Unprecedented transport properties of monolayer TMD devices: Experiment and theory
DIPC Seminars
- Speaker
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Kristen Kaasbjerg, Technical University of Denmark
- When
-
2018/09/14
14:00 - Place
- Donostia International Physics Center
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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.