Abstract
We have performed systematic investigations of transport through graphene on hexagonal boron nitride (hBN) substrates, together with confocal Raman measurements and a targeted theoretical analysis, to identify the dominant source of disorder in this system. Low-temperature transport measurements on many devices reveal a clear correlation between the carrier mobility $\mu$ and the width $n^*$ of the resistance peak around charge neutrality, demonstrating that charge scattering and density inhomogeneities originate from the same microscopic mechanism. The study of weak-localization unambiguously shows that this mechanism is associated to a long-ranged disorder potential, and provides clear indications that random pseudo-magnetic fields due to strain are the dominant scattering source. Spatially resolved Raman spectroscopy measurements confirm the role of local strain fluctuations, since the line-width of the Raman 2D-peak --containing information of local strain fluctuations present in graphene-- correlates with the value of maximum observed mobility. The importance of strain is corroborated by a theoretical analysis of the relation between $\mu$ and $n^*$ that shows how local strain fluctuations reproduce the experimental data at a quantitative level, with $n^*$ being determined by the scalar deformation potential and $\mu$ by the random pseudo-magnetic field (consistently with the conclusion drawn from the analysis of weak-localization). Throughout our study, we compare the behavior of devices on hBN substrates to that of devices on SiO$_2$ and SrTiO$_3$, and find that all conclusions drawn for the case of hBN are compatible with the observations made on these other materials. These observations suggest that random strain fluctuations are the dominant source of disorder for high-quality graphene on many different substrates, and not only on hexagonal boron nitride.
Highlights
Hexagonal boron nitride substrates enable the fabrication of graphene devices [1,2,3,4], exhibiting extremely high carrier mobility values, and leading to the observation of new, interesting physical phenomena [5,6,7,8,9]
The importance of strain is corroborated by a theoretical analysis of the relation between μ and nà that shows how local strain fluctuations reproduce the experimental data at a quantitative level, with nà being determined by the scalar deformation potential and μ by the random pseudomagnetic field
Whereas most of our work has focused on graphene-on-hexagonal boron nitride (hBN) devices, we have looked at devices on SiO2 and SrTiO3 substrates and found that the observations made on these devices are fully compatible with the conclusions drawn for hBN, which points to the relevance of random strain fluctuations under rather broad experimental conditions for high-quality graphene devices on different substrates
Summary
Hexagonal boron nitride (hBN) substrates enable the fabrication of graphene devices [1,2,3,4], exhibiting extremely high carrier mobility values, and leading to the observation of new, interesting physical phenomena [5,6,7,8,9]. For all charge carrier densities, τiv is much longer than τ, the elastic scattering time extracted from the carrier mobility This finding directly establishes that the mobility is limited by intravalley scattering caused by long-ranged potentials, confining the possible microscopic mechanisms to charged impurities and random strain fluctuations in the graphene lattice. We directly probe local strain fluctuations with confocal Raman experiments [21] and show experimentally that larger strain fluctuations limit the maximum mobility that can be observed in transport measurements Based on this evidence, we analyze theoretically the linear relation between 1=μ and n×which was previously observed in devices exposed to potassium atoms and taken to be an indication of charge impurity scattering—and show that such a relation can be explained quantitatively invoking random strain fluctuations only. Whereas most of our work has focused on graphene-on-hBN devices, we have looked at devices on SiO2 and SrTiO3 substrates and found that the observations made on these devices are fully compatible with the conclusions drawn for hBN, which points to the relevance of random strain fluctuations under rather broad experimental conditions for high-quality graphene devices on different substrates
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