Exploring carrier dynamics in van-der-Waals heterostructures with shot noise spectroscopy, thermoelectricity and opto-electronic study

Abstract

In the last two decades, there has been tremendous progress in two-dimensional (2D) - material research. It is mainly due to the availability of a large selection of 2D-materials and their van-der-Waals heterostructures. The 2D-material based heterostructures, in particular, have significant fundamental and technological importance due to their tunable electrical and optical properties. In this thesis, we have studied three types of 2D-heterostructures: graphene pn junction, magic-angle twisted bilayer graphene, and MoTe2-MoS2 based hetero-pn junction. We investigate the carrier transport and its dynamics using cutting-edge probes like shot noise spectroscopy, thermoelectricity, and optoelectronic studies.

In the first part of the thesis, we will present our research on equilibration dynamics of quantum Hall (QH) edge states in graphene pn junction (PNJ). Graphene pn junction with co-propagating spin-valley polarized QH edges is a promising platform for studying electron interferometry. Though several conductance measurements have been attempted for such PNJs, the edge dynamics of the spin-valley symmetry broken edge states remain unexplored. In this study, we present the measurements of conductance together with shot noise, an ideal tool to unravel the dynamics, in dual graphite gated hexagonal boron nitride encapsulated high mobility graphene devices. The conductance data show that the symmetry broken QH edges at the PNJ follow spin selective equilibration. The shot noise results as a function of both p and n side filling factors reveal the unique dependence of the scattering mechanism. Remarkably, the scattering is found to be fully tunable from incoherent to a coherent regime with the increasing number of QH edges at the PNJ, shedding crucial insights into the velocity-dependent phase coherence of the QH edges at a pn junction. Furthermore, we study the bias-dependent tunneling between the ν = ±1 QH edges of the pn junction. At zero bias, we observe finite tunneling (t ∼ 0.5), which remains almost constant up to a bias energy of few hundreds of µV . The tunneling sharply falls with further increasing bias voltage and exhibits repeated smaller peaks at discrete energies before completely vanishing. The tunneling at zero bias is anomalous as it is expected to be zero between ν = ±1 edges due to their orthogonal spin polarization. Thus, from tunneling to fully blockade regime suggests bias-controlled switching of spin polarization at graphene pn junction.

In the second part we will discuss the interaction driven resetting of the band-structure of a Magic-angle twisted bilayer graphene (MtBLG) using thermoelectricity as a probe. MtBLG has proven to be an extremely promising new platform to realize and study a host of emergent quantum phases arising from the strong correlations in its narrow bandwidth flat band. In this regard, thermal transport phenomena like thermopower, in addition to being coveted technologically, is also sensitive to the particle-hole (PH) asymmetry, making it a crucial tool to probe the underlying electronic structure of this material. In this study, we have carried out thermopower measurements of MtBLG as a function of carrier density, temperature and magnetic field, and report the observation of an unusually large thermopower reaching up to a value as high as ∼ 100µV/K at a low temperature of 1K. Surprisingly, our observed thermopower exhibits peak-like features in close correspondence to the resistance peaks around the integer Moire fillings, including the Dirac Point, violates the Mott formula. We show that the large thermopower peaks and their associated behaviour arise from the emergent highly PH asymmetric electronic structure due to the cascade of Dirac revivals.

In the last part, we will present the optoelectronic properties of MoTe2-MoS2 hetero pn junction. MoS2 and MoTe2 are, respectively, n-type and p-type transition metal di-chalcogenides with band-gaps of ∼ 1.8 eV and 0.8 eV. Over and above the antiambipolar transfer characteristics observed similar to other hetero pn junction, our experiments reveal a unique feature as a dip in the transconductance near the maximum. We further observe that the modulation of the dip in the transconductance depends on the doping concentration of the 2D flakes and also on the power density of the incident light. We also demonstrate high photo-responsivity of ∼ 105A/W at room temperature for a forward bias of 1.5V. We explain these new findings based on interlayer recombination rate-dependent semi-classical transport model.