Scanning Probe microscopy of van der Waals heterostructures and non-equilibrium magnetotransport in graphene

Abstract

Graphene is a two-dimensional semimetal that has linear dispersion in energy-momentum space. When graphene is subjected to a perpendicular magnetic field, the dispersion is no longer linear, resulting in discrete energy levels because of the formation of cyclotron orbits of different energies. This energy discretization leads to quantum oscillations in longitudinal magnetoresistance known as Shubhnikov de-Haas oscillations which provide a plethora of properties, including the effective mass of charge carriers and topological properties like Berry phase. Furthermore, the transverse resistance in the magnetic field is quantized, making it useful for resistance metrology. The quantization effects have been realized in graphene in the ohmic regime, i.e., with a small current density < 0.01 A/m passing through the channel. Non-equilibrium magnetotransport studies in two-dimensional electron gas systems based on GaAs-AlGaAs quantum wells have been intensively investigated under high current densities, demonstrating the effect of carrier heating, magnetophonon-oscillations, and Hall field-induced magneto-oscillations in longitudinal resistance. However, the effect of high current densities on magnetotransport in graphene has not been thoroughly investigated. In this thesis, we have explored the magnetotransport in graphene Hall bar devices under non-equilibrium conditions by introducing a high current density (> 1 A/m) through the channel, which produces a strong Hall field across the channel and results in tilting of the Landau levels. For the experiments aimed at realizing electron transitions between two cyclotron orbits in the presence of a magnetic field, the width of the channel becomes crucial. We have fabricated large-width Hall bar devices, which ensures the number of cyclotron orbits in the bulk is significant, and edge scattering will have less contribution, making it more sensitive to magnetotransport in the bulk of the channel. Making extra-large width devices becomes a significant step that requires a sizeable clean xii area of graphene to ensure high mobility. The dry pick-up and transfer method to fabricate hexagonal boron nitride (hBN)- encapsulated graphene is a standard technique to achieve high-quality devices. Sandwiching graphene between hBN often leads to folding, wrinkling, and the formation of air pockets between hBN and graphene, which limit sample quality. There fore, it becomes essential to identify the geometrical extent of clean graphene. Here we have developed a non-invasive sub-surface electrical scanning probe technique to identify a clean and significant area of graphene encapsulated by 20-30 nm thick hBN. We have used Electrostatic Force Microscopy (EFM) to identify the region of interest. This method reveals the effect of substrate and ambient environment on the doping of graphene. We have conducted elaborate measurements on various encapsulated layered materials and observed that the EFM phase acts as a clear fingerprint of the constituent layered materials in complex heterostructures involving graphene, hBN, and transition metal dichalcogenides. In addition to providing visually striking images of buried layers, the technique is also useful in probing the electrical properties of the constituent layers. We have extended the technique to other van der Waals heterostructures of transition metal dichalcogenides such as MoS2 and WSe2 encapsulated in hBN. We expect our findings to advance reliable and high throughput device architectures for various nano and optoelectronics applications. To explore the non-equilibrium transport properties in graphene we have exploited the EFM technique to identify the homogeneous and residue-free region of graphene encapsulated by hBN. Hall bars with device widths ranging between 12 µm to 18 µm were made to investigate the non-equilibrium magnetotransport. In addition to expected carrier heating effects, we observe two branches of novel magnetoresistance oscillations near the charge neutrality point when plotted as a function of carrier density and dc current at magnetic field ranging between 1 T to 5 T. These oscillations show linear dispersion as a function of dc current and carrier density. The drift velocity of carriers associated with dispersion matches well with the TA, and LA phonon modes in graphene, indicating phonon-assisted intra-Landau level transitions aid in these oscillations. The novelty of these results are expected to stimulate further studies that can help unravel a unified picture of the various resonant processes in this regime, not only in graphene but also in related Moiré heterostructures