetimes of Photoexcited Carriers in Few-Layer Transition Metal Dichalcogenide MoS2

“…Exfoliated samples were characterized electrically and optically, using transmission/reflection spectroscopies, to determine the electronic and optical conductivities 18 . The monolayer samples were found to be moderately ndoped (> 10 12 1/cm 2 ) and the multilayer samples were found to be lightly n-doped (2-4×10 15 1/cm 3 ), consistent with previously reported results 10,12,18,44,45 . The thickness of the samples was measured by AFM to determine the number of layers 1 .…”

“…The refractive index response is given by the changes in the imaginary part of the optical conductivity which can affect the probe transmission through the term containing the product processes in which carrier capture by defects occurs via Auger scattering have been presented by the authors in previous works 12,18,25 , and used successfully to model the carrier recombination dynamics in monolayer and bulk MoS 2 samples 12,18 . A prominent feature of Auger scattering is recombination times that are independent of the temperature but depend on the carrier density (or the pump fluence).…”

“…Recently, defect-assisted electron-hole recombination was proposed as the dominant non-radiative recombination mechanism in monolayer and bulk MoS 2 12,18,25,35 . The carrier density and the temperature dependence of the observed recombination dynamics suggested that photoexcited carriers are captured by defects via Auger processes 12,18,25 . It is reasonable to expect that surface layers of few-layer TMDs have far more defects and impurities than the inner layers and, consequently, if defect-assisted processes are indeed responsible for electron-hole recombination in TMDs, then the recombination time would scale in some meaningful way with the number of layers in few-layer TMDs.…”

“…Indirect bandgap few layer and bulk TMDs are well suited for high quantum efficiency photodetection applications because of their large optical absorption coefficients and long photoexcited carrier lifetimes [12][13][14] . Few layer TMDs and their heterojunctions are promising for flexible photovoltaic and solar cell devices with thicknessadjustable bandgaps [14][15][16][17] .…”

Gate-Controllable Magneto-optic Kerr Effect in Layered Collinear Antiferromagnets

“…Note that due to the oscillatory behavior of σ xy (ω), the size of the Kerr angle has a strong dependence on the smearing parameter, and can be made larger in high-quality samples [23]. The smearing parameter η = 0.1 eV chosen here corresponds to a carrier relaxation time of 6.5 fs, which is in the realistic range for layered transition metal chalcogenides [31]. The generation of the MOKE in a magneto-optically inactive material using gate voltage is an important distinction from previous work [1].…”