Identifying suitable substrates for high-quality graphene-based heterostructures

Abstract

Abstract:We report on a scanning confocal Raman spectroscopy study investigating the strain-uniformity and the overall strain and doping of high-quality chemical vapour deposited (CVD) graphenebased heterostuctures on a large number of different substrate materials, including hexagonal boron nitride (hBN), transition metal dichalcogenides, silicon, different oxides and nitrides, as well as polymers. By applying a hBN-assisted, contamination free, dry transfer process for CVD graphene, high-quality heterostructures with low doping densities and low strain variations are assembled. The Raman spectra of these pristine heterostructures are sensitive to substrate-induced doping and strain variations and are thus used to probe the suitability of the substrate material for potential high-quality graphene devices. We find that the flatness of the substrate material is a key figure for gaining, or preserving high-quality graphene. IntroductionFor over a decade, graphene has been in the spotlight of solid state research. Its high charge carrier mobilities 1-4 and long spin diffusion lengths, 5,6 as well as its optical 7 and mechanical properties 8 promise a wide range of applications ranging from spintronics 9 to high frequency electronics, 10 ultra-sensitive sensors 11,12 and flexible optoelectronics. 13 In order to advance prototype devices to true applications, large effort has been put into growth 14-18 and contamination-free transfer 3,4,19,20 of high quality graphene based on chemical vapour deposition. However, as graphene and other two-dimensional (2d) materials consist only of surface atoms, the choice of substrate material has a large influence on their structural and electronic properties. 2,[21][22][23][24][25] In this work, we investigate strain, doping and the strain uniformity of high quality CVD graphene/hBN heterostructures placed on different substrate materials. Here, we follow a recently reported, contamination free, dry transfer process, where exfoliated hBN is used to pick up CVD graphene directly from the growth substrate. The obtained stack is subsequently placed on different target substrates. 4 This fabrication process yields high quality heterostructures with little intrinsic overall doping and low nanometre-scale strain variations. As the graphene crystal is covered, i.e. protected by hBN on the top side, modifications in doping and strain are purely due to the substrate at the bottom side of graphene, making our approach suitable for benchmarking the substrate suitability.

Mobility Enhancement in Graphene by in situ Reduction of Random Strain Fluctuations

“…Although the hBN surface is typically much flatter, height fluctuations are still present in hBN-supported graphene devices [13], which can result in RSFs. These RSFs have been confirmed in Raman spectroscopy measurements [40,41]. Here we demonstrate in a direct experiment that RSFs can be the mechanism limiting the mobility of encapsulated devices.…”

Suppression of intrinsic roughness in encapsulated graphene

“…Using Raman spectroscopy on separately prepared samples, we estimate the strain in our samples to be tensile with a value between 0.08% and 0.22%, assuming purely biaxial and uniaxial strain, respectively (see Supplemental Material, Fig. S1 [24]), using published methods [45,46].…”

Magnetotransport in heterostructures of transition metal dichalcogenides and graphene

“…Bringing graphene into the proximity of transition metal dichalcogenides (TMDCs) has been predicted theoretically [6,7] and observed experimentally [8][9][10][11] to increase SOC in graphene. Furthermore, transport measurements [12] and recent Raman measurements indicate the suitability of these substrates for high mobility graphene [13]. This is in contrast to previously explored methods for increasing SOC in graphene, such as hydrogenation [14,15], fluorination [16], or the attachment of heavy atoms [17,18], as these methods have the disadvantage of increasing the scattering and therefore decreasing the mobility of graphene.…”