Unprecedented transport properties of monolayer TMD devices: Experiment and theory

DIPC Seminars

Kristen Kaasbjerg, Technical University of Denmark
Donostia International Physics Center
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Unprecedented transport properties of monolayer TMD devices: Experiment and theory Experimental low-temperature transport in monolayer transition metal dichalcogenides (TMDs; MX2) is – like in conventional semiconductor heterostructure based 2DEGs – typically found to be limited by Coulomb disorder scattering by, e.g., charged substrate impurities with mobilities not exceeding μ~5000 cm2/(V.s) [1]. Here we demonstrate unprecedented transport properties in TMD devices based on p-type monolayer WSe2 showing record-high low-temperature mobilities as high as μ~25.000 cm2/(V.s) [2]. The mobility surprisingly decreases with the carrier density n, which is not in accordance with charged impurity scattering for which a μ~nα scaling with α>0 is anticipated [3]. Using an atomistic density-functional based method for modeling scattering by realistic atomic-scale defects with the T-matrix formalism, we investigate the effect of point defects on (i) quasiparticle scattering and spectral linewidths, (ii) midgap states, and (iii) transport in 2D TMDs. We demonstrate that the observed density dependence of the mobility is consistent with short-range disorder scattering off point defects, such as, e.g., atomic vacancies. However, as we have recently argued [3], vacancies in 2D TMDs may act as combined Coulomb and short-range scatterers due to filling of their associated midgap states upon doping. To exclude the existence of the former, we show that atomic-vacancy disorder in WSe2 – in contrast to many other TMDs – does not give rise to shallow filled midgap states above the valence-band edge, and does therefore not give rise to Coulomb disorder scattering for p-type doping. This points to strongly material and defect dependent (i.e., existence of filled/empty midgap state) as well as doping (n vs p) dependent transport properties monolayer TMD devices. Furthermore, our results points to a concomitant break down of the widely used Born approximation which severely overestimates the effect of vacancies on carrier scattering and yields a μ~n0 behavior for the mobility in 2D TMDs systems. In conclusion, our combined experimental and theoretical study has demonstrated unprecedented transport properties in monolayer TMD devices with record-high mobilities limited by short-range disorder scattering. This indicates extremely clean TMD monolayers with defect densities as low as 1011 cm-2 as well as high-quality vdW heterostructure devices free of residual charged impurity scatterers. [1] B. W. H. Baugher et al., Nano. Lett. 13, 4212 (2013); H. Schmidt et al., Nano. Lett. 14, 1909 (2014); X. Cui et al., Nature Nano. 10, 534 (2015); X. Cui et al., Nano. Lett. 17, 4781 (2017). [2] P. Kim et al., unpublished. [3] K. Kaasbjerg, T. Low, and A.-P. Jauho, arXiv:1612.00469v1.