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Open Access Publications from the University of California

Vertical Tunneling Hot Carrier Transport in 2D van der Waals Material-Based Devices for Optoelectronic Applications

  • Author(s): Torres Jr., Carlos Manuel
  • Advisor(s): Wang, Kang L.
  • et al.

For over half a century, Moore’s law has driven the silicon electronics industry towards smaller and faster transistors. However, as the scaling limit of silicon Complementary Metal-Oxide-Semiconductor (CMOS) technology draws to an end, novel materials and device concepts have been eagerly sought out and investigated with hopes to augment the next generation of information processing. In this dissertation, I introduce a new paradigm of device concepts based on a vertical tunneling transistor structure incorporating individual 2D van der Waals (2D vdW) materials, such as graphene and MoS2, in the active region. The essential physics relies upon the injection of non-equilibrium, or hot, electrons via the quantum tunneling process through a vertical heterostructure.

For the electronics aspect, we demonstrate 2D vdW material-based ambipolar hot carrier transistors (2D vdW-AHCTs), in which the injected hot electrons traverse vertically through the 2D vdW material in the base region and either reach the collector or back-scatter into the base region. For the optoelectronics aspect, the hot electrons are injected into the conduction band of the specific 2D vdW material in the base region where they relax and emit photons via hot carrier luminescence. Furthermore, it will be shown that the application of a top-gate voltage offers several functionalities. In the case of the 2D vdW-AHCTs, the top-gate/collector voltage controls the filter barrier height at the collector-base oxide.

We discovered that by choosing MoS2 and HfO2 for the filter barrier interface in addition to implementing a non-crystalline semiconductor such as ITO for the collector electrode, allows for the simultaneous emergence of ambipolar transport, an unprecedentedly high and voltage-tunable current gain (α ~ 0.95, β > 15), and a voltage-tunable recombination current in the base region of MoS2. Depending upon the collector electrode’s bias polarity, either a hot electron mode of operation or a hole mode of operation dominates the transport mechanism of the 2D vdW-AHCTs. In the case of the 2D vdW wavelength-agile light-emitting transistors (2D vdW-LETs), the top-gate voltage can tune the wavelength of the emitted photons via the band-filling effect in the Graphene Broadband Infrared Light-Emitting Transistor (GBILET) or by tuning the direct bandgap of monolayer MoS2 in the MoS2 Visible Light-Emitting Transistor (MoS2-VLET) with the application of a perpendicular electric field.

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